Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 2 |
Subtitle: The Power Supply Unit - From Wall Power to Clean DC |
Introductory Summary (Extended Section 2 Preview) |
Every barcode printer starts with a simple act: you plug it into the wall. But what happens between that wall outlet and the delicate logic chips inside is nothing short of electrical alchemy. The power supply unit, or PSU, is the unsung hero of the printer. It takes the chaotic, noisy, high-voltage alternating current from your mains and transforms it into smooth, stable, low-voltage direct current that the electronics can actually use. This chapter pulls back the curtain on that transformation. We will explore why a printer needs multiple voltages, how switching power supplies work in plain language, and why efficiency matters not just for your electricity bill but also for the printer's reliability. We will look at real-world designs from major companies: how Mean Well builds rugged off-the-shelf supplies for industrial printers, how Texas Instruments integrates everything into a tiny chip for portable models, how CUI Inc. addresses the challenge of worldwide voltage compatibility, and how Delta Electronics tackles electromagnetic interference at the source. We will also examine the critical role of bulk capacitors, the trade-offs between linear and switching regulators, and the safety features that protect both the printer and the user. By the end, you will never look at a power brick the same way again - and you will understand why a poorly designed power supply can ruin an otherwise perfect printer design. |
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Chapter 1: Why a Printer Needs More Than One Voltage |
If you open up any barcode printer, you will find not one but several voltage rails running through the circuit board. The most obvious is the high-voltage rail - typically 24 volts DC - that powers the thermal printhead and the stepper motors. The printhead needs this high voltage because it must push a large current through tiny resistors to generate heat quickly; lower voltage would mean longer heating times and slower printing. The motors also benefit from higher voltage because it allows them to spin faster and with more torque. But the logic circuits - the CPU, memory, sensors, and communication chips - cannot survive 24 volts. They operate at 5 volts, 3.3 volts, or even 1.8 volts. Feeding them 24 volts would instantly fry them. So the power supply must produce multiple outputs: one or more high-voltage outputs for the power-hungry actuators, and one or more low-voltage outputs for the delicate brains. |
But it is not just about having the right voltage. Each rail must be stable - meaning the voltage should not fluctuate more than a few percent even when the load changes suddenly. When the printhead fires, it can draw 10 amps in a microsecond. That is like a car suddenly accelerating from zero to sixty. The power supply must respond fast enough that the voltage does not dip, because a dip would cause the motors to lose torque and the logic to see a brown-out. Similarly, when the printhead stops firing, the voltage must not overshoot, because an overshoot could damage the MOSFETs. Achieving this stability requires careful design of the power supply's feedback loop and its output capacitors. |
Design Example: Mean Well RPS-120S Series |
Mean Well, a Taiwanese power supply manufacturer, offers the RPS-120S series specifically for medical and industrial equipment, including printers. This series takes an 80 to 264 volt AC input - which covers every country from Japan's 100V to the UK's 240V - and outputs 24 volts at 5 amps continuously, with a peak of 6.5 amps for up to 10 seconds. The RPS-120S has three separate outputs: the main 24V rail, a 5V standby rail that is always on, and a 12V rail for optional accessories like an external label rewinder. The 5V standby rail is important because it keeps the CPU's real-time clock and the EEPROM alive even when the printer is 'off' - the CPU uses this rail to detect a wake-up signal from the host computer. The RPS-120S also has a built-in fan control output that varies the fan speed based on the load, reducing noise when the printer is idle. Mean Well's design uses a flyback topology with a switching frequency of 65 kHz, which is a sweet spot between efficiency and electromagnetic noise. The supply is certified to UL/EN/IEC 60950-1 and has a mean time between failures of over 500,000 hours - meaning it is more likely to outlast the printer itself. |
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Chapter 2: The AC to DC Conversion - A Simple Analogy |
Think of the AC input as a river that flows back and forth. The DC output is a lake with a steady, constant level. The power supply is a combination of a dam and a filtering system. The first stage is the rectifier - a set of diodes that act like one-way valves, allowing the current to flow in only one direction. This turns the back-and-forth AC into a pulsing DC that still goes up and down sixty times a second. The second stage is the filter - a large capacitor that acts like a reservoir. When the pulsing voltage rises, the capacitor charges; when it falls, the capacitor discharges into the load, smoothing out the dips. The result is a bumpy DC that still has a noticeable ripple - a small AC component riding on top of the DC. The third stage is the regulator, which actively fights the ripple and maintains a precise voltage. In a linear regulator, this is done by dissipating excess voltage as heat - like a valve that restricts flow. In a switching regulator, it is done by rapidly turning the power on and off and averaging the result - like a faucet that pulses open and closed so fast that the flow appears constant. |
For a barcode printer, the switching regulator is the clear winner. Linear regulators are simple and quiet, but they waste a lot of power. If you take 24 volts down to 5 volts at 2 amps, a linear regulator dissipates (24-5)*2 = 38 watts of heat - enough to melt the plastic enclosure. A switching regulator, on the other hand, achieves 85-95% efficiency, dissipating only a few watts. The downside is that switching regulators generate electrical noise - the fast switching creates high-frequency harmonics that can interfere with sensitive analog circuits and radio communications. The engineer's job is to balance efficiency and noise, using careful layout and filtering. |
Design Example: CUI Inc. VOF-120C Series |
CUI Inc., an American power supply manufacturer, produces the VOF-120C series, which is an open-frame AC-DC converter used in many desktop printer designs. This series has a built-in power factor correction (PFC) circuit - a feature that ensures the printer draws current from the wall in a way that does not distort the AC waveform. PFC is required for equipment above 75 watts in Europe and Japan. The VOF-120C outputs 24V at 5A and has a built-in 5V standby output. It also has a 'DC OK' signal - a logic-level output that goes high when the 24V rail is stable. The printer's CPU monitors this signal and only starts printing after it is asserted. This prevents the printer from trying to print before the power supply is ready, which could cause the printhead to underheat and produce faint labels. The VOF-120C has a hold-up time of 20 milliseconds - meaning if the AC input drops out for 20 ms, the output remains within regulation. This is important because the printhead strobe can last 500 microseconds, and the printer might be in the middle of a strobe when a brief power dip occurs. The hold-up time ensures the printer finishes the strobe cleanly. |
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Chapter 3: The Flyback Converter - Workhorse of the Printer World |
The most common topology for low-to-medium power AC-DC converters in printers is the flyback converter. It is simple, cost-effective, and provides isolation between the input and output - meaning there is no direct electrical connection between the high-voltage AC side and the low-voltage DC side. This isolation is crucial for safety; if the AC side fails, the DC side does not become live. In a flyback converter, the transformer does double duty: it stores energy during the switch-on phase and releases it during the switch-off phase. The primary side has a switching transistor that turns on and off at a high frequency (typically 60 to 100 kHz). When the transistor is on, current builds up in the transformer's primary winding, storing energy in the magnetic field. When the transistor turns off, the magnetic field collapses, inducing a voltage in the secondary winding that is rectified and filtered to produce the DC output. The output voltage is regulated by controlling the on-time of the transistor - longer on-time means more energy stored and a higher output voltage. |
The flyback converter's transformer also provides the multiple outputs - by adding multiple secondary windings, you can get 24V, 5V, and 12V from the same core. But there is a catch: the cross-regulation between outputs is not perfect. If you draw a lot of current from the 24V rail, the 5V rail might dip slightly. This is why many printers use a flyback for the main 24V output and then use post-regulators - small switching or linear regulators - to derive the lower voltages from the 24V rail. This gives better regulation for the logic rails at the cost of a few extra components. |
Design Example: Power Integrations TOP264 Chip |
A design from a British printer manufacturer uses Power Integrations' TOP264, which is an integrated flyback controller with a built-in power MOSFET. This chip simplifies the design by incorporating the oscillator, feedback, and protection circuits in a single package. The TOP264 operates from a 85 to 265V AC input and can deliver up to 62 watts. The design uses a transformer from Wurth Elektronik with a primary inductance of 800 uH and a turns ratio of 10:1 for the main 24V output and a separate 5:1 winding for the 5V standby. The feedback is taken from the 24V output through an optocoupler - this keeps the isolation barrier intact while allowing the controller to sense the output voltage. The TOP264 also has a soft-start feature that gradually increases the switching duty cycle at power-up, reducing the inrush current that could blow the fuse. The manufacturer chose this chip because it has a built-in line voltage compensation - as the AC input varies, the chip adjusts the switching parameters to maintain a constant output, ensuring that the printer works equally well in a Japanese office (100V) and a European warehouse (230V). |
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Chapter 4: The Bulk Capacitor - Energy Reservoir for the Strobe |
The 24V output of the flyback converter is not smooth enough to directly power the printhead. During the strobe pulse, the printhead draws 10 to 15 amps for 500 microseconds. The power supply itself cannot deliver that peak current - its current limit is around 5 amps. The solution is a bulk capacitor - a large electrolytic capacitor placed at the output of the power supply. This capacitor charges up during the idle periods and discharges during the strobe pulse, providing the peak current. The capacitor acts like a battery that is continuously recharged. |
The value of the bulk capacitor is a trade-off. A larger capacitor provides lower voltage droop during the strobe, but it takes longer to charge and costs more. A typical value for a 4-inch printer is 4,700 uF at 35V. This capacitor stores about 0.5 joules of energy, which is enough to fire a 500 us strobe with a 2V droop. The capacitor must have a low equivalent series resistance (ESR), because high ESR causes the capacitor to heat up internally during the high-current pulses, reducing its lifetime. Many designers use capacitors from the Panasonic FC series or the Nichicon UHE series, which are known for their low ESR and long life - up to 10,000 hours at 105C. |
Design Example: Nichicon UHE Series in Zebra Printers |
Zebra Technologies, a leader in industrial barcode printers, uses Nichicon UHE series capacitors in their high-end models. These capacitors are rated for 35V, 4,700 uF, and have an ESR of just 18 milliohms at 100 kHz. The capacitor is placed within 2 centimeters of the printhead connector to minimize the parasitic inductance of the trace - inductance would resist the rapid current change and cause a voltage spike. The PCB layout includes two 4,700 uF capacitors in parallel, giving a total of 9,400 uF - this is overkill for a 4-inch head, but it allows the printer to operate at 14 inches per second without any voltage sag. The capacitors are placed on the bottom layer of the board, with thermal relief pads to the ground plane to allow heat from the capacitor's internal resistance to dissipate. The design includes a pre-charge circuit: a small resistor in series with a relay that limits the inrush current when the capacitor is initially charged. Without this pre-charge, the capacitor would look like a short circuit at power-up, potentially blowing the main fuse. |
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Chapter 5: The Inrush Current Limiter - Soft Start |
When you plug in a printer, the bulk capacitor is completely discharged. It charges up very quickly, drawing a huge surge of current - sometimes 100 amps or more for a few milliseconds. This surge can weld the contacts of the power switch, blow the fuse, or trip the circuit breaker. To prevent this, the power supply includes an inrush current limiter. The simplest type is a negative temperature coefficient (NTC) thermistor placed in series with the AC input. When cold, the NTC has a high resistance, limiting the inrush. As current flows through it, it heats up and its resistance drops to a low value, allowing normal operation. But the NTC has a downside: it wastes power as heat during normal operation, reducing efficiency. A more advanced solution is a relay that bypasses the NTC after the capacitor is charged. |
Design Example: Ametherm SL22 2R512 Inrush Limiter |
A design from a South African printer manufacturer uses an Ametherm SL22 2R512 NTC thermistor. This part has a resistance of 2.5 ohms at 25C and drops to 0.05 ohms at full current. It is placed on the live line, before the bridge rectifier. The design also includes a 50 ohm resistor in parallel with the NTC to provide a path for the input filter capacitors to discharge when the printer is unplugged - this protects the service technician from a shock. The NTC is rated for a maximum steady-state current of 12A, which is more than enough for the printer's 5A rating. The manufacturer tested the inrush current and found it to be limited to 15A peak, compared to 80A without the NTC. This allowed them to use a 5A slow-blow fuse instead of a 10A fuse, reducing the cost and improving the protection. The NTC is mounted with its leads long enough to allow it to cool by natural convection - it is not placed too close to other heat-generating components like the MOSFETs. |
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Chapter 6: The Bridge Rectifier - Turning AC into Pulsing DC |
The first active stage after the fuse and inrush limiter is the bridge rectifier. This is a set of four diodes arranged in a diamond shape. During the positive half of the AC cycle, two diodes conduct; during the negative half, the other two conduct. The result is that the current always flows in the same direction through the load, but it is a series of positive humps - like a camel's back - rather than a smooth DC. The bridge rectifier is rated for the printer's input current, but it must also withstand the peak inverse voltage - the maximum reverse voltage that a diode can tolerate without breaking down. For a 240V AC input, the peak voltage is 340V, so the diodes must be rated for at least 400V. |
Design Example: Vishay GBU10K Bridge Rectifier |
A design from an Australian printer OEM uses the Vishay GBU10K bridge rectifier. This is a 10A, 800V device in a standard GBU package. It is mounted on a small heatsink because the power dissipation in the rectifier is about 2 watts at full load - the forward voltage drop of each diode is about 1V, and the average current is 2A, so two diodes conduct at a time, dissipating 2*1*2=4W in total. The heatsink is a simple aluminum clip that also holds the rectifier to the PCB. The manufacturer chose an 800V rating instead of 400V to provide a safety margin - in areas with unstable grids, voltage spikes can exceed 400V, and the 800V diode survives them. The rectifier is placed before the main filter capacitor, and its output connects directly to the capacitor's positive terminal. The PCB traces are sized to carry the peak current without excessive voltage drop - 2 mm wide for the AC input traces and 3 mm wide for the DC output. |
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Chapter 7: The Switching Transistor - The Heart of the Flyback |
The flyback converter relies on a high-voltage transistor that switches on and off at high speed. This transistor is typically a power MOSFET (metal-oxide-semiconductor field-effect transistor) with a voltage rating of 600V to 800V and a current rating of 5A to 10A. The MOSFET is driven by a controller IC that generates a pulse-width modulated (PWM) signal. The on-time of the MOSFET determines the energy transferred to the secondary side. The off-time allows the magnetic field to collapse and deliver that energy to the output. The MOSFET must have a low on-resistance (Rds(on)) to minimize heating - typically below 0.5 ohms. It also must have a fast switching speed to reduce switching losses, but fast switching also generates more electromagnetic noise - another trade-off. |
Design Example: Infineon CoolMOS P7 Series |
A design from a German printer manufacturer uses Infineon's CoolMOS P7 series, specifically the IPD70R600P7, which is a 700V, 600mOhm MOSFET. This is a super-junction device that offers low on-resistance and low gate charge, enabling efficient operation at 65 kHz. The MOSFET is driven by a gate driver IC that provides a 15V gate signal - the gate-source voltage must be high enough to fully turn on the MOSFET. The gate driver is powered from a separate winding on the transformer, called the auxiliary winding. This winding also provides the power for the controller IC itself, making the converter self-sufficient once it starts. The MOSFET is mounted on a copper pad on the PCB - not a separate heatsink - because its power dissipation is only 1.5W at full load, which is within the capability of the copper pad. The manufacturer reports that this MOSFET survived a short-circuit test where the output was shorted and the controller cycled into 'hiccup' mode - the MOSFET's temperature rose to 110C but stayed within the 150C maximum. |
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Chapter 8: The Feedback Loop - Keeping the Output Stable |
The power supply must maintain a constant 24V output despite variations in the input voltage and the load current. This is done with a feedback loop. A small sample of the output voltage is compared to a precision reference voltage (typically 2.5V). The difference, or error, is amplified and used to adjust the switching duty cycle. If the output voltage drops, the error increases, causing the controller to increase the on-time of the MOSFET, delivering more energy. If the output voltage rises, the opposite happens. The feedback loop must be fast enough to respond to the load transients from the printhead, but not so fast that it becomes unstable - instability can cause the output to oscillate, which can damage the printhead. |
Design Example: Texas Instruments TL431 and Optocoupler |
The most common feedback circuit in printer power supplies uses the TL431 programmable shunt regulator and a linear optocoupler. The TL431 is connected to a voltage divider from the 24V output. When the 24V output is at its nominal value, the divider voltage equals the TL431's 2.5V internal reference, and the TL431 conducts just enough current to keep the optocoupler's LED emitting light. If the 24V output rises, the TL431 conducts more current, making the LED brighter. The optocoupler's phototransistor on the primary side receives the light and sinks more current, pulling down a compensation pin on the PWM controller. This reduces the duty cycle, bringing the output back down. The loop compensation - the resistors and capacitors around the TL431 - determines how fast the loop responds. In a typical printer design, the loop is tuned for a crossover frequency of about 1 kHz, which is fast enough to correct for the 100 Hz ripple from the AC input but slow enough to avoid ringing. The manufacturer often recommends specific values for the compensation network based on the output capacitor ESR. |
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Chapter 9: The Standby Supply - Always-On Power |
As we mentioned earlier, the printer needs a small amount of power even when it is 'off.' This is provided by a standby supply - a separate, low-power converter that is always on. In many designs, the standby supply is derived from the same flyback transformer, using a dedicated secondary winding. But this winding is not regulated as tightly as the main output, so a separate linear regulator (like a 78L05) might be used to clean up the 5V standby. The standby supply must draw very little current when the printer is completely idle - typically less than 1W - to meet energy efficiency standards like Energy Star. |
Design Example: On Semiconductor NCP1028 Standby Controller |
A design from a Brazilian printer manufacturer uses the NCP1028 controller from On Semiconductor for the standby supply. This controller integrates a 700V MOSFET and is specifically designed for low-power applications. It operates in a frequency-foldback mode - at light loads, it reduces the switching frequency to reduce losses. The standby supply outputs 5V at 300mA, which is enough to power the CPU's real-time clock and the USB transceiver. The NCP1028 also has a built-in over-voltage protection that latches off if the output exceeds 6V, protecting the sensitive logic. The standby supply is completely independent of the main 24V supply; it has its own transformer winding and its own feedback loop. This means that even if the main 24V rail is shorted, the standby supply continues to operate, allowing the CPU to detect the fault and display an error message. |
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Chapter 10: The Post-Regulators - Deriving Low Voltages from 24V |
The main 24V rail is great for the printhead and motors, but it is too high for logic. The lower voltages are derived from the 24V rail using DC-DC converters placed on the main board, close to the chips they power. The most common approach is to use a switching buck converter for the 5V rail (which powers the motor drivers and USB), and then use a linear regulator from 5V to 3.3V for the CPU, or a separate buck for 3.3V. The choice between switching and linear for the 3.3V rail depends on the current. If the CPU draws less than 500mA, a linear regulator is acceptable - its efficiency is 66% (3.3/5), but the power loss is only 0.85W, which is manageable. If the CPU draws more, a switching buck is preferred. |
Design Example: TI TPS54335A for 5V Rail |
A very common design uses the TPS54335A from Texas Instruments - a synchronous buck converter that can deliver 3A at 5V from a 24V input. It operates at 500 kHz and achieves 95% efficiency. The input is the 24V rail, and the output is 5V. The converter uses a 4.7 uH inductor and a 22 uF ceramic output capacitor. The switching frequency is chosen to be above the audio band (20 kHz) to avoid audible noise. The TPS54335A has an enable pin that is connected to the CPU - the CPU can turn off the 5V rail when the printer is in deep sleep, saving power. However, the 5V rail for the USB host must remain on to detect a wake-up, so a separate 5V regulator is used for that purpose. The TPS54335A also has a soft-start feature that ramps up the output voltage over 1 ms, reducing the inrush current into the downstream capacitors. |
Design Example: Analog Devices ADP150 for 3.3V Logic |
For the 3.3V logic rail, a design from a Danish printer company uses the ADP150 linear regulator from Analog Devices. This regulator has an input voltage range of 2.2V to 5.5V (so it operates from the 5V rail), and it delivers up to 150mA. Its output is 3.3V with an accuracy of (+-)1%. The ADP150 has a low dropout voltage of only 50mV at 150mA, meaning the input can be as low as 3.35V and the output stays at 3.3V. This is important because the 5V rail might sag to 4.5V under heavy load, and the linear regulator still works. The ADP150 also has a noise reduction pin that allows the user to add a capacitor to reduce output noise to below 10 uV - this is overkill for a printer, but it is a nice feature. The manufacturer chose the ADP150 because of its low quiescent current (10 uA) and its small package (5-lead SOT-23), saving board space. |
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Chapter 11: The High-Side Switch - Protecting the Printhead |
The 24V rail to the printhead is not always on. It is controlled by a high-side switch - a MOSFET or a load switch IC that sits between the 24V rail and the printhead connector. The CPU turns on this switch only when a print job is in progress. This has two benefits: it prevents the printhead from accidentally heating during startup, and it allows the printer to shut off the printhead power in an emergency - for example, if the over-temperature sensor trips. The high-side switch must be able to handle the peak current of 15A and must have a low on-resistance to avoid dropping too much voltage. |
Design Example: Infineon BTS5012S High-Side Switch |
A design from an American printer manufacturer uses the Infineon BTS5012S, which is a smart high-side switch with built-in protection. This device has an on-resistance of 12 milliohms, so at 15A, it drops only 0.18V - negligible. It has a diagnostic output that the CPU can read to detect open-load, short-circuit, and over-temperature conditions. The switch is turned on by a logic-level signal from the CPU (3.3V). The BTS5012S also has a very fast turn-off time (less than 10 microseconds), which is important for emergency stops. The design includes a freewheeling diode across the printhead - when the switch turns off, the inductive load of the printhead cabling would otherwise cause a voltage spike; the diode clamps the spike to a safe level. The BTS5012S is placed near the printhead connector to minimize the high-current trace length, reducing both voltage drop and EMI. |
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Chapter 12: The Motor Power - A Separate Path |
While the printhead draws its power from the 24V rail through the high-side switch, the motors often draw their power directly from the 24V rail, without a separate switch. The reason is that the motors are driven by H-bridge drivers that have their own enable pins - the CPU can disable the H-bridge outputs without cutting the main power. However, the motor drivers have large capacitors on their inputs to decouple them from the printhead's bulk capacitor. If the motors and the printhead shared a single capacitor, the printhead's massive current pulse could cause the motor voltage to dip, resulting in a loss of motor torque and a potential misstep. So there is a separate capacitor bank for the motors - typically 1,000 uF at 35V, placed close to the motor drivers. |
Design Example: Panasonic EEE-FK Series for Motor Decoupling |
A design from a Japanese printer OEM uses Panasonic EEE-FK series capacitors for motor decoupling. These are surface-mount electrolytic capacitors with a 1,000 uF, 35V rating and an ESR of 40 milliohms. Two of these capacitors are placed in parallel near the A4988 motor driver IC. The capacitors are connected to the 24V rail through a ferrite bead (600 ohms at 100 MHz) to isolate the motor noise from the rest of the system. The ferrite bead prevents the high-frequency switching noise of the motor driver from coupling into the printhead's strobe signal. The capacitors provide the peak current for the motor during micro-stepping, smoothing out the current waveform and reducing audible noise. |
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Chapter 13: The Linear Regulator for Analog Circuits |
Some parts of the printer need very clean power - the analog-to-digital converter (ADC) that reads the printhead temperature, and the sensor comparators that detect label gaps. These circuits are sensitive to the switching noise from buck converters. For these sensitive analog circuits, a linear regulator is often used, even though it is less efficient. The linear regulator's output is virtually noise-free because it does not switch; it simply passes the input voltage through a pass transistor and dissipates the excess as heat. A separate linear regulator is used for the analog supply, with its own input filter - a pi-filter consisting of a ferrite bead and two capacitors. |
Design Example: Micrel MIC5205 for Analog Supply |
A design from a French printer company uses the MIC5205 low-dropout regulator from Micrel (now Microchip). This regulator provides 3.3V at 150mA with an output noise of 20 uV typical. It is powered from the clean 5V rail (not the 24V rail, because the 24V rail is too noisy). The output is used solely for the ADC reference voltage and the sensor comparators. The input has a 10 uF tantalum capacitor and a 0.1 uF ceramic capacitor for bypassing. The output has a 1 uF ceramic capacitor. The MIC5205 also has a shutdown pin that allows the CPU to turn off the analog supply when the printer is in sleep mode, reducing power consumption. The manufacturer measured the ADC readings with and without this clean supply and found that the noise was reduced by 80%, improving the temperature measurement accuracy from (+-)3C to (+-)1C. |
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Chapter 14: The Pi-Filter - Silencing the Switching Noise |
Every switching regulator produces noise at its switching frequency and its harmonics. This noise can couple into other circuits through the power lines. To prevent this, designers use pi-filters - a capacitor, an inductor (ferrite bead or small inductor), and another capacitor - in series with the power line. The pi-filter acts as a low-pass filter, attenuating frequencies above its cutoff. For a printer, the pi-filter is used on the 24V rail before it goes to the printhead, on the 5V rail before it goes to the analog circuits, and on the 3.3V rail before it goes to the CPU's analog pins. |
Design Example: Murata BLM21 Ferrite Bead |
A design from an Italian printer manufacturer uses Murata BLM21 ferrite beads in the pi-filters. This bead has an impedance of 600 ohms at 100 MHz and can handle 4A of DC current. The pi-filter for the analog 5V rail consists of a 10 uF capacitor, the ferrite bead, and a 1 uF capacitor. The ferrite bead is placed as close as possible to the input of the linear regulator. The designer chose a ferrite bead instead of a wire-wound inductor because the bead is smaller and cheaper, and it works well at high frequencies. The bead's DC resistance is only 0.05 ohms, so it does not drop much voltage. The manufacturer measured the noise on the analog 5V rail and found it to be less than 5 mV peak-to-peak - well below the 10 mV noise tolerance of the ADC. |
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Chapter 15: The Power Supply's Undervoltage Lockout (UVLO) |
If the input voltage drops too low, the power supply cannot maintain regulation. The output voltage will sag, and the CPU might enter an undefined state. To prevent this, the power supply includes an undervoltage lockout (UVLO) circuit that shuts down the converter when the input voltage is too low. For an AC-DC converter, the UVLO monitors the rectified DC voltage after the bridge rectifier. If the AC input drops below 80V (for a 100-240V design), the UVLO turns off the switching transistor, preventing the output from browning out. The converter restarts automatically when the input voltage returns to normal. |
Design Example: STMicroelectronics L6566A with UVLO |
A design from a Spanish printer manufacturer uses the STMicroelectronics L6566A, a controller that has a built-in UVLO with a hysteresis of 7V. The UVLO threshold is set by a resistor divider from the rectified DC input. When the rectified voltage rises above 30V (corresponding to about 85V AC), the controller starts. If the voltage drops below 23V (about 60V AC), the controller stops. This hysteresis prevents the controller from oscillating on and off when the input is marginal. The UVLO also protects the controller itself - the controller's internal circuits are powered from a startup resistor, and the UVLO ensures that the controller only operates when the supply voltage is sufficient. The manufacturer found that this feature was essential in regions with unstable power grids; without it, the printer would sometimes print half a label and then stop. |
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Chapter 16: Over-Voltage Protection (OVP) - Saving the Printhead |
The printhead is the most expensive component in the printer. If the 24V rail ever exceeds its maximum rating (typically 26V), the printhead's resistors can be damaged. The power supply must have a robust over-voltage protection circuit. In a flyback converter, OVP is usually implemented by monitoring the output voltage and, if it exceeds a threshold, latching off the switching transistor. The latch can only be reset by removing and reapplying the AC power. This is a 'crowbar' protection - it essentially short-circuits the output to blow the fuse, but that is too violent for a printer. Instead, the controller stops switching, and the output voltage slowly decays. |
Design Example: Texas Instruments UCC28600 with OVP |
A design from a Canadian printer manufacturer uses the UCC28600 controller from Texas Instruments, which has an OVP feature. The controller monitors the output voltage via an auxiliary winding on the transformer - this is an isolated method that does not require an optocoupler for OVP. If the output voltage exceeds 26V, the controller stops switching and enters a fault state. The fault state latches; only a power cycle clears it. The manufacturer tested this by intentionally shorting the feedback optocoupler (simulating a failure) and found that the output voltage rose to 25.8V before the OVP kicked in - within the 26V limit. The OVP also protects the MOSFET from a secondary breakdown - if the output is open-circuit, the voltage on the MOSFET drain can ring to high levels; the OVP indirectly limits this because the flyback stops delivering energy. |
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Chapter 17: Over-Current Protection (OCP) - Preventing Fire |
If the printer's motor stalls or the printhead develops a short circuit, the current can rise to dangerous levels. Over-current protection limits the current to a safe value. In a flyback converter, OCP is typically implemented by sensing the current through the primary MOSFET using a small resistor in series with the source. The voltage across this resistor is fed to the controller; if it exceeds a threshold, the controller reduces the duty cycle or stops switching. This is a pulse-by-pulse current limit - it acts on each switching cycle. |
Design Example: ON Semiconductor NCP1380 with OCP |
A design from a Korean printer manufacturer uses the NCP1380 controller from ON Semiconductor, which has a built-in OCP with a threshold of 0.5V. The sense resistor is 0.2 ohms, so the peak primary current is limited to 2.5A. At the maximum input voltage (265V AC), this corresponds to a maximum output power of about 80W, which is within the supply's 60W rating. The OCP also includes a 'short-circuit protection' mode - if the output is shorted, the primary current reaches the limit on every cycle, and the controller enters a hiccup mode: it stops switching for 1 second, then tries again. This repeats indefinitely until the short is removed. The manufacturer chose a 0.5W sense resistor to ensure it can handle the power dissipation. The OCP is a hardware protection, meaning it operates even if the controller's firmware (if any) fails. |
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Chapter 18: Thermal Protection - Shutting Down Before Melting |
The power supply components - the MOSFET, the diode, and the transformer - can overheat if the printer is operated in a hot environment or if the cooling fan fails. The power supply should include thermal protection that shuts down the converter if the temperature exceeds a safe limit. This is usually implemented with a thermistor or a thermal cutoff device. Some controllers have a built-in temperature sensor on the chip. |
Design Example: NXP TEA1755 with Temperature Sensing |
A design from a Dutch printer manufacturer uses the NXP TEA1755, which is a combination PFC and flyback controller with a dedicated temperature sense pin. A negative temperature coefficient (NTC) thermistor is placed near the output diode - the diode is the hottest component. The NTC forms a voltage divider with a fixed resistor; the voltage is read by the controller. If the temperature exceeds 110C, the controller reduces the switching frequency to lower the power, and if it exceeds 130C, it shuts down completely. The controller also has a soft-recovery feature - when the temperature drops back to 100C, it restarts automatically. This prevents nuisance shutdowns from brief temperature spikes. The manufacturer used a 10 kohm NTC from Vishay and placed it on the PCB with a small amount of thermal adhesive to ensure good contact with the diode's heatsink. |
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Chapter 19: The Fuse - The Ultimate Sacrifice |
Despite all the protections, something catastrophic could happen - a lightning strike, a massive surge, or a component failure. The fuse is the last line of defense. It is a thin wire that melts when the current exceeds its rating for a sufficient time, breaking the circuit and preventing a fire. The fuse must be rated for the input voltage and the inrush current. A slow-blow fuse is used because the inrush current of the power supply can be several times the steady-state current for a few milliseconds. |
Design Example: Littelfuse 0218 Series |
A design from a Taiwanese printer OEM uses a Littelfuse 0218 series slow-blow fuse rated for 250V AC and 5A. The fuse is placed in the live line, before the MOV and the inrush limiter. The fuse has an interrupting rating of 100A at 250V - meaning it can safely break a circuit with up to 100A of fault current. The fuse holder is mounted on the PCB with a 5 mm pitch. The manufacturer selected a 5A fuse because the printer's maximum input power is 100W (about 0.4A at 240V), and the inrush current is limited to 15A by the NTC, which is below the fuse's 10x current rating for 10 ms. The fuse is placed in a clip that allows replacement without soldering - this is a serviceable part. The manufacturer also added a varistor in parallel with the fuse to absorb small surges, preserving the fuse for larger events. |
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Chapter 20: The Common-Mode and Differential-Mode Filters |
Before the AC power even reaches the bridge rectifier, it passes through an EMI filter. This filter consists of a common-mode choke (two windings on a single core) and several capacitors. The common-mode choke attenuates noise that is common to both lines - noise that appears on the live and neutral lines simultaneously, which is typical of high-frequency switching noise from the power supply itself. The differential-mode choke (or a simple inductor) attenuates noise that is differential - appearing between the lines. The capacitors are of three types: X capacitors (across line and neutral) for differential noise, and Y capacitors (from line to ground and neutral to ground) for common-mode noise. The X capacitors are rated for the full AC voltage and are designed to fail open-circuit, while Y capacitors are rated for high voltage and are designed to fail short-circuit, but they are connected to ground so a short does not create a shock hazard. |
Design Example: Schaffner FN2040 EMI Filter |
A design from a Swiss printer manufacturer uses a Schaffner FN2040 integrated EMI filter module. This module contains the common-mode choke, the differential-mode inductor, and all the X and Y capacitors in a single encapsulated package that mounts on the PCB. The module is rated for 10A at 250V and has an attenuation of 40 dB at 100 kHz. The manufacturer chose this module because it simplifies the design and ensures consistent performance - the capacitors are matched to the choke, and the leakage inductance is controlled. The module also has a built-in bleeder resistor that discharges the X capacitors when the printer is unplugged, preventing a shock to the user. The module is placed immediately after the fuse, before the inrush limiter, because the inrush limiter can generate noise as its resistance changes. |
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Chapter 21: The Power Supply's Efficiency - Why It Matters |
Efficiency is the ratio of output power to input power. A 90% efficient power supply at 100W output consumes 111W from the wall - the extra 11W is wasted as heat. That heat must be dissipated, which requires a heatsink or a fan, adding cost and noise. Also, wasted heat reduces the lifetime of the components inside the supply. In many countries, energy efficiency standards (like Energy Star or the EU ErP directive) mandate a minimum efficiency for power supplies. For a printer, typical efficiency requirements are 85% at full load and 80% at half load. Achieving these numbers requires careful design of the transformer, the choice of low-loss MOSFETs, and synchronous rectification on the secondary side. |
Design Example: Delta Electronics Power Supply |
Delta Electronics, a Taiwanese company, is one of the largest power supply manufacturers in the world. Their printer power supplies are known for high efficiency, often exceeding 90%. In a design for a large industrial printer, Delta uses synchronous rectification on the 24V output - instead of a Schottky diode, they use a MOSFET that is turned on when the transformer's secondary voltage is positive. This MOSFET has a much lower voltage drop (e.g., 0.05V) compared to a Schottky diode (0.4V), reducing the loss by about 0.35V * 5A = 1.75W. The synchronous rectifier is driven by a dedicated controller that detects the transformer's voltage waveform. Delta also uses planar transformers instead of wire-wound ones - planar transformers have lower leakage inductance and better thermal performance, improving efficiency by about 2%. The result is a power supply that runs cool enough to be fanless, reducing the printer's acoustic noise. |
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Chapter 22: The Hold-Up Time - Riding Through Brownouts |
The hold-up time is the time that the power supply can maintain the output voltage after the AC input is removed. For a printer, a hold-up time of 10 to 20 milliseconds is typical - enough to ride through a momentary power dip or a generator transfer switch. The hold-up time is determined by the energy stored in the bulk capacitor and the output power. A larger bulk capacitor gives a longer hold-up time, but it also increases the inrush current and the cost. |
Design Example: TDK EPCOS Capacitor in Zebra Printers |
Zebra's high-end printers use a hold-up time of 25 milliseconds. To achieve this, they use a TDK EPCOS B43564 series capacitor of 6,800 uF at 35V. This capacitor stores about 1.6 Joules of energy. At full load (80W), the capacitor can supply power for about 20 ms before the voltage drops below the 24V minimum. The capacitor is placed in parallel with the main bulk capacitor, but it has a slightly larger value. The designer also added a 'DC OK' signal that indicates when the output voltage is within regulation. If the AC input drops, the DC OK signal goes low after 5 ms, giving the CPU time to finish the current line and enter a safe state before the voltage collapses. |
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Chapter 23: The Transformer - More Than Just a Coil |
The flyback transformer is the most critical component of the power supply. It stores energy, provides isolation, and steps the voltage up or down. The transformer's core is made of ferrite - a magnetic material that conducts magnetic flux well at high frequencies. The primary winding has many turns; the secondary winding has fewer turns for a step-down. The transformer's design involves many trade-offs: more turns increase the inductance and reduce the peak current, but they also increase the copper losses. A larger core reduces the flux density and the core losses, but it also increases the size and cost. |
Design Example: Wurth Elektronik 750810001 Transformer |
A design from a Swedish printer manufacturer uses a standard off-the-shelf transformer from Wurth Elektronik, part number 750810001. This transformer is designed for flyback converters with an input of 85-265V AC and an output of 24V at 5A. It has a primary inductance of 750 uH and a turns ratio of 12:1. The transformer is wound on an EFD25 core, which is a flat profile that is suitable for low-profile printer designs. The secondary winding is center-tapped to provide both 24V and 12V outputs. The transformer has a built-in thermal fuse that opens if the winding temperature exceeds 130C - a safety feature required by UL. The manufacturer selected this transformer because it is pre-qualified for the controller IC they are using, saving development time. |
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Chapter 24: The Snubber Circuit - Absorbing Voltage Spikes |
When the flyback's MOSFET turns off, the leakage inductance of the transformer causes a voltage spike on the MOSFET's drain. If this spike is too high, it can exceed the MOSFET's breakdown voltage and destroy it. A snubber circuit absorbs this spike. The most common snubber is an RCD (resistor-capacitor-diode) network - a diode that conducts when the drain voltage exceeds a threshold, charging a capacitor through a resistor. The capacitor's voltage is discharged by the resistor between switching cycles. |
Design Example: RCD Snubber with 47k Resistor and 1nF Capacitor |
A design from a Chinese printer manufacturer uses a standard RCD snubber with a diode (UF4007), a 47k ohm resistor, and a 1 nF capacitor rated for 1 kV. The snubber is placed across the primary winding of the transformer. The resistor's value is chosen to limit the snubber's power dissipation to about 0.5W. The capacitor is chosen to limit the peak voltage to less than 600V. The designer used a fast recovery diode to ensure the capacitor charges quickly. The snubber also slightly reduces the efficiency of the power supply (about 1%), but it is a necessary trade-off for reliability. |
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Chapter 25: The Startup Circuit - Getting the Controller Going |
The flyback controller needs power to start, but the main power supply is not yet running. A startup circuit provides this initial power. The simplest startup circuit is a resistor connected from the rectified DC to the controller's VCC pin. The resistor charges a small capacitor; when the voltage reaches the controller's UVLO threshold, the controller starts. Once the power supply is running, an auxiliary winding on the transformer takes over the VCC, providing a stable voltage. The startup resistor must be large enough to limit the power dissipation (it dissipates about 0.5W continuously) but small enough to start the controller within a second. |
Design Example: Startup Resistor of 150k Ohm |
A design from a US printer manufacturer uses a 150k ohm, 1W startup resistor. The resistor is a flameproof type, because it could overheat in a fault condition. The resistor charges a 10 uF capacitor to about 15V within 0.5 seconds at 120V AC. The controller starts and begins switching. The auxiliary winding then provides 12V to keep the controller running. The startup resistor remains connected, but its current is negligible compared to the power consumption. The manufacturer noted that a higher value resistor (220k) would reduce the power dissipation but would increase the startup time to 2 seconds - they chose 150k as a compromise. |
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Chapter 26: The Auxiliary Winding - Self-Sustaining Power |
Once the power supply is running, the controller's power is derived from an auxiliary winding on the transformer. This winding is separate from the main secondary winding and is referenced to the primary ground. The voltage from the auxiliary winding is rectified and filtered, providing a stable DC voltage (usually 12V to 18V) that powers the controller and the gate driver. The auxiliary winding is also used for feedback in some designs - the controller can sense the output voltage indirectly from the auxiliary winding, eliminating the need for an optocoupler. However, this 'primary-side regulation' is less accurate than using an optocoupler. |
Design Example: Auxiliary Winding with 12V Output |
A design from an Indian printer manufacturer uses the auxiliary winding to produce 12V, which powers the controller and also a small fan. The auxiliary winding has 5 turns, while the main secondary has 10 turns. The 12V is rectified with a 1N4148 diode and filtered with a 47 uF capacitor. The 12V rail also powers a 5V linear regulator for the front panel display - this is a cheap way to get a clean 5V for the LCD, separate from the noisy main 5V rail. The manufacturer ensured that the auxiliary winding could provide at least 100mA, which is enough for the controller and the fan. |
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Chapter 27: The Soft-Start - Avoiding Inrush and Overshoot |
When the power supply first starts, the output capacitor is discharged. If the controller started with a large duty cycle, the output voltage would overshoot and possibly damage the load. A soft-start circuit gradually increases the duty cycle from zero to the normal operating value over a period of several milliseconds. The soft-start is usually achieved by an internal ramp generator in the controller that slowly increases the reference voltage for the feedback loop. |
Design Example: Soft-Start Time of 5 ms |
A design from a Brazilian printer manufacturer uses a controller with a 5 ms soft-start time. The controller's internal soft-start capacitor is 1 uF, which, together with an internal current source, creates a ramp from 0V to the reference voltage. The designer chose 5 ms because it is long enough to limit the inrush current into the bulk capacitor to less than 1A, but short enough that the printer can start printing within 2 seconds of power-up. The soft-start also prevents audible clicks from the transformer, which can occur if the transformer is driven with a high-duty-cycle pulse at startup. |
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Chapter 28: The Remote Sense - Compensating for Cable Loss |
In some printers, the power supply is located in a separate brick, connected to the printer by a long cable. The cable has resistance, so the voltage at the printer's connector is slightly lower than the output of the power supply - especially at high currents. To compensate, some power supplies have a remote sense feature: they have separate sense wires that measure the voltage at the load and adjust the output to compensate for the cable drop. This is more common in laboratory power supplies than in printers, but some high-end printers use it. |
Design Example: Remote Sense in Zebra Industrial Printers |
Zebra's industrial printers often use a separate power brick with remote sense. The power brick has a 4-pin connector: two pins for power (24V and ground) and two pins for sense (sense+ and sense-). The sense wires are connected to the printer's main board, close to the bulk capacitor. The power supply's feedback loop is based on the sense voltage, not the output voltage at the brick. The sense wires carry only a small current (microamps), so their voltage drop is negligible. If the cable has a resistance of 0.1 ohms and the printer draws 5A, the voltage drop is 0.5V. The remote sense compensates by raising the supply's output to 24.5V, ensuring the printer sees exactly 24V. The manufacturer uses a 5-pin connector to include the sense wires, and the cable is shielded to prevent noise coupling into the sense lines. |
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Chapter 29: The Power Factor Correction - Efficiency and Compliance |
Power factor correction (PFC) is a technique that aligns the input current waveform with the input voltage waveform. Without PFC, the current is drawn only near the peaks of the voltage waveform, which results in a high peak current, harmonic distortion, and poor power factor (typically 0.6). With PFC, the current is drawn throughout the entire AC cycle, resulting in a power factor close to 1.0. PFC is required for equipment above 75W in Europe and Japan. It also reduces the stress on the input components and improves the efficiency. |
Design Example: NXP TEA19161T with PFC |
A design from a German printer manufacturer uses the NXP TEA19161T controller, which includes a PFC boost converter followed by a flyback converter. The PFC stage takes the rectified DC and boosts it to a regulated 400V DC. This 400V rail is then fed to the flyback converter. The PFC stage operates in 'critical conduction mode' - the inductor current returns to zero each cycle, which simplifies the control. The PFC stage adds about 5% to the total cost, but it is necessary for compliance. The manufacturer also noticed that the PFC stage reduced the inrush current because the PFC inductor limits the current rise. The PFC stage has its own controller and MOSFET, which are rated for 600V and 6A. |
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Chapter 30: The Hold-Up Capacitor for PFC |
With PFC, the bulk capacitor is placed after the PFC stage, at 400V DC. This capacitor is much smaller in capacitance than a 24V capacitor because the voltage is higher - the energy stored is 0.5*C*V^2. A 400V, 100 uF capacitor stores about 8 Joules, which is enough for a 20 ms hold-up time at 80W. However, these capacitors are physically larger and more expensive than 35V capacitors because they must withstand a higher voltage and have a lower ESR at high frequencies. |
Design Example: Rubycon 400V 100 uF Capacitor |
A design from a US printer manufacturer uses a Rubycon 400V 100 uF capacitor for the PFC hold-up. This capacitor is a snap-in type with a diameter of 22 mm and a height of 35 mm. It is rated for 10,000 hours at 105C, which is sufficient for a printer's expected lifetime. The capacitor is placed on the bottom of the PCB, with a clip to secure it during shipping. The designer added a bleed resistor of 470k ohms across the capacitor to discharge it to a safe voltage within 5 seconds of unplugging - this is a safety requirement. |
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Chapter 31: The Synchronous Rectification - Boosting Efficiency Further |
In a flyback converter, the secondary side uses a diode to rectify the output. The diode has a forward voltage drop of about 0.4V for a Schottky diode, which at 5A results in a loss of 2W. This is significant in a 60W supply - it represents over 3% efficiency loss. Synchronous rectification replaces the diode with a MOSFET, which has a much lower voltage drop when turned on. The MOSFET is controlled by a driver that detects the voltage across the MOSFET and turns it on when the body diode conducts. |
Design Example: Texas Instruments UCC24610 Synchronous Rectifier Driver |
A design from a Korean printer manufacturer uses the UCC24610 driver from Texas Instruments. This driver is designed for flyback converters up to 10A. It senses the drain-to-source voltage of the secondary MOSFET and turns on the MOSFET when the body diode starts conducting. The MOSFET used is a 60V, 5mOhm device (e.g., CSD18504Q5A), which has a voltage drop of 25mV at 5A - compared to 0.4V for a Schottky diode. The loss is reduced by 0.375V * 5A = 1.875W, improving efficiency by about 3%. The driver is powered from the output voltage (24V) and has a quiescent current of 2mA. The manufacturer reported that the efficiency of their power supply increased from 88% to 91% with synchronous rectification. |
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Chapter 32: The EMI Filter on the DC Side |
Not all electromagnetic interference is conducted back to the AC line. Some noise is radiated from the PCB traces and the cables. In addition, the power supply's output can have high-frequency noise that interferes with the printer's logic. An EMI filter on the DC side - a ferrite bead or a common-mode choke - can reduce this. The DC-side filter is usually a ferrite bead on the 24V output, placed right at the connector. |
Design Example: Wurth Elektronik 74279251 Ferrite Bead |
A design from a Swedish printer manufacturer uses a Wurth Elektronik 74279251 ferrite bead on the 24V output. This bead has an impedance of 120 ohms at 100 MHz and a DC resistance of 5 milliohms, so it drops only 25mV at 5A. The bead is placed in series with the 24V line, just after the bulk capacitor. The bead attenuates high-frequency noise from the switching converter, preventing it from reaching the printhead and motors. The manufacturer also placed a 10 uF ceramic capacitor after the bead to create a low-pass filter. The combination of the bead and the capacitor reduced the high-frequency noise on the 24V rail from 200 mV peak-to-peak to 30 mV, which is within the tolerance of the motor drivers. |
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Chapter 33: The Status LEDs on the Power Supply |
Some power supplies have built-in status LEDs that indicate the state of the output. A green LED might indicate that the output is within regulation, and a red LED might indicate a fault. This is useful during manufacturing test and for service technicians. The LED is usually driven by the 'DC OK' signal. |
Design Example: Green and Red LEDs in Mean Well Supply |
The Mean Well RPS-120S has two LEDs: a green one labeled 'DC OK' and a red one labeled 'Fault.' The green LED is on when the 24V output is within 10% of nominal. The red LED turns on for faults like over-voltage, over-current, or over-temperature. The LEDs are visible through a hole in the printer's enclosure. The manufacturer added these LEDs as a direct request from their customers, who wanted a quick way to check the power supply without using a multimeter. The LEDs draw about 5mA each, which is negligible. |
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Chapter 34: The Connector - Bringing Power into the Printer |
The interface between the power supply and the printer is usually a DC barrel jack or a specialized connector. The connector must be rated for the current and must have a positive locking mechanism to prevent accidental disconnection. The center pin is typically positive, and the outer barrel is ground, but some printers use the opposite polarity to prevent using the wrong adapter. |
Design Example: CUI PP-002D Barrel Jack |
A design from a North American printer manufacturer uses a CUI PP-002D barrel jack with a 2.1 mm center pin and a 5.5 mm outer diameter. The jack is rated for 5A at 24V. It has a threaded collar that allows the power plug to be screwed in, preventing it from pulling out during operation - this is important in industrial environments where the printer might be dragged by the cable. The jack also has a normally closed switch that disconnects the internal battery when the adapter is plugged in, preventing the battery from being charged by the adapter without proper control. |
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Chapter 35: The Secondary Protection - TVS Diodes on the DC Rail |
Even with the power supply's over-voltage protection, a voltage spike could reach the printer through the connector - for example, from electrostatic discharge or from a faulty power supply. A transient voltage suppressor (TVS) diode on the 24V rail clamps any voltage above 26V, protecting the printer's components. |
Design Example: Littelfuse 1.5KE30A TVS Diode |
A design from an Australian printer manufacturer uses a Littelfuse 1.5KE30A TVS diode. This diode is rated for 1500W peak pulse power and clamps at 30V. It is placed across the 24V rail and ground, right at the power input connector. The diode is bidirectional, meaning it clamps both positive and negative transients. The manufacturer selected this diode because it has a fast response time (less than 1 nanosecond) and can handle the energy from a typical ESD event. The diode is a through-hole component, placed on the bottom of the PCB so that it is not blocked by other components. |
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Chapter 36: The Filtering of the Standby Rail |
The standby 5V rail must be particularly clean because it powers the CPU's real-time clock and the USB transceiver. A small LC filter (pi-filter) is added to the standby output, using a ferrite bead and two capacitors. This ensures that the USB controller does not emit excessive noise, which could affect the communication. |
Design Example: TDK MPZ1608 Ferrite Bead for Standby |
A design from a Japanese printer manufacturer uses a TDK MPZ1608 ferrite bead (1000 ohms at 100 MHz) on the standby 5V rail. The bead is followed by a 10 uF capacitor and preceded by a 1 uF capacitor. This pi-filter has a cutoff frequency of about 10 kHz, effectively removing the switching noise from the standby converter. The standby rail's noise is reduced to less than 5 mV, which is important because the USB transceiver uses the 5V rail as a reference for the differential signals. The manufacturer measured the USB eye diagram with and without the filter and found that the filter improved the eye opening from 70% to 90%, ensuring reliable USB communication. |
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Chapter 37: The Design Review Checklist for Power Supplies |
When an engineer designs a power supply for a printer, they go through a detailed checklist: Is the input voltage range correct for all target marketsIs the output voltage within toleranceIs the hold-up time sufficient for the printer's strobe pulseIs the EMI filter adequateAre the safety clearances (creepage and clearance distances) sufficient for the isolation voltageAre all components derated (operated below their maximum ratings)Is the thermal management adequateThis checklist ensures that the power supply will be reliable. |
Design Example: Power Supply Checklist from UL/EN Standards |
A design from a UK printer manufacturer follows the UL/EN 60950-1 standard for information technology equipment. The checklist includes: primary-to-secondary isolation of at least 3.75kV AC, creepage distance of at least 5 mm, clearance distance of at least 3 mm, and a temperature rise of less than 40C on all components. The manufacturer uses a safety-approved transformer and a flame-retardant PCB material (FR4). The checklist also includes a test for 'abnormal operation' - for example, shorting the output or the feedback optocoupler - to ensure the supply does not catch fire. The manufacturer sends the power supply to an independent test laboratory for certification. |
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Chapter 38: The Future - GaN, USB-PD, and Digital Power |
The power supplies of tomorrow will be smaller, cooler, and smarter. Gallium nitride (GaN) transistors allow switching at much higher frequencies (e.g., 1 MHz) than silicon MOSFETs, reducing the size of the transformer and the capacitors. USB Power Delivery (PD) allows printers to be powered from a USB-C port, using a universal power source that can negotiate up to 240W. Digital power supplies use a microcontroller to control the switching, enabling adaptive control algorithms that optimize efficiency for different loads. |
Design Example: GaN Systems GS-065-011-1-M |
A design from a Canadian startup printer manufacturer uses a GaN Systems GS-065-011-1-M, a 650V, 11A GaN transistor, in their prototype power supply. The switching frequency is 500 kHz, allowing a transformer that is 1/3 the size of a conventional 65 kHz transformer. The supply achieves 94% efficiency at full load. The manufacturer uses a digital controller (a TI C2000 microcontroller) that implements a predictive control algorithm - it anticipates the load change from the printhead and adjusts the switching parameters in advance, reducing the output voltage droop from 2V to 0.5V. The supply also includes a USB PD controller (STUSB4500) that negotiates a 24V, 5A output from a USB-C adapter. This eliminates the bulky AC adapter, making the printer truly portable. This is a glimpse of the future, and it is exciting to see how the humble power supply is evolving. |
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Detailed Summary - Tying It All Together |
We have journeyed through the entire power supply of a barcode label printer, from the AC inlet to the final clean DC rails. We started with the big picture: a printer needs multiple voltages - 24V for the printhead and motors, 5V and 3.3V for logic, and a standby rail for always-on functions. We learned that the flyback converter is the workhorse of the printer world, providing isolation, multiple outputs, and low cost. We saw how companies like Mean Well and CUI Inc. produce off-the-shelf supplies that many printer manufacturers use, and we saw how Texas Instruments, Infineon, and others provide the controller ICs and MOSFETs that make these supplies work. |
cs@easiersoft.com