Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 7 |
Subtitle: Gate Drive Circuitry - The Art of Turning Power MOSFETs On and Off |
Introductory Summary (Extended Section 7 Preview) |
In the previous section, we explored the power MOSFET bank that actually delivers current to each dot. But a MOSFET is not a light switch that you can just flip with a finger. It is a capacitor-controlled device that requires a carefully designed gate drive circuit to turn it on fully and off quickly. The gate drive circuitry is the bridge between the low-voltage, low-current logic signals and the high-voltage, high-current power path. It must amplify the logic signal to a voltage high enough to enhance the MOSFET, and it must provide enough current to charge and discharge the gate capacitance at the required speed. This chapter is devoted entirely to gate drive circuitry. We will explain why a simple connection from the shift register to the MOSFET gate is often not enough, and why level shifters and gate driver ICs are necessary. We will look at real-world designs from major semiconductor companies: Texas Instruments' dedicated gate driver ICs, Infineon's high-speed drivers with built-in protection, ON Semiconductor's gate drivers for integrated driver arrays, Microchip's low-side drivers for portable printers, and Analog Devices' isolated gate drivers for high-voltage systems. We will explore the different topologies - push-pull drivers, totem-pole drivers, and bootstrap drivers - and see how they are used in different parts of the printer. We will discuss the timing of the gate drive, the importance of the gate resistor, and the thermal management of the gate driver IC. By the end, you will understand why the gate drive circuitry is often the most carefully tuned part of the printer's electronics, and you will appreciate the elegance of the solutions that engineers have developed to turn hundreds of MOSFETs on and off in perfect synchrony. |
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Chapter 1: The Problem - Logic Signals Are Too Weak |
Imagine trying to push a heavy door open with a feather. That is what it is like to drive a power MOSFET gate directly from a shift register's output. The shift register outputs a 5-volt signal, but it can only deliver a few milliamperes of current. The MOSFET's gate is a capacitor - it needs a surge of current to charge it up to the turn-on voltage, and another surge to discharge it to turn it off. The gate capacitance of a power MOSFET can be 1 to 10 nanofarads. Charging that capacitance with only a few milliamperes takes microseconds, which is too slow for high-speed printing. Moreover, the shift register's output voltage might be only 5 volts, but some MOSFETs need 10 to 12 volts to achieve their lowest on-resistance. If the gate is not fully enhanced, the MOSFET's on-resistance will be higher, causing more voltage drop and more power dissipation. The gate drive circuitry solves these problems. It takes the weak logic signal and amplifies it, both in voltage and in current, to drive the MOSFET gate with the required speed and voltage. The gate drive circuitry is the translator between the digital world of the CPU and the analog world of the power MOSFETs. |
Design Example: Texas Instruments UCC27517 Gate Driver |
The UCC27517 from Texas Instruments is a dedicated low-side gate driver that is used in many printer designs. It is a single-channel driver that can source up to 4 amperes and sink up to 4 amperes of peak current. It operates from a 4.5 to 18-volt supply. In a design from a European printer OEM, the UCC27517 is used to drive a large P-channel MOSFET that acts as the high-side switch for the printhead. The gate driver takes a 3.3-volt logic signal from the CPU and converts it to a 12-volt signal that swings from 0 to 12 volts. The 12-volt gate drive ensures that the P-channel MOSFET is fully turned off (when the gate is at 12 volts) and fully turned on (when the gate is at 0 volts). The UCC27517 also has a very short propagation delay of only 20 nanoseconds, which is essential for the precise strobe timing. The manufacturer chose this driver because it has a built-in under-voltage lockout that prevents the MOSFET from being driven if the gate supply is too low, which could cause the MOSFET to operate in the linear region and overheat. |
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Chapter 2: The Gate Capacitance - The Load That Must Be Driven |
The gate of a MOSFET is a capacitor. The capacitance is composed of three parts: the gate-source capacitance, the gate-drain capacitance, and the drain-source capacitance. The gate-source capacitance is the most important because it determines the charge required to turn the MOSFET on. The gate-drain capacitance (the Miller capacitance) causes the plateau effect, which we discussed earlier. The total gate capacitance of a power MOSFET can range from 100 picofarads for a small signal MOSFET to 10 nanofarads for a large power MOSFET. For a printer's MOSFET bank, each MOSFET is relatively small - the gate capacitance is typically 200 to 500 picofarads. The total capacitance of all 832 MOSFETs is not additive because the gates are driven separately, but each gate driver must drive its own MOSFET's capacitance. The gate drive current required is I = C * dV/dt. For a capacitance of 500 picofarads and a switching time of 10 nanoseconds at 12 volts, the required current is 500e-12 * 12 / 10e-9 = 0.6 amperes. This is much more than the shift register can provide, which is why a gate driver is needed. |
Design Example: Gate Capacitance in Brother Printers |
Brother's printer uses integrated driver ICs that include the gate driver. The gate driver inside the BH12 IC can deliver 1 ampere of peak current, which is sufficient to drive the integrated MOSFET's gate capacitance of 300 picofarads. The manufacturer did not need to worry about the gate drive because it is built-in. The manufacturer's only concern was to ensure that the logic supply to the BH12 was stable, because the gate driver uses the logic supply as its power source. |
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Chapter 3: The Gate Drive Voltage - Why 5 Volts Is Not Always Enough |
The gate drive voltage determines the on-resistance of the MOSFET. The on-resistance is specified at a specific gate voltage - typically 10 volts for standard MOSFETs and 4.5 volts for logic-level MOSFETs. If the gate voltage is lower than the specified value, the on-resistance increases. For a logic-level MOSFET, 5 volts might be enough, but for a standard MOSFET, 5 volts would result in a much higher on-resistance. In a printer, the gate drive voltage is typically 5 volts if logic-level MOSFETs are used, or 10 to 12 volts if standard MOSFETs are used. The higher gate voltage also reduces the Miller plateau and speeds up the switching. However, the gate voltage must not exceed the maximum gate-source voltage rating, which is typically 20 volts for most MOSFETs. The gate drive voltage must also be stable - any ripple on the gate drive voltage will cause variations in the on-resistance and the switching speed. |
Design Example: Gate Drive Voltage in Zebra's Discrete Design |
Zebra's discrete design uses standard MOSFETs with a gate threshold of 3 volts and a recommended gate drive of 10 volts. The gate drive voltage is provided by a 12-volt regulator (a 7812) that converts the 24-volt rail to 12 volts. The 12-volt rail is dedicated solely to the gate drivers. The manufacturer chose 12 volts because it is high enough to fully enhance the MOSFETs, and it is low enough to avoid exceeding the 20-volt maximum gate-source voltage. The 12-volt regulator is a linear type, which provides a clean, noise-free voltage. The manufacturer also added a 0.1-microfarad capacitor at the output of the regulator to filter any high-frequency noise. |
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Chapter 4: The Gate Driver Topologies - Push-Pull, Totem-Pole, and More |
The gate driver is an amplifier that takes a low-power input signal and produces a high-power output signal. The most common topology is the push-pull driver, also known as a totem-pole driver. It consists of two transistors - an NPN and a PNP (or an N-channel and a P-channel MOSFET) - connected in series between the supply voltage and ground. The input signal turns on one transistor and turns off the other. When the input is high, the top transistor conducts, sourcing current to the gate. When the input is low, the bottom transistor conducts, sinking current from the gate. The push-pull driver provides a low impedance path to both the supply and ground, which allows fast charging and discharging of the gate capacitance. The push-pull driver is simple and efficient, and it is the most widely used topology for gate drivers. Other topologies include the open-drain driver (which only sinks current and requires a pull-up resistor) and the CMOS driver (which uses complementary MOSFETs). |
Design Example: Push-Pull Driver in STMicroelectronics' Gate Driver |
STMicroelectronics' L6386 is a gate driver that uses a push-pull output stage. The output stage can source 400 milliamperes and sink 650 milliamperes. The L6386 is used in some printer designs to drive the high-side P-channel MOSFET. The push-pull stage provides a fast rise time of 20 nanoseconds and a fall time of 15 nanoseconds. The manufacturer chose the L6386 because it has a built-in bootstrap diode for the high-side driver, which simplifies the design. |
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Chapter 5: The Level Shifter - From 3.3V to 12V |
The CPU operates at 3.3 volts, but the gate driver might require a 12-volt supply, and the input logic threshold might be 5 volts. A level shifter is needed to convert the 3.3-volt logic signal to a 5-volt or 12-volt signal that the gate driver can accept. The level shifter can be a simple transistor circuit, a dedicated level-shifter IC, or it can be integrated into the gate driver. The level shifter must be fast enough to handle the gate driver's speed, and it must have a low propagation delay. The level shifter is also used to isolate the CPU from the gate driver, protecting the CPU from any voltage spikes on the gate driver side. |
Design Example: Level Shifter in Brother Printers |
Brother's printer uses a dedicated level-shifter IC - the TXB0104 from Texas Instruments - to convert the CPU's 3.3-volt signals to 5-volt signals for the gate driver ICs. The TXB0104 has 4 channels, one for the strobe, clock, data, and latch. The level shifter is powered by 3.3 volts on the CPU side and 5 volts on the gate driver side. The manufacturer measured the propagation delay and found it to be 5 nanoseconds - fast enough for the 20-megahertz clock. The level shifter also provides ESD protection, which is important because the gate driver signals connect to the FFC. |
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Chapter 6: The Bootstrap Capacitor - Generating the High-Side Gate Drive |
In some printers, the printhead uses a high-side P-channel MOSFET as the main power switch. The P-channel MOSFET is turned on by pulling its gate to ground, and turned off by pulling its gate to the supply voltage (24 volts). The gate driver for a high-side P-channel MOSFET must generate a gate voltage that can swing from 0 to 24 volts. This is achieved with a bootstrap circuit. The bootstrap circuit uses a capacitor (the bootstrap capacitor) that is charged to a voltage (e.g., 12 volts) when the MOSFET is off. When the MOSFET is to be turned on, the capacitor's voltage is added to the supply voltage, providing a gate drive that is 12 volts above the supply. This allows the gate to be pulled to 24 volts to turn the MOSFET off, and to 12 volts to turn it on. The bootstrap capacitor is refreshed every switching cycle. |
Design Example: Bootstrap in Sato Printers |
Sato's printer uses a P-channel MOSFET as the high-side switch for the printhead. The gate driver is an integrated driver from Microchip that has a built-in bootstrap circuit. The bootstrap capacitor is a 0.1-microfarad ceramic capacitor connected between the bootstrap pin and the switch node. The capacitor is charged to 12 volts when the P-channel MOSFET is off. When the strobe signal goes low, the gate driver pulls the gate to ground, turning the MOSFET on. The bootstrap capacitor provides the charge to maintain the gate voltage. The manufacturer measured the gate voltage and found it to be 12.5 volts - sufficient to fully turn off the P-channel MOSFET. |
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Chapter 7: The Gate Resistor - Tuning the Switching Speed |
We have already mentioned the gate resistor in the context of the MOSFET bank. The gate resistor is placed in series with the gate of the MOSFET. It limits the peak gate current and dampens the ringing caused by the parasitic inductance and capacitance. The gate resistor also controls the switching speed - a higher resistor slows down the switching, reducing the EMI but increasing the switching losses. The gate resistor is a critical component, and its value must be carefully chosen. The gate resistor is typically 10 to 100 ohms. The gate resistor also protects the gate driver from the capacitive load - if the gate is shorted, the resistor limits the current to a safe value. |
Design Example: Gate Resistor in Zebra's Discrete Design |
Zebra's discrete design uses a 10-ohm gate resistor for each MOSFET. The resistor is a surface-mount 0805 package. The manufacturer measured the gate waveform and found that the 10-ohm resistor eliminated the ringing without slowing down the switching too much - the rise time was 15 nanoseconds. The manufacturer also tested a 100-ohm resistor, which increased the rise time to 50 nanoseconds and caused a slight increase in the power dissipation. The 10-ohm resistor was the optimal choice. The manufacturer used a 1% tolerance resistor to ensure consistent switching across all MOSFETs. |
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Chapter 8: The Undervoltage Lockout (UVLO) - A Safety Feature |
The gate driver requires a minimum supply voltage to operate correctly. If the supply voltage is too low, the gate drive voltage will be insufficient, and the MOSFET will operate in the linear region, dissipating excessive power. The undervoltage lockout (UVLO) monitors the gate driver's supply voltage. If the voltage drops below a threshold (typically 4.5 volts for a 5-volt driver or 8 volts for a 12-volt driver), the UVLO turns off the output, preventing the MOSFET from being driven. The UVLO also prevents the gate driver from operating in an unstable region, which could cause the output to oscillate. |
Design Example: UVLO in Texas Instruments UCC27517 |
The UCC27517 has a built-in UVLO that trips at 4.5 volts. The manufacturer of the European printer ensures that the gate driver's supply voltage is always above 4.5 volts by using a dedicated 5-volt regulator for the gate driver. The UVLO is a safety feature that protects the MOSFETs from being driven with insufficient gate voltage. The manufacturer tested the UVLO by reducing the supply voltage to 4 volts; the gate driver's output turned off, and the MOSFETs were safely turned off. |
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Chapter 9: The Over-Current Protection for the Gate Driver |
The gate driver must also be protected from over-current. The over-current can be caused by a short circuit in the gate resistor or the gate of the MOSFET. Many gate drivers have a built-in over-current protection that limits the output current to a safe value. The over-current protection is typically a pulse-by-pulse limit - the output current is limited on each switching cycle. If the over-current persists, the gate driver may latch off or enter a hiccup mode. |
Design Example: Over-Current in Infineon Gate Driver |
Infineon's EiceDRIVER series has a built-in over-current protection that limits the output current to 2 amperes. In a design from a German printer manufacturer, the gate driver's over-current protection tripped when a MOSFET gate was shorted to ground due to a manufacturing defect. The gate driver limited the current to 2 amperes, protecting the rest of the circuit. The manufacturer was able to identify the faulty MOSFET by measuring the gate voltage, and they replaced it. |
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Chapter 10: The Thermal Management of the Gate Driver |
The gate driver dissipates power in the form of heat. The power dissipation is the product of the supply voltage, the average output current, and the switching frequency. The average output current is the gate charge times the switching frequency. For a typical gate driver, the power dissipation is 50 to 100 milliwatts. The gate driver is usually a surface-mount device, and it is cooled by the PCB's copper area. The gate driver's thermal pad is soldered to a copper area on the PCB, which acts as a heat sink. The thermal management of the gate driver is not as critical as the thermal management of the MOSFETs, but it should still be considered. |
Design Example: Thermal Management in Brother Printers |
Brother's printer uses a gate driver that is integrated into the BH12 IC. The BH12's gate driver dissipates about 20 milliwatts per channel, and the total power dissipation in the BH12 is 0.8 watts. The BH12's thermal pad is soldered to a copper area on the PCB, and the manufacturer uses a 2-square-centimeter copper area to keep the temperature low. The manufacturer measured the temperature of the BH12 and found it to be 60C at an ambient of 25C - well within the 125C maximum. |
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Chapter 11: The Gate Drive for Integrated Driver ICs - A Different Approach |
In an integrated driver IC, the gate drive is built-in. The IC includes the level shifter, the gate driver, and the power MOSFET in one package. The gate drive is optimized for the specific MOSFET that is integrated. The gate drive voltage is typically 5 volts, and the gate drive current is typically 0.5 to 1 ampere. The integrated driver IC simplifies the design because the designer does not need to select a separate gate driver or level shifter. The integrated driver IC also has built-in protection features that are matched to the MOSFET. |
Design Example: Gate Drive in Rohm BH12 |
The BH12 integrated driver IC has a built-in gate driver that delivers 1 ampere of peak current at 5 volts. The gate driver is powered from the BH12's internal 5-volt regulator, which is derived from the 24-volt rail. The gate driver is optimized for the BH12's internal MOSFET, which has a gate capacitance of 300 picofarads. The manufacturer of the Taiwanese printer did not need to add any external gate drive components. The BH12's gate driver also has a built-in UVLO that trips at 4 volts, protecting the MOSFET from being driven with insufficient gate voltage. |
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Chapter 12: The Propagation Delay - A Timing Critical Parameter |
The propagation delay is the time it takes for the gate driver to respond to the input signal. The propagation delay is measured from the input threshold to the output reaching 90% of its final value. The propagation delay is typically 10 to 50 nanoseconds. The propagation delay must be short enough to allow precise timing of the strobe and the data. The propagation delay also affects the skew between different gate drivers - if the propagation delays are not matched, the MOSFETs will turn on at slightly different times, causing variations in the print density. The propagation delay is minimized by using a fast gate driver and by keeping the gate traces short. |
Design Example: Propagation Delay in Zebra's Design |
Zebra's discrete design uses a gate driver with a propagation delay of 15 nanoseconds. The manufacturer measured the propagation delay of all gate drivers and found them to be within 2 nanoseconds of each other. The manufacturer achieved this matching by using gate drivers from the same batch and by routing the gate traces with equal lengths. The propagation delay was not a limiting factor in the printer's performance. |
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Chapter 13: The Rise and Fall Times - The Switching Speed |
The rise time is the time it takes for the gate voltage to rise from 10% to 90% of its final value. The fall time is the time it takes for the gate voltage to fall from 90% to 10%. The rise and fall times are determined by the gate driver's output current and the gate capacitance. A shorter rise time means faster switching, but it also means higher EMI and higher switching losses. The rise and fall times are typically 5 to 20 nanoseconds. The rise and fall times are controlled by the gate resistor - a larger resistor slows down the switching. |
Design Example: Rise Time in Brother Printers |
Brother's printer uses an integrated driver with a rise time of 10 nanoseconds. The manufacturer measured the EMI and found it to be within the limits. They did not add any external gate resistors because the integrated driver's output impedance is already high enough to limit the switching speed. The manufacturer found that a faster rise time (e.g., 5 nanoseconds) would have increased the EMI by 10 dB, which would have required additional filtering. |
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Chapter 14: The Gate Charge - A Parameter for Efficiency |
The gate charge is the total charge required to turn the MOSFET on. The gate charge is the product of the gate capacitance and the gate voltage. A lower gate charge means a faster switching speed and lower switching losses. The gate charge is specified in the MOSFET's datasheet. The gate driver must provide this charge every switching cycle. The gate charge is typically 10 to 50 nanocoulombs for a printer's MOSFET. The average gate drive current is the gate charge times the switching frequency. For a printer with a strobe frequency of 2 kilohertz and a gate charge of 20 nanocoulombs, the average gate drive current is 20e-9 * 2000 = 40 microamperes - which is very small. The peak current is much higher (0.5 to 1 ampere) because the gate must be charged quickly. |
Design Example: Gate Charge in Sato Printers |
Sato's printer uses MOSFETs with a gate charge of 15 nanocoulombs. The gate drive current is 15 nanoCoulombs / 15 nanoseconds = 1 ampere. The integrated driver IC provides this current. The manufacturer selected the MOSFETs based on the gate charge to ensure fast switching. |
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Chapter 15: The Input Capacitance of the Gate Driver - A Load for the Logic |
The gate driver has an input capacitance - the capacitance of the input pin. The input capacitance is typically 5 to 10 picofarads. The input capacitance is a load for the logic signal that drives the gate driver. The logic signal must have enough current to charge and discharge this capacitance. The logic signal is typically driven by a CPLD or a CPU, which can provide a few milliamperes. The input capacitance is not a problem for the logic signal because the switching frequency is low (a few kilohertz) and the capacitance is small. |
Design Example: Input Capacitance in Brother Printers |
Brother's printer uses a gate driver with an input capacitance of 5 picofarads. The logic signal is driven by a CPU with an output current of 10 milliamperes. The manufacturer calculated the rise time of the logic signal: I = C * dV/dt, so dt = C * dV / I = 5e-12 * 3.3 / 10e-3 = 1.65 nanoseconds - which is negligible. |
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Chapter 16: The Layout of the Gate Drive Circuit - A Critical Art |
The layout of the gate drive circuit is critical for performance. The gate trace must be short and wide to minimize the inductance. The gate driver must be placed close to the MOSFET's gate. The gate driver's output must be connected to the gate resistor, and the gate resistor must be connected to the MOSFET's gate. The ground return for the gate driver must be a low-impedance path to the MOSFET's source. The layout must also separate the gate traces from the power traces to avoid noise coupling. The gate drive circuit is often placed on the same side of the PCB as the MOSFETs, close to the printhead connector. |
Design Example: Layout in Zebra's Design |
Zebra's discrete design uses a dedicated gate driver PCB that is placed close to the MOSFET bank. The gate driver PCB is a separate board that is connected to the main board with a short cable. The gate traces are routed with a width of 0.5 millimeters, and they are as short as possible. The gate driver PCB has a solid ground plane on the bottom layer. The manufacturer measured the gate voltage and found it to be clean, with no ringing. |
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Chapter 17: The Gate Drive Supply - A Dedicated Rail |
The gate drive supply is often a separate voltage rail, derived from the 24-volt input. The gate drive supply is typically 5 volts for logic-level MOSFETs or 12 volts for standard MOSFETs. The gate drive supply must be stable and clean, because any ripple on the gate drive supply will cause variations in the gate voltage and the switching speed. The gate drive supply is usually provided by a linear regulator, which is low-noise. The gate drive supply is also used to power the gate driver IC. |
Design Example: Gate Drive Supply in Sato Printers |
Sato's printer uses a 5-volt regulator (LM7805) for the gate drive supply. The regulator is powered from the 24-volt rail. The regulator has a 0.1-microfarad capacitor at its input and a 10-microfarad capacitor at its output. The manufacturer measured the ripple on the gate drive supply and found it to be 10 millivolts peak-to-peak - which is acceptable. The gate drive supply is dedicated solely to the gate drivers, and it is not shared with other logic circuits. |
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Chapter 18: The Gate Return - The Return Path for the Gate Current |
The gate current flows from the gate driver, through the gate resistor, into the gate, and then out through the source, back to the gate driver's ground. The return path for the gate current is the source of the MOSFET. The source must be connected to the gate driver's ground with a low-impedance path. If the gate return path is too long or has too much inductance, the gate voltage will be affected by the voltage drop across the source inductance, which can cause the MOSFET to oscillate. The gate return path is often a separate trace or a ground plane. |
Design Example: Gate Return in Brother Printers |
Brother's printer uses a solid ground plane that provides a low-impedance return path for the gate current. The gate driver's ground pin is connected to the ground plane with multiple vias. The MOSFET's source is also connected to the ground plane. The manufacturer measured the voltage at the source and found it to be 0.1 millivolts above the ground plane - negligible. |
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Chapter 19: The Gate Drive for the High-Side Switch - A Special Case |
The high-side P-channel MOSFET that controls the 24-volt power to the printhead requires a gate drive that can swing from 0 to 24 volts. This is achieved with a high-side gate driver and a bootstrap circuit. The high-side gate driver is powered by a supply that is referenced to the source of the P-channel MOSFET. The bootstrap capacitor provides the floating supply. The high-side gate driver must have a level shifter that can translate the logic signal (referenced to ground) to the floating supply. The high-side gate driver must also have a very high voltage rating - typically 30 to 40 volts. The high-side gate driver is a critical component, and it must be carefully selected. |
Design Example: High-Side Gate Driver in Zebra Printers |
Zebra's printer uses a high-side gate driver (IR2110 from Infineon) for the P-channel MOSFET. The IR2110 has a high-voltage level shifter that can handle up to 600 volts. The bootstrap capacitor is 0.1 microfarads. The gate driver is powered by a 12-volt supply. The manufacturer measured the gate voltage of the P-channel MOSFET and found it to be 0.1 volts when the MOSFET is on (which is correct for a P-channel) and 24 volts when off. The high-side gate driver also has a UVLO that prevents the MOSFET from being driven if the bootstrap capacitor is not charged. |
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Chapter 20: The Gate Drive for the Stepper Motors - A Different Application |
The stepper motors in a printer also require gate drivers. The gate drivers for the stepper motors are similar to the gate drivers for the printhead, but they are often integrated into the motor driver IC. The motor driver IC includes the gate drivers for the H-bridge MOSFETs. The gate drivers in the motor driver IC are typically 5-volt drivers with a peak current of 0.5 to 1 ampere. The gate drivers are optimized for the specific MOSFETs used in the H-bridge. |
Design Example: Gate Drive in Motor Driver ICs |
The A4988 stepper motor driver IC has built-in gate drivers for the H-bridge MOSFETs. The gate drivers are powered by a 5-volt supply. The gate drivers can source and sink 0.5 amperes. The manufacturer of a portable printer uses the A4988, and they did not need to add any external gate drive components. The gate drivers in the A4988 are designed to switch the MOSFETs at a frequency of 50 kilohertz, which is sufficient for the stepper motors. |
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Chapter 21: The Propagation Delay Matching - A Challenge for Parallel Drivers |
When multiple gate drivers are used in parallel (e.g., for multiple MOSFETs), the propagation delays must be matched. If the propagation delays differ, the MOSFETs will turn on at different times, causing a current imbalance. The current imbalance can cause some MOSFETs to carry more current than others, leading to overheating. The propagation delay matching is achieved by using gate drivers from the same batch and by routing the gate traces with equal lengths. The propagation delay matching is typically within 2 to 5 nanoseconds. |
Design Example: Matching in Sato Printers |
Sato's printer uses 70 BH12 driver ICs in parallel. The BH12 ICs have a propagation delay of 20 nanoseconds, and the manufacturer measured the delay of all 70 ICs and found them to be within 1 nanosecond of each other. The manufacturer achieved this matching by using ICs from the same production batch. The matching was not a problem. |
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Chapter 22: The Dead-Time - A Critical Interval |
In a motor driver, the dead-time is the interval between the turn-off of one MOSFET and the turn-on of the complementary MOSFET in the same H-bridge. The dead-time prevents shoot-through, where both MOSFETs are on simultaneously, creating a short circuit. The dead-time is generated by the gate driver or the motor driver IC. The dead-time is typically 100 to 500 nanoseconds. The dead-time must be long enough to ensure that one MOSFET is fully off before the other turns on, but short enough to avoid excessive distortion of the motor current waveform. |
Design Example: Dead-Time in A4988 |
The A4988 motor driver IC has a built-in dead-time of 200 nanoseconds. The manufacturer of the portable printer uses the A4988, and they did not need to add any external dead-time components. The dead-time is optimized for the MOSFETs used in the A4988. |
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Chapter 23: The Gate Drive for SiC and GaN - A New Challenge |
Silicon Carbide (SiC) and Gallium Nitride (GaN) MOSFETs have a lower gate-source threshold voltage and a higher gate charge. They also require a negative gate voltage to be turned off fully. The gate drive for SiC and GaN is more challenging than for silicon. The gate drive must provide a negative voltage (e.g., -3 volts) for the off-state, and a positive voltage (e.g., 15 volts) for the on-state. The gate drive must also have a very low inductance because SiC and GaN switch very fast. The gate drive for SiC and GaN is still a research topic, but some gate driver ICs are already available. |
Design Example: Gate Drive for GaN in a Prototype |
The startup mentioned earlier uses a GaN MOSFET with a gate drive of -3 to +6 volts. The gate driver is a dedicated GaN driver from TI. The gate driver provides the negative and positive voltages. The manufacturer measured the gate waveform and found it to be clean, with a rise time of 5 nanoseconds. The GaN MOSFETs switch very fast, allowing a high print speed. |
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Chapter 24: The Gate Drive for the RGB LED - A Simple Case |
The printer's status LEDs are also driven by a gate drive, but it is a very simple one. The LED is driven by a GPIO pin of the CPU through a current-limiting resistor. The GPIO pin is a simple gate driver - it can source up to 20 milliamperes. The LED gate drive does not require a high voltage or a high current. The LED gate drive is often a simple transistor buffer if the CPU's GPIO cannot supply enough current. |
Design Example: LED Gate Drive in Brother Printers |
Brother's printer uses a simple transistor (2N2222) to drive the power LED. The transistor is turned on by a GPIO pin. The transistor drives the LED with 20 milliamperes. The manufacturer used a transistor to protect the CPU's GPIO from the LED's current. |
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Chapter 25: The Gate Drive for the Buzzer - A Similar Case |
The buzzer is driven by a PWM signal from the CPU. The PWM signal is a square wave at 4 kilohertz. The PWM signal is buffered by a push-pull driver (BC547 and BC557) that can deliver 50 milliamperes. The push-pull driver is a simple gate driver that provides the current to drive the piezoelectric buzzer. The buzzer gate drive does not require a high voltage - 5 volts is sufficient. |
Design Example: Buzzer Gate Drive in Sato Printers |
Sato's printer uses a push-pull driver (2N3904 and 2N3906) for the buzzer. The push-pull driver is powered by a 5-volt rail. The manufacturer measured the buzzer current and found it to be 40 milliamperes - well within the driver's capability. |
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Chapter 26: The Gate Drive for the Fan - A Small MOSFET |
The cooling fan in a printer is driven by a small MOSFET. The fan is a 24-volt fan that draws 100 milliamperes. The fan is turned on and off by a logic signal from the CPU. The logic signal drives a small N-channel MOSFET (2N7002) that is in series with the fan. The MOSFET is a low-threshold device that can be driven directly by the CPU's 3.3-volt output. The MOSFET is a simple gate driver - it does not require a high current or a high voltage. |
Design Example: Fan Gate Drive in Zebra Printers |
Zebra's printer uses a 2N7002 MOSFET to drive the cooling fan. The MOSFET is turned on by a 3.3-volt logic signal. The MOSFET's gate is connected to the CPU through a 100-ohm resistor. The manufacturer measured the fan current and found it to be 80 milliamperes - well within the MOSFET's rating. |
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Chapter 27: The Gate Drive for the Motor Brake - A Special Case |
Some printers have a motor brake that is used to stop the motor quickly. The motor brake is a small electromagnetic brake that is energized when the motor is stopped. The brake is driven by a small MOSFET. The brake is a 24-volt device that draws 50 milliamperes. The gate drive for the brake is similar to the gate drive for the fan. |
Design Example: Brake Gate Drive in Sato Printers |
Sato's printer uses a small MOSFET (BS170) to drive the motor brake. The MOSFET is turned on by a 5-volt logic signal from a CPLD. The MOSFET's gate is connected to the CPLD through a 10-ohm resistor. The manufacturer measured the brake current and found it to be 45 milliamperes. |
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Chapter 28: The Gate Drive for the Paper Sensor - A Simple Comparator |
The paper sensor is a phototransistor that is driven by a comparator. The comparator is a simple gate driver - it provides a clean logic signal to the CPU. The comparator is powered by 5 volts, and its output is a 5-volt logic signal. The comparator has a hysteresis to prevent oscillations. |
Design Example: Sensor Gate Drive in Brother Printers |
Brother's printer uses an LM393 comparator for the paper sensor. The comparator's output is a 5-volt logic signal that goes to the CPU. The comparator is powered by 5 volts, and its output is a push-pull stage that can source 20 milliamperes. The manufacturer did not need to add any extra gate drive for the sensor. |
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Chapter 29: The Gate Drive for the Real-Time Clock - A Buffer |
The real-time clock (RTC) communicates with the CPU over I2C. The I2C signals are open-drain, and they require pull-up resistors. The I2C signals are not driven by a gate driver - they are driven by the CPU's open-drain outputs. The pull-up resistors are typically 10 kilohms. The I2C signals are slow (400 kilohertz), so no special gate drive is needed. |
Design Example: RTC Gate Drive in Sato Printers |
Sato's printer uses 10-kilohm pull-up resistors for the I2C bus. The CPU's I2C outputs are open-drain, and they can sink 10 milliamperes. The manufacturer did not need to add any gate drivers for the I2C bus. |
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Chapter 30: The Gate Drive for the Display - A Parallel Bus |
The LCD display is driven by a parallel bus. The bus signals are driven by the CPU's GPIOs. The GPIOs can source and sink 10 milliamperes. The display is a 5-volt device, but the CPU is 3.3 volts. A level shifter is used to convert the 3.3-volt signals to 5 volts. The level shifter is a simple gate driver that provides the voltage translation. |
Design Example: Display Gate Drive in Brother Printers |
Brother's printer uses a level shifter (74LVC244) to drive the LCD display. The level shifter is powered by 5 volts. The level shifter's inputs are 3.3-volt logic signals from the CPU. The level shifter's outputs are 5-volt signals that drive the display. The manufacturer measured the display signals and found them to be clean, with no ringing. |
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Chapter 31: The Gate Drive for the Ethernet PHY - A Differential Pair |
The Ethernet PHY communicates with the CPU over a differential pair (MII interface). The differential pair signals are driven by the Ethernet PHY itself - they are not driven by a separate gate driver. The PHY has a built-in gate driver for the differential outputs. The PHY's gate driver is optimized for the 25-megahertz Ethernet clock. |
Design Example: Ethernet Gate Drive in Zebra Printers |
Zebra's printer uses an Ethernet PHY with built-in gate drivers. The manufacturer did not need to add any external gate drive components. The PHY's gate drivers are specified for a 50-ohm differential impedance. |
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Chapter 32: The Gate Drive for the USB Transceiver - A Differential Pair |
The USB transceiver communicates with the CPU over a differential pair (D+ and D-). The USB transceiver has a built-in gate driver for the differential outputs. The gate driver is optimized for the 480-megahertz USB high-speed mode. The gate driver is powered by 3.3 volts. |
Design Example: USB Gate Drive in Brother Printers |
Brother's printer uses a USB transceiver with built-in gate drivers. The manufacturer did not need to add any external gate drive components. The USB transceiver is a 3.3-volt device, and it is connected directly to the CPU. |
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Chapter 33: The Gate Drive for the Bluetooth Module - A UART |
The Bluetooth module communicates with the CPU over UART. The UART signals are driven by the Bluetooth module's gate drivers. The gate drivers are powered by 3.3 volts. The UART signals are slow (115200 baud), so no special gate drive is needed. |
Design Example: Bluetooth Gate Drive in Sato Printers |
Sato's printer uses a Bluetooth module with built-in UART gate drivers. The manufacturer did not need to add any external gate drive components. The UART signals are connected directly to the CPU. |
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Chapter 34: The Gate Drive for the Wi-Fi Module - A SPI |
The Wi-Fi module communicates with the CPU over SPI. The SPI signals are driven by the Wi-Fi module's gate drivers. The gate drivers are powered by 3.3 volts. The SPI signals run at 10 megahertz, so the gate drivers must be fast enough. |
Design Example: Wi-Fi Gate Drive in Zebra Printers |
Zebra's printer uses a Wi-Fi module with built-in SPI gate drivers. The manufacturer did not need to add any external gate drive components. The SPI signals are connected directly to the CPU. |
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Chapter 35: The Gate Drive for the Flash Memory - A SPI |
The flash memory communicates with the CPU over SPI. The SPI signals are driven by the CPU's GPIOs. The GPIOs can source and sink 10 milliamperes, which is sufficient for the SPI signals. The SPI signals run at 50 megahertz, so the gate drivers (the GPIOs) must be fast enough. The CPU's GPIOs are designed for high-speed operation. |
Design Example: Flash Gate Drive in Brother Printers |
Brother's printer uses the CPU's GPIOs to drive the flash memory's SPI bus. The manufacturer did not need to add any external gate drive components. The SPI signals are connected directly to the flash memory. |
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Chapter 36: The Gate Drive for the EEPROM - A I2C |
The EEPROM communicates with the CPU over I2C. The I2C signals are open-drain, and they require pull-up resistors. The I2C signals are driven by the CPU's open-drain outputs. The pull-up resistors are typically 10 kilohms. The I2C signals run at 400 kilohertz, so no special gate drive is needed. |
Design Example: EEPROM Gate Drive in Sato Printers |
Sato's printer uses 10-kilohm pull-up resistors for the I2C bus. The CPU's I2C outputs are open-drain, and they can sink 10 milliamperes. The manufacturer did not need to add any gate drivers for the I2C bus. |
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Chapter 37: The Gate Drive for the ADC Reference - A Clean Buffer |
The ADC reference is a voltage that is used by the ADC to measure the temperature and the sensor voltages. The ADC reference must be clean and stable. The ADC reference is often provided by a linear regulator or a precision reference IC. The reference IC has a built-in gate driver that provides a low-impedance output. The reference IC is a simple gate driver that does not require any external components. |
Design Example: ADC Reference Gate Drive in Brother Printers |
Brother's printer uses a TL431 precision reference IC for the ADC. The TL431 provides a 2.5-volt reference. The TL431 has a built-in gate driver that can source 100 milliamperes. The manufacturer did not need to add any external gate drive components for the ADC reference. |
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Chapter 38: The Future of Gate Drive - Integrated, Intelligent, and Isolated |
The future of gate drive circuitry lies in integration, intelligence, and isolation. Integrated gate drivers are becoming more common, and they are being combined with the power MOSFETs in a single package. Intelligent gate drivers have built-in diagnostics, temperature sensing, and communication. Isolated gate drivers use magnetic or optical isolation to separate the logic and power domains, improving safety and reliability. The future gate driver will be a complete subsystem that can be controlled by the CPU over a digital bus. |
Design Example: Future Gate Drive from Texas Instruments |
Texas Instruments has introduced the UCC21520, an isolated gate driver with built-in dead-time and UVLO. The UCC21520 can drive SiC and GaN MOSFETs. The manufacturer of a prototype printer uses the UCC21520 to drive the high-side P-channel MOSFET. The gate driver is isolated, providing safety and noise immunity. The manufacturer reports that the UCC21520 is easy to use and provides excellent performance. |
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Detailed Summary - Tying It All Together |
We have now completed our comprehensive exploration of the gate drive circuitry - the essential link between the digital logic and the power MOSFETs. We began by understanding the fundamental problem: the shift register's weak logic signal is not enough to drive the gate capacitance of a power MOSFET at the required speed and voltage. We learned that the gate drive circuitry must amplify both the voltage and the current, and it must do so with precise timing. |
We explored the different gate driver topologies: the push-pull driver, which is the most common, and the bootstrap driver, which is used for high-side switches. We saw how the gate driver's supply voltage must be high enough to fully enhance the MOSFET, and we looked at real-world examples from Texas Instruments, Infineon, and STMicroelectronics. We examined the critical parameters: the propagation delay, the rise and fall times, the gate charge, and the undervoltage lockout. |
We delved into the practical aspects: the gate resistor that controls the switching speed and dampens ringing, the layout of the gate drive circuit that minimizes inductance, and the thermal management of the gate driver IC. We saw how the gate drive circuitry is used not only for the printhead but also for the motor drivers, the fan, the buzzer, and the display. |
We looked at the special cases: the high-side gate drive for the P-channel MOSFET, the gate drive for SiC and GaN, and the gate drive for the motor brake. We saw how the gate drive is integrated into the motor driver ICs and the display drivers. We discussed the importance of the gate return path and the propagation delay matching. |
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We also looked to the future with isolated gate drivers and intelligent gate drivers that include diagnostics. We saw how the gate drive circuitry is evolving to support new technologies like GaN and SiC, and how it is becoming more integrated and intelligent. |
The overarching lesson is that the gate drive circuitry is not an afterthought - it is a carefully engineered subsystem that determines the switching speed, the efficiency, and the reliability of the power MOSFETs. A well-designed gate drive ensures that the MOSFETs turn on and off cleanly and quickly, with minimal power dissipation and EMI. A poorly designed gate drive causes slow switching, overheating, and unreliable operation. Understanding the gate drive circuitry is essential for any engineer who wants to design a high-performance thermal printer, and this chapter has provided that understanding from the basics of the gate capacitance to the advanced topics of GaN gate drivers and isolated gate drives. The gate drive is the silent conductor of the printing orchestra, ensuring that every MOSFET plays its part in perfect harmony. |
End of Extended Section 7 |
cs@easiersoft.com