POWER DESIGN Selecting gate drivers for silicon and wide-bandgap devices

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A power semiconductor, whether made from silicon or wide-bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN), is a switch that rapidly turns on and off to conduct current and convert electrical power. While silicon MOSFET and IGBT switches win on cost and maturity, wide-bandgap switches offer enhanced thermal performance beyond bill of materials (BOM) file costs. All switches, whatever they’re made of, need a responsive device that lets them turn on and off quickly. That’s where gate drivers come in!

Power transistors need to turn on and off during the power conversion process to keep efficiency high and losses low. A microcontroller (MCU) or a controlled pulse-width modulation (PWM) integrated circuit (IC) sends a signal to the gate terminal to turn the transistor on and off. An MCU logic pin at high can give 5V or 3.3V to the gate pin, turning on the transistor. On the other hand, an MCU logic pin at low/0V turns off the transistor.

High currents and voltages are applicable in power electronics. Thousands of small power transistor cells are connected on the die. MCU alone cannot supply the required signal to turn a power transistor on. An optimal solution is to use a driver circuit to handle the gate signal. Simply put, a gate driver is a power amplifier that accepts a low-power input signal from the MCU and produces an appropriate voltage output for the gate.

The gate acts more like a capacitor. It uses this high current/voltage signal to quickly charge or discharge the gate capacitance. That’s how the power transistor rapidly turns on and off. The gate driver, MCU, and switch never work alone; they sit between the DC bus power supply at the input side and the load on the other. There are other critical components in the system, such as snubbers to clamp unwanted signals and DC-link capacitors that keep the bus steady.

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Choosing the switch first: Si vs WBG applications

When in power electronics, before betting on a gate driver, engineers must choose a switch in accordance with their application and tolerable ratings.

Silicon MOSFET: Medium-frequency and low-to-medium current applications. They were historically in demand for high-power applications, including DC-DC conversion, variable speed motor drives, welding, fluorescent lights, HVDC power distribution, and induction heating.

IGBT: IGBTs, again made from silicon, are suitable for medium- to high-power applications, such as switching power supplies in high-power electronics, variable-speed refrigerators, variable-speed air conditioners, uninterrupted power supplies, hybrid inverters in renewables, EVs, and industrial motors.

The story takes a different turn with SiC and GaN; wider bandgap allows them to withstand higher voltages and currents, exactly what power electronics need.

SiC MOSFET: With “safe” thermal performance, SiC MOSFET covers high-power applications from 650V to 3.3kV in EV drivetrains, EV on-board chargers, photovoltaics, and data center chargers.

GaN FET: GaN, however, dominates high-frequency space in RF applications and consumer chargers.

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How to select a suitable gate driver?

According to experts, gate driver selection is an iterative process. A designer needs to pick a potential gate driver IC and resistor, analyze datasheet values, and validate on the bench to refine the process.

Start with the power switch: Note the switch’s threshold voltage, gate charge (Qg), and device type.

Silicon MOSFET: +10V to 15V on, 0V off

IGBT: +15V on, -5V to -15V off

SiC MOSFET: +18V to +20V on, -3V to -5V off

This means that silicon MOSFETs want 10V to 15V on the gate. IGBTs and SiC MOSFETs, however, require a voltage swing between negative and positive to improve noise immunity and avoid gate voltage spikes.

Choose driver type: A gate driver can send input signals to drive the gate of one or various switches.

• Low-side gate driver sends an input signal to drive a single switch near the ground or negative side of the power supply.

• High-side gate driver drives a switch near the positive voltage rail.

• Half-bridge driver combines the capabilities of high-side and low-side gate drivers to drive two switches.

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Set the gate-drive voltage rail by choosing a turn-on and turn-off level.

Determine peak drive current: It is the maximum instantaneous current a gate driver delivers to the switch’s gate terminal during the switching process. It’s important to note that peak drive current runs for nanoseconds as it’s only a maximum value, not a continuous current. Once designers have the peak current value, they must choose a driver rated above that value to avoid failure.

Select the gate resistor: The gate resistor is a small series resistor placed between the output of the gate driver and the switch’s gate terminal. It controls how fast the switch turns on and off and also prevents false turn-on. Instead of choosing a single gate resistor, designers recommend using two separate resistors for turn-on and turn-off process control.

Voltage rating: Designers recommend picking a driver with a voltage rating 1.5-3x the operating voltage.

Add isolation: If the application needs to separate control and power stages, an optocoupler or transformer could provide isolation.

In high-power applications, isolated gate drivers provide galvanic isolation between the low-power MCU signal stage and the high-power switching stage.

SiC MOSFETs can swing at 150 V/ns. So, designers recommend choosing an isolated gate driver with a high common mode transient immunity (CMTI) rating, more than 100 kV/us, that prevents failures and data errors.

In low-to-medium power applications, non-isolated gate drivers share a common reference between the MCU control and power switches.

Bias supply: Provide a stable supply. If the driver’s supply voltage drops too low, it stops working. It doesn’t turn the switch off as a response. This is a well-documented problem in power electronics and is known as under-voltage lockout (UVLO). Gate drivers for SiC MOSFETs are built with UVLO.

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Timing: Check the timing to assess whether the gate driver switches at the right speed.

• Rise time shares how long the signal takes to go from low to high during the turn-on process.

• Fall time shares how long the signal takes to go from high to low during the turn-off process.

• Propagation delay shows the small lag between the MCU sending the command signal and the driver executing it. For SiC MOSFETs, values should be low.

• Delay matching shows how internal propagation between channels should match. It just occurs between the top and bottom, the switch with the ground and another with the voltage rail.

• Dead time shows when both switches are off during handover.

Check for built-in safety features: For protection, the driver detects DESAT or overcurrent in a short circuit. It turns off the switch at this stage. The driver slowly turns off, known as soft turn-off, to avoid a voltage spike.

For SiC MOSFETs, negative turn-off bias from -3V to -5V can block high dv/dt and prevent false triggering.

Calculate the expected power dissipation and junction temperature range: Designers recommend estimating power dissipation from gate charge, frequency, and voltage swing. On average, SiC MOSFETs can withstand a junction temperature greater than 175 degrees Celsius, whereas IGBTs reach up to a maximum of 150 degrees Celsius.

A double pulse test (DPT) can evaluate the switching performance of power semiconductors like power MOSFETs, IGBTs, and SiC MOSFETs with gate drivers. DPT uses function generators, oscilloscopes, voltage probes, a power supply, and a load to assess switching performance. Another testing method is to run simulations on different gate drivers with power switches on software such as SPICE and PLECS.

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