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Gate Drivers Keep Up with GaN’s Fast Switching Speed

March 7, 2024
We take a look inside Analog Devices' first gate-driver IC specifically designed for GaN power switches.

Check out our coverage of APEC 2024. This article is part of the TechXchange: Gallium Nitride (GaN).

The GaN FET is becoming widely preferred in power systems such as high-frequency DC-DC converters.

As a result, most major players in power electronics have rolled out gallium-nitride (GaN) power FETs that use the unique power-handling properties of the material—reduced gate charge (Qg), output capacitance (COSS), and reverse-recovery losses—to switch faster and save more power.

The very fast switching speeds of GaN FETs make it possible to use smaller passive components as well as transformers, inductors, and other magnetics, saving space and costs. But the unique characteristics of GaN also create tough challenges.

Without gate-driver ICs specifically designed to control GaN power FETs, it’s more likely that you will leave performance on the table or even raise the risk of dealing permanently damage to the FET, said AI Wu, managing director for multi-market power division at Analog Devices.

To deliver more robust and reliable control, Analog Devices debuted a gate-driver IC that delivers source currents of 4 A at its peak and sink currents of up to 8 A, as well as fast propagation delays, to tap into the potential of GaN power FETs. As the company’s first gate driver designed from the ground up for GaN, the LT8418 can also tolerate the high-voltage transients (dv/dt) that may occur at fast switching speeds, up to 50 V every nanosecond, said Wu. Analog Devices introduced the gate driver at APEC 2024.

On top of supplying large source and sink currents, the 100-V gate driver delivers 0.6-Ω pull-up and 0.2-Ω pull-down resistance to adapt the turn-on and turn-off rates of the FET. The IC integrates a pair of driver stages so that it’s able to control both GaN power FETs in the half-bridge topology widely used in high-density power supplies. It can also be configured into the full-bridge or buck, boost, and buck-boost topologies.

Wu said the gate-driver IC is primarily a fit for high-frequency DC-DC converters. It’s also appropriate for motor drives and power supplies in data centers upgrading to GaN to save space and power.

Gate Drivers: The Key to Unlocking Faster Switching Speeds

The gate driver plays a central role in power electronics. It serves as the interface between the MCU or pulse-width-modulation (PWM) controller that pumps out signals to control a power supply’s duty cycle, frequency, and dead time, among other features, and the FET. Since the quality of the power output by a FET can impact the performance of the power supply, care must be taken when choosing gate-driver ICs.

While GaN power FETs feature faster switching speeds than silicon, they have to handle high transient voltages (dv/dt), which represents the rate of voltage change over time, while switching. The high dv/dt between the drain and source of the power FET can cause a wide range of issues: excess power losses, false switching, or even permanent damage to the device, said Wu. It’s the gate driver’s responsibility to carefully control and drive the FET and prevent any unintended consequences.

The more time a FET remains between on and off states, the more power is lost to switching transients. The current used to drive the gate in the device determines how long it takes to transition. If sufficient current is forced into the FET, the voltage rises to the point where the device turns itself on. The gate driver must be able to supply high current during the switching process to handle fast slew rates. Doing so reduces dead time and, thus, the power losses experienced—and the heat generated—by the FET.

The LT8418 is a split gate driver with separate turn-on and turn-off paths. This allows for adjustment of the turn-on and turn-off slew rates of GaN power FETs to limit ringing and reduce the electromagnetic interference (EMI) that can throw a wrench into the system at fast switching speeds, said Wu. The high- and low-side outputs have separate pull-up (0.6 Ω) and pull-down (0.2 Ω) resistances that can be used to tune the turn-on and turn-off times.

Wu said the inputs and outputs on the gate driver also have a default "low-state" to prevent the power FETs from falsely turning themselves on. Depending on the dv/dt, excess voltage can be induced at the gate input of the GaN FET. The voltage is based on the ratio between its gate-drain capacitance (Cgd) and gate-source capacitance (Cgs) or a current flowing to the resistor placed in front of the gate. The voltage can cause inadvertent turn-on of the FET.

The other challenge with GaN power FETs is the fact that the gates inside them are vulnerable to voltage spikes, specifically the FET on the high side of the power supply topology, causing permanent damage, warned Wu. During the dead-time interval between turn-off and turn-on of the gate inside it, the GaN power FET can experience a voltage drop of 2 to 3 V from the source to the drain—or even higher in some cases. The voltage drop will then be added to the supply voltage and can cause overvoltage.

Since it’s necessary to send voltage into the GaN FET very accurately to prevent any damage, a form of supply regulation is required. In general, the gate-driver IC is paired with a separate dc-dc bias supply or bootstrap diode to deliver a bootstrap voltage that helps drive the gate of the GaN power device on the high side of the power stage. Instead, the LT8418 integrates a “smart switch” that can fully control output of a balanced bootstrap voltage from VCC with a minimum dropout voltage, said Wu.

The “smart” bootstrap switch inside is used to regulate the high-side gate drive voltage to operate within the safe operating area (SOA) of the GaN FET, ensuring sufficient reliable operation under all conditions.

Gate Drivers Keep Power Losses Under Control

To keep up with fast switching frequencies in excess of 1 MHz, the LT8418, as mentioned, pumps out a peak source current of 4 A and a sink current of up to 8 A to enable rapid turn-on and turn-off of the GaN power FET.

With a supply voltage of 3.85 to 5.5 V, Analog Devices said the gate driver features a fast propagation delay of 10 ns and maintains a delay matching of 1.5 ns between the top and the bottom channels.

This allows for a more accurate reproduction of the desired power switching waveform with higher fidelity in response to a PWM signal from the MCU or other controller at the heart of the power supply.

One of the other unique characteristics of GaN power FETs is the lack of a body diode inside the device, which eliminates reverse-recovery losses (Qrr). By eliminating these power losses, GaN FETs can deliver better power efficiency in high-frequency, hard-switched topologies, said Wu.

Despite that, GaN power FETs may still experience power losses in the form of reverse-current conduction. Such conduction losses can be minimized by reducing the amount of dead time between on and off cycles as much as possible.

A short propagation delay is preferable because it enables shorter and less variable dead times between the on and off cycles of the FET, which reduces power losses—and heat—that can add up during rapid transitions. The gate driver contains a pair of PWM inputs that are fully independent (one each for the high and low sides of the half-bridge). They’re also logic-compatible in case even more precise control is desired.

The LT8418 is housed in a compact WLCSP BGA package measuring 1.67 × 1.67 mm to reduce parasitic inductance, enabling its widespread use in high-performance and high-density power systems.

The gate driver is equipped with several internal protections, including overvoltage and undervoltage lockout (UVLO) protection to prevent GaN power devices from turning on with insufficient drive voltage.

Check out more of our coverage of APEC 2024. Also, read more articles in the TechXchange: Gallium Nitride (GaN).

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