Moving On to GaN Switches in Switch-Mode Power Supplies

Abstract

GaN switches being used in switch-mode power supplies is relatively new. They promise higher efficiency and higher power density power supplies. This article discusses the readiness of this technology, mentions the challenges, and gives an outlook on the future of GaN as a replacement for silicon in switch-mode power supplies.

A common question today among power management design engineers is:

Is It Time to Switch from Silicon Power Switches to GaN-Based Ones?

Gallium nitride (GaN) technology offers many advantages over traditional silicon-based MOSFETs. As a wide band gap semiconductor, GaN allows power switches to operate at high temperatures and achieve high power density. It has a high breakdown voltage, making it suitable for applications above 100 V. Even below 100 V, GaN’s high power density and fast switching capability provide benefits like higher power conversion efficiency in various power supply designs.

The Challenges

There certainly are some challenges when replacing silicon-based MOSFETs with GaN devices. First, GaN switches typically have lower gate voltage ratings. Hence, it is important to ensure that driver stages are strict in limiting the maximum voltage to avoid damage to the GaN device.

Then one must deal with the fast voltage change (dv/dt) of the switch node of the power supply. This may cause false turn-on of the bottom switch. To solve this problem, a separate pull-up and pull-down pin and a carefully designed printed circuit board layout are needed.

Lastly, GaN FETs have a higher conduction loss during dead times. One way of solving this problem is to strictly minimize dead times. This needs to be done without generating overlapping times of the high-side and low-side switches to avoid a short circuit to ground.

How to Get Started

With the alluring promises of GaN in power supply design, a basic question is how to get started. A simple way is to select a switch-mode power supply controller IC, such as Analog Devices’ LTC7891 single phase, step-down (buck) GaN controller. Selecting a dedicated GaN controller makes a GaN power supply design simple and robust. All the challenges previously mentioned are addressed and solved with such controllers. Figure 1 shows the simplicity of a step-down power design using GaN FETs controlled by a dedicated GaN controller like the LTC7891.

Figure 1. A dedicated GaN controller yields a robust and dense power supply circuit.

Using Any Controller IC

If an existing power supply with an existing controller IC needs to be repurposed to control a GaN-based power supply, it makes sense to use a dedicated GaN driver. It takes care of solving the challenges with GaN and allows for a simple and robust design. Figure 2 shows the power stage of a buck regulator implemented with an LT8418 driver IC.

Figure 2. A dedicated GaN driver controlling a power stage based on logic PWM signals from a heritage silicon MOSFET controller.

Taking the First Steps

Once suitable hardware, controller IC, and GaN switches have been selected, a great way to get the first evaluation results is to use a detailed circuit simulation. ADI’s LTspice^® offers complete circuit models that may be used for simulation free of charge. This is a convenient way to learn about using GaN switches. Figure 3 shows a simulation schematic with the LTC7890, a dual-channel version of the LTC7891.

Figure 3. LTspice, a useful simulation tool for GaN power supplies.

Conclusion

GaN technology for switch-mode power supplies has reached a solid development state in which many power supply applications can be designed with them. However, there will be further development with each new generation of GaN switches. The existing switch-mode power supply controllers and drivers for GaN from ADI are a flexible, yet dependable way to work with GaN FETs from different vendors now and in the future.