Applications of Bidirectional GaN Switches

Bidirectional GaN power ICs are designed for various applications, from motor drives and renewable energy inverters to USB chargers, portable electronics, E-bikes, and more. This article presents examples of the application possibilities with bidirectional GaN switches.

GaN high electron mobility transistors, HEMTs, or HFETs, have been fabricated using a 600 V process that produces devices with on-resistances down to 70 mΩ. Unidirectional and bidirectional devices were fabricated on the same wafer for performance comparison. A p-GaN gate is used to obtain normally-off properties in the devices.

To produce the bidirectional HFET, a second gate (G2) is added to the drift zone close to drain D1 and is used as the controlling gate for one direction of operation (Figure 1). The bidirectional GaN HFET is symmetrical and can carry the same current in both directions.

Figure 1. Cross-sections of unidirectional (left) and bidirectional (right) GaN HFETs. The bidirectional HFET can be operated from left to right with S1, G1, and D1 as the source, gate, and drain, respectively, or from right to left using S2, G2, and D2. (Image: Ferdinand-Braun-Institut)

The addition of the second gate didn’t materially impact device performance. The specific on-resistance for the bidirectional FET is the same in both directions and 3% higher than for the unidirectional devices. The off-state drain leakage current up to 600 V was below 5 µA for both devices. In a 10 A, 400 V clamped inductive switching test, both devices demonstrated slew rates of 60 V/ns during turn-off.

In another case, a device has been developed for topologies like Vienna rectifiers in power factor correction circuits, current-sources inverters in renewable energy systems, and solid-state circuit breakers. On an equivalent performance basis, the device is 4x smaller than a SiC bidirectional FET, 3x smaller than two uni-directional GaN devices, and 9x smaller than a traditional silicon solution (Figure 2).

Figure 2. A significant reduction in solution size is possible when using bidirectional GaN switches. The schematic symbol is on the right. (Image: Navitas Semiconductor)

USB port OVP protection

A 40 V, 4.8 mΩ bidirectional, normally-off, enhancement mode (e-mode) GaN transistor is available in a wafer-level chip-scale (WLCSP) package for lower power and voltage applications. It’s designed to replace back-to-back Si MOSFETs in battery management systems (BMS), overvoltage protection (OVP) applications, switching circuits for multiple power sources in standby and backup power systems, and high-side load switches in bidirectional converters. In addition to their low on-resistance, these devices are characterized for operation at 1 MHz.

ARPA-E CIRCUITS

The Advanced Research Projects Agency-Energy (ARPA-E) Creating Innovative and Reliable Circuits Using Inventive Topologies and Semiconductors (CIRCUITS) program sponsored the development of a GaN-based four-quadrant switch (FQS).

An FQS replaces two FETs, two IGBTs, and two diodes with a single device that provides bidirectional voltage control and current flow. It uses two gates to block the voltage of either polarity and supports current flow in either direction. The use of FQSes will reduce parts count, resulting in higher power densities, higher reliability, and lower-cost solutions.

Lateral GaN technology enables the fabrication of FQSes because the voltage-blocking region can be shared between the two sections of the device. That’s not possible with vertical structures used for Si and SiC devices. Prototype FQSes have been fabricated using 650 V GaN technology with a threshold voltage of 4 V. Being fabricated with GaN; these devices support fast switching and low losses for compact and high-efficiency solutions.

E-bikes and more

For battery-powered devices like electric bicycles (E-bikes) and energy storage systems, a 100 V bidirectional GaN-on-Si e-mode device is offered in a 4.0 x 6.0 mm flip-chip quad flat no-lead (FCQFN) package. These devices are rated for 100 A and have a maximum RDD(on) of 3.2 mΩ with a VG of 5 V and a typical QG of 90 nC with a VDD of 50 V.

This device’s applications include protection in a battery management system, (BMS) high-side switch in bidirectional converters, and switching circuits in multiple power supply systems. A BMS application can be paired with an analog front-end (AFE) IC that supports cell balancing and a gate driver (Figure 3). With proper heatsinking, this single-gate device is rated for 120 A.

Figure 3. BMS application with the AFE on top, the bidirectional GaN device on the bottom, and the gate driver IC in between. (Image: Innoscience)

Summary

Bidirectional GaN switches with ratings from 40 to 650 V have been developed. They can simplify and shrink a variety of applications, from portable electronics to e-bike battery systems, multi-kW motor drives, and renewable energy inverters.