eGAN-FET Driver Simplifies Switching Design

As switching devices for high-voltage power-conversion circuits, GaN (gallium-nitride) FETs appear poised to eat into silicon FETs’ market share. GaN-FET technology has a lot going for it. For example, the FETs boast a lower on-resistance than competing units, resulting in lower switching loss, a lower gate charge, and, in turn, faster switching. They also have much smaller footprints than do silicon devices.

Conventionally, a GaN device operates in depletion mode, meaning it is normally on: When you initially apply power, the GaN FET is in its on state—the opposite of silicon FETs. This situation is potentially hazardous.

To address this problem, EPC (Efficient Power Conversion) manufactures eGAN (enhancement-mode-GaN) FETs. EGaN FETs behave in a similar way to MOSFETs, use a bulk-CMOS process, and can take advantage of the inherent advantages of the process. The technology poses some design challenges for gate drivers, however. For example, they are more sensitive to gate overstress than are MOSFETs; their absolute gate voltage must be less than 6V, and their gate-threshold voltage, at less than 1.4V, is much lower than that of MOSFETs. They are also prone to fault turn-on due to dV/dt (change in voltage over time) during turn-off, and they require a low-impedance driver for turn-off.

To counter these challenges, National Semiconductor recently introduced what it claims is the industry’s first 100V halfbridge gate driver for use with eGaN FETs in high-voltage power converters. The new LM5113 high-side and lowside GaN-FET driver reduces component count by 75 percent and uses as much as 85 percent less PCB (printed-circuit-board) area than discrete-driver designs.



According to the company, the device uses proprietary technology to regulate the high-side floating bootstrap-capacitor voltage at approximately 5.25V to drive eGaN power FETs without exceeding the maximum gate-source voltage rating. The LM5113 also features independent sink and source outputs for flexible turnon strength with respect to the turn-off strength.

A 0.5Ω-impedance pulldown path provides a fast, reliable turn-off mechanism for the low-threshold-voltage eGaN power FETs. The LM5113 also integrates a high-side bootstrap diode, further minimizing PCB real estate, and provides independent logic inputs for the high- and low-side drivers, enabling flexibility for use in a variety of both isolated- and nonisolated-power-supply topologies. The LM5113 comes in a 10-pin 4×4mm LLP.

National Semiconductor