Industry's first eGaN FET Driver simplifies switching design

GaN (Gallium-Nitride) FETs appear poised to eat into silicon FETs market share as switching devices for high-voltage power conversion circuits. GaN FET technology has a lot going for it: The FETs boast a lower Rdson, resulting in lower switching loss, and a lower gate charge, resulting in higher switching speeds, and have a tiny footprint when compared to silicon devices. Conventionally, a GaN devices operates in depletion mode, meaning it is normally on: When power is initially applied the GaN FET is in its ON state. This is potentially hazardous, and is the opposite of silicon FETs.  Efficient Power Conversion Corp. (EPC) is the only manufacturer of enhancement mode GaN, or “eGaN” FETs. (EPC has trademarked the term eGaN.) Enhancement mode GaN FETs behave similarly to MOSFETs and, because they are based on a bulk CMOS process, can take advantage of the dost advantages of a bulk CMOS process.

Enhancement mode GaN poses some design challenges for gate drivers however. They are more sensitive to gate overstress than MOSFETs; their absolute gate voltage must be less than 6 V; Their gate threshold voltage is much lower than MOSFETs (< 1.4V); They can be prone to fault turn-on due to dV/dt during turn-off; They require a low impedance driver for turn-off.

National Semiconductor has just introduced the industry’s first 100V half-bridge gate driver optimized for use with enhancement-mode GaN FETs in high-voltage power converters.  The new LM5113 is a high-side and low-side GaN FET driver that the company claims reduces component count by 75 % and shrinks pc board area by up to 85% compared to discrete driver designs.

From National’s press release today: “National’s LM5113 is a 100V [half-bridge gate driver] for enhancement-mode GaN FETs.  Using proprietary technology, the device regulates the high side floating bootstrap capacitor voltage at approximately 5.25V to optimally drive enhancement-mode GaN power FETs without exceeding the maximum gate-source voltage rating.  The LM5113 also features independent sink and source outputs for flexibility of the turn-on strength with respect to the turn-off strength.  A low impedance pull down path of 0.5 Ohms provides a fast, reliable turn-off mechanism for the low threshold voltage enhancement-mode GaN power FETs, helping maximize efficiency in high frequency power supply designs.  The LM5113 features an integrated high-side bootstrap diode, further minimizing PCB real estate.  The LM5113 also provides independent logic inputs for the high-side and low-side drivers, enabling flexibility for use in a variety of both isolated and non-isolated power supply topologies.”

The LM5113 comes in a 10-pin 4 mm by 4 mm LLP package and cost $1.65 each in quantities of 1,000.  Samples are available now and production quantities will be available in September.