Bernhard Zojer

A new driving concept for normally-on GaN switches in cascode configuration

Normally-on high-voltage (HV) power transistors are usually operated in a series connection with low-voltage (LV) MOSFETs to ensure safe operation. In the widely used cascode configuration (CC) the status of the combined switch is controlled via the MOSFET gate, whereas the alternative direct drive (DD) method controls the gate of the HV switch directly and utilizes the LV transistor as a safety switch only. Both concepts have their respective benefits: CC allows simple standard driving schemes, while DD excels in low switching losses. This paper investigates a third approach: by controlling both HV and LV gates an optimization of switching performance particularly in hard-switched half-bridge applications can be achieved. However, a straight-forward implementation of such a “dual drive” (2D) concept would obviously require a sophisticated driving scheme to allow independent control of both gates. The key idea of this paper is thus the substitution of a separate HV gate driver by a charge pump circuit connected to the LV gate driver. The new concept finally modifies CC by simply adding 3 passive components (resistor, capacitor, diode - “RCD” concept), yet it significantly lowers both losses and voltage stress. In a nutshell, the proposed cascode is able to combine direct drive switching performance with standard driving schemes.