A control scheme to optimize efficiency of GaN-based DC-DC converters

Abstract

Half-bridge switch cells are frequently employed in DC-DC converter circuits to improve efficiency. However, under light-load conditions, employing both switches degrades efficiency as the switching and driver losses become dominant. In metal-oxide-semiconductor field-effect transistor (MOSFET)-based converters, the sync field-effect transistor (FET) is turned OFF to eliminate these losses and during this duration sync FET's body diode enables reverse conduction. In a gallium nitride (GaN) FET, reverse conduction is possible through its 2-D electron gas (2DEG) channel when the voltage across source-to-drain is higher than the gate threshold voltage. Unlike MOSFET's body diode, it has no reverse recovery loss. However, the voltage drop across source-to-drain during reverse conduction is significantly higher than forward drop of the MOSFET's body diode. The voltage drop can be reduced using a Schottky diode in anti-parallel configuration with the sync FET. MOSFET-based converters also use zero current detection (ZCD) circuitry to enable discontinuous conduction mode (DCM) of operation using the sync FET. Both the above-mentioned techniques use additional components to realize DCM; therefore, the techniques are not cost effective. To optimize the efficiency under light load conditions, this chapter proposes a novel control scheme that can emulate the discontinuous conduction mode (DCM) of operation without using a Schottky diode or a ZCD circuitry. Furthermore, the proposed control technique is simple to implement and it can be easily combined with the existing light-load control techniques. A prototype of the GaN boost converter is designed and a field-programmable gate array (FPGA) device is used to implement the proposed scheme.