Diagnosing for Cross-conduction in GaN Half-Bridge

Abstract

GaN E-HEMTs eliminate the inherent parasitic diodes compared to Si MOSFETs and have faster switching speeds and slew rates [1]. These and other improved characteristics of GaN E-HEMTs have reduced system size and weight, have delivered lower system costs and have increased efficiency [2]. However, advancements in the GaN HEMTs used in switching power supplies have made characterizing the performance of these power supplies increasingly challenging. A particularly demanding measurement is measuring the high-side VGS in a half bridge, which is a traditional way to diagnose the cross-conduction for Si MOSFETs. Although special probes and measurement systems with galvanic isolation (optical isolation) have been developed for this purpose, GaN users are discouraged by the cost and are seeking a lower cost method to diagnose GaN cross-conduction. By taking advantage of GaN’s unique feature (no body diode and reverse recovery loss), this paper is proposing a practical way to diagnose the cross-conduction /potential cross-conduction without sensing the high-side VGS: i. Calculating the high-side Coss charge during the voltage commutation; ii. Implementing a set of double pulse tests and recording the low-side drain currents; iii. Integrating the low-side drain current bump area above the load current and iv. Confirming that the result should match the COSS charge value. The proposed diagnostic method has been demonstrated by using the 25 mΩ RDS_on 650V GaN HEMT GS66516T as an example.