Non-Linear Capacitance of Si SJ MOSFETs in Resonant Zero Voltage Switching Applications

Abstract

The parasitic capacitances of modern Si SJ MOSFETs are characterized by their non-linearity. At high voltages the total stored energy Eoss(VDC) in the output capacitance Coss(v) differs substantially from the energy in an equivalent linear capacitor Coss(tr) storing the same amount of charge. That difference requires the definition of an additional equivalent linear capacitor Coss(er) storing the same amount of energy at a specific voltage. However, the parasitic capacitances of current SiC and GaN devices have a more linear distribution of charge along the voltage. Moreover, the equivalent Coss(tr) and Coss(er) of SiC and GaN devices are smaller than the ones of a Si device with a similar Rds,on. In this work, the impact of the non-linear distribution of charge in the performance and the design of resonant ZVS converters is analyzed. A Si SJ device is compared to a SiC device of equivalent Coss(tr), and to a GaN device of equivalent Coss(er), in single device topologies and half-bridge based topologies, in full ZVS and in partial or full hard-switching. A prototype of 3300 W resonant LLC DCDC converter, with nominal 400 V input to 52 V output, was designed and built to demonstrate the validity of the analysis.

Keywords

• Hard-switching
• non-linear capacitance
• resonant converter
• wide band gap
• zero voltage switching