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 E oss (V DC) in the output capacitance C oss (v) differs substantially from the energy in an equivalent linear capacitor C oss(tr) storing the same amount of charge. That difference requires the definition of an additional equivalent linear capacitor C oss(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 C oss(tr) and C oss(er) of SiC and GaN devices are smaller than the ones of a Si device with a similar R ds,on. In this work, the impact of the nonlinear 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 C oss(tr) , and to a GaN device of equivalent C oss(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.