Hard-Switching Losses in Power FETs: the Role of Output Capacitance

For certain field-effect transistors in soft-switchingoperation, the large-signal behaviour of their output capacitance(Co) has shown to deviate considerably from the datasheet values.This can have a significant effect on hard-switching losses, if the output charge value is also different for a given voltage. However, standard hard-switching tests are incapable of fully setting apart the contributions from Co, whereas existing methods tailored to characterise losses in soft-switching operations subject Co to a fundamentally different chargedischarge process, and hence, might not predict the correct behaviour for hard switching. To address this, first we analyse and establish the particular chargedischarge conditions that Coundergoes in hard-switching circuits by considering a half bridge at no-load conditions. We show that the channel of the switching device incurs a fixed energy loss during the turn-on process, which is separated into co-energy and stored energy components of the top and bottom devices, respectively. Exploiting this, a new measurement technique is developed to obtain charge versus voltage (QV) curves of devices subjected to actual hard switching. Experimental results for commercial Si, SiC and GaN devices show that the effective charge-capacity of Co for hard switching can considerably vary from the values based on datasheets or the SawyerTower circuit; this could greatly undermine efficiency and thermal optimizations in design phases.