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

For certain field-effect transistors (FETs) in soft-switching operation, the large-signal behavior 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 value of output charge is also different for a given voltage. In addition, standard hard-switching tests are incapable of fully setting apart the contributions from Co , whereas existing methods tailored to characterize Co losses in soft-switching operations subject Co to a fundamentally different charge–discharge process, and hence, might not predict the correct behavior for hard switching. To address this, first, we analyze and establish the particular charge–discharge conditions that Co undergoes 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 (complementary) and bottom (switching) devices, respectively. Exploiting this, a new measurement technique is developed to obtain output-charge versus voltage curves of devices subjected to actual hard switching. The experimental results for commercial Si, SiC, and GaN devices indicate that the effective charge-capacity of Co for hard switching can considerably vary from the values based on datasheets or Sawyer–Tower circuit; this could greatly undermine efficiency and thermal optimizations in the design phase of power converters.