June/July 2021

www.infineon.com/coolsic-mosfet-discretes POWER SEMICONDUCTORS 23 www.power-mag.com Issue 3 2021 Power Electronics Europe bridge is turned off, the additional dissipation from the forward conduction of the diode can be minimized. Bi-directional converters such as the PSFB or versions of the ‘LLC’ arrangement operate with zero voltage switching (ZVS) for highest efficiency, in which mode the reverse recovery of the body diodes is not critical, as the applied reverse voltage rises resonantly. However, there are situations where the converter may transiently enter a ‘hard’ switching mode such as on start- up, shut-down or with load steps, during which periods high voltage is present during recovery, leading to possibly damaging stress. Device failure can also result if recovery is not complete during the on-period of the associated MOSFET channel. Problems can also occur if the MOSFET switch in a bi-directional converter has too high output charge, Q OSS . In a hard- switched converter, the current resulting during switching transitions circulates within the primary circuit of a converter causing losses. The output capacitance C oss also varies strongly with drain-source voltage resulting in high Q oss . If it is the dominant charge to be removed in a soft- switched resonant converter, then it can be difficult to maintain ZVS and high efficiency under worst case conditions. Minimum dead time between high- and low-side switches must also be increased as a function of Q oss , resulting in a significant duty cycle loss at high switching frequencies. With lower Q oss , the circuit can be ‘tuned’ for better efficiency. For all these reasons therefore, stable and low output capacitance, low Q OSS and minimum body diode reverse recovery energy and time are vital for high efficiency and reliability. In some topologies such as the totem-pole PFC, which is hard- switching, current Silicon superjunction MOSFET technology yields body diodes which are simply not good enough for a viable circuit. SiC MOSFETs are a better solution Wide bandgap silicon carbide (SiC) MOSFETs are now mainstream and are used for their better figures-of-merit (FOMs) for efficiency at high frequency, compared with Silicon. They have a range of additional advantages as well, such as inherent high temperature operation, low gate charge, lower increase of on- resistance with temperature, or robustness. Importantly for this discussion, their body diodes have much lower recovery charge, along with output capacitance that varies much less than that of Silicon MOSFETs with drain-source voltage. Additionally, for the same RDSON, a SiC MOSFET has around one sixth of the Q OSS of a Silicon superjunction MOSFET. As a comparison, we can take a Si- based 600 V CoolMOS™ CFD7 superjunction MOSFET (IPW60R070CFD7) and a CoolSiC™ SiC MOSFET 650 V (IMZA65R048M1H) from Infineon. These are both TO-247 packaged devices with similar voltage and on-resistance ratings at 25°C. The general body diode reverse recovery waveform for both is shown in Figure 2, with total reverse recovery charge noted as Q RR . For the CoolMOS™ device, the figure is typically 570 nC and for the CoolSiC™ MOSFET just 125 nC at twice the forward current and 10x the rate of change of current dIF/dt. Figure 3 shows the variation in output capacitance of the two MOSFET technologies, with a range of CoolSiC devices shown compared with the CoolMOS CFD7 superjunction MOSFET. SiC devices show lower C oss at low voltages, with both types low at high voltages. Note however that the IMZA65R048M1H CoolSiC MOSFET changes by a factor of around ten between saturation and full blocking voltage whereas the superjunction MOSFET changes by a factor of about 8000. Although low C oss is good for low loss from charge and discharge currents, a non-zero value for C oss at high voltages with SiC is helpful – it reduces the need to slow switching speed with a gate resistor, to keep drain-source voltage within recommended derating from its maximum value. Otherwise with Si devices, a higher value resistor is needed to limit peak drain voltage, resulting in less controllability. Reference design shows high efficiency As a demonstration of the advantages of SiC MOSFETs in a bi-directional converter, Figure 2: MOSFET body diode reverse recovery waveform. SiC exhibits QRR of about 20 % the value of Si MOSFETs Figure 3: SiC devices show far less variation in output capacitance with drain voltage