During turn off at no-load or light-load condition, when the inductor current is negative, if both MOSFETs are turned off, this negative current can only find a way through the body diode of Qrect to the input dc source. In an isolated converter, this produces voltage stress on the MOSFET as this current cannot find an immediate path for the current to continue to flow. The result is higher voltage stress on the devices. This reverse current eventually charges up the MOSFET output capacitance and then this reverse current can get reflected to primary side and can cause voltage stress on the primary-side MOSFET, eventually leading to its failure.