Improving the full-bridge phase-shift ZVT converter for failure-free operation under extreme conditions in welding and similar applications

Most published material on the popular high power soft-switched full bridge phase-shift converter has focused on its use in telecommunications and similar power supply applications. In such service the load tends to be resistive and nonfluctuating in nature, so published literature, with one notable exception has not probed the behavior of the topology under extreme conditions of near short-circuit or open-circuit load. In arc welding applications, it is relatively commonplace for the converter to function either with a very reduced load, during pauses between welds for example, or with a virtual short-circuit as when an electrode "sticks" to the work piece. This paper identifies a mechanism that induces power MOSFET failure when the converter operates into a short-circuit load, a condition that leads to the MOSFET being turned off while its own intrinsic diode is conducting reverse recovery current at the same time as normal channel current. The fast rising drain-source voltage ramp linked to this attempted turn-off can then induce second-breakdown in the parasitic NPN transistor associated with the MOSFET, in that the device is still full of minority carriers. The physics of this operating mode are examined in detail. Laboratory experiments described establish a correlation between reverse recovery behavior of the intrinsic diode and converter immunity to failure. Circuit techniques to alleviate the failure-inducing stress are also suggested.