A novel technique to reduce the reverse recovery charge of a power diode

Abstract

Power diodes are required to have short reverse recovery time, soft recovery, high breakdown voltage and a low forward voltage drop at the rated forward current. Manufactures introduce recombination centres in the device to achieve these parameters; however, a trade-off between the on-state losses and breakdown voltages, and a trade-off between the on-state losses and switching speeds is still significant. In this paper, a post manufacturing technique that reduces the reverse recovery charge of a power diode is proposed. The reverse recovery characteristic of the power diode was simulated by reconstructing a test circuit similar to the one previously published. To implement the technique, a current injection circuit, which injects an additional current from a pre-charged capacitor into the power diode prior to current zero, was added into the test circuit with results indicating a significant reduction in the power diode's reverse recovery charge. This 'helps' to cancel the reverse current due to the stored electronic charge in the wide drift region and therefore prevents the device from conducting large reverse current. Nevertheless, the technique requires precise timing for injecting the appropriate forward current pulse seeing that premature injection will increase the forward current, and belated injection will either lengthen the power diode's forward conduction period or switch the already turned off diode into its low impedance state. Two methods were developed to detect the falling anode current. The first method uses a digital signal processing (DSP) kit to predict the time where the falling anode current reaches zero based on its average gradient. The second method uses an analogue circuit to trigger the current injection circuit at a reference current level. Of the two methods, the DSP kit is limited by the processor's response time; and the analogue circuit is restricted by its inability of predicting current zero. Therefore, the limiting factor of this technique is the electronic delay in its components.