Simplified Open-Loop Transfer Functions to Analyze Influential Parasitic Parameters for Oscillation Caused by Parallel Connected Transistors

This study suggests a method to derive an approximate but sufficient formula of an open-loop transfer function for examining the oscillation seen for power devices connected in parallel. The stability of parallel-connected power devices is evaluated by an equivalent circuit and its open-loop characteristics are analyzed using simulation. The simulated Bode diagram is classified into bands by the estimated poles and zeros, and accordingly, the open-loop transfer function is successfully expressed as a simplified analytical form. This calculation makes it clear that while gate-to-drain capacitance prevents self-turn-on and enhances high-speed switching, it also causes parallel oscillation. This insight achieved by simulations was experimentally confirmed.