A Multi-Level Active Gate Driver for Achieving Thermal Balance in Parallel Connected Power MOSFETs

In high power applications, the temperature differences between parallel connected power MOSFETs can be significant if the cooling condition is non-uniform. Although power MOSFETs have an inherent negative temperature coefficient to balance their drain-to-source currents in parallel connection, their temperatures may still be different due to non-uniform cooling air or liquid flow, compromising the system reliability. With the adoption of multi-level active gate driver, applied gate voltage of the MOSFETs can be dynamically changed to adjust their on resistances. As a result, thermal balance can be achieved with a minimal efficiency reduction. For parallel connected MOSFETs, the cooler MOSFET can be switched with a nominal gate drive voltage of 15 V. For the warmer MOSFET, a lower gate drive voltage, such as 11 V, can be applied. Under this condition, the on-resistance of the warmer MOSFET will be increased, leading to decreased current flow. The conduction loss of the warmer MOSFET is then reduced, lowering its temperature. In this multi-level active gate driver, the power MOSFET's gate-to-source capacitance and the flying capacitor form a voltage divider. The gate drive voltage can be switched between the nominal 15 V to a designed lower voltage, e.g., 11 V. By activating and deactivating this multi-level switching function, a thermal balance can be achieved between the parallel connected MOSFETs without any noticeable degradation in power conversion efficiency.