Intelligent gate drive for high power mosfets and igbts
Authors: Fang Zheng Peng
Venue:  (2008)

An intelligent gate drive has been investigated for the gate control of power MOSFETs and IGBTs. The implemented smart functions include closed-loop switching speed control, fully-equipped fault protection, dead-time elimination, self-powered supply, and self-diagnosis. To overcome the drawbacks of the conventional gate drives, a closed-loop gate control method is proposed to actively control the switching speeds according to preset references. As a result, both the transient voltage overshoot and current overshoot can be effectively controlled. With the proposed gate control, both the power electronic circuit's steady-state and transient behaviors can be effectively controlled by the system modulator. The relationships between the voltage overshoot, current overshoot and associated energy losses as functions of controlled switching speed are derived and can provide guidelines for practical design. Also the power device's equivalent circuit is developed and the closed-loop gate control is modeled and mathematically analyzed. For over-current or short circuit protection, the proposed closed-loop gate control can very effectively limit the voltage overshoot with minimized energy loss regardless of the dc bus voltage and fault current level. Neither an additional hardware circuit nor a soft shutdown process is needed. Even if a fault condition cannot be detected, this method still can automatically control the turn off switching speed and the resultant voltage overshoot. This robust feature allows full use of the capability of the power devices. Based on decomposition of a generic phase-leg into two basic switching cells, a dead-time elimination method is implemented in the proposed intelligent gate drive. Unlike approaches using expensive current sensors, this method precisely determines the load current direction by detecting which anti-parallel diode conducts in a phase-leg. In comparison with complicated compensators, this method features simple logic and flexible implementation. This method provides a complete solution to the dead-time issue and significantly reduces the output distortion as well as gate drive power. A self-powered and contained solution is proposed for the intelligent gate drive to obtain power directly from the main circuit dc bus. Whenever the main circuit is powered, the gate drive circuits are powered. No electrical connections are needed for the driven power devices to any other circuits of the power conversion system. Based on monitoring of the gate charge and discharge currents, a self-diagnostic function is proposed to detect abnormal conditions that occur either within the gate drive circuit itself, the driven power device or the connection between them. This smart function also provides a new method to detect degraded power devices. Finally the integration of the proposed intelligent functions into a gate drive circuit is discussed. A prototype circuit is introduced and the circuit design is explained in detail. Also, the ability to bypass the integrated functions is explained. The versatility of optionally adding and disabling the smart functions makes the proposed intelligent gate drive generic and all-purpose.