Cost-Effective Method to Discharge DC Link Capacitors with SiC Power Modules

Abstract:
The high bus voltages commonly used in electric vehicles (EVs) present a safety hazard to vehicle operators and technicians. As the number of EVs on the road increases, it is essential to mitigate these risks by implementing safety features to prevent contact with high-voltage conductors within the vehicle. The primary source of shock risk within the vehicle is from the DC link capacitors that source energy from the EV batteries to the inverter. These capacitors must be discharged when the vehicle is not in operation, such as when the vehicle is turned off or in the event of a crash. This paper presents an active discharge technique that dissipates the DC link energy through the switching semiconductors already present in the system, thus not requiring additional components or control circuitry. The proposed method works by commanding the inverter to alternate between zero sequence states at a high switching frequency. The output capacitance (C (OSS)) of the power devices are charged and discharged each switching period to dissipate the energy stored in the DC link capacitors across the power module. This method is advantageous because the discharge rate is easily controlled and limits thermal heating within the MOSFETs. This paper demonstrates the operation of this method using a SiC power module and presents an analytical model for predicting the discharge time of the DC link capacitor. The analytical model is shown to accurately predict the discharge time for a given system. The power dissipated in the modules during the discharge event is also evaluated and shown to be well within the allowable operating conditions of the power module.