8 DC Link Active Discharge

Every EV traction inverter requires a DC link active discharge as a safety-critical function. The discharge circuit is required to discharge the energy in the DC link capacitor under the following conditions and requirements:

• In an emergency situation or during repairs, the voltage in the system must be safe to touch in less than 2-5s. The emergency situation response must run without an MCU and locally, for example, inside the traction inverter.
• At vehicle key-off, where all systems remain operational, the discharge happens within minutes.
• System-level safety requirements of ASIL D

TI has several active discharge designs targeted for different system-level requirements. In general, the active discharge dissipation method can be separated into three categories; resistive discharge, discharge through the power stages, or discharge through the motor windings.

In resistive discharge, a bleeding resistor can potentially work but draws current at all times and is potentially too slow. Therefore, a switched resistor can be used. Using a switched resistor, both ON or OFF control and PWM switching control, can be realized as:

• Power transistor on or off control using the TPSI3100-Q1 device. The TPSI3100-Q1 reinforced isolated switch driver has an integrated 17V gate supply that can drive the discharge power switches without requiring a secondary bias supply, as the device integrates a secondary bias supply from power received on the primary bias supply. When combined with an external power switch, the device forms a complete isolated solid-state relay configuration. Internal, dual, high-speed comparators enable a communication back-channel with multiple diagnostic features.
• Controlled PWM using the AFE539F1-Q1 device. The AFE539F1-Q1 smart AFE device has built-in non-volatile memory for PWM and custom waveform generators. The device has added programming ability and logic to enable operation without an MCU. Figure 8-1 shows a scope shot of a DC link discharge using the AFE539F1-Q1 device. Channel 1 (yellow) is the AFE359F1-Q1 output, channel 2 (pink) shows the voltage drop from 950V to 0V. Channel 3 (blue) shows the EN signal input at the gate driver, which initiates active discharge, and channel 4 (green) is the SiC FET drain-to-source current.

Figure 8-1 DC Link Active Discharge Based on the Smart AFE (Left) and Testing Waveforms (Right)

Energy can be discharged through the motor windings. Dividing a winding-based discharge into multiple stages is possible. These stages include a rapid discharge stage or a bus voltage regulation stage. Generating large negative d-axis current quickly reduces the DC link energy, while the q-axis current must be at zero. Fast loop control from TI’s Sitara™ or C2000™ MCUs and safety isolated gate driver include serial peripheral interface (SPI) programming ability, while six ADC channels provide a reliable and smoothly controlled discharge. Although this design is potentially cost-efficient, the design requires a fully functional system, from the MCU to the bias supply and gate drivers.

A coming trend in traction inverter designs is discharge through the power stages. Here, a pulsed operation mode of the power stages enables discharge through the linear region of the MOSFET, where the MOSFET behaves as a resistor. For this to work, the gate driver requires very precise gate control and a high frequency pulsed operation to not overstress the MOSFET. Alternatively, the gate driver can drive a pulsed short circuit operation mode to discharge a DC link voltage.