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When the vehicle is in motion and the permanent magnet synchronous motor (PMSM) is spinning without actively being controlled, the motor will generate electrical power and supply it into DC-link like a generator. The MOSFET body diodes act as a rectifier circuit for the motor-induced back EMF voltage. System designers must make sure that the DC-link voltage does not increase to the point of system damage or failure.
Figure 1 Shorting the motor with DC positive (high-side switches)ASC provides a controlled freewheeling path for the current induced by the back EMF voltage of the PMSM.
ASC on the gate driver is used to short the motor to either the DC positive or DC negative by turning on all top MOSFETs or all bottom MOSFETs in a three-phase inverter. The above figure shows the flow of current between high side switches (when turned on) and the PMSM motor, thereby dissipating the inductive energy when the motor is not actively driven by the inverter as shown in Figure 1.
1.1 ASC functionality in EiceDRIVER™ gate
Some electric vehicles use a three-phase PMSM drive for propulsion. Such a system is mainly composed of a battery, a PMSM, and a three-phase inverter which comprises three half-bridge power switches as Figure 2.
Figure 2 Three-phase PMSM drive
ASC pin, for example in 1EDI3031AS as shown Figure 3, is required for PMSM to prevent an unintended charge-up of the DC-link voltage when in . Free-wheeling occurs when the wheels begin to move without the need of fuel or power, and occurs on downward inclination. The engine is mechanically cut off from the rear wheels when free-wheeling.
Figure 3 Block diagram of 1EDI3031AS
Infineon’s allow transition into ASC from the primary as well as secondary side of the gate driver. Primary side ASC takes over when the MCU fails and the safety logic needs to take over. ASC on secondary side is required when there is a system fault on the primary side of the inverter.
References:
ASC pin functionality in Infineon EiceDRIVER™ gate drivers