3 How to Control the Slow Frequency MOSFETs

Generally, to further improve efficiency, MOSFETs will be used as the slow frequency FETs (driven at grid frequency, normally 50Hz or 60Hz), due to lower conduction loss compared to diodes, as shown as Q3 and Q4. Since it is driven at grid frequency, with always high or low state, some users might select GPIO to control the slow frequency MOSFETs. However, for the reliability consideration, it is still suggested to use ePWM output as the control signals, so that the fast protection response could be ensured for the MOSFETs during the fault conditions. Otherwise, the delay caused by the ISR to set low status for the GPIO might pose risks to the system.

To simplify the configuration, the continuous software force action of the AQ submodule can be used. For example, if using ePWMxA for Q3 and ePWMxB for Q4, in the positive cycle, the below settings can be applied.

EPWM_setActionQualifierContSWForceAction(base, EPWM_AQ_OUTPUT_A, EPWM_AQ_SW_OUTPUT_LOW);
EPWM_setActionQualifierContSWForceAction(base, EPWM_AQ_OUTPUT_B, EPWM_AQ_SW_OUTPUT_HIGH);

Note that when enabling the actions with TZ submodule for the slow frequency MOSFETs during fault conditions, one-shot trip mode should be used instead of CBC mode, since generally the MOSFETs selected for the slow frequency FETs are not suitable for hard switching conditions. Besides, after trip low with one-shot mode, the TZ flags could be cleared at the next zero-crossing point so as to enable the slow frequency MOSFETs with the least effect for the system.