TIDA-00364: Soft switching

Dear Mohana Krishnan M,

thank you for your interest in TIDA-00364 48-VDC Battery Powered 5-KW Inverter Power Stage Reference Design for Forklift AC Traction Motor.

A common PWM operated 3-phase inverter with dead time (three half-bridges) driving a brushless motor is inherently hard and soft-switching.

Depending on the current direction, the upper or lower switch (FET) of each half-bridge will be hard switching (hard turn-on or hard turn-off), while the complementary switch in that half-bridge will soft-switching (soft turn-on or soft turn-off). The reason for the soft-turn off the is reverse body diode or free-wheeling diode parallel to each FET (or IGBT). 

During an e.g. 16 kHz  PWM cycle the current does not change significantly. The motor's time constant (stator inductance over stator resistance) acts as a lowpass filter to attenuate the PWM carrier frequency. 

For example:

a) The motor phase current of the corresponding half-bridge is positive and the lower FET is turned off: Due to the dead time, the body diode of the low-side FET will continue to conduct the current, so the low-side FET is soft turned-off. After the dead time of the complementary PWM the high-side FET is hard-turned on.

b) If the motor phase current is negative, the low-side FET is hard turned off and the motor phase current will commute to the high-side FET's body diode. After the dead time of the complementary PWM the high-side FET is soft turned-on.

This is explained in more detail the TIDA-00364 design guide chapter: 2.4 Hard and Soft Switching of Inverter, with figures 15 and 16.

Regards,

Martin Staebler

Dear Martin,

Thanks for the clarification.

Regards,

Mohana Krishnan M