Bridge and drive power arrangement for PWM power control of AC Induction motors. Switching AC at high speed with fets is done by placing a bridge rectifier into the AC circuit, then switching the rectified current, creating a Bidirectional Switch.
change the pulse width of the square wave in the centre to control the power through the load. longer pulses mean more power.
in this circuit there are two bidirectional switches. the top switch controls main drive current through the AC Motor, and the bottom switch handles the emf current generated when the motor is deenergised. PWM then controls the on time of each switch.
the gate drive power supply for each circuit has to be referenced to the source terminal of each fet to create the voltage difference necessary to drive the gate while also keeping that voltage within the Vgs of the fet. this means that both power supplies to the gates are floating and can reach line voltages. this is especially prevalent when switching the top drive switch during the negative phase of the AC cycle. the gate drive signal has the be referenced to the source terminal, not to the circuit ground.
The PWM signal is fully isolated from the gates using optocouplers. there must not be any dead time between both switches, or very large EMF spikes start happening.
notes here:
https://www.google.com.au/url?sa=t&source=web&rct=j&url=https://www.nxp.com/files/microcontrollers/doc/ref_manual/DRM039.pdf&ved=0ahUKEwiIhImc29bRAhXLjLwKHS6aBFsQFghdMAw&usg=AFQjCNFon2xgKOMSITM6jNS4NQ667h_MXQ&sig2=Ls6pRg15lKgBPZKsNiODJw
