I'm designing a drive to control a Brushed DC treadmill motor. The drive is going to be current controlled PWM via comparitor in PIC.

I have a 600V 70A IGBT that I plan using to switch the motor (PWM), however I have some issue figuring out how I can protect it from back EMF.

The supply is going to be just over 300Vdc and I'll need to supply pulses of upto 15A to the motor, so back EMF will probably be several thousand volts.

I would ordinarily just put a diode across the motor, however in this application the motor can be driven backwards, and the torque the motor is providing needs to remain constant, A diode across it will not allow this to happen.

So far my thoughts are to use a gas discharge arrester across the motor (not sure will be fast enough), switch both sides of the motor and use diodes to throw the EMF back to the supply, or actively clamp the emf with a seperate FET or IGBT that triggers when the other turns off (I suppose this could be automatically done with a zener diode onto the gate to the negative side of the motor)

I'd rather stay away from switching the high side if I can,

any thoughts ideas or even bashing my ideas are welcomed as I'm a little stuck and only half know what I'm doing.

Thanks