inductive motor kick back

[perts]

im driving a motor with a FET. I am PWMing it. There is a bunch inductive kick back that is coming back and making my FET real hot.

How i do protect against that. Do i use a recirculate diode, if so then where.
BATT
|
__
-PWM---| (N-FET)
|
--
|
|
O motor (25 A)
|
|
GND

 

[ScottyUK]

Perts,

Referring to your diagram above, connect the freewheel diode's cathode to the node joining the FET and motor, and the diode anode ground. Choose the diode carefully - you need a fast type, and its voltage and current ratings should be similar to those of the FET.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[electricpete]

I don't know much about motor electronic drives.

I am curious how you determined that it was kick which was causing the overheating, rather than load or switching rate.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[cbarn24050]

Hi, you are not driving the fet correctly, the voltage at the motor will tend to turn the fet off increasing its resistance. You need to swap the fet & motor positions and put in a catch diode for the back emf.

 

[sreid]

ScottyUK is correct. The indutive kick goes negative in your circuit and causes the FET to avalanch.

 

[mc5w]

You need an inverse connected diode across the FET and a freewheeling diode across the motor. The motor acts as a current source which needs the freewheeling diode. The inverse connected diode is needed because during part of the PWM cycle the motor will try to regenerate current back to the battery.

For a good book on switching power converter theory get Switching Power Converters by Peter Wood.

Mike Cole, mc6w@earthlink.net

 

[mc5w]

Opps, I meant mc5w@earthlink.net not mc6w.

Mike cole, mc5w@earthlink.net