protecting FET from inductive kickback

[movingbait]

inductive kickback

Dear People,

I have a FET turning on a motor, its a 24V, 3A motor, so its not overly big;

the FET keeps dying, as well as a cap I placed, 600V also keeps dying on me.

I need the thing to turn on quickly using a FET, can anyone recommand protection techniques for it (i dont mind doing reading if there are books)?

Best Regards,
movingbait Ü

 

[Mr.Cool]

inductive kickback diode

do you have a schematic of your circuit? i'm trying to picture how a single mosfet can run a motor? maybe it is acting only as a switch, an On/Off switch so to speak. is the mosfet rated for 60V or more? is the mosfet pulse rated to handle the inrush?

if the cap is dying before the motor even begins to rotate, and it is clearly rated for the voltage, the only thing that comes to mind is charge/discharge rate problem.

anyway, please post a schematic you'll probably get more useful advice as we narrow down to your *real* problem. if you don't have a schematic, draw a block diagram.

Mr.Cool

Added after 2 minutes:

by the way, if it is truely an inductor Bemf (kickback) problem use a mosfet that has a faster body diode quickly free-wheel stored energy. if that is not good enough, place a schottky diode (forward current rating 2x greater than what you think you need) across the mosfet drain/source

Mr.Cool

 

[movingbait]

inductive kick back

Dear Mr.Cool,

as per your request, the fet is indeed used as a switch,

abit about the schematic,


d4 is reverse protection for the FET
d6,d5,c21 are the absorbers of the flyback

i have a feeling the Fet switching may be to fast for the whole circuit to toggle on time.

as for the fet
its a 24v system, and its rated for 30V

Best Regards and thank-YOU,
movingbait Ü

 

[arsenal]

inductive kickback protection

use an rs latch to avoid the turing on the switches at the same time

 

[Mr.Cool]

inductive kickback energy

hmm... i can not seem to find a datasheet for your chosen mosfet.

how do you ensure that the current in the motor is less than 3Arms? your switch must be PWM applied to the opto?

how do you size the mosfet to allow for current seen by the switch?
keep in mind V = L*di/dt

your mosfet is rated for this di (peak current) as the mosfet's continuous rating must allow for the peak current as per formula above. the actual RMS current delivered to the motor will be MUCh less.

one more observation, though i am less sure on this, is when your inductor free wheels when mosfet is Not Conducting it might pull the voltage below ground reference at node location between D4 and inductor. this would voltage stress your mosfet and cause it to fail immediately.

Mr.Cool

 

[edf]

kick back in inductor current

The capacitor has no path to discharge. Usually there is a resistor in snubber circuits to dissipate the flyback.

 

[Ahmed Ragab]

inductive kickback voltage

Actually i don't know what your motor is like or how much current does it need, so here is a circuit i used which was quite easy to implement. Only needed a ULN2003 Darlington Pair which included flyback diodes allowing the current to pass through instead of burning out the transistors.
Here it is, Enjoy.

p.s. It can be extended to be used with any motorin the same manner ... just tell me what you need and if it can deliver the range of power for your operation.

 

[monnoliv]

kickback 15 schematic

What's a schematic ! It's not good at all. Try this one:
Add a 15V zener diode between gate and source pins.

 

[andy2000a]

inductive fet

ULN2003 inside with Diode , and motor add D4
i think maybe need shunt a Cap on motor