soft start MOSFET switch

Thread Starter

fjb2069

Joined Nov 16, 2016
5
I am having issues damaging N channel mosfets that control CHARG and DISCHARGE current of a battery pack.

When I drive the DISCHARGE port with a PWM signal generated by a micro processor the fast switching immediately damages the mosfet even under light load. IF the mosfet is turned ON first and then a large load is connected, there is no problem.

Circuit creates switch to ground.

Am I using to larger of pullup on BJT? How can I increase switching speed, if that is the issue?

VBAT is 16 volts. LOAD connects load to VBAT.

See attached circuit.
 

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crutschow

Joined Mar 14, 2008
34,281
I don't understand how that circuit can work. :confused:
You have two back-to-back MOSFETs connected between a "load" and ground.
You have their gates connected to their drains which turns them on.
You have D2 connected between the battery and ground(?).
Where is the path between the battery and the MOSFETs?
 

Thread Starter

fjb2069

Joined Nov 16, 2016
5
I don't understand how that circuit can work. :confused:
You have two back-to-back MOSFETs connected between a "load" and ground.
You have their gates connected to their drains which turns them on.
You have D2 connected between the battery and ground(?).
Where is the path between the battery and the MOSFETs?

I messed up when I copied schematic. See attached
 

Attachments

AnalogKid

Joined Aug 1, 2013
10,986
Those are N-channel MOSFETs, so each gate needs to be 5-10 V above the source for "saturation". R31 is what pulls up the Q10 gate. Since it is connected to Vbat (same as the load), where does the extra gate drive voltage come from? Also, are you sure about the control signal logic polarities?

Plus all of that Wally stuff.

ak
 

SLK001

Joined Nov 29, 2011
1,549
You have a 10μF cap holding the base of your switching xsistors to ground. That is the LAST thing that you want to do, because it prevents the MOSFETS from rapidly switching from ON to OFF, and OFF to ON.

Also, unless you are CHARGING and DISCHARGING at the same time, there is no low impedance path from your load to ground in either condition.

Discard this circuit and start over.
 

Thread Starter

fjb2069

Joined Nov 16, 2016
5
You have a 10μF cap holding the base of your switching xsistors to ground. That is the LAST thing that you want to do, because it prevents the MOSFETS from rapidly switching from ON to OFF, and OFF to ON.

Also, unless you are CHARGING and DISCHARGING at the same time, there is no low impedance path from your load to ground in either condition.

Discard this circuit and start over.

I just made that same change to the circuit and replaced with a 0.1uf along with a 1k pull up and so far it is working. The 10uF was a carry over from a similar P-channel design that is not needed in this circuit.

Thanks for your help.
 
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