# [SOLVED]Battery charge balancing circuit mosfets getting burnt

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#### ashugtiwari

##### Full Member level 4
Hello,
I have a battery charge balancing circuit which has issue with MOSFET always getting burnt when I connect both the batteries.
As from figure, if G1>G2, PWM1 is on with 12.5% duty cycle, during this time the current from G1 and input current goes through U$10 to charge G2 for balancing charge for both batteries. if G2>G1, PWM2 on with 12.5% duty cycle during which, current through G1 and from G2 will pass through U$11 to balance charge.
if G1==G2, mosfets are off and batteries will charge as usual.
L7 is used to limit current spike and CT is used to find current value.
The PWM is running at 250kHz.

Now, in any of above conditions, even if both mosfets are off; as soon as i connect both batteries, the mosfets get hot and burnt within few seconds. If only one mosfet is present in circuit, the circuit works fine with charge balancing.PWM's are driven through mosfet driver.

What can be the issue?

#### crutschow

##### Advanced Member level 5
What are the measured gate voltages of the two MOSFETs when they are supposedly "off".

#### ashugtiwari

##### Full Member level 4
The gate source voltage for both mosfet is 0 but with respect to ground, gate voltage for upper mosfet is around 12V and lover mosfet is 0V.

#### FvM

##### Super Moderator
Staff member
PWM's are driven through mosfet driver.
What kind of MOSFET driver? Bootstrap drivers like IR2110 don't work for this circuit without special provisions.

As from figure, if G1>G2, PWM1 is on with 12.5% duty cycle
Means the inductor current is commutating through MOSFET body diode with respective losses? But I guess it would be O.K. in discontinuous current mode.

I calculate inductor peak current of only 33 mA in specified operation point which should do no harm. So there's probably either a driver problem or incorrect pwm waveform.

Show your driver circuit.

#### ashugtiwari

##### Full Member level 4
@FvM, the driver im using is MIC4101. It is a bootstrap driver using 4.7uF of bootstrap capacitor. But, why would it not work for this situation? Please explain...

Yes, it is fine with diode carrying current to prevent inductor just storing energy and go to saturation. But how did you calculate 33mA current?
Actually, the max current from boost converter is 5.4A and batteries im using is quite high capacity. I have used 10A of fuse in series to batteries to prevent any damage due to higher than rated current.

#### FvM

##### Super Moderator
Staff member
DCM, inductor current is rising linearly during on time, starting at zero

12V*0.125/(180µ*250k) = 33mA

MIC4101 high side driver doesn't work if the low side switch isn't periodically turned on to charge the bootstrap capacitor.

ashugtiwari

### ashugtiwari

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