Popped a FET in PWM Solar Controller

[metrologist]

In a cheap PWM solar charge controller - I reverse poll'd it 

There are a pair in common positive with switched negative. One popped, but no marking except for an embossed

 D
005

in a tiny circle.

This is for 24V solar system and rated 20A, so each is probably a 10A FET. I removed the shorted one and it works fine now, but I'd like to get a new pair ( ).

I wanted to clarify that common positive is for solar panel, battery, and output. I was looking at this topic and see they might be in series, so now I'm not sure.

https://electronics.stackexchange.com/questions/306836/pwm-solar-charge-controller-mosfet/306843


Any recommends or should I measure the gate pin?

[fzabkar]

AIUI, the usual case is that the first MOSFET is configured as an ideal blocking diode (to prevent the battery from discharging into the panel). The second MOSFET charges the battery via PWM, while the third MOSFET is the load switch.

Could you upload photos of your PCB?

[metrologist]

I am not sure if that block diagram is the same as this one. I know there is a 10A and 20A version. I figured there is a reference design that everyone copied.

This is removed fet and bottom trace is common positive.

I reversed the battery. The trace side has output on left side, panel on right, and battery center.

[fzabkar]

ISTM that there are 4 parallel MOSFETs for charging the battery, 2 parallel MOSFETs for supplying the load, and a single MOSFET configured as an ideal blocking diode. The really cheap (AU$10 - $20) PWM controllers have only a single MOSFET for each function.

That said, I'm wondering how the circuit manages to share the current equally between the parallel connected MOSFETs. Could the failure have been due to unequal current sharing? It would be interesting to measure the RDSon in charge mode. You could do this by measuring the voltage drop across the current sense resistor pairs (R005) and the voltage between drain and source.

[floobydust]

Q1, Q2 are in parallel, I think the drain goes to the load (-) and assume they are N-ch and for the the load switch.
I see four MOSFETs are in parallel Q3, Q4, Q5, Q10 with the drain going to panel (-) and assume they are N-ch and for the PWM and backfeed diode.
You could check the gate-drive Q9 and DZ2.
Is it working or still shorted with Q5? pulled

[metrologist]

With the shorted part, it was dumping panel voltage to the battery (measured), and the charger indicated that voltage. Removed, the system showed battery voltage and seemed to function, but I really did not test much.

The panel terminals measure +1M or 45K ohm depending on polarity.