Electronics > Repair

SMPS failure

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eblc1388:

--- Quote from: johnmx on April 10, 2024, 07:01:32 pm ---I measured the capacitor C10 with an LCR meter and it had 9.8uF with 4.5 Ohm series resistance. A somewhat high value, but I don't have any of these capacitors to replace it.

--- End quote ---

The ESR value of C10 is too high. The MOSFET inside the PWM IC was probably destroyed via high back emf voltage from the switching.

The datasheet of FD9020D calls for 2x 10uF 450V filtering capacitors but the SMPS vender skimmed on budget and use only a single 10uF 400V capacitor. You should replace it by two 10uF 450v in parallel or a single 22uF 450V capacitor instead. Consider mounting the capacitor horizontally if space is insufficient.

I attach a scope measurement of the back emf when the filtering capacitor is faulty in another similar SMPS I have repaired earlier.

wraper:

--- Quote from: eblc1388 on April 15, 2024, 03:26:35 am ---The ESR value of C10 is too high. The MOSFET inside the PWM IC was probably destroyed via high back emf voltage from the switching.

--- End quote ---
Such ESR can be normal for some brand new capacitors, especially miniaturized. No way it's high enough to kill the IC. Not to say OP measured at 1kHz which is not correct and will show higher ESR value.

--- Quote ---The datasheet of FD9020D calls for 2x 10uF 450V filtering capacitors but the SMPS vender skimmed on budget and use only a single 10uF 400V capacitor. You should replace it by two 10uF 450v in parallel or a single 22uF 450V capacitor instead. Consider mounting the capacitor horizontally if space is insufficient.
--- End quote ---
:palm:

rteodor:
It may be the optocoupler.

MathWizard:
So when it burned out, did you have a secondary load hooked up ? I see in the datasheet that the FB voltage gets divided down and compared to 3.7V. So if the secondary or opto had some problem, the chip should reset or shutdown.

But yeah I would try and scope the input drain of the chip, surely some overvoltage or over current into that pin or the StartUp voltage pin, or even on the low voltage Vcc winding, if that get too high for some reason, or it's cap fails, would kill the chip pretty quick.

johnmx:

--- Quote from: fmashockie on April 13, 2024, 01:14:45 pm ---What is the resistance between pins 1 and 8 on the board with FD9020 removed?  I'd be suspecting D7 or D5 if you measure low resistance without IC soldered in.
(...)
EDIT: My bad I see you said it is 'pratically' the same.  Could you possibly trace the schematic out a bit?  Maybe notate some more of the differences? That might help people to better help you

--- End quote ---
The resistance between pins 8 (+) and 1 (-) is 2.3 MΩ and >50MΩ between 1 (+) and 8 (-).
The differences on the primary side are: C11 does not exist, C1 is 22 uF and in parallel with a ceramic cap of 100nF, R11 is 10Ω.


--- Quote from: Audiorepair on April 13, 2024, 01:54:12 pm ---Its possible the transformer core is cracked, or shorted turns.

--- End quote ---
The primary coil resistance is 2.15Ω and 0.58Ω for the auxiliary coil.


--- Quote from: Audiorepair on April 13, 2024, 05:23:53 pm ---Personally, I would never rule out anything bought from Ali Express wasn't fake or reject.

--- End quote ---
What makes me not suspect this is because the original controller also broke in the same way.

I want to replace all capacitors, but it may take a while to buy them. Additionally, I wanted to check the circuit with an oscilloscope using a new controller, but for that, I want to power the board with lower voltage (< 100Vac) to avoid damaging the scope. I don't have high-voltage probes. I have a Variac with an Isolation Transformer for this type of measurements, but it's in another location and I may only have access to it next month.

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