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GR8874 PWM controller ic - need help

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Chriss:
Hi!
I have to repair a power supply where is a GR8874 pwm controller ic which controls the G of a fet.
That is nice and cool.
The schematic is very similar (almost the same) like on this link on page 9.
http://www.grenergy-ic.com/d/datasheet110314/eng/GR8874-Datasheet-V1.2-20101022.pdf

This is the GR8874 datasheet.

The PSU is not powering.
I checked the rectifier diodes, FET, capacitors, zener on the primary side.
Nothing is bad.

Also searched for some short on the secondary side and there I also didn't find anything bad.

What I suspicious is the pwm ic on the primary side.

When I measure for short on the GR8874 between pin 4 and pin 2 I have a short, the ic is out of circuit until measuring.
Actually I have a resistance aroun 22 Ohm between the pins.
I assume it is in short and that is the reason why the psu didn't work.

Can somebody approve my thinking?

Thanks.
Best regards.

danadak:
Based on part block diagram that sure looks like a part failure
as that pin has a comparator on it looking at pin voltage, and 22
ohms will not provide any significant signal for comparison.

That being said measure the pin with + of ohmmeter on pin 2,
ground of meter on pin 4. If you did opposite that would turn
on parasitic substrate diode and you could see a low impedance.


Regards, Dana.

Chriss:
Makes no difference how I put my pins of my ohm meter.
The reading is almost the same, around 22 Ohm.

I will try to get a new ic.
I'm almost sure does the ic if bad.

Thanks for commenting.

My best regards.

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