Author Topic: SMPS repair (probably bad PWM controller)  (Read 2344 times)

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Offline dav

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SMPS repair (probably bad PWM controller)
« on: February 05, 2017, 09:46:21 pm »
Hi,
this morning I'm attempting to fix a little 12V 5A SMPS.
I suspect a bad PWM controller.
There is this text on it:
10Q=704
I cannot find any datasheet for it.
It's in TSOP-6 package and I just deducted its pinout analyzing the circuit:
1 GND
2 Feedback
3 Frequency setting
4 Current sense input
5 VDD
6 Gate control

I removed it from the PCB and I measure 21ohm between its VDD and GND pins. Is it normal?? I suppose no.
 

Offline Neshi

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Re: SMPS repair (probably bad PWM controller)
« Reply #1 on: February 05, 2017, 11:26:21 pm »
Part is manufactured by Richtek:
10Q has to be then product code
704 date code

I was not able to find 10Q product code, but 0Q stands for R7731(GE) Series.

You can also have a look in Richtek_Marking_Code.PDF document for other product codes.
 

Offline dav

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Re: SMPS repair (probably bad PWM controller)
« Reply #2 on: February 07, 2017, 04:01:39 am »
Thank you!
It should be an OQ=704 (first character is not an "1"; it seems more a long vertical line)
so R7731 Burst Triple Mode PWM Flyback Controller.

Any opinion about the low resistance I measured between its VDD and GND pins? (just 21 ohms)
Could it be broken?
 

Offline Neshi

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Re: SMPS repair (probably bad PWM controller)
« Reply #3 on: February 07, 2017, 04:35:33 am »
I'm not an expert in electronics, but inside R7731 seems to be Zener diode across VDD and GND and OPamps connected to VDD, so I would expect no resistence at all.
Supply voltage is 30V, so 21Ohm means 1.43 A (43W) which also seems too high.

I suspect that PWM Controller may be just victim of some other faulty component.
 

Offline dav

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Re: SMPS repair (probably bad PWM controller)
« Reply #4 on: February 07, 2017, 04:58:19 am »
Thanks for reply!
I checked all the other components on the PCB and they seem to be good (capacitors, diodes, MOSFET, optocoupler, resistors...)
 

Offline JFJ

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Re: SMPS repair (probably bad PWM controller)
« Reply #5 on: February 07, 2017, 06:47:21 am »
I removed it from the PCB and I measure 21ohm between its VDD and GND pins. Is it normal?

A low resistance reading in just one direction might be normal. The output voltage of your meter's ohms range could be sufficient to forward bias the controller's internal zener diode (which is connected directly between VDD and GND). However, if the VDD-GND resistance measures 21ohm in both directions (test leads reversed), then the PWM Controller has to be defective.
« Last Edit: February 07, 2017, 06:56:44 am by JFJ »
 

Offline dav

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Re: SMPS repair (probably bad PWM controller)
« Reply #6 on: February 08, 2017, 02:15:22 am »
Yes, 21ohm in both directions
 

Offline Armadillo

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Re: SMPS repair (probably bad PWM controller)
« Reply #7 on: February 08, 2017, 05:22:13 am »
I removed it from the PCB and I measure 21ohm between its VDD and GND pins. Is it normal?

internal zener diode (which is connected directly between VDD and GND).


The datasheet shows functional block diagram and cannot be read as a schematic.... I don't think the zener diode is directly connected from VDD to GND.  Some passive components are not shown...
 

Offline Armadillo

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Re: SMPS repair (probably bad PWM controller)
« Reply #8 on: February 08, 2017, 10:36:14 am »
Normally VDD is 15Vdc, if you measured 21ohm in both direction while the chip is out of circuit, then it can be highly suspected that the chip is defective.

However, if you want to experiment further, put the chip back in circuit, hook up external power supply, 15v~19vdc to VDD and Gnd, Check Pin 1 for pulse train. If there is no pulse at all, then the chip is confirmed dead. if there are pulses, then you will need to check elsewhere.
[note: do not connect to the AC line. With the external isolated power supply, you can hook the oscilloscope directly to pin 1].
 

Offline dav

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Re: SMPS repair (probably bad PWM controller)
« Reply #9 on: February 11, 2017, 03:29:48 am »
So no AC line, direct external power supply between VDD and Gnd.
Does the controller output pulses without any feedback and any sense signals?

No AC -> no output -> no feedback signal
No AC -> no drain-source current -> no sense signal
 

Offline Armadillo

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Re: SMPS repair (probably bad PWM controller)
« Reply #10 on: February 11, 2017, 04:34:40 am »
I copied response from Rasz which I think is appropriate reply here;  :D

yes, in the words of our prophet


 

Offline dav

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Re: SMPS repair (probably bad PWM controller)
« Reply #11 on: February 11, 2017, 04:41:27 am »
I did it before my last post and no got pulses; this is the reason I asked...

So:
bad controller
or
no pulses due to no feedback and no sense signals?
 

Offline Armadillo

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Re: SMPS repair (probably bad PWM controller)
« Reply #12 on: February 11, 2017, 04:44:41 am »
Sorry, I think the chip is defective.
 


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