SMPS repair (probably bad PWM controller) - Page 1
Forum section: electronics / repair

author: dav
author_rank: Regular Contributor
timestamp: February 05, 2017, 10:46:21 am
content: 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=704I cannot find any datasheet for it.It's in TSOP-6 package and I just deducted its pinout analyzing the circuit:1 GND2 Feedback3 Frequency setting4 Current sense input5 VDD6 Gate controlI removed it from the PCB and I measure 21ohm between its VDD and GND pins. Is it normal?? I suppose no.

author: Neshi
author_rank: Newbie
timestamp: February 05, 2017, 12:26:21 pm
content: Part is manufactured by Richtek:10Q has to be then product code704 date codeI 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.

author: dav
author_rank: Regular Contributor
timestamp: February 06, 2017, 05:01:39 pm
content: 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?

author: Neshi
author_rank: Newbie
timestamp: February 06, 2017, 05:35:33 pm
content: 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.

author: dav
author_rank: Regular Contributor
timestamp: February 06, 2017, 05:58:19 pm
content: Thanks for reply!I checked all the other components on the PCB and they seem to be good (capacitors, diodes, MOSFET, optocoupler, resistors...)

author: JFJ
author_rank: Regular Contributor
timestamp: February 06, 2017, 07:47:21 pm
content: Quote from: dav on February 05, 2017, 10:46:21 amA 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.

author: dav
author_rank: Regular Contributor
timestamp: February 07, 2017, 03:15:22 pm
content: Yes, 21ohm in both directions

author: Armadillo
author_rank: Super Contributor
timestamp: February 07, 2017, 06:22:13 pm
content: Quote from: JFJ on February 06, 2017, 07:47:21 pmThe datasheet showsfunctional block diagramand 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...

author: Armadillo
author_rank: Super Contributor
timestamp: February 07, 2017, 11:36:14 pm
content: 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].

author: dav
author_rank: Regular Contributor
timestamp: February 10, 2017, 04:29:48 pm
content: 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 signalNo AC -> no drain-source current -> no sense signal

author: Armadillo
author_rank: Super Contributor
timestamp: February 10, 2017, 05:34:40 pm
content: I copied response from Rasz which I think is appropriate reply here;Quote from: Rasz on February 09, 2017, 01:12:41 pm

author: dav
author_rank: Regular Contributor
timestamp: February 10, 2017, 05:41:27 pm
content: I did it before my last post and no got pulses; this is the reason I asked...So:bad controllerorno pulses due to no feedback and no sense signals?

author: Armadillo
author_rank: Super Contributor
timestamp: February 10, 2017, 05:44:41 pm
content: Sorry, I think the chip is defective.