Please help me repair a SMPS - Page 1
Forum section: electronics / repair

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 07:50:58 am
content: I'm trying to repair this SMPSIt shorts when I plug it in with a bulb in series.I replaced Mosfet and switching transistors, and also sense resistor because they were blown. also replaced a few zener diodes and shorted zener diode on the secondary side.I even bought a scope and found out that the switching frequency is 20k and the MOSFET is almost always closed, the PWM pulses on the gate are almost 100% duty cycle. What could be the reason?Attaching a schematic in the PDF

author: capt bullshot
author_rank: Super Contributor
timestamp: November 10, 2017, 08:25:05 am
content: Missing feedback or shorted/overloaded outputCheck the circuitry around D660 and the optocouplers.

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 04:06:59 pm
content: I checked and actually replaced D660 it works fine.Will check the optocouplerThe output voltage seem to be correct +21V but voltages on the optocoupler (on the secondary side) are 1.2-3.2v so it looks like D660 is pulling it to the ground.

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 04:36:08 pm
content: Hi,You should check the ohms of R654 and R655 with power off.

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 04:39:53 pm
content: R654 and R655 are good. I think I checked every single resistor on the primary side and around feedback area on the secondary

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 05:08:54 pm
content: I now checked the optocoupler (it was bad) and replaced it.The same result.Checked the optocoupler and it still works.Any ideas?

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 05:19:06 pm
content: Quote from: zape on November 10, 2017, 04:39:53 pmYeap, the secondary side should be In order, 50mA flowing across the resistors is well within the capability of SL431A.Edit:You sure you checked D605? How did you check it?Is the output voltage correct?

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 05:29:58 pm
content: I checked it in-circuit with multimiter like a normal diode. It conducts one way. I did not check the Zener voltage.The output voltage is +21V and it is correct.

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 06:05:24 pm
content: Quote from: zape on November 10, 2017, 05:29:58 pmSo your question is actually on the PWM, Why it's on 100% duty as you purported by measuring the voltage of 4.9volts. The gate is always power "ON" by the resistors divider and zener diode at the gate. You can see for yourself. So voltage measurement at the Gate is surely incorrect. It will be pulsed "OFF" by the transistor.No it is not 100% duty, You should actually see it on a oscilloscope probe it onto R611 [with isolating transformer].The wave is actually by number of pulses. As you load it more, more pulses rises to appear. As you load it less, less pulses will appear.This power supply is BAD, at most 55% efficient and will fail horribly.Edit: To put it simply, this is not a PWM switching power supply, it is a PFM switching power supply.

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 06:36:55 pm
content: Here are some photos:I used X10 probe (and I use Backup UPS instead of isolation transformer, because I don't have one)

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 06:48:21 pm
content: Load up the power supply say 0.5A, 1,A, 2A [check whether it is able] and probe across R611.Then you see the PFM properly.Edit: I am not sure, no knowledge about backup UPS or whether it's isolated..

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 07:11:29 pm
content: Ok the only load I was able to find around was electric boiler which measures around 10OhmThe output voltage drops from 21 to around 3v when I connect it.Attached scope photos:

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 07:21:39 pm
content: Quote from: zape on November 10, 2017, 07:11:29 pm"Electric Boilder" and its 100 ohm?How did you measure the ohm? By multimeter?Edit: Measure the voltage at the opto-coupler, whether the SL431 releases the current.? or same as before?

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 07:23:48 pm
content: 10, not 100Yes used multimiter

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 07:26:14 pm
content: Quote from: zape on November 10, 2017, 07:23:48 pmMeasure the voltage at the opto-coupler, whether the SL431 releases the current.? or same as before?BTW: what is the Max current of this power supply?

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 07:38:58 pm
content: Yes it does.With load connected the opto-coupler is off. No voltage drop on it, and all voltages are equal to ~3v output.The TV is rated at 90W so I guess the max current is about 2-3A x 21v

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 07:46:04 pm
content: Quote from: zape on November 10, 2017, 07:38:58 pmSo is a TV power supply... There are the command On signal from the TV. I suggest you plug this power supply back into the TV to test it.There are also the 3.3v, 13v etc...

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 07:50:44 pm
content: I forced the PSU to turn on.I don't think it should drain so much power when powered so the bulb lights up

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 07:54:43 pm
content: you should measure all the secondary power whether is OK, e.g 13V, 3.3 V, 21 V etc.....Anyone shorted.?Edit: What bulb lights UP? What is so much power?Edit: I know this type of power supply is 50% efficient. From your scope trace, I deduced 19W is expended somewhere, don't know if you have connected something consuming 19W or secondary voltage of one of them is shorted.

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 08:03:40 pm
content: Nothing shorted on the secondary side. all voltages are fineI learned this "bulb trick" here: you connect your device not directly to mains but through a 100W light bulb in series that acts as protection in case the power supply is draining too much power (as in my case)If I connect it directly to the mains the fuse will blow up and probably mosfet and others (I learned this the hard way lol)

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 08:06:27 pm
content: Quote from: zape on November 10, 2017, 08:03:40 pmThe trick is for initial testing of power supply incase short circuit. If is not short circuit anymore, you should take the series light bulb out.

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 08:09:18 pm
content: but it is shorted - the bulb is very bright

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 08:09:28 pm
content: Quote from: zape on November 10, 2017, 08:03:40 pmIf all the voltages are fine, just plug it back to the TV.WITHOUT THE SERIES LIGHT BULB...... hahahahahah LOL.

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 08:12:28 pm
content: Quote from: zape on November 10, 2017, 08:09:18 pmHello, how many watts is the series light bulb you used.? What bulb is that? Is it incandescent bulb?The mosfet is only firing at 0.25A. [from your scope].What is the AC current flowing through, can your backup power supply show the current, if not, measure the AC current.Don't let the bulb trick you.

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 08:19:52 pm
content: Man!..... to think that you want to do a load test with a series light bulb dragging the voltage in circuit......

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 08:24:35 pm
content: I have two bulbs 40W each so it is 80WUPS shows 73W and it does not change if I connect the loadSo the AC current is 0.6A

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 08:30:26 pm
content: Quote from: zape on November 10, 2017, 08:24:35 pmIt is not short circuit. the bulb does light up in normal situation. You can take out the light bulb and plug it directly into the main to see what brightness is short circuit intensity.Yes you learnt the bulb trick, but the teacher was not complete...... hahahahahahah LOL.Edit: You are not using incandescent light bulb right...? hahahahaahahah LOLEdit: And then maybe your backup power supply is charging the internal battery....... hahahahahaahha LOL....Oops, sorry,..... just a lighter moment for all of us...I have never used a UPS before, hence cannot recognise all the problems associated with it... example power limit etc...

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 08:43:48 pm
content: It is an incandescent light bulb of course.HMM let me check one thing

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 08:45:16 pm
content: Quote from: zape on November 10, 2017, 08:43:48 pmit depends before the UPS or after the UPS.Do you think 0.6A is a short circuit?Edit: 0.6A is not a short circuit. the series light bulb is only used for initial testing for short circuit. In this case, the light bulb as you used it with the UPS cannot discern or provide you with a useful cue between short and 0.6A.So, you should plug back to the TV and turn it on.

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 08:53:53 pm
content: Quote from: zape on November 10, 2017, 08:43:48 pmDon't say you wired it in parallel?

author: zape
author_rank: Contributor
timestamp: November 10, 2017, 09:17:15 pm
content: IT WORKS!So how should I properly use this light bulb trick then?

author: Armadillo
author_rank: Super Contributor
timestamp: November 10, 2017, 09:38:24 pm
content: Great news.The light bulb trick is for initial testing in case of short circuit.Purpose is to limit the voltage and current through the device under test.So for me I would size the light bulb to be 20% of the full load power of the device under test.So, if for example the DUT is 20 watts, so the light bulb should be 100W [around to your discerning ability of shorts].The sizing is not for the light bulb to lights up or maybe abit.the light bulb must be incandescent light bulb.you must wired in after the isolating transformer [after any device that consume power] and in series to the DUT.The isolating transformer must have the power sufficient for the DUT X 2.I have 1500VA isolating transformer to meet the occasion.

author: xavier60
author_rank: Super Contributor
timestamp: November 11, 2017, 12:41:24 am
content: The function of Q621 makes no sense. Its Collector would have to connect to the Gate of the MOSFET to reduce the MOSFET's on time, in case of excessive line voltage.

author: Armadillo
author_rank: Super Contributor
timestamp: November 11, 2017, 04:02:42 am
content: Quote from: xavier60 on November 11, 2017, 12:41:24 amits "over voltage protect" not for "output over voltage" but for Q602 if the induced voltage at the Base continue to rise even after it had pulsed.

author: xavier60
author_rank: Super Contributor
timestamp: November 11, 2017, 07:31:55 am
content: Quote from: Armadillo on November 11, 2017, 04:02:42 amYes, that's the way it might seem from the way it's drawn. Not useful at all.D621 rectifies the + half cycle or the forward half of the waveform rather than the fly-back half. The voltage rectified by D621 will be proportional to the incoming line voltage.