Troubleshooting a cycling SMPS

Thread Starter

Yami

Joined Jan 18, 2016
352
Especially with TO92 or similar transistors you need to check that the pinout is the same. Those two do seem to have the same connections so it should be OK as a replacement.
Thanks, there is more numbers and letters on the shorted transistor I'm not 100% sure that it is actually is a SA673AB.
 

Irving

Joined Jan 30, 2016
2,313
thats a 2SA673 transistor, M204 is the date code

Type Designator: 2SA673 PNP Si
Maximum Collector Power Dissipation (Pc): 0.2 W
Maximum Collector-Base Voltage |Vcb|: 35 V
Maximum Collector-Emitter Voltage |Vce|: 35 V
Maximum Emitter-Base Voltage |Veb|: 5 V
Maximum Collector Current |Ic max|: 0.5 A
Max. Operating Junction Temperature (Tj): 125 °C
Transition Frequency (ft): 40 MHz
Forward Current Transfer Ratio (hFE), MIN: 40

Package: TO92, E-C-B


2SA1015 was a reasonable match..
 

Thread Starter

Yami

Joined Jan 18, 2016
352
thats a 2SA673 transistor,

Type Designator: 2SA673 PNP Si
Maximum Collector Power Dissipation (Pc): 0.2 W
Maximum Collector-Base Voltage |Vcb|: 35 V
Maximum Collector-Emitter Voltage |Vce|: 35 V
Maximum Emitter-Base Voltage |Veb|: 5 V
Maximum Collector Current |Ic max|: 0.5 A
Max. Operating Junction Temperature (Tj): 125 °C
Transition Frequency (ft): 40 MHz
Forward Current Transfer Ratio (hFE), MIN: 40

Package: TO92, E-C-B
Phew thanks!
Do you think I should try what you have suggested after what has happened?
Thanks again.
 

AlbertHall

Joined Jun 4, 2014
11,610
Ok umm heres an update, I replaced the IC 3842 turned on the power there was a pop sound and the light bulb turned on to full brightness.
I quickly disconnected the power supply - the mosfet has shorted out and the new IC is also shorted out (all the pins to ground)
Yes, they do that sort of thing if you don't find all the faulty parts. It's very annoying...
 

Thread Starter

Yami

Joined Jan 18, 2016
352
In addition to post #19 - is it safe to assume that when the MOSFET shorts out like that, its an issue on the HOT side? rather than on the secondary side.
 

Irving

Joined Jan 30, 2016
2,313
Thanks for the help everyone - here is what I have done after the shorting of the mosfet and the control IC.

1. I replaced the IC
2. Another transistor (picture attached) SA673AB was shorted. So I replaced it with a A1015 thats what I had available - its a pnp transistor in the path of the Mosfet's gate. I couldn't find a similar schematic with a pnp transistor to share here. Also could someone please confirm that I have the part number correct, the markings were a bit odd I did a lot of searching online to come to the pnp conclusion.
3.Replaced the capacitor at the VCC pin.
4. I powered the circuit with out the mosfet first, to see whether anything blows etc. Powered up nothing blew. I did measure the voltage at VCC pin. It didn't stabilise never went above 8V. Should it have stabilised without the Mosfet?
5. Soldered the Mosfet back
6. Same thing happened again. IC,MOSFET,pnp transistor shorted.

I measured the resistance of the transformer windings which connects to the mosfet, read almost 0 ohms. I know it'll be a low value but does 0 ohms sound right?

So finally can I conclude that the transformers winding has shorted out and if that is the case why didn't the mosfet and everything shorted out before. I should remark that the original IC was AS3842 when I replaced it, I replaced it with UC3842A.

@Irving - sorry I was typing this message while you posted. Do you think I should do the steps you mentioned provided what has happened thus far? Thanks
p.s the mosfet was an IRF840
Yes, but we need to think it through a bit more...

Its strange, putting that PNP in there, as the 3842 has a 1A gate drive capability...

Its unlikely the winding are shorted. You could try measuring the windings on the good one. They'll be small enough to measure in situ. but you'll be looking at milliOhms
 

Thread Starter

Yami

Joined Jan 18, 2016
352
Yes, but we need to think it through a bit more...

Its strange, putting that PNP in there, as the 3842 has a 1A gate drive capability...

Its unlikely the winding are shorted. You could try measuring the windings on the good one. They'll be small enough to measure in situ. but you'll be looking at milliOhms
Thanks I shall do that, I am feeling a bit tired/frustrated (I thought it would be a simple fix) I shall start fresh in the morning. Will report back.
Once again thanks so much
Thanks everyone :)

p.s I'll share a picture of the power supply. I am using jumper wires to connect to the breakaway board. I double checked the connection before powering up each time.

IMG_1982.JPG
 

Irving

Joined Jan 30, 2016
2,313
Here's another circuit, primary side regulation only. The primary winding is 45 turns of #26 - that's going to be <0.5m of wire, say< 70mOhm

What outputs does your supply have?

1594313994878.png
 

Irving

Joined Jan 30, 2016
2,313
OK, thanks,
5 x 5 = 25
12 x 2.5 =30
12 x. 7 = 8.4
Total = 73.4W

The interesting info is just below where it says Max load 40w!
Or 55W with forced air cooling.

How much were you loading it?
 

Thread Starter

Yami

Joined Jan 18, 2016
352
Hi guys, so here is an update.
I was looking around the transformer I slightly tugged on one of the wires wasn't sure if it was broken or not. Removed the transformer and yes it was broken - it was the aux winding!. I'm not sure if it was me or it was broken before. Anyway its soldered back on now and I get a low ohms value. I was able to remove the conformal coating on the board with IPA and now I'm able to take measurement more easily.
I replaced the MOSFET, the pnp transistor, and I soldered the original AS3842 chip which I removed from before.
Powered back on - nothing blew! but I'm back at square one. The P/S keeps cycling. I measured the outputs without a load connected and all the outputs come up to around 5V when it turns back off. Thinking that the P/S needed some loading I connected some resistors to the power rails, but then the output voltage does not go above 1V before the P/S shuts off.
There are two of these unknown components on the secondary side (shown in the picture). Its suppose to be a diode according to the silkscreen. When I measured across them in diode mode I get a voltage drop of 0.007V in both directions. Its the same for both of them.

@Irving - initially I was not loading it.

Thanks everyone for the help much appreciated.
IMG_2002.JPG
 

Irving

Joined Jan 30, 2016
2,313
Can you redo the measurements around the chip as before again please.

Thats no diode I've ever seen... could be some sort of PTC, whats its resistance and what's in the good supply at that location?
 

Thread Starter

Yami

Joined Jan 18, 2016
352
Can you redo the measurements from before again please.

Thats no diode I've ever seen... could be some sort of PTC, whats its resistance and what's in the good supply at that location?
I shall repeat the measurements around the chip and report, should I remove the MOSFET before doing it?

With regards to the odd looking 'diode' - the working replacement power supply does not have it, I would have to do some tracing to see whats at the same location because the two supplies are not 100% identical.
Is there any way to confirm whether its a PTC?
Thanks
 
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