SMPS keeps cycling

[Yamin]

Hi guys,
I am trying to troubleshoot a switch mode power supply which keeps cycling, the output voltage jumps around. The model of the power supply is ASTEC LPT42 - https://datasheet.octopart.com/LPT42-Artesyn-datasheet-8318749.pdf
Its a power supply based around the IC AS3842. I looked for any faulty components on the secondary side bulged caps etc. Couldn’t find any obvious faults. Following is the result of voltage measurement I took at the control IC
COMP =   0V
VFB = 0V
ISENSE = 0.260V Varies
RT/CT = 0.02V
GND = 0V
OUT = 0V
VCC = 11V to 18V keeps varying a lot
VREG = 0.08V
The power supply keeps trying to start up, voltage measured across the main filter cap also varies slightly during the failed start ups.
I was hoping that you could give me some more ideas on what to look for.
Thanks in advance for the help.

[TheMG]

Second rule of troubleshooting: check the capacitors!

Such problems with SMPS are most of the time caused by one or more bad capacitors. They should be checked for ESR and capacitance. Check all electrolytic capacitors, including the smaller ones.

A common culprit would be the capacitor on the VCC pin of the switching IC, if that loses too much capacitance or has a too high ESR, the power supply will not start correctly. It needs to hold a charge long enough to allow the power supply to start up and produce enough voltage to power itself through the auxiliary winding on the primary of the transformer.

[Yamin]

All the capacitors I checked with the ESR meter on the secondary side checks out ok. The board is covered in conformal coating and to test anything I have to scrape it off  .
I shall try and check all the capacitors.
Another question that I wanted to ask was - how can I be sure that its an issue with the secondary (cold) side, or an issue with the controller IC. I noticed that the VCC for the chip kept varying a lot, varied so much that I was not able to read the values. Here is what my thought process was - if there was an issue on the secondary side that is a short or an issue with the feedback etc... at least the VCC for the chip would be stable. Am I on the right track?

[Audiorepair]

Wire up a lightbulb in series with the mains input.  A fault on the secondary is usually a shorted something or other, the bulb will flash briefly as the excess current is drawn.

A controller starting up and then immediately shutting down (cap maybe) may not draw any current at all and the bulb will not flash.

It will give you some basic knowledge you wouldn't otherwise have.

[Yamin]

Wire up a lightbulb in series with the mains input.  A fault on the secondary is usually a shorted something or other, the bulb will flash briefly as the excess current is drawn.

A controller starting up and then immediately shutting down (cap maybe) may not draw any current at all and the bulb will not flash.

It will give you some basic knowledge you wouldn't otherwise have.

I have the light bulb connected. Here is how it behaves -
On initial switch on the bulb flashes once (with the inrush current), and then nothing, absolutely no light. No flashing etc.

[Audiorepair]

When you are saying "The output voltage jumps around", do you mean that on each start attempt the low voltage output capacitors are charging briefly? 
That would indicate the main FETs are firing and the transformer appears to be in circuit.

Otherwise, perhaps the main FET's aren't being fired at all.

[Yamin]

By jump around I mean - it doesn't show any stable or meaningful values at the outputs. But let me double check this.

[tautech]

VCC = 11V to 18V keeps varying a lot

The power supply keeps trying to start up, voltage measured across the main filter cap also varies slightly during the failed start ups.

This tells a lot.
Like:
HV resistor dropper to VCC is OK.
UVLO is working.

Unknowns:
VCC series diode to the Aux primary winding....possibly faulty.
VCC cap tired.
Faulty diode or caps secondary side.

Some SMPS require a secondary side load.

[Yamin]

VCC = 11V to 18V keeps varying a lot

The power supply keeps trying to start up, voltage measured across the main filter cap also varies slightly during the failed start ups.

This tells a lot.
Like:
HV resistor dropper to VCC is OK.
UVLO is working.

Unknowns:
VCC series diode to the Aux primary winding....possibly faulty.
VCC cap tired.
Faulty diode or caps secondary side.

Some SMPS require a secondary side load.

Series diode from the Aux winding checks out ok. The cap and the IC is on a breakaway board so accessing the cap with ESR meter's leads is a challenge, however I did check the capacitance value with a DMM,it showed the correct value. I have had issues with the same cap on other faulty p/s. Every time I checked it with a DMM it showed a low value. Anyway I am going to desolder the board for closer inspection.
With regards to the secondary side load - I have got an almost identical working power supply sent by the same manufacture it doesn't require a load to be connected to work. So I am assuming this faulty one doesn't need one either.

I have take voltage measurements from the working P/S at the control IC:
COMP = 2.1V
VFB = 2.4V
ISENSE = 0.1V
RT/CT = 2.1V
GND = 0V
OUT = 0.094V
VCC = 11.02
VREG = 4.9V

[tautech]

One thing to help diagnose SMPS problems is to power only the SMPS IC with a PSU and scope the gate drive with a scope.
But if your keeps ticking it's signalling there's an overload on the secondary side of the FB circuit is faulty.
High Isense could also indicate the transformer is shorted. Check for signs of hot spots.

It's also important to study theory of operation and the 'typical application circuit' in the datasheet. Try to find other datasheets for the same IC as some are very short on info.

[Yamin]

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) 

[Yamin]

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

[TheMG]

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?

That's normal. There is a resistor which charges up the capacitor on the VCC from the high voltage rectified DC, but this resistor is a large value and does not provide enough current to run the IC. Normally, when the power supply starts up, there is an auxiliary winding which is rectified and provides the VCC for continued operation, and the resistor only charge the capacitor for the initial startup.

If you want to check the operation of the IC without the MOSFET in place, do not apply AC power, but instead apply the necessary voltage (refer to datasheet) to the VCC pin from a bench power supply. That should allow you to check for proper gate drive using an oscilloscope. You can do this with the MOSFET in place too, since without AC input there won't be any high voltage DC to let the smoke out.

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.

The low reading is normal. Since the switching frequency is very high, there is only a very small number of turns in both the primary and secondary, so it will be less than 1 ohm for sure. Many DMMs are not very good for sub-1ohm measurements and will just read 0. It would be quite unusual for a transformer in a SMPS to develop shorts between turns of a winding, since there are few turns and they do not usually overlap. I have never personally seen a transformer fail with a short of any kind in a reasonably well designed SMPS, which Astec would be. Open-circuits are more common, especially in high vibration environments the winding wire can break at the pin.

[xavier60]

When a  UC3842 doesn't start, the output pin stays Hi-Z. The Gate pin can float up if here is no bleed resistor.

[Yamin]

Thanks everyone, what can cause the MOSFET and the IC the PNP transistor to short all of a sudden now? What I changed was the control IC only.
I forgot to include the picture of the PNP transistor, for some reason I cannot upload it.

[eblc1388]

... I have got an almost identical working power supply sent by the same manufacture it doesn't require a load to be connected to work. So I am assuming this faulty one doesn't need one either.

Chances are the SMPS is fine to begin with, it just need a minimum load to operate correctly.

[Yamin]

Ah ok, power supply was definitely faulty the unit which it was in did not power up. The manufacture sent a replacement but it was a smaller foot print. Thats why I'm trying to repair this one.
Would not connecting a load cause the MOSFET to short out?

[Audiorepair]

It doesn't say whether this SMPS requires a minimum load to operate, or whether a minimum load is required to guarantee the quoted specs.

I have never come across an SMPS that won't run without a load, but have seen a couple that have higher than normal output voltges without  load.


I only deal with SMPS's typically found in audio equipment, however.

[asis]

Hi
It seems to me - the best way to understand the operation of a power source is to look at its circuit diagram.
Only in this case can something be explained in words.
I have not seen your specific PSU. If you post his photo and a list of components, I can correct the presented scheme.
Vladimir.

[Yamin]

Hi
It seems to me - the best way to understand the operation of a power source is to look at its circuit diagram.
Only in this case can something be explained in words.
I have not seen your specific PSU. If you post his photo and a list of components, I can correct the presented scheme.
Vladimir.

The model is ASTEC LTP42, I couldn't find any schematics online. I have seen similar ones. Something I noticed was there was a pnp transistor connected to the gate of the MOSFET, I didn't any schematic which shows the same.
I couldn't upload the photo so here is the datasheet :
https://datasheet.octopart.com/LPT42-Artesyn-datasheet-8318749.pdf
 

[tautech]

Would not connecting a load cause the MOSFET to short out?

No but the substitution of another incorrect SMPS controller might.

These PSU's are finely tuned machines and their operating principles are quite simple and generally work very reliably until components age, generally the VCC and secondary side caps suffering from current ripple close to their design capability. VCC diodes and dropper resistors are common failure points too.
A few cents saved on the BOM is mostly responsible for their weaknesses.

[MathWizard]

OP do you have an oscilloscope ? When it was working did you capture any of the startup and then shutdown into fault mode of the controller chip ? Is there a supervisor IC on the DC output side monitoring all the rails ?

I also have some broken or semi-broken SMPS, I haven't worked on them in ages, I can't wait for another try at them.

Also about the noisy Vcc for the controller, is that driven off an auxiliary 10-15V that's generated by a mini-SMPS that creates 5V standby-PWR on the cold side from 1 winding, and 10-15V or so auxiliary voltages on the hot side ?

[Yamin]

OP do you have an oscilloscope ? When it was working did you capture any of the startup and then shutdown into fault mode of the controller chip ? Is there a supervisor IC on the DC output side monitoring all the rails ?

I also have some broken or semi-broken SMPS, I haven't worked on them in ages, I can't wait for another try at them.

Also about the noisy Vcc for the controller, is that driven off an auxiliary 10-15V that's generated by a mini-SMPS that creates 5V standby-PWR on the cold side from 1 winding, and 10-15V or so auxiliary voltages on the hot side ?

Unfortunately I haven't got access to an oscilloscope as its at work, we are still in a lock down so the movement is a bit restricted.

Vcc is taken from an aux winding from the main transformer (hot side). There are no other special ICs on the cold side.

[MathWizard]

So I'm familiar with a typical computer SMPS, and usually they have another little 8-pin DIP that will control/create the aux-voltages.

Now yes the output cap's on that rail should be checked, but as for what counts as too noisy to these chips, I have no idea yet.

But as for blowing up the mosfets and stuff, IDK. 

OP if you could get soon enough, even a little $20 ebay scope w/200kHz BW would let you see a lot more.

If the main chip is working, and deciding to shutdown/restart/cycle, yup u need a scope.

[Yamin]

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. 

Thanks everyone for the help much appreciated.