Switching Mode Power Supply half output problem

Thread Starter

wardjk

Joined Jun 20, 2019
10
I’m trying to fix a Switching Mode Power Supply for an audio component that is producing only half of the designed output voltage. It has 12V and 5V outputs that are now only 6V and 2.5V. I replaced an obviously bad capacitor on the output of the bridge rectifier and the unit initially worked fine for about a minute until it went off again with the current symptom. It is switched by a Power Integrations TOP245PN (datasheet here). The design of the SMPS is similar to the one in Figure 44 but stripped down to just the 12V/5V outputs with a few component value differences.

I don’t know what the output of the transformer should normally be but it appears that the duty cycle may only be half of what it should be according to what I’ve seen with an oscope.

The 5V output (blue) is overlaid on the 12V output (green) measured at the transformer secondary. The time between the yellow/magenta markers is ~25ms (40Hz).

Could this apparently half-duty cycle be normal or be the problem?
 

Dodgydave

Joined Jun 22, 2012
8,409
The output is loaded to much or it's in current overload, disconnect the optocoupler pin to U3 zener, see if it gives out maximum voltage, and check the output diodes for shorts, D6 to D11 this more likely the problem and capacitors faulty.
 
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LesJones

Joined Jan 8, 2017
2,325
I suggest that you start by measuring the voltage across R12. It should be very close to 2.5 volts. If it is less than 2.5 volts then there should not be enough voltage across LED in U2 (Opto isolator.) to cause it to conduct. This is how the feedback loop controls the output of the switch mode chip (U1)
This should help you to decide if the fault is on the input side or output side of the transformer.

Les.
 

AlbertHall

Joined Jun 4, 2014
8,381
The output is loaded to much or it's in current overload, disconnect the optocoupler pin to U3 zener, see if it gives out maximum voltage, and check the output diodes for shorts, D6 to D11.
Do not do this. The output voltage will be very high and most likely damage something else. Follow @LesJones recommendation.
 
Could this apparently half-duty cycle be normal or be the problem?
The 40Hz confuses me, maybe your horizontal resolution is off or some other horizontal timing issue. If it actually is 40Hz then it could be some response time issue with the controller. Perhaps a bad capacitor in the feedback loop.
I would also check the bridge rectifier, one of the diodes may be bad. That would explain the bad capacitor on the bridge output.
 

Thread Starter

wardjk

Joined Jun 20, 2019
10
@LesJones: The actual design deviates from the example schematic. For the equivalent measurement across R12, the voltage at the cathode of the LED in the photocoupler is 1.8V. At the anode, it is 2.6V. These are relative to RTN/GND.

@CharlesWMcDonald: The 40Hz pulses seemed odd to me but I don't know if it was by design. A bad capacitor in the feedback loop seems a plausible cause. I didn't measure continuity of the bridge rectifier diodes but the output seemed fine based on the math: 120VAC in x √2 ≅ 170VDC which is about what I measured. A SMPS should work over a wide range of input voltages; the nameplate shows 100-240VAC in this case.

The unit had been powered on for most of its 10 years+ of life (>90,000 hours). The capacitor on the output of the bridge rectifier was swollen and replaced. Replacements for all other electrolytic capacitors have been ordered but I wanted to understand the problem. This is somewhat of an academic exercise for me.
 

Thread Starter

wardjk

Joined Jun 20, 2019
10
The SMPS is in a specialized tuner that is part of a more extensive and expensive audio system. It's no longer available nor are replacement parts at the board level which is why I'm having to repair at the component level. I could fit a non-matching but electrically compatible SMPS in the case but I'd rather keep it nearly original.
 

LesJones

Joined Jan 8, 2017
2,325
If the schematic you posted is not the schematic of your power supply we will only be guessing at what the problem might be. The TL431 in the schematic that you posted provides the reference voltage to be compared with the output voltage. (Via a potential divider.) You at least need to trace out that part of the schematic for YOUR POWER SUPPLY to see if the feedback loop is causing the problem. After reading some other members comments on this (And the ETO forum) I think the theory of it shutting down due to some fault condition and restarting every 25 mS is a more likely theory than the possible feedback loop fault that I was trying to get you to check.

Les.
 

Marc Sugrue

Joined Jan 19, 2018
60
I’m trying to fix a Switching Mode Power Supply for an audio component that is producing only half of the designed output voltage. It has 12V and 5V outputs that are now only 6V and 2.5V. I replaced an obviously bad capacitor on the output of the bridge rectifier and the unit initially worked fine for about a minute until it went off again with the current symptom. It is switched by a Power Integrations TOP245PN (datasheet here). The design of the SMPS is similar to the one in Figure 44 but stripped down to just the 12V/5V outputs with a few component value differences.

I don’t know what the output of the transformer should normally be but it appears that the duty cycle may only be half of what it should be according to what I’ve seen with an oscope.

The 5V output (blue) is overlaid on the 12V output (green) measured at the transformer secondary. The time between the yellow/magenta markers is ~25ms (40Hz).

Could this apparently half-duty cycle be normal or be the problem?
45 to 50 % Duty cycle would be normal for a flyback converter at full load operating in discontinuous mode. When your doing the test what are you using as a load? The PSU will need something to give DC paths. Under no load it would probably go in some form of low power hiccup mode due to it otherwise overvolting but if its overloaded it could be the fault is elsewhere.
 
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Thread Starter

wardjk

Joined Jun 20, 2019
10
While waiting to receive the replacement capacitors, I prepared to continue troubleshooting based on the advice given. It was then that I discovered that the circuit deviated from the example even more than I had initially mentioned, particularly with the addition of Q1. Because of the kind efforts of those who took the time to read about the problem and especially of those who offered advice, I felt obligated to begin the laborious task of partially reverse-engineering the board to have a more accurate schematic, which is shown below.

In the schematic, all components are accounted for but I didn’t go as far as examining the value of each component, which isn’t always necessary for troubleshooting anyway. By the time I got this far, I had the replacements. I first replaced C11 and C12 because C12 seemed most likely to cause my symptom. They were also identical in value and were adjacent to each other so for convenience, I replaced them at the same time before testing. This fixed the problem. Unfortunately, because I did both at the same time, I don’t know which one fixed the problem so I partially lost my academic exercise in properly troubleshooting this circuit. Most importantly though, the unit works and is in service. While I was at it and had new electrolytic caps, I replaced the others as well which wasn’t absolutely necessary because the old ones still worked fine but maybe the replacements will postpone another repair.

Thanks for the troubleshooting tips. For others who may benefit from this, this schematic is of the switched-mode power supply (SMPS) for a NuVo tuner model NV-T2DFG. I was unable to find a schematic of it from other sources. The tuner is discontinued and the company was sold to Legrand.
 

dendad

Joined Feb 20, 2016
3,025
The first port of call with switch mode supplies is usually the caps.
Over time, they dry out and no longer work as they should. For example, many video monitors can be resurrected by replacing thecaps.
A handy cheap tester is ...
ComponentTester.jpg
While not a precision device, they work really well for the price.
Just make sure the caps are discharged before connecting them.
Have a look on Ebay. These will give a capacitance reading and the ESR too. As well as testing resistors and semis.

Have a look at

(The price has gone up from the original $7 )

It could be interesting to test the removed caps.
 

Thread Starter

wardjk

Joined Jun 20, 2019
10
@Dodgydave:
I think you have drawn the transformer 12v output wrong
Because the 12V winding is connected to the 5V diode cathode?
..and the opto-coupler feedback is wrong..
I've compared the board and schematic and can't find the difference. Please help.

@dendad: Wow! Some are even under $5 now! It’s an open-source project that has dozens of sellers and variants. I ordered a slightly more expensive assembled variant with a molded case.
 

AlbertHall

Joined Jun 4, 2014
8,381
I think you have drawn the transformer 12v output wrong..and the opto-coupler feedback is wrong..
The only thing I can see wrong with the schematic is that when first switched on IC1 will have no power supply and so nothing will happen. I suspect that R10 is actually connected from IC1 (M) to the rectified mains, anode of ZD1. This then supplies a little power to IC1 when first switched on to get things going, then D4 & C12 can take over.
 

BR-549

Joined Sep 22, 2013
4,936
The voltage at C6 will tell you to check up or check down the circuit. If circuit can not be energized, I would advise checking diodes. And possibly the input filter elements.
 
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Dodgydave

Joined Jun 22, 2012
8,409
@Dodgydave:
Because the 12V winding is connected to the 5V diode cathode?

I've compared the board and schematic and can't find the difference. Please help.

@dendad: Wow! Some are even under $5 now! It’s an open-source project that has dozens of sellers and variants. I ordered a slightly more expensive assembled variant with a molded case.
Yes the output side is drawn wrong and the supply for the chip is missing on pin L, normally there are two separate outputs for 5 and 12v, R2 is for the Current limit.. here is the circuit in the datasheet you can see how it's done...


Screenshot_20190628-093325.png
 
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