Solving a few power supply cold cases

 I spent a week or so clearing a few power supplies off my bone piles recently.

The case of the phone charger

I have a flip phone USB charger that went out of regulation outputting 7V.  I did the usual check on the electrolytic capacitors and found nothing wrong with their values.  I left it on my bone pile for scraps.

Top side of the PCB

In the left side of the PCB below, there is a slot (for isolation) between primary and secondary circuits.  Normally you would find an optoisolator used as a feedback bridging both sides.  There was a footprint for a 4-pin device, but it isn't hooked up correctly.  It is probably a left over from a previous version of the design.

 

I hooked up my bench supply (max out about 63V) to the AC input of the charger.  It was sufficient to get 6V at the output..  Feeling a bit brave (or foolish?), I poke around a bit with my multimeter and found another spot across the capacitor measuring 6V at the primary side.  I traced the circuit and annotated in the picture below.

There is winding for a feedback for the oscillator.  There is another branch of it with a simple rectifier + capacitor.  The negative voltage across is an approximation of the output voltage.  A Zener diode is connected between this voltage and the base of the flyback driver transistor.  At some point when the output voltage is high enough, the Zener diode conducts and pulls the base towards ground.  This stops the oscillation and closes the negative feedback loop for the output voltage.

Bottom side of the PCB

Funny that the old diode didn't fail completely but ended up having a higher Zener voltage.  I fool around putting resistor and Zener diodes in parallel (4.7K, 10K) to see if that has an effect on the output.  Yes!

I managed to find one scrap Zener diode that sets a reasonable voltage: No load: 5.2V and 4.76V at rated load of 350mA.  The diode drop difference between no load to full load across the output Schottky diode would probably account for most of the droop.  It is good enough for the USB specs.

The case of the flickering LED

Back in 2017, I installed a LED driver (Modding 220V LED for 110V - LED rectrofitting).  It is the outer ring that is only switched on for extra brightness.  It became the main light when the inner rings failed in 2019.  

Recently the LED starts to flicker once every few hours.  The bulk capacitor has started to fail under heat.  The PCB is discolored (around the driver chip) due to heat.  This Chinese no-name brand capacitor was actually a quality part as it survived about 3 years of abuse under heat.

It still have about 1/2 of its value, so the LED flickers when there is a slight dip AC . I don't have an exact replacement (400V 10uF) nor was I looking for one.  I replace it with a 47uF 450V capacitor that I have in stock.  

I leave a bit of a gap between the base of the capacitor and the PCB.  As the capacitor replacement has a higher value and lower ESR, I also added in a NTC Inrush current limitor to protect the tiny bridge rectifier.  (There is a 0.5A fuse in series I added for protection.)

 

LED driver PCB + defective cap (desoldered)

The case is there to cover the high voltage circuits.  It is discolored over time due to the high heat.  The case is mounted horizontally with standoffs. There is space above and below the case to allow for convection.   I have decided to improve the cooling by drilling some holes in the case.  I used an EMI shield as template for the holes.

 I kept the holes small  to prevent flying insects getting into the case.  Part of the case crack because it was too thin, brittle and weak.  I stitched the cracks together with some dental floss.

I hope this would last for another few years.

The case of the blow input stage PSU

This is the donor of the PSU case for my previous article.  There was a bag with a blown bridge rectifier diode and two flyback transistors with it and a missing fuse.  I assumed that those were the broken parts from my previous diagnostic a long time ago.  I don't remember whether I blew it or it was already DOA when I picked up a few PSU from a surplus place.  Scrapping it was the correct decision. 

I found a $20 store brand PSU in my box.  It was the replacement I bought for the one that caught fire.  Funny thing about that PSU as its parts still lives on in a few things.

It lived long enough to its retirement.  So why bother fixing the other one?  When I opened it up to clean it, I realized that the bean counters have been working overtime as a lot of the EMC and filtering parts on the PCB have been jumpered off.   The heatsink are about 1/3 as thick vs the your usual computer store silver box special.

So I took a chance by taking the flyback transistors from this board.  In the process of removing the input filter cap from my dead PSU, I also found a loose pin in the main filter capacitor.  I finally have a rough idea of what happened.

Simplified generic 200W PSU schematic showing the damaged parts

They used a bridge rectifier rated for 2A (max) in a "250W" PSU. We use 115V here, so the part is under spec.  Even in the cheap out PSU, they use larger diodes (probably 3A rated ) for the 115V.

So basically what might have happen is that the diodes were overloaded and at some point they failed shorted together shorting all the AC inputs and the negative output together.  This blew up the lower filter capacitor, two flyback transistors and the 5A fuse.

In general, I would use all part at least rated for the fused value (5A) for the input circuit.  I replaced it with a 600V 7A part that fits the mounting hole.  I replaced the two filter caps with the ones from the PSU that caught fire.  The two flyback transistors were from the accountant optimized PSU.  They have slightly lower rating.

I used the old trick of using a 40W incandescent light bulb in series of the AC input for testing the "fixed" power supply.  The light grows briefly and went out.  I shorted the enable pin to ground with the "PSU tester" block that came with one of my PSU and the fan spin up.  The PSU is fixed!

Now the fun part of finding a case for it as I have repurposed its old one.  I foolishly think that I would fit inside a Dell 200W PSU as the PCB are the same dimensions.  The Dell case is nice to work with as it allows access to the trace side of the PCB. Wrong!  Dell decided that they want mounting holes in a slightly different locations than the "standard" ones used in generic PSU. I leave the EMC ground pad for the high voltage as is but offset the rest of the mounting hole, drill some new ones.  I have to trim off the excess metal bits so that they don't short circuit the PCB traces on the secondary side.  I soldered some nuts to the frame to make life a bit easier.  I clean up and redid all the wiring.  I added an On/Off switch from the el Cheapo PSU.

Fixed PCB fitted into a Dell PSU case

Component side of the PCB

The case of the leaky capacitors

This one is build by a certain consumer electronics/entertainment corporation in Japan for another certain fruity electronics/entertainment corporation.  I modified its output from 7.5V output to 5V a long time ago.  I took it out of storage and it died after a few minutes.

Bad leaky capacitors is not what I would expect to see here.  There are 3 electrolytic capacitors that leaky and they spill oily black liquid that attacked the solder mask and cause some minor corrosion to copper traces and wires. I had to remove some of the components to clean and assess the damages.

Two caps and an inductor (top)

Trace side of the PCB showing corrosions and dirty build up due to the leak

I have decided to use tantalum capacitors as  I don't want the same part to fail again.  They handle heat a lot better as they don't dry up.  They cost a lot more, but I have a big pile.

The three fail capacitors are located right next to the heatsink for the rectifier and heat is trapped between the subassemblies or corner of the case.

The 2 replacement capacitors, bad main capacitor and common mode filter PCB

The PSU with case with no vents  = heat trap

82uF cap

for feedback circuit power.  It is non-critical, so I used two 22uF 25V in parallel.

180uF cap: Output cap after LC Pi filter.  

I used three 47uF 10V caps in parallel:: 140uF,  ESR of 0.266R and ripple current of 1.3A

1200uF cap: Main capacitor

This is the capacitor that I missed in my initial replacement.  The capacitance value was roughly correct and I don't have the right equipment to measure its ESR.  The power supply fails to reach its correct value and would fail at 50% load.  It took me a few days to figure out that it might be the problem.  I soldered in a 1000uF 50V caps and the output was fine.

I used twenty 47uF 10V caps in parallel: 940uF, ESR of 0.04R and 8A of ripple current. I only need 2-3A or so to match the original spec. I used heat shrink around the capacitor to prevent short circuit. (Stacking caps warped in heat shrink and operating in a heat trap would likely derate the ripple current rating a lot.)

I have some much higher value caps in parallel, but their ESR might be too low and affect the feedback circuits.  I also have a lot less of them.

stack of 20 capacitors 

47uF 400V:  Input filter capacitor

It has not failed, but I replaced it anyway.  Caps next to heatsink is not good.  I use 47uF 450V cap from my left overs.

Testing

5V: no load 5.2V

5V @ 1.9A (2A rated): 4.7V

Verdict: poor regulation. It has same regulation as the phone charger that doesn't even have secondary feedback. I traced out the feedback circuit and it is much more complex than it needed to be.   It uses 1 dual opamp, a lot of passives and a TL431 just for voltage and current feedback.  All that extra junk for poor load regulation.  Almost tempted to rewire the feedback circuit...

Voltage and Current Feedback circuit (daughtercard)

As a point of comparison, I hacked a 12V +5V module into a 17Vcurrent limited charger/adaptor for my old laptop in my younger foolish days.  It worked fine and I managed to get the proper one on ebay.  So yeah, I know what I am talking about.  

I wounded a small current transformer, a transistor and a few resistors for the current limit. It measure the AC current from the secondary winding to trigger the transistor to fool the feedback circuit to lower the output voltage.  The existing voltage feedback uses a TL431, a few passive and an optocoupler would look like circuit on the right hand side.  It is very simple and work well.  See TI app note sluaa66 (.pdf) for how to design the circuit on the right hand side.