A simple repair of a small power supply, plus a little theory

I hadn't even thought about writing about this repair. Then I figured I'd just write a little and add a few photos, but I got carried away. In the end, it turned into an article about how power supplies behave when the input voltage is too high, with a bit of extra theory about chip circuitry.

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It all started when a friend called and asked if I could take in a monitor for repair. He'd brought it from a customer. Symptoms: it doesn't work. Backstory: during yet another power outage caused by a missile strike on “military facilities,” there was a voltage surge and the monitor stopped working.


The monitor is very thin, although I should say that about 15 years ago I made power supplies about this thick for a friend. They usually use EFD-series transformers with low-profile cores for this kind of thing.


It's a fairly ordinary, inexpensive video door phone monitor, and it's even still under warranty. Unfortunately, after yet another shelling of exclusively “military facilities,” the store was gone.


Inside, the damage is pretty much what you'd expect: clear signs of soot.


I'll take a brief detour here and post a few photos of the monitor's circuit board. Experience shows that sometimes they can help someone with a repair.


Signal switches, presumably


Digital section; the memory card slot is on the left.


Power supply and controller


We take out the power supply and get the full picture... But I've already shown something like this before, so I doubt I'll scare anyone.


The power supply is actually put together reasonably well: it has an input filter and even an output inductor. The only oddity is that the Y capacitor is an SMD ceramic one.


But we're interested in the power supply's input, or “hot,” side.
There are soot marks, so something clearly burned. It even left soot on components that were relatively far away.


First checks.
1. There's practically a short across the input.
2. After the bridge rectifier, the meter shows infinite resistance and a 1 V drop when testing in diode mode with the probes connected in one polarity.
This power supply isn't really ideal for demonstrating diagnostic techniques, and I'll come back to that. For now, I'll just say that the expected drop is around 0.5–0.6 V, whereas here we're seeing the drop across the bridge rectifier. Well, at least it's not shorted.


1. We desolder the small capacitor, which serves as an X capacitor across the power supply input.
2. The resistance across the input immediately jumps to infinity.
3, 4. Testing the bridge rectifier showed that it's working, so there's a very good chance the PWM controller is alive. Test the bridge rectifier in four steps:
L to positive
L to negative
N to positive
N to negative.


And now for the theory.
Here's what I checked right away:
1. A short across the input: the EMI suppression capacitor had failed short.
2. A short after the bridge rectifier. Check either points 1-1 or 2-2; they're essentially the same, since the transformer winding has very low resistance. To be fair, though, the winding can sometimes go open circuit, albeit very rarely. So if there's no short at points 1-1, check points 2-2 as well.

This uses a DK125 chip; here's the circuit from its datasheet


Why was I almost certain the power supply had survived and the problem was only in the input circuit?
The thing is, chips like this usually have not only a high-voltage transistor, but also an overvoltage protection circuit that stops operation if the voltage is above the limit.
But you'll say, what about the transistor? Won't it break down? No, it usually survives because it's rated for 600–700 V, while even with a 380 V input you'll get a maximum of 530 V. Yes, the input electrolytic capacitor or varistor will probably blow up, unless the fuse blows first, but the PWM controller usually survives. That's exactly why, if the input capacitor is rated for 450 V rather than 400 V, as is often the case, the power supply has an even better chance of surviving.
The capacitor is cheap, of course, and easy to find, but the bigger problem is that it can leak electrolyte all over the board, which then has to be cleaned.


The datasheet also included a block diagram showing the high-voltage transistor, and this is where I noticed two unusual features. Not unusual to me personally, but unusual to see in a very ordinary Chinese chip.
First, I think you noticed that there are two transistors. That's why, when checking continuity, we saw a voltage drop across the bridge rectifier rather than across the parasitic diode of the high-voltage MOSFET. That's also why the drop was 1 V instead of 0.5–0.6 V.


This circuit topology is very similar to a BIMOS switch. When I tried to find information about it online, I discovered there was almost none; this topology is used so rarely. I did find some article from abroad that shows this clever switch itself:


But I recognized it as soon as I saw the block diagram. Back in the late 1990s, I bought a very useful book that was essentially a translation of a collection of application notes from International Rectifier. It discussed this kind of switch, in application note AN-946B. The same book describes the features of this circuit topology, which are also covered in the link above. The upper switch is a high-voltage bipolar transistor; the lower one is a low-voltage MOSFET.
This circuit topology allows for faster switching with lower switching losses.



By the way, if you're getting interested in designing and repairing power circuits, I highly recommend this book. Yes, it's almost 30 years old, but the basic principles it explains will save you more than a bucketful of transistors. Unfortunately, I gave my copy to a friend quite a while ago and haven't seen it since :( There is a scanned copy, but the quality is terrible.


The second unusual feature is that the lower transistor isn't just a MOSFET; it seems to have a built-in current sensor, something like this:


In simplified terms, these are two transistors: one switches the current, while the other acts as a shunt. That's a very simplified explanation, though. IR once actively promoted transistors like these, but they never became very popular because they cost more, and finding a replacement during a repair could be a challenge.


Since everything else seems to be working, I start by desoldering the input capacitor and cleaning the soot off the board. Fortunately, there are no burn marks, and although the soot wasn't always easy to remove, I managed to clean it all off.


I connect a 15 W incandescent bulb in place of the fuse.


I apply power, and everything looks good. By the way, intercom power supplies are often rated for more than 12 V—13–14 V.
The bulb stays off, the power supply works, and everything is fine.


All that's left is to put the protection components back, because I think it's better to have them. Unfortunately, I don't have any high-voltage ceramic capacitors on hand, and ordering just one would take a long time and cost too much, so we'll do without it.
In the end, I replaced the fuse with a larger one and soldered in a 470 V varistor in place of the capacitor. Heat-shrink tubing won't hurt either; at least I won't have to pick up the pieces if anything happens, and the bridge rectifier will be protected too.
The board ended up with the same overall height as before.


The second step is cleaning the case. I couldn't get it as clean as the board; some marks remained, including noticeable marks on the backlight film behind the LCD panel.


I put the power supply back in its original place.


I turn it on and check it.


And that's it. The repair was simple, and the customer got lucky—you could say they got off lightly. Most of the time, that's how these repairs go. But if, for example, the damage was caused by a lightning strike, things can be much worse. I have three practically new monitors at home from the same site, but repairing them isn't worthwhile, especially since finding parts for them is very difficult.

That's all for now. I hope this was useful, and that this information might help someone—not necessarily with repairing power supplies, but at least with broadening their horizons.