
I came across the schematic of a power supply at my work. The "engineer" that designed it doesn't work here anymore and as far as the company is concerned the design is fine. There was an "anomaly" that prompted my attention. I spent several days trying to figure out what was going on to no avail but had to return back to my project. Meanwhile another person added some bandaids, which caused another problem, and another bandaid was added. As I mentioned, the company thinks the design is fine but personally I think there is something seriously wrong with it. It's really been boggling my mind, so I'm sharing it because I'm really interested in what some of you might have to say about it.
The only connection this board has to the outside world is AC mains. The first confusing part is that the 3.3V for the microcontroller is at the same potential as the NEUTRAL from the AC mains. For whatever reason this needs to be the case.
In the grand scheme that would put 3.3 at Ground, and it would put GND at -3.3V. That's the first layer of confusion to me. Because the board has no other connections, I decided that it's safe to ignore this part and look at it from a local perspective of the board, so that GND is Ground, and 3.3V is 3.3V and so forth.
On the board you will see a 4V rail. This is dedicated to a wireless communications module. Apparently during the design, this module wasn't transmitting but now the module is updated and transmitting, bringing us to the "anomaly".
So what happened is that when the module is transmitting it will draw about 1A from the 4V supply. At that same time, the 3.3V would dip down to about 1.5V, and recover as soon as the module stopped transmitting.
The finger was immediately pointed at RF interference, and while possible, I was skeptical of that. Something tells me there is a different issue that is with the design itself.
When I looked at the 3.3V regulator, the 12V input was staying strong but the 3.3V output dipped. I concluded that maybe it's not the 3.3V that's coming down, but rather the GND coming up.
I started trying to figure out the return currents and it was really confusing to me.
If you look at the schematic you will see two switchers, one creates the 12V, the other the 4V (from the 12V). For the return current I imagine there are 4 basic paths depending on the combination of which switch is on or off. In addition there are capacitors but I think they are quickly drained to a lower level when the 1A is drawn from 4V.
Perhaps one of the issues is that when the switch for the 4V is off, the inductor L39 is supplying the current. The return current must go through D31 before reuniting with the source at L39. This may be causing GND to rise.
Btw. I noticed that Q1 was in series with U3, instead of U3 driving the gate of Q1. This doesn't have anything to do with the issue here but that tells me that the engineer may not have known exactly what he was doing; so I think it's likely there are other mistakes.