My current project includes an on-board switcher that produces 5V from the mains input to run some analog circuitry on the mains side. It doesn't work. I've already decided that the next revision won't have a "mains side" at all and will be externally powered, but I can't stand the thought of leaving it non-working. So maybe someone experienced with switching power supplies will recognize the symptoms and give me some pointers.
I'm using a LNK305DG from Power Integrations in a high-side buck configuration with direct feedback. I've basically cut-and-pasted the reference design and just tweaked the reference voltage divider to 5V (instead of 12V) and replaced parts with ones more easily attainable from Digi-Key, but the values are otherwise identical. The behavior I'm seeing is that the supply won't start when fed with anything more than about 30VDC. It's "hiccuping" which means the IC is seeing an overload condition either due to too-high switching current or because the feedback is not working. With 16-30VDC input the supply starts and roughly holds regulation but is switching at 2KHz or less (66KHz is normal). This doesn't surprise me too much since the part requires 70VDC minimum to operate, but at least it shows that the fault is sensitive to input voltage. Under all conditions I see no more than 50mA average current at the input but I don't have a handy way to see what the peak current into the switch is.
With light load (e.g. a 1K resistor, still more than required per the datasheet) once it starts at < 30VDC I can crank up the voltage a little and it doesn't stop, but the output voltage also increases with it. This seems to indicate a problem with the feedback. With moderate load (100 ohm resistor) it immediately starts hiccuping when I increase the input voltage. With no load the output voltage increases with each hiccup, but it still keeps hiccuping and does not start. Again, this is not a normal operating condition so maybe that's not too indicative of a problem.
Also worth noting is that both diodes get fairly hot. In particular, the feedback diode gets to 50C which seems really odd. If I measure the voltage across it, the peak forward voltage is only 0.68V which according to the datasheet corresponds to a forward current of only 1mA. The flyback diode sees 1.36V peak forward voltage but doesn't get as hot (35-40C). These readings were taken with the 30VDC input, since that's the only condition where the supply will operate continuously.
I've tried reducing the capacitance by a factor of four but it makes the output rougher and doesn't otherwise change the behavior. I've also tried increasing it by a factor of four, again with no improvement. I originally specified placed a ceramic cap for the feedback capacitor but it made quite a lot of noise at 2KHz so I replaced it with a tantalum. I can still hear switcher noise if I touch either diode, or the main inductor.
Here's the schematic:
http://hg.partiallystapled.com/circuits/ctservice/raw-file/6b2146b43551/out/production.PDF (see page 5)
Switcher datasheet:
http://www.powerint.com/sites/default/files/product-docs/lnk302_304-306.pdfAny ideas?