Has anyone seen any good write-ups on, or figured out for themselves, just
> how the self-oscillating dirty - i.e. smoothing cap-less - switchers that
> you find in use as lamp ballasts (or 'electronic transformers' that they
> seem to be sometimes known as) work ?
They're capacitor-less because the load doesn't care, and it makes their power factor much nearer to 1 without needing any additional power factor correction circuitry. Some of them will also work with phase control dimmers. This is the design of most 12V electronic lighting transformers.
I recently acquired a fairly sophisticated disco lighting fixture that has
> such a supply to run the 24v 150 watt lamp. It seems to have two main
> switching devices, which I suspect are FETs or IGBTs, but can't tell for
> sure so far, as the numbers have been ground off them. On the mains side, it
> has a bridge but no filter cap, so dirty DC is applied directly to the
> collector / drain of one of the devices. No sign of any control IC, but
> there is a small vertical sub-pcb that has a few small transistors and other
> bits and bobs on it. It does have a preset pot on the main board, which I'm
> guessing sets the output voltage. There are lots of 1 and 2 watt resistors
> scattered around, as well as quite a lot of diodes, but overall, the
> topology is not one that I immediately recognise as being any type of common
> switcher. The output to the lamp is taken straight from the secondary side
> transformer. No rectification or filtering.
>
> Thus far, I've only quickly run an ESR meter over the small electros on
> there, but nothing particularly bad looking amongst them. It just sits
> there, pretty dead. Nothing exploding or smoking. Fuse intact. Nothing
> obviously short on the 'power' side of things. Probably going to finish up
> being an open polyester cap or some such, but it would be nice to understand
> a bit more, what kicks these designs off, and keeps them running.
Power semiconductors can explode without showing it on the device package. Look carefully around the component for remains of the ejected guts, often just a dark mark on the inside of a case lid, or similar. Often ejected where the plastic package bonds to the heatsink tab, or where the leads enter the package.