Need a DC +12v to -15v converter good for at least 50mA. Any ideas?

[anotherjim]

That IR2155  does seem to be the right stuff. I think there are also MOS half bridge drivers that have high-side drive for the top FET built in.

[Rob Strand]

One scary element of this design is that it ought never be powered up with the 555 absent. Without its grip on the BJT base drive the pair will both turn on and sit there getting very hot. I usually socket IC's, but it might be wise in this case to solder it in.

Not much of a big deal.  Like if you lifted the transistors (or IC's) in a power amp is would wreak havoc as well.

it's probably too late for me to have thought of it by now, or it's outside the scope of "parts in the bin" [although I have one in mine], but what about something like the IR2155? hi and low side drivers, and with dead-time inbuilt, might be a thing.

There a few chips around like that.  They have nice strong drive as well so they can switch large MOSFETs quickly.  Hard to beat purpose-build stuff.

With this device you still have to use diodes in the multiplier which is the main source of loss.

The ICL7660 looked interesting, but can't quite produce the goods in this case. They can be parallel and/or series ganged, but that's still more parts.

The big advantage of those negative output devices is the don't need any diodes to get negative outputs.  They use synchronous switches which have low loss.  (The ICL7660 is only 10V, the  ICL7660A is 12V (?), and the LT1054 is 15V.)

[amptramp]

A negative buck-boost converter looks like this:




The first thing you will see is that there is only one diode, so efficiency is better than capacitor superposition converters like the 7660.  Instead of the sudden voltage change from stacking one capacitor on top of another then another noise burst from removing the capacitor from circuit, the buck-boost current drain starts at zero and builds up to a sudden shutoff so it only has an abrupt current change during shutoff.  The inductor current continues to increase until it shuts off and when it shuts off, the junction of the coil and diode goes extremely negative, just like the ignition system in a car.

[Rob Strand]

The first thing you will see is that there is only one diode, so efficiency is better than capacitor superposition converters like the 7660.

Yes a modern switch-mode device can offer much better efficiency.   You can keep noise issues down with good layout and (magnetically) shielded inductors. 

Not a lot of chips on offer for < 1A  + negative output  + 12V rails.  (The efficiency often degrades when the load is < 10% rating. )

The other way to get negative output is with a transformer instead of an inductor.