On Fri, 1 Jan 2010 11:56:28 -0600, "Tim Williams"
wrote:

I see circuits like these in VCRs a lot... not really anywhere else, for
some reason. Not the greatest efficiency, but the parts are a lot cheaper
than a whole UC3842 or TL494 deal. Maybe not worthwhile against TOPswitches
and such these days.

Efficiency as shown is around 67%. Runs at ~200kHz full power, much slower
at lighter loadings (it fires a full cycle, then stays off for a while)

Tim

Here's my +12 to -12 converter. The IC is just a schmitt-trigger gate
driver.

ftp://jjlarkin.lmi.net/Inverter.jpg

At powerup it self-oscillates at a calculated duty cycle, 75% maybe,
until the -12 comes up, turns on the common-base transistor, and backs
down the duty cycle.

John