J
Joerg
- Jan 1, 1970
- 0
John said:[...]
Another option would be to use a CD40106 or something similar as a
Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can
drive a little FET, a simple logic level device like a 2N7002 as long
as doesn't cost much. Pipe Vref out of your MCU (hoping it has that
...) and use a cheap opamp to pull the Schmitt oscillator input "to
the side" when the target voltage is reached. That reduces the duty
cycle as much as needed to maintain regulation, pretty much like the
throttle on a gasoline-powered generator. If the target voltage
doesn't have to be very precise you could also use a NPN plus zener
for that, without a reference source. Probably a TL431-type device
would work as well and those are quite cheap, in the penny range.
Hi Jeorg,
Are you talking about a SEPIC/flyback arrangement here? I.e., 2N7001
drives a transformer? How do you stop the opamp railing during startup
or a load step? The schmitt osc would stop and .. how does it go
... Phut!
If you use an opamp a simple trick to avoid this is a diode. That way
the enchilada can pull the Schmitt to one side but cannot push it to the
other. If you are really brazen use a comparator with O/C -> no diode,
saves 1-2 cents. If you use a transistor or TL431 the problem goes away
on its own, sans diode. Now, set the max duty cycle to whatever worst
case demand you calculated, at the minimum expected input voltage, plus
20% margin or so.
Also of course by the time he's actually bought a SEPIC transformer it
could all approach his 1.8 Euros. Or are there some super-cheap ones
now?
Yes, but only when you buy in Asia. For some reason you can't get them
much under a buck inside the US even though I could imagine those are
also made in China. Coilcraft is typically the best deal for coupled
SEPIC coils if it has to be domestic.
Using two individual inductors gets you around the problem.