On Wed, 03 Oct 2012 08:29:37 -0700, John Larkin
wrote:
On Wed, 03 Oct 2012 07:42:51 -0700, Jim Thompson
wrote:
On Tue, 02 Oct 2012 21:20:47 -0700, John Larkin
wrote:
On Tue, 02 Oct 2012 21:11:23 -0700, Jim Thompson
wrote:
On Tue, 02 Oct 2012 20:31:25 -0700, John Larkin
wrote:
On Tue, 02 Oct 2012 17:50:14 -0700, Jim Thompson
wrote:
On Thu, 27 Sep 2012 12:44:00 -0700, Jim Thompson
wrote:
AC Cap Dropper DC Power Supply
Anyone know of a clever way to regulate other than the typical
brute-force zener clamp?
...Jim Thompson
I ended up with this...
http://www.analog-innovations.com/SE...00056_MOD3.pdf
Using a depletion-mode NMOS FET.
I have a version I personally liked better that used a cheapy TRIAC
under DDROP3, but I could not find a way to get some "free" power for
the gate... needed while pumping, goes away when overcome by the
switcher coming on line.
(This is a wild high PFC, high efficiency dude, where the customer is
squeezing me for milliwatts :-)
...Jim Thompson
That's crazy. Not only is it expensive, once the 24 volt supply kicks
in, it dissipates most of a watt, doing nothing.
Boy, did I just save you some embarassment.
You're so ignorant you don't know how it works. Thanks for the
exhibition!
...Jim Thompson
Well, how much power do R1, R2, the TL431, and ROFF dissipate once the
main supply is up?
It was "my bad" to have emulated the main supply with a pulse
source... doesn't work that way at all, but I can't show the
customer's circuitry.
I was thinking about that, specifically where that idealized 300
volt-peak full-wave-rectified waveform came from. If it is through
diodes, there's no pull-down to pump the series RC thing; if it also
drives a 24-volt non-PFC switcher front-end, you won't get that
waveform. If the startup circuit has to be working before a main PFC
switcher starts up, there are all sorts of interesting tangles. If the
switcher starts up on its own, what's this circuit for? Standby power
for some controls? It would still have to work when the PFC was down.
In actual practice a PFC/PWM chip comes alive and the switcher starts.
But that "20V" sags since the charge pump can only support ~24mA.
The switcher takes over at 14.5V (about 250ms into the action), the
FET gate is pulled low to shut off the wasted power. The gate
resistor is actually 220K, but the demo TL431 won't work (because of
Iq) to show how "regulation" might be accomplished.
Right. ROFF is obviously over-constrained, hence my question about
power consumption, which you elected to not answer. There's no ROFF
value that works sensibly. A much simpler circuit would work.
Does this rig have to work from 90 to 264 volts AC? That, and the Idss
spread, gets interesting.
Harder question: what's the worst-case dissipation of the fet if the
main supply doesn't come up, like if the load is shorted or something?
You are trying to amuse yourself by throwing sand in the air. But you
don't understand how it works. Will some first year EE student
explain it on Larkin ?:-)
I understand how it doesn't work. And so far, it doesn't.
You've made a pretty quick transition from "I ended up with this..."
to "my bad." But you're not done yet.
ps - the answer is just about 1 watt lost, with your values of R1, R2,
the TL431, and ROFF. That is, for the record, about 1000 of your
customer's milliwatts.
Keep trying.
My fault for trying to demo without telling ;-)
There's roughly 400mW consumed while bringing up the switcher. Drops
to ~1mW when turned off.
If the load is shorted ?:-) 212mW in RDROP, 31.9mW in the FET. See,
you clearly don't understand how these things work :-)
...Jim Thompson
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