Re: undervoltage lockout for flyback powersupply input

On Mon, 03 Dec 2007 11:26:49 +0000, Eeyore
<rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:


Thanks for the ideas guys, also is there a way to shut off the circuit
when the voltage drops below 75V? I think the DIAC and comparator
circuits will only prevent startup until 75V, but won't disable the
circuit if the voltage drops below 75V.

Correct.

Stopping it again is slightly more tricky, since once started up the switcher IC gets its supply
from the 'boost in' winding.

Maybe something could be kludged using the Isense input to inhibit switching ?

There are a number of ways to inhibit the 384x:

- Pull the Ct pin low (z dependant on Rt)
- pull the Comp pin below 1V2 (<800uA)
- pull the Fb pin above 2V5 (z dependant on V-divider)
- pull the Isense pin above 1V (z dependent on sensor decoupling )
- crowbar the Vref line.
- crowbar the Vcc line.

Most of these show up in app notes for the part.They all have
different advantages and disadvantages. Normally you'd want to tie
inhibition into a slow-start circuit, but I don't see one in your
schematic. At this power level, you may think you don't need one.

By the way, what prevents R4 (or anything else) from popping under
overload conditions? You'd have to disable the start-up bias it if you
wanted to prevent the rest circuit from drawing juice early, or
intended it to hiccough under overload - the latter being the usual
cheap method of overload protection with simpler flybacks.

It's beginning to look complicated, isn't it? That's one of the
reasons you won't see it used often, any more, at low power levels.
Simplification would involve combining circuit functions to reduce
parts count, instead of just continuing to add more parts every time
you think of something new that may or may not need attending to.

RL
.

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