Re: undervoltage lockout for flyback powersupply input



legg wrote:

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.

I had a feeling an expert on the part might eventually show up !

Graham

.

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