Re: undervoltage lockout for flyback powersupply input



Jamie Morken wrote:

Eeyore wrote:
Jamie wrote:
Jamie Morken wrote:

I have a flyback powersupply that operates from about 20V to 450V+ (AC
or DC input) and I would like to reduce the operation to 75V to 450V,
what is a possible circuit to use on its input to keep it from starting
up until this threshold is reached?

For simplicity reasons, Get your self a DIAC rated around 75 volts
connect one end to the cathode of D34 and the other end will be
the V+ supply for R5 instead of connecting R5 directly to V+.

Neat idea actually Jamie. 30-40V diacs are more commonly available though
(I've never met a 75V diac) so perhaps 2 of those in series.

http://uk.farnell.com/jsp/search/browse.jsp?N=500004+1000013&Ntk=gensearch_001&Ntt=diac&Ntx=


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 ?

Graham

.

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