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Soft start circuit for SMPS

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Advanced Member level 5
Jun 13, 2021
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Why is the attached, ringed soft start cct not more popular with SMPS?
R13 is needed, (along with D3) so as to ensure that after start up, the soft start capacitor, C10, does not get involved in converter feedback loop dynamics.
LTspice sim and jpeg attached

Softstart cct.jpg


  • buck 24v to 12v at
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To me, what you have circled is not the soft start. It is a rail
dV/dt trigger applied to that comparator and that then to
some loopy BS circuitry. The ideal sources are a nice touch,
for startup in a standalone supply. I am suspicious of it in

I'd bet there's a POR chip that does the job of that C/R-D
comparator bodge for a lot less BOM cost. We build it in
when we do IC products, do you use controllers without
built in SS? If so which and why?
Thanks, the basis for this is AN-70, page 13 (top LHS)

....they detail a cct like this there....(to delay entry into auto-restart).
They describe simply using a cap across the upper divider resistor...and then say beware because it may mess up the feedback loop......but with the two diodes and resistor that i show in the top post...., there need be no mess-up of the feedback you agree?

LNK3209 is the Buck controller which i dont have an LTspice model i simply show it with other topology as in the top post, as the principle is the same..

Thanks, yes you are right, but we are not using the LT1243.....we are using a controller which doesnt exist in LTspice, so we are using LT1243 in the sim purely for representation.......the principle being demo'd is the diode/cap/res..........and especially so, since power integrations suggest just the cap...which has the problem of creating a follow-on feedback loop stability issue.

Sorry but, As you know, one commonly has to simulate with different chips than the ones that are used, because sim models dont exist.

As you will be knowledgeable, The PI only is not good enough for use with LNK3209 and high value the PID has to be used...otherwise if it doesnt get into regulation before 50ms, it goes into auto-restart...and the off time is 1.5 seconds.

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