Short Circuit Detector on SMPS

[omid2006]

Hi Dears,

While I was working on a DC/DC converter. I've obtained that an additional circuit was added to detect short circuit and disable output but I can't find it's logic. it uses current mode controllers to control this buck converter. even I'm doubtful about that part U4 with marking of "VW". I'll attach schematic circuit. I appreciate if anyone can help me to solve this circuit and find it's purpose.

 

[danadakk]

U4 I think is simply there to detect the present of 12V .....which then turns off Q2 if 12V is present.

So if its absent Q2 is turned on, turning off Q1. Maybe this is considered short circuit protection.....?
V trip is set by the V divider and input threshold of the inverter.


Regards, Dana.

 

[omid2006]

if Q2 turns off the output will be disabled because Q2 is controlling main IC VDD. U4 is 74LVC1G18GW that is non-inverting multiplexer.

 

[dick_freebird]

That power path looks like it'd be hard pressed to
keep up with gate drive on any decent sized FET.

Consequences of removing controller power must
be understood (what happens on the way to the
"protected" state, especially if (say) there's a gate
driver looking at the controller output, which might
get a "bum steer" during the series of events?).

 

[omid2006]

That power path looks like it'd be hard pressed to
keep up with gate drive on any decent sized FET.

Consequences of removing controller power must
be understood (what happens on the way to the
"protected" state, especially if (say) there's a gate
driver looking at the controller output, which might
get a "bum steer" during the series of events?).

it doesnt have any gate driver. the controller output controls FET directly. in protected state the output voltage will be zero because there is no gate pulse on FET. just I have problem in that non-inverting multiplexer logic. If anyone can find other ICs with Marking (VW) I appreciate.

 

[dick_freebird]

If that "U4" is a multiplexer, where are the S (elect) pins?
And the 1B, 2B alt inputs? I think it's misidentified.

I take the P/N as a 2-channel "7404" (inverter). Then
12V absent, pushes Q2 base high, pulls Q1 base
low and lights up "U1 VCC" with the crude 12V.

Still, that's not trivial current if you're driving a FET
gate through the controller "directly". While C10
might cover a single or N gate charge cycles, what
does your VIN and your series R of 10K imply for
time averaged DC current and where do you stand
then, re the 12V zener " shunt regulator" dropping
down to UVLO?

If I picked a nominal 28V line and a 8V UVLO, a 20V
max droop and 10Kohms means 2mA. That's like
standby current, not active.

Perhaps this is part of a "hiccup" startup scheme
and is meant to -remove- an AUX supply from the
controller's power tree once "legit 12V" (like from
an AUX winding, rectified) has come up. Then the
PNP switch turns off (but you still burn the ~2mA,
only into the zener instead of the controller VCC)?

It seems likely to me that the function is not short
circuit protection.