SG3525 PWM controller OCP cell

ym58

1 mW
Joined
Jul 23, 2022
Messages
10
How does that cell work exactly ?
Does it produce some kind of delay (47uF/220nF ?) to the 'dis' output once a current limit has been reached on R9 (current sense) ?

question_forum_SG3525_Current_Sense_Cell.png

---
I am myself triggering OCP thru a sense resistor in a Meishile 1000W/12V/87A SMPS (that I want to limit to 50A) but my OCP feedback cell is much simpler than this one (I am using an opamp comparing the voltage across the current sense resistor to a REF voltage then sending a High signal to the shutdown input if Vshunt > threshold) and it works as far as current limiting is concerned.
But my cell keeps inducing an annoying 'click-click-click' sound somewhere within the SMPS !
--
Will such a cell prevent this drawback ?
 
This is the datasheet for the chip
https://www.onsemi.com/pdf/datasheet/sg3525a-d.pdf
which should explain it's operation.

I don't know the circuit of the board the chip is in for your specific unit, so I don't know exactly how that will react to the chip's output, or what feedback/input it is giving the chip.

If you happen to have a circuit diagram, you could attach it to the post using attachments tab and I could make some educated guesses (but i"m no expert on SMPS).
 
ym58 said:
I am myself triggering OCP thru a sense resistor in a Meishile 1000W/12V/87A SMPS (that I want to limit to 50A) but my OCP feedback cell is much simpler than this one (I am using an opamp comparing the voltage across the current sense resistor to a REF voltage then sending a High signal to the shutdown input if Vshunt > threshold) and it works as far as current limiting is concerned.
But my cell keeps inducing an annoying 'click-click-click' sound somewhere within the SMPS !
If your system isn't lowering the voltage when current is over it's limit, then it's probably just turning the whole system on and off in response to overcurrent. That could "click" each time this cycle occurs.

If you're trying to make the SMPS be a constant-current supply, like a charger, or LED PSU, then the feedback needs to be in the form of voltage control, to lower the voltage until the current drops to the limit.

That requires the feedback circuit be able to drive the FETs that are switching the transformer on the input side, to control the PWM to their gates (just like a motor controller).
 
The schematic that I posted in my OP is something that I've found here and there.
Starting from there, I just wondered if I could adapt this type of cell to the current PSU I am working on !
The PSU I am working on is based on a half-bridge topology and, actually, there is no RS (shunt for sensing current) in the source of the bottom Mosfet, nor in the ouput DC stage of the PSU !
However, there is a small transformer T3 "before" the AC/DC final transformer (T4) and IMHO this T3 senses the current by ways of a rectifier that feeds the shutdown pin#10 of the SG3525 through a simple transistor Q1.
That's something I am not really familiar with.
I am currently doing some reverse engineering on this PSU and so far here's what I have been able to find ... I think that the circle named 9 is the key issue and, as you can see, it only involves pin#10 (shutdown) of the PWM controller :

question_forum_SG3525_Current_Sense_Cell%285%29.png
 
I don't see any schematics or other images. If I quote your post I get links to a dropbox site but it won't open for me.

If you attach the images to the post, anyone that can see the post can see the images.
 
On my side, I can see the image both in my preview and in the thread ... strange.
Let me repost it (see below) :
post5-copy.png
 
Back
Top