SMPS problem

[amitak]

i found a working smps circuit and made a few pcb samples.
i modified it to get variable output. it worked great.
then i tried to modify it to make it more compact and use some leds at output.
the pcb was 50 %smaller than the first one.
but it worked only for a short time .. and only the first sample.
i assembled more samples - no good.
the fuse blows off when i power ON.
i tried to short the fuse, and then the pcb ground tracks blow off !
it is clear that the circuit is drawing too much of current !!!
does anybody have any clues ?

 

[Mr RB]

Post your schematic and PCB layout. Photos might help too.

 

[amitak]

yes sure ..

**broken link removed**

**broken link removed**

 

[summitville]

First, remove all modified components and replace with original values.
Now, does the new PCB still blow the fuse?
If yes? Then your new PCB design/layout is faulty.
If no? Then substitute a variable resistor (preset at ~2K) for R9 not R10.

I think ...
1) R10 is used in the Voltage Setpoint & the FEEDBACK Compensation Gain/Phase calculations.
2) R9 is only used in the Voltage Setpoint calculations.

You do not want to change the Feedback Compensation circuit with a variable resistor as this can make the PWM unstable.

What is the new Voltage that you are attempting to regulate to?
Is your new Duty Cycle exceeding 50% ?

 

[amitak]

Thank you for the reply
yes it seems to be bad bad PCB layout - the fuse blows even with same component values
With smaller pcb i made the tracks thinner .. this could be one possible problem
another possibility is that current loop maybe - it was taken care of in the first version - but not in the second ..
which ones do u think is more relevant ?
i can try with another layout and reorder samples if i can find a reasonable cause ..
else i shall stick to the existing working design

Hope this experience helps others struggling with smps design
For the first design - all may use it freely - it is nice and stable .. a good circuit without any PWM control IC - a cheap design too