General discussion

smps repair

By libya123456. ·
Tags: Hardware
i have fixed almost every smps that come in my hand,
the only few times i failed in smps repair is where the board is of the type that does not allow you to desolder components with out damaging it ,here is my approach to smps repair


1 fuse is blwon

in this case you should look for shorts in the following components before installing new fuse
(power fet-bridge rectifier-400v cap-cermaic line filter caps) if one is bad it will **** the fuse again
install all bad components and make the light bulb trick (search google or buy my ebook it is 10 usd only!!)
if bulb still glows brightly and you replaced all shorted components then you have missed the pwm-chip ic !!

yes yes some bad pwm chips will putout dc not ac dc will cause the mosfet stuck on forever ..alot of current will pass in the fet and it will **** again ..make sure to check the pwm chip output it is always the last missed bad component


now case 2 fuse not blown

if your fuse is not blown and you cant test any output voltage at the output then go to the dc 400 cap in the primary
test it for 300 dc ?? (i assume line is 220 ac) if the 300vdc is missing and fuse is good then you have bad connection between ac input and the 400vdc cap check it .....if 300vdc is good then go to the pwm chip test its vcc for 12to 20vdc is it there? if not then look for opened start-up resistor if it is goood then you have either blocked smps or bad chip

blocked smps means that your smps wont work because it has either over current or over voltage shutdown
this signals come from the feedback of the ic and will shut it in a split of second now here is what to do to discover if it A BLOCKED SMPS or BAD CHIP

bring your bench power supply adjsut it to 15 vdc ..make sure your smps is power off and the high voltage 400v cap is dischrged now make the power supply ground to hot ground and 15vdc to the pwm chip vcc pin

now the PWM MUST make and output ..there is now no FEEDBACK to shut it down

if you have pulses at the mosfet gate then you have confirmed that chip is good and you have over-current or over-voltage shutdown

for over current shutdown look for bad or shorted diodes at the secondary or look for shorted loads in the secondary...
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