Electronics > Repair
strange behaviour of the damaged smps with uc3845
excalibur_the_sword:
i am trying to fix a 400w rainproof smps that uses a uc3845 controller ic.
it just will not turn on when plugged into the mains.
but:
when i connect 12v to the output, the transistor sides of the two optocouplers show a resistance of under 500 ohms. in the picture, they are next to the main transformer. (So the feedback section seems to be OK).
when an external voltage is applied to the controller IC, it turns on at ~8.6V. Vref=5V and the output is stabilised at 105kHz and reaches the gate of the mosfets. (So the controller IC and the path to the gates are fine).
But when I connect it to the mains power, it does not turn on. The voltage on the Vi pin of the UC3845 is 8.9V (no fluctuation), but it will not start. Vref is ~0.5V and no square wave on the output pin.
The strange thing is: when i disconnect the mains and the bulk capacitors are charged, and i apply an external voltage of 8.6V to the controller ic, it starts pulsing and smps turns on and stabilises some voltages at the output. even the output led turns on. the voltage decreases until the bulk capacitors are fully discharged.
can anyone help me find the problem?
Thank you.
Harry_22:
Hi!
Could you make two good PCB photos from top and bottom.
dustooff:
Hi,
if Vref is being held low, the controller will stay inactive.
Vref is only able to source 30mA max.
from the data:
When VCC is greater than 1 V and less than the UVLO threshold, a 5-kΩ resistor pulls VREF to ground. VREF
can be used as a logic output indicating power-system status because when VCC is lower than the UVLO
threshold, VREF is held low.
inse:
Are you sure it’s not a supply issue of the controller?
It usually has two supply sources: for startup via resistive divider from mains and by an auxiliary winding on the transformer during operation.
Typical faults are: startup resistor high ohmic, Vin filter cap dead.
excalibur_the_sword:
Thank tou all.
main problem solved by replacing Vcc and output capacitors.
but still the output fluctuates, and needs more investigation.
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