UC3825 Vref expect 5.1 but measure 11Vdc.

[RHeTTRoNiCS]

Hello again team...am troubleshooting an smps power supply...really running out of options after replacing all semiconductor parts from the primary area. Problem is the same. No output, fan spin intermittently. I saw UC3825 pwm, measured the Vcc read 11Vdc. Then I measured Vref and it read also 11Vdc. The specs shows Vref should be 5.1V. So anyone can share ideas what influences this Vref to reach 11V? Any tips on troubleshooting this smps? Thanks again for continual support.

 

[dick_freebird]

I guess I'd be looking at the GND pin, since outputs
equal positive supply.

 

[RHeTTRoNiCS]

Done replaced the PWM, not helping...Gnd pin connects to pin10. Actually VCC is 11.4V, and Vref is 11.13V.

 

[ahsan_i_h]

If pin10 is disconnected from circuit board ground (may be due to blown pcb trace or blown series resistor) and if you try to measure voltages respect to circuit board ground, you may get near supply voltage (11.4V) at Vref. So make sure that circuit board ground and pin10 is shorted.

 

[RHeTTRoNiCS]

Ah ok...I noticed that the pin10 gnd of uc3825 is not connected with the gnd at primary side. Should be connected?

 

[ahsan_i_h]

Further analysis is required. You need to find out why it is not connected.

 

[RHeTTRoNiCS]

Hi all...yes correct found grounding problem due to a trace broken...Now when I check the Vref I measured only 3.5Vdc. Expected 5.1Vdc. The measured Vcc for this PWM id 10.10Vdc. Did experiment with good UC3825 but still at Vcc 8.5Vdc the Vref read 5.1Vdc....
Any idea what factors or problem may affect the Vref to drop? Thanks a lot.

 

[ahsan_i_h]

The UC3825 may be starting and stopping repeatedly. As a result you are getting 3.5V on an average.

The UVLO threshold voltages of UC3825B is different from UC3825/UC3825A. You may check these in Under-Voltage Lockout section of the datasheet. Is the part number simply UC3825 ?

 

[RHeTTRoNiCS]

The original part is UC3825N, same part I replaced. Please see photo when I turn on the power supply. Hoping help..thanks.