Supply not reaching PWM ic, but standby is working (Switched mode power supply)

[??? ????? ??????]

Supply not reaching PWM ic but standby working ( Switched mode power supply )

Hello every one on board .

I am right now trying to analyse a power supply as an assignement from a teacher , he gave me a computer power supply of a switched mode type .

So after checking the primary side , i can see that it is doing its job with a correct 300 V dc on the main filtering capacitor .

Now i am moving to the secondary side , and there is no main output voltage at all ( 12 volts), but the standby voltage is there (5 v) that is given by a separate standby power tiny switcher 3.

I have decided to see if the PWM IC ( s2153) is outputing a correct wave form but i noticed that its supply voltage is not there at all .

Further ahead , i have checked the circuit that feeds the IC and found a bad 150 ohm resistor that was giving 750 ohms , i then changed it but the problem is the same .

Am sure the problem lays somewhere between the primary and the IC supply circuit .

Any ideas are very welcomed

 

[D.A.(Tony)Stewart]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

Check rectifiers? CRx or Dx

 

[??? ????? ??????]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

Check rectifiers? CRx or Dx

Hi and thank you

I have checked the mains rectifier it gives a normal reading but the reading from negatif to positif should give 2*diode drop i guess , mine is giving only 600 mv drop .

On the other side there is a tvs diode witch gives a reading of 1400 mv on diode mode , is that normal ?

link for the diode :

https://pdf1.alldatasheet.com/datasheet-pdf/view/132675/STMICROELECTRONICS/1-5KE440A.html

 

[chuckey]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

On some PCs, the on/off switch is at LV, the aux 5V needs to be connected back to the PSU circuit to bring on the main inverter. Also the main 5V might need a certain minimum load on it or the PSU shuts down.
Frank

 

[FvM]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

On some PCs, the on/off switch is at LV, the aux 5V needs to be connected back to the PSU circuit to bring on the main inverter.

In case you are talking about ATX power supply, PS_ON# is an active low signal, in so far no 5V return is necessary. Shutdown in no load state is permitted by the ATX specification (but uncommon, I think).

 

[??? ????? ??????]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

I have to mention that i was able to get the output at 12 volts without connecting it to PC .

Its really weird .

I have the same situation with another xbox 360 PSU too .

- - - Updated - - -

If i manage to fix that , i will hit two birds with one stone .

 

[??? ????? ??????]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

BUMP , any idea ?

 

[D.A.(Tony)Stewart]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

Hi and thank you

I have checked the mains rectifier it gives a normal reading but the reading from negatif to positif should give 2*diode drop i guess , mine is giving only 600 mv drop .

On the other side there is a tvs diode witch gives a reading of 1400 mv on diode mode , is that normal ?

link for the diode :

https://pdf1.alldatasheet.com/datasheet-pdf/view/132675/STMICROELECTRONICS/1-5KE440A.html

600mV indicates no load current
1400mv for which polarity? or is it bidirectional?
If HiVdc exists then does circuit oscillate to provide buck operation or is it disabled?
Record voltages on all component pins between AC and DC and compare with component specs for violations

 

[??? ????? ??????]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

Hi again , i have actualy found that the psu main output should be activated by shorting enable pin and 5 vsb pin in orther to fire up the PSU .
That what i did and the psu is working , lol it wasnt even out of service .

 

[FvM]

Re: Supply not reaching PWM ic but standby working ( Switched mode power supply )

As previously mentioned...