PWM VDD Voltage Question

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  • transformer
    New Member
    • Mar 2011
    • 4
    #1

    PWM VDD Voltage Question

    Hello,

    A simply Question about SG6840 PWM Controller, must the VDD Voltage on Pin 7 measurable after PWM starts or also before and even if the PWM is defective ?
    Its an external TFT SMPS with no power failure(no secondary output), i checked all the parts and replace some caps and also the MOSFET, but i cant find the failure, bridge rectifier and cap is working normal, on the middle Mosfet Pin is 320V, on the PWM Vin Pin is 5V, but on Pin 7 VDD i read only 1.7V, (with black probe on Hot Ground) should be about 16V ?, where comes the VDD Voltage normaly, from the bridge recifier, or only from the MOSFET after the PWM starts ?
    Sorry if that is an stupid Questions, i´m not an expert in such things.
    All smd resistors are ok, also the 10uF/50V Cap(replaced by new one), Z-Diode include.

    Thanks,
    Thomas
    Tags: None
    • Dgtech
      E. Technician
      • Apr 2009
      • 1462
      • Steeler
      #2
      Re: PWM VDD Voltage Question

      Post a picture of the board in question. I have a feeling you may have a shorted diode in the prior part of the circuit. If you want to be sure, pull that chip and then check the Vdd pin for the voltage. If it's there, I'd change out that chip. If it's not, then the problem is before that.
      The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

        Comment

        • transformer
          New Member
          • Mar 2011
          • 4
          #3
          Re: PWM VDD Voltage Question

          Hm, a shorted diode, there are only 2 Diodes on the primary side, i´ve soldered out both and tested it for good.
          I´ve read in the Data Sheet that the VDD comes from the transformers primary winding, maybe there is an interrupt there, i miss the Voltage on the diode that goes to the VDD.


          Last edited by transformer; 03-09-2011, 04:36 PM.

            Comment

            • Dgtech
              E. Technician
              • Apr 2009
              • 1462
              • Steeler
              #4
              Re: PWM VDD Voltage Question

              Did you take the bridge rectifier out of circuit when you checked it? There isnt much before this PWM chip in this configuration. One diode in the rectifier could be bad making it half work without blowing the fuse.
              The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                Comment

                • transformer
                  New Member
                  • Mar 2011
                  • 4
                  #5
                  Re: PWM VDD Voltage Question

                  Okay i´ve replaced the bridge rectifier with a new one, no effect.
                  What about the Varistor ?, i cant read any Ohms(solderd out) there shout be any reading in high Ohm range, or not ?, but also on 2M Ohm range o my Multimeter there shows nothing.

                    Comment

                    • transformer
                      New Member
                      • Mar 2011
                      • 4
                      #6
                      Re: PWM VDD Voltage Question

                      It was a really bad idea to solder out the PWM IC and turn on power, SMPS goes up in smoke ! RIP

                        Comment

                        • Dgtech
                          E. Technician
                          • Apr 2009
                          • 1462
                          • Steeler
                          #7
                          Re: PWM VDD Voltage Question

                          Originally posted by transformer
                          It was a really bad idea to solder out the PWM IC and turn on power, SMPS goes up in smoke ! RIP
                          Sounds like you found your problem. Smoke is good when you are looking for a defective component. Change it out, solder back in PWM and report back.
                          The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                            Comment

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