• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 1226 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28740: UCC28740, start-up

kwnag woon Lee
Prodigy 170 points
Part Number: UCC28740

I am developing a 70W ac/dc converter using UCC28740.

Although the circuit diagram is configured as below, the initial start is not performed.

 

300V is applied to HV pin but 2V is generated on VCC pin. No voltage is applied to the Vs pin.

Np to Naux is 67:10.

After the initial inrush current occurs, nothing happens. Where should I check?

Best regards.

  • 0
    TI__Expert 8210 points

    Kwnag Woon Lee

    For normal startup, UCC28740 should trickle charge the VDD capacitor from the JFET connected at the HV pin to VVDD(on) (21V typical).  Once on it reaches this turn on threshold UCC28740 will generate 3 PWM pules.  If a fault condition is triggered UCC28740 will enter a fault shutdown mode.  If no fault condition is measured then UCC28740 will startup in normal operation.  Details are in section 7.3.3 (page 12) of the UCC28740 datasheet.

    On your schematic it appears that you have quite a bit of external circuitry that is also powered off of VDD.  Please note that the trickle charge startup current thru the HV pin is only 250 µA, so if these external circuits consume more than this they will load VDD and prevent the voltage from reaching VVDD(on).  We recommend that you either modify these circuits to consume less current to allow proper startup, or add a blocking diode so the trickle charge current for startup does not power this external circuitry (the aux winding will still power it).

    While this app note is for our PSR controller family, UCC28740 uses the same base control operation as the PSR family of devices so many of the debug techniques described in this app note are applicable http://www.ti.com/lit/an/slua783/slua783.pdf.  I recommend you review it to help with debug.

    Best Regards,

    Eric