• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 62 subscribers
  • Views 545 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.

Original question:

LM5023: Startup circuit issue

LM5023: Problem with startup circuit

vtolent
Prodigy 30 points
Part Number: LM5023
Other Parts Discussed in Thread: UCC24612

Hello,

I have an ACDC converter which uses the LM5023 as the flyback controller and UCC24612 as the secondary rectifier.

Even though the flyback controller is supplied with the required voltage, neither its gate driver (out pin) nor its soft start pin seem to be working as expected – They are always low.

Already at start-up, the circuit seems to be operating in an overload  scenario, as the VSD pin intermittently controls the charging (12.5V) and discharging (7.5V) of the VCC cap as depicted in the attached image, which leads me to believe that the internal logic is actually working.

 The same behaviour repeats itself in two different samples, so I assume it is a systematic error and since the schematic is very much similar to the reference design one, I wonder what went wrong.

Attached is the schematic and the measurement results.

Any help is appreciated.

ACDC_Schematic_V1.pdf

  • 0
    TI__Guru* 85616 points

    Hello,

    LM5023 has OVP and OCP protection.  If either of these get triggered the device will cycle VCC on and off.  You just might not be seeing the gate drive pulses because you resolution is no high enough.

    The aux winding will trigger and OVP trips when VS is roughly 3V.  If there is too much ringing on the Aux winging it can trigger an OVP.  This can happen any time while the flyback is delivering energy.  If this is the case putting a snubber across the output rectifier will help resolve this issue.

    There  is an evaluation module for the UCC24612 that has a user guide that describes how to set up an RC snubber for this purpose in section 7.  The users guide can be found at the following link.  http://www.ti.com/lit/ug/sluubp3/sluubp3.pdf

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points

    Hello,

    I found out the problem, which was actually related to the under voltage shutdown circuit, which was affecting the soft start. Now the board is at least starting up.

    Thanks,

    Victor