Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 41 subscribers
  • Views 175 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

PMP4529: Secondaty side Circuit

Irene Perez
Prodigy 150 points

Part Number: PMP4529

Hi,

I have a question related to the way of switching on and off of the mosfets of the secondary side.


1-) I understand the way of driving Q3 is a turn on controlled by the gate resistance and a faster turn off through the pnp transistor. Why is not managed the mosfet Q2 in the same way?
2-) Why is is added to the circuit the additional diode D101?

Additionally I would like to know: Why is choosen a transformer with a small magnetizing inductance? as per my understanding for a proper peak current control the current ramp of the output inductor should be higher(seen from the primary side) than the magnetizing current ramp, but in this case is the opossite: 226mApp versus 429mApp(due to Lmag)

Many thanks for your support

  • 0
    TI__Guru 58985 points

    Hi,

    1) Add Q2 or not depends on needed or not from a design. This particular design does not need to add.

    2) D101 is to help free wheeling at the beginning of each free wheeling interval, to help reduce loss of Q1 body diode.

    3) smaller magnetizing inductance is to help set up proper magnetizing current for active clamp function. You can increase magnetizing inductance as long as your design meet your sepcs. 

  • 0 in reply to Hong Huang
    Prodigy 150 points

    Hi Hong,

    Thanks for your answer,

    1) Could you please give more detailed about why the turn on/off circuit is not the same for Q2 and Q3? thanks

    2) Perfect, thanks

    3) In my design im trying to keep the inductor slope higher than the magnetizing, but dead times become around 200ns(that I think is too much), what is your criteria for selecting the Lmag?

    Thanks,

  • 0 in reply to Irene Perez
    TI__Guru 58985 points

    Hi,

    1) Q3 is to help Q2 faster turn-off. The designer found Q2 turn-off is not fast enough so added Q3 circuit. Q1 turn-off is fast enough then no need to add such a circuit.

    2) ok.

    3) 2pi sqrt (Lmag Ccl) > tOFF(max), as a rule of thumb for design