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PMP20195: Questions for D3/D10 purpose

Xiaobing (Eric)
Intellectual 895 points
Part Number: PMP20195

Hi Teams,

 I have a questions in PMP20195 reference design. how is D3/D10 purpose in this schematic? If I remember correct, if this power supply is two switch forward converter, D3/d10 is necessary. I double check the schematic, which is two switch flyback power supply. Can you help to understand the D3/D10 purpose？

 By the way, the transfromer core is PQ32, right?

 Tks, teams. 

Regards,

Xiaobing (Eric)

  • 0
    TI__Expert 5425 points
    Hi Eric,

    D3/D10 are diodes for the RCD clamp. When the Primary FETs are on and the transformer is energizing, D3/D10 conduct and dump the ringing energy in the caps. When the primary FETs turn off and D5/D7 start conducting, D3/D10 are reversed biased, and the energy stored in C7/C16 dissipates in the parallel resistor.

    You can download TI's free Power Stage Designer www.ti.com/.../POWERSTAGE-DESIGNER and review the 2-Switch Flyback operating waveforms.

    For transformer information, please contact Ian Rensing <IRensing@rencousa.com> at Renco.

    Thanks,
    Ryan
  • 0 in reply to Robert Taylor1
    Intellectual 895 points

    Hi Ryan,

     Nice to see you. Tks for your feedback and suggestion.

     I'm run a two switch DCM flyback converter simulation in PSIM and checked parallel diode (D3 location) current, looks like NO current through D3. Anyhing I missing? What's real purposes in above two switch flyback schematic?  Pls correct me if any where is wrong. Tks.

    Regards,

    Xiaobing (Eric)

  • 0 in reply to Xiaobing (Eric)
    TI__Expert 5425 points

    Eric,

    It is a bit confusing are you referring to the two diodes on the primary side that are labeled D3 and D2 in your simulation?

    Those two diodes are used to reset the transformer in the off time.  They clamp the switching voltage back to the input voltage once both FETs turn off.

    Or are you referring to the original schematic that has D3 and D10 as snubbers on the secondary side?

    Thanks,

    Robert

  • 0 in reply to Robert Taylor1
    Intellectual 895 points

    Robert,

     Tks for your feedback.

     The question is D3/D10 in original schematic on the secondary side. Refer simulation result there is no current through D3 location diode during two switch flyback converter work at DCM. How to understand D3/D10 purpose in original two switch flyback schematic? Appreciate you feedback. 

    Regards,

    Xiaobing (Eric)

  • 0 in reply to Xiaobing (Eric)
    TI__Expert 5425 points

    Eric,

    There is no current through those parts in the simulation because you have no parasitics. The snubber is used to dissipate the leakage energy from the transformer. Since your simulation has no leakage inductance, you will not see any current there.

    Hopefully this helps.

    Thanks,

    Robert

  • 0 in reply to Robert Taylor1
    Intellectual 895 points

    Noted with tks. Robert. 

     I'm double checked simulation file and transformer doc, the leakage inductance has been defined in above simulation result (No current through D3 location diode in original schematic). I put up the simulation doc pls help to find anything is missing. Tks.

    Two switch flyback_single loop-Vo feedback_closed loop.rar

    Regards,

    Xiaobing (Eric)

  • 0 in reply to Xiaobing (Eric)
    TI__Expert 5425 points

    Eric,

    The circuit you are trying to simulate represents the parasitic behavior of the entire system. It is going to be very difficult if not impossible to even estimate those values. I addition to the leakage inductance there are other parasitic capacitances and inductances that cause ringing on the switching node. The purpose of D3/D10 circuit is to damp that ringing.  I don't think you will be able to generate meaningful results with the approach you are taking now.

    Also, you have 1n listed for the leakage inductance vs. 80u (20u if you take into account the turns ratio) magnetizing. The leakage is such a small percentage you will probably not see any effect from it. Try increasing it to 1uH or something like that.

    Thanks,

    Robert

  • 0 in reply to Robert Taylor1
    Intellectual 895 points

    Noted with tks. Good day, Robert. 

    Regards,

    Xiaobing (Eric)