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PMP21479: Some questions about schematic diagram

Sergio Tassinari
Prodigy 10 points
Part Number: PMP21479
Other Parts Discussed in Thread: UCC28780, , UCC28782, UCC24612

Hello!

I'm designing a 72W power supply/battery charger (with CV/CC output control) based on UCC28780, and I'm looking at the PMP21479 as a starting point.

Can you please explain me the purpose of transistors Q100 and Q101 and diode D101? Do the auxiliary winding of the transformer need to be "reset" when the controller is not switching?

Why the auxiliary winding is center tapped and the VS pin is connected to the mid-point of the winding? Would it be the same to connect R20 to the anode of D4 (and changing  the values of R20 and/or R23)?

What is the purpose of Q102 and related components? It seems that it creates a sort of hysteresis on the BUR pin, but shouldn't this function already be managed internally to the UCC28780?

Thank you so much in advance.

Best regards,

Sergio

  • 0
    TI__Expert 6670 points

    Hi Sergio,

    I have asked the main designer to take a look at the questions that you have. In the meantime, I would suggest to look at UCC28782 it is an updated version of the Active Clamp Flyback controller and incorporates some of these extra components that are needed.

    Thanks,

    Robert 

  • 0
    TI__Intellectual 2885 points

    Hi Sergio,

    I agree with Robert that using our latest generation active clamp controller, UCC28782, will greatly reduce the amount of components because it integrates much more of these functions.  To answer your questions:

    1. Q100, Q101 and D101 eliminate the parasitic ringing on the the transformer windings when the power FETs are idle.  This was added to this design because the winding voltage on the secondary (pins A and B of T1) could sometimes ring high enough to cause the body diode of Q7 to conduct.  This could cause UCC24612 (U7) to falsely turn-on, resulting in some undesirable behavior.  This circuit is not usually needed.

    2. The split winding auxiliary structure is used accommodate the wide output voltage swing (5V to 20V.)  When the output voltage is lower, VDD is derived through D4 and Q3.  When the output voltage is higher, VDD is derived through D3.  You are correct, that the VS resistor divider could be connected to either of the auxiliary windings, but resistors would need to be adjusted if this connection is moved.  The UCC28782 includes an integrated, low-power boost converter to eliminate the need for this external circuit.

    3. You are correct. The circuit of Q102 adds more hysteresis to the BUR pin.  We found that the internal hysteresis was not quite enough to provide crisp transitions between some of the operating modes.  The UCC28782 has increased, adjustable hysteresis, eliminating the need for this circuit.

    Additionally, the UCC28782 integrates the low-side driver.

    thanks,

    Brian

  • 0
    Prodigy 10 points

    Hi Brian and Robert,

    many thanks for your prompt reply!

    Regards,

    Sergio