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PMP10195: VOR reflected flyback voltage

Xavier Pacheco
Prodigy 75 points
Part Number: PMP10195
Other Parts Discussed in Thread: UCC28711, LM5020

Hi,

I'm using this reference design for a flyback design. My input voltage is 100 to 1200 VDC. I have two questions:

First: I see D1 is 110V. So for the VOR I'm saying:

With two 800 V mosfets, let's leave 100V safety margin when at max voltage, so not to exceed  700V on each drain, i.e., in total 1.4kV.

Vinmax + Vz = 1400 + Vz <= 1200

Vz <=1400 – 1200 = 200V

Let’s pick Vz = 110V

Vz/VOR = 1.4 optimum ratio

VOR = 77 is this good? 

Second: I replaced LM5020 with UCC28711 which max frequency is 100 kHz. I don't want to hit the minimum on time since I'm dealing with a wide input range 100 - 1200VDC and output windings 48V and 12V. Would UCC28711 work properly in this case?

  • 0
    TI__Expert 5305 points

    Hi Xavier,

    Your calculations are correct.

    In my case, I selected the LM5020 because the customer wanted to have a fixed frequency Flyback. Then I selected low switching frequency (32 KHz) in order to keep low switching losses at 1260Vin, because also the board had to be small.

    Regarding the Flyback reflected voltage (VOR), the LM5020-2 is limited to 50% duty cycle, so, according to the turns ratio of the transformer (2.53:1) and the minimum DC input (65V), it comes automatically to set VOR ~ 65V.

    If you want to work with UCC28711, this controller has maximum duty cycle close to 50%, so the VOR voltage can be selected accordingly.

    Please have a look to the excel calc-sheet of this controller, available in its product page.

    So, I also believe the UCC28711 can be used here. Please take into account that, due to the extra wide input voltage range, from Vin(min) to Vin(max) the negative current through the VS pin is too high. In this case we suggest to add a Zener + silicon diode (anode to anode) in parallel to VS and ground (see D10 and D9 in the attached PMP8646 schematic).

    Please let me know if you need further support.

    Best regards,

    RobertoPMP8646 Rev_B_pads.pdf