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LM5021: PMP20809 module design question

Part Number: LM5021
Other Parts Discussed in Thread: PMP20809, LM5012

Hi Sir,

       The AC to DC module of PMP20809 is suitable for my customer design except the Vout, is it possible that just change the feedback resistor to let Vout down to between 14V and 22V (original Vout is 24V). it will let our customer test it in PMP20809 and follow this board schematic and layout to accelerate the project. 

  • KJ

    I assume that your customer wants to be able to adjust Vout in the range 14 – 22 V?

    It should be possible to adjust the output voltage of PMP20809 from 24 V to 22 V by changing the output voltage divider resistors R11 & R13. The input voltage to the LM5012 VIN pin from the aux winding will drop, but should still be ok to keep the VIN pin high enough to generate adequate VCC level.

    But if you want to adjust Vout down to a low level ~ 14 V, this will be a problem, since the transformer sec to aux turns ratio is 2:1 - that means that the aux voltage level to the VIN pin is ~50% of Vout, which would be too low, VCC will drop below the UV turn off threshold.

    To ensure that the VIN pin is high enough to maintain VCC regulation when Vout = 14 V, the transformer turns ratio will need to change, more turns are need on the aux winding.

    To ensure at least 10 V at VIN (to allow for the internal 1.7-V dropout to VCC) when Vout = 14 V, the aux turns will need to increase to 9T (assuming secondary stays at 12T). At maximum Vout of 22 V, the VIN pin will rise to ~17 V, but this is still well below the VIN max level of 30 V.

    I hope this helps answer your question.