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Original question:

LM5026: Vin 48V to Vout 12V(120W) circuit design.

LM5026: PMP22075 - Modify for 10A Output - Current Sense components

Fraser Lawson1
Prodigy 240 points
Part Number: LM5026
Other Parts Discussed in Thread: PMP22075

Tool/software:

Hello, T.I. team.

This question is related to LM5026: Vin 48V to Vout 12V(120W) circuit design.

We have updated the transformer and output inductor to cater for the greater load current.

However, we need some advice on the Current Sense components and how to calculate them. Refer to partial PMP22075 schematic below:

The PMP22075 current sense circuit is designed for a 5A load.

Therefore, please advise how to calculate the correct values for R29, R7, R8 etc to change this for a 10A load.

Thanks,

Fraser

  • 0
    TI__Guru 74115 points

    Hi,

    Did you check that current transformer ok for 10A change? Please provide the transformer specs for this. You can always make these design changes based on you current transformer specs.

    Also you need to provide your transformer specs.

  • 0 in reply to Hong Huang
    Prodigy 240 points

    Hello, Hong Huang.

    We have the same Current transformer as the original PMP22075 design:

    • Part number PA1005.050NLT
      • Current rating 20A
      • Turns ratio 1:50

    Our main transformer is PA0937NL configured with both secondary windings connected in series.

    • Primary - 7 turns, Secondary 2 x 7 turns 
      • Therefore Np/Ns = 1:2
      • Power rating 250W
      • Primary Inductance : > 165uH
      • Leakage Inductance : < 0.3uH

    I hope this helps.

    Therefore, please advise how to calculate the correct Resistor values for the Current Sense circuitry values R29, R7, R8 (on PMP22075) to change this for a 10A load.

    Thanks,

    Fraser

  • 0 in reply to Fraser Lawson1
    TI__Guru 74115 points

    Hi,

    R29 is called current transformer core reset resistor. You can keep using the same resistor. If you want to find how to design this resistor, you can contact the transformer vendor to help. Or in general, you can search to get current transformer core reset resistor

    R7 is current sense resistor with the current sense transformer. Its formula is like the below:

    You can use the below application note to make design. The above is from this application note, page 31.

    https://www.ti.com/lit/pdf/slua535

    R8 combined with C15 is a low pass filter to help remove the current signal high frequency noise. You can keep using these then tuning up during the bench test - as long as the noise is ok to tolerate then the filter constant is better to small to cause less delay.

  • 0 in reply to Hong Huang
    Prodigy 240 points

    Hello, Hong Huang.

    Thanks for your reply.

    We will give this a try. Hopefully, we are on the correct path.

    Best regards,

    Fraser