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Original question:

LM5025A / utilize both CS1 and CS2 to monitor the main switch FET current

LM5025AEVAL: Vout issue

Ting YI
Prodigy 140 points
Part Number: LM5025AEVAL

Tool/software:

Hello,

I have encountered two issues:

  1. I am currently trying to adjust the output voltage to 5V. However, after adjusting the voltage divider resistor R27, the maximum voltage I can achieve is only 4.3V. I suspect this might be due to an excessive duty cycle. Should I adjust the resistor R9 at the Ramp pin to allow for a larger duty cycle?
  2. When I adjust R9, the maximum value I can set is 110k. If I increase it to 120k, the IC burns out. What could be the reason for this?

Original specifications:

Input voltage: 36-78V

Output voltage: 3.3V

Output current: 30A

  • 0
    TI__Mastermind 32935 points

    What is the frequency, max on-time, max duty clamp setting, etc? What was the max duty before you made changes and what changes were made? What design is this and what are the specs? Please provide details such as changes made, calculations, waveforms to help demonstrate/explain the issue?

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 140 points

    I apologize for the incomplete information.

    Here are additional parameters for the active clamp forward converter.

    Before Modification:

    • Frequency: 233kHz
    • TON: 2.45us
    • Dmax: 57%
    • Vout: Vout=D⋅(Ns/Np)⋅Vin​
    • Vin: 48V
    • Ns/Np (turns ratio of transformer T2): 1/6

    Calculations:

    • Vout,max: 4.56V (not able to achieve 5V output)

    Required for 5V Output:

    • Therefore, to achieve Vout,max=5V
      • Dmax=5V/48V⋅6=62.5%
      • TON=4.3us⋅62.5%=2.69us
      • RFF⋅CFF=(48V⋅2.69us)/2.5V=5.165×10−5
      • If we keep CFF=470pF, then RFF>110kΩ

    Issue:

    Going back to the initial issue, after adjusting RFF to 110k and then adjusting the voltage divider resistor, the maximum Vout still couldn't be adjusted to 5V. Therefore, I further adjusted RFF to 120k, and as a result, the IC burned out.

  • +1 in reply to Ting YI
    TI__Mastermind 32935 points

    For a 2:1 input voltage range, the transformer turns ratio of 6:1 is too high. For 36V<VIN<72V, your input is only varying by 2:1 so there is no way you should be operating at such high duty cycle. I would recommend to set the Dmax to something like 45%-50% at VIN=36V, then recalculate the turns from there.

    Steve