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UC3825A: UC3825A output issue

Aniket Band
Prodigy 20 points
Part Number: UC3825A

Output waveforms on OutA and OutB are skipping for some time.

  • 0
    TI__Expert 7900 points

    Hi Aniket,

    Skipping pulses is not uncommon or unexpected for a closed-loop system.  If the output voltage, and FB pin, are higher than the setpoint the error amplifier will force the controller to skip pulses to bring Vout back to regulation.  Pulse skipping is very common too if the load is very, very light.  Also, if the current is above the pulse-by-pulse current limit there will be missing pulses too.

    Regards,

    Eric

  • 0 in reply to Eric Reicher
    Prodigy 20 points

    Dear Sir,

    I Understand what what you want to say.

    But here problem is somehow different The unit which we are testing is OK tested unit return from field.

    SMPS Details:

    Input 180-265VAC

    Output 30V 40A

    Testing at 30V 20A which is not light load.

  • 0 in reply to Aniket Band
    Prodigy 20 points

    We too getting Sine wave on Non Inverting (PIN 2) which is connected to the Optocoupler.

  • 0 in reply to Aniket Band
    TI__Expert 7900 points

    OK, having that additional information is helpful, but it's difficult to pin-point the problem without 'scope pictures or a schematic.

    First, I would examine how the UC3825A is operating:

    1) When it is not pulse skipping, is the switching frequency correct according to RT/CT values?

    2) How does the VREF pin look? Stable at 5.1 VDC?

    3) How does the VCC pin look? Stable, or cycling between UVLO Start and UVLO Stop?

    4) How does the ILIM pin look?  Does the ramp on ILIM remain below 1.0V?

    5) How does the SS pin look?

    If the UC3825A is operating OK then I would check the components around the controller.  MOSFET OK?  Are the COMP components correct?  Transformer issue?  Output capacitor and output inductor OK?

    Regards,

    Eric