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LM5023: Over load protection issue

SATHISHKUMAR vishwanatha
Intellectual 495 points
Part Number: LM5023

Hi,

We have designing AC/dc converter 100W power supply.

input 84 to 264V AC 50hz

Output 24VDC 4.2A

Switching frequency 58KhZ

Here we facing Overload protection issue @ different input voltages.

We have place 0.1_ohm current sensing resistor.

Issue: Controller turned closed loop(overload protection)

@ 120VAC input - Output current @ 3.6A output controller turned off

@ 240VAC input - Output current @ 5.1A output controller turned off

Find the attached schematic.  We need at 4.2A controller turn off the circuit at 84V - 264V  input.

Kindly give the solution for this issue.

Regards,

Sathishkumar V.

  • 0
    TI__Mastermind 40120 points

    Hello Sathishkumar,

    This issue appears to be a classic case of current limit variation due to delays in the MOSFET control.  This situation is described in the LM5023 datasheet on page 21 through 24, along with a design procedure to correct it.  Please follow that procedure to find the proper value for R12 in your design.

    Increasing the value of R12 will add an offset to the CS voltage that reduces the peak current attained at high line.  The offset voltage is generated by a current out of CS that is mirrored from the QR input (see block diagram, page 9).  This current varies proportionally with the input voltage, so it is higher at high line and adds a higher offset then.

    Please note 2 things:

    1.  The time constant of R12*C16 is part of the total delay, so it must be included in the term "tprop" in your calculations (eqns 20, 21).  To keep the RC value constant, you must reduce C16 after finding the correct value for R12.

    2.  Since it is seen that current limit already functions at 3.6A at 120Vac input, it indicates that the values for R13 and R14 are too high.  Increasing R12 will reduce the high-line primary peak current (and consequently reduce the overall current limit) to more closely match the peak at low-line, but it does not increase the low-line peak.  So if you are getting only 3.6A then R13 & R14 are not adjusted correctly to obtain a 4.5-A limit at 85Vac input.   Fix R13, R14 first, then find R12.

    Regards,
    Ulrich