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LM431: Output Voltage oscillates by 20mV against Set point of 10V.

Sudharsan K
Intellectual 385 points
Part Number: LM431
Other Parts Discussed in Thread: TL431,

Output Voltage oscillates by 20mV against Set point of 10V.

Scheme is attached here...

We are using BD240C transistor where as simulation is done with BD238.

The output is expected to be stable @ 10V where as we encounter swing of 20mV [0.2% Variation which is unacceptable]

Kindly suggest any modifications in scheme for getting stable output.

  • 0
    TI__Mastermind 19550 points

    Hi Sudharsan,

    In this type of application I feel the device can experience a very low phase margin. I would increase C1 or remove C1 and see how it affects the circuit.

    -Marcoo

  • 0 in reply to Marcoo Z
    Intellectual 385 points

    We tried removing the capacitor and note the stability of output, the output dips by 29mV in time span of 90 minutes.

    Let us check the effect of increasing the C Value.

  • 0 in reply to Sudharsan K
    TI__Mastermind 19550 points

    Hi Sudharsan,

    Can you tell me the voltage and current going into the cathode of the device from your simulation?

    -Marcoo

  • 0 in reply to Marcoo Z
    Intellectual 385 points

    Voltage @ Cathode is 9.47 V & Current through it is 6.89 mA. For the given scheme.

    As per the the technical Application Report Document Reference: SLVA482A, the "recommend capacitance is less than 1/10 of the left boundary line or greater than 10 times the right boundary line."

    As per our loading conditions Vka = 9.47 V & Ik = 6.89 mA, the right boundary line is falling to 0.6 uF hence we tried with 10uF as the final C value, but we observed upto 30mV dip in the regulated voltage.

    We tried 100 uF and the o/p seems to be stabilized.

    This is our observation. Let us know if there is some grey areas in the design?

  • 0 in reply to Sudharsan K
    TI__Mastermind 19550 points

    Hi Sudharsan,

    The grey area I see with the design is the inclusion of the transistor. The stability is more sensitive with a transistor. I the stability graphs were tested with the CLoad directly connected to the Vk and are a good baseline for a transistor less design. With a transistor extra precaution must be taken as the stability will be degraded.

    -Marcoo

  • 0 in reply to Marcoo Z
    Guru 140200 points
    Hi Sudharsan,

    as Marcoo Z already mentioned the stability plot is only valid for the basic circuit without transistor! By introducing R5 in the cathode line of 431 you get a total different circuit.

    Unfortunately the TINA model of TL431 doesn't simulate the stability issue very well. I don't see any instabilty when simulating the basic circuit which is used for the stability plot. But that's not very unusual, because these model are very limited.

    So, the only way to find out whether your circuit is stable or not is to test it experimentally. Bias your circuit with 15V input voltage and your desired load current and introduce little abrupt changes, first of input voltage, then of load current. Observe with the scope whether there's any ringing or instability at output. Next step is to test the circuit at different ambient temperatures. And finally check several identical circuits, each with a fresh LM431 and BD238, best from different lots or manufacturers.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 385 points

    Hi Kai & Marcoo,

    Thanks for your support. We would try the recommended testing conditions.