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TL431: Maximum Input voltage to TL431 cathode with series resistor.

selvam perumal
Intellectual 515 points
Part Number: TL431

Hi,

I am using TL431 as Error Amplifier in my circuit,

Cathode current is sourced from my converter o/p voltage of  50V through a series resistor of 8 Kohm.

If the voltage at the ref pin goes below 2.5V(i.e O/p Voltage goes below 50V,say 48V),TL431 will not drive the output transistor as per TL431 block diagram.

If above condition occurs,What will be the voltage across Vka.

From my knowledge,If there is no collector current,Output voltage of 48V will appear across Vka.Is it right?

Refer the attached image for circuit.

Expecting your valuable reply.

Thanks in advance.

P.Selvam.

  • 0
    TI__Mastermind 19370 points
    HI Selvam,

    The cathode of the TL431 cannot go above 36V because that violates ABS Max. In the conditions that the voltage of the feedback is below 2.5V then the TL431 will violate ABS max. In this condition the collector current is very small but still exists because the internal amplifier still needs to be powered.

    Is the purposes of this circuit to just be an isolated comparator that shows if a voltage is above or below ~51V?

    -Marcoo
  • 0 in reply to Marcoo Z
    Intellectual 515 points

    Is the purposes of this circuit to just be an isolated comparator that shows if a voltage is above or below ~51V?

    I am using this circuit as an isolated voltage feedback to my controller for an isolated converter.

    My doubt is whether TL431 can work at above condition by clamping to max Vka =36V or it will fail.

    If  so,Do you have any shunt voltage reference IC for a working voltage of 50V?

  • 0 in reply to selvam perumal
    TI__Mastermind 19370 points
    Hi Selvam.

    There is no TL431 or '431 like device that works greater than 36V. This does not mean that you can't use a TL431 in isolated flyback. The solution to doing this is to add a diode in series with the TL431 to give a voltage drop. The diode must be 50V - 36V = 14V minimum diode.

    The following external link shows a schematic example that you can use.
    ac-dc.power.com/.../


    -Marcoo