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TL1431: TL1431 Designed to shunt current (upto 3A) if Vin > 34V will current limit in input current in limited to 1/2A or less

Part Number: TL1431

I have a voltage limiter/current shunt that when the power supply input is current limited to 1/2A or less the TL1431 will shunt the entire 1/2 A and drop the power supply voltage to 8V.   If I power on the supply with the current limit above 1/2A the ckt will not shunt until Vin is > 34V (the designed value) where it regulates correctly.  I can power on the supply with curent > 1/2A then drop the current and the shunt will still operate properly. 

Any Idea as to the cause?

  • Hello Ron,

    Can you help to provide more details about the circuit?
    I am not sure where the 1/2A and 3 A currents are going or what the rest of the circuit is doing. What is limiting the current in the circuit?
    Please note that the TL1431 is rated for a maximum cathode current of 100mA, so going beyond that may cause permanent damage to the device.

    The key here also is to ensure that at all times the TL1431 has a minimum of 1mA cathode current to ensure that it is regulating. If the load is consuming all of the current in the system, the TL1431 may not have enough current to regulate.

    If you can provide a circuit schematic or any additional details, I would be happy to help further.

    Best,
    Michael
  • Michael ,

    I’ve provided a schematic of the shunt circuit.   The J1 and J2 inputs are the inputs from the Solar Array Simulator. The J3 and 4 inputs are returns, J5 and 6 are load resistors for shunted current and the emitter node  of TIP2955 is connected  to a reset circuit (not shown) the reset circuit will  switch power to the electrical power system after  ~10 seconds. So when the system is powered up there should only be the shunt circuit and an RC circuit with ½ meg input as the load.

    The circuit was designed to limit the power bus to 34V.   When I power on at 32V and ½ A (which is 2x what the system needs initially to power on and boot a embedded processor) the current draw is ½ A and the voltage drops to ~9V.

    At 32V the cathode voltage should be > 16V which should bias the TL1431 at > 1mA.  

    If the system is powered on at 32V and 0.6 A or above it operates fine. No current draw for at least 10 seconds then the system boots.

    Ron

  • Hello Ron,

    If I am understanding correctly, your input power supply is limiting to .5A, and therefore the power supply is limiting itself to 8V.
    Is the TL1431 causing this current limit?
    What are the voltages at the Reference and Cathode of the device?
    What is the voltage at the base of the Q3 transistor?

    Best,
    Michael
  • I Don't have that information yet. I don't have direct access to the hardware. I will post when I know further
  • Ron,

    I understand. Yes, please do help to provide additional information, and I can help further.

    Best,
    Michael
  • Michael,  

    What happens to the TL1431 if it doesn't get enough bias current to operate?  Could it cause the TL1431 to trigger the shunt circuit?

    There is a long umbilical (10ft) connecting the power supply to the hardware. Any chance this could contribute? 

    Ron

  • Michael, 

    The circuit seems to lock up and does not recover if the current is boosted.  Can the TL1431 get into a mode that only a power cycle will fix?

  • Ron,

    If the TL1431 is not getting enough current to operate, then it can't regulate its output, but in this case, it can't regulate until a threshold is exceeded, so it just needs to have enough current at this point, which would be more than 1mA.
    When the threshold isn't exceeded, it isn't sinking current, so no issues when it isn't acting as a shunt.

    Please help to provide measurements or oscilloscope shots of what is happening. If there is a long cable connected to the output, it may be possible that the capacitance is causing an oscillation on the output.
    I am just wondering what is happening at the terminals of the TL1431 so that I can see if it is behaving oddly.

    Best,
    Michael
  • The emitter of the TIP2955 supplies power to some electronics and it also connects to some solar panels so there is considerable wiring. The solar panels are diodes isolated so the diodes present their cathodes to the shunt regulator circuit. 

  • Hello Ron,

    I don't think there should be any issues with the TL1431 in this case, but I am wondering if there might be a stability issue or something due to capacitance. 

    The TL1431 should only shunt current when the output voltage tries to go above the set point. This is why I am asking to see the voltages/scope shots at each of the pins so that I can check to see if the TL1431 is operating in a manner I do not expect.

    Best,
    Michael

  • Is the capacitance issue you mentioned related to the TL1341?  From the schematic I have supplied, you can see there are no capacitors connected to the TL1341 what type of capacitance could be causing this issue? 

    There is a long umbilical between our equipment rack and the electronics  ~10 ft. 

    I disconnected a Solar Array Simulator from the umbilical and connected an HP Bench supply. I scoped the current and voltage. The results were better. I was able to power on successfully at lower currents ~100mA whereas before it was 500mA and lower caused an immediate problems.

    My next step is to connect a bench supply directly to the hardware and bypass the umbilical and take measurements. 

    Right now getting access to the shunt circuit would require disassembling the hardware. It would be my last choice. 

  • Hello Ron,

    Thank you for the update. I was only mentioning capacitance because the long cord could be a source of capacitance causing instability in the TL1431. Although since there is a good bit of series resistance, this scenario is unlikely.

    It seems you are making progress, and I can't say for sure that the TL1431 is causing the problem, but without more information, it is hard to speculate.

    If you are able to get more information that suggests that the TL1431 is causing the problem, then please let me know and I would be happy to help.

    Good luck with your current work, I hope you are able to find the cause.

    Best,
    Michael