Semiconductor Current Limiting

Discussion in 'General Electronics Chat' started by Skeebopstop, Jun 3, 2011.

  1. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    Hi All,

    just curious what the reference diode in the attached circuit is doing?

    Cheers,

    James
     
  2. Maged A. Mohamed

    Member

    Apr 6, 2011
    18
    1
    Hi,
    I think set up a maximum value to the output voltage less than the input level
    e.g. if you use TTL gates as load and want no more than 5 volts at all current levels
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The circuit is a current-limited voltage regulator. The reference diode sets the unloaded output voltage at Vref (the ref diode's voltage) minus the Vbe (≈0.7V) of the transistor.
    As the load current increases, the output voltage drop due to current through Rsense. When the drop across Rsense≈0.5V, the diodes start to conduct. As the load current increases further, the current through the diodes steal base current and ref diode current. This causes the output voltage to drop precipitously if the load current tries to increase even more. This drop in voltage will limit the output current.
    Below is a plot of a 5 volt regulator with current limiting that begins at about 60mA.
    I also included a simulation of the same circuit, but with a transistor as the feedback element. As you can see, it has superior current limiting characteristics.
     
  4. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    Thanks for the nice analysis.

    In the original image it didn't show a zener symbol hence the confusion. If they had of I believe it would have been more evident.

    I guess for the application I am looking to use it for, there is no reference voltage and we are talking some fairly significant currents (500mA @ 24V).

    I can't afford to chunk off much power as it'll mess with my efficienty target but I don't want to pulse by pulse current limit either so that soft starting anything connected isn't a big issue. In reality, a foldback would have been nice but I was hoping for a nice 24V current foldback circuit that employs some simple PWM mechanism.

    Thanks for the response.

    Regards,

    James
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What would you use PWM for?
     
  6. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    Was more thinking of a current limited type of PWM regulator operating like a current cut-off, where 100% duty cycle is the norm up to full load (i.e. foldback limit) but starts to chop up after.

    Above mentioned solution has its shortcomings also, requiring some LC type filtering to smooth out the current waveform during pulse limiting, especially in the short circuit case and any LC type filtering is compromised by the fact the power bus is externally exposed so the C will always be changing.

    Unless I were to come across something that can do current limiting for 24V @ 500mA that doesn't require the ability to dissipate heaps of energy when it starts to enter into current fold back mode, I think I'm just going to have to drop this one. I've only come across people using it for soft starting or for small currents. I want there to be a sustained short circuit on the second rail of a flyback design that doesn't cause the first rail to flicker.

    Cheers
     
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