SMPS Bleeder Resistor

Discussion in 'General Electronics Chat' started by Robin Mitchell, Nov 3, 2014.

  1. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009
    Hi all,

    I have decided to design a switch mode power supply and so starting with the basics. But there is one thing that confuses me, the bleeder resistor and its function.

    If you switch the input to the filter circuit the capacitor will charge. If there is no bleeder resistor to remove the charge from the output capacitor then the control circuit will eventually need to stop switching the input to the output. Does this mean that if a SMPS is not connect to anything then once the desired output voltage is reached the controller will set the PWM controller to have a duty cycle of 0%?

    Also what type of switches are used? MOSFETS? solid state relays?

    Many thanks,
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Depends on the SMPS. Some start and stop. Others run continuously. There is usually a voltage divider across the output used to feed back a fraction of the actual output voltage. That voltage divider provides a minimum load (bleeder).

    FETS are used for switches. The catch diode is a special Schottky (not a garden-variety Silicon).
  3. nigelwright7557

    Senior Member

    May 10, 2008
    On low output currents a SMPS will regulate poorly.
    To aid this a bleeder resistor is put as a load across the output.