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Switching Power Supplies

Discussion in 'General Electronics Discussion' started by quantumtangles, Feb 24, 2014.

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  1. quantumtangles


    Dec 19, 2012
    As I understand it, the essential requirements for a switching power supply are:

    Gated oscillator (anywhere north of 100kHz) x 1
    Operational amplifier x 1
    Voltage reference
    Resistors x 2 (voltage divider)
    NPN (darlington pair) series pass transistors x 2
    Filtering Caps x 2 for input and output
    Catch diode x 1
    Inductor x 1

    Has anyone here actually built a switching power supply circuit using gated oscillators etc and if so, what is the trade off between using a super high frequency gated oscillator (to move packets around really quickly) and the downside of high frequency packet transference.

    Is it simply the case that high frequency gated oscillators in the 'on' state result in fully saturated series pass transistor pairs where the respective bases are almost continually supplied with the required voltage to allow collector/emitter action (and therefore it consumes more electrical energy because it is in the 'on' state most of the time), or is it more subtle than this?

    In other words, why not use a 1Ghz gated oscillator to move the packets around ever faster. Is there a downside to ever increasing packet speed, and if so, what is it?

    Thanks in advance for any responses:D
    Last edited: Feb 24, 2014
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You can have a lower frequency.

    Well, you need something with gain, not necessarily an op-amp

    This is only required if you want a regulated output. Some switchmode power supplies do not require this.

    There is no need for a voltage divider. Even though one is often used.

    Well... you only need one pass element and it doesn't necessarily need to be a BJT.

    Only the output one is required.


    In essence all you need is a switch, an inductor, a diode and a capacitor.

    For an example of a SMPS lacking several of the things you indicate above, see here:

    It has no input capacitor, voltage reference, the frequency is under 100kHz.

    The first design has a voltage divider, but the final design does not.

  3. quantumtangles


    Dec 19, 2012
    Thanks Steve. Awesome answer. The thread on SMPS is fantastic. Beginning to find switching power supplies fascinating :D
    Last edited: Feb 26, 2014
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