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Switchmode power supply question

  1. Feb 5, 2010 #1
    Why are SMPS smaller than linear power supplies?
    I understand when you switch at high frequency, the transformer,inductor and capacitor size reduces. Usually, there are 2 transformers in a smps. One at the AC input and one after switching.
    How do they rectify the 60Hz using the first transformer. I am trying to understand this as the transformer is really small.
    Is the freq upconverted some how?
  2. jcsd
  3. Feb 5, 2010 #2
    First 120V AC input is directly rectified with a bridge rectifier and filtered with capacitors to get something like 168 Volts DC.

    The DC voltage is now used to supply power to oscillators which generate the actual high frequency AC at high voltage - there is one oscillator for every transformer.

    A transformer down converts high voltage AC to a lower voltage AC. Then the lower voltage AC is rectified and filtered with smaller capacitors (due to high frequency) and that generates the DC voltage.

    The smaller transformer is sometimes used to generate a low power DC standby voltage that supplies power to the PWM (Pulse Width Modulator) chip. The PWM in turn precisely regulates pulses which control the main transformer.
  4. Feb 6, 2010 #3
    So there is no isolation at the line(60Hz AC)?

    By high voltage AC, you mean switched AC or the 60Hz?

    oh! the pwm does load regulation.

    The whole reason I got the doubt about transformer bcoz I was looking at roomba power supply and it had 2 transformers. Its a 32W power supply. But the transformer near the AC input is so small, it is in no way remotely close to 32w.
    3rd pic here - http://www.schneordesign.com/Avi/irobot/roomba_mod1.htm
    I just checked my roomba PS, the transformer near the AC line is a common mode filter transformer.

    But I wonder how they switch the 168V DC. They probably use mosfets. But I am not sure how to drive the mosfets.
    Last edited: Feb 6, 2010
  5. Feb 6, 2010 #4


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    If you use high frequencies, the transformer size can be considerably less. Also, a linear regulator needs to dissipate power in what is, effectively, a variable resistance, whereas a switching regulator will dissipate a vanishingly small amount of power - depending on how fast the switching is achieved (rise and fall times) - because the switch is either on or off (no dissipation in either case) and not some intermediate resistance. Hence it can be built smaller because there is less heat to dissipate.
  6. Feb 6, 2010 #5
    In the pic the smaller transformer near the AC input is not really a transformer but it's an EMI filter. It kills any interference entering the circuit.

    Isolation occurs after the main transformer.

    I meant high voltage, and high frequency switched AC at this stage.

    Some designs use mosfets. Most PC power supplies I've seen use NPN transistors.

    You could verify this quickly with a multimeter.
  7. Feb 6, 2010 #6
    waht, how do the drive the NPN transistors or mosfets? All they have on-board is 168V DC.
  8. Feb 6, 2010 #7
    That's the job of an oscillator. Supply DC voltage, and it outputs AC. And this can be achieved in a number of ways.

    Sometimes a relaxation type of oscillator is used. The transistor is switched by energizing and dumping current in the transformer, and the frequency depends on the inductance of the windings.

    Or an independent PWM chip can supply pulses to switch the transistors for better load regulations.
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