Basic question about PPU

Discussion in 'General Electronics Chat' started by norstudent, Mar 18, 2014.

  1. norstudent

    Thread Starter New Member

    Feb 26, 2014

    I have a question about the imagie below. Keep in mind im failry new to power electronics & electric circuits in general. I understand what's going on here and understand the function of the rectifier as well as the switch-mode converter, what I don't understand however, is why is there a capacitor in the middle?

    What is the role of this capacitor?

    Lets say the load on the right end is a dc motor, you get AC current from the utility, the rectifier converts it to DC and the switch-mode converter uses PWM to scale it up or down, what does the capacitor do?


  2. wayneh


    Sep 9, 2010
    The rectifier actually makes pulsing DC, not smooth DC like a battery. The capacitor would serve to reduce the size of the ripple seen by the SM converter, which likely helps it operate better.
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    The utility is AC, when the AC is in the positive section of the cycle, the diode is forward biased, it is conducting and there is current flow into the circuit. When AC is in the negative section of the cycle, the diode is reverse biased, the capacitor tries to discharge, but the diode is blocking the current from exiting the circuit, so the current goes to the motor.

    So basically instead of building a full wave rectifier, you have a half wave rectifier, the part of the wave that is missing and does not provide current is replaced by the capacitor. So the positive half of the wave gets rectified. The negative part of the wave is blocked, and at this time the capacitor discharges into the motor.
  4. #12


    Nov 30, 2010
    Both of those guys are right, but I have some doubt about the drawing. It might show only one diode because it is convenient to draw it like that. Still, in the concept, it doesn't matter how they drew it.
  5. norstudent

    Thread Starter New Member

    Feb 26, 2014
    Thanks for the answer guys!

    I should probably have mentioned:
    In this course we basically view rectifiers as black-boxes without going into much details, so I think #12's intuition might be spot-on, it's probably a full wave rectifier represented with a simplified symbol

    So if we assume it's a full bridge rectifier, does that mean the capacitor actually takes a pulsated signal and smooths it out into a constant signal, then then the switch-mode converter takes this smooth signal and uses pulse with modulation to adjust it to whatever voltage we need?
  6. magnet18

    Senior Member

    Dec 22, 2010
    yep, that sounds right, although pwm won't directly go to voltage, it would need smoothed with another circuit
  7. norstudent

    Thread Starter New Member

    Feb 26, 2014
    Ok, thanks guys!