Discussion in 'Homework Help' started by GreenSheep, Apr 19, 2010.

  1. GreenSheep

    Thread Starter New Member

    Apr 19, 2010

    Been reading the site for a while and need to ask a question that I hope someone can help with...

    I'm trying to model a basic PSU in PSPICE with the following setup:

    ac input (230 V rms at 50 Hz) > transformer (s:p = 10:1) > full wave bridge rectifier (assume voltage drop per diode of 0.7 V) > shunt capacitor (1000uF) > load resistor (25 ohm)

    Now I've calculated the average load voltage to be 24.91 V and the ripple voltage peak to peak to be 12.45 V.

    The trouble is, I've stuck this all on PSPICE and only get 23 V average and 5 V peak to peak ripple.

    I'm now getting to the point where I cannot really troubleshoot this anymore (I'm currently running without a transformer using a sine wave supply straight onto the bridge to simplify and I've tried lots of other bits to try and work this out...) and I've started to question whether I've calculated the ripple voltage correctly:

    I've used the formula:

    Vr(pk-pk) = (1/((2fs)(C)(Rl)))Vc(max)
    [Ripple peak to peak voltage = (1/((2 frequency)(Capacitor)(Load resistance))Capacitor maximum voltage]
    Vr(pk-pk) = 1/((2(50))(0.001)(25))31.13
    Vr(pk-pk) = 12.45 V


    Vc(max) = Vs - 2Vd
    [Capacitor maximum voltage = (Voltage peak from secondary of transformer) - 2(Voltage drop at diode)]
    Vc(max) = 32.53 - 2(0.7)
    Vc(max) = 31.13 V

    I have little experience with PSPICE (SIMetrix/SIMPLIS).

    Can anyone see anything wrong it what I've done about or offer a pointer as to why the PSPICE simulation differs from my calculation so much.
    I was expecting some error but not this much.

    Cheers, GreenSheep