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PS-Filter

Discussion in 'Electronic Basics' started by Samuel Groner, Jun 12, 2004.

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  1. Hi

    I want to design a C-L-C (or is L-C-L better?) filter for a 30 V/4 A
    linear power supply. What is a good value for L if C is around 1 uF?

    Thanks for input
    Samuel
     
  2. Something near infinity (assuming you are talking about the rectifier
    filter).
    If you think of your full load as a resistor, it would have a value of
    30/4=7.5 ohms. If your line frequency is 60 Hz, at the fundamental
    ripple frequency (120 Hz) of a full wave rectified output, a 1 uf cap
    would have an impedance of about 1300 ohms. This is essentially
    insignificant as a filter in parallel with 7.5 ohms. So the inductor
    is going ot have to do the whole job. I would suggest you start with
    capacitors that have a 120 Hz impedance less than 7.5 ohms before you
    start to worry about the inductor. I would normally think in terns of
    1000 to 5000 uf per amp for a rectifier filter. So a 4700 uf 50 volt
    electrolytic would be a ball park start. And the input cap has to
    have a ripple current greater than 4 amperes, also, if you use the CLC
    form.

    What is the design goal of the filter?
     
  3. I want to design a C-L-C (or is L-C-L better?) filter for a 30 V/4 A
    Typo - should read "1 mF" or "1000 uF"...
    The filter will sit right after the rectifier and in front of the
    regulator (IC & ext. transistors). Goal of the CLC-filter would be
    lowest ripple for C=1 mF.

    Samuel
     
  4. I have not seen a linear regulator that likes to be fed with an
    inductance, so I think your choices are CLC or LC or C. The first and
    last (capacitive input) tend to charge the caps up to the peak voltage
    of the rectified AC somewhere approaching 1.4 times the AC voltage of
    the secondary, while the LC (inductor input) tends to average the
    rectifier output. This averaging effect only takes place if the
    inductor current is continuous throughout the rectification cycle. So
    for light loads or insufficient inductance, the voltage will rise.
    Indictor input filters are good if the transformer voltage is a bit
    too high, and they also spread the diode conduction time throughout
    the cycle, lowering the transformer copper losses, line harmonics and
    diode peak current. The CLC (pi) filter) doesn't put much AC voltage
    across the inductor, so can work with smaller inductance. You just
    want ot avoid having a resonance at any rectified line harmonics, so
    the fundamental resonance should be lower than the second line
    harmonic (the fundamental of the rectified line). But in all cases,
    more inductance implies lower ripple, so your design goal of minimum
    ripple still implies infinite inductance. By the way, what is the
    load that you are so worried about rectifier filtration, if you have a
    linear regulator? after the filter? The output ripple will be
    dependent on both the filter and the regulator, assuming the filter
    ripple is not so deep that the regulator has nothing ot discard at the
    minimum points on the ripple.

    What is the secondary voltage from the transformer you are using to
    drive the 30 volt 4 amp linear regulator?
     
  5. Thanks for the help, link etc. so far!

    The PS is for a multiple headphone amp. I'm not worried about the
    performance of this design, but the PS should be a prototyp for a
    mic-preamp where it really matters.
    Samuel
     
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