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Line filtering

Discussion in 'Electronic Design' started by Tim Williams, Mar 16, 2007.

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  1. Tim Williams

    Tim Williams Guest

    So what kind of filtering does an induction heater need?

    Potentially salient points...
    10kW (240V, 50A USA supply) ultimate goal
    ~20kHz resonance (may go as low as 5kHz in the future)

    Currently, DC filtering is 44uF (2 x (10 x 2.2uF film), filtering +V/GND/-V)
    after inverter, then electrolytics at FWB (2 x 8 x 470uF 200V). Measured HF
    ripple of 4% at 50V supply, 15A peak output IIRC.

    I'm supposing a chunky inductor of some uH (or mH!) suitable to put a few
    ohms in the way of >2kHz, at enough current that it doesn't saturate due to
    line current, which is ZERO, for a common mode choke... The wire at least
    will have to be 6 or 8AWG.

    And as a wider question, what is the *point* of a common mode choke? I can
    see differential mode, which is how I'm drawing the current for pete's
    sakes, but I don't particularly intend to be swinging around wildly in
    space, leveraging my chassis' potential against--air? I mean, how does a
    grounded box like a computer produce HF hash when the case isn't also
    radiating in some obvious manner? I don't see how to calculate the
    inductance for a common-mode choke, out of the blue. And yet I've seen them
    in switchers with two- and three-wire cords.

    Tim
     
  2. Tim Williams wrote:
    (snip)
    (snip)

    They are there to put an impedance in series with a signal
    that is in common with both sides of the line. For
    instance, assume you have a rectifier and capacitor down
    stream of the filter, and the DC supply feeds a switching
    mechanism that produces rapid voltage swings that feed
    capacitive current into ground (through a transformer inter
    winding shield, perhaps). That current completes a loop
    through ground, back to the power lines and through the
    common mode inductor back to the rectifiers and capacitor,
    every time the rectifiers conduct. This causes a pulse of
    high frequency in common on both sides of the power line
    twice per cycle. The common mode choke gets in the way of
    that current completing the loop. Without it, the radio
    noise radiated by the pair of power lines because of the
    current they each carry half of, acting as a single
    conductor at RF, would be much worse.
     
  3. Lionel

    Lionel Guest

    Harmonic leakage from the cables & any gaps on the metalwork, mostly.
     
  4. I would be thinking in terms of using a choke input DC
    filter. That way, you not only clean up the differential
    noise on the line, you improve the power factor by
    eliminating the high current peaks each time the rectifiers
    turn on. Think in terms of the choke used on the output of
    a simple DC welder. Small shoe box size.
     
  5. Tim Williams

    Tim Williams Guest

    So why don't I just put a big fat choke on the ground line? It's not like
    it has to carry any current! <g,dr>

    How about the other points: two-line supplies and my situation?

    Tim
     
  6. Tim Williams

    Tim Williams Guest

    Ouch, there goes rail regulation. Seems kind of a waste to run 600V IGBTs
    from +/-100V. I don't even have much capacitance, at least yet.

    Besides, that's gotta be what, 0.1H at 50A? Yech! Could buy the IGBTs to
    build a PFC for the price of the *wire* for that sucker!

    Tim
     
  7. Terry Given

    Terry Given Guest

    PI-filter on your DC bus then. But without some line inductance (before
    or after rectifier, no major diff) then as JJ points out, current peaks
    will be HIGH. A Pi-filter also means the cap lifetime calcs are easier -
    rectifier-side caps see BRUTAL 100/120Hz ripple, but no HF. vice-versa
    for SMPS-side caps.

    Anecdote:

    testing a range of 230V and 400V AC motor controllers. Tech measures RMS
    input current. 4.7kW drive has 30% *more* Irms than 12kW drive. WTF says
    I, then goes and does calcs. large drive has 5% line chokes, small
    drives none. Voila, math = measurement. ouch.

    Cheers
    Terry
     
  8. Terry Given

    Terry Given Guest

    no worries. thanks for the pics. hairy-faced gits flock together?
    rather than a WAG, use a scientific WAG (SWAG). this is for the DM filter.

    you ought to know what the HF ripple in the DC bus capacitance is. its
    usually easier to set up a crude simulation than an analytic expression.
    Then do a reasonabole (include ESL,ESR) model of the DC bus caps, so
    you have a reasonable idea of the ripple voltage.

    now you can look at a roughly accurate spectrum of said ripple voltage.

    I typically do the design with a copy of the relevant EMI spec - that
    way you know what your allowable DM current into 50R is. Because EMC is
    all about parasitics, I pick the lowest allowable level, and use that to
    design the filter. but I include reasonable values of parasitics for the
    DM filter components.

    I usually have 2 models - a switching-level model, so I can get fairly
    accurate RMS currents, and a cruder model for the filter, using some
    sort of a vandalised current source to represent the current into the
    filtering network.

    the rectifier complicates things somewhat. but it tends to look pretty
    capacitive, so is a short at EMC frequencies. which is what I use.

    then fiddle with your components (AC sweep) until you have something
    that looks like it will meet the requisite EMC standards. inductor
    end-to-end capacitance is hugely important - a massive inductor does
    naff all at EMC frequencies because of this capacitance, which is
    dominated by the start wire and the final layer of turns + the finish
    wire. Adding more L does naff all, the C doesnt really change but the
    corner frequency (at which the L stops helping) gets lower...... if you
    make the inductors, you can control this - eg bank winding, with N+0.5
    turns so the start and finish stay the hell away from each other.....

    with reasonably large caps that work at HF (ESL, ESL, ESL) and a
    moderate switching frequency, you can probably use a fairly small L, <<
    1mH in your PI filter. the sims/calcs will show this.


    CM noise occurs because of parasitic capacitance to anything that is
    earthed. like say the metal plate upon which the LISN sits at the EMC
    lab, if you have no earth wire. or all the metalwork if you do. so plan
    on having a CM choke, and Y caps.
    for a reasonable AC (or DC) inductance, use 5% chokes. 3% is the
    critical value below which not much happens; 5% gives a huge reduction
    in peak current, for 5% drop in AC volts across said chokes at 100%
    rated current (do the calc ignoring the nasty(ish) current waveshape) so
    the line regulation is ~ 5%.

    5% for a 10kW 240V single-phase supply is:

    Vbase = 240Vrms

    Pbase = 10kW

    Ibase = 41.7Arms

    Zbase = 5.8R

    Wbase = 120*pi

    Lbase = 15.3mH

    5% L = 763uH

    Epeak = 1.32J - and that DOESNT INCLUDE the evil current peaks. its a
    ~2J inductor.

    again its easy to simulate the overall rectifier behaviour for AC line
    current, but hard to calculate. try stepping L, have a look. then use
    the actual peak current to calculate the required energy, so your L
    doesnt saturate at the current peaks (thereby defeating its purpose)

    HTH

    Cheers
    Terry
     
  9. Fred Bartoli

    Fred Bartoli Guest

    Tim Williams a écrit :
    Sure there is. A PFC will handle all that nicely :)
     
  10. Tim Williams

    Tim Williams Guest

    Thanks for the reply.

    So I should say, split my electrolytic cap bank with a few microhenries (or
    maybe as much as mH, I forget) to decouple the HF from one side? Or just
    toss on more film caps (maybe even one of those swanky "film-lytics") for
    the HF side of the filter?

    So, at the line, I take it there's no way to get away from high ripple or
    bad regulation or bad power factor (PFC aside).

    Tim
     
  11. Tim Williams

    Tim Williams Guest

    ....

    Bugger... I don't like simulation. I don't have a simulator on hand,
    either.
    Don't have any idea what kind of EMI spec I want. I mean, I'm building this
    for myself, is the FCC going to give a damn about a couple kilowatts of
    radiated hash running for at most a couple hours?

    Tim
     
  12. Terry Given

    Terry Given Guest

    LTspice. I dont use it, but its free and good (a rare combination)
    In that case, size your rectifier caps for the 120Hz ripple current, and
    the caps on the HF side of the Pi filter to prevent bus overshoot and
    device destruction. then stick as much inductance as you can be bothered
    making between the two; 20dB is a good start wrt attenuation. remember
    to put something HF in parallel with the LF electros.

    then smack in as much CM inductance in the AC line as you can be
    bothered winding. an X2 cap across the AC of the rectifier, and a pair
    of Y caps from P,N to E at the same point - so any capacitive coupling
    to chassis flows thru these caps and back into your smps.

    or do nothing. after all, your work coil is air-cored so spews flux
    everywhere (although its pretty sinusoidal). this is the quickest option.

    Cheers
    Terry
     
  13. Tim Williams

    Tim Williams Guest

    If you don't recall, I have electrolytics from computer supplies, 16 x 470uF
    200V. Any bearing on the capacity of them? I recall typical specs for
    similar product are around 2-4A ripple, and I have no recollection of ESR,
    ESL...

    Any recommendations on film caps? It's looking like I'll have to buy some
    more... I've got Cornell Dubilier DMEs (2 x 10 x 2.2uF 400V) on there right
    now. Seems to be the cheapest, densest mylar line available, at least at
    the time I checked. No good for current (I have a clip of some 0.1uF's on a
    tank circuit and one starts oozing smoke) and notable ESL (series resonant
    circa 300kHz for...0.47uF I think?), so basically bulk nonpolar capacitance.

    With some nice polypropylenes on the inverter itself, HF crud should be low
    enough by the time the ripple gets to the film cap bank, huh?

    Oh...do I have links to any recent pictures? I forget...
    This is about the most recent I have online...
    http://webpages.charter.net/dawill/Images/Induction806sm.jpg
    Except the big coupling capacitor is now split in two, so it makes a fake CT
    off the supply voltage.
    Got it. Common mode should be pretty easy, although I don't really have a
    core big enough to loop all that 6AWG through. Or the #6, for that
    matter...Maybe another pi-wound dealie from 1/2 x 0.04" copper (like the
    Lmatch)...
    Yeah, but that's cheating. The power line conducts crud, while the coil
    drops off as what, inverse cubed distance?

    Tim
     
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