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What type of SMPS is this? (Harbor Freight Plasma Cutter)

Discussion in 'Electronic Design' started by Jeremy Samuels, Feb 5, 2006.

    Page 14

    Looks like a half bridge driven through an isolation transformer? Also
    at a loss to describe why a half bridge is used in a 2.5+kw
    application...but I guess that would be explained at how incredibly
    bottom of the line this unit is.
  2. Looks like a two-transistor (IGBT) forward converter with the diodes
    shown wrong.

    Best regards,
    Spehro Pefhany
  3. Pooh Bear

    Pooh Bear Guest

    To keep it cheap. There's nothing wrong with a half bridge.
    What's 'wrong' about the diodes ?

  4. The bottom connection on BG3 and the top connection on BG4 should go
    to the opposite sides of the primary from where they are shown.

    Best regards,
    Spehro Pefhany
  5. I believe that BG3 and BG4 are needed as shown to protect emitter to
    collector breakdown voltage (V(br)ECS) of the IGBT's which is about 15V. The
    question is why they do not need the diodes you suggest Spehro, clamping
    transformer primary to the opposite rails. When the IGBTs are turned off
    there is a lot of residual energy remaining in the primary that will flyback
    and the additional diodes will clamp this voltage at the opposite rails.
    Maybe they use 1200V IGBTs and the transformer primary has a lot of
    parasitic capacity to absorb that energy before 1200V is reached. IGBTs can
    absorb lots of reverse voltage avalanche energy (Earv) so maybe just a big
    heat sink will do. I as you would add those diodes.
  6. Tim Williams

    Tim Williams Guest

    Hmm, if it's supposed to be half bridge, it's missing a ground. As is, the
    transistors act in series; why they would draw a switch at each end is far
    beyond me. Engrish schematic, as it were, y'think?

  7. Normally inverse-parallel diodes are built into the IGBT half-bridge
    module. The parts shown are outside of the module, based on the part
    numbering and the quantity of diodes shown in the assembly drawing,
    unless some of the diodes shown are paralleled.

    Also, why use two switches and two sets of driving circuitry, doubling
    the cost and power dissipation?
  8. I had just assumed that the only way of making a 2+kw SMPS was full
    bridge (or half bridge), and just assumed the schematic to be a
    simplification/mistranslation. I found a near top of the line plasma
    cutter schematic here with similar ratings:
    page 26
    Wish I could get the full schematics...but from what I see, there's no
    PFC. Would a PFC stage be too expensive/have too little benefit?

    I'm sure this is a stupid question, but could anyone explain the
    purpose of R1,2 C1,2?

  9. It's a "simplified" schematic.
  10. Fred Bartoli

    Fred Bartoli Guest

    Make the switches switch together.
    When the switches are on you transfer power to the output, and also rise
    current into the magnetizing inductance.
    Then open both switches. The primary polarity reverses and with the right
    pair of (omitted here) diodes the core resets by sending back its stored
    energy to the supply.
  11. If the inverse parallel diode is not built into the module it must be
    placed externally as shown to protect the collector breakdown voltage
    (V(br)ECS) of the IGBT's which is about 15V.
    The rail clamping diodes (D1+D2) in your link are not need if the IGBTs can
    absorb the flyback energy at turn off. Lots of primary transformer capacity
    will help.
    Two switches are used as your article should point out to reduce the
    voltage stress on the IGBTs. One of the IGBTs may also function as the pilot
  12. This is a different subject, but are they actually *needed*? The info
    I have implies that it's just a power dissipation thing with IGBTs. I
    don't see where much reverse current would come from, either way.

    Best regards,
    Spehro Pefhany
  13. Bingo.

    Best regards,
    Spehro Pefhany
  14. That is probably the same circuit, but with MOSFETs rather than IGBTs.
    That's the power supply filter, or voltage doubler, depending on how
    the input voltage is set up. The resistors are to equalize the voltage
    across the caps when they are used in series. RL2 controls the 115/230
    configuration (see CP). A similar arrangement is used in PC power
    supplies with a manual switch on "CP".

    Best regards,
    Spehro Pefhany
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