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Linear PSU Power Stage

Discussion in 'Electronic Basics' started by Costas Vlachos, Jul 14, 2003.

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  1. Just to ask for some comments on the following power stage scheme for a
    linear PSU:

    Everything in the circuit is single-supply (no negative voltages anywhere,
    and single-supply op-amps used). The BC556B/TIP126 pair provide a constant
    1A current sink to aid programmability for capacitive loads. Due to the
    single-supply operation, the current sinking circuit is only effective down
    to about 2V. The BDV67A pair provide the high current sourcing (up to 4A),
    and are mounted on the same heatsink for good current sharing. Load current
    is sensed on the high side, a 1V/A signal is generated and fed back to the
    LT1014 for current limiting (constant-current operation). For high frequency
    compensation, there are 22pF and 270pF caps between (-) and OUT of U1A and
    U1B, respectively (not shown).

    So far it seems to work alright, but I haven't done extensive tests. What do
    you think? Does it look OK? Any comments are appreciated.

    Many thanks.

  2. Jim Stockton

    Jim Stockton Guest

    The design seems dangerously close to the SOA limit at worst case
    conditions for the BDV67A pair. Have you set the output to max voltage
    out , input to max voltage in, current limit to max output current, and
    shorted it. I have been bitten by SOA on darlingtons in the past as most
    seem to be optimized for switching applications. Perhaps a third BDV67A
    to give more SOA margin.
    Good Luck
    Jim Stockton

  3. Thanks for the comments Jim. Yes, I've been bitten by SOA too. That's why I
    used two BDV67A in parallel. If you check the SOA graphs, you'll see that
    *each* can take 4A at about Vce = 50V, so I assumed two would be enough.
    Note that there seem to be many varieties of BDV67A around! The ones I'm
    using are made by SEMELAB and are rated for 200W continuous power each. I've
    found others made by MOSTEK (again BDV67A) but rated at 125W (?!?!?). The
    data sheet for the SEMELAB ones is here:

    I think the SOA for them should be OK for my specs. Let me know if you still
    think there is a problem.

    On another issue, what concerns me is the BC546B/BC556B push-pull pair
    (before the BDVs). Initially I didn't have them in the design, but had to
    put them in for reasons relating to the limited current sinking ability of
    the LT1014. I'm not very experienced in analogue design, so I wonder if this
    "triple darlington" configuration has any dangerous pitfalls in terms of
    speed/stability for the feedback loops. Anyone have any comments on that? It
    seems to work OK, but I want to make sure.

    Thanks again for the reply.

  4. Jim Stockton

    Jim Stockton Guest

    The curve I looked at showed 2A at 40V Continuous.
    Your part should be better.
    Yes the triple darlington could have problems but there are resistors
    internal to the BDV67A that should help. The degenerative feedback in
    the emitters also helps with high frequencys. Perhaps others can comment
    on stability, but I don't see anything glaring.
    Good Luck
    Jim Stockton
  5. Greetz

    Greetz Guest

    Greetz }:)~~~

    Thiz circuit is bumhole becoz yu hav not explaned it rather well. So I
    cannot be boffered to do much more than throw it up in the bin wiv my over

    4tuneatlee, I am not implied buy the cumpanee you hav just bean sakd frum so
    I dunt hav to figur out wut yu thogt yu wer doing n'ee'*way*. Perps you do
    kno and kneed to disc the Manical.


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