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Diode ratings in push pull converter

Discussion in 'Electronic Design' started by Franc Zabkar, Mar 16, 2005.

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  1. Franc Zabkar

    Franc Zabkar Guest

    I have a generic "400W" ATX PSU which I suspect is a fake. This is its
    circuit diagram:

    "LC-B250ATX ch. Y-B200-ATX ver. 2.9 JNC Computer Co."

    The label on the cover reads as follows:

    L&C Technology Inc
    Model: LC-B400ATX

    +3.3V +5V +12V -12V -5V +5VSB
    28A 40A 17A 0.8A 0.3A 2A

    +5V & +3.3V combined load 220W
    +5V & +3.3V & +12V combined load 380W
    Total output is 400W max.

    The +3.3V and +5V rails are each rectified by a S10C40C dual diode,
    and the +12V rail is rectified by two FR302 fast recovery diodes.

    According to their datasheets, the FR302 is rated at 3A, and the
    S10C40C is rated for 10A.

    Printed on the PCB is the following legend:

    F5A/250V (for 180W -- 235W)
    F7A/250V (for 300W -- 350W)
    F6.3A/250V (for 250W)

    The fuse is 5A. :-(

    The PCB is labelled "Y-B200 ATX Ver 2.8" which also suggests a 200W

    Can anyone offer an opinion as to the real rating of this PSU? How do
    the diode ratings in the push pull converter affect the PSU spec? Can
    one assume that the total wattage is close enough to ...

    Pt = (3.3 x 10) + (5 x 10) + (12 x 6) + (5VSB x 2) + (-5/-12 wattage)
    = 175W approx

    - Franc Zabkar
  2. Terry Given

    Terry Given Guest

    they probably parallel both diodes. in theory this gives twice the
    current rating, if they share equally (which they probably do as they
    are on the same die so at the same temperature)
    the diode drop is a large chunk of the 3.3V output power - say 1V at
    10A, so the actual power from the 3V3 winding would be 4.3V*10A = 10W
    more than you calculate. not so bad for the 5V, even less so for 12V.
    But its a good start. And you can see why people use synchronous
    rectifiers (ie FETs) for really low output voltages like 1.5V.

    read the diode drop off the datasheet (taking account the individual
    diode current where 2 are paralleled) and add it to each supply voltage
    in your calc, and you'll be a lot closer. It doesnt account for xfmr and
    choke Cu losses, switching losses or any primary FET conduction losses,
    but you'll be fairly close.
    in a push-pull or forward or half- or full-bridge converter the average
    diode current is the output current.

    I have no idea where you got your current values from....

    without specifying the input voltage, it is not much use looking at the
    fuse rating. Even then its not too helpful (unless you take into account
    the pulsating nature of a rectifier/diode filter, not that hard if you
    like maths)

    but if its a 5A fuse it probably wont like more than 3Arms long term -
    the fuse will eventually fail at any more than that.

  3. Alan Turner

    Alan Turner Guest


    This does raise the interesting question of whether the power rating
    stamped on the PSU is the input power or the output power.

    If it's the input power then 200W power supplies can easily be
    upgraded with a resistor :)

  4. Franc Zabkar wrote...
    Did you trace the drawing yourself to know it matches your supply?
    You know this how, from taking apart your power supply?
    Most Pacific Rim PC supplies are NOT rated for continuous power,
    especially for the +12V line, etc. The reasoning is because in
    actual use the power-consumption peaks when the HD motor spins up,
    etc., and you want to know it won't sag under the transient.

    They are also not rated for simultaneous full load on all outputs.
    Again, that would be an unrealistic application.

    So, who knows exactly what the PSU rating spec means? Ask them.

  5. Indeed you have been swindled. Unfortunately you are not the first nor
    surely the last person who will be swindled by "L&C Technology" power
    supplies. That company clearly has no moral standards, they simply slap
    stickers on their products with whatever fanciful ratings they want with no
    regard to the actual powersupply construction.

    As I write I have a L&C technology so called "300W" powersupply sitting in
    my lap. I might add the powersupply failed after maybe 3000 hours of
    operation (at much less than 300W output). The powersupply has three
    substantially discolored regions on the PCB indicating prolonged extreme
    heat. This powersupply also uses the same basic output rectifiers as yours
    does (two 10A center tapped TO-220 schottky diodes and two 3A FR302
    devices). It has two 330uF 200V input capacitors.

    To be within specification of just the output rectifiers both of our
    powersupplies should be rated for an absolute maximum of no more than 10A
    5V, 10A 3.3V, 6A 12V, for a combined total on these rails of only 155W.
    This assumes perfectly ideal duty cycle, perfect cooling, no other
    significant diode losses, and no component derating of any kind.

    Now adays I don't normally buy computer powersupplies unless I get a chance
    to at least peek through the holes in the side with a flashlight first.
    Signs of reasonably decent construction I specifically look for are large
    input capacitors (two 470uF or more for 250W, 680uF or more for 300W+), and
    a large main power transformer that uses a round center post. Transformers
    that use a round center post are inherently superior to those that use
    rectangular center posts. For a given cross sectional area the circle has
    the smallest perimeter, which directly translates into better copper wire
    utilization. These types of cores also normally have a much taller than
    wide window area which also improves proximity effect losses for high
    frequency operation.

  6. Well okay, but a low voltage schottky diode operated at reasonably decent
    current density has a typical forward voltage more in the range of 0.4-0.5V.
    In your typical computer power supply which uses a half bridge topology the
    secondaries are center tapped. So rectification for each rail is normally
    achieved using a single TO-220 or TO-247 packaged dual diode in the typical
    full wave rectifier (for center tapped secondaries) configuration. So only
    one diode is effectively in series with the output at any given moment in
    time. The 12V rail usually uses a TO-220 packaged 100-200V ultrafast
    recovery diode with a forward voltage approaching 1V.
  7. Fritz Schlunder wrote...
    When Franc made his post, I questioned his determination of the
    FR302 diodes, but now you confirm it in another L&C Technology PS.
    Two 3A half-wave rectifier diodes could be claimed to deliver 6A
    continuously (although without a proper temperature derating). The
    L&C Technology 17A rating doesn't speak well for company's honesty.

    The diodes would appear to drop nearly 2V at 17A, at 25C, less when
    hot. Let's call it 1.5V; that's about 13W dissipation (50% duty).
    It's doubtful they can do this for more than say 30ms, starting cool
    from a current *much* lower than 6A. Perhaps this is long enough to
    do something useful, spin up a disk drive, etc., but the discrepancy
    between the 6A datasheet and 17A L&C spec'd numbers boggles the mind.

    Another thing that boggles the mind is using el-cheapo $39 power
    supplies with thousand-dollar processors, display monitors, etc.
  8. Terry Given

    Terry Given Guest

    I have seen both approaches in pc smps high-current outputs - often a
    pair of dual diodes on opposite sides of the same heatsink. the
    datasheet for the particular schottky mentioned specs 0.6V at 10A, ie
    6W, which is still a sizeable error wrt 33W - about +18%. Besides, the
    word "say" is my get-out-of-imprecision-free-card :)

  9. Terry Given

    Terry Given Guest

    Jolly good points.

    I have a mate who has several hundred videogame machines. He has used pc
    smps for a long time, as they are cheap and readily available. About 6-7
    years ago he stopped buying really cheap ones, and started buying
    (comparatively) expensive ones from a reputable manufacturer. His smps
    failure rate plummeted to almost zero (his techs were replacing one
    every week or two), and the logic board failure rate likewise became

    I think the answer to "how can you make a good smps for so little money"
    is simply that you cant. Cheaper is invariably nastier.

  10. O.T.O.H.:

    That power supply probably costs less than USD 5,00 per piece in lots of
    1000 at the dockside, which is, I suspect, The *Real* Requirement.

    PeeCee equipment is notoriously shitty - it's amazing how often it happens
    to work in the general case while anyway near the "edge" - Boom!
    (literally)!! Probably the very same principles are used here as in the olde
    days with power transistors for switchmode: Everybody *knows* that the
    salesperson lies through his teeth, after all his lips are moving, so we
    just de-rate by 60% to compensate.
  11. Franc Zabkar

    Franc Zabkar Guest

    No, the anodes of each diode attach to opposite ends of a center
    tapped winding. The rated average forward current is 5A per diode,
    giving a total rating of 10A for the package. As each diode only
    conducts during alternate half cycles of the oscillator, the rating of
    that particular rail would be 10A.
    I'm not concerned with the power output from the *winding*, except
    insofar as it reflects the efficiency of the PSU. Instead I'm trying
    to determine the power available to the external load, ie I'm
    comparing this figure to the one on the PSU's rating sticker.
    That's what I've always thought, but after seeing several "200W" PSU's
    with 150W parts I was left to wonder whether there was something I
    didn't understand.
    I'm not sure which current values you mean, but one set was printed on
    the ratings sticker, the others came from device datasheets.

    Here is a diagram of a push pull converter:

    A diode rated at X amps should be able to carry 2X amps during each
    half cycle of the oscillator. The other diode carries 2X amps during
    the alternate half cycle.
    The fuse ratings were printed on the PCB. I included this information
    to confirm that this particular configuration of the PCB was intended
    for a 180W-230W application. The suitability of the fuse was of no
    particular concern to me.

    - Franc Zabkar
  12. Franc Zabkar

    Franc Zabkar Guest

    I was careful not to say that this was actually an L&C PSU. The
    sticker says L&C, but who knows who really made it? I posted several
    links to what may be the real (?) L&C unit. These look entirely
    different. Even the low wattage L&C enclosures look different to mine.
    Who knows what a quality PSU really costs to make, especially in the
    PRC, and by prison labour? One wonders if a genuine 400W generic PSU
    is really significantly inferior to a branded product. Remember, mine
    a 180W PSU with a 400W label, so it never really had a chance.

    - Franc Zabkar
  13. Franc Zabkar

    Franc Zabkar Guest

    Yes, I did. In fact I'd traced it before I found the online circuit.
    I'd even determined the pinouts of the mysterious combo PWM
    controller/supervisor/3V3 regulator IC. Any differences in my
    implementation of the circuit are in unpopulated sections (missing
    filter caps, linked inductors), and in the wiring of the fan. My fan
    is fed directly from the +12V rail, whereas the circuit diagram shows
    a thermistor controlled version (this is available on my PCB, but not
    Meticulously. :)
    Yeah, right. :)

    The PSU label has a separate C-tick certification number. I've emailed
    the relevant authority.

    - Franc Zabkar
  14. Franc Zabkar

    Franc Zabkar Guest

    As stated previously, this PSU does not look like any of the L&C PSUs
    in the URLs I posted elsewhere in this thread. Maybe it's a fake L&C
    unit put together by a third party. After all, a fake version of a
    generic brand would not be subject to the same scrutiny as a fake
    Enermax, for example.
    I believe the various diodes are good for their rated current up to 75

    - Franc Zabkar
  15. Terry Given

    Terry Given Guest

    Paralleling a pair of diodes is not uncommon. If I was sneaky I would
    have said "possibly" not probably :)

    as you have discovered, the rail "rating" doesnt have bugger all to do
    with the diode rating. And the rated average current is given at some
    (unspecified) setup having a thermal resistance which keeps the diode at
    a suitable temperature. Depending on how you cool the device, it is
    possible to run at much higher currents than the data sheet suggests
    (Ifsm being the single-shot limit, usually constrained by adiabatic
    heating, or perhaps bond wire fusing). Conversely if you did a dreadful
    job of cooling the diode, reliable operation (now theres the snag) would
    mandate a lower-than-rated current. My experience with cheap smps is
    that they beat the snot out of the diodes, which often fail.

    for a reputable manufacturer, its what the label says. For a cheap
    manufacturer, its often a lot less - depends on how long you want it to
    live for, and under what conditions. Cheap smps ratings are a bit like
    audio amp power ratings in that regard - wildly optimistic if not
    complete fiction.

    when a design is "optimistically spec'd" (thats being kind) like this,
    its the losses that determine the medium- and long-term power available
    to the load, because they are the bits that will die. But you cant
    really calculate that, you pretty much have to measure it. I have done
    that on several occassions when evaluating OEM parts - the results are
    sometimes pleasing, often horrifying.

    cap ripple current ratings affect long-term operation too, and are
    usually woefully inadequate. These are wonderful for techs, they are a
    never-ending source of income.
    the mentality of the lying bastards that wrote the label. But as Fritz
    pointed out, dynamic and static loads are often quite different, and
    marketers usually use the former in place of the latter.
    maybe. depends on overall thermal performance. The S10C40C datasheet
    gives [email protected] = 2.4W and [email protected] = 6W, so the peak die temperature
    rise is 2.5 times larger with a 2x increase in current. again, thermal
    considerations dictate whether or not this is a terribly bad thing, but
    it will probably decrease the life of the part, due to the effect of
    temperature cycling on the die-Cu interface.
    if you know the ac input voltage and dc bus capacitance, you can
    calculate the 100/120Hz ripple and diode conduction angle, then
    calculate the rms current it will draw - assuming a diode/cap rectifier.
    I doubt a cheesy smps like this will have a UPF universal input.
  16. Terry Given

    Terry Given Guest

    belabouring the point: 75C case temperature. Add to that the drop across
    the heatsink, then internal ambient, which will likely be a fair bit
    hotter than external ambient.
  17. legg

    legg Guest

    The schematic indicates that a dual diode body is to be laid out for
    the 12V rail at D14 - not FR503s. FR503s might be used in the negative
    rectifier, as these are inicated as being discrete.

    A lot of power is easily deliverable by the higher voltage rail, with
    a TO220 dual rectifier employed. This is also where the extra power is
    usually needed.

    An SB1040 is also not necessarily a Mospec S10C40 - this could be an
    autocad designation simply identifying a footprint or bin number.

    I suspect that the correspondence between the schematic and the unit
    you're disassembling is not as perfect as you believe.

    A single schematic can easily represent a number of models that differ
    in power rating - just as the same board art can be populated with
    components producing different total output ratings - you'd realy need
    to read the BOM in question or identify individual parts present in a
    correctly labelled unit.

  18. Franc Zabkar

    Franc Zabkar Guest

    The artwork can accept either a TO-220 package or the bigger TO-3P. It
    appears the PCB can be configured for 180W operation or 350W,
    depending on the choice of caps, inductors, transformers, and power
    semis. Another difference is that my PSU has 4 rectifier diodes at the
    AC input whereas the circuit diagram shows a bridge. The artwork is
    designed to accommodate either. If anything, the circuit diagram of
    the LC-B250ATX PSU shows a fully populated PCB - my "LC-B400ATX" has
    missing parts.
    That should be FR302, and they *are* used in the +12V rectifier. Do
    you know of any PC PSU that uses 3A diodes on an 0.8A rail? :)
    An SB1040 is a single diode in a TO-220 package. An SB1040CT is a dual
    I traced most of the circuit before I discovered it on the Net. The
    circuit references appear to be identical. Even the transformers bear
    similar part numbers, ie EI-33ASG, EE-19N, EE-16(TOP). The former is
    shown on the schematic as an EL-33ASG - I don't know if this is
    significant, or if it is a typo.
    Agreed. I've identified the parts and they clearly cannot come within
    a bull's roar of the spec claimed on the label.

    - Franc Zabkar
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