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Paralleling Three Full Wave Rectifier Bridges

Discussion in 'Electronic Design' started by Anand P. Paralkar, Dec 27, 2013.

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  1. Hi everyone,

    I am trying to build a relatively high voltage-high current DC source.
    The scheme is simple and uses no regulation (therefore no feedback
    control). The scheme is as follows:

    Utility mains supply => Variac => Three full wave rectifier bridges in
    parallel => Huge capacitor bank => Load.

    Variac: something similar to this one:

    Bridge: KBPC3510

    Capacitor bank: 6800uF, 400V

    I could not find a full wave bridge rectifier with a sufficiently high
    current rating and therefore, I thought of paralleling three that were
    readily available.

    I start (slowly) increasing the output AC voltage of variac so as to
    increase the DC supply to the load. However, the variac fuse blows-up
    at around 10V AC output!

    Paralleling three bridges may not be the most elegant way to build a
    high-current DC source, but I do not understand what could cause the
    fuse in the variac to blow-up. (Please note, everything works fine with
    a single bridge rectifier. This ofcourse limits the amount of load
    current I can draw out from the source.)

    Thank you for your help and greetings for festive season. Wish everyone
    a new year full of good health and prosperity!

  2. Maybe one of your rectifiers is bad. Check them with a meter on the
    diode scale and mayb try them one at a time.

    Best regards,
    Spehro Pefhany
  3. miso

    miso Guest

    On 12/26/2013 9:39 PM, Anand P. Paralkar wrote:

    I have no idea on the variac fuse issue, but you really can't parallel
    diodes due to current hogging.

    Maybe there is a scheme that isn't too complicated to use power mos
    devices instead of diodes, i.e. active rectification.
  4. Guest

    If your mains supply is US style 120 V, it's not common to have more
    than about 20 A available. You *can* get up to 50 A on the right
    circuit, but not from a "normal" outlet. For Europe-style 230 V or
    240 V, you probably have 16 A or less available.
    That size Variac is only going to be good for something like 10 A.
    Bigger Variacs do exist.

    Both the mains supply and the Variac will limit how much current you can
    get. Neither one has much to do with your fuse-blowing problem, though.
    35 A, 700 V bridge rectifier. OK.
    Probably OK. If this is made out of multiple capacitors, you might try
    using just one at a time - maybe you have a bad one.
    This may or may not work. The diodes in each bridge will turn on at
    very slightly different points in the AC cycle. When an AC cycle
    begins, one bridge rectifier may temporarily be carrying all the
    current, until the AC voltage rises a little more.

    Do you have the bridge rectifier(s) on a heat sink?
    Like Sphero said, test each bridge individually. Maybe you have a bad
    one, or maybe the terminals are incorrectly labeled.

    You also have a lot of capacitors to charge up; they will appear as a
    dead short at the beginning of the first AC cycle and draw a large
    current for a brief time. (On the other hand, it does work if you use
    just one bridge - but does it work with each of the three bridges used

    If the Variac has a fast-acting fuse in it, this may be part of the
    problem; try a "slow blow" fuse of the same current rating and physical
    size. Be aware that you might blow the next fuse in line (like the fuse
    or circuit breaker for the building mains) instead when you do this.

    I know this *can* work, as I've helped do something similar to charge
    a 288 V battery pack: 240 V mains, isolation transformer (1:1), Variac,
    bridge rectifier, small capacitor (around 1000 uF), battery pack. I
    only used one bridge, though. The control system was a student that
    watched a voltmeter and an ammeter and turned the Variac knob. :)

    Matt Roberds
  5. Jasen Betts

    Jasen Betts Guest

    Are they identical? parallelling disparate rectifiers is likely to
    fail due to current hogging,
    have you got a clamp meter to probe for fault currents with?
    only one thing: too much current.
  6. Artem

    Artem Guest

    1. Check Capacitor polarity.
    2. Connect any tungsten lamp between variac and rectifier. It will prevent future explosions.
    3. Use only one rectifier. Google "diode parallel".
    4. Do not forget discharge you capacitors after.
  7. Artem

    Artem Guest

    On Friday, December 27, 2013 8:49:47 AM UTC+2, Artem wrote:
    tungsten lamp
    220 Volt!!!
  8. Chris Jones

    Chris Jones Guest

    I agree with Spehro. Perhaps one rectifier is bad or wrongly connected.

    Also, regarding putting rectifiers in parallel:

    Whilst it is not the cause of the present problem, it may be worth
    keeping in mind that the forward voltages of diodes tends to decrease
    when they get hotter. This means that whichever of the three rectifiers
    is hottest will tend to hog more than its fair share of the current, and
    may get hotter still, possibly leading to damage. The usual way to fix
    this problem is to mount the semiconductor devices in good thermal
    contact with each other, and then add individual resistors in series
    with each device so that if there were any current imbalance, it would
    tend to self-correct due to the voltage dropped in the resistance being
    large enough to swamp any thermal mismatch. These balancing resistors
    are most often seen when multiple bipolar transistors are used in
    parallel, in which case each transistor has the resistor in its emitter.
    In the case of the rectifiers, it might turn out to be the case that
    just using individual long wires for each rectifier, rather than joining
    them together with short thick wires, would be enough balancing
    resistance. It might even be the case that the rectifiers themselves
    have enough parasitic internal ohmic resistance to prevent thermal
    runaway. Anyway, checking the current sharing (e.g. with a hall effect
    clamp meter) under full load, especially after you artificially heat or
    cool one device, might be a test worth doing once you get the circuit

  9. Artem

    Artem Guest

    I'm not sure that capacitor is necessary. DC Choke will be more better solution.
    I prefer use capacitor in series of primary coil for current stabilisation.
  10. Guest

    Why do you use multiple bridge rectifiers instead of four discrete
    sufficiently big rectifiers ?

    Anyway, the specifications for most rectifier bridges seem to be quite
    "optimistic". In a single phase bridge rectifier two diodes connect
    all the time, causing two Vf voltage drops and the power dissipated
    P = 2 x Vf x I, thus at 35 A, one could expect about 70 W heat

    One data sheet for the KBPC3510 claimed Iav at Ta=55 C with 3/8"
    leads, which would suggest that the bridge was suspended in air
    without heatsink. Of course, this is ridiculous.

    The derate curve makes more sense, in which the derate should start at
    55 C _case_temperature, which is believable. With Tj(max)=150 C and
    Ta=20 C, the thermal resistance from junction to case is 1.4 C/W and
    from case to ambient air 0.5 C/W. To achieve such low heat sink
    thermal resistance, you would need huge fans to circulate cold air or
    have a constant supply of (ice)water :). And as a reality check, who
    would design a product to operate at the maximum junction temperature
    anyway ?

    Assuming some more realistic design parameters, say Tj=120C, Ta=40 C
    (inside an equipment) and Rth=3 C/W (from junction to air) a 27 W
    dissipation would be tolerated thus the load current should be kept
    below 15-20 A.

    Perhaps a diode has been failed short in your previous experiment.

    If you do not have sufficient balancing resistance (or inductance) in
    _each_ feed to the rectifiers, if one rectifier gets hotter
    (insufficient cooling in the middle unit etc.), the Vf drops, more
    current flows through this bridge, while the current in the other
    drops. Finally the hot diode overheats and breaks.

    Use small separate resistors from the AC input point to each rectifier
    AC input to balance the current even if there is a temperature
    imbalance. Of course you cold use such resistors also between each
    bridge (+)-side and storage capacitors or if you have three storage
    capacitors, feed each with a dedicated bridge and combine the output
    from each capacitor to the final load.

    Of course, balancing series resistor dissipate some power and cause
    voltage sag. One way to reduce the dissipation is to use series
    inductors from the main AC input to the individual AC inputs of the
    bridges. This is also gentler to the rectifiers and electric network,
    since it reduces the peak current during startup and during each mains
  11. Jasen Betts

    Jasen Betts Guest


    the lamp only sees the difference.

    a 12V 100W lamp might be a good start.
  12. Guest

    I have not seen any specification for the actual thermal resistance
    from juction to ambient air without heat sink, but for packages of
    similar sizes, this is somewhere between 10-30 C/W, take 10 C/W,
    Tj=150, Ta=25 C, the maximum power dissipation would be 12.5 W.
    assuming Vf=0.9 V/diode, which would allow 7 A continuous current, For
    Rth(ja)= 30 C/W, we are down to 2.5-3.0 A.

    Looking at data sheets from various manufacturers, the Vf=1.2 A at 35
    A rather than 1.0 V that I had assumed in a previous post. The Rth(jc)
    is 2 C/W, while I had assumed 1.4 C/W based on the derate curve. Thus,
    even my calculations gave a too optimistic view of the situation.
  13. Fred Abse

    Fred Abse Guest

    What current? The pictured variac you referred to looks like less than 5
    amps rating. What fuse does it have? It certainly doesn't look capable of
    the 35 amps that one KBPC3510 is rated at.

    Did you test it under load, or unloaded?

    Bear in mind that, with the rectifier-capacitor configuration you
    describe, the peak current drawn can be much higher than the average
    current, dependent on the impedance of the supply. Variacs have low
    resistance windings, and also, being autotransformers, the currents in
    each half, either side of the wiper are very unequal. Variacs are not
    really suitable for reservoir capacitor rectifier operation.

    A 5 amp variac will only *deliver* 5 amps at whatever voltage. You can't
    use them for current step-up, I've seen melted windings where people have

    3*35 amps = 105 amps, which is a BIG variac, which would weigh hundreds of
    pounds, and cost thousands of bux. The only ones that big that I've seen were
    motor driven, three phase, and needed a crane.

    Paralleling block bridge rectifiers is never a good idea, unless you can
    guarantee a close match on forward voltage and temperature coefficient thereof,
    and ensure they're all at the same temperature, under all conditions. Individual
    diodes can be paralleled with a suitable resistor in series with each.

    If you can't find a suitable bridge, you'll just have to make one out of
    individual diodes. They're available up to hundreds of amps.

    Check your rectifier connection (easy to get wrong), and your capacitor
  14. Guest

    On Friday, December 27, 2013 12:39:11 AM UTC-5, Anand P. Paralkar wrote:

    The variac in your link does not supply enough current to stress that KBPC3510 bridge you're using, not even close. Get real!
  15. Fred Abse

    Fred Abse Guest

    Most RF power transistors I know of are a single transistor, with multiple

    From memory, the humble 2N3866 has 9 emitters.
  16. Guest

    The nasty thing with that configuration is that the inductor will also
    carry DC, requiring a quite big choke with an air gap (size comparable
    to the output transformer in audio applications). Putting the
    inductance prior to the rectifier and you do not have to handle the DC
    current and no air gap needed.
  17. mike

    mike Guest

    If the inductor is sized to carry the peak current without saturating,
    why does it matter which side of the rectifier?
    What am I missing?
  18. Guest

    It definitely is NOT standard practice to parallel semiconductor diodes.
  19. You've wired something up wrong.

    Bridge rectifiers can be purchased in enormous sizes meant for powering
    heavy machinery. There's no need to gang them up. Check any online
    parts catalog.

    Micro Commercial MB354W-BP
    Fairchild GBPC3504W
    400V 35A continuous at 55C. The surge current charts shows they can
    survive a 60Hz 4 cycle inrush current of 200A. That inrush current
    rating is at least as high as normal-grade electrolytic caps allow.
  20. Jasen Betts

    Jasen Betts Guest

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