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A More Efficient Bridge Rectifier?

Discussion in 'Electronic Design' started by D from BC, Feb 10, 2010.

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  1. D from BC

    D from BC Guest

    My offline smps design has 3 parts making the most heat.
    A power diode, a mosfet and a bridge rectifier.

    I'm interested in any cookbook designs that replaces a bridge rectifier.
    Any pointers?
     
  2. An active(synchronous) rectifier? i.e., mosfets.

    for 100A, say, your looking at about 70W dissipation per diode verses a
    5mOhm resistor of about 50W. It gets much better for smaller currents. For
    10A it is 0.5W. Similarly you can parallel the mosfets to reduce the
    dissipation even further.

    Obviously it adds to the complexity and cost...
     
  3. Jon Kirwan

    Jon Kirwan Guest

    Ancient, but really huge, selenium ones with stacked fins
    were cool. Four of those in a bridge would take some space
    but also double as a nice hand-warmer.

    Then there is germanium and leaky but lower voltage Schottky.

    But what about synchronous rectifiers using MOSFETs? I have
    to guess they are NOT simple. But in concept they seem as
    though they should be efficient.

    ....

    Just for giggles, here's a synchronous rotary mechanical one:

    http://home.freeuk.net/dunckx/wireless/bridge/bridge.html

    It would be a kick to one day open up a stereo amplifier and
    see one of those inside it!

    Jon
     
  4. Joerg

    Joerg Guest

    Maybe you can cook something up around this chip but I've not tried that
    yet:

    http://cds.linear.com/docs/Datasheet/1698f.pdf
     
  5. Hammy

    Hammy Guest

    I havent seen any FET's capable of blocking line voltages in the
    400-600V range with that low of an Rdson. The few sub 0.1 ohm ones
    I've seen are expensive. They would also likely need a beefy drive
    circuit.

    If you know of a cheap supplier of even 0.05 ohm 400V-600V Fets I'd be
    intrested.

    The rdson doubles at working temperature so even 0.05 ohm would be 0.1
    ohms. So you would probably be better off with the diodes both cost
    and complexity wise and power wise.

    The only time I've seen Fets used is after a transformer steps the
    voltage down.

    But I'll be following the thread to see if there are alternatives. I
    dont think there is though.
     
  6. Joerg

    Joerg Guest

    Well ...

    http://ixdev.ixys.com/DataSheet/99496.pdf

    About $13 in qties.
     
  7. Tim Williams

    Tim Williams Guest

    How many volts, amps, what efficiency?

    Tim
     
  8. Hammy

    Hammy Guest

    You can buy a massive heatsink for your bridge build an aux supply for
    a fan and maybe even liquid cool the works for what one of those would
    cost let alone the other 3.

    But IXYS makes nice ones.To rich for my blood though.

    The lowest Rdson 500Vds Fet I have is the FQA24N50 and thats 0.2 ohms
    90nC gate charge.

    They were on sale for a buck a piece when I got them.

    http://www.fairchildsemi.com/ds/FQ/FQA24N50.pdf
     
  9. Joerg

    Joerg Guest

    Sure, diodes screwed onto a large heatsink is the usual way to do this
    but there are situations where you can't. Like tightly enclosed areas.


    Normally those run almost 3 bucks a pop and abs max is 24A at 25C. So it
    won't be cheaper when you parallel enough of those. Unless there is a
    buy one get one free deal again :)
     
  10. D from BC

    D from BC Guest

    Line voltage here is 120VAC 60Hz. The bridge rectifies 120VAC 60Hz.
    Diode current is complicated.. Say 2amps to 4amps.

    Efficiency: Just wondering how good it gets and/or how complicated it'll
    get.
     
  11. You might look into Silicon Carbide Schottky rectifiers. Here is one from
    Sensitron that has a 1.75V forward drop at 10 amps and possibly 1.2V at 2-4
    amps. Rated at 600V, too!
    http://www.sensitron.com/data_sheets/5008.pdf

    There is a 1A 600V SiC device in the LTC SwitcherCad library for a
    MicroSemi UPSC600 but I could not find any data sheet or availability. Also
    the simulation shows it drops 1.2V at 500 mA.

    Diode drop on a 120 VAC rectifier circuit should be negligible in terms of
    overall efficiency. Just use a bridge rated at a lot more than you need.

    Here is a 35A rated bridge that exhibits less than 1 volt per leg drop at
    2-4 amps (25C):
    http://www.rectron.com/data_sheets/t4ars3505m-t4ars3507m.pdf

    And for a dual diode rated 200V and 30A with about 0.75V drop at 2-4A:
    http://www.rectron.com/data_sheets/her3001c-3003c.pdf

    Your application seems to be for a PSU rated about 300W, so 5W or so in the
    primary rectifier bridge is not even 2% losses.

    Paul
     
  12. Tim Williams

    Tim Williams Guest

    Average or peak? (PFC'd?)
    Depending on how you count the current, that's around 2-10W for an average
    silicon bridge. If schottkies aren't good enough, your only choice is an
    active FET rectifier.

    I think silicon stops at ~100V, but SiC schottkies go higher, although I
    forget what Vf gets to. Lesse...
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMuIUjt4yeP9c75E3XT9Wj25V7zpLgYK/V0=
    Ouchie, $3.11 in singles...
    Aha! 2.4V max. at 4A! Even worse conduction than high speed silicon! I
    guess the only reason for them is high voltage at high frequency = low
    switching loss in exchange for conduction loss.

    Neat, PTC for currents over only 1.8A. Looks resistive for anything over
    1A, no wonder the 10ms surge current is so small. It almost acts like a low
    voltage vacuum tube.

    If it had better semiconductor characteristics (i.e., still exponential, as
    in the <0.5A range), it might be okay... Vf < 1V in that range. That's
    equivalent to maybe 0.4V in silicon... gee, I wonder what a proper PN
    junction in SiC runs... 3V or so?

    Too bad they still make them in cheapass epoxy stuff. I suppose the mil
    spec ceramic and gold, spot welded packages are awfully expensive, but this
    does happen to be one diode which *will* work as hot as the package can!

    As for MOSFETs, you'll need under 0.25 ohms Rds(on) to beat silicon diodes,
    and you already figured 0.05 plus operating temperature doubling. If these
    are driving a big filter capacitor, that's rather annoying, but if they're
    driving PFC instead, that's actually quite simple, since you can synchronize
    it to the AC line. You might even get away with those PV gate drivers,
    since you only need switching times under 1ms. Zero crossing detect and
    you've got a fairly simple, high efficiency rectifier.

    Heyyy... since it's synchronous... do the same to the PFC converter diode.
    So it's buck or boost, depending on which port is delivering power. And do
    the same at the output side. Now, you've got a power supply, or an AC line
    interface. Put batteries and solar panels on the output and power the grid
    in reverse! ;-D

    (Boy, that's a fun input characteristic... not only is it negative
    resistance due to the switching regulator behavior, but in reverse, it's
    actively delivering power, like a real negative resistor. The PFC won't
    drive 0V very well though.)

    Tim
     
  13. I have a little M-G set I got for a few dollars at a Hamfest fleas market.
    That's another way to generate DC from AC mechanically. It can be done
    fairly efficiently, but is costly in terms of size, weight, expense,
    maintenance, mechanical noise, and other factors. And a DC generator has a
    commutator which essentially acts as a mechanical rectifier.
    http://en.wikipedia.org/wiki/Motor-generator
    http://en.wikipedia.org/wiki/Dynamo

    Paul
     
  14. Joerg

    Joerg Guest

    It's not silly. I have recently done that on a project. There just
    wasn't any place where we could have dumped the heat into.
     
  15. Fred Abse

    Fred Abse Guest

    Real men use IGBTs ;-)

    Vceo 1200V, Vce(sat) 2-3 volts at 1000 amps if you really need it.
     
  16. Hammy

    Hammy Guest

    Yep when you start pushing close to 1kW IGBT's are pretty well the
    best alternative.

    I've never built anything requiring anywhere near that power though. I
    start counting micro amps at the start of anything I do so I can keep
    the power supply requirements small and inexpensive.

    Here in Canada they are going to start charging us a premium for
    electricity during peak usage hours. They just installed the new meter
    on my house a couple of weeks ago. So I'm not likely to go anywhere
    near a kW anytime soon. I think the rate almost doubles during peak
    hours!! I would imagine the next step is to gouge us for poor PF that
    will likely be the next cash cow. The US probably pays less for the
    electricity we generate here and sell to them.

    I don't know if the US or anywhere else for that matter charges a
    premium for electricity during peak hours or if we are the only
    country?
     
  17. Joerg

    Joerg Guest

    Plus a 3/4-horse motor to drive the cooling propeller :)


    Oh yeah, probably depends on the political leanings in the area. Here in
    California we pay roughly $0.15/kWh. But the millisecond you exceed a
    rather modest baseline qty for the month it shoots up, big time. So
    people who want to start a business that uses some energy will think
    twice about it and maybe do it elsewhere.
     
  18. Hammy

    Hammy Guest

    Here are utilities have to approach a regulatory body to ask for rate
    increases. When they don't get what they want they have a cute trick
    where they just jack up delivery charges and some debt surcharge and a
    couple other things I cant recall.

    My electrical usage comprises about half my bill the rest consist of
    whatever charges they can dream up. Soon they will tack on another
    surcharge for the new meter as well as the infrastructure for
    implementing the new system.

    That doesn't really give one to much incentive to conserve.

    We should be spending money on Hydro generation which we have massive
    amounts of land with water suitable for that as well as nuclear.

    I just remembered another charge on my hydro bill it's for a green
    incentive it's supposed to go towards green generation projects (solar
    and wind). What a joke.
     
  19. Joerg

    Joerg Guest

    Reminds me of when the telephone tax for funding the Spanish-American
    war rescinded because the war was, ahem, sort of over for a while. Missy
    Bell jacked up some fee which ate it right up. But they have a monopoly
    so you must buy at the company store.


    Be glad you don't live in some parts of Europe where they give solar
    panel owners huges sums per kWh. Guess who ends up paying that hidden tax?
     
  20. tm

    tm Guest

    Same thing goes on here in the US.
     
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