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Half-Bridge or Full-Bridge SMPS?

Discussion in 'General Electronics Discussion' started by eem2am, Sep 3, 2011.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    I think that Offline Half-Bridge converters are a complete waste of time....am i right?

    The full-bridge is cheaper and better......

    FET RMS current will be much higher with a half-bridge than a full bridge, and the half-bridge fets will need to be more pricey with bigger heatsinks.
    -this is due to the unfortunate rail-splitting capacitors in a half-bridge, which divide the input voltage by two.

    Also, the Half bridge needs these highly expensive , extremely low dissipation factor, rail-splitting film capacitors.....these are more expensive than a mosfet....so the extra mosfets in a full bridge, would be payed for by the fact that the full bridge doesn't need the expensive film capacitors.

    FULL BRIDGE COSTS:
    4 FETs, Two NCP5181 Bootstrap fet drivers.

    HALF-BRIDGE COSTS SO FAR
    2 FETs, one NCP5181 Bootstrap fet driver, 2 Pulse rated film capacitors



    ...Already the half-bridge is looking awry...........but there's more woes for it........

    the half bridge cannot utilise low-side resistive current sensing, because when the high side fet conducts...most of the power current would not go through the low-side current sense resistor.......so the half-bridge must use an expensive current sense transformer in order to be able to sense any primary overcurrents.

    ..the Full-Bridge can just use a cheap current sense resistor.


    Also, in the event of overload, the half-bridge's rail-splitting capacitors will end up with one discharged , and the other charged all the way up to the rail........which means that each rail-splitting capacitor must be rated for the full Vin of the DC Bus.....so thats two, expensive 400V rated pulse capacitors for the half bridge.

    Also, the film capacitors for the half-bridge take up a large amount of space in the half bridge.


    Is anyone seriously going to defend the half-bridge?


    Even more woes for the half-bridge in that it cannot do peak current mode control without blowing itself up, or at the very least running in an unstable way...this is mitigated by adding a balancing winding, but thats more expense and complexity.

    The full-bridge might need more heatsinks...but actually, no it won't...because if you use Insulated tab fets you can simply fix the four fets of a full bridge to the same heatsink, side-by-side.

    Its already been said that primary current is much higher for a half-bridge than a full-bridge......so there's more woes for the half-bridge at the EMC testing stage.

    So, Is anyone seriously going to defend the Half-Bridge vs the Full-Bridge?...Surely not?
     
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