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Driving primary isolated mosfets

Discussion in 'General Electronics Discussion' started by boots_n_braces, Apr 25, 2012.

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  1. boots_n_braces

    boots_n_braces

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    Feb 24, 2012
    Evening all, im wondering how people have dealt with gate supply to isolated mosfet drives? Im building an isolated full bridge supply and im struggling with supplying the mosfet drivers there 15v logic supply. They need to be on the primary side of the windings (obviously so they can drive the mosfet gates) but 15v is way below the primary dc voltage. However due to the magnitude of the output current of the low side driver which uses the Vcc supply to drive the low side mosfet im struggling to figure out how to do it.

    The best i can come up with is floating the ground of the half bridge driver and associated logic to 15v below primary voltage but this still leaves me with how i deal with sinking such a substanial current which really rules out anything simple other than using a pass transistor setup to hold ground at the required level and just suffer the (15v x 3a) 45w losses

    Alternativley i could try a non isolated buck converter with a voltage divider to start it running??

    Any ideas or enthusiasm greatly recieved :D
     
  2. Rleo6965

    Rleo6965

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    9
    Jan 22, 2012
    Please post circuit diagram.
     
  3. boots_n_braces

    boots_n_braces

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    Feb 24, 2012
    I dont have a schematic i can upload but its simply 4 mosfets in h bridge configuration across the transformer primary the trouble is driving the high side n type mosfets and keeping the isolation. I can use opto isolators but the high side fets need there above supply voltage which means the mosfet driver needs to go on the primary side which inturn means i need a 15v supply for the driver on the primary side. However due to the gate current (3 amp) a simple voltage divider etc wont work. I think its going to have to be a seperate primary side buck converter.
     
  4. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    You need a high side mosfet driver chip. They supply the voltage above the high side voltage by using a switched capacitor. They are also designed to give a high peak current to the gate.

    Bob
     
  5. boots_n_braces

    boots_n_braces

    16
    0
    Feb 24, 2012
    Hi bob
    Im using an IRF high side low side driver however this also requires a 15v supply for the logic and the low side gate drive (I think most of these drivers do) And generating that voltage is proving to be a pain:D
     
  6. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Last edited: Apr 28, 2012
  7. boots_n_braces

    boots_n_braces

    16
    0
    Feb 24, 2012
    The chip isi similar to this fairchild version http://www.farnell.com/datasheets/1526487.pdf

    The problem has nothing to do with boot strap operation, the driver requires a 15v supply for the low side drive and the logic however without a dedicated smps to generate this its proving problematic as its so much lower than the line voltage
     
  8. TedA

    TedA

    156
    16
    Sep 26, 2011
    boots_n_braces,

    You need to find some design examples. I would look on the IR website for ap notes. I think Power Integrations, ON and TI also might be good places to look, as well as Fairchild.

    Have you been looking at SMPS controller ICs? The data sheets for these often have application circuits that show how the control power can be provided.

    It is possible to just incorporate a small low voltage DC power supply to power the primary side circuits. This could be a smaller SMPS, or an iron core transformer and a couple of diodes.

    Many SMPS designs power the primary side control and driver circuits from an extra, low-voltage, winding on the main power transformer.

    To get the supply started, they use a high value resistor from the raw DC to charge-up a capacitor on the low voltage supply bus. This capacitor must be is large enough to run the circuits for a moment as the supply begins to run. This scheme counts on the control circuit doing nothing and drawing very little power until the voltage gets high enough.

    Some designs then disconnect the start-up resistor, after start-up.

    Powering the primary control circuits off the main supply may not work if the main output must be adjustable over a wide range, or operate with no load. In this case, the independent auxiliary supply is the answer.

    Ted
     
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