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  1. #1
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    MOSFET Body Diode vs Schottky Diode

    I am trying to build a simple DC to DC boost converter.
    I am concerned with the body diode of the MOSFET (IRFZ44N) used as a switch.

    How can I make sure that the body diode does not conduct when the inductor kicks?
    The objective is to make the schottky diode conduct to charge up the capacitor.

    I am new to this design. One idea that has popped up is to put a simple diode in series with the MOSFET to delay its response.
    During that time, schottky will start conduction and all of the inductor current will flow through the capacitor.

    I am using this approach:

    http://www.electronicdesign.com/file.../figure_01.gif

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  2. #2
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    Re: MOSFET Body Diode vs Schottky Diode

    I guess the 1st question is, why? The body diode of the FET should be more than adequate to conduct the boost charge current.
    Any way, if the shottky diode is faster at turning on than the body diode it will do all the conduction without the added series diode as it has a lower voltage drop. And will do all the conduction at some point because of the lower voltage drop.



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  3. #3
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    Re: MOSFET Body Diode vs Schottky Diode

    In normal circumstances the coil does not generate a high-voltage spike. It does generate current for a time after the switching device stops conducting.

    Volt level will rise according to several factors, including load. With little or no load, the coil produces however high a volt level is necessary to overcome the charge on the capacitor. The output stage can soar to a hazardous level.

    In normal circumstances the volt level will not endanger the mosfet/transistor as long as you use one which is rated for your anticipated levels (since the body diode will be rated that high as well). Also as long as you keep a load attached to the output stage.

    To see the inner workings of a boost converter, you might look at my video which has animated simulations of switched-coil converters (including buck, boost, and buck-boost).

    It portrays current flowing through wires. It shows the flux field building and collapsing. It shows capacitors charging and discharging.

    www.youtube.com/watch?v=FT_sLF5Etm4



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  4. #4
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    Re: MOSFET Body Diode vs Schottky Diode

    Quote Originally Posted by BradtheRad View Post
    In normal circumstances the coil does not generate a high-voltage spike. It does generate current for a time after the switching device stops conducting.

    Volt level will rise according to several factors, including load. With little or no load, the coil produces however high a volt level is necessary to overcome the charge on the capacitor. The output stage can soar to a hazardous level.

    In normal circumstances the volt level will not endanger the mosfet/transistor as long as you use one which is rated for your anticipated levels (since the body diode will be rated that high as well). Also as long as you keep a load attached to the output stage.

    To see the inner workings of a boost converter, you might look at my video which has animated simulations of switched-coil converters (including buck, boost, and buck-boost).

    It portrays current flowing through wires. It shows the flux field building and collapsing. It shows capacitors charging and discharging.

    www.youtube.com/watch?v=FT_sLF5Etm4
    Good Point, I forgot about a minimum load on a boost converter. It will go to destruction without some minimum load on the converter all the time.



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