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  1. #1
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    Simple MOSFET switch design

    Her’s the problem of MOSFET switcher that allows to swith between two power supplies with logic signal 5V. First supply is 3.3V at maximum 5A , second is 12V with 1A.
    Trouble is that theory ( working diagrams) are sometimes strongly different than practical process.
    Assumption was to get small element with low power waste, as low average current drawn .
    Problem is with that 3.3V as it is value that MOSFET start to work.
    Important is to get logic level, that starts to work at very low voltage, so with 3.3 works strongly. Solution for that can be rdsOn that are in SO8 pkg.
    If you check some MOSFET specification, you will find that threshold can be between 1-3.5V. But only about 250 uA of drain current has 10Vds at 25 degrease C. at higher temperatures it is worst. No one can quarantine what is going to happened.
    Althought that good solution is to take the most similar specifically values.
    I prefer TO-220 packages as they are easier to heatsink, and I used FDP6030L. Parameters of it is 12mOhm RdsOn at 10Vgs, so close to 12V that I wanted and 1A output. Nice is that power consumption is only 13mW.
    When you need device for slow operation you don’t have to be very punctual with calculation, but if you need fast switching with high currents and voltages you need pay attention on that.
    If you are worry about producers data sheet, you may buy more elements, hoping that one of them will be very close to your assumption ( and to data sheet parameters too)

    Alternative solution is to connect two mosfets together. I saw that in laptop motherboard with two 4407. it really works. Source of first of them was plugged to drain of second. But in this case you need to use SMD element between them.

    •   Alt13th May 2010, 14:38

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  2. #2
    FvM
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    Re: Simple MOSFET switch design

    I don't exactly understand the problem you're referring to. True "logic level" MOSFET have a maximum gate threshold
    voltage of e.g. 0.95V rather than "1-3.5V", also a guaranteed minimum drain current or maximum Rds,on with e.g. 2.5V
    Vgs. They can be used to reliably switch 3.3V or 5V supplies without auxilary supply voltages. There may be of course
    differences in preferred package styles and other parameters, that make you want to use standard MOSFET.

    P.S.: If you are particularly referring to TO220 package, there are a few devices with guaranteed parameters at Vgs=2.8V,
    e.g. IRF3706 or IRF3708. But for switching applications up to several A, you won't need a heatsink.

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