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    NMOSFET On high side?

    Hello, I need place NMOSFET ON High side and control it with PWM approx. 400Hz.

    Q5 switch 25V to about 8Ohm Load. In device is 50V too.
    My solution
    Becouse Gate to Source volatge on Q2 is allowed max +-20V.
    Zener D1 subtrack 12V On 38V.
    If MCU is High 3.3V Q3 and Q4 is On and Q2 is Off On G Q5 is 0V On Source 0 too.
    If MCU is Low 0V Q3 and Q4 is Off and Q2 is On On G Q5 is 38V On Source 25V.

    Did I miss something?
    Is there any simpler solution?

    Click image for larger version. 

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    Re: NMOSFET On high side?

    Hi,

    A dedicated high side driver is more simple. Less parts, more rugged, faster....
    It needs less parts and it does not need an extra 50V power supply.
    That's why it's used many million times every year.

    Klaus
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    Re: NMOSFET On high side?

    Here's the LTspice simulation of a discrete bootstrap high-side driver that requires no additional voltage supply.

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    Zapper
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    Re: NMOSFET On high side?

    Thank you for your suggestions
    1. Use integrted High Side MOSFET driver.
    Why not, but where can I find driver that
    - it suffices with supply voltages +5V or +25V or +50V
    - have price max aroud 1$
    Low cost driver as IRS10752LPBF need supply +10V - +20V

    2.bootstrap high-side drive
    this is also an option
    - but include 7 components as my driver
    - the question is how much current will seep through C1
    In real is Load heating powered over thermocouple and when the transistor is closed is measured volatge on TC for control temperature of heating elements



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    Re: NMOSFET On high side?

    Low cost driver as IRS10752LPBF need supply +10V - +20V
    If you don't have that voltage available then you can easily generate it from the higher MOSFET voltage using a resistor and a 15V zener along with a filter cap.
    The circuit only requires 100uA at that voltage so will require less than a mA through the resistor and zener.
    the question is how much current will seep through C1
    Why is that of concern?
    No current "seeps through" the capacitor but it has to provide charge to the MOSFET gate during turn-on.
    The current required to supply the charge for the MOSFET gate at the operating frequency is Q*f where Q is the gate charge and f is the operating frequency.
    At 400Hz it will be quite low.
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    Re: NMOSFET On high side?

    If you don't have that voltage available then you can easily generate it from the higher MOSFET voltage using a resistor and a 15V zener along with a filter cap.
    Yes, it is possiblem but,
    1xR + 1x zener + 1x C +1x expensive high side mosfet driver
    advantages faster switchung time (switching us only 400Hz)
    disadventage significantly higher price, higher consumption

    current will seep through C1
    in real everyone capacitor have any DC leakagecurrent, yes if will be use smal ceramic C , wiil be DCL normally of the order nA but over timem un temperature and humidity can increase . Just yesterday I changed, yes defective C, 0603 with parallel resistance, who drained the 2x AAA battery in a month.



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    Re: NMOSFET On high side?

    1xR + 1x zener + 1x C +1x expensive high side mosfet driver
    The cost of those items is quite low, so I don't see how that makes it "expensive"?
    in real everyone capacitor have any DC leakagecurrent,
    Normal leakage current is usually negligible, but if you are worried about a pathological case, then the answer to the question is that the amount of current that can seep through the capacitor is -- any amount.
    Last edited by crutschow; 3rd September 2019 at 01:42.
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    Re: NMOSFET On high side?

    Hi,

    You asked about an N channel Mosfet.

    May I ask why you don't want to use a P channel Mosfet?

    Klaus

    Added:
    Or the most simple and cheapest solution: use a logic level N Mosfet with low side switching?
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    Re: NMOSFET On high side?

    The cost of those items is quite low, so I don't see how that makes it "expensive"?
    expensive is High side mosfet driver, IRS10752LTRPBF have price about 0,5$ , roughly 2x more than a solution with three transistors.
    Or the most simple and cheapest solution: use a logic level N Mosfet with low side switching?
    May I ask why you don't want to use a P channel Mosfet?
    It is simple, my load is heating resistance in series with thermocouple (if I'm not mistaken Type N 39uV/C) . when the transistor is OFF is measured voltage of the thermocouple.
    Why not P mosfet?
    I tried three different p mosfet transistors and all have significant Zero Gate Voltage Drain Current and especially this current is significant, changed with the temperature of the transistor (that is, calibration will not help).
    I am not saying that it can not relate to some N MOSFETs, but in IPP057N06N3 we did not notice this problem



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    Re: NMOSFET On high side?

    I tried three different p mosfet transistors and all have significant Zero Gate Voltage Drain Current
    How much current?
    If it's significantly more than the spec sheet value, then I suspect the gate-source voltage is not really zero.
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    Re: NMOSFET On high side?

    Hi,

    It is simple, my load is heating resistance in series with thermocouple
    We can't see this in your circuit.

    Klaus
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