24v driven MosFET


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  1. #1

    Default 24v driven MosFET

    Hi Everyone

    I've got a samll problem of driving a MosFET (P or N) in a 24v DC environment.
    Before this day, I happily worked in a 12v DC environment (cars). But I have come across several trucks (24v DC) and now industrial automation systems that are 24v DC religiously.
    So three things to modify (for a start):
    * 5v reg isn't a problem as a lot of switch-mode regulators will take 7 - 40V.
    * MosFET drivers aren't a problem as MICROCHIP produces FET drives with 4.5 to 30v supply.
    * FETs are a bit of a problem because of their Vgs limit. The highest I could find is 20V max (other characteristics being realistic).

    Can you please share your experience and ideas in working with 24V DC

    Thanks in advance.

  2. #2
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    Default Re: 24v driven MosFET

    P or N?

    It is different thing.

    About the Microchip drivers, which is the one that is up to 30 Volts?n

    Ioannis

  3. #3

    Default Re: 24v driven MosFET

    TC4431
    TC4432
    P and N in one application (to be more general) say for an H bridge

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    Default Re: 24v driven MosFET

    I will give you an idea on P driver because I had a similar problem. Needed to drive a P-Mosfet from a PIC and the load was operating on 24V battery (up to 28,5 volts fully charged).

    I ended with a small mosfet from PIC output driving a resistive divider to the P-Mosfet,because of the Vgs as you noted.

    Name:  P-Mosfet driver.png
Views: 10975
Size:  4.9 KB

    Ioannis

  5. #5

    Default Re: 24v driven MosFET

    Thanks a lot Ioannis for waking me up. I've completely forgotten that often Mosfets do not need much current to switch them on/off.Name:  STP60  12v 7_52A load.jpg
Views: 5183
Size:  212.7 KB Attached is the oscillogram showing some delay on switch_OFF. The FET is driven by PIC through 10k and 330R respectively.
    I've got a question then. In what situation would one need a 3A FET driver. Considering 5/330 = 0.015A does a pretty good job as seen above.

  6. #6
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    Default Re: 24v driven MosFET

    I cannot understand how is your circuit. Please attach it to see what you have done.

    About the driver FET, the one I select was based on price, availability, size and speed.

    Remember that when you try to switch ON the Power mosfet, you try to bring low the gate. The gate is at that moment a capacitor charged at 24 volts and discharged through the small FET and the 470 or 330 resistor.

    Also remember that the resistor on my schematic should be at least 1/2 watt because when the small MOSFET is ON, they draw a current of at least 16mA and this leads to power loss of 0.391 Watts!

    Ioannis

  7. #7

    Default Re: 24v driven MosFET

    Name:  STP60NFL.jpg
Views: 7498
Size:  650.5 KBHere it is. Sorry I didn't mention it is an N FET. Yellow line is 2V/div and is the drain against the earth, purple line is 1V/div and is the gate against the earth. Zero level is where the little purple arrow on the left is (one div of the bottom). There are two oscillograms merged together: left was obtained with the 10k resistor to the gate, right one with 330R to the gate. Both oscillograms are taken on the OFF. The left graph (yellow line) shows that it takes 70 uS to drain the gate charge, when the right one (330R to the gate) only around 10uS.
    Long story cut short - even 330R (very high compare to the internal impedance of a TC4424 3A peak 0.6A realistic continuous) is enough to drive the FET at quite a considerable load (the switching time is affected by the load current), if 10uS is acceptable switching time. Just to compare the time to something a 30A relay takes 5mS to switch ON and 2mS to switch OFF.
    I think your schematic shows that it is possible to avoid using high current driver and simplify the drive circuit to suit the 24v environment.

  8. #8
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    Default Re: 24v driven MosFET

    Quote Originally Posted by netstranger.nz View Post
    ... is enough to drive the FET at quite a considerable load ...
    That's why I love Mosfets!

    Ioannis

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