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I'd like to build a high side N channel MOSFET as I want to make a BLDC ESC. I need one on the high side and one on the low side so I can change the polarity.

I thought N channel MOSFETs were the best choice so I bought some IFRB3207.

The low side switch works fine but this schematic for the high side doesn't work:

schematic of bootstrap circuit

The load remains on even when the gate is at ground.

I then tested this without the capacitors or diodes here:

enter image description here

For some reason, the LED is on even though the gate is at gnd and the source is at +5V. For an N channel MOSFET, I thought the gate had to be 5V above the source. In this case, it is 5V below.

Here's the circuit on a breadboard. The orange wire was connected to ground and the blue to +5V. They were disconnected here so the LED didn't blind the camera:

enter image description here

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  • \$\begingroup\$ "For some reason, the LED is on even though the gate is at gnd and the source is at +5V." – In your schematic, the gate is at GND and the drain is at 5 V. The source is connected to the load, so it should be at GND, meaning that Vgs is 0 V here. \$\endgroup\$ Commented Jan 3 at 19:10

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You have your source and drain swapped. If we look at the datasheet accessed here

enter image description here

You can see the swap. The MOSFET will always conduct with this wiring due to the internal diode. (thanks Chris)

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    \$\begingroup\$ "The MOSFET will be always on with this wiring" ...due to the internal body diode always being forward biased. \$\endgroup\$ Commented Jan 3 at 14:07
  • \$\begingroup\$ Thanks, that fixed the issue but it does seem counter intuitive to have the power supply plugged into the drain and the output into the source. \$\endgroup\$
    – 8MB_SSD
    Commented Jan 3 at 17:41
  • \$\begingroup\$ It is confusing, but the names are based on the source and drain of electrons rather than conventional current. \$\endgroup\$
    – Finbarr
    Commented Jan 3 at 19:53

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