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The circuit I am building uses a capacitive voltage divider. The switches are used to toggle between the two voltages across the divider as can be seen in the schematic below. I tried it in hardware but on the output I just get voltage spikes whenever the switch gate pulse transitions from low to high, the voltage appears to be just noise.

I don't have any problem with the voltages. as the capacitor dividor performs its job well (because of an auxillary active voltage balancing system). However the switches even if they turn on the voltage on the source side of the MOSFET doesn't appear. It just idles at zero with noise and the spikes as mentioned above.

I am using a micro-controller to provide the gate signal which is stepped up to 10V and used to drive the MOSFET gates. The switching frequency is also very low around 100Hz.

So, what is going on here. I would like to know why the circuit is failing to perform its task and how to correct it.

I can't replace the MOSFETS with BJTS because of current constraints enter image description here

The Battery is 24 Volts. So I would expect voltage at the output alternating between around 12 Volts and 24 Volts.

! - The NOT gate is not present in the real design. The actual design uses a single transistor inverter.

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  • \$\begingroup\$ You need a special driver circuit to do this with N-channel MOSFETs. So don't. Use p-channel MOSFETs. Also, please replace the inverter with the actual circuit you use. Your "simplifications" prohibit a proper answer. \$\endgroup\$
    – scorpdaddy
    Commented Apr 23, 2019 at 13:22
  • \$\begingroup\$ What's the point of this hi-side XOR output? \$\endgroup\$
    – D.A.S.
    Commented Apr 23, 2019 at 14:23
  • \$\begingroup\$ @scorpdaddy, can you please give some example of the driver circuit. I'll update the schematic \$\endgroup\$
    – user94963
    Commented Apr 23, 2019 at 15:51
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    \$\begingroup\$ You could google "high side gate driver". A specific example: IRS2004. While possible, it is not a good idea. Use a P-FET. \$\endgroup\$
    – scorpdaddy
    Commented Apr 23, 2019 at 16:04
  • \$\begingroup\$ @scorpdaddy, thank you for the example. I guess its simpler to switch to the P-channel MOSFET, to avoid complexity for a small project \$\endgroup\$
    – user94963
    Commented Apr 23, 2019 at 16:34

1 Answer 1

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Yet another example of trying to use an N-channel MOSFET as a high-side switch. It simply doesn't work.

Use P-channel MOSFETs and pull the gates to ground to turn them on.

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  • \$\begingroup\$ I dont know, why using N-channel MOSFET as a high side switch fails to make the setup work. Is it because of something to do with references?. So, Should I replace the high side MOSFET with a p-channel. Will having both the MOSFETs as p-type present the same problem?. Appreciate your help \$\endgroup\$
    – user94963
    Commented Apr 23, 2019 at 16:19
  • \$\begingroup\$ An N-channel device wired the way you have it functions as a source follower, not a switch. The source terminal cannot rise any higher than the gate voltage minus the threshold voltage. \$\endgroup\$
    – Dave Tweed
    Commented Apr 23, 2019 at 16:44
  • \$\begingroup\$ ok, now I get it. SO it basically prevents the MOSFET from turning ON. I guess I better switch to P-channel to simplify the task at hand. Thank you for your help \$\endgroup\$
    – user94963
    Commented Apr 23, 2019 at 16:53

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