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What are the key features or rather benefits/downsides of these discrete switching circuits in comparison:

  1. Low-side NMOS
  2. High-side NMOS
  3. Low-side PMOS
  4. High-side PMOS
  5. Boot-strap NMOS circuit

The basic functionality of those circuits is rather the same. #3 doesn't make that much sense from my understanding, as the gate-source voltage must be greater than the threshold voltage and the load voltage to close the DS-path in the NMOS. Vice-versa for the #4? For #5 only possible, if th load can be PWM controlled.

Besides that, what are the other basic benefits/downsides, i.e switching speed (rise and fall time), safety ("high-side always grounded"), cost and resistance (NMOS vs PMOS), etc.

It would be great to have an inside on to why sometimes circuits are built/preferred in one way or the other.

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  • \$\begingroup\$ What does "low-end" actually mean? Normally it means cheaper or poorer quality. Is this what you mean? \$\endgroup\$
    – Andy aka
    Commented Feb 28, 2023 at 15:10
  • \$\begingroup\$ Sorry, question is not that well described. High-/Low-End in terms of controlling and switching VCC vs. GND is meant. Not in terms of quality. :) \$\endgroup\$
    – Dakalaom
    Commented Feb 28, 2023 at 15:12
  • 2
    \$\begingroup\$ electronics.stackexchange.com/questions/455693/… \$\endgroup\$
    – DKNguyen
    Commented Feb 28, 2023 at 15:41
  • 2
    \$\begingroup\$ electronics.stackexchange.com/questions/424193/… \$\endgroup\$
    – DKNguyen
    Commented Feb 28, 2023 at 15:41
  • 1
    \$\begingroup\$ Is this homework? \$\endgroup\$
    – winny
    Commented Feb 28, 2023 at 16:03

1 Answer 1

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The term is low-side and high-side, not end.

No one does low side PMOS because its the worst of all worlds. NMOS is more efficient than PMOS, and while low-side is more convenient drive for NMOS, NMOS always needs a floating drive for high-side.

PMOS only has more convenient drive on the high side if pulling the gate to gnd does not exceed the max Vgs. At higher voltages you exceed the max Vgs pulling the gate to ground so you need more complex drive circuits and if you need those then you might as well just use an NMOS.

Boot strap circuits use a floating capacitor and therefore can't do 100% duty cycle without extra circuitry like charge pumps. Without a charge pump, they need some other means of refreshing the cap which is usually done by other transistors switching frequently enough during normal bridge operations. Or the entire bootstrap cap can be replaced with an isolated, floating supply. PMOS on high-side does not have this problem.

PMOS on low side has the inefficiency of PMOS as well as all the drive issues of high-side NMOS. No one does this.

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  • \$\begingroup\$ Edited. Mixed the two words up. Thanks for the correction and the explanation. \$\endgroup\$
    – Dakalaom
    Commented Feb 28, 2023 at 16:02

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