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schematic

simulate this circuit – Schematic created using CircuitLab

I want to use two MOSFETs to supply power to two different circuits. However only one of these MOSFETs can be on at a time. One FET for the first circuit and the other FET for the second circuit.

The circuit needs to be analog, so no micro-controllers. I was thinking of using a 555 timer that outputs a square PWM at around 10Hz. Then feed this into a half-bridge gate driver IC which then feeds the gate of the two FETs. So when the PWM is high (5V) one of the FETs turns on, and when the PWM is low (0V) the other FET turns on while the first FET that was on, now turns off. This process then repeats indefinitely. I'm not sure if this will work with or if someone has a better idea in order to do this ...?

The load that the FETs will be switching is powered using 24V DC and draws about 3A

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    \$\begingroup\$ An inverter gate could be your new best friend \$\endgroup\$
    – Andy aka
    Feb 21 at 19:54
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    \$\begingroup\$ You can always feed pwm into RC and then have it as an input to 2 comparators set to different trigger voltages. RC will slow down rise/fall, one comparator can trigger at 1/3 PWM_high and lower, another at 2/3 PWM_high and higher \$\endgroup\$
    – Ilya
    Feb 21 at 20:02
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    \$\begingroup\$ Not all H-bridge drivers include dead-time; if you need this, ensure yours includes it by reading it's datasheet. \$\endgroup\$
    – rdtsc
    Feb 21 at 20:04
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    \$\begingroup\$ It all depends on your implementation. I don't see a reason for it not to work. You can even put a potentiometer instead of RC and tune rise/fall time \$\endgroup\$
    – Ilya
    Feb 21 at 20:24
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    \$\begingroup\$ @JoeyB yes :) (both) \$\endgroup\$
    – Ilya
    Feb 21 at 20:27

1 Answer 1

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You can feed a PWM signal into RC connected to two comparators, which will trigger at 1/3 V and lower and at 2/3 V and higher, which will leave some dead zone (of course you can tune the values/levels). Also, you can tune RC as you wish as well.

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  • \$\begingroup\$ In that case comparators could need hysteresis. Without that, with long RC constatns (slow rise / fall times) you will be facing multiple transitions betwen low and high state at output around threshold voltage at the input. Depending on the circuit, that could create some issues \$\endgroup\$
    – klokaj
    Feb 21 at 21:00
  • \$\begingroup\$ why would comparators need hysteresis if their trigger points are 5V apart? \$\endgroup\$
    – Ilya
    Feb 21 at 21:14
  • \$\begingroup\$ I am not sure if in case of that particular system comparators needs hysteresis, but I think that it is something worth being consider. Without hysteresis we could end up with switching between '0' and '1' for voltages near the threshold (because noise and long RC constant). And that could violate driver or load specification. \$\endgroup\$
    – klokaj
    Feb 21 at 21:47
  • \$\begingroup\$ There are two thresholds. For 12V supply one mosfet can turn on if RC is at 2V or less, another one turns on with RC at 10V or more. You're really considering some edge case. Unless there is some reason something edgy could happen, I don't see it as any significant risk \$\endgroup\$
    – Ilya
    Feb 21 at 21:53
  • \$\begingroup\$ you mean that multiple transitions for long RC are edge case or that they will create an issue? \$\endgroup\$
    – klokaj
    Feb 21 at 22:05

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