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this is a power circuit I'm trying to complete: schematic

the button at top turns the curcuit on, supplying VCC to mcu, and the MCU will output a high signal to a pin (marked in pink) which turns the N-MOSFET on, and the P-MOSFET on as well. (right?) when the button is released, the mcu signal still keeps the VCC supply on, through the IO pin. MCU is able to turn itself off (by turning the N-MOSFET off, which turns the P-MOSFET off, thus cutting the VCC supply)

  1. should I place the filters (the caps and choke in the yellow rectangular) before or after the mosfets? (I placed them before mosfets, so that the load isn't capacitive. so that I can get rid of C1 and R1. is it good practice ?)

  2. Do i need to limit the inrush current? (Vout is going into a switching regulator, and from that to an LDO, then to MCU. do these component qualify for being "capacitive load" ?)

  3. if so, how values of C1 and R1 are calculated ? (which are supposed to limit inrush current)

Elementronics
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  • 1) Why do you (think you) need so much filtering? I would remove L1 and the 22u F cap and maybe replace the zener with a simple 1N4001 so that there's only reverse voltage protection. What supply will be used that validates this much filtering? 2) what value will C1 be? I would suggest 100 nF. Why worry so much about inrush current? It depends on what the 12-16V supply is if you need to care about inruch current. The PMOS can handle 10A so I do not see an issue, the inrush current peak should be less than that. – Bimpelrekkie May 10 '21 at 11:44
  • The input supply is not in my control. The end user will supply it. (I thought it's better to be prepared for a noisy 12v). Even with less number of caps and chokes, the question is about the order of components. Vin-filter-mosfet-Vout or Vin-mosfet-filters-vout? Btw, isn't the p-mosfet protecting against reverse voltage by itself? – Elementronics May 10 '21 at 11:59
  • A zener diode without a limit resistor is a dead zener. – Marko Buršič May 10 '21 at 13:05
  • I assumed the PPTC will serve as the current limiting resistor. (have seen many zener fuse combo circuits for overvoltage protection) – Elementronics May 10 '21 at 13:12
  • *isn't the p-mosfet protecting against reverse voltage by itself?* The PMOS has the diode included in its symbol. What happens when the input voltage is negative? – Bimpelrekkie May 10 '21 at 13:27
  • *I thought it's better to be prepared for a noisy 12v* I'm an idiot user and I supply your circuit with a supply that's so poor and noisy that your device doesn't work, the filters aren't "fixing" my poor supply. Who's to blame? Me for using a poor quality supply or you for not making your circuit capable of handling my lousy supply? My point: there's only so much you can do. The danger is that you **over design** which is what you're doing now. That can be OK but realize that you're making things more expensive than they need to be. – Bimpelrekkie May 10 '21 at 13:30
  • The button will bypass the mosfets, and the reverse voltage will damage the circuit. Using a generic diode (or preferably another p-mosfet) will guard against Reverse Polarity issues. I'll add the second pmos. My question is about the order of components. – Elementronics May 10 '21 at 13:33
  • @Bimpelrekie this circuit will be used as an unlocker. It's safety (electrically and environmentally) is vital or an undoable situation will happen. That's why I'm kinda deliberately over-protecting it and mostly think of worst case scenarios. I'll add a 220v protection later for when the user supplies directly from mains. i.e the price and BOM is not important here – Elementronics May 10 '21 at 13:45

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