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There is a well-known reverse polarity protection technique using a P-channel MOSFET like so:

schematic

simulate this circuit – Schematic created using CircuitLab

If I use the common BSS84 for this task, it can only handle 130mA continuous current through it. Lets say I want to use the BSS84 to reduce the number of unique parts on my board, but I need to support 200mA continuous current. The battery voltage could be anything from 2.5 to 4.5V. Is there any significant concern with doing the following to retain the circuit protection properties, and increase the current support?

schematic

simulate this circuit

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  • \$\begingroup\$ It will work, though the currents through each device will not be equal due to the variations of RDS(ON). \$\endgroup\$ – Jack Creasey Jan 6 at 1:41
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130mA is abs. max. so you shouldn't use it that high. It will more or less work under ideal conditions.

Note that you can use an N-channel as well if there is no common ground on the battery- just flip the circuit- maybe you have a more appropriate N-channel transistor for this particular application.

Also note that as the battery dies the current may split even less evenly because you will be approaching Vt, so the circuit may only live to work once per set of batteries- many circuits (those with switchmode converters at the input, notably) draw more and more current as the voltage drops so you may have a perfect storm.

On the other hand, the drop across the MOSFET cannot exceed about 1V because of the body diode (the transistor is being used "backwards" in this application).

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  • \$\begingroup\$ Can you elaborate on "...so the circuit may only live to work once per set of batteries"? I don't quite follow. \$\endgroup\$ – vicatcu Jan 6 at 5:05
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    \$\begingroup\$ The current draw and even-ness of sharing may tend to get worse as the battery dies, which could kill one or more of the MOSFETs. Or not, just a cautionary note. \$\endgroup\$ – Spehro Pefhany Jan 6 at 5:07
  • \$\begingroup\$ Thanks, got it, because R_dson is typicallly 1.9 Ohms, but can be as high as 17 ohms... so worst case, there could be a pretty significant imbalance. \$\endgroup\$ – vicatcu Jan 6 at 5:28

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