| Electronics > Beginners |
| P - Channel Switching |
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| TheDood:
I'm trying to understand P - Channel switching. Is it possible to use a P - Channel enhancement FET in the middle of a cct? If I apply 5V to the Base of the NPN transistor, will the load before the FET (R1) act on how the FET is switched? How about after (R2)? With N - Channel FETs, if there's a load after the FET, the Gate V will have to be greater than the total cct VCC (VCC + Vgs). I'm not sure if I'm comprehending P - Channel, it seems operation is less dependent on where the FET is placed? As long as the P - Channel FET Gate is tied to GND, the P - Channel FET should be turned on? This is just a mock schematic to gain a principle understanding. |
| Dave:
--- Quote from: TheDood on November 22, 2019, 09:15:22 am ---I'm not sure if I'm comprehending P - Channel, it seems operation is less dependent on where the FET is placed? As long as the P - Channel FET Gate is tied to GND, the P - Channel FET should be turned on? --- End quote --- No. It's the exact same story as with an N-channel transistor, except the voltages are reversed. You need to establish a high enough gate-source voltage to turn the transistor on (the voltage being negative with a P-channel transistor). If you're putting the load between the power rail and the transistor, you're effectively reducing the VGS when the transistor begins to conduct, partially closing it. Your schematic is drawn incorrectly, by the way. The arrow inside the transistor symbol should point towards the source terminal, not drain. |
| Peabody:
As Dave said, setting the gate to ground will turn the P-channel on ONLY if the voltage at the source pin is high enough to produce a Vgs high enough to turn on the mosfet. In your drawing, having the load at R1 would reduce the source voltage, which would tend to shut down the mosfet even if the gate is at ground. Typically, P-channels are used to switch the load's connection to the positive rail, and N-channels are used to switch the load's connection to ground. That way the P-channel's source will always be at the positive rail, and the N-channel's source will always be at ground. |
| TheDood:
Thanks guys, attached is the reference schematic I was modeling after (except I added a 2nd load), taken from here: https://youtu.be/vRiF_GghBY8 Is there a way to switch FETs placed between loads without using charge pumps (are these what charge pumps are used for)? |
| TheDood:
Could you switch string B, utilizing a different component or IC, in place of FET M2, so that you didn't need to use a charge pump to offset the voltage (am I comprehending charge pump use correctly)? I appreciate the help guys, thanks. |
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