MOSFET high side switching vs low side | battery

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Hi, might be a trivial question but putting this through your expertise would help a newbie!

I am using a deepsleeping ATtiny85 on standby for external interrupt to wake up Arduino Uno and perform certain tasks and go back to sleep. To power the Uno upon interrupt, I plan on using a MOSFET for now (later might replace with a relay but might stick with MOSFET for compactness) to switch on/off. I’m personally a fan of grounding everything so was wondering if there’s a downside of using high gate switching using a P-MOSFET.

Rather, would you say in such cases that high side switching is better than low side switching?

PC

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Hi Prithul
I think in this case a high side switch using a P channel Mosfet device would be the way to go. If you use a low side switch you will have a ground conflict as all UNO grounds will be above the “real” ground due to the small voltage drop across the N Mosfet. Having several grounds at different potentials will cause no end of problems overall.
Cheers Bob

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Hi Prithul,

I have also gone down this path - albiet with WS2812 strip’s and poor grounding.

I came across this product in Core’s new product feed: Pololu Isolated Solid State Relay/Switch, SPST, 30V, 11A | Buy in Australia | POLOLU-5421 | Core Electronics
And have been meaning to deploy something similar to your project for ESP32’s, much like Core’s Nano power timer, but with a variable (controllable) sleep.

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Hi Liam
That is a pretty useful device. Could be ideal for this project. Very small too. If you dropped it at my place it would be lost. Still needs to be high side switch though. The ground problems don’t go away.
Cheers Bob

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Hi Bob,

Have been meaning to get my hands on one since I saw it come out.
Yeah didn’t hone in on the measurements until you mentioned, quite a bit smaller than Cores current FET offering.

In the description it does mention that this product is capable of high-side switching,

(for example, this board can be used for high-side or low-side switching). The board can be used as a replacement for a mechanical relay in many situations.

I’ll pick one up and report back with my findings when I have a moment.

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Thanks for sharing @Liam120347! Pretty exciting, especially 'cause it costs almost as much as a MOSFET.

Ordering a few to play with

PC

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Hi @Robert93820 and @Liam120347,

Since my application has very limited space, I will have to use a conventional 3 legged MOSFET to make the high-side gate. The application has 2 switches actually- one for a 5V sensor and another for a 12V illuminator. I have IRF4905PBF that’s working fine for the 5V sensor when the 3V HIGH pulse is provided to turn off, but unfortunately not for the 12V switch.

I suppose I’ll have to buy a different MOSFET like this one that can trigger gate with lower voltage than that on source. Do you have a suggestion on a better option by any chance?

Regards,
PC

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Hi Prithul

If you are going to just switch the gate to 3.3V you are a bit lucky to switch the 5V one off.
Th turn off the gate has to be returned to source potential. At the moment you are leaving the gate about 2V negative WRT to source which could leave the Mosfet ON. You just got a bit lucky.

With the 12V switch the gate will never get anywhere near the source. About -9V actually which will leave the Mosfet well and truly ON.

Normally you would use a transistor to switch the gate to ground to turn the Mosfet ON. About 20kΩ source to gate then about 1kΩ gate to transistor collector then transistor emitter to ground. Switch the transistor ON using a suitable base series resistor to switch the Mosfet ON.

Transistor can be a general purpose NPN.

By the time you get all this put together you will not save much space

Cheers Bob
PS: A screen clip from the Data sheet. You really did get lucky.
image
In reality it might not have turned completely off.

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Hi @Prithul271176,

This is a good example of the circuit configuration that @Robert93820 is talking about above:

image
image608×396 10.8 KB

Note that the IRF9530 can be replaced with either the IRF490 or IRF5305. The 5V can be replaced with 12V also. Resistors can be changed depending on your circuit also.

This configuration is best practice because it safely and reliably enables low-voltage microcontrollers (like an ATTiny85) to control high-side P-channel MOSFETs, ensuring full switch-on without exposing GPIOs to high voltages.

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Thanks a ton @Robert93820 and @Ryan, that’s really helpful. I’ve been following posts and tutorials on the same as well this week and I guess I’ll end up using a lot of the space on my PCB going this way. Perhaps, the best way would be to use Pololu Isolated Solid State Relay/Switch, SPST, 30V, 11A | Buy in Australia | POLOLU-5421 | Core Electronics as recommended earlier.

PC

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Hi All
You would probably have to leave R3 out. Even at a load of 10mA this would drop 1.5V which in most cases would be unacceptable. 10Ω would probably be enough. In the event of a short on the load side this would fuse and fail thus isolating the faulty load.
Cheers Bob

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Hi Prithul
Yes probably would be easiest. Not as much experience as rolling your own though. But unless you have the facility and expertise to have a PCB made an exercise like this could end up being fairly large physically.
Cheers Bob

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