Controling Mosfet by 3,3V Raspberry Pi GPIO Pin

Question

So I'm currently working on a project where I need an IR camera with two IR Leds attached to it. The camera looks something like this: https://rads.stackoverflow.com/amzn/click/com/B01ICLLOZ8 These LEDS are powered by the camera board an run on 3V with up to 0.3A per LED. Because these LEDS get very hot over time I only want them on power when I really need them. To do this I bought this logic level Mosfet: https://www.arrow.com/en/products/ipb048n15n5lfatma1/infineon-technologies-ag IPB048N15N5LFATMA1 by Infineon Technologies AG which should have a gate threshhold voltage of 3.3V according to the datasheet.

https://www.infineon.com/dgdl/Infineon-IPB048N15N5LF-DS-v02_00-EN.pdf?fileId=5546d4625b3ca4ec015b573da84a026c

But when I connect the wires to the mosfet, the LED won't turn on. The Drain-Gate Voltage is About 2,7V, the Gate-Source Voltage at About 3,3V and the Drain-Source at 0. The boards are wired up like this: (sorry for the bad wiring diagramm, never worked by Fritzing or something like this)

I really suck at stuff like this especially when transistors or mosfets are involved, so any help is appreciated. Thank you!

Answer 1

First, you need to give the LED it's own circuit, like this:

^{simulate this circuit – Schematic created using CircuitLab}

The MOSFET you have chosen, though, will not work for this circuit; from the datasheet:

If you look closely, the threshold voltage (which may be as high as 4.9V) is specified at a drain current of \$255 \mu A \$.

To fully turn this device on, you need 10V \$ V_{gs} \$ drive so it is not really a logic level FET.

There are devices that will do what you need; in manufacturer parametric tables such devices have \$ R_{DS}{(ON)}\$ tabulated against various \$V_{gs}\$ drive levels.

Look for a device that has a 2.5V \$V_{gs}\$ drive capability.

Likely sources are Vishay and Diodes Inc to name but two.

Answer 2

I would be surprised if that FET did work with 3.3V on the gate.

Diagram five, page 7 of the datasheet doesn't show the FET turning on at all for a 3.3V gate, and is only just starting to turn on with 5V on the gate. The 3.3V threshold voltage is the minimum turn on value, so the FET is only just starting to conduct. You need to find a different FET or drive it differently.

The diagram five on page 7 is the output characteristics graph. This is usually a lot more useful than numbers in the tables, as you don't operate FETs at fixed values, the real world is analogue. This graph shows how it operates with different gate voltages depending on your drain-source voltages and currents.

You say that this is a logic level FET, I can't see this claim in the datasheet. Did you choose "logic level" from your FET supplier? Catalogue suppliers are really bad at organising their components, it's something to watch out for.

Answer 3

First, that MOSFET is not a logic level MOSFET. That unit will only just barely start to conduct at 3.3V, if at all. According to the datasheet the threshold voltage is typically 4.1V. You will need to source a proper MOSFET. Don't just look at threshold voltage, that is just where the thing starts to conduct. You need to look at the voltage curves, or at least find one advertised to switch with 3.3V.

Second, you need to use low-side switching for this. Put the MOSFET between the LED's cathode and ground. The reason for this is so you can get the gate high enough over the source to turn on the transistor.

You need to wire it up like this:

^{simulate this circuit – Schematic created using CircuitLab}

You need to ensure that the camera, Pi, and MOSFET all share the same ground.