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I am using several MOSFETs (STP16NF06 datasheet here) controlled by a shift register (74HC595 datasheet here) which is controlled by a microcontoller.

The spec sheet says the gate threshold voltage is between 2 V and 4 V. When controlling directly via the MCU, the voltage on the pin is 3.3 V.

However, I have switched to a shift register (to save pins) which is connected to the same 5 V source that the microcontroller uses for power. That means the output pins of the shift register are 5 V, which is 1 V higher than the maximum of the MOSFET gate.

My multimeter reads 5 V across the shift register Vcc and GND, and 5 V at the shift register pin out/MOSFET gate pin.

Is this something I should correct?

I am going to run the whole thing off of a 24 V power supply, so I already plan to use a boost/buck converter that will convert it to 5 V for the microcontroller and shift registers, while the 24 V will be used for the solenoid valves. I don't really want to add another boost/buck converter just to power the shift registers; while they are not large, they still take a fair bit of space (5 cm x 2.5 cm). (The shift register works with 2V minimum to 6 V maximum and 5 V typical.)

The MOSFETs seem to be running fine at the moment, powering a simple LEDs off the same 5 V power source. But once I wired it up, I realised the shift register was outputting 5 V and not the 3.3 V of the microcontroller. Do I need to worry about being over the maximum voltage? If I do, is there a better way to get the 4 V and 5 V other than a boost/buck converter?

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    \$\begingroup\$ It's not usually very important. What matters is the gate voltage at which RDS(on) is measured or guaranteed. Which would be ... 10V for this FET. Meet that and performance is guaranteed : ignore it and performance is a matter of luck. \$\endgroup\$
    – user16324
    Commented Nov 12, 2020 at 13:40
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    \$\begingroup\$ Ignore Vgsth if you are using the MOSFET as a switch. \$\endgroup\$
    – DKNguyen
    Commented Nov 12, 2020 at 16:37
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    \$\begingroup\$ the important thing here, which may not be immediately obvious from the answers, is: you were using your mosfet wrong before (at 3.3V) and you are still running them dangerously close to the threshold (at 5V). as brian says, if you really want this fet, you should drive it at >10V and <20V if you really want to be safe, or choose another mosfet. \$\endgroup\$
    – BeB00
    Commented Nov 12, 2020 at 21:57

2 Answers 2

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The Gate Threshold Voltage VGS(th) refers to the voltage below which the FET stops conducting almost completely, defined by some manufacturers as the gate voltage at which the drain current crosses the threshold of 250 μA.

It is not a maximum gate voltage (and nor is it the minimum gate voltage at which the FET can be used as a switch for the larger amounts of current you probably need).

There are graphs in the datasheet in which you can look up what the FET does at various Vgs (the transfer characteristics, figure 6 in the datasheet), which is usually more interesting for designing. Figure 5 shows the I/V characteristics for various Vgs.

For further reading: there is a Vishay Application Note here that explains it quite well (no affiliation).

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    \$\begingroup\$ I had a very basic "youtube demo" understanding of how they worked. These are great answers and comments which answered the several other questions I had about them. Now off to experiment on them some more. \$\endgroup\$ Commented Nov 15, 2020 at 23:54
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    \$\begingroup\$ "the voltage below which the FET stops conducting completely" This seems a contradiction, because 250uA is hardly "not conducting". Subthreshold operating mode is usable if low currents are OK. And so on. Sometimes you have a light load and a fraction of a mA does the job - but then, sometimes that's not the case. \$\endgroup\$ Commented Apr 16, 2022 at 6:06
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The Vgs(th) aren't maximum, it's minimum actually as already noted in other answer.

Maximum Vgs are stated in "Absolute maximum ratings" table of the datasheet. Which is rated at 20V for this particular MOSFET. Don't exceed that and you'll be fine.

Moreover, gate voltage close to Vgs(th) aren't good because it will have higher Rds. This value means that the transistor will "barely open". Check figure 7 of the datasheet for details. At 5V between gate and source it can conduct around 5A drain-source of 6V and above. And 3.3V are even not on the graph, lowest are 4V.

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