Mosfet V_GS question.

[Rob Northen]

I've made a another little project, including a LCD. This time, I thought it could be interesting to control the back light with PWM. I'ts done with a PWM signal from a Atmega 328 MCU, which goes to a IRFZ46N mosfet. It is working like a charm.

My question is, IRFZ46N V_GS (th) voltage is defined 2-4v. Its a power Mosfet. "Logic" Mosfet's usuallt has V_ GS (th) 1-2v.
According to other forums, it should not be possible to drive a IRFZ46N from logic level, but it seems to work. The logic level is 5V.

What do miss ?

[Th3_uN1Qu3]

Re: Mosfet V_GS question.

5v on a 4v Vgs(th) mosfet is a bit on the edge, but your backlight presents a very low load, so it doesn't matter much. A few mW extra dissipated in the fet... Unless your device runs on batteries i wouldn't be concerned. When you get to switching high currents, that's where the level of the gate drive comes into play.

[Rob Northen]

Re: Mosfet V_GS question.

No no, it runs from mains.
What is the recommended voltage for Vgs(th) ?

[PeteS in CA]

Re: Mosfet V_GS question.

Not clear what is puzzling you. A standard MOSFET will turn on at ~4V, but at 5V G-S it may or may not be in saturation. A "Logic-Level" MOSFET will be in saturation at 5V G-S. If the gate drive signal from your MCU is 5V, a standard MOSFET will be on, and the Drain current in your circuit may be low enough that not being in saturation (if that is the case) isn't causing a problem. That your switch MOSFET is on the primary (mains) side of the circuit doesn't mean much, unless it gets ~300VDC on the Gate, in which case you'll have a brief, bright, noisy and bad-smelling event.

[Rob Northen]

Re: Mosfet V_GS question.

Not clear what is puzzling you. A standard MOSFET will turn on at ~4V, but at 5V G-S it may or may not be in saturation. A "Logic-Level" MOSFET will be in saturation at 5V G-S. If the gate drive signal from your MCU is 5V, a standard MOSFET will be on, and the Drain current in your circuit may be low enough that not being in saturation (if that is the case) isn't causing a problem. That your switch MOSFET is on the primary (mains) side of the circuit doesn't mean much, unless it gets ~300VDC on the Gate, in which case you'll have a brief, bright, noisy and bad-smelling event.

What is puzzeling is these line from the IRF46N datasheet:

-------------------------------Min Max Units
VGS(th) Gate Threshold Voltage 2.0 ––– 4.0 V

I read it as max 4.0v. But I've been told that is not correct, and I read it wrong.

[budm]

Re: Mosfet V_GS question.

You may want to read this to understand the Vgs and Vgs (th).Page 22 of Fairchild app notes:
Threshold Voltage (VGS(th))
This is the minimum gate bias which enables the formation of the channel between the source and
the drain. The drain current increases in proportion to (VGS–VGS(th))2 in the saturation region.

[PeteS in CA]

Re: Mosfet V_GS question.

You may have spotted the point of confusion, bud. The V(GS) th specifies the range in which the part is guaranteed to start to turn on. The absolute maximum G-S voltage is 20V (Datasheet front page). Also, a MOSFET is not a perfect switch, either fully on or fully off. There is a voltage range between its turn-on threshold and the gate voltage at which it goes into saturation. In that range the MOSFET can be used as an amplifier rather than a switch.

[budm]

Re: Mosfet V_GS question.

Agree, It confuses a lot of people when they see the MIN and Max numbers of Vgs(th) without looking at the Vgs spec.

[Rob Northen]

Re: Mosfet V_GS question.

Thanks to both of you, it made more clear. I did notice the Vgs +-20v, but VGSth
comfused me...

[tom66]

Re: Mosfet V_GS question.

In a robot competition, we used a power MOSFET to control a pump. The IO output gave 3.3V but this was fine. The MOSFET dropped about 0.5V at 500mA so this is about 1 ohm rds(on). Was just a prototype .