MOSFET switching voltage Vgs

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MOSFET switching voltage Vgs [Copy link]

 

As the title says, which parameter in the MOSFET manual can tell me the Vgs voltage when it is fully turned on as a switch?

My colleague told me that by default, 12V can fully open most of the NMOS. If it is lower than 12V, it is a bit dangerous. The MOS will not be fully opened, which will cause the Rds resistance to be too large and the MOS tube to heat up. So the circuit diagrams he designs now basically open the MOS tube at 12V. For example, as shown in the figure below

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The MOS tube is fully turned on to see if the ID is the maximum value.

What my colleague said makes sense.

But you need to look at the manual data of the MOS tube.

Take the low-power MOS tube 2N7002 in the picture of the OP as an example.

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There are also many manufacturers that make MOS tubes 2n7002 low power.

Just find an NXP

Look at its internal resistance parameters

Rds

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Rdson internal resistance is the internal resistance between D and S when the MOS tube is turned on, which will produce a voltage drop. According to Ohm's law, the smaller the better.
When the current Id between D and S is the largest, it is fully turned on.
The screenshot is the 2N7002 manual.
In the figure, Vgs=10V, the current is the largest, it is fully turned on, Rds is 5 ohms, the current ID is 500 mA, and the voltage drop is 2.5V.

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However, when Vgs=4.5V, Id=75mA, it should not be fully turned on. At this time, the internal resistance Rds=5.3 ohms. Because it is not fully turned on, the voltage drop Vds=0.4 is the minimum.

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That is, when using MOS tubes, the size of the ID overcurrent is not considered, that is, in the signal control circuit, only the conduction voltage drop is considered. It does not necessarily require a very high Vgs. It depends on your own circuit. When there is a small conduction voltage drop, the signal can be turned on. Among low-power MOS tubes, the 2N7002 is often used for signal switch control.

For high-power MOS tubes in power supply circuits, when the maximum ID is required and the MOS tube is to be fully turned on, it is indeed better to use a larger Vgs than the MOS tube manual.

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The first post said that it is higher than 12V. According to the NXP 2N7002 manual, there is no need to use 12V.

maychang

The above is part of the IRF460 tube manual. Note that the test condition of the on-state voltage in the red box is a drain current of 250uA. 250uA is a very small current, so the on-state voltage is the voltage at which the MOS tube starts to conduct.

Another parameter in the red box is transconductance. This parameter indicates how much the drain current can change with a unit gate voltage change under certain conditions. The minimum transconductance of this model of MOS tube is 13S (Siemens), that is, a 1V change in gate voltage can cause a 13A change in drain current.

Then, to make the drain current reach 60A (the pulse drain current of this type of MOS tube is allowed to be 84A), the gate voltage should reach 60A/13S+4V=8.6V, where 4V is the conduction voltage. For the sake of reliability, some margin is added, and the gate voltage should reach about 10V.

maychang

The two lines above the red box are tested under the gate voltage of 10V. This also means that the gate voltage of this model should reach 10V when it is "fully turned on".

Your colleague said that to achieve 12V, it is nothing more than making the margin larger.

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qwqwqw2088 Published on 2020-11-11 22:27 That is, when using MOS tubes, the overcurrent of ID is not considered, that is, in the signal control circuit, only the conduction voltage drop is considered, and no...

I understand. Thank you for the detailed explanation. Vgs should be determined according to the actual usage to get the appropriate Rds and Id.

maychang

Of course, what is said on the 9th floor is only for a certain type of power MOS tube. For other types of power MOS tubes, it depends on the specific parameters of the MOS tube.

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qwqwqw2088 posted on 2020-11-11 22:32 The first floor said that it is higher than 12V. According to the manual of NXP's 2N7002, there is no need to use 12V.

As shown in the figure below, the curves of Vgs, Rds, and Id of 2N7002. There is no necessary connection between these three, right? In addition, the maximum continuous current of Id in the manual of 2N7002 is 0.1A, and the test curves of these two figures are far more than 0.1A. So how do you see this?

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maychang published on 2020-11-12 08:47 The above is part of the IRF460 tube manual. Note that the test condition of the on-state voltage in the red box is a drain current of 250uA. 250uA is a very small current, so...

Thank you teacher, I learned the parameter of transconductance

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maychang published on 2020-11-12 08:51 The two lines above the red box, the test condition is that the gate voltage is 10V. This also means that when this model of tube is "fully turned on", the gate voltage should be...

Teacher, the gfs of 2N7002 is 80ms. The maximum continuous Id is 0.115A, and Vgs-th is 2.5V. According to this calculation, 0.115A/0.08S+2.5V=3.75V. It does not require 10V to reach the maximum current. But the curve in the manual does not show this conclusion.

maychang

Pingpiaoliu published on 2020-11-12 11:28 Teacher, the gfs of 2N7002 is 80ms. The maximum continuous Id is 0.115A, and Vgs-th is 2.5V. According to this calculation, 0.115A/0.08S+2.5V=3.75V ...

"The maximum continuous Id is 0.115A"

It cannot be calculated using the continuous current. The drain current in pulse form can be much higher than the continuous current, but of course, it can only exist for a very short time.

maychang

maychang posted on 2020-11-12 11:57 "The maximum continuous Id is 0.115A" It cannot be calculated using the continuous current. The drain current in pulse form can far exceed the continuous current...

In addition, different types of tubes have different parameters. If you are using 2N7002, then of course you have to calculate according to the manual of that model.

Also, transconductance is not a constant, it varies with drain current, etc.

Specifically for this type of tube, from the curve in the figure after your 12th floor, in order to make the on-resistance small enough, the gate voltage also needs to reach 6V or even 9V.

maychang

Pingpiaoliu published on 2020-11-12 11:07 qwqwqw2088 published on 2020-11-11 22:32 The first floor said that it is higher than 12V. According to the manual of NXP's 2N7002, there is no need to use 12V. If...

There is a mark in the upper right corner of both figures: Pulsed, indicating that the test was conducted under pulse conditions, so the current can be far greater than 0.1A.