# I need a tutorial on MOSFET gate resistor selection

Discussion in 'General Electronics Discussion' started by bonedoc, Jan 17, 2012.

1. ### bonedoc

122
0
Dec 21, 2011
I am really trying to understand why certain things are necessary when choosing the proper mosfet. But, things are a little confusing. Specifically, resistor and protection diode size. Here are two different situations that I am looking at, and my confusion, if anyone can tutor me on it:

case 1

http://skory.gylcomp.hu/alkatresz/IRF840.pdf

-Say I use an IRF840 to act as a slow switch, maybe once a minute, to discharge 100V/5A. The datasheet says that turn on voltage is 2-4V. If my microchip pin has a pin voltage of 5V, should this properly switch the mosfet? If I raise the voltage to, say 20V, will it discharge the load better? Does more gate current cause a better discharge? As far as protection of at gate with a zenner that is revered biased from gate to source....how does one choose the value? If I am switching with a pic pin at 5V, do I need to do a 5V zenner? I understand that a 5V zenner may start conducting at 3V...wont that interrupt my switching?

case 2

http://www.nteinc.com/specs/2900to2999/pdf/nte2987.pdf

-I have the same questions on a logic level mosfet, like the NTE2987. Say the the voltage being switched is 30V at 200kHz...the data sheet says there is a Gate-Source voltage of +-15V. Wont this kill the mosfet? Say I drive one with a 5V microchip pin. Is a gate resistor needed since it is logic level? How to I pick a protection zenner?

I really appreciate some explanations. Im trying to construct a flow chart in my head. I have read about this stuff for hours and hours, but need to ask certain questions to fill in the blanks. Im close to getting it....but not there yet.

2. ### duke37

5,364
769
Jan 9, 2011
Case 1 IRF840
The maximum source-gate voltage is +- 20V and the voltage necessary to turn it fully on is more than 6V. So choose a Zener of about 12V. The Zener is not necessary if you are sure that the gate is not overdriven.

Case 2 NTE2987
Similar to case1. A driving voltage of 10V would be ideal but 5V would be adequate for moderate currents. See the data.

Gate resistors are included to suppress oscillation during the switching time. The need for these will be very dependent on the circuit and layout.

3. ### bonedoc

122
0
Dec 21, 2011
Well, I am stupid ;-) I forgot that source = ground. I doubt that the voltage will ever exceed 5V if driven directly by a microchip.

-In the case of a motor or an inductive load, does the zener still go from source to gate? Or, does something need placed on the drain?

-In the case of logic level mosfets, I dont believe a microchip ever puts out 10V, so why is the optimal voltage on the gate when most situations will never see over 5V?

-Lets say I need to pull 5A on the IRF840. According to the charts, a 5V gate signal will pull about 2.5A. So, that wont happen. Let say my system has a 12V rail and I want to use an optocoupler to switch the IRF840. Do I just go ahead and use 12V at the gate? This is not close to the 20V GS voltage.

-In the case of the logical level, if I am using 12V via an optocoupler, is the mosfet safe? It has a GS voltage of +-15V. I know 12V is close, but if I am pulsing it at 200kHz and a 30% duty, is it really even seeing the 12V?

4. ### duke37

5,364
769
Jan 9, 2011
It is good practice to put a diode across an inductive load to suppress high inductive pulses.

Logic level
The optimal voltage may be 10V but 5V should be good enough.

12V rail
Go ahead and use 12V. Do not under any circumstances use over 20V.

Pulsing
The gate may not see 12V if not driven by a low impedance supply to charge and discharge the gate input capacitance. At 200kHz you will need a low impedance to turn the fet on and off rapidly to keep fet dissipation low.

5. ### BobK

7,682
1,686
Jan 5, 2010
Case 1:

The gate threshold of a MOSFET is NOT the voltage you need to apply in order to saturate it. If you look at the datasheet, the threshold of 2-4V that you quote is the voltage at which the MOSFET will pass just 250uA, not much help when you want it to carry 5A. If you look at the figure 5 in the datasheet, you will see that with a gate voltage of 5V it is barely able to pass 2A. So you cannot drive the gate directly from the PIC. The full current is achieved with a gate voltage of 10V, which is typical.

You do not need a zener to protect the gate unless you are in danger of placing more than the maximum Vgs on it. In this case 20V on it. If you were driving it from a PIC output (which you cannot) it would not be necessary since the PIC could never output 20V without letting the smoke out.

case 2:

For this one, the full current is still only achieved at a gate votage of 10V, but if you look at the spec for Rds(on) it is characterized at 5V on the gate and 10A. So this one can pass 10A with a gate voltage of 5.

Gate current is irrelevant to how much current a MOSFET will pass. Unlike a BJT, a MOSFET is a voltage driven device, it is the voltage on the gate that determines the resistance of the D-S channel.

On the other hand, the gate current determines how fast it will turn on. Why? The gate is essentially a capacitor. So when you apply a voltage it starts charging, just like a capacitor, and the more current you pump in, the faster it charges to the desired gate voltage. Once it is charged, it pulls practically no current. So to switch a MOSFET quickly on and off, the gate resitance must be low. A power mosfet may prove too much of a capacitive load for a PIC output, so in this case you might need to use a gate resistor to limit the current surge when turning the MOSFET on or off.

There are dedicated MOSFET gate driver chips. These can supply a short pulse at large currents ~1A or more, in order to charge / discharge the gate capacitor quickly and therefore switch the MOSFET quickly. For any high power application I would use of these.

Hope this helps.

Bob

6. ### bonedoc

122
0
Dec 21, 2011
Ok, In the case of an inductive load like:

-a 12v motor that connects to the drain

-a inductor in a DC to DC converter where the high side of the primary is 12v and the low side of the primary is going to the drain, BUT with a 1:10 ratio, the primary may experience 30V at times.

Do I put the zener reversed biased from the ground to the DRAIN?

If so, how do I chose such values? Somewhere close to the drain/source voltage limit?

7. ### BobK

7,682
1,686
Jan 5, 2010
In a DC to DC converter no diode is needed (except the ones in the circuit itself). The circuit is already designed to direct the current to the load when it is switched off.

In the case of a motor, you want a diode (not zener) directly across the motor inputs, placed in the reverse polarity (so it does not conduct when the motor is powred). This gives the current a place to flow when the motor is turned off.

Bob

8. ### GonzoEngineer

321
2
Dec 2, 2011
TC427 Mosfet Drivers......