Drive a power mosfet with a gate driver or logic level mosfet

Thread Starter

bkhan10000

Joined Jan 18, 2017
29
Hey guys,

I am trying to drive a mosfet using pwm from an arduino, however, the low 5 volts is not enough to fully conduct for the mosfet I am using. My other option is to then use a gate driver to drive the mosfet. My question is, why can't I just use a logic level mosfet to control the regular mosfet I am using to turn it on and off? What are the drawbacks of doing so? What are the benefits of using a gate drive? Also, I have never used a gate driver before. Is it correct that to get a driving voltage of say 18 volts, I must also have a 18 volt voltage source? All help is appreciated. Thank you!

-Khan
 

#12

Joined Nov 30, 2010
18,210
Why don't you buy a logic level mosfet that will switch the current and frequency you need with a 5 volt gate signal?
 

crutschow

Joined Mar 14, 2008
25,633
A gate driver is designed to rapidly switch the large gate capacitance of a power MOSFET.
Even if you use a logic-level MOSFET that can be controlled by a 5V signal, the signal still needs to provide enough current to rapidly charge and discharge the gate capacitance to minimize switching losses.
 

Thread Starter

bkhan10000

Joined Jan 18, 2017
29
Why don't you buy a logic level mosfet that will switch the current and frequency you need with a 5 volt gate signal?
I don't know. Can I ? Is this sarcastic? I am just trying to learn.

A gate driver is designed to rapidly switch the large gate capacitance of a power MOSFET.
Even if you use a logic-level MOSFET that can be controlled by a 5V signal, the signal still needs to provide enough current to rapidly charge and discharge the gate capacitance to minimize switching losses.

Hi Crutchshow. Thanks for your response. So a gate driver is used basically because it can work best with the capacitance and switching speed characteristics of a mosfet? So the concensus basically is that it is more efficient to use a gate driver than a logic level mosfet to perform switching on a mosfet through a logic level controller. Are there any other pros and possibly cons of a gate driver? Appreciate all the responses!
 
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shortbus

Joined Sep 30, 2009
8,043
Are there any other pros and possibly cons of a gate driver?
I'm just a hobbyist but can see no 'cons' with using gate drivers. They put many separate components in a small package. I don't understand all of the resistance by people for using them. People would never consider making an opamp from separate components, but try to drive mosfets that way.
 

#12

Joined Nov 30, 2010
18,210
I don't know. Can I ?
Lots of people come here wanting to know how to make a solution they already decided on instead of telling what the problem is. That's how we end up going through strange, convoluted design work, and arrive at parts that haven't been invented yet, like a DC transformer or 50,000 farads of capacitance, when the whole thing could be fixed by re-arranging the circuit or using 50 cents worth of common parts. If you would just say what frequency and how much current you need, I could find out if there is a single mosfet that will do the job with no helper parts.

I asked what you need in post #5. You have chosen to not answer. I can't look up that part.
 

Thread Starter

bkhan10000

Joined Jan 18, 2017
29
Lots of people come here wanting to know how to make a solution they already decided on instead of telling what the problem is. That's how we end up going through strange, convoluted design work, and arrive at parts that haven't been invented yet, like a DC transformer or 50,000 farads of capacitance, when the whole thing could be fixed by re-arranging the circuit or using 50 cents worth of common parts. If you would just say what frequency and how much current you need, I could find out if there is a single mosfet that will do the job with no helper parts.

I asked what you need in post #5. You have chosen to not answer. I can't look up that part.
Thanks for the reply but I don't have a specific mosfet in mind. I just want to learn the concept so I can apply it to different circuits.

Lots of people come here wanting to know how to make a solution they already decided on instead of telling what the problem is. That's how we end up going through strange, convoluted design work, and arrive at parts that haven't been invented yet, like a DC transformer or 50,000 farads of capacitance, when the whole thing could be fixed by re-arranging the circuit or using 50 cents worth of common parts. If you would just say what frequency and how much current you need, I could find out if there is a single mosfet that will do the job with no helper parts.

I asked what you need in post #5. You have chosen to not answer. I can't look up that part.
In essence, what you are saying, is that in some cases it would be easier to use a logic level mosfet rather than a gate driver? In my case I am using an arduino which switches at 490hz. How would I use the parameters of current and frequency to then determine if it is better to use a logic level mosfet or a gate driver? Definitely appreciate that you would go out and find the appropriate solution, but my question is how would one make the determination for themselves? (This is so I can go out and do it by myself rather than ask someone specifically everytime)
 
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shortbus

Joined Sep 30, 2009
8,043
Thanks for the reply but I don't have a specific mosfet in mind. I just want to learn the concept so I can apply it to different circuits.



In essence, what you are saying, is that in some cases it would be easier to use a logic level mosfet rather than a gate driver? In my case I am using an arduino which switches at 490hz. How would I use the parameters of current and frequency to then determine if it is better to use a logic level mosfet or a gate driver? Definitely appreciate that you would go out and find the appropriate solution, but my question is how would one make the determination for themselves? (This is so I can go out and do it by myself rather than ask someone specifically everytime)
A logic level mosfet will also be helped with a gate driver. A gate driver allows you to source(turn on) and sink(turn off) a mosfet at a higher amperage, this allows it to switch faster and stay cooler. The switching time of a mosfet is what creates heat in them, more than the current through them. The gate driver takes power from the main input supply and allows it to be switched by the small output of the controller.
 
Last edited:

ronv

Joined Nov 12, 2008
3,770
Thanks for the reply but I don't have a specific mosfet in mind. I just want to learn the concept so I can apply it to different circuits.



In essence, what you are saying, is that in some cases it would be easier to use a logic level mosfet rather than a gate driver? In my case I am using an arduino which switches at 490hz. How would I use the parameters of current and frequency to then determine if it is better to use a logic level mosfet or a gate driver? Definitely appreciate that you would go out and find the appropriate solution, but my question is how would one make the determination for themselves? (This is so I can go out and do it by myself rather than ask someone specifically everytime)
The reason we are having dificulty is that there is no specification. So we don't know exactly what you need. But here are some things you should look for to drive FETs. This example is for P Channels from another thread, but the principal is the same for N FET.
Fets are voltage controlled devices (mostly:)) So if you want to be able to turn it on and off with 5 volts you need to look at these graphs.
upload_2017-1-19_16-1-0.png
upload_2017-1-19_16-4-25.png
The one on top is a normal FET. It has a much higher current level, but ignore that part. The one on the bottom is a logic level FET. As you can see the one on the bottom turns on at a much lower voltage to supply it's maaximum current. While the one on top is barely turned on with 4.5 volts.

The second thing that happens is that the gate looks like a capacitor so the more current your driver can handle will determine the switching time.
upload_2017-1-19_16-12-29.png


upload_2017-1-19_16-14-9.png
So to estimate switching time you can divide the nC value by the gate driver current. This will give you the switching time in ns.
In your case I think the micro can sink and source 40ma. So for the larger standard FET the switching time would be about 1575 ns - pretty slow, but for the smaller one 200ns (8/.040) Not to bad. The problem with slow switching time is that for that period of time the FET is 1/2 on and 1/2 off so the power in it is quite high. This may not make to much difference if the switching frequency is low (like your example) but it can be a problem when the switching is fast.
Hope this helps.
 

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