# how to drive half bridge mosfets

#### baggio2

Joined Dec 25, 2008
60
hi...i have a project called d class amplifier but i cant drive mosfets...can anyone help me please?

#### beenthere

Joined Apr 20, 2004
15,819
The image is much too small to make out. Can you say why you can't drive mosfets?

#### SgtWookie

Joined Jul 17, 2007
22,227

Image shack hosted images require clicking on them a few times before you can view them full size; even then this one is still hard to read.

Baggio2,
I see you're using N-channel MOSFETs on both the high and low side of your H-bridge. While this is fairly common nowadays, the method you're using won't work.

U3A appears to be an opamp, though I can't make out what specific model. Usually, opamps are not sufficient to drive a power MOSFET's gate at more than a couple of kHz.

But let's look at what happens to Q1's drive. If Q1's source were connected to the negative rail, then the output of U3A would be able to vary Q1's gate voltage from somewhere above the negative rail to somewhere below the positive rail. An IRF540 MOSFET has a maximum Vgs of +20v/-20v. If your + and - rails were more than 20v apart, you would likely exceed the maximum Vgs of +20v and destroy the MOSFET.

Let's look at Q2's drive. Basically, you want Q2 off if Q1 is conducting, and vice versa. However, you've connected both gates together, so they'll try to be ON together, and OFF together. If they were both conducting simultaneously, you would effectively create a direct short between your positive and negative rails.

What's not quite so obvious is that Q2 is fully OFF when Vgs=0v, and fully ON when Vgs=10v. You must always reference the gate voltage to the source terminal of the MOSFET. As things are now, you would never be able to raise Q2's gate above the positive rail, which will be a necessity in order to get Q2's source terminal near the positive rail.

#### baggio2

Joined Dec 25, 2008
60

#### SgtWookie

Joined Jul 17, 2007
22,227
If I did it for you, you would not really learn anything.

It's more important that you learn something.

Why don't you do some research on H-bridge driver circuits, in particular high side MOSFET drivers?

#### baggio2

Joined Dec 25, 2008
60
i did it...but couldnt find anything...actually my time is limited..i have to finish it till tomorrow and i have been trying to do it for 2 weeks...i learnt a lot of things..the one problem is to drive mosfets...please help me...or tell me how to do it...

#### baggio2

Joined Dec 25, 2008
60
the input signal is 20 khz and carrier is 100 khz...i can generate a pwm output from U3A opamp( +12 to -12) ..mosfets must amplify pwm but it doesnt work..i dont know what i am gonna do...

#### SgtWookie

Joined Jul 17, 2007
22,227
Why don't you look at Elliott Sound Products's D-class page for a tutorial on D-class amplifiers and some ideas?

http://sound.westhost.com/articles/pwm.htm

I'm not certain what opamp you're using, but it will not be sufficient to drive the gate of an IRF540 MOSFET at 100kHz.

#### baggio2

Joined Dec 25, 2008
60
here another shematic...i tried to drive mosfets but fail...

#### baggio2

Joined Dec 25, 2008
60
i read it times and times...i tried what it says....but i couldnt make it...i am using mosfets as aplifier...i mean they must amplify pwm...please man...here it is 20:35 and i have to do it till tomorrow morning

#### SgtWookie

Joined Jul 17, 2007
22,227
You need to look at that page again. Particularly down around diagram 5b.

You will need BJT drivers for both your high side and low side.

#### baggio2

Joined Dec 25, 2008
60
i am trying to do 5b...there is something wrong..what is it like an opamp?(used to drive low side)

#### SgtWookie

Joined Jul 17, 2007
22,227
He's using a MOSFET driver.

There are quite a few on the market. Some of them can output several amps of current.

In your case, you do not have time to research and model such drivers. You will have to make discrete bjt drivers for both the upper and lower, and they will have to be opposite in polarity so that when Q1 is conducting, Q2 is off, and vice versa.

#### baggio2

Joined Dec 25, 2008
60
thank you so much....you are my man i will try it but if it doesnt work?

#### baggio2

Joined Dec 25, 2008
60
no..i couldnt make it...it doesnt work...i am doing something wrong....thanks for ur help...maybe i am insufficient for electronic...probably i wont be able to finish the school...could u please help me...do u have pspice or protesus?

#### SgtWookie

Joined Jul 17, 2007
22,227
I don't have either of the products you mentioned.

You haven't been trying the circuits for very long - maybe 45 minutes.

Why don't you look at the example again, and try to see where you are making mistakes?

#### baggio2

Joined Dec 25, 2008
60
i tried..i have the page you sent to me...i have been trying for days on it and on others...but no result...it takes just 5 minutes for you...you know it well....let me send the file just look at it...maybe you can see my mistake...

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#### SgtWookie

Joined Jul 17, 2007
22,227
I'm wondering why you didn't ask your instructor for more help before now, or why you didn't start asking for help earlier.

Asking for help at 21:00 on the night before it's due is a bit irresponsible, don't you think?

If I DID complete it for you, then the only thing you would learn is that it's OK to be irresponsible, and that you can con someone to do your work for you at the very last minute.

However, if you take that big fat "F" for failure, you will learn that being irresponsible is not acceptable, and that if you find yourself "over your head" in a project, you should more diligently seek out resources to gain the understanding you need to complete the project.

Failing will often teach you more than succeeding. It's what you learn that counts.

#### baggio2

Joined Dec 25, 2008
60
i asked my instructor but he is a bad teacher becos didnt help...i know my responsibilites and i did my best....i searched and tried for hours...if u helped me it would be great but believe me i did my best...thanks for everything

#### SgtWookie

Joined Jul 17, 2007
22,227
Why don't you re-post the schematic, but larger, so that we can actually read what the component values are?

You don't need to use Imageshack - click the "Go Advanced" button, and then "Manage Attachments".

.PNG format images work best. .JPG format images are too "lossy".

In the 2nd schematic you posted, it looks like you're trying to use an LM324. That will never work at 100kHz. They're limited to just a few kHz. You need a much faster opamp.