Why do Mosfet drivers always show 2 Mosfets?

Thread Starter

clan

Joined Oct 27, 2018
3
I am trying to find out how a Mosfet driver is wired. I want to drive a universal motor with a full wave recitifier and a Mosfet and set it up as a chopper to control the speed of the motor. Every time I try to find circuit examples of a Mosfet driver, they seem to always show 2 Mosfets being driven instead of a single Mosfet. Can anyone explain this to me? (Newbe to Power Mosfets)

Thanks
Clan
 

Papabravo

Joined Feb 24, 2006
21,157
This is because a single device cannot be used to source current into a load AND sink current from a load. It takes two devices to do this.
 

ebp

Joined Feb 8, 2018
2,332
You need to explain a little more about the circuits you are finding. Are they circuits to drive a load directly or are they circuits to drive the gate of a FET that drives the load?
 

shortbus

Joined Sep 30, 2009
10,045
Most not all mosfet drivers are called "half bridge drivers" also. By using two of them you can make a motor run in forward and reverse. That said, you do not need to use both mosfets at the same time in one of the half bridge drivers. The mosfet drivers have what is called a "high side" and a "low side" driver in them. If you Google those terms you can get more understanding and information about what they mean.

To do what you are asking about is also called PWM, pulse width modulation. By putting the motor between the lower mosfet and the power source, you can do the PWM by switching it on and off at a set frequency to control the motor speed. They put both a "high side" and a "low side" mosfet driver in one chip/package to reduce the number of IC's that need to be made and the size of a PCB when doing the circuit.
 

Thread Starter

clan

Joined Oct 27, 2018
3
Thank you for your answers. I did a little more research and now I understand a little more about how these work. A lot more complicated than I first believed! Going to take me a while to wrap my head around switching losses, gate charges, enhancement and depletion Mosfets .. etc.
I now understand there are many ways to charge and discharge the gate and some require 2 transistors and other parts. (Looking more into how that works)
Sorry to have asked such a newbie question without doing a little more leg work myself.
;)

I have one more question. Mosfets have a rating as below:
ID @ TC = 100°C Continuous Drain Current, VGS @ 10V 21 A
I take this to mean that at a junction temperature of 100C, the Mosfet can safely handle 21 Amps DC if I feed the Gate with 10V.
I am trying to control the speed of a universal motor in a Weed Trimmer. It is rated as 120V, 10 Amp.
I will make a full wave rectifier and feed it pulsing DC @ 120V. For the above Continuous Drain Current rating, do I de-rate my pulsing DC to .707 * 10A = 7.07A? )
In other words, do I use RMS of my voltage because it is not continuous DC?

Thanks again for all your help.

Clan
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
I am trying to control the speed of a universal motor in a Weed Trimmer. It is rated as 120V, 10 Amp.
Clan
Using a DC voltage instead of the original AC may run the motor at a higher RPM and increased current.
What do you have against the Triac method?
Max.
 

Thread Starter

clan

Joined Oct 27, 2018
3
From what I have read, the Mosfet is much more efficient at lower speeds. I am not yet sure what speeds I need, but right now the Weed Trimmer is running so fast it is flinging rocks around like confetti. So, if the weed trimmer is runing at 10,000 rpm, then I may want the speed to be dropped to 5000 rpm or maybe even lower.
I don't really have a problem with triacs and if I could find a good design (or somehow manage to design one myself), I would give it a try. Right now, I am just exploring my options. If you know of a good triac design, I definitely would be interested.

Thanks
Clan
 

shortbus

Joined Sep 30, 2009
10,045
So, if the weed trimmer is runing at 10,000 rpm, then I may want the speed to be dropped to 5000 rpm or maybe even lower.
If you just want it dropped by half, put a single diode, of the proper value in the main lead. That will give it half voltage.

But think your heading for more problems down the road with your trimmer. They are designed to use a certain speed, to be able to cut the weeds. At a lower speed they don't work as good, the cord will not cut as good because it bends instead of cutting the weed. Borrow a gas powered one and run it a half speed, and you will see what I'm talking about. Or get a router speed controller and plug your trimmer into it, a no fuss, pretty cheap way of doing it.
 
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