The Simplified Model
If you don’t know the basics behind H-Bridges, this section will go over the principles of how an H-Bridge controls the direction of rotation of a DC motor. If you already know the basics of how H-Bridges work, you can skip ahead to page 3.
Imagine an H-Bridge as an array of four switches, as shown below:
Figure 2.1 - Simplified H-Bridge
You can alter the states of these four switches in order to manipulate the voltage across the motor, and through that the direction of current flow and rotation of the motor. In Fig 2.1, you can see that all the switches are open, and the motor terminals are disconnected from the circuit. This state will allow the motor to spin freely.
Figure 2.2 - S1 and S4 closed
If we close two of the switches, S1 and S4 as show in Fig 2.2, the motor terminals are connected to the voltage supply, and a potential difference across the motor is created, which will cause the motor to begin rotating in one direction.
Figure 2.3 - S2 and S3 Open
Now, if we open S1 and S4 again, and close S2 and S3 as in figure 2.3, the voltage across the motor is switched around, and this will cause the motor to rotate in the opposite direction.
Figure 2.4 - Active Braking
What happens if we leave S1 and S2 open while S3 and S4 are closed? The motor terminals will be short circuited. This will cause the motor to brake, and rapidly slow down.
Figure 2.5 - Short Circuiting
Now, what will happen if we close S1 and S3 at the same time? This will cause a short circuit from V+ to ground. This is known as “Shoot Through” The motor will not spin, and excessive current will flow through the switches, wires, and power supply. In a Mosfet H-Bridge, this can cause Mosfets and batteries to overheat and free trapped smoke, or even explode!! This is dangerous, and you should avoid short circuiting your h-bridge like this at all costs.
- Introduction
- H-Bridge Basics
- The Ideal MOSFET H-Bridge
- MOSFET Imperfections
- Power Supplies, Noise and Cooling
- Design Considerations and Conclusion
#1 by Rodrigo at January 19th, 2009
You say LM293 and LM298. I think you mean L293 and L298. Regards.
#2 by Roko at January 19th, 2009
Good catch, thanks!
Updated..
#3 by Frank at April 6th, 2009
The mosfets I want to use have their heatsink connected to drain. Is it wise to connect them together on a heatsink ? I want to use irf9450 (P) and irf620 (N) but put them all on the same heatsink. Will this produce smoke or am I safe?
#4 by Roko at April 6th, 2009
Frank;
Since all the drains are connected to heat sink pad on the FETs, connecting them all directly to the same external heat sink would cause you to short out your left and right legs if the heat sink is electrically conductive.
There are available special heat sink insulators available for this purpose:
http://en.wikipedia.org/wiki/Mica_insulator
#5 by Romeu at April 10th, 2009
hey!
Thanks a lot for all this informations about H-bridge! I’m trying to make one but it’s not easy when we dont know a lot of things… Now i hope it will be a bit better:)
Good luck in your hobbies!
#6 by nhaan at April 14th, 2009
thanks for your informations.
Can i use a 12-220V transformer instead of a motor in full h-bridge to make a inverter? I use PWM method with a micro controller.
#7 by djallel at May 20th, 2009
hi guys,
i’m working in a project whitch contient an H-bridge for driving my DC motor with I load =10A.and i have my battery voltage =6V.
so in the first step i used 4 N channel mosfets to build my H bridge with PWM input toin crease mu current load to 10A.
the problem i didn’t find a good idea to wire my H bridge with the microcontroller (it’s power supply is 3V) .
i tried some H-bridge driver can be work in this project but i didn’t.
can some one help me.
thanx.
#8 by Roko at May 20th, 2009
Using 4 N channel Fets is a good idea, as it is cheaper and N channel fets tend to have a lower RDS On resistance, the challenge is that you need to switch the upper N-channel fets with a voltage higher than your bridge supply voltage.
I’ve used the MC33883 bridge driver from Freescale successfully. It requires a 5.5 volt supply, so you could power it directly for your battery, but the I/O will work with 3 volt inputs, so you should be able to wire up your micro directly to the bridge driver I/O and a common ground.
#9 by djallel at June 3rd, 2009
Hi;
Thank you very mush for your replay,
Regarding your suggestion,is a good idea but the problem that i haven’t Pins enought in my microcontroller there is just 2 Pins (one for hight mosfets and another for the down mosfets ).
I found some H-bridge driver like L99H01 with voltage supply of 6V and operate logic supply between 3 to 5.3.
But I have a doubt about voltage supply will decrease during use.
Thank you again.
#10 by vpunk at July 17th, 2009
Thanks for this very practical and informative guide! You’ve answered all the questions I had and some I hadn’t even considered. Keep up the good work!
#11 by Matt at July 20th, 2009
Hey, this is a damn good article. As an ee, I really appreciate the analysis. I have looked at some websites out there, riddled with foolishness like paralleled BJT’s
. I have spent a lot of time recently studying mosfets and h-bridges, so I know what I’m talking about when I say excellent Job!
I’ll be looking forward to upcoming posts.
#12 by aditya at July 28th, 2009
i’cant see the picture! i only see a black line… please send the picture to my e-mail. i need their picture to help me to understand how H-bridge work!thx..
#13 by Hugh at August 2nd, 2009
Very nicely written article. It is informative and understandable! Thank you so much
#14 by sam at August 5th, 2009
Thank you for this great article. I’m building my first H-bridge now
#15 by Jose at October 12th, 2009
I’m think you might be confused names of the operating areas. The triode/linear region for a fet is where you WANT to be for switching this area is similar to a BJT’s saturation reagion (this is where i think the confusion started). Now the fets saturation region is typically used for amplification NOT SWITCHING this is similar to the BJT’s active region.
#16 by Roko at October 14th, 2009
Thanks for the note! It’s been a while since I wrote this, so I’ll check it over and correct that when I get a chance.
#17 by ashish at November 5th, 2009
heyy all,
im making a solar boat.. the solar panel can deliver a max 12 v. can u help me to make a motor driver circuit which can drive a 12 v , 5A motor ..plz give me the details
#18 by ashish at November 5th, 2009
itz realy urgent and important for me…plz send me the circuit layout on a pcb….my email id is ashishr_singh@yahoo.co.in
#19 by Omer Malik at November 7th, 2009
can some one tell that waht are the chances of burnout in L298/293. I have heard that they are quite often burnt and dont prove to be reliable
#20 by Omer Malik at November 9th, 2009
……. i am using pitman motor.
#21 by Roko at November 9th, 2009
Omer, the chances of burning out an L298/L293 depends a lot on your motor an application. Smaller motors that they are designed for will run well, and won’t burn them out. However, larger motors that draw a lot of current are what often causes problem. To reliably use them, you will need to ensure that your motor’s stall current is lower than the maximum rated current for the chip, and that you are properly cooling the chip with a large heat sink.
#22 by Guy at November 13th, 2009
Hi,
I was just reading through this design and was curious as to whether this design requires that motor supply voltage be at the same voltage as the output of the micro ? Otherwise if say the motor supply was 12 V and the micro outputs 5 V, wouldn’t there still be a negative potential across the p-channel Vgs? hence not turning it off when we want it to. Or perhaps I am misunderstanding something?
Cheers
#23 by Roko at November 14th, 2009
Hi Guy,
You are correct that by itself a 5 volt MCU would have trouble switching a 12v bridge. To get around this, a mosfet driver should be used to translate the low current 5v signal to a higher current signal capable of sourcing 12v to the fets.
I briefly mention this on page 4, but when I get a chance I’ll elaborate upon this in the article. Thanks for the note!
#24 by Guy at November 15th, 2009
Thanks man,
Its just a small area of confusion.
#25 by Guy at November 15th, 2009
So should I some how amplify the signals from the micro to the Mosfets to match that of the load?
Would that work?
#26 by XTL at November 16th, 2009
“Turning either A or B high will cause the motor to rotate in one direction or the other. If they are both high or both low at the same time, then the motor will actively bake.”
#27 by Martin Kynde at December 2nd, 2009
Hallo
I am studying electronics. The interface circuit would not work! Q1 and Q4 has to work together, and Q2 and Q3, else the motor would not turn in any direction. Just swap Q1 with Q2 on figure 3.2, then it would work
#28 by Roko at December 2nd, 2009
Hi Martin,
I believe you’re confusing the operation of P and N channel FETs. As the paragraph below figure 3.2 states, the P and N channel fets are activated by opposite polarities.
Thus, the interface circuit is correctly set up to provide a high signal to Q4, and a low signal to Q1 to switch them both on, while ensuring that the others are off.
#29 by Martin Kynde at December 13th, 2009
Hallo Roko
I think you have right. Sorry for the confusing
#30 by John Essen at January 12th, 2010
A clearly written article on this subject, it answered all my questions.
Thanks Roko
#31 by genotio at January 19th, 2010
Note that you can put 2 or more mosfets in paralell. Rdson will be smaller, but driver will be more loaded.
#32 by arash at January 24th, 2010
hello
my friend
please help me
please send pdf or shematic file circuit driver dc motor 24 v for automatic door(left or right fasrt speed and low speed by pwm and countering rpm by shaft encoder ) by mosfet and L99H01 chip or masfet and ir 2102 chip
thank you
please send now
i am a waiting
#33 by tuan anh at March 16th, 2010
Hi, thanks much for your good explanation of H-bridge. I just want u to explain more about operation of the 4 high-speed Schotky diodes. As u said, we use them to prevent back EMF (electro-magnetic field – right?), in other words, to prevent back current flow. Could u make it clear about how the back current flows inside the circuit?