H-bridge

Thread Starter

Dritech

Joined Sep 21, 2011
901
Hi,

I want to control a motor (12V, 2A) using an h-bridge from a PIC microcontroller. To do so, i am going to use the IRFZ44N transistors. Is it a good decision to use these transistors? will i need to use heatsinks for them?

Is it ok if i wire the h-bridge as the picture attached?

Thanks.
 

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hgmjr

Joined Jan 28, 2005
9,027
You are going to find that the upper mosfets will never turn on properly, since their gates can never be taken higher than their sources by the necessary Vgs.

For the configuration you have chosen, you will need a high side drive IC.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,027
Here is a link to an example IC. I have provided this link for reference only. There are other devices that perform the same function.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,027
The circuit to which you link should work fine. You will notice that it uses p-channel mosfets in the upper legs and n-channel mosfets in the lower legs. This allows it to get around the need for a highside driver IC.

One thing that you will need to do is carefully structure your software so that it avoids turning on the upper and lower mosfet on the same side of the bridge. You should be able to provide a software controlled dead-zone in which you make sure that both upper devices are off before you switch between the lower mosfets.


hgmjr
 

Potato Pudding

Joined Jun 11, 2010
688
You can run a high drive charge pump with a couple of diodes and capacitors, and an NPN transistor or small mosfet, pulsed by a spare pin of the Microcontroller.

That will work very well if you are only running forward and reverse. If you are running PWM for speed control the High Side Drive will work harder to keep up. Pulsing makes it more complicated because you need to be sure it can keep the gates charging repeatedly instead of only once. It might still be easy enough.

It also depends on your 12Volt power supply. If your main supply drops to 9 Volts under a high motor load then the high side drive might drop to as low as 16Volts.

This kind of highside is just a charge pump that doubles the Power Rail voltage minus diodes and some droop.

IRFZ44N would probably survive with only 7 volts of drive overhead even if we are looking at highest load situations. You say your motor is 2 Amps, so assuming 10 Amps for stall is safe we can hope. At 7 Volts drive the FET would be 10 Amps and less than half a volt Drain to Source. 5 watts should be in its survival zone even without a heat sink.

But this is all quick look guesses and I am very frequently wrong.
 

Thread Starter

Dritech

Joined Sep 21, 2011
901
Hi,
I decided to design an h-bridge myself. The attached figure shows the design. For the transistors i am going to use the IRFZ44N and 1N4501 for the diodes.

Is the design correct and will i work? and what opto isolator can i use?

Just as a reminder, the motor is supplied with 12V and consumes approx 2A.
 

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Wendy

Joined Mar 24, 2008
23,415
The one thing I noticed is a serious lack of a discharge path for the gate. Remember, the gate of a MOSFET is a capacitor. You need to charge it fast and discharge it just as fast.

The MOSFET driver chips have this feature, in the form of push pull transistors.
 

RRITESH KAKKAR

Joined Jun 29, 2010
2,829
I want to control a motor (12V, 2A) using an h-bridge from a PIC micro controller.
I will suggest you to use L298 for 2Amps if you want to control large current go for MOS and don't forget to add a series DIODE in path of power supply (VCc and GND) to avoid damage of internal diode in MOS in case of wrong connection of power source.( i have also damage my H_BRIDGES thats why...(IRF9540/540))
 

Thread Starter

Dritech

Joined Sep 21, 2011
901
Sorry i thought you said 'i think so'.

I bought the L298 but when i give signal to its inputs, the PIC (which is connected to the same supply) is restarting. I noticed that i am having voltage on ground when the h-bridge is active (this only happen at start up of the motor).
Do you know what can be causing this??

Thanks in advance.

EDIT: By the way, i am using the attached circuit diagram.
 

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Thread Starter

Dritech

Joined Sep 21, 2011
901
I noticed that when i disconnect the motor, the PIC is working fine. The problem is when i connect the motor. The PIC is also restarting when i connect the motor directly to the supply.
BTW is motor is a small DC one so its should not trip the supply.
 
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