Designing 3V DC Motor H-Bridge with Power FETs

[kthouz]

Hello!
Lastly i was designing a toy car moving in both direction foward and backward using relays and switches. I thank everybody who helped me.
Now i want to move to h-bridge. i have read many books about the h-bridge but one problem that i still have is how to chose power FET transistors. Actually, i want to design a H-bridge for a 3V DC Motor. What type of Power FET am i going to use?
Thanks in advance.

 

[vk6kro]

The main problem is availability.

I did a quick search on Google and found lots of circuits for H Bridges and also one-chip devices that could drive small motors directly.
So, you could have a look through those circuits and pick one that seems OK for 3 volts.
Then look for suppliers of the parts.

Incidentally, this post might do better in the Electrical Engineering section. There have been a few H Bridge queries there over the last few months.

 

[Bob S]

The real problem with FETs is getting enough voltage to turn them on. Here is one solution:
http://www.pololu.com/catalog/product/120
This Allegro chip will go down to 1.8 volts, but I don't knoe how to install it.:
http://www.allegromicro.com/en/Products/Part_Numbers/1448/
I usually prefer discrete NPN and PNP collector outputs at 3 volts. Avoid Darlingtons.
[added info] This TI H-bridge DRV592VFP works from 2.8 V to 5.5 V. See
http://www.chipcatalog.com/Datasheet/05E85EF564800B2FB1F1FC107941937B.htm
I have seen some on ebay.

 

[vk6kro]

I found this one on Google. It seems like a very good presentation, although I haven't tried it, of course.

http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html

 

[Bob S]

I found this one on Google. It seems like a very good presentation, although I haven't tried it, of course.

http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html

This looks like a nice choice, until one looks into the details. This H-bridge uses collector outputs (common emitter configuration) which is nice, but the four output transistors are all darlingtons, and V_CE sat is about 2 volts or more, so they won't work on a 3 volt power supply. A better choice for 3-volt operation is to drive a discrete NPN common emitter (collector output) with a PNP transistor, and vice versa.

 

[vk6kro]

Not a problem. The motor has to run off 3 Volts, not the H Bridge. Still, the less voltage needed the better.

The circuit was misleading though. The transistors were shown with normal symbols not Darlington ones.

I liked the tutorials in that article.

 

[kthouz]

This looks like a nice choice, until one looks into the details. This H-bridge uses collector outputs (common emitter configuration) which is nice, but the four output transistors are all darlingtons, and V_CE sat is about 2 volts or more, so they won't work on a 3 volt power supply. A better choice for 3-volt operation is to drive a discrete NPN common emitter (collector output) with a PNP transistor, and vice versa.

Nice tutorial. I liked it too. But actually, i would like my Hbridge be driven by power FET. I have already designed one with BJT but i don't find it more efficient so, as i think FET are the powerful ones, i would like to replace those BJT by FET. Just as hint, how does one chose FET to drive a motor according to its voltage needs?

 

[Bob S]

FETs are nice. When they are turned on, they look like a low resistance in series with the motor. However, if the voltage across the motor is too high, an external resistance is required between the FET and the motor (which is actually being driven by two FETs in an H-bridge) to limit current. For H-bridges operating on a low voltage power supply, finding a FET that can operate on say 3 volts (2 D cells) can be a problem. The problem is getting enough gate voltage to turn the FET fully on. Thus using a PNP bipolar transistor with emitter tied to Vcc and the collector tied to an N-channel FET gate gives the best voltage swing.

 

[matandi]

DeviceCraft has a nice H-Bridge controller.
http://www.devicecraft.com

It is too high a voltage 10 to 50v but
It is pretty much the standard design recommended by IRF plus the microcontroller and Temp/Current protection.
The onboard PIC microcontroller is in a socket and can be reprogrammable.
The device has a lots of cool features like fast hall effect overcurrent protection, thermistor over temperature protection, limit stopping, forward/reverse, 50amp MOSFETS mounted with mica to heatsink, mounting plate, LEDS, and thick copper traces.

There are a zillion uses for this thing.

The part is listed on EBAY now.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250498364685&ssPageName=STRK:MESELX:IT