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nevada
05-28-2008, 08:40 PM
I've been looking around the place for specific recommendations for a MOSFET which can be used for the following in a H Bridge

- DC Motor Driving (at 15 V, Stall current of about 15-20 Amps)
- Running above motor with PWM
- Preferably the MOSFET should come with inbuilt diodes for back current protection

Also, if you could suggest an appropriate MOSFET driving circuit to go along with the above, it'll be great. As far as I understand, its not recommended to drive one using 5 Volts Gate Voltage, because of the relatively high drain source resistance.

If you know Lamington Road in Bombay, perhaps you could guide me for the shop where the above may be purchased as well.

I don't have much of a clue on the design of the circuit, so if you could link me up on that too, I'd really appreciate it.

PS : Am I to assume that a motor of lesser power rating will be able to run on the above circuit just as easily? I'm guessing it will, but i suppose there is no harm in confirming.

Thanks.

MohitM
05-29-2008, 12:09 AM
There are some things to take care while designing around MOSFETs.

Chiefly, the gate has a reasonable capacitance that causes it not to be driven high or low quick enough using just a microcontroller pin. Remember, you'll most probably be PWM switching these MOSFETs at fairly high frequencies, where this capacitance comes into play. If not switched quick enough, the MOSFET will spend quite some time in the linear region and will therefore get a bit hot. You can design a MOSFET driver circuit using some discretes and transistors. But you can also choose the easy way out and go for dedicated MOSFET drivers available in the market, in which case your task becomes fairly simple and you'll avoid the proverbial black smoke - you can burn a lot of MOSFETs with a 15A motor.

I have done H-bridge design using TC4429 series MOSFET drivers from Microchip. You could also try TC4426 series. These will be available at Silicon Components in Mumbai. Google for them. About Rs. 80 for each. Supply this with 15V so that the MOSFETs get 15V at their gates. But drive it with 5V from the microcontroller. So interfacing is kind of easy. But put a resistor between the TC4429 and the microcontroller (~100 ohm). For best results use an optocoupler like 6N137 to isolate the motor supply and microcontroller supply.

For MOSFETs, use IRF3205 (n-channel) and IR4905 (p-channel). In-built diodes in MOSFETs are standard these days. In case you are not using the H-bridge and simply PWMing the motor I'll suggest MBR3045 across the motor terminals. These will be available with Cirkit Electro at Lamington Road. All three in the range Rs. 30-60. This may sound slightly high, but just check their specs, they are uber-cool! Rds = 8 milliohms for the IRF3205. At 15A you probably won't be needing a heat sink even.

nevada
05-31-2008, 03:03 PM
Thanks for the comprehensive reply.

I still have a couple of doubts though :

1. I intend to use this to drive a motor both as a H bridge and with PWM, so is it going to give me similar performance as the L298 motor driving IC?

2. Instead of using a MOSFET driving IC, is it possible to use the L293D motor driving IC to provide gate voltage? I have quite a few of those lying around, so it'll be useful.

3. Are there any specific PWM modified MOSFETs, which are meant to be used for the same ops? Any ideas on the same and if they make for superior H bridges?

MohitM
05-31-2008, 11:20 PM
1. I intend to use this to drive a motor both as a H bridge and with PWM, so is it going to give me similar performance as the L298 motor driving IC?

2. Instead of using a MOSFET driving IC, is it possible to use the L293D motor driving IC to provide gate voltage? I have quite a few of those lying around, so it'll be useful.
Absolutely no idea of L293D, so can't help you there.


3. Are there any specific PWM modified MOSFETs, which are meant to be used for the same ops? Any ideas on the same and if they make for superior H bridges?
Not sure I get you. Most MOSFETs these days can turn on and off very fast. So they can be used for PWM at high frequencies. What you need is some driver circuit that can switch them quick enough.

joprinz
10-13-2008, 10:15 PM
Hello Neveda, Mohit,

I am thinking of using FET H-Bridges to drive steppers. For a 2 phase Bipolar Stepper, I be needing 2 Full Bridges.

I need to drive the steppers at about 8A @ 50V. Current Limiting will be done by PWM @ about 20kHz. The Stepper Sequencing will be done using an AtMega48 programmed to do Full as well as Half step. Step rate will be around 2000 - 3000 Pulses per second.

I was wondering what FET and FET Driver should i choose?

Please Advice,

Thank You.

Joe!