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  #1  
Old 03-16-2011, 02:15 AM
BrIDo BrIDo is offline
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Default AUV Project - H Bridge Conundrum

Hi there.

I'm a mechanical engineering student tackling a group project at the minute. It's the design and build of an underwater vehicle. I volunteered to do the electronics (expand the mind and all that) and am in need of some advice.

Basically, the craft will have 6 motors. They'll be controlled using an arduino microcontroller, which as I understand it puts out 5v logic. The motors will be drawing up to 6.5 amps, and each motor must be bidirectional and be capable of running independently, which means 6 motor controllers. I've struggled to find a commercial driver I could buy 6 of, and still stay reasonably within our budget. So, I've opted to build my own.

I breadboarded one yesterday and got it working, but this was using 4.5v from three 1.5v batteries to drive a small test motor and to configure the logic of the bridge. I then tested the bridge using a 12v lead acid battery for the bridge, connected one of the 'big' motors, and again used the three 1.5v batteries to configure the logic of the bridge, and... it shorted the right hand side of the bridge, and my high and low side mosfets melted, on that one side.

I've had a bit of advice elsewhere that, if I understand it properly, suggests that the 4.5v wasn't sufficient to 'open' the p types, so they were always closed, the n types came online, and shoot through occurred, thus frying the mosfets. This, I can live with.

The advice I'm looking for is regarding the schematic I've attached. I basically have three key questions:

Will using the mosfet driver, in all likelihood, be able to activate the gates of both the P and N types sufficiently?

Are the diodes I've opted for, with a forward current rating of 7.5A, necessary, and if possible, could I use smaller rated diodes?

I've sometimes seen capacitors used in the power rail in H bridges to provide extra capacitance, and 1k pulldown resistors in the base leg of the mosfets to set a default state for the bridge. Are these required given I'm using a mosfet driver?

I should probably also state we're not looking to utilize PWM control, and the motors are unlikely to be switching on and off very fast...

Cheers folks...
Attached Images
File Type: jpg initial_design.jpg (40.5 KB, 19 views)
File Type: jpg IMAG0112.jpg (285.2 KB, 14 views)
Attached Files
File Type: pdf HBRIDGE SCHEMATIC (Schematic).pdf (17.0 KB, 21 views)
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  #2  
Old 03-16-2011, 02:18 AM
BrIDo BrIDo is offline
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I've attached the datasheets for the components to save looking them up...
Attached Files
File Type: pdf TC4426A_driver.pdf (414.8 KB, 8 views)
File Type: pdf Ptype-FQP27P06X.pdf (746.7 KB, 7 views)
File Type: pdf 7.5A diode.pdf (134.4 KB, 3 views)
File Type: pdf Ntype- IRLU8726.pdf (354.6 KB, 3 views)
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  #3  
Old 03-16-2011, 10:29 AM
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eblc1388 eblc1388 is offline
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Quote:
Originally Posted by BrIDo View Post
if I understand it properly, suggests that the 4.5v wasn't sufficient to 'open' the p types, so they were always closed, the n types came online, and shoot through occurred, thus frying the mosfets. This, I can live with.
No. The MOSFET gates are driven by +12V because the Vdd of the driver IC is hooked up to +12V.

The way in your design that both P type and N Type MOSFET in one leg are driven by a common drive signal would mean they can both be conducting when the drive signal change state, albeit very briefly. This will probably destroy the MOSFETs as now there is a direct short circuit between +12V and 0V and hugh current will flow. I don't understanding why you said you can live with that?

You would probably need *two* driver ICs to drive all the 4 MOSFET individually so that no two MOSFETs from the same leg can be turned ON at the same time.

The TC4428A might be a better choice as it contains one normal and one inverted output.
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Last edited by eblc1388; 03-16-2011 at 10:36 AM.
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Old 03-16-2011, 01:22 PM
BrIDo BrIDo is offline
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Thanks for the detailed reply.

When I said I could live with it, I meant, if that's all it was, it seems fixable. I was worried there was maybe a more fundemental problem which would take longer to fix.

I should probably clarify that when I tested (and melted) my bridge, I wasn't using any mosfet driver, only 3 x 1.5v batteries. I've attached a JPEG to show what I did and what I think is a possible solution.

By using a TC4426A I think the mosfets will open and close as expected but...

What you're saying is - if I understand correctly - is that as one P type opens and one N type closes, due to switching times, they are both in the active region for a period of time and at risk of shoot through. I've read about this problem, but I've also read that, as I'm not utilizing PWM, I should get away with it. No?
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Old 03-16-2011, 01:55 PM
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I see.

You did it wrongly in the second test as 4.5V is insufficient to turn OFF the P MOSFET(requires over 10V w.r.t 0V, when its Source is at +12V) but it might certainly turn ON the N MOSFET. That's why both of your MOSFETs were toasted.

The proposed modified circuit will not work either. It is even worse than the original one. With 4.5V as your control voltage, you cannot even turn ON fully a NPN or N-ch MOSFET if its collector/drain is already at +12V. The emitter or Source, which is also the gates of the H-bridge MOSFETs you want to drive, get only a few volts w.r.t. 0V.
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Old 03-16-2011, 02:06 PM
John P John P is offline
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I suggest doing a web search on H-bridge and see what you come up with. These things are tricky, and you're reinventing a wheel that has made many revolutions over the years.
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Old 03-16-2011, 02:28 PM
BrIDo BrIDo is offline
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Quote:
The proposed modified circuit will not work either. It is even worse than the original one. With 4.5V as your control voltage, you cannot even turn ON fully a NPN or N-ch MOSFET if its collector/drain is already at +12V. The emitter or Source, which is also the gates of the H-bridge MOSFETs you want to drive, get only a few volts w.r.t. 0V.
Thanks for the help.

Assuming you're referring to the most recent diagram I posted, I was told that would be a potential fix for the circuit. I don't understand then. If I had a mosfet on it's own with 12v at drain, you're saying I wouldn't be able to turn it on using 5v logic? If not, what is the point in mosfets?

However, that aside... the circuit I actually WANT to use is not the one in the recent diagram, but in the schematic of the original post, which uses the TC4426A mosfet driver, labelled 'H bridge schematic.pdf.'

Do you think that circuit would work, with the risk of shoot through due to overlapping active times?
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Old 03-16-2011, 02:43 PM
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Will it work?

As they always said, "the proof is left for the reader as an exercise."
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Old 03-16-2011, 03:09 PM
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Why use a dual mosfet driver, instead of a 'half-bridge' mosfet driver? The half-bridge driver is made for making up a H-bridge, it uses two N-mosfets and has the shoot-through delay built in.
The driver you picked (TC4426A) is not really a H-bridge driver. It uses a P and a N mosfet that if not using a 'complimentary' pair type of mosfet can have two differing switching times.

A half-bridge driver allows you to use two N mosfets of the same type which have the same characteristics and the switching times match. The N mosfet is usually cheaper with better specs. Here is a well documented half-bridge driver data sheet; http://www.irf.com/product-info/data...ata/ir2110.pdf
While it's rated at up to 600V it works just as well at 12V. And the it uses logic level commands down to 3.3V

Here is a link to other half-bridge drivers too; https://ec.irf.com/v6/en/US/adirect/...793+4294837535
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Old 03-17-2011, 01:57 PM
BrIDo BrIDo is offline
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Hmm.

I still think that mosfet driver will work.

What about the diodes?

Do I need diodes that highly rated? Would smaller ones be ok?
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