H bridge mosfet choice?

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techno
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H bridge mosfet choice?

Post by techno » Sat Apr 24, 2004 8:10 am

The motor drives for my boat are about 3 hp x 2 and this means 200 amps at 12 volts! I can't find any pre-schematic and not sure how to choose a mosfet for this power level.
Does the thing need to be rated for this 200 amps or less?
If this don't work I'll use plan A and have servo controlled switches for the power instead. Fwd-off-reverse. Rather have the H-bridge with an Oopic for speed control though-less barbaric.

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Bob Scott
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Re: H bridge mosfet choice?

Post by Bob Scott » Sat Apr 24, 2004 11:03 am

You are going to have a hard time finding P channel MOSFETs for the H bridge that meet the requirement for Low on resistance and hence low heat dissipation. A compromise would be to forget reverse and use a single N channel MOSFET for forward only, with source connected to battery ground and drain connected to the negative terminal of the motor. I may be wrong, but those electric motors are so steerable that you just spin them around backwards to obtain reverse. This forward-only way you also avoid having to design around H bridge shoot-through. I've done it and patented my method, but why add complexity if you don't need it?<p>Do a product selection at irf.com. My choice for an N channel MOSFET would be the 25V 210A 230W IRF3703. Wire a few of these in parallel. Remember, every time you double the number of MOSFETs in parallel you cut the total MOSFET dissipation to half, and cut the dissipation of each device to 1/4. Another good one would be the IRL3713. Also remember that dissipation of the MOSFET in Watts is I squared R(ds on).<p>I use a lot of the 3703. You can get them for less than 4 bucks at DigiKey, less than 3 bucks at Newark.<p>Bob :cool:
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rshayes
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Re: H bridge mosfet choice?

Post by rshayes » Sat Apr 24, 2004 12:13 pm

All four of the devices in the H-bridge can be N-channel devices if you can provide gate drive above the level of the input power. This could be done by building a small, transformer coupled, 12 V to 12 V inverter and floating the output on the plus 12 V lead of the input power.<p>The control circuits will need a little care to avoid turning on both transistors on the same side of the H-bridge at the same time. Just guarentee a little dead time when the drive direction is changed. The 12 V battery can probably provide over 1000 A when shorted, and this type of fault current can do spectacular things very rapidly (wiring melts, transistor cases crack, etc.). You very definitely need a fuse or circuit breaker in the circuit. Current can also get high if the motor is stalled, even without an electronic fault.

techno
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Re: H bridge mosfet choice?

Post by techno » Sat Apr 24, 2004 1:10 pm

Thanks for the leads I'll have to do some more search and learn. I haven't studied the H-bridge just assumed to copy it :D
Thats why I was stumpified by the amp/wattage deal.<p>Do need reverse though. A motor in each sponson is the setup and on-off- reverse is the only control over them. No steering the motors.<p>My limited searching only found very expensive items. Maybe I can use them for the P channel.
Although the above 12 v all Ns system seems cheaper.<p>This whole thing is just an auxillery drive for my Outboard. It isn't really needed but a must have. Those 2 added together mean cheap as possible. The first method were massive manual switchs activated by an RC system, not all that cheap.

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jwax
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Re: H bridge mosfet choice?

Post by jwax » Sun Apr 25, 2004 6:00 pm

I used a Curtis Instruments MOSFET controller for a 48V, 175A electric go-kart.
http://www.curtisinst.com/
There's many concerns about throttling 5-10 kilowatts that the amateur might do well to leave to a pro! (Heat, emergency shut-down, runaway, acceleration limiting, to name a few) Just my 2c.

toejam
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Re: H bridge mosfet choice?

Post by toejam » Wed Apr 28, 2004 6:57 am

maxium makes a high side mosfet driver chip. i forgot the number but it should be on thier website
im starting to get my notes together if you can,t find it mention the fact and i,ll dig it out.
tj

techno
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Re: H bridge mosfet choice?

Post by techno » Wed Apr 28, 2004 5:15 pm

maxium? I couldn't even find thier site!

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jwax
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Re: H bridge mosfet choice?

Post by jwax » Mon May 03, 2004 3:46 pm

tj may have meant Maxim.
www.maxim-ic.com

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