MOSFET and H-Bridge related questions

[reSpawn]

Hello,
I have 2 UCC37325 MOSFET Drivers from TI ( http://focus.ti.com/docs/prod/folders/print/ucc37325.html ) and I want to drive 2 motors. I build this circuit http://www.robotroom.com/HBridgeSchematic.gif with one MOSFET and it gets damn hot while driving 2 motors. And as I have 2 of these I'm wondering if I can put them in parallel using the same 4 diodes by connecting the two OUTB1 with OUTB2, OUTA1 with OUTA2, and the same for the inputs.

[ArcMan]

That schematic is just wrong.
The device that you linked is a low-side MOSFET driver, not a motor driver.
#1, you can't drive a motor with it, you drive a MOSFET with it.
#2, you can only drive the low-side MOSFET.

What you need is a motor driver, like an LMD18201 or an L298.

[reSpawn]

can you please explain why it is wrong? I made it and the motor spins when I leave both inputs to low( as per datasheet ). I haven't tested yet with different inputs, I just wanted to see if it's working, and it is. I'll hook it up to my atmega later.

[szhang]

You CAN drive a motor with a dual-MOSFET driver (some drivers even list that in their datasheet), but you have to realize that mosfet drivers don't have some of the features used in dedicated H bridge chips.

However, if it gets really hot, it might mean you have just killed it.  The MOSFET drivers don't have as much protection from flyback as H bridge drivers, so using a high current motor will quickly kill them.

The driver you have can only dissapate 500mW of power, which is about .1A @ 5V, you won't be able to drive any big motor with that.

I am using a 9A dual-mosfet driver to drive the smaller steering motor on my robot (.5A stall), since my current consumption is low it works fine.  But if I use them to drive my main motos (7A stall) they quickly get really hot and stop working.

The motor shouldn't spin if both side is low (or both side is high), rather it should brake the motor.

You can stack them in parallel to double the current rating, but make sure you have good protection diodes.

[reSpawn]

Thanks szhang.
When you say that you can put the outpurs in parallel, do you mean something like this:

also, those 4 holes are to attach the heatsink, so I will be able to drive bigger motors

[szhang]

Yes, you pretty much connect the corresponding pins on both chips.  But the power dissipation will be your main problem, and PDIP components can't dissipate very much heat.

What are the current ratings for the motor you are using?

Even with 2 mosfet drivers you can only dissipate around 1W of power, so unless your motors are really small, you might be better off using LMD18200 or L298 as drivers. 

[reSpawn]

as I said I will be attaching a heatsink (those 4 holes are to attach the heatsink). What do you say? My motors(there is one on each side, but their wires are connected) draw ~3-4 A on startup, and like 2 A on normal load( just the robot chasis)

[szhang]

PDIPs don't benefit very well from heatsinks (it is covered in plastic, not the best conductor of heat).  If you're making PCB's then look at the other packages, the drivers I use also come in TO-220 (transistor) package, which has a mounting hole for a heatsink.  Other surface mount packages also have exposed metal pads for mounting heatsinks.

2A on normal load?!?!  I don't think you can use the mosfet drivers, no way they are designed to dissipate 10-20Watt of power.  What is your stall current (the current draw when you stop the motor from rotating)?

if you're powering the motor from a >12V supply, I would recommend using two LMD18200 instead.

[reSpawn]

I'm powering the motors from a 7.2V pack. Also on the chip will be a thermoconductive paste. I hope I will be able to cool them

[szhang]

Thermal paste isn't going to help much because there is a slab of plastic between your heatsink and the die.  You are dissipating 14.4Watts, the package can only dissipate 500mW, and because of the plastic the heatsink is barely going to change that.

[reSpawn]

Hello szhang, I realised that those mosfets can't give enough power to my motors. I now have some LMD18245T and I want to make a motor controller with them. Have you worked with them before? Can I use a lower supply voltage than 12V ?

[szhang]

No.

Supply Voltage Range (VCC) +12V to +55V

Check the datasheet.

[reSpawn]

Do you have a electronic schematic or PCB image for LMD18200? Seems like they don't need external diodes for protection

[szhang]

eagle has footprints for it i think, and no, no external diodes needed.