Thanks guys, all great points!

This is a controller for my latest endeavor.=A0 I'm putting a small 12= volt trolling motor on the back of a kayak and controlling the steering vi= a a joystick.=A0 I'm using a 12 volt dc 14 Nm gear motor to turn the tr= olling motor.=A0 This gear motor has a stall current of 26-32 amps and a no= minal current of about 8 amps.=A0 The trolling motor will be mounted in suc= h a way that the torque will not be much.=A0 The spec says peak power is 36= .5 watts and nominal is 24 watts but if I'm pushing 8 amps at nominal p= ower 12 volts aren't those h-bridges seeing 96 watts?

Having worked in gte's government systems environmental lab f= or a few years I've seen first hand how vibration and shock play a roll= in system reliability.=A0 The vibration shouldn't be too bad especiall= y where I'd mount the board on rubber mounts but I'm thinking the T= O-263 package will be better with the beefiest heatsink I found for that pa= ckage DV-T263-101E-ND.=A0 And this way I can make use of the big pad for th= e output.

Thoughts?

On Sun, Jul 21, 2013 at 12:01 PM, Stuart Brorson <sdb AT cloud9= .net> wrote:

Good points. =A0I agree that environment -- = shock & vibe -- also play a
role in deciding whether to stand the part up, or lay it down. =A0I
would imagine that the amount of space you have over the board will
also play a role.

As for standing the part up, there are heatsinks which are mounted to
the PCB, and the tab is screwed to the heatsink. =A0What do you think
about them w.r.t. the shock/vibe issue you raise? =A0I think you have
expertise in design for extremely harsh environments, and know more
about this than I do.

The one problem with laying the part down is that in general I don't know how to calculate the heat dissipation of a large PCB pad. =A0With
standing parts and commercial heatsinks, the heatsink comes with a
rating allowing you to compute whether it will handle all the power
dissipated by the part. =A0For computing the dissipation allowed by a
PCB pad I generally look around for an app note with a recommended
footprint. =A0Maybe there is some commerical thermal analysis program
out there which can compute the thermal dissipation of an aribtrary
pad?

The good news is that this Infineon part comes in a TO-263 variant.
It is designed to lay down, and use the tab as the heat sink. =A0I found a TI app note which gives specs for the thermal conductivity of the
TO-263 package, and a clear drawing of the recommended footprint.

http://www.ti.com/lit/an/snva328a/snva328a.pdf

Maybe Rob should consider this package?

Stuart

On Sun, 21 Jul 2013, Bob Paddock wrote:

On Sun, Jul 21, 2013 at 10:27 AM, Stuart Brorson <sdb AT cloud9 DOT net> wrote:

1. =A0Since the output is on both pin 4 and pin 8 (the tab), you can
stand the part upright, with the tab in the air,

2. =A0You can lay the part down on the board,

Which one you want to do will be largely determined by how much power
the part will dissipate.

The other major determining factor for stand/lay, be sides space and
power dissipation, is the system environment.

If the unit is subject to high vibration levels (this is a motor boat contr= ol?)
or high G levels (dropping the motor when mounting), standing parts
tend to break (or be ripped in the case of G forces) their leads at
the solder junction after a while.
Alas no way to know how long "a while" might be.

I've seen this failure most with standing electrolytic caps. =A0Some have had the leads break *inside* the case so the part looked fine.
:-(

I've learned the hard way it is best to lay parts down when you can. Don't overlook the stress of bending the leads to get that done, as
that can also lead to system =A0failures.

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