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Electronics design for cash
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Valen
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Joined: 07 Jul 2004
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lower PWM frequency is better for the fets but worse for the motor.
Given the size of the motor your driving you might get away with something in the 500Hz range (more inductance means lower ripple), The switching period Vs the "charge" period is such that the "stiffness" of the power supply is largely irrelevant.

Your also going to want to add some nifty diodes across the motor to help with the freewheeling current when the switches are off (rather than passing it through the fet body diodes) High speed and low forward voltage is what your after but the current doesn't matter that much, you can probably overrate them by double or more.

In terms of fet selection your main interest is in RDS on. The lower that is the less heat the thing is going to generate. Thats why there are 300A fets in 50A packages.

I have a design for a 6x 1405 setup thats about as big as your thumb, its been working on the bench (well it was a while ago), I haven't run it as a low side switch but it should be good for 200-300A or so if you can get the heat out adequately. Thats with 3khz PWM. It just needs a brain, power supply and the software written for it, but that should be pretty straight forward. Whole thing will probably look a bunch like our current bolt switch, 2x ring terminals with some LED's and a servo plug.

I'll see how I'm going later in the week, work is a bit full on at the moment.

A thing to look for would be the power supply chip, I think something in the way of an automotive grade voltage regulator, around 100-300ma output at 14 volts or so.
The hard part will be finding one thats happy with a maximum input voltage of around 28-36 volts and an absolute max of 50.
Otherwise we'll just get a high voltage regulator and roll some protection circuitry in front of it.
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Post Tue Jun 24, 2008 10:52 pm 
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Nick
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I just searched thru Digikey and this part looked useful: IRFP2907, http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=IRFP2907-ND

Compared to the 1405, it has a higher voltage rating, higher theoretical current rating, lower RDS and barely half the die to heatsink thermal resistance, so they can be pushed harder in a high ambient temperature. The TO247 case is about 5mm larger all 'round than the TO220, so four to six of them will not take up all that much more space. These FETS are more expensive, but I think the bang for the buck is about the same. We can get things going with the 1405 FETs and up-size later on.
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Post Tue Jun 24, 2008 11:09 pm 
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Valen
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they look ok yeah.
Mk 1.5's perhaps lol
I have a tube of 1405s to get through first ;->

The thing i have nfi on is the power supply chip at this point in time.
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Post Tue Jun 24, 2008 11:15 pm 
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Nick
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I will look around for something with the specs you gave. If they come in low quantities, Which supply company do you find the easiest to deal with? Anything else we need?
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Post Tue Jun 24, 2008 11:17 pm 
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Valen
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everything else is probably jaycar able.
If you find one we can probably hit the manufacturer up for samples
I have accounts with most of the big ones.
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Post Tue Jun 24, 2008 11:19 pm 
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Valen
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these dont look so bad
http://www.national.com/mpf/LM/LM2574HV.html
switch mode which i would have rather avoided for this but still given the input power requirement, i havent found anything > 50ma thats a linear regulator
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Post Wed Jun 25, 2008 11:09 pm 
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Nick
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Hey, that's a very versatile regulator - could be used for many other things.
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Post Thu Jun 26, 2008 9:56 am 
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Nick
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Was your FET driver chip the TC4452 or similar? Its a very clever IC; I looked at the data sheet and with a low switching frequency it will be dissipating quite low power, so the 8 pin dip version should work. I also looked at using multiple smaller FETs like the IRF1405 VS several large modules - the combined input capacitance is quite a bit higher for the 1405s, so switching them fast would be harder.

For version one of this switch, we should just get the FETs, driver and opto-isolator going to make a basic SSR. The brains can be added in Ver. 2 as a separate module.
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Post Fri Jun 27, 2008 1:45 am 
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