Electronics gurus needed - mosfets, PCBs etc

[Archer]

I really should know all this stuff, but not working with the stuff since forever equals letting out the magic smoke

So I've got a few questions with a project I'm working on. Its basically a microcontroller, with various IOs and PWM outs. PWM is running at ±500Hz

• Mosfets, I know should use a gate resistor if connected directly to the controller right (controller maxes out at 40mA). But that'll slow down the fet turn on time iirc. If thats an issue I will use a driver IC. Or can I use a simple transistor, any timing or power disadvantages to these solutions?
• Where the blue blazes can I get library parts for PCB design software? Started with Eagle, now on DesignSpark. Or is there something better that I should be using (freeware)?
• Potentially running 12V, 5A (assume 50°C ambient) through one of the fets (IRFP064N). Shouldnt be an issue from my calcs, only 0.2W across the fet, so no heatsink right? Now one other must be able to withstand a max of 20A. So thats 3.2W, meaning Tj of around 180°C, but with a heatsink it'll be roughly 100°C right? No magic smoke escapades? Might even run two in parallel to be safe.

Thats all for now, thank goodness I didnt forget how to code

 

[R13...]

How about SPICE? Admittedly haven't used it in a very long time.

 

[biena]

Can at least help you with one, I personally use Eagle to find parts/libraries I go here
First stop
http://www.cadsoftusa.com/downloads/libraries?language=en

Next
http://www.opencircuits.com/SFE_Footprint_Library_Eagle
http://www.esawdust.com/blog/eagle/eaglepartsearch.php

Getting desperate
google: eagle library + part number

Fed up
Copy another mosfet with same package and create your own part.

For your part there should exist a part with the name "mosfet" in the correct package that you can use.

Eagle is a learning curve (steep one at that).

For the rest I will not trust my answers to you my experience is also years old and I just do simple stuff for a hobby.

 

[biena]

How about SPICE? Admittedly haven't used it in a very long time.

Spice is a simulator not schematic/pcb. Well that is what my version is would be very surprised if they add the rest of the cad stuff.

 

[kayvee]

Something to be aware of is the gate threshold voltage of the particular FET you select, in your case the datasheet specifies min 2V max 4V, which is probably fine if you are running your micro from 5V, but if you are running at 3.3V or lower, probably not. Note that most parameters are specified at a VDS of 10V, you are going to have 5V. There are FET's available in "L" versions, which are better for lower voltage drive, without the need for an additional FET driver before it.

A series gate drive resistor not requires as it is a capacitive load, unlike a conventional transistor. If added it will slow down the switching of the FET as you will have created an RC netwok on the gate pin. It may be wise to add a pulldown resistor on the gate if there is any likelyhood of you IO pin being left in the floating condition, intentionally, or otherwise.

As for the pinout, it's just as easy to use an existing library footprint like a 7805 regulator, and ensuring you rename the net names in either the schematic or the pad numbers of the footprint in PCB, and double check their positions against the datasheet after!

CAD wise, the 2 packages you mentioned are the main freeware ones out there, but Eagle is just IMO, and not a very intuitive interface. Word is Altium Designer may have a free version coming which will be great, but right now it costs major $$$.

 

[Mr.Jax]

What biena said for parts.

Eagle is not so hard to learn, its just that some stuff aren't so obvious; I'm sure there are some tutorials that will guide you in making your own library parts. It really isn't difficult, once you know what to do


Good luck with your project and have fun!