Note the CL next to the 2500pf capacitor, and no fet!!!!
CL = C load. They assume a fet with ~2500pf gate capacitance.
So DON't use that cap if you use a fet.
Using TWO mosfets could be worse than usning ONE.
Two mosfets is double the gate capacitance, so double the gate drive is needed for the same switching time.
Without sufficient drive, the mosfets switch slower, and get hotter.
You could try ONE 30A logic mosfet, e.g 30N06L, with a 100ohm gate resistor, directly driven from the Arduino.
Add a 100k resistor from gate to source for security during startup.
That would probably be ok for the 12A peltier.
Guestimation: ~6-9watt losses/heat in the mosfet and about 500mv max S/D when "on".
Did you calculate the 15uH inductor, or just grab one.
Value seems very low for 31Khz.
What is the size.
I will remove that cap, I really appreciate that!
The calculated size is actually 8uH at 31Khz, Switching Converter Power Supply Calculator is the calculator I used. But I did just grab one, I salvaged inductors from old ATX PSU's, I'm reusing the main inductors, 5 wire, originally for 3.3, 5 and 12 in a PSU, I just run 12v through all the lines, I have an inductance meter so that's how I know it's 15uH. I'm not sure of it's saturation amperage, but I'm reasonably sure that it should handle the max of 25a. At least I really hope so....
The driver I'm using is rated at 6 amps, I wish I had screen shots but I still haven't repaired from my 'oops' on the electrolytic... It did seem to be driving dual FET's with no issues, I had nice clean pulses. (even with a bit of ahem extra capacitance...)
The FET I am swapping to is a logic level - AUIRLI2505
http://www.mouser.com/ds/2/200/auirli2505-321030.pdf which was one of the better ones that I found. A bit expensive, but I was shooting blind, I've never designed a circuit before and you are the first person who's replied with more than a "you should use this" and then never shows back up! (this is in different forums mind you)
I really appreciate the advice and help! I'm not sure I have a large enough heat sink to dump the heat if I run a single FET though. (again, salvaged heat sinks from PSU's, cut in half to fit on a 2x3 board)
Here are my thoughts on using the driver:
- If I screw up, I blow up the driver, not the Arduino (and I WILL screw up at some point, 3 drivers later I'm absolutely positive of that )
- Running multiple MOSFETs, the driver is only a single output, but has more than enough current to sink Vds on multiple FET's.
- I had another reason here..... But I lost it. I'll think of it later!
The FET that I chose should run just fine off the Arduino and it would really really simplify my circuit! I'm just not sure that I have the cooling capacity for running just one (which was why I went parallel, testing the IFRZ44N it reached 50c running a 10a load with a single FET and heatsink, figured that would go up as I increase the amperage across it).
Again, I'm all ears for advice and I really appreciate everything you've gone over so far! I'm not by any stretch of the imagination an engineer, so I am leaning towards the conservative side on my heat dissipation and amperage ratings of the components. I would rather over build then under build....