OK, I'm pulsing the low side ,so I thought that low side sensing was the way to go. But... Here is a sim with resistors on the high side. The current through R1 is showing the same as L1. I like the idea of high side sensing because of the added short circuit protection.

 

Remember, commonly available comparators (like LM311, LM393, LM339) can sense to ground, but cannot get within more than about Vcc-1.5v.

High-side sensing is sometimes the way to go, but it does somewhat complicate things.

If you really want to get away from the Rsense power losses and difficulty of high-side sensing, you might consider looking into using Hall-effect sensors.

 

I was looking at this, http://datasheets.maxim-ic.com/en/ds/MAX4376-MAX4378.pdf

Does it look like a possible solution?

 

Interesting find.

Don't really have time to examine it further today.

 

One thing I noticed was that table 1 only goes up to 10 amps. I'm wondering if this will be a problem.

 

You might use the resistance of a trace on the board as a resistor. You really can't use a "normal" SMT/SMD resistor for that much current, as it would vaporize.

There are calculators for copper weight vs trace width vs thermal rise vs resistance.

For example, 200mil wide trace, 2 Oz copper on board, 19A current, 3.62" long trace, 4.99mOhms, 94.8mV drop.

PCBtemp is a useful trace calculator available for a free download. Google's your buddy here.
http://www.smps.us has various calculators available.

 

It's been a long while since I have had time to look at this. The circuit is better off now than before, no components are getting killed anymore. The problem now is that the high side mosfets aren't being turned on fully. The voltage from gate to source on these is only hitting about 7.5 volts. Vcc/Vdd is dropping down to about 8.2 volts when the lower fets start switching. C1 and C2 are .68uF at the moment, not .1 like the diagram shows. Any ideas on why the voltage is drooping so much? I have attached an updated drawing and the data sheet for the voltage regulator I am using.

 

I can see the datasheet, but you only posted 1/2 to 2/3 of your schematic.

Can you post the whole schematic?

 

I didn't even notice that.

 

I believe it's fixed now, it looks like there was a bad solder connection on the board. The Vcc/Vdd voltage is holding now. Gate-source voltage for the upper mosfets is reading ~12 volts.

 

Glad you figured it out.

Sorry I didn't get back to you earlier; many things going on both here on the board and here in my house.

 

Yeah it's pretty busy here too. Thanks everyone for all the help. So far it's running good in my test tank. Now I need to get this thing on the water for some real world testing.