# Newbe needs help MOSFETs and PCB

Discussion in 'Electronic Basics' started by Dan, Jun 28, 2007.

1. ### DanGuest

Greetings everyone,
I am a rank amateur with electronics. I have found a Battery Isolator
schematic at Discover Circuits.com and I would like to build one of
these. I understand what I’m doing but not sure how to do it. I
downloaded Express PCB software and designed a few PCBs. The problem I’m
having is with the amp load on the PCB.
This battery isolator uses P Channel MOSFETs is rated for 60 amps. My
alternator is rated at 35 amps, but when I had it tested, he stated it
put out 40 amps. He did not specify the voltage he tested it at. So I’ll
call it 40 amps to be on the safe side.
I designed a PCB where the all the components will fit
I used a trace width calculator on the Internet at it’s telling me that
I need trace widths of 1.3” or so for the drain and source traces at 40
amps. They are huge, and it makes sense because the traces are so thin.
So in order to get very thick traces, I am thinking of using sheet
copper. I figured .036” thick copper .250” wide is roughly the same
cross section as #8 copper wire. Solder these to the bottom of the PCB
right over the traces. Am I thinking in the right direction? How do you
normally put thick copper on the bottom of PCBs? How do you normally
handle large amp loads on PCBs?
I am also worried about localized heat where the transistor’s pins are
soldered to the copper sheet. Should I be concerned with this?
I am going to put a heat sink on each MOSFET to handle the heat.
Dan
http://www.discovercircuits.com/H-Corner/bat-iso.htm

2. ### EeyoreGuest

You would normally use thicker copper on the pcb. 'Standard' copper is called
one ounce (that's the weight of copper per square foot or something, I've
forgotten now) and is ~ 35 um thick. 2 oz (70um) is readily available and you
can get thicker too if you ask.

Graham

3. ### kellGuest

Because the mosfet has a tab electrically connected to the drain, you
can screw the mosfets directly to a metal heatsink using no insulators
and screw a wire onto the heatsink. The wire will then be connected
to the drains of the mosfets. You must isolate the heatsink
electrically from other parts of the circuit in this case. Also, you
need a separate heatsink for each battery's mosfet bank. Then you only
need a fine trace from the drain to the chip.
As for the mosfet sources, consider leaving the source pins long so
they stick out the other side of the PCB and solder wires on the
pins. That way you wouldn't have any heavy currents going through