Hi,
I am trying to build an H-bridge using MOSFETS (how many threads start with this?).
I have decided to use P-channel FETs for the high side rather than N-channel (I will admit to not being able to deal with the additional gate drive complexity for an all N-Channel bridge).
I am trying to determine which MOSFETS to purchase with an aim to being able to handle 12 Volts and 30 Amps (and minimal dollars).
Looking at the data sheets for various MOSFETS, I was delighted to find that, in many cases, Ids max will often far exceed my expectations for a mere $1.50 !
Then I discovered this:
http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html
and have been depressed ever since
.
Given that, for most of the “Through-hole” N-Channel MOSFETs that I have looked at:
- Rtheta Junction to Ambient is 62°C/W
- T junction max is 175°C
- 25°C ambient is almost typical for southern California
am I really doomed to spending $5.00 - $10.00 for a MOSFET with and Rds_on of <2 miliOhms in order to meet my 30 amp goal?
(This is based on:
P = (Tj max – T ambient)/Rtheta Junction to Ambient = 2.4 Watts
Using P = I^2R and a factor of 2 to cover for the increase in Rds_on at Tj max:
2.4 Watts = 30^2 Amps^2 x (Rds_on x 2) Ohms
Which gives a target Rds_on of 1.3 milliOhms.)
I understand that I could “stack” the MOSFETS to try to over come this but I was hoping to keep the final package as small as possible.
Thanks in advance for your thoughts / comments.