MOSFET ratings

[Jon Slaughter]

Does the current rating of a mosfet take into account all maximum
circumstances under normal operating circumstances? e.g., basically if it
says X amps and I run it < 80%X or so will everything be ok? i.e., does it
essentially include the power dissipation in that value too(in the sense
that P_max = X*Rds_ON)?

Just not sure if I actually need to calculate anything else if I'm running
it under "normal operating conditions"? I'm just trying to find some decent,
but not too derated, mosfets(actually H-Bridge) for motor control of about
15A but not sure if they can handle the power dissipation but not sure if
the max current rating takes that into account or not. I do see the max
power dissipation on the data sheet but I'm a bit confused at how they got
it:

Dual N & P-Channel PowerTrenchŽ MOSFETN-Channel: 40V, 20A, 24m?
P-Channel: -40V, -20A, 54m?
Features
Q1: N-Channel
??Max rDS(on) = 24m? at VGS = 10V, ID = 9.0A
??Max rDS(on) = 30m? at VGS = 4.5V, ID = 7.0A
Q2: P-Channel
??Max rDS(on) = 54m? at VGS = -10V, ID = -6.5A
??Max rDS(on) = 70m? at VGS = -4.5V, ID = -5.6A
??Fast switching speed
??RoHS Compliant

How can it be both 20A and 9A?

Thanks,
Jon

 

[Joerg]

Jon Slaughter wrote:

Does the current rating of a mosfet take into account all maximum
circumstances under normal operating circumstances? e.g., basically if it
says X amps and I run it < 80%X or so will everything be ok? i.e., does it
essentially include the power dissipation in that value too(in the sense
that P_max = X*Rds_ON)?

Just not sure if I actually need to calculate anything else if I'm running
it under "normal operating conditions"? I'm just trying to find some decent,
but not too derated, mosfets(actually H-Bridge) for motor control of about
15A but not sure if they can handle the power dissipation but not sure if
the max current rating takes that into account or not. I do see the max
power dissipation on the data sheet but I'm a bit confused at how they got
it:

Dual N & P-Channel PowerTrenchŽ MOSFETN-Channel: 40V, 20A, 24m?
P-Channel: -40V, -20A, 54m?
Features
Q1: N-Channel
??Max rDS(on) = 24m? at VGS = 10V, ID = 9.0A
??Max rDS(on) = 30m? at VGS = 4.5V, ID = 7.0A
Q2: P-Channel
??Max rDS(on) = 54m? at VGS = -10V, ID = -6.5A
??Max rDS(on) = 70m? at VGS = -4.5V, ID = -5.6A
??Fast switching speed
??RoHS Compliant

How can it be both 20A and 9A?

Because the marketing guy sez so? Seriously, look at the guaranteed
Rdson at your drive level and nothing else. Graphs are just typical data
and I never design by those. And the marketing glitz, well, you know.

Be conservative. If in doubt take a biger FET.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Eeyore]

Jon Slaughter wrote:

Does the current rating of a mosfet take into account all maximum
circumstances under normal operating circumstances? e.g., basically if it
says X amps and I run it < 80%X or so will everything be ok? i.e., does it
essentially include the power dissipation in that value too(in the sense
that P_max = X*Rds_ON)?

There's more to power dissipation that that alone if it's switching such as turn
on and off losses during which it's operating in a linear way.

Graham