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Sci.Electronics.Basics -> MOSFET ratings
There are 3 messages in this thread.
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Author: Jon SlaughterDate: 21:23 07-04-08
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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 PowerTrench® 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
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Author: JoergDate: 21:28 07-04-08
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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 PowerTrench® 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/
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Author: EeyoreDate: 00:26 08-04-08
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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
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