Is My LED Circuit Safe? Understanding the FET Safe Operating Area

[rbelli1]

I am designing a circuit to control an LED. The current will be under 40mA and the 2n7000 FET has a maximum operating current of 200mA continuous. See attached schematic. The switch in my final circuit is actually another IC.

I looked up the http://www.fairchildsemi.com/ds/2N%2F2N7000.pdf" and everything looked fine until I got to the Maximum Safe Operating Area on page 6.

20 Volts and 40mA are above the safe operating area with 10 Volts on the gate. Vds will be 20V in the off state but very small after the FET turns on. The solid DC line looks like the maximum continuous power dissipation of the device. The dashed line indicates the power dissipation of Rdson with the specified voltages applied.

Will the attached schematic violate the safe area? If so, why? If not what does the the graph mean?

Many thanks to anyone that can clear up my understanding of this FET

BoB

 

[es1]

Those plots are just trying to show how much power you can dissipate safely in the FET. X-axis is V, y-axis is I, multiply together to get P. With the right voltage even a current below Ids(max) can be "unsafe" if it causes the FET to over heat due to excess power in the FET.

You notice as the pulse time gets shorter the peak power of the pulse can go up.

Your circuit is below the "safe" line (meaning there will not be too much power in the FET) thus it is safe.

 

[rbelli1]

So as long as the on state is below the line I will be safe?

BoB

 

[es1]

In terms of power rating, yes.

You have the other specs covered as well though so I don't think your design will have an issue.

 

[rbelli1]

Thanks. I was worried that there was some aspect of the care and feeding of FETS that I was unaware of.

BoB