Electronics > Projects, Designs, and Technical Stuff
My constant current load - 90W , 3A 30V
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LEECH666:
http://www.maximintegrated.com/app-notes/index.mvp/id/1957

This article by maxim is a bit more detailed, but I am not sure how large the impact of an undefined unused gate really is. I guess it's worth a try.
LEECH666:
This thread is also informative ...

https://www.eevblog.com/forum/beginners/dave's-constant-current-load-traps-for-young-players/
Spikee:
So i replaced the LM324N whit a L2722 1A opamp and i terminated the unused pins. Now it works perfectly.
I will now continue testing whit a lead acid battery to achieve higher currents.
Jay_Diddy_B:
Hi Harvs and the group,

Harvs asked:


--- Quote from: Harvs on December 12, 2012, 04:50:21 am ---
--- Quote ---Note: It sis not safe to operate this MOSFET at high voltages and low currents.
--- End quote ---

Sorry I'm probably missing something here, but how did you come to that conclusion?

For DC loads the mosfet naturally has a Vds limit (the right hand line), a power dissipation limit (as marked on the line sloping down to the right), and a limit of the Rds(on).  Apart from these things, what's not safe about operating this mosfet at say 80V, 0.15A?

Cheers

--- End quote ---

It comes from looking at the SOA curve for the Renesas 2SK3481  MOSFET



If you study the DC line.

At 5V Vds the maximum current is 11A a dissipation of 55W
At 9V Vds the maximum current is 6A a dissipation of 55W

At 20V Vds the maximum current is 1.8A a dissipation of 36W
At 40V Vds the maximum current is 0.65A a dissipation of 26W

You can see how the maximum Dissipation is reduced at higher voltages.

Some MOSFETs do not like being operated in this part of the SOA graph at all.

For further reading on this:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100014777_2010016074.pdf

or

http://powerelectronics.com/power_semiconductors/power_mosfets/power_trench_fets_fragile/


Note Bipolar transistor have similar de-ratings, but the reason for the failure is very different. Google 'Second Breakdown BJT'

JDB


Harvs:
Thanks JDB, I understand what you're talking about now.

Cheers
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