Electronics > Projects, Designs, and Technical Stuff
My constant current load - 90W , 3A 30V
Jay_Diddy_B:
Hi,
I have been following this thread for a while. It is time to jump in and offer some advice that will lead to successful loads.
Here is a simple load:
The circuit is based on the LT1635. This is a combination of a 200mV reference and an op-amp.
The circuit use a 0.1 Ohm resistor in the source of the MOSFET to give a maximum current of 2A.
The datasheet for the LT1635 can be found here: http://www.linear.com/product/LT1635
MOSFET selection
Some MOSFETs that have been optimized for switch mode power supplies will not work well in the Linear Region. We can tell this by looking at the Safe Operating Area curve on the datasheet. Here is a Vishay IRF530:
This curve shows where it is safe to operate the MOSFET. This MOSFET is only limited by power dissipation.
The second SOA curve shows a Renesas 2SK3481
Note: It sis not safe to operate this MOSFET at high voltages and low currents.
MORE Power, More MOSFETs
To increase the power handling more MOSFETs are required. It is not a good idea to parallel MOSFETs.
Why??
The transfer graph:
shows that at currents below 10A the drain current will increase with temperature for a given gate source voltage. The hot MOSFET will hog the current and get hotter. We now have the possibility for thermal run away.
Matching the MOSFETs may help a little, but the better way is to use an op-amp per MOSFET. This removes need to match the MOSFETs.
This technique is found in most commercial loads.
JDB
Harvs:
--- 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
Spikee:
As i stated in my previous post the lm 324n wont regulate at all and the output will jump to the top rail. It looks like the lm324 cant drive the mosfet any more. Ill have to look if i have a higher power mosfet to drive the fet.
LEECH666:
--- Quote from: Jay_Diddy_B on December 12, 2012, 04:10:32 am ---Hi,
[...]
shows that at currents below 10A the drain current will increase with temperature for a given gate source voltage. The hot MOSFET will hog the current and get hotter. We now have the possibility for thermal run away.
Matching the MOSFETs may help a little, but the better way is to use an op-amp per MOSFET. This removes need to match the MOSFETs.
[...]
This technique is found in most commercial loads.
JDB
--- End quote ---
Hi JDB,
The Master/Slave design is exactly what I am trying to build in my "own variant" of a dummy load.
I've linked to my thread previously but anyway here is the link again
https://www.eevblog.com/forum/beginners/derpy-load-is-this-dummy-load-design-any-good/
Cheers,
Florian
//EDIT:
Spikee, since the LM324 is a quad OP: Did you properly terminate the unused gates? I read that if you leave the unused gates at undefined levels that this might interfere with the used gates.
For example here: http://www.electro-tech-online.com/general-electronics-chat/42965-beginner-unused-half-dual-opamp.html#post353639
and here
http://twilightrobotics.com/wiki/Properly_Terminating_Unused_Op-Amps
Spikee:
I will try your suggestion leech. I had them floating all the time. I will try it now.
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