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Build a DC electronic load

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Eazy:
Hello guys,

I need to build a DC electronic load, but my electronic skills are not very good to understanding what I will do.
I have done some researches, but I don't know how to select components.

Here is the specs that i need: 28Vdc and 6A. but it doesn't really matter for now.

I have already watched the Eevblog video of doing an electronic load, and found lots of documents.
Before going further on how control my DC load I need to understand the differences between the basic op amp follower (saw on Eevblog video) and this op amp with resistor and cap (see attachment).

In the basic op amp follower I undersand that the op amp outpout will try to follow the V+ voltage by doing the comparison with the feedback on V-.
On the attachment file, the resistors and capacitor added are to compensate the capacitor of the mosfet. But now the op amp doesn't look like a follower but an integrator, am I right?

MikeK:
I think Dave had a video some years ago about building an electronic load.  That would be good to at least familiarize yourself with the idea.

EDIT: It's EEVblog #102.

Eazy:
Thank you for your answer.

I have already watched this video (as I said in my first post, but I haven't mentionned the video number).
It was a good start, but after some researches, I have found a "upgrade" schematic" with resistors and capacitor, into the op amp feedback, which are there to reduce the oscillation of the output op amp due to the capacitor of the mosfet (If i understood). But with that added, is the op amp still in follower? Because for me it looks like an integrator.

magic:
Well, yes, it's an integrator, but you have to look closely at what it is integrating: the current through R3, or alternatively the voltage across R3.

The integrator will keep IN- equal to IN+, by slewing the output up when R3 conducts to the right (R1 voltage too low) and down when R3 conducts to the left (R1 too high).

The circuit will stabilize when R1 voltage equals IN+. So it is a follower.

blackdog:
Hi Eazy

C1 ensures that the phase margin remains good.
R2 and the MOSFet input capacitor makees that the signal from the opamp output is delayed to the -input of the opamp.
This delay should not be too large, if the delay becomes too large, and exceeds 180 Degrees then the opamp will generate.

Many circuits on the Internet do not sufficiently take this into account.
But at least you have C1 in your circuit!  ;)
The high frequencies now have a short path through C1 to the inverting opamp input and helps the phase margin to be good

Yes, you can say that C1 forms and integrator with the resistor R3.
The point is to leave the high frequencies sufficiently outside R2 and the MOSFet to keep the circuit stable enough.
A good starting point is to choose the filter corner for C1 and R3 around 30KHz and this can be done with R1 of 4K7 and C1 of 1nF.

This 30KHz is not a fixt value, the properties of the MOSFet vary at different Gate voltages and the current through the Drain.
So the combination of R3 and C1 must be such, that the whole circuit is always stable, but not more!
It is a beginner's mistake to make C1 to large!

It is also important to know if you want to use the DC Load only, or for DC or e.g., modulate it as well.
So with modulation you have a Dynamic Load and then you want wide frequency responce.
C1 should then be just large enough to always be stable at different loads and also the best frequency response.

The next step is the sense resistor value and the offset value of your opamp to be used.
It seems attractive to take the sense resistor value nice and small,
but as I mentioned above, you have to take into account the offset voltage and drift of the opamp you're going to use.
The offset voltage/drift of the LM324 or the LM358 is not so good.

Modern opamps are a lot better at that, and the opamp you're going to use should be able to go to "0" with its inputs.
A modern opamp is e.g. the Texas Instruments OPA140 Series, has a reasonably low offset voltage and is also reasonably fast so that if you choose to modulate  the DC Load, 
so that it can become reasonably fast.

There is much more to tell about this type of circuit,
but with a little searching you can also find a design by Jay_Diddy_B on this forum.
Even LT SPice files he has made available, so you can experiment extensively with this.

It is going a bit too far to go explain all this now, I would first test for a basic DC Load, use an LM324 or an LM358 IC and a Sense resistor of 1-Ohm.
At 1V on the + input of the opamp, there will then be 1-Ampere running through the Drain of the MOSFet.

Do some testing with this, see/measure what the limitations are with this setup.

Another tip, take a MOSFet in a large case, these have a low thermal resistance.
A commonly used MOSFet is the IRFP150 for this type of application.
You want a MOSFet that can dissipate quite a lot of power and has low thermal resistance, not to mention low Gate capacitance.

Kind regards,
Bram

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