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Programmable Electronic Load: Dynamic Behaviour: Part 4 Effect of Output Voltage Change

jc2048

This series of posts relates to the work of  and  in their WIP Programmable Electronic Load  blog.

 

In the previous parts, I looked at the effect of changes to the control input. Now I want to look at

the effect of holding the control input constant and varying the voltage of the output. That change

of voltage will result in the circuit having to vary the resistance of the MOSFET, to compensate and

bring the current back to the desired value, and the way it deals with that change may be different

to the way it handles a change to the control input.

 

Here's the circuit, much the same as before.

 

image

 

Initially, I'm going to try inductance of 1mH in each

lead, which is high but will show clearly what the effects are. I'm going to step the voltage of VG2,

the voltage source for the output loop, by 2 volts up and down between 8V and 10V. That's quite

arbitrary - I just want to get a feel for how it behaves. I'm going to have that voltage slewing

rather than a step change because any real power supply would have capacitance associated with it.

 

Here it is with the original compensation/servo components (C2=4.7nF and C3=100p)

 

image

 

And here with my alternative values (C2=1nF and C3=220pF)

 

image

 

I find this fascinating. Which is better? My immediate reaction was that my alternative values, which

so obviously ring, are worse than Peter's original values. And yet the excursion from the desired

value is less at its extremes and it settles quicker than the original values which, to a first

glance, looks much more controlled and graceful.

 

One thing that is evident to me now is that tuning a servo loop for a real system (rather than a

simple textbook example) isn't at all straightforward. Where there is more than one variable that can

change it get even more complicated because we're then making decisions about what is important to us

and doing trade-offs.

 

I'm going to leave this up in the air a bit - there are probably component values that would reduce

the excursion of the first waveform a bit and get it back on course quicker, and it would be natural

to experiment with that for a while, but I want to move on to looking at the open-loop response and

how a traditional Bode plot approach to stability views this.

 

Programmable Electronic Load: Dynamic Behaviour: Part 1 Overview

Programmable Electronic Load: Dynamic Behaviour: Part 2 The Servo Loop

Programmable Electronic Load: Dynamic Behaviour: Part 3 Effect of Output Inductance

  • in reply to jc2048

    Hi Jon,

     

    Thanks for the update.

     

    What I wanted to address was possible oscillation effects of inductance and capacitance from the leads running into the load.

     

    It looks like you have a robust solution for your testing and as long as your leads are short it is probably not an issue.

     

    DAB

  • in reply to DAB

    Hello DAB, thanks for showing an interest and commenting.

     

    I really didn't explain very well what I was doing and made a bit of a mess of that one. I should have gone further with the simulation and shown you the traces for different amounts of inductance in the output circuit. Part of the problem here is that I don't know the material well and often have no idea what I should be be doing - you're looking over my shoulder as I learn and discover things.

     

    The 1mH I chose for the blog is very large. You wouldn't see it in practice with just having connection leads, where the inductance would be more like a few uH (depending on diameter, length, and where the two wires were in relation to each other).

     

    Here's what happens if I step the inductance in each lead from 0 to 1mH in steps of 200uH (this is with the 4.7n/100p combination). This is with the same step change in voltage from 8V to 10V with a slew time of 50uS. Unlike the situation where we are demanding a change to the output current, which is what I was looking at in previous blogs, here the inductance actually helps us. That's because now the servo is trying to maintain the same current, which is also what the inductor wants. These higher values of inductance smooth out the change somewhat.

     

    image

    Now here it is for the case of 0 to 10uH (stepped in increments of 2uH) for each lead (which is a more realistic figure for thick leads of a few metres in length):

     

    image

     

     

    Now the inductance isn't having any real effect and the response is largely down to the servo. Initially the circuit is caught unaware and the current rises. Then the differentiator gets on the job and start to haul it back. The slewing stops, and again it's caught unaware and there's a sudden lurch down, before the differentiator sorts itself out and then there's the longer response of the integrator as it then hunts for the set current again.

     

    This next shows it for different slew times - this is for 1uH inductance in each lead and the slew varied from 10uS to 50uS. The servo manages to limit the excursion to about the same in each case whilst the change is occuring.

     

    image

     

    Whether that's good or not I've got no idea, never having done this before and not having any experience of professional instruments. The good news for me is that it's not frightening - whatever I do to the output, the current excursion is only a few tens of milliamps, so although it may look a bit messy it isn't going to harm anything.

     

    I don't think capacitance is really an issue here. The only capacitance in the output loop that really affects the loop response is the intrinsic capacitance of the MOSFET. That does have an effect, but it's swamped by the integrator (the time constant of the deliberate integrator is longer than that of the MOSFET and so dominates in the circuit response). Any capacitance across the voltage source powering the output loop is the problem of the power source to handle.

  • in reply to DAB

    I don't know if testing with an inductive load represents a real use case, because this instrument is the load.

  • Yes, reality always finds a way to make us reassess theory.

     

    I agree that your ringing is probably fine as long as you do not exceed any of the voltage tolerance of the other components.

     

    Though I would check it on an inductive and capacitive load, just in case.

     

    DAB