Tuning the Negative Feedback Loop
The basic functional block
Abstracted portion of the feedback under investigation
The current-shunt pickup amp
Using an opamp with extremely low quiescent draw because I'm running on supercapacitor energy, I want to detect a load via a shunt resistor measuring battery current, and provide power to offset that current.
High-Side linear PMOS switch
The Optoisolator pulls this PMOS transistor's (Q4's) gate towards ground which turns it on and begins reducing/offsetting the current that turned it on to begin with - a classic negative-feedback scheme
Load Current and Batt Current
To test, I connected a circuit that would draw current from the battery in a pulse fashion (upper trace) and observed the net battery current (lower trace)
Gate drive
At greatly-expanded time-scale, each pulse of the load can be seen to be causing the opamp amplifier circuit to call for several pulses of offsetting current
Unless you KNOW you want multiple pulses, this WILL cause problems
Increased Load Time
To understand the parameters better, I increased the length of time the load was drawing power
Increased Load Time Gate Drive
This divulges that the problem IS a sluggish response to a load, an overcorrection, a saturation, a re-overcorrection to the saturation....etc.
Adding a differentiating component
Placing a capacitor in parallel with a resistor increases the mathematical 'differentiation' component to the feedback. If the pulsations were caused by a delayed response, this should help. Turn on the power in response to 'a change in load'
Load current and Net Batt current
Gate Drive with differential component having been added to primary side of Optoisolator
Not Solved. Now the batt looks to be being CHARGED at every pulse of the load.
We want that lower trace to have only a glimpse of a load, but almost flat
Gate Drive with differential component having been added to primary side of Optoisolator
Gate Drive with differential component having been added to primary side of Optoisolator
Gate Drive with differential component having been added to primary side of Optoisolator
While this waveform could be fine in the context of a gate-drive response to a pulsed load, we see from the previous slide it resulted in net battery charging - that could be dangerous if it overrode other battery voltage-limiting circuitry
An Integrating Component
Gate Drive with differential component having been added to primary side of Optoisolator
Load and Batt Net with Integrating component
With the response being so forceful and proactive, even affecting a net CHARGING in the face of pulsed discharging, it's likely an Integrating component is needed to smooth the initial hyperactive response
Load and Batt Net with Integrating component
Gate Drive with differential component having been added to primary side of Optoisolator
Load and Batt Net with Integrating component
This looks pretty good. Batt discharge is a small fraction of total load. Let's look at the gate drive.
Gate Drive with Integrating and Differentiating components
Gate Drive with Integrating and Differentiating components
Gate Drive with Integrating and Differentiating components
A DC offset? Is it really staying on over the whole period? It makes sense in terms of an integrator that's not getting reset, but it's harder to understand how this could result in the desired regulation.
Adjusting the RC values
Gate Drive with Integrating and Differentiating components
Gate Drive with Integrating and Differentiating components
If it results in the proper cancelling of the current, we wouldn't care THAT the gate drive has a DC offset, but we would want to go back and understand WHY it occurs
Wider load interval
Wider load interval, tailored RC tuned feedback
Wider load interval, tailored RC tuned feedback
I widened the load interval and it flattened out between the 2 transients, so that seems good for absence of oscillation. Integrals above and below the reference line on the batt current (lower) trace looks good, although battery DC voltage should be measured. Area below can be slightly greater than area above, because that represents battery discharge. We DON'T want this area to be positive because that would charge the battery uncontrolled
Wider load interval, tailored RC tuned feedback
Wider load interval, tailored RC tuned feedback
Wider load interval, tailored RC tuned feedback
Although there is still the unexplained DC offset, this looks perfect. A DC offset could be explained by the threshold voltage of the Optoisolator diode. The slight pulse downward suggests it's enforcing the prevention of net charge which we want. The fact that there's the single pulse per period, and no additional ringing or oscillation is a good sign this can move forward to higher order testing.
Here I re-did the above experiments.
Smooth Power Group
Senior Design Project
Demo Setup
