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0.13x vs 1.0x gain op amp regulated power supply - fail
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HendriXML:
Hi,

I did some characterization of a IRLZ44N via a power supply https://www.eevblog.com/forum/testgear/this-graph-brings-tears-to-my-eye/msg2190639/#msg2190639.
Did also some curve fitting and came to the hypothesis that it should be possible to drive such a reactive component with very low gain. Here is an ongoing little experiment in which I will examine the possibilities.

First about the schematics:
It is essentially a differential 1 gain amplifier, with the reference voltage at the gate! To keep it from being a fast positive feedback, there is are a few capacitors to suppress that from happening. Given more time the ref voltage settles and Vsg become about the same.

What are the benefits of this solution compared to high gain feedback ones?
There is/should be less overshoots. There's no (relatively) large inrush current to the Mosfet gate (the gate capacitance), because this will be smoothed by a short and fast feedback loop (via R4). Resistor R12 which kind of separates the loops is therefore critical. At first I didn't have this resistor in the circuit and the outcome was rubbish.

The negatives of the circuit I will explore. It probably drops in voltage with a very sudden increase in current. Like going from 1mA to 1A. I’ll do some measurement later on.
It’s uses also quite some idle current because of the Zener diode reference voltage.

One of the attachments shows the noise while driving a DC low RPM motor at 700mA.
Feel free to discuss and comment on this experiment. (I will eventually use it as a 5 x AA battery to 5V regulator). It is with persistence on, so a 5 sec representation of the noise.

Kleinstein:
The amplifier (e.g. OP) is only one of the noise sources in the voltage regulator. It may be relevant at higher frequencies.
The other important noise source is the voltage reference itself, especially at lower frequencies, as essentially all voltage references have quite some 1/f noise.

In the current circuit there is a resisive divider (:2) and the relatively large resistors (47 K that add to the noise -  so it would not be very low noise).

The circuit uses an output stage that controls the current (the MOSFET controls the current via the gat voltage). Because of the rather high trans-conductance of the MOSFET, the overall loop gain may not be that small. The rather simple loop structure with mainly the output capacitor as the bandwidth limiting part may work well with some loads, but may not work well with other loads. The MOSFET is also rather nonlinear and will respond stronger at high current. So the response is to be expected very slow at low current and may be acceptable (and possibly to fast resulting in oscillation) at high current.

The ESR of the output capacitor can have quite some effect on the response - too little may be a bad idea.
HendriXML:

--- Quote from: Kleinstein on February 13, 2019, 03:58:38 pm ---The circuit uses an output stage that controls the current (the MOSFET controls the current via the gat voltage). Because of the rather high trans-conductance of the MOSFET, the overall loop gain may not be that small.

--- End quote ---
Hi, thanks for reacting!

In the supplied link I did some measurements on the Mosfet voltage rise on source-drain vs needed Vgs compensation ratio.

What came out of the graph that the regulation of current might not lineair, the voltage rise isn’t as well. If one compares them then the compensation ratio seems not that widely spread.

I spoke of noise, but I mean all the stuff you don’t want on the output.  :-+

From what I’ve seen it does a great job, but I wouldn’t know how to do a proper benchmark. I know for example the output is cleaner than that of my bench power supply. But it’s got longer leads to the DC motor load.

The posts are also about how to investigate things (with limited means) and learn from the comments.

I guess power supplies are a great way of doing this kind of exploration.
Kleinstein:
A voltage regulator looks like a simple beginners circuit and it kind of still is.  However when looking at the theory behind it a voltage regulator can be quite a complicated beast, one can write books about. There is a lot of theoretical background that may be needed to really understand it in detail. If one really understands a lab supply it something like half way to the EE degree  :popcorn:.

For the "noise" one normally uses this word for the noise coming from the regulator when used with a simple resistive or similar well behaved load. The extra AC part seen with a more nasty load like a DC motor is normally described with by the output impedance. So how much change in output voltage when the load current changes. So it is more about applying an AC current (superimposed to the DC load) and measuring the AC voltage.

For doing this one frequency at a time it would need something like a function generator and a Lockin amplifier or similar instrument. The voltage may well be to small to directly measure with the scope - though with a simple not that good regulator the scope may be sufficient.

The more common test is in the time domain, looking at the step response when the load current changes from something like 10% to 60% of the nominal load (The load levels vary - not sure there is kind of standard case) and back.  Ideally this may need a special switchable load. With a modern DSO to measure one may get away with just 2 resistors and a suitable switch / relay.  Some older lab supplies (e.g. 1960s-1970s) use such curves to show there performance. It is still a good test.
HendriXML:

--- Quote from: Kleinstein on February 13, 2019, 03:58:38 pm ---The ESR of the output capacitor can have quite some effect on the response - too little may be a bad idea.

--- End quote ---
One thing I’ll try to realize is that better specs don’t alway perform better. Voltage needs to drop (maybe a tiny bit) to get a regulation happening.
A capacitor that withstands a voltage drop, also withstands a compensatory voltage rise. One thing I was a bit disappointed in was that the circuit needs a capacitor (a 330uF was good as well). But I did take the best I could find and even measured it having a low esr. So something to keep in mind.

My other  design https://www.eevblog.com/forum/projects/bi-regulated-power-supply/ seems not to need one.
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