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Lab Power Supply - The Lost Current
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C:
If you look at that HP supply you will see a whole bunch of differential connected op amps.

And you can create a dual differential mixer using an op amp
I call this a mixer as used in audio but it is actually a summer.

Think differential output -  differential reference.

In addition to working it also lowers noise some.


Kevin.D:

--- Quote from: C on February 27, 2018, 12:49:20 pm ---
One of the big problems for a power supply is the unknown load.


--- End quote ---
That's the challenge really :

Designing (not copying or combining a few integrated regulator's) a DECENT bench power supply really takes a fair amount of knowledge and skill, many try but few succeed in creating anything decent, the web is full of them.
It's far more challenging  than designing a C.V/C.C source (linear or switcher) for a 'known' fixed load.


--- Quote from: xavier60 on February 26, 2018, 01:58:17 pm ---
No, not at all. In my previous post I explained that I had eliminated  instability when C6 is 1uF. I can even get it to be stable with no output capacitor. But I will be putting a 47uF on the output when I finish it.

--- End quote ---

To be sure a design is up to the job you have to test the stability and frequency response under the most 'unfavorable' loads you can expect.
knowing what the most unfavorable conditions possible to test under then is crucial in a design/test process.

Here's a few suggestions to try :-
For C.V Mode:-  test at a set voltage that gives lowest voltage across  Mosfet (which gives max Cgd) and  near the max current rating of the supply using a current sink as the load (so high Z load) and load the output with ceramics, say 2uF worth to emulate a load which may have lot's of ceramic decoupling cap's (so this test has created the lowest possible frequency gate and load pole's using very low esr caps).
Then also test with just a low ohmic load and no load capacitance and at high current and at max/min voltages (now there is no load pole and a high current (= higher FET Gfs) so the Vcntrl loop should be at max gain/speed).
For CC mode test. Try load stepping at high current with various pure inductive loads (10uH,1mH,100mH), but the large capacitor you generally find on the output of supplies though makes C.C mode very stable and ruins it's transient load response .

The large cap on the output of almost all bench supplies is a bit of a fudge really but it gives a very good load transient response in CV mode and makes for easy stability (rely's on large size and ESR) so the trade off is  sacrificing  CC mode performance in favor of CV mode performance.

 
Best Regards   . Kevin
xavier60:

--- Quote from: Kevin.D on February 27, 2018, 06:49:28 pm ---For C.V Mode:-  test at a set voltage that gives lowest voltage across  Mosfet (which gives max Cgd)

--- End quote ---
That is a problem, when the regulator goes into drop out. The Gate gets over charged causing a longer than usual delay when either op-amp tries to turn off the MOSFET later. Ill try some higher current op-amps later like TLC072. Reducing the control supply rail voltages should have some advantages also.
C:

Most op amps have a limited current output.
Here the op amps only have one direction control with the other by resistor.
With limited current output of op amps in mind, think a two transistor current booster would be a better choice.
This could be placed next to the gate of Q2 and give a better increase/decrease of output.

If you have a signal source need to check the booster for speed.
Here you can do a non linear voltage to current curve of booster with the center more action of op amp and current increasing for larger changes.
have booster powered from 12v rails.
Make up side match the down side of booster.
Note Booster not just a high current driver.

You need to check the range needed to adjust booster properly.
Take measurements of Q2 gate to base.

Make an xy chart of the values. One axis output voltage and other current.

Then test that the booster will function over the range need with a little extra margin.
Speed is important as you can easily slow the speed but not boost it.





xavier60:
I have experimented with an Emitter follower buffer and have found little advantage in turning the MOSFET on faster than the 2ma from the pull up resistor. While Vds is above 2V, the Gate is fairly easy to drive. 
The TLC072 is suppose to output about 50ma which will turn the MOSFET off fast enough and I can increase the pull up current if needed also.
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