Electronics > Projects, Designs, and Technical Stuff
Is 550uF too big for a power supply that has CC limit?
bloguetronica:
So, I've been doing some mods to the FAU201 power supply too, which is somewhat similar to the one in the OP. Definitely, I'll have to put a capacitor between the output and the inverting input of the op-amp, if I want to use a capacitor in parallel with R5. The last capacitor (in parallel with R5) will increase the high frequency response, so the former is required in order to decrease the high frequency noise.
In the case of FAU201, a 10nF from the op-amp output to the inverting input would greatly reduce the HF noise, although it was only a slight improvement comparing to the effect of a 3.3nF cap in the same place.
Edit: Added some measurements before and after the mods. They are full of noise because of the cable that I've had to use. The scope probe doesn't fit well between the banana plugs. Even though, you can notice the improvement.
P.S.: Unshielded cables make good antennas
Kind regards, Samuel Lourenço
xavier60:
I have learned much from experimenting with this circuit, https://dangerfromdeer.com/2016/04/06/bench-power-supply-build-part-ii/#jp-carousel-898
Notice that the author gets it totally wrong with the compensation.
After adding proper compensation and other changes, it is now very stable. I did some experimenting with the output capacitor last night.
CV mode is stable with a 1uF MLCC in series with 0.5Ω. CC mode is stable with no output capacitor because the output stage itself is a current source, controlled by the error amplifiers.
I have mostly solved the CC op-amp's windup problem. The CC op-amp is allowed to slew at its full speed until it has taken control of the output stage.
The schematic is on this page, https://www.eevblog.com/forum/projects/linear-lab-power-supply/300/
I don't believe that it has turned into an over complicated monster yet.
I recently changed the sharing resistors to 50mΩ, reducing dropout to 1.65V @ 4A.
BTW: the Darlington at the output is not an Emitter Follower. It is driven by the voltage drop across the 2.2K caused by the current sourced by the PNP transistor, Q1.
bloguetronica:
--- Quote from: xavier60 on January 31, 2019, 08:28:07 am ---I have learned much from experimenting with this circuit, https://dangerfromdeer.com/2016/04/06/bench-power-supply-build-part-ii/#jp-carousel-898
Notice that the author gets it totally wrong with the compensation.
After adding proper compensation and other changes, it is now very stable. I did some experimenting with the output capacitor last night.
CV mode is stable with a 1uF MLCC in series with 0.5Ω. CC mode is stable with no output capacitor because the output stage itself is a current source, controlled by the error amplifiers.
I have mostly solved the CC op-amp's windup problem. The CC op-amp is allowed to slew at its full speed until it has taken control of the output stage.
The schematic is on this page, https://www.eevblog.com/forum/projects/linear-lab-power-supply/300/
I don't believe that it has turned into an over complicated monster yet.
I recently changed the sharing resistors to 50mΩ, reducing dropout to 1.65V @ 4A.
BTW: the Darlington at the output is not an Emitter Follower. It is driven by the voltage drop across the 2.2K caused by the current sourced by the PNP transistor, Q1.
--- End quote ---
I must say that I've learned plenty in the last few days, while reading the comments, seeing some videos and making the mods. Definitely, increasing the speed and compensating (increasing phase margin) is the way to go.
I've took new measurements regarding the AC noise at the output of the FAU201 power supply when loaded in the same conditions. This time, I've used the scope probe and a short piece of wire. One of the measurements was done after the mod, the other before. The noise reduction is incredible. Both capacitors are instrumental to increase phase margin.
I'll apply the same principle to the project in the OP, in an effort to reduce the output capacitance and take out or reduce some capacitors that are masking the issue. Type I compensators (TI terminology) near IC4B and IC7 are definitely needed. Perhaps, I should use a type II compensator near the latter op-amp.
Thanks for all the help so far!
Kind regards, Samuel Lourenço
xavier60:
My main point is, it is difficult to make Current Control work properly with a Voltage Follower output stage.
chickenHeadKnob:
--- Quote from: Zero999 on January 29, 2019, 12:23:04 pm ---It's all but impossible to design a PSU with constant current and constant voltage and have good load regulation for both. A capacitor will improve the transient response of the CV mode, at the expense of CC. No capacitor an a large inductor will improve the CC mode, at the expense of CV mode.
Normally we optimise for CV mode and accept that current surges beyond the current limit can occur. I've toyed with the idea of building a CV/CC power supply with a three position switch which adjusts the filtering to be optimum for CC, CV or the best of both, because no PSU I've seen offers this, although I've never got round to it.
--- End quote ---
Well the HP 6632B and related family members have a fast/normal mode switch , located inconveniently on the back. Quoted from the user manual:
Fast/Normal Operation
A switch on the rear of the unit lets you switch between operating in either Fast mode or Normal mode.
When set to Fast mode, this switch disconnects the output capacitor that is located inside the unit. Fast
mode lets you improve or enhance certain operating characteristics; while at the same time degrading
other operating characteristics.
1. In Fast mode, the programming time for voltage programming is faster than for normal operation,
however, output noise is greater.
2. In Fast mode, the absence of the internal output capacitor results in increased output impedance and
therefore, greater stability when driving inductive loads. Conversely, the addition of external
capacitive loads in Fast mode will reduce the stability of the unit during constant voltage operation.
3. In Normal mode, the internal output capacitor helps control peak voltage excursions away from the
the nominal value for sudden changes in load current. In Fast mode, larger peak voltage excursions
will show up at the output of the unit during sudden load current changes.
Capacitive Loading
In Normal mode, the dc source will be stable for many load capacitances, however, large load
capacitances may cause ringing in the dc source’s transient response. If this occurs, the problem may be
solved by increasing or decreasing the total load capacitance.
In Fast mode, the dc source can maintain stability only for small capacitive loads. These limits are:
Agilent 6631B 2.2 μF.
Agilent 6632B and 66322A 1.0 μF.
Agilent 6633B .22 μF.
Agilent 6634B 0.10 μF.
Looking at the schematic I am lost but it appears the capacitance that is switched is 100 mu Farad for the 6632b, 50 or 22 for the other models :-//
I think having such a feature enabled by a front panel switch on a hobby purpose built supply is not a bad idea, however it doesn't eliminate the need to design for minimum output capacitance though.
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