Electronics > Projects, Designs, and Technical Stuff
Is 550uF too big for a power supply that has CC limit?
bloguetronica:
--- Quote from: xavier60 on February 01, 2019, 12:16:22 am ---My main point is, it is difficult to make Current Control work properly with a Voltage Follower output stage.
--- End quote ---
What do you mean? I think there is some confusion. IC7, the op-amp responsible for the CC regulation, is not a voltage follower. The circuit in the OP works, but is not very well tuned.
There are two DACs: IC3 sets the voltage and IC5 sets the current limit. The only voltage follower here is IC4A, and it is needed as the IC3 DAC buffer. IC4B is the error amplifier, IC6 amplifies the sensed voltage in order to "read" the current and IC7 acts as a comparator of sorts that sets the CC mode when the current is exceeded.
Kind regards, Samuel Lourenço
xavier60:
--- Quote from: bloguetronica on February 01, 2019, 03:03:08 am ---
--- Quote from: xavier60 on February 01, 2019, 12:16:22 am ---My main point is, it is difficult to make Current Control work properly with a Voltage Follower output stage.
--- End quote ---
What do you mean? I think there is some confusion. IC7, the op-amp responsible for the CC regulation, is not a voltage follower. The circuit in the OP works, but is not very well tuned.
There are two DACs: IC3 sets the voltage and IC5 sets the current limit. The only voltage follower here is IC4A, and it is needed as the IC3 DAC buffer. IC4B is the error amplifier, IC6 amplifies the sensed voltage in order to "read" the current and IC7 acts as a comparator of sorts that sets the CC mode when the current is exceeded.
Kind regards, Samuel Lourenço
--- End quote ---
I mean the output stage transistor Q4. I don't fully understand how everything works , mainly what I have discovered from experimenting and somewhat from what others have said.
Having the output transistor functioning as a voltage follower makes voltage regulation easier because the transistor itself is performing voltage regulation in having the Emitter voltage follow the Base voltage. The CV error amp only needs to make small corrections.
Current regulation is easier when the output transistor/s are configured to function as a voltage controlled current source or transconductance amplifier. The CC error amp only needs to make small corrections.
When one has the choice of only one of the output stage types, the transconductance type is the best compromise, from my experience.
It can be made to work well for CV and CC modes.
bloguetronica:
Today, I've been doing some tests, and concluded that a capacitor in parallel with R5, R7, or in parallel with any resistor that is located in series after the output of a compensated op-amp, is a bad idea. A capacitor from the output to the inverting input is generally a good idea and works well. In my case, when testing modifications to the FAU200 and FAU201 boards, I found out that a type I compensator consisting of a 1K resistor at the inverting input and a 3,3nF capacitor from the output to the former input works well. The noise generated by the DC-DC converter is around 60KHz, and gets reduced.
--- Quote from: xavier60 on February 01, 2019, 03:40:18 am ---
--- Quote from: bloguetronica on February 01, 2019, 03:03:08 am ---
--- Quote from: xavier60 on February 01, 2019, 12:16:22 am ---My main point is, it is difficult to make Current Control work properly with a Voltage Follower output stage.
--- End quote ---
What do you mean? I think there is some confusion. IC7, the op-amp responsible for the CC regulation, is not a voltage follower. The circuit in the OP works, but is not very well tuned.
There are two DACs: IC3 sets the voltage and IC5 sets the current limit. The only voltage follower here is IC4A, and it is needed as the IC3 DAC buffer. IC4B is the error amplifier, IC6 amplifies the sensed voltage in order to "read" the current and IC7 acts as a comparator of sorts that sets the CC mode when the current is exceeded.
Kind regards, Samuel Lourenço
--- End quote ---
I mean the output stage transistor Q4. I don't fully understand how everything works , mainly what I have discovered from experimenting and somewhat from what others have said.
Having the output transistor functioning as a voltage follower makes voltage regulation easier because the transistor itself is performing voltage regulation in having the Emitter voltage follow the Base voltage. The CV error amp only needs to make small corrections.
Current regulation is easier when the output transistor/s are configured to function as a voltage controlled current source or transconductance amplifier. The CC error amp only needs to make small corrections.
When one has the choice of only one of the output stage types, the transconductance type is the best compromise, from my experience.
It can be made to work well for CV and CC modes.
--- End quote ---
I've far more concerned with the CV performance. The CC mode is accessory in this project, and was only added as a safety feature in order to protect the loads.
Kind regards, Samuel Lourenço
David Hess:
--- Quote from: bloguetronica on February 01, 2019, 09:27:56 pm ---I've far more concerned with the CV performance. The CC mode is accessory in this project, and was only added as a safety feature in order to protect the loads.
--- End quote ---
This points to a difference between common power supplies which have an adjustable current limit and more specialized power supplies like those found in source-measurement instruments. Good constant current performance requires low output capacitance and low output capacitance also limits surge current when a lower voltage device is connected to a higher voltage output.
bloguetronica:
Well David, my objective is to make this supply as versatile as possible, while stable. I'm planning to add just the needed capacitance and a little extra. But CV performance is primordial. It must be low noise and high precision in terms of voltage. The current setting is not nearly as precise.
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