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Is there any variable DC-DC converter suitable for pre-regulation?
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bloguetronica:
Thanks Tim! I'll have to consider all the options. Gain is not important. Probably, I could sort of track the DAC voltage to have a faster response (so that the voltage after the pass element doesn't have to depend on the voltage of the DC-DC converter, which in turd depends on the former).

However, I'll add current limiting, and I have to create an unusual circuitry to compute the "should be" output voltage (which is limited to a value when the current is "limited" to a set value, according to the load resistance - or is equal to the DAC voltage when the current is not limited). Probably, on second thought, I'll track that computed voltage indirectly without the use of the funny transistor, although tracking the output voltage directly via that transistor would be much simpler. So, if I add a voltage to that computed value, via an op-amp or even a Zener pulled up, I'll have the tracking reference for the DC-DC converter. Thus, the computed value would be the reference for the op-amp that controls the pass element, and the added value (via the Zener) would be the reference for the DC-DC converter.

Lets see where this goes.

Kind regards, Samuel Lourenço
T3sl4co1l:
You don't want the SMPS output to track DAC/setpoint output, because it won't follow under a current limit condition.  That's why it's made to follow the actual output instead. :)

(But you can get just as stable DC with a servo amp setting the SMPS correctly; and, a reasonably fast, stable and well-filtered SMPS can do as well on AC noise, as the postreg can, without the extra loss).

Tim
bloguetronica:
I'm planning to do something as represented in the attached sketch.

There are some things that were not represented there:
- The amp circuitry that controls the pass transistor, in this case a Darlington, will include lead impedance/resistance compensation (and the possibility of having an external voltage sensing at the load). This is fairly easy to implement, and it only requires 6 precision resistors and a few protection diodes.
- The DC-DC converter control is not that straightforward, and probably is grossly misrepresented here. The voltage that controls this stage is the same voltage that controls the final stage, but with a Zener drop added. Probably instead of the Zener, I'll use the PNP transistor and drive its base with the voltage from the "computation block".
- The "computation block" is fairly simple. You have two DACs, one that sets the voltage, and one that sets the current. It the voltage resulting from the current sensed at the output equals or exceeds the voltage given by the "Iset" DAC, the compute block will output the voltage so that these previous voltages are equal. If not, the "compute block" will output the exact same voltage as the "Vset" DAC.

This will require some extra precision op-amps and comparators, but it allows for a very precise voltage and current control, which is the main goal of this project. As for taking out the post regulator altogether, and replacing it by a filter, well, you will need a huge filter. I want to keep the noise as low as possible, and on any load conditions.

Kind regards, Samuel Lourenço
prasimix:

--- Quote from: bloguetronica on September 08, 2018, 10:32:13 am ---- The amp circuitry that controls the pass transistor, in this case a Darlington, will include lead impedance/resistance compensation (and the possibility of having an external voltage sensing at the load). This is fairly easy to implement, and it only requires 6 precision resistors and a few protection diodes.

--- End quote ---

Hm, if you are talking from experience than you'll be well aware that is not "fairly easy to implement" since there is a lots of places where things can goes wrong. Many of them are even easily visible in simulation and you can spent hours and hours in front of simulation just to address various "use cases" and still not be 100% sure that everything can be fine with real circuit.


--- Quote from: bloguetronica on September 08, 2018, 10:32:13 am ---- The DC-DC converter control is not that straightforward, and probably is grossly misrepresented here. The voltage that controls this stage is the same voltage that controls the final stage, but with a Zener drop added.
- The "computation block" is fairly simple. You have two DACs, one that sets the voltage, and one that sets the current. It the voltage resulting from the current sensed at the output equals or exceeds the voltage given by the "Iset" DAC, the compute block will output the voltage so that these previous voltages are equal. If not, the "compute block" will output the exact same voltage as the "Vset" DAC.

This will require some extra precision op-amps and comparators, but it allows for a very precise voltage and current control, which is the main goal of this project. As for taking out the post regulator altogether, and replacing it by a filter, well, you will need a huge filter. I want to keep the noise as low as possible, and on any load conditions.

--- End quote ---

It seems that you're really interesting in making a pre-regulator (mind the prefix in that name :)) control overcomplicated. All what you need is a simple PNP tracker connected to output as you are already instructed in some of previous posts. That tracker will faithfully follow output voltage regardless of mode of operation, i.e. CV-constant voltage or CC-constant current when due to current limitation voltage starts to drop and your pre-regulator output voltage will follow that change!
bloguetronica:

--- Quote from: prasimix on September 08, 2018, 10:45:07 am ---
--- Quote from: bloguetronica on September 08, 2018, 10:32:13 am ---- The amp circuitry that controls the pass transistor, in this case a Darlington, will include lead impedance/resistance compensation (and the possibility of having an external voltage sensing at the load). This is fairly easy to implement, and it only requires 6 precision resistors and a few protection diodes.

--- End quote ---

Hm, if you are talking from experience than you'll be well aware that is not "fairly easy to implement" since there is a lots of places where things can goes wrong. Many of them are even easily visible in simulation and you can spent hours and hours in front of simulation just to address various "use cases" and still not be 100% sure that everything can be fine with real circuit.
...

--- End quote ---
Well, I've used the concept already in another project, but I'm yet to test it, truth to be told. Not complicated at all. If you open the schematic attached to the OP, you'll see four precision resistors near the op-amp. That is for internal lead compensation.


--- Quote from: prasimix on September 08, 2018, 10:45:07 am ---...

--- Quote from: bloguetronica on September 08, 2018, 10:32:13 am ---- The DC-DC converter control is not that straightforward, and probably is grossly misrepresented here. The voltage that controls this stage is the same voltage that controls the final stage, but with a Zener drop added.
- The "computation block" is fairly simple. You have two DACs, one that sets the voltage, and one that sets the current. It the voltage resulting from the current sensed at the output equals or exceeds the voltage given by the "Iset" DAC, the compute block will output the voltage so that these previous voltages are equal. If not, the "compute block" will output the exact same voltage as the "Vset" DAC.

This will require some extra precision op-amps and comparators, but it allows for a very precise voltage and current control, which is the main goal of this project. As for taking out the post regulator altogether, and replacing it by a filter, well, you will need a huge filter. I want to keep the noise as low as possible, and on any load conditions.

--- End quote ---

It seems that you're really interesting in making a pre-regulator (mind the prefix in that name :)) control overcomplicated. All what you need is a simple PNP tracker connected to output as you are already instructed in some of previous posts. That tracker will faithfully follow output voltage regardless of mode of operation, i.e. CV-constant voltage or CC-constant current when due to current limitation voltage starts to drop and your pre-regulator output voltage will follow that change!

--- End quote ---
Probably I'm over-complicating. But having a voltage (from the computation block) that is equal to the output voltage, and deriving the DC-DC control from there provides faster response, right? Then I could drive the PNP tracker with the voltage from that "computation block" instead of using the output voltage for that. Yes, I dropped the Zener idea, meanwhile.

Without using any "computation block", I could track the output voltage directly, and implement CC-constant by starving the base of pass transistor. I'm not sure about the precision if I do that. Probably I can get away with just another DAC, an instrumentation amplifier for the current sensing (or a dedicated amplifier), a comparator and another NPN transistor. The NPN transistor would be wired in an analogous fashion as depicted in the OP schematic, but its base would be connected to the comparator. I think this last approach is more simple and could be tested first.

Kind regards, Samuel Lourenço
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