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Is there any variable DC-DC converter suitable for pre-regulation?
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bloguetronica:
Hi,

I can't use a off-the-self DC-DC converter without any kind of "postreg", since I want both low noise and accuracy. Hence the topology with the Darlington being served with a precision DAC and a precision op-amp. The voltage after the DC-DC doesn't need to be accurate at all. It only needs to have a certain difference to mitigate the dissipation on the Darlington., yet still providing enough overhead.

Kind regards, Samuel Lourenço
T3sl4co1l:
Ah, so you're not going to attempt an SMPS design yourself, only looking for off-the-shelf?

There's still a lot you can do (e.g., servo the output of an inaccurate module with a more accurate reference and opamp), though the noise levels of COTS modules are probably going to suck no matter how you cut it (passive or active filter).

Tim
prasimix:

--- Quote from: bloguetronica on September 06, 2018, 04:01:06 pm ---Thanks prasimix,

It seems that I'll have to use a buck converter. Can't find any isolated DC-DC converter with a suitable output voltage and/or with external feedback.

I've noticed that the boost or buck converter have a fixed output voltage of 5V, on their standard application. Is there any reason? Does that have any influence? This is hard to understand, BTW.

Kind regards, Samuel Lourenço

--- End quote ---

Yes, in almost 100% of all app. notes and eval. boards output voltage is fixed since it's easier in that case to ensure stability of the circuit despite the load connected and input voltage variations. But, many of them can be make stable regardless of broad output voltage range. So far, I didn't find any example of converter presented as pre-regulator. Usually people mentioned a couple of Jim Williams LTC app. notes that include some basic switching pre-regulator but nothing based on "modern" SMPS controller. If you're going to design your own SMPS pre-regulator take care about PCB layout and try to use as much as possible SMT parts since they are smaller and can be better organized for lower EMI and circuit stability. If possible (don't know your budget and final goal) try to use 4-layer PCB instead of min. two layers.
bloguetronica:
Well, I will have to weight all the options carefully. Probably a self made DC-DC converter will do, although I saw an application based on the LM2576 from Texas Instruments. In my application, I don't require a sophisticated DC-DC converter. However, I was not expecting that the DC-DC converter would be one having a fixed voltage, which is counter intuitive. With that feedback loop, the output voltage is now variable?

Difficult to grasp for me, IMO, for now. I want to get there and fully understand this concept from head to toe. Only then, when I understand the whole concept, and when I can customize it, I can call myself a designer of this project. Probably, a discretely designed DC-DC converter, customized to the task at hand, having minimal parts, would be ideal. A design simple, elegant, and one that I can understand. I have to research, research, research... and reinvent the wheel if necessary.

As for the PCB, you gave a good advice. A 4-layer board is definitely in the requirements for this project.

Kind regards, Samuel Lourenço
bson:
You could relay switch between a few different converters for different ratios.  Then each of them can be optimized for that voltage and a specific load current.  For very low currents you can bypass all of them.
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