Electronics > Projects, Designs, and Technical Stuff
Is there any variable DC-DC converter suitable for pre-regulation?
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bloguetronica:
Hi,

I'm currently thinking of doing a variable power supply. In order to be more efficient, I wanted to use a variable DC-DC converter before my pass Darlington element, basically to keep the input voltage a couple of volts above the output voltage, and to reduce the dissipation on that pass Darlington. I don't have a circuit yet, but it would be something in the likes of the one attached, but adding lots more power besides that DC-DC converter before the pass element (in this case will be a Darlington and not a simple NPN).

I wonder, is there any DC-DC converter controlled directly by voltage? Say, a tracking DC-DC converter? I think I can make a control circuit that just adds 2 or 3 volts and outputs the voltage to be tracked, no problems there. I want this to be very accurate, and that's why I'm not planning to use an off the self DC-DC converter + LDO in the same package or something.

Kind regards, Samuel Lourenço
David Hess:
DC-to-DC converters which use an external feedback divider can be used to do this.  One complication however is that the DC-to-DC converter will need to operate over a wide voltage range.
bloguetronica:
Thanks David. But using a DC-DC converter like this wouldn't have the opposite effect? I mean, if the DC-DC converter sees a too low voltage on its feedback loop (from the DAC, or the output - to be decided), won't it attempt to ramp up its output voltage to compensate?

Kind regards, Samuel Lourenço
David Hess:

--- Quote from: bloguetronica on September 06, 2018, 02:30:52 pm ---Thanks David. But using a DC-DC converter like this wouldn't have the opposite effect? I mean, if the DC-DC converter sees a too low voltage on its feedback loop (from the DAC, or the output - to be decided), won't it attempt to ramp up its output voltage to compensate?
--- End quote ---

Isn't that what it is suppose to do?  The idea is to use the DC-to-DC converter to regulate the voltage across the linear regulator while the linear regulator regulates the output voltage.  If the output voltage rises, then the output voltage from the DC-to-DC converter rises to compensate and maintain the same voltage across the linear regulator.

bloguetronica:
Hi David. That was the idea. The DC-DC converter has to keep a certain voltage difference from the linear output, or from the DAC. I see that my logic was flawed. I was thinking about the feedback return seen on many DC-DC converters. So, what I have to use is a variable DC-DC converter that has its voltage set by a voltage divider, right?

Kind regards, Samuel Lourenço
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