Switching supply topology

Thread Starter

eblc1388

Joined Nov 28, 2008
1,543
Hi,

Is there a configuration that would produce a constant output voltage with input voltage ranging from below to over the desired output voltage?

E.g. A 5V output voltage for 3~10V input voltage.

Links on info or schematic examples welcome.

Thanks.
 

hgmjr

Joined Jan 28, 2005
9,029
The topology that you are referring to is a referred to as a buck-boost mode smps.

hgmjr
 

thatoneguy

Joined Feb 19, 2009
6,349
Your description is of a "Buck/Boost" Switching Power supply. They are available in IC format, or as discrete circuits, but not a quick and easy circuit to design and build, compared to a linear regulator.
 

jpanhalt

Joined Jan 18, 2008
7,699
National Semiconductor has a design tool on its web site for its "simple switcher" series. I don't have the links handy. You will find a lot of chips numbered like LM2679; be sure to look at the LM22679, etc. chips as well.

John
 

Thread Starter

eblc1388

Joined Nov 28, 2008
1,543
Thanks thatoneguy and hgmjr. The configuration is indeed "buck-boost" and with that term google search turns up quite a lot of interesting information.

There is one by TI called tpic74100-q1 which provides +5V output for 1.5V to 40V input voltage. The link of the datasheet is here:

http://focus.ti.com/lit/ds/symlink/tpic74100-q1.pdf

A study of the IC internal reveals that MOS switches were used to switch the inductor connecting position to achieve either buck or boost configuration, depending on input reaching a crossover voltage. Interesting IC.




@jpanhalt,

The LM2679 and LM22679 appears to be a buck(Step down) converter IC only.
 

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jpanhalt

Joined Jan 18, 2008
7,699
I understood that the part numbers I gave were for buck regulators. I gave them to illustrate that National is adding a 2 to many of its regulator products for newer versions. I did not have access to my files at the time to find a better example.

Another comment re. National: Its WEBENCH can be helpful in addition to its design tool. I used WEBENCH to get some parts I did not have and could not meet the minimum at DigiKey for ordering. It may be a little more expensive for some items, but not outrageous. They shipped fast and appeared to be in DigiKey packaging. I suspect that WEBENCH creates design-specific orders and DigiKey fulfills them, at least in the USA.

I think it is worth checking out, but of course, TI is pretty good too.

John
 

Thread Starter

eblc1388

Joined Nov 28, 2008
1,543
think it is worth checking out, but of course, TI is pretty good too.

John
Thanks for the pointer John.

It appears that it will not be easy to rig up a circuit with discrete component because a mode change in the configuration topology is required.

I think using those special ICs which incorporate the mode switching hardware internally or with signal pins for external MOSFETs connection would be a much easier solution.
 

Thread Starter

eblc1388

Joined Nov 28, 2008
1,543
Hi Bill,

I'd be interested in a step up SMPS
The boost topology of a SMPS produces an output voltage higher than the input voltage. It is not much different to the usual "Buck" type SMPS except in the way the inductor and the On/Off control switch is connected.

Can you elaborate a bit more on your comment?
 

Wendy

Joined Mar 24, 2008
21,840
It's funny, I'd never heard the term "buck" until I joined this site. I have plenty experience with SMPS, and studied them in college, but I don't think the term had been invented yet.
 
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