Ideas for wasting current.

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I have a buck converter that I am working on. Based on the inductor size that I use, the switching freq and input vs output voltage, I need about 200ma to get the buck converter into continuous mode. My app only uses about 100ma, so I am looking for ideas on how to soak up aleast another 100ma to get me into continuous mode.

My app is not battery powered. The case for the project is a transparent blue, so no crazy bight LEDs.

I am looking for a way that:
-generates minimum heat
-low component count
-cheap to implement
-minimal board space required

I am currently getting about a 100mv pk-pk ripple on my output. I am hoping to get this much lower by getting into the continuous mode. Maybe putting an LC filter on the output is a better idea.

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Is the cup half full, or half empty?

To the pessimist, the cup is half empty.
To the optimist, the cup is half full.
To the engineer, the cup is simply too big!

Being the engineer type, I say that your power supply is too big.

It makes no sense to waste energy in idle heat - which is what you will ultimately be doing.

Look at designing a power supply that better fits the application requirements.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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The issue is of board space and cost.

I use a lm2575 because it is dirt cheap, ~$1.50 and has an internal switch. The problem is that it needs a huge inductor to go into continuous mode @ 100ma. I use a 330uh inductor, it is as big as I can go.

If someone can recommend a switching regulator that is not much more expensive and has better performance then that would be even better.

I was using a linear regulator before but it just got too hot for my liking.

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What is your desired input and output voltage?

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Well, if it is 5V output and you need to "waste" another 100ma, that is 0.5W, any way you cut it. Resistors, transistors, skyhooks, whatever. It will still be 0.5W. That will make a total of at least 1W inside your box, no matter what, and that is not good. I strongly recommend that you look for a different switcher.

Also, discontinuous mode is not all that bad!

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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CirMicro wrote:
What is your desired input and output voltage?

Input is ~12v-16v output is 5v.

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ka7ehk wrote:
Well, if it is 5V output and you need to "waste" another 100ma, that is 0.5W, any way you cut it. Resistors, transistors, skyhooks, whatever. It will still be 0.5W. That will make a total of at least 1W inside your box, no matter what, and that is not good. I strongly recommend that you look for a different switcher.

Also, discontinuous mode is not all that bad!

Jim

Discontinuous is not bad, you are right. This is my first time working with a switching regulator, so I am not sure what is the acceptable ripple for typical uC applications.

It does make me cringe to waster power, that is the last thing this world needs. If anything I just wanted to make amends with myself that I did put in effort into running the switcher in continuous mode.

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You may want to check out the L5980 from ST - Mouser sells them for about $2.

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toalan wrote:

I am looking for a way that:
-generates minimum heat
-low component count
-cheap to implement
-minimal board space required

A resistor. You'll find no way to generate less heat and still soak up as much current, and no way to use fewer components or anything cheaper.

Other than that, pick a converter that operates at a higher frequency. Pick one of the units that operate between .5 and 2MHz, and get away with something like 10uH and 10uF.

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I like the LM2671 and LM2672 devices- higher frequency and a fet for the switch yields higher efficiency. I would like them even better if they were synchronous rectifier types, but they are not. I don't notice any practical deleterious effects when operating in discontinuous mode, even in sensitive analog systems. Mr. Ballantine reveals very low ripple either way.

Tom Pappano
Tulsa, Oklahoma

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Run a small pump and pump water to a high storage tank, then you can install a small hydro-electric facility and recover some of that energy in a few years time when we run out of fossil fuels. As others have pointed out, it's going to end up as heat whatever method you use.

Four legs good, two legs bad, three legs stable.

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CirMicro wrote:
You may want to check out the L5980 from ST - Mouser sells them for about $2.

Thanks for the info, that switcher looks to be fantastic value. I am reading the datasheet right now, looks to require more external components than the simple type of switchers. Great price for a 1mhz switcher.

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tpappano wrote:
I like the LM2671 and LM2672 devices- higher frequency and a fet for the switch yields higher efficiency. I would like them even better if they were synchronous rectifier types, but they are not. I don't notice any practical deleterious effects when operating in discontinuous mode, even in sensitive analog systems. Mr. Ballantine reveals very low ripple either way.

I am liking the LM2671, pretty much the exact same datasheet as the lm2575 I am used to but with different values.

I am going to continue to use the lm2575 in discontinuous mode for now. If I run into problems, I will break out the L5980 or LM2671.

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Stay with discontinuous mode, but add an additional LC filter on the output to reduce the ripple further.

If you think education is expensive, try ignorance.

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Wasting the 100mA will create heat, no way around this, if you don't want to send somthing to the outside. If you would really waste 100 mA, its about as bad as using an 7805 instead, this would waste about half the voltage.

An MC34063 or similar regulator would be an even cheaper alternative. May not be as efficient, and needs more external circuit, but is very felxible.

What is the reason of not going to more than 300µH ?

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Dont forget that the "wasted" 0.5W cost about 5 cent a month.

/Jesper
http://www.yampp.com
The quick black AVR jumped over the lazy PIC.
What boots up, must come down.

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And contributes to global warming. :mrgreen:

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If the world give in and we all die, we'll know who to blame! ;-)

/Jesper
http://www.yampp.com
The quick black AVR jumped over the lazy PIC.
What boots up, must come down.

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One could always build a circuitry to push that wasted current back to the power grid. That way somebody else can use the energy.

That's the way they do it when a 500 kVA UPS must be tested in the factory.

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you can use a fan to cool everything and just consume 100ma!

I love Digital
and you who involved in it!

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Ali_dehbidi wrote:
you can use a fan to cool everything and just consume 100ma!

Of course! No more global warming! Use fans instead of resistors!

Four legs good, two legs bad, three legs stable.