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'[EE] Talking about power supplies..'
2005\08\26@155408 by Mauricio Jancic

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Hi,
       I have a question. When should one consider choosing a switching
power supply over a standard AC/DC + CAP + Voltage regulator?

       I have a product that has a motor (12V and 2A max) and a control
circuitry (5V and <100mA). I have a transformer with 2 windings and two
separated power supplies (meaning 2 rectifiers, 2 capacitors, and actually 1
regulator, since the motor needs no regulation on this application).

       I would like to know why should I want to or consider to change to a
switching power supply... is it usually more expensive, right?

Thanks,

Mauricio Jancic

2005\08\26@165045 by Marcel Duchamp

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Mauricio Jancic wrote:
> Hi,
>        I have a question. When should one consider choosing a switching
> power supply over a standard AC/DC + CAP + Voltage regulator?
>

>        I would like to know why should I want to or consider to change to a
> switching power supply... is it usually more expensive, right?

1. Where does the power come from?
If it comes from a limited source such as a battery, you may want to
maximize power delivery so even when the battery drops low, a switcher
can boost it back up.  But if the circuit is power by AC mains, then a
transformer/linear regulator is often all that's needed.

2. Where does the power go?
If a large portion goes into heating your linear voltage regulator, then
a switcher might make sense.

3. Do you need absolutely the lowest amount of power supply noise?
Switchers generally make more noise than linears. You really should test
this theory first with your circuits however.  I have seen plenty of
sensitive analog circuits run from switchers with no problems.

4. Expense?  Depends on how much power you are dealing with and how you
will remove heat.  In a linear supply that generates a lot of heat, you
may need large heat sinks, fans, etc.  These are not free of course.
With so many switcher chips on the market, switchers don't have to be
expensive.  Plus, you can make your own with stuff like the PIC10F222,
especially for low power designs.

Good luck!

2005\08\26@171036 by PicDude

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Physical space, cost, efficiency/heat-dissipation, etc.  I usually do DC-DC,
and usually start with a linear supply, but I start looking into switchers
when I can no longer dissipate enough heat or if the physical contraption
required to dissipate the heat gets subjectively complicated.

For DC-DC, I would say it is generally more expensive, but can't necessarily
say that for an AC-DC supply.  Physically mounting and heat-dissipating
components can add to cost really quickly.

Cheers,
-Neil.



On Friday 26 August 2005 02:54 pm, Mauricio Jancic scribbled:
{Quote hidden}

2005\08\26@180543 by Bob Axtell

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I prefer switching supplies over standard pass-element regulators
because:

-- Switchers can provide a very precise DC output while the input
can be very noisy.  I am stating that in a very noisy input environment,
switchers can provide a very quiet output. For example, in automotive
systems, switchers are much less expensive that smoothing-choke
battery supply filtering systems.

--Dramatically reduced (or no) heat generated.

--Capable of enormous secondary load swings while maintaining regulation.

--Reduced need for expensive and physically large filter electrolytic caps.

Since most of my designs are automotive or battery managed, switchers
allow the designer a way to load-manage a battery system to wring out
the absolute MOST from a LiIon battery.

Here is a good example. The design needed to get maximum battery life
from LiIon cells. The 3.8V output had to be precisely regulated over a
range of 10uA to 1A within 20mV. It had to be able to do this while the
battery cell could range from 2V to 4.2V.  We took two identical LiIon
cells in series, having a range of 4V to  8.4V, which was buckdown regulated
to 3.8V, and operated when the battery ranged from 8.4V downto 4V. In
addition, the 8.4V battery pack fit the switching charger device operating
on an automotive source ranging from 17V down to 9V. The switching
charger generated so little heat that it was mounted on the product itself.

Without switchers, none of these schemes would work.

--Bob

Marcel Duchamp wrote:

{Quote hidden}

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2005\08\26@180852 by olin piclist

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Mauricio Jancic wrote:
> I have a question. When should one consider choosing a switching
> power supply over a standard AC/DC + CAP + Voltage regulator?
>
> I have a product that has a motor (12V and 2A max) and a control
> circuitry (5V and <100mA). I have a transformer with 2 windings and two
> separated power supplies (meaning 2 rectifiers, 2 capacitors, and
> actually 1 regulator, since the motor needs no regulation on this
> application).
>
> I would like to know why should I want to or consider to change to a
> switching power supply... is it usually more expensive, right?

Yes, but only when you look at parts cost.  Linear supplies are certainly
simpler, but dissipate more power.  Usually switchers become competitive
when the linear supply would require physically larger components to
dissipate the heat.  Since switchers are more efficient, they dissipate
little heat and can therefore be small.  Switchers look good when the input
voltage is substantially higher than the output voltage or the input voltage
is unknown or must be allowed to vary over a wide range.

Another aspect is the effeciency itself.  Sometimes it's worth spending a
little to save some watts, regardless of the cost of getting rid of the
heat.


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