Operation of the Sepic Regulator on the NCE Radio Boards.
By Mark Gurries on Aug 5 2006 – NCE Yahoo Group..
Marcus Ammann wrote on Aug 4 2006:
The radio boards have a "Sepic" regulator on board. It is a
special type of switching regulator. This provides the necessary voltage to the
board so long as the batteries can supply the required current. This is why any
combination of batteries can be used with the throttles. This enables 2 only
AAA NiMH batteries that have a total of 2.4 volts in
the Cab04s to operate correctly. So battery voltage is not an issue. It is all
about providing the required current and until the batteries a nearly
discharged, they can supply the current.
I pulled the back of my Procab and found it. Designated a LTIZ, it is a
5 terminal surface mount chip on the board, approximately 2 x 1 mm in size. For
more details see:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1031,C1060,P1775,D3577
or click on the following Linear
Technologies LT1615.
Mark Gurries could enlighten us a little more with a more precise simple
explanation, as his company makes these Sepic regulators.
Mark replied:
Simple...that’s a challenge.
Yes I worked with Jim on getting the design going. The part of concern is called a LT1615 using
the Sepic Configuration as shown in the back of the datasheet in the
applications section.
A regulator is nothing more than a device that can take some input
source of unregulated power and makes a stabilized source out of it. Think of a air
pressure regulator but in this case, we typically mean voltage.
There are two type of regulators in electrical
world, Linear and Switcher.
Common Linear regulators allow the excess voltage (pressure) to drop
across it. It loses a lot of power
(energy) in the process in the form of heat.
This heat has to be managed and in many cases one must have a heat sink
of some other method of getting this heat to the outside air where it can
dissipate. In other words, the circuit
itself is very simple to understand and use, but it is very wasteful of your
input power. IF you have a limited input
power source such as a battery, using this type of regulator would be throwing
away some of the battery's energy away to run the load. In other words, it will give you less battery
life than what is possible with a Switching Regulator.
Common Switching regulators use magnetic fields in the form of inductor
or transformer to help regulate power.
The easiest way to describe this type of power converter is to think of
it as transforming Electrical Energy to Magnetic Energy and then back to
Electrical Energy. In other words,
Watts IN to Watts OUT via a neutral form of energy in the middle. The excess
energy wasted (heat) in a linear regulator circuit is instead captured and
reused to run the load with a switching regulator. So battery life is much longer with a
switching regulator than a linear regulator.
The problem with switching regulators is they have lots of parts and are
complicated to design.
Which to use? That depends on
your priorities for the device. In
almost all applications involving hand held products, battery life is #1
followed by low heat (both are interrelated to each other)!
******** More Detail ********** (Using KISS principles)
If you remember,
A Sepic converter/regulator is a special type of Switching Regulator
that have some unique properties relative to most of it cousins. (It is in the
same class of converters often referred to as a fly back converter)
This switching regulator is designed to not have any DC circuit path
between the input and the output under any conditions such that power could
find a way to flow through it. Instead
this converter relies 100% on the magnetic field to bridge the power from the
input to the output.
This in turn permits the input voltage and current to be completely
independent of the output voltage and current just a long as the power going in
is equal to the power going out. (I'm
ignoring efficiency issues.)
Analogy: (Thinking this up on the spot..) Think
of a Bucket-Brigade system emptying a lake of water to fill a small pond. It does not matter if the pond is higher or
lower in elevation relative to the lake for the Bucket-Brigade is the only
method the water is permitted to flow to reach the other side. Gravity is not involved at any time since
there are dams all around the lake and pond to prevent gravity having an effect
on the water flow.
So how does this apply to NCE?
This permits NCE to use 2 or 4 alkaline cells in series (1.8V to 6V
range) for input and still get
the same fixed and regulated 5VDC output to power the cab electronics. Only when the battery
voltage falls to a level such that the Sepic IC can no longer regulate the 5V
output, will the cab die, due to a dead battery.
There is much more to this, but I think you get the main idea.
Best Regards,
Mark Gurries
Linear Technology
Power Supply & Battery Charger Applications Engineer/Manager
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