Dc Converters

Jaycar Electronics Reference Data Sheet: DCDCCONV.PDF (1)

DC-DC CONVERTERS: A PRIMER
DC-DC converters are electronic devices used whenever
we want to change DC electrical power efficiently from
one voltage level to another. They’re needed because unlike
AC, DC can’t simply be stepped up or down using a
transformer. In many ways, a DC-DC converter is the DC
equivalent of a transformer.Typical applications of DC-DC converters are where 24V
DC from a truck battery must be stepped down to 12V DC
to operate a car radio, CB transceiver or mobile phone;
where 12V DC from a car battery must be stepped down
to 3V DC, to run a personal CD player; where 5V DC on a
personal computer motherboard must be stepped down to
3V, 2V or less for one of the latest CPU chips; where the
340V DCobtained by rectifying 240V AC power must be
stepped down to 5V, 12V and other DC voltages as part of
a PC power supply; where 1.5V from a single cell must be
stepped up to 5V or more, to operate electronic circuitry;
where 6V or 9V DC must be stepped up to 500V DC or
more, to provide an insulation testing voltage; where 12V
DC must be stepped up to +/-40V or so, to run a car hifiamplifier’s circuitry; or where 12V DC must be stepped up
to 650V DC or so, as part of a DC-AC sinewave inverter.
In all of these applications, we want to change the DC
energy from one voltage level to another, while wasting as
little as possible in the process. In other words, we want to
perform the conversion with the highest possible efficiency .
An important point to remember about all DC-DCconverters is that like a transformer, they essentially just
change the input energy into a different impedance level. So
whatever the output voltage level, the output power all
comes from the input; there’s no energy manufactured
inside the converter. Quite the contrary, in fact — some is
inevitably used up by the converter circuitry and
components, in doing their job.
We can thereforerepresent the basic power flow in a
converter with this equation:

P in = P o u t + P l o s s e s
where Pin is the power fed into the converter, Pout is the
output power and Plosses is the power wasted inside the
converter.
Of course if we had a ‘perfect’ converter, it would behave
in the same way as a perfect transformer. There would be
no losses, and Pout would be exactly the same as Pin. Wecould then say that:

Vin x Iin = Vout x Iout

or by re-arranging, we get:

Vout/Vin = In/Iout
In other words, if we step up the voltage we step down the
current, and vice-versa.
Of course there’s no such thing as a perfect DC-DC
converter, just as there are no perfect transformers. So we
need the concept of efficiency, where:

Efficiency(%) = Pout/Pin
Nowadays some types ofconverter achieve an efficiency of
over 90%, using the latest components and circuit
techniques. Most others achieve at least 80-85%, which as
you can see compares very well with the efficiency of most
standard AC transformers.

Many different types

There are many different types of DC-DC converter, each
of which tends to be more suitable for some types of
application than for others. Forconvenience they can be
classified into various groups, however. For example some
converters are only suitable for stepping down the voltage,
while others are only suitable for stepping it up; a third
group can be used for either.
Another important distinction is between converters which
offer full dielectric isolation between their input and output
circuits, and those which don’t. Needless tosay this can be
very important for some applications, although it may not
be important in many others.
In this data sheet we’re going to look briefly at each of the
main types of DC-DC converter in current use, to give you
a good overview. We’ll start first with those which don’t
offer input-output isolation, and then progress to those
which do.

Non-isolating converters

The...