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DC to DC
converters explained
Bill Naylor, Electronworks Ltd
This article explains how a dc/dc converter works.
These are the latest highly efficient regulators
used in all modern electronics including everything
battery powered. You can use one of our electronic
kits to evaluate the theory.
If you have any suggestions for improving this
application note, please drop us a line at:
enquiries@electronworks.co.uk
Introduction
In many applications in electronics the designer
needs to convert one voltage to another. In a car,
the battery voltage is typically 12V while most of
the electronics runs at 5V or lower. A method of
converting one voltage to another is needed.
Voltage Regulator Techniques
A simple way of converting from a higher voltage to
a lower voltage is to ‘throw away’ the spare voltage
in the form of heat. These devices are called linear
regulators and represent a low cost method of
regulating a voltage. However heat can cause damage
to components, shorten their lifetime and is
wasteful. To dissipate the heat, a heatsink is often
needed which makes the circuit bulky. This is not
too important in a car as there is lots of power
available from the battery. However in a mobile
phone where battery life equates to talk time,
converting from a high voltage to a lower voltage in
an efficient way is very important. This is where
‘switched mode’ power supplies are needed. A
switched mode power supply (or dc to dc converter)
uses a switch to regulate the voltage.
A device dissipates heat if it has current flowing
through it at the same time as it has a voltage
across it.
Consider a resistor of 1 Ohm with a voltage applied
to it of 1 volt. It will develop a current through
it of 1A (from V = I x R). The resistor will
dissipate power (heat) according to:
Where P is the power (in Watts), I is the current in
Amps and V is the voltage across it in Volts. In our
case, the resistor will dissipate 1W of power
(heat).
With a car battery, if you have a circuit requiring
5V with a current of 0.5A (like a car stereo or a
satellite navigator) and you are powering this from
the 12V battery, the voltage across the regulator is
7V and the current flowing through it is 0.5A,
implying a heat dissipation of 3.5W. This is a lot
of heat to get rid of and would normally be achieved
by using a heat sink which is big and bulky and will
get hot.
Now, consider the operation of a switch. When it is
closed, a switch has a current flowing through it
but with no voltage across it. When it is open it
has a voltage across it, but with no current flowing
through it. In both cases the product of (current x
voltage) is zero, so the heat dissipation is zero.
This is the basic principle of operation of a
switched mode power supply.
Thus a circuit in FIG 1 can be considered.
FIG 1
To smooth the current and voltage in a switched mode
circuit, inductors and capacitors are used.
This is a standard ‘step down’ regulator that
produces a lower output voltage from a higher input
voltage.
When the switch closes, the input voltage is
connected to the inductor. The current in the
inductor ramps up thus storing energy. When the
switch is opened, the energy in the inductor
discharges into the capacitor and the capacitor
charges up slightly. As this process is repeated,
the capacitor charges more and more until the
desired output voltage is reached. If the capacitor
voltage is connected back to the component that
controls the switch, the switch oscillation can be
stopped, thus regulating the output voltage at any
desired level (as long as it is less than the
input).
With a circuit similar to that in if FIG 1, we can
regulate the 12V to 5V with very little heat
dissipation hence no heat sink and thus design a
smaller, more efficient, more elegant circuit.
For a more detailed insight to the operation of
dc/dc converters, please see: ‘More Detailed
Operation of dc/dc Converters.’
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