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More Detailed
Operation of a dc/dc Converter
Bill Naylor, Electronworks Ltd
This article explains the operation of dc/dc
converters in more detail. 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
Before reading this article, please refer to ‘dc/dc
converters explained’ to get an insight into the
basics of dc/dc converter operation.
We have established so far that a device dissipates
power if it has a current flowing through it at the
same time a voltage is across it. We have also
established that a switch has theoretically zero
power dissipation.
We will now look in more detail how a switched mode
power supply works.
Firstly, we need to consider the basic operation of
an inductor. Inductors behave according to the
equation:
Where V is the voltage across the inductor, L is the
inductance value and di/dt is the change in current
with time. To make things easier, a constant di/dt
looks like a current ramping up over time.
Thus if a fixed voltage is applied to a fixed value
inductor, the current will ramp linearly over time
according to:
The basic circuit of a dc/dc converter is shown in
FIG 1.
FIG 1
When the switch is closed, assuming the output
voltage is constant (which at any instant we can
assume this will be approximately true), there will
be a fixed voltage across the inductor and the
current will ramp linearly over time. If we switch
off this current, there will be a certain amount of
energy stored in the inductor.
Using real life component values, if the input
voltage is 12V and the output voltage is 5V, when
the switch is closed there will be 7V across the
inductor. If the inductor has a value of 100uH (100
x 10-6H) the current will ramp up at
Or 70,000 Amps per second.
This seems a lot, but if we open the switch after
10us (10 x 10-6 seconds), the current
will only rise to 700mA (not so bad!).
Now, inductors do not like having the current going
through them suddenly turned off. If we open the
switch, this is exactly what we are doing. They try
to maintain the current going through them
immediately before the switch was opened (in this
case 700mA). They do this by turning themselves into
a (kind of) battery. The end connected to the switch
develops a negative voltage and the other end
develops a positive voltage. Thus the ‘positive’ end
of the inductor pushes current out of the inductor
and the ‘negative’ end pulls current into the
inductor. Now, the ‘positive’ end of the inductor is
clamped to the output voltage, leaving the
‘negative’ end to fly negative. This has the effect
of maintaining the current magnitude and direction
that was present immediately before the switch was
opened.
You can see that if the switch is open there is no
current path connected to the negative end of the
inductor so no current can flow. The effect of this
is that this end of the inductor flies negative
until something blows up. Yes the voltage will get
bigger until something arcs to an adjacent track.
If we want to generate a high voltage, this is one
way of doing it. However if we want to have control
over the operation of the circuit, we put a fast
acting ‘Schottky’ diode into the circuit to catch
the negative spike as shown in FIG 2.
FIG 2
Referring to FIG 2, when the inductor tries to fly
negative, the diode conducts clamping the junction
of the diode and the inductor at about -0.5V. This
causes a current to flow in a clockwise direction
through the inductor, into the capacitor and up
through the diode. In doing this, we are pumping
current into the capacitor and the capacitor
charges.
If this is repeated continually (the switch is
closed, the inductor charges etc), the voltage on
the capacitor increases. If we monitor the voltage
on the capacitor we can stop the switch oscillating
when we have reached our desired voltage and have
hence produced a power supply that regulates a
higher voltage down to a lower voltage.
Since the switch has a very low ‘on’ resistance,
very little power is dissipated in this type of
circuit and we have managed to convert 12V to a
lower voltage with very little power being wasted.
In a mobile phone, this means more power is saved
hence longer talk time. Since hardly any heat is
dissipated in the circuit, we do not need a heat
sink so can make the mobile phone very small.
The above theory is true for small power supplies
(inside mobile phones) to large power supplies used
in cars.
In practice the switch and inductor have a very
small resistance, so there will always be a small
amount of heat wasted and the main area of research
for these companies is in getting the resistance
down while maintaining small package sizes.
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