With the introduction of the transistor in the early 1950's, and
especially with the development of integrated circuits from the
early 1960's onwards (16), designers of electronic equipment,
computers and instrumentation have increasingly demanded smaller,
more efficient power sources to supply their equipment. Therefore,
to meet this demand, the power supply itself has become more and
more sophisticated. In fact the developments in power supply technology
can be directly linked to the introduction of various power semiconductor
devices, even though the theory, in many cases, was already well
The regulated power supply technology can really be divided into
two distinct forms; firstly, the linear or series regulator and,
secondly, the switched-mode conversion technique. Switched-mode
technology is multi-facetted with a wide variety of topologies
achieving the end result of providing a regulated DC voltage.
The main differences between the linear and switched-mode regulator
are in the size, weight and efficiency. The linear regulator,
Figure 1, utilises simple techniques of controlled energy dissipation
to achieve a regulated output voltage independent of line and
load variation. It is, therefore, inherently inefficient, especially
when a wide input voltage range has to be catered for. When linear
techniques are applied to regulating a low voltage from the mains
11OV or 24OV AC source, then the disadvantages of the technique
A typical linear power supply is shown in Figure 2. The step
down, low frequency mains transformer is very bulky, large heat-sinking
is required to dissipate the heat generated by the regulating
element and very large filter capacitors are required to store
enough energy at the voltage to maintain the output for a reasonable
length of time when the mains source is removed. Switched-mode
techniques, on the other hand, offer the possibility of theoretically
loss-less power conversion. The switched-mode regulator, Figure
3, employs duty cycle control of a switching element to block
the flow of energy and thus achieve regulation. It has the added
advantage of when applied to off-line applications of giving significant
size reduction In the voltage transformer and energy storage elements.
Since a switched-mode converter can operate at significantly high
frequencies, then a smaller transformer using ferrite cores can
be used. Also since the high rectified mains voltage is chopped,
then energy storage for hold-up can be accomplished on the primary
side of the step-down transformer and so much smaller capacitors
than the linear counterpart can be used. As shown in Figure 4 below.
Although the benefits of switched-mode techniques are great, there
is a penalty paid in the increased noise present at both input
and output of the supply due to the power switching techniques.
Also the associated control circuitry is much more complicated
than its linear counterpart.
Historically, the linear was very common during the late 1950's
and early 1960's when power supplies using switching techniques
were very rare. The ascendency of switched-mode power supplies
is directly linked to the development of fast high voltage switching
power transistors in 1967 (49) and, to
a lesser extent, on developments
in ceramic ferrite materials and capacitor technology. Even so,
linear regulators and power supplies still have their areas of
application to-day, such as low power below 50W, where the costs
of the different technologies is comparable and also in stabilised
bench power supplies. The linear is also preferred where a virtually
noiseless supply is required.
2 Origins and the Development of Switched-Mode Techniques
The origins of switched-mode converters are linked with the developments
in inverter circuitry. An inverter is a processor for generating
AC from DC and is, therefore, a constituent of some forms of switched-mode
power supplies. The earliest described inverters were developed
before the first transistors appeared and, therefore, employed
valves as switching elements, such as a push-pull inverter described
by Wagner (1). After the first bipolar
junction transistor was produced in 1948 (2),
there was a proliferation in inverter/converter
circuit designs. In 1952 a low power high voltage DC supply was
described by Bryan (3) for applications
such as Geiger counters.
This utilised a transistor in an oscillator with power delivered
to the output in a flyback mode via a transformer.
From 1952 onwards, Germanium power transistors increasingly became
available (26,) so giving impetus
to inverter/converter developments.
The various forms of transistor switching circuits developed during
the 1950's were categorised into three main groups by the end
of the decade (11), namely Ringing
Choke, Self-Oscillating, Push-Pull and Drive Push-Pull Converters.
The Ringing Choke Single-Ended Converter, illustrated in Figure
5, employs a single transistor in a transformer coupled relaxation
oscillator which turns the transistor on and off periodically.
Energy is transferred to the output during the switch off stroke
and so its modern equivalent is the Flyback Converter.
The circuit was only suitable for low power applications and was
mainly used to supply a DC voltage. A paper by Light and Hooker
(6) is a well-known reference
on Ringing Choke Converters, dealing with all aspects of the circuit
including methods of feedback control and regulation.
The Self-Oscillating Push-Pull Converters are found in the majority
of medium power applications, 10-100W, although a 250W converter is
described in a paper by Uchrin (8).
The circuit uses a pair of transistors in a symmetrical square-wave oscillator.
The transistors switch on and off alternately, connecting the DC input voltage
to one or other side of a centre-tapped transformer winding. This
inverter produces a square-wave voltage across the transformer
secondary which can be rectified to produce a DC output voltage.
There are two main methods of causing the circuit to oscillate.
The basic form shown in Figure 6 employs a feedback winding
on the main transformer, and has been described by Royer
(7) and Uchrin and Taylor
(4). The two transistors' conduction alternates
when the transformer, usually made of square hysteresis loop core
material, saturates. Other self-oscillating forms, Figure 7,
employs a separate feedback drive transformer which controls transistor
switching. Such a circuit is described in a paper by Jensen
Driven Push-Pull circuits, Figure 8, are more commonly used
in high power applications, where the frequency stability over
self-oscillating forms is an advantage. The push-pull circuit
is driven by a separate master oscillator which controls the frequency
of operation and can be used to supply either DC or AC depending
on the presence of output rectifier or otherwise. Of all the early
forms of power switching circuit, it is the driven push-pull which
is still commonly used as part of some power supplies at the present
The 1960's heralded the development of the modern forms of switching
regulators and switched-mode power supplies. During the early
1960's three forms of non-dissipative switching regulators were
developed for low voltage DC to DC applications. They are the
buck, boost and buck-boost (14)
regulators. The buck regulator steps down the input voltage to a lower
regulated output voltage.The boost regulator steps-up the input voltage
to a higher regulated level. The buck-boost regulator, also referred to
as a flyback regulator, is used to regulate a negative voltage at a level
higher or lower than the positive input voltage. The method of regulator
control in all cases is achieved by varying the duty ratio of
the electronic switch, most commonly by pulse width modulation
(PWM). These switching regulators will be dealt with further in
The main drive in switched-mode converter developments was the
aerospace industry (15,
16), where the advantages of small size
and high efficiency are of great importance. Power supplies working
from the mains for the commercial market were still largely linear
forms, with isolation, a necessary requirement for safety, being
achieved by using a bulky mains frequency transformer. The major
breakthrough for the commercial power supply section occurred
at the end of the 1960s with the introduction of high voltage
silicon switching transistors (19,
The original impetus for the development of these devices came
from the television industry. Power supply design engineers were
now able to implement switching regulator and inverter technology
into stabilised power supplies operating from the mains. In the
USA, with the advantage of a lower 110V rms. mains supply than
Europe, manufacturers of power supplies were the first to introduce
switched-mode converters, notably Trio. In Europe too, switched-mode
power supply developments took place using the new high voltage
One of the first commercially available ranges of switched-mode
power supplies were the 'MG' single output units introduced by
Advance Electronics Ltd. in 1972 (25).
Several other companies also entered the market soon afterwards with their
own switched-mode power supply ranges, including Farnell Instruments Ltd.
The converter topology favoured in these first designs was the half-bridge circuit
controlled using pulse width modulation, Figure 9. This topology
enables the best utilisation of switching device in terms of voltage
rating for a two transistor system. The voltage rating of switching
transistors need only be greater than the peak of the input AC
voltage, since high voltage transistors above 500V rating were
not readily available (31).
3 The Modern Switched-Mode Power Supply Topologies
3.1 The Switching Regulator Family
While the commercial switched-mode power supply manufacturing
industry was beginning to grow during the 1970s, the theory and
technology of switched-mode conversion was being relationalised
as part of the academic discipline of Power Electronics. By far
the greatest contribution made within the discipline has been
the work done by R.D. Middlebrook and his colleagues in the Power
Electronics Group of Caltech in California, USA. The initial work
of the Caltech Group, started in 1970, was aimed at developing
models for the three basic dc-to-dc switching regulator topologies
already developed in the 1960s, namely the buck, boost and buck-boost
29). From this work stemmed the modelling and
analysis method called state-space averaging (31,
34) which allowed
the theoretical prediction of a converters frequency response,
and therefore a better understanding of a switched-mode regulator's
feedback loop and stability criteria. Further work at Caltech,
especially by Cuk in his PhD Thesis, produced a fourth member
of the basic dc-to-dc switching regulators which has been described
as an optimum topology (35) because
of its symmetrical structure
and non-pulsating input and output currents. The new optimum topology
dc-to-dc switching regulator is now commonly known as the Cuk converter
after its inventor and completes the family of single-switch non-isolated
switching regulators. The family are shown in Figure 10 together
with the steady-state current waveforms and transfer equations.
3.2 Isolated Topologies
In many applications, such as operating off the AC mains, isolation
is a necessary requirement within the converter between input
and output. By inserting an isolation transformer into the four
basic switching regulator topologies, then the four single-ended
isolated switching converters are derived as shown in Figure 11
The isolated buck and isolated buck-boost topologies are more
commonly referred to as the forward and flyback converters respectively,
and are the most popularly topologies used in switched-mode power
supplies manufactured commercially.
3.3 Multiple Switch Topologies
The main disadvantage of the single switch topologies is the need
for the high voltage blocking capability of the transistor switch
(twice the DC input voltage), especially when operating from a
rectified AC mains supply. Also the single switch topology is
not an ideal solution for higher power converters, where the current
rating of the transistor switch needs to be much greater. Therefore
another group of isolated converters utilising more than one switch
can be identified. Figure 12 illustrates the three multiple
switch topologies, namely the half-bridge, full-bridge and push-pull
converters. All three converters are buck derived due to the nature
of switching involving pulsating input current and non-pulsating
current and also having an identical ideal voltage gain of the
The original self-oscillating push-pull converters described in
the 1950s by Royer and Jensen are really members of this group.
These topologies also have an added advantage over the single-ended
forward and flyback converters in that full flux excitation of
the transformer core occurs instead of only a half core flux capability.
This makes these multiple switch topologies more suited to higher
3.4 Alternative Topologies
The single-ended and push-pull groups of SMPS represent the basic,
most common converter topologies used in commercial applications
from the mid-seventies up until the present day. These "square-wave"
switching converters do have drawbacks when their design is realised
in practice. Switching Bipolar Junction Transistors (BJTS) do
not turn-on and off immediately but do exhibit finite rise and
fall times in their switching behaviour (48)
which can lead to a number of undesirable affects in conventional converter topologies.
In multiple switch converters, transistor switching overlap can
occur which could cause catastrophic failure to the converter
by effectively applying a short circuit to the supply source.
A number of researchers have worked on alternative topologies
and protection circuitry to eliminate this problem. One such alternative
topology worthy of note is the Weinberg push-pull converter
Figure 13, which is inherently self-protecting when there is
any possibility of component imbalance or conduction overlap.
In all basic switched-mode topologies, the finite duration of
the switching transitions will cause a high peak pulse power dissipation
in the device, as shown in Figure 14. This produces a degradation
in converter efficiency and worst of all, can lead to transistor
destruction during the turn-off transition due to the inherent
BJT second-breakdown phenomenon. Therefore the greatest amount
of research into alternative switched-mode topologies has been
in the field of resonant converters. These converters have tuned
circuits as part of the power conversion stage and exhibit sinusoidal
voltages and/or currents, so leading to transistor switching transitions
at the ideal conditions of zero stress.
The development of Resonant conversion is discussed more fully in
a separate document.
3.5 Control Methods
In the majority of converter topology applications, Pulse Width
Modulation is used for controlling the converter's output voltage
through feedback control of the switching transistor.Other forms of control are becoming
increasingly popular. One such technique is current-mode control
(45) which utilises the switching
transistor current as a control parameter and has the benefit of providing
an inherently more stable closed loop response. Another control method
finding favour with power supply designers is feed forward
(30) which improves
the transient load and line response of mains driven power supplies.
3.6 Switched-Mode Topology Applications
The switched-mode power supply market is now well established
within the electronics sector (43,
44), with a large number of
power supply manufacturers world-wide providing a wide range of
units for the commercial and military markets. The main end-user
systems for switched-mode supplies, are computers, both large
mainframe and smaller, personal and word processors, and the various
telecommunications systems. A typical system often requires a
number of output voltages from its power supply and therefore
the majority of power supplies tend to be multiple output forms.
Typical supply voltages are +5V for Bipolar logic, +2V, -5V, for
ECL logic, +12V for CMOS logic, +12V, +15V for operational amplifiers,
and +24V for DC motors such as disc drives.
The topologies and control methods used to achieve the desired
output voltages in the various power ranges tends to vary from
manufacturer to manufacturer. The topologies reviewed previously
in this section all have their more favoured applications. In
general switching regulators are usually used as secondary regulators
on multiple-output units, Isolated single-ended configurations
are used in low power single or multiple output ac to dc converters
and multiple switch topologies are used for higher output power
applications. Also used as secondary regulators in some multiple
output power supplies are linear regulators, mainly three terminal
integrated circuits in low current outputs and magnetic amplifiers
(21) for higher current outputs.
Overall, the various converter topologies can be arranged in an
applications 'tree' as illustrated in Figure 15.
3.7 Future Trends
Most commercial switch-mode power supplies in the market to-day
operate in the frequency range 10kHz to 50kHz
(43). There is now
a growing trend in research work and new power supply designs
in increasing the switching frequencies upwards to 100kHz and
above (36). The reason being to
reduce even further the overall
size of the power supply in line with miniaturisation trends in
electronic and computer systems. Several new developments in circuitry
techniques and components have helped the drive towards higher
operating frequencies and size reduction.
The introduction of the power Metal Oxide Semiconductor Field
Effect Transistor (MOSFET) (47,
49), at the end of the 1970s, has
provided the power electronics industry with a device capable
of switching at much greater speeds than BJTS. This is due to
the MOSFETs' inherent lack of storage and fall time affects when
turned off. Therefore MOSFETS are now increasingly replacing BTJs
in new designs operating at much higher frequencies. However,
MOSFETs still have their limitations especially in voltage rating
and cost. The device has a much higher fabrication cost compared
with BJTs due to it being more akin to the integrated circuit
than a discrete component. This has prohibited its application
where the component cost of the power supply is paramount. The
intrinsic characteristics of the MOSFET produce a large on-resistance
which increases excessively when the devices' breakdown voltage
is raised. Therefore, the power MOSFET is only useful up to voltage
ratings of 500V and so is restricted to low voltage applications
or in two-transistor forward converters and bridge circuits operating
off-line. Improvements in fabrication techniques and device characteristics
are still in progress and so the MOSFET is likely to replace BJTs
in most applications especially as the cost per device is reduced.
Another new device likely to displace the BJT in many high power
applications is the Insulated Gate Transistor (IGT)
device combines the low power drive characteristics of the MOSFET
with the low conduction losses and high blocking voltage characteristics
of the BJT. Therefore the device is highly suited to high power,
high voltage applications. However, since current transport in
the device is by the same process as the BJT, its switching speed
is much slower than the MOSFET, so the IGT is at present limited
to lower (<50kHz) applications.
The growth of the switched-mode power supply market during the
late 1970s has stimulated many semiconductor component manufacturers
to introduce special control ICs for switched-mode power supplies
(38). The majority of these highly
integrated control and regulating
circuits contain the necessary constituents of the Pulse Width
Modulator for a switched-mode power supply. Other control ICs
offer the current mode control method or feed forward capabilities.
These control ICs also include many protective and monitoring
circuits such as current limit, overvoltage protection and soft-start-up.
The implementation of a control IC within a power supply should
reduce the development costs associated in designing discrete
control circuitry and also save circuit board area occupied by
the control and monitoring circuitry.
An extension of the integrated control circuit philosophy is the
amalgamation of the same drive and control circuitry with a power
device especially the MOSFET. In the future (49),
more and more integrated power devices will be introduced so simplifying board
layout and reducing component count.
Overall, the operating frequency of the conventional switched-mode
topologies such as the half-bridge will be increased by improvements
in components. Also developments in new topologies, especially
resonant converters, will aid the move to higher operating frequencies
and so reduce the overall size of the switched-mode power supply.
Above all the speed with which these developments will take place
will be governed by market forces within the commercial sector.
The driving force in every manufacturer's design will always be
the combined component and production costs. Therefore, any new
device or topology will have to justify its implementation based
on mainly commercial criteria.