Ateneo de Naga University
College of Science and Engineering
In Partial Fulfillment of The Requirements for the Subject
INDUSTRIAL ELECTRONICS
SWITCHED-MODE POWER SUPPLY
RESEARCH ASSIGNMENT
Submitted By: Jaime Josef N. Intia
Submitted To: Engr. Cornelio Labrador II
Power Electronics
The field of power electronics is concerned with the processing of electrical power
with the use of electronic devices. In specific, power electronics involves the study of
electronic circuits designed to control the flow of electrical energy wherein the key
element here is the switching converter. Power electronics covers the conversion of
its available input form into the desired output form taking in consideration the
efficiency, control and the conditioning of the electric power. Typical examples that
meets this definition are rectifier circuits. Common applications include inverters (a
general term for dc-ac converters) and dc-dc converters for power supplies used in
many mobile devices, such as cell phones or PDAs (Personal Digital Assistant).
Switched mode power supply also known as switch-mode power supply or
switching-mode power supply is an electronic circuit that uses switching devices that
are turned on and turned off at high frequencies when converting power and uses
storage components such as inductors or capacitors when supplying power whenever
the switching device is in its non-conduction state
Switch-mode power supply are generally used in variety of electronic equipment.
Popular in terms of size, weight, cost, efficiency and stable power supply for sensitive
equipment these power supplies are commonly used in computers. The very first
switch mode power supplies dating back to 1958 were designed by IBM. These power
supplies were based on vacuum tube technology. Several patents were filed for a
‘transistor oscillation’ by General Motors Corporation around the same time. By these
sudden inflow in technological advancements comes the potential groundbreaking
designs for designers where given multiple components and manufacturers to choose
from.
APPLICATIONS
Switched mode power supplies have applications in different areas. Little did we
know that this type of power supply is used everywhere. A switched-mode supply is
chosen for an application when its weight, efficiency, size, or wide input range
tolerance make it preferable to linear power supplies. Some common applications are
listed below
•
Personal Computers
Switched-mode PSUs in domestic products such as personal computers
usually has inputs available for anywhere around the world. With
frequencies ranging from 50 – 60 Hz and voltages from 100 – 240V AC,
these PSUs can accept power from most main electricity all throughout
the world. Modern laptops and desktops also have their voltage
regulator module, a dc to dc converter that step down the voltage from
the power supply to a value that may be appropriate for the CPU core
ratings.
•
Battery chargers
First chargers were linear power supplies but because of the efficiency
they saw in switched mode power supplies, they quickly moved to the
cost-effective ringing coke converter having the configuration of a
SMPS.
•
Central power distribution
SMPS came to power distribution when there was a need to escape on
the integration of capacitors for stabilization and batteries as an energy
storage. SMPS may be essential for efficient conversion of electric DC
energy. SMPS may be essential as well for AC applications where the
primary source can no longer produce frequency and voltage.
•
Vehicles
Normally, ordinary trucks use nominal 24V DC but may need 12 V DC
these is where the application of SMPS comes in. Ordinary cars use
nominal 12 V DC and may need to convert this to drive equipment.
•
Consumer Electronics
Applications may be found on some television receivers. Some TV
models uses switch-mode power supply wherein it operates when the
voltage reaches a specific value. At that moment, one can vary the
voltage with the variac.
•
Lighting
With switched-mode power supply setup as a constant current source,
powering of led circuits can be achieved while taking in consideration
the efficiency.
Power Supply
Power supplies are electrical device that supplies electricity to electrical loads. The
main purpose of power supply is to vary the electrical current from an input source
to the accurate voltage, frequency and current to supply the load. The power supply
circuits are classified into different types based on the power they utilize for providing
for circuits or devices
Figure 1 Power Supply Block Diagram (Retrieved from: elprocus.com)
Types of Power Supply and its Operation
Regulated power supply using zener diodes, means the cheapest approach in
regulating small amounts of current. When dealing with higher currents, it is okay
to use high power zeners but it is recommended to use dedicated IC’s
Figure 2 Schematic of Regulated Power Supply using Zener Diodes
The step-down transformer reduces AC supply to 15v AC and four 1N4007 diodes
connected as bridge rectifier this will give regulated DC supply. This DC filtered by
using C1 (470uF/16v) capacitor and then filtered DC supply fed into 12v zener diode
by the way of zener break down the DC supply regulated in 12v. The transistor
2sc1061 drives the output supply to the load.
Linear power supply is another type of supply widely used because of the
advantages they offer in terms of overall performance. Although they may not be as
efficient as other types of power supply, it offers the best performance and are
therefore used in many applications where noise is of great importance. Often audio
amplifiers and many other items of electronic equipment use linear power supplies
to obtain the best performance.
In terms of the overall make-up of a linear power supply, it can be split into several
parts; input transformer, rectifier, smoothing, and linear regulator.
Figure 3 Power Supply with a Linear Regulator (Retrieved from:www.autodesk.com)
AC voltage is first lowered by the transformer and then rectified by several diodes.
It’s then smoothed into a low DC voltage by a pair of large electrolytic capacitors. This
low DC voltage is then regulated as a steady output voltage with the use of a
transistor or integrated circuit. The voltage regulator in a linear power supply acts
as a variable resistor. This allows the output resistance value to change to match
output power requirements. Because the voltage regulator is constantly resisting
current to maintain a voltage, it also acts as a power dissipating device. This means
that useful power is constantly being lost in the form of heat to keep the voltage level
constant.
Switched mode power supply also known as switch-mode power supply or
switching-mode power supply is an electronic circuit that uses switching devices that
are turned on and turned off at high frequencies when converting power and uses
storage components such as inductors or capacitors when supplying power whenever
the switching device is in its non-conduction state. Switch-mode power supply are
generally used in variety of electronic equipment. Popular in terms of size, weight,
cost, efficiency and stable power supply for sensitive equipment these power supplies
are commonly used in computers.
Figure 4 Switched-Mode Power Supply (Retrieved from:www.autodesk.com)
Switch-mode power supply regulates an output voltage using pulse width modulation
(PWM). The process involves high efficiency rating in a small form factor in exchange
for high frequency noise. First stage is to rectify the ac input and smoothed by the
sets of diodes and capacitors providing high dc voltage to be inputted in the next
stage. Then, the high DC voltage is then lowered using a small ferrite transformer
and set of transistors. The lowered dc voltage finally is converted to steady DC voltage
output by the another set of diodes, capacitors, and inductors.
Category of SMPS topologies
Isolated power supply is a type of power supply wherein the circuit is separated
from the AC line in order to prevent electric shock. This type of power supply provides
protection against dangerous voltages. Usually a transformer is used to obtain the
isolation. Disadvantage for this supply would be low in efficiency and that the size for
this power supply would be much bigger compared to that non-isolated due to the
transformer present in the circuit that is doing the isolation. Applications for this
type of supply includes medical devices where the load can be a human.
Figure 5 Transformers used in Isolated Power Supply (Retrieved from: resources.altium.com)
Non-Isolated power supply on the other hand are not shielded from the AC power
source, allowing the possibility of electrocution. Efficiency from this type of power
supply usually is below 95%. Without transformer, non-isolated power supplies are
more compact than isolated. They are commonly board mounted near the load they
are supplying and are pertained to point of load (POL) power supplies. Several
consumer devices typically are not isolated, from refrigerators to washing machines.
Figure 6 Non-Isolated supplies always hold the risk of electrical shock through the design.
Types Of SMPS
Buck Converter
Buck Converters are the most common type of dc to dc converter. This type of
converter is common due to its cheap part components and is one of the simplest.
Buck converters are ideal to be used as a DC to DC converter used to step down
voltages not to be used for applications where isolation is required. Not only can you
achieve high efficiency levels, but also high-power levels using a buck converter. The
down side to buck converters is that the input current is always discontinuous,
resulting in higher EMI. However, EMI issues can be addressed with filter
components such as chip beads, common mode chokes and filter chokes.
Figure 7 Buck Converter
It consists of dc input voltage source VS, controlled switch S, diode D, filter inductor
L, filter capacitor C, and load resistance R. The state of the converter in which the
inductor current is never zero for any period is called the continuous conduction mode
(CCM). It can be observed that when the circuit is conducting, the diode is reversed
biased and when not conducting, the diodes conducts forward in order to provide
uninterrupted current in the inductor.
Boost Converter
Unlike the buck converter, Boost converter are non-isolating and voltages are being
stepped up rather than stepping it down. This type of converter is ideal choice for
Power Factor Correction circuits because of the continuous current it draws even
when operating in continuous conduction mode. Like the buck topology, there are
many choices for the inductor used in boost circuits.
Figure 9 Boost Converter
Figure 8 Boost Converter waveforms
It consists of dc input voltage source VS, boost inductor L, controlled switch S, diode
D, filter capacitor C, and load resistance R. Looking at the circuit, having the switch
on increases the current in the inductor while maintaining the diode off. Having the
circuit switch off, lets the diode provide a path for the inductor current.
Forward Converter
Forward converter is just a transformer isolated buck converter. Like the flyback
configuration, the forward converter is best suited for lower power applications. One
advantage of using this converter is having an extra inductor present on the output
which made it not possible to be use on high voltage outputs. Since the output current
is non-pulsating, it is well suited for applications where the current is over 15A.
Figure 10 Forward Converter
Given the diagram above, when switch S is turned on, diode D1 conducts turning off
diode D2. On the other hand, when the switch is off, diode D1 is off and diode D2
conducts. Energy is being passed through from the input, through the transformer,
and lastly to the output filter. In this converter, since the energy-transfer current
flows through the transformer in one direction, to prevent transformer saturation, an
additional winding with diode D3 is needed in order to zero the magnetization
current.
Flyback Converter
Flyback Converter are just the buck boost converter only that isolation is done by
using transformer as the storage inductor. The transformer provides the output
voltage by varying the turns ratio. Since a transformer is used, multiple outputs are
possible. The flyback is the simplest and most common of the isolated topologies for
low-power applications. While they are well suited for high-output voltages, the peak
currents are very high, and the topology does not lend itself well to output current
above 10A.
One advantage of the flyback topology over the other isolated topologies is that many
of them require a separate storage inductor. Since the flyback transformer is the
storage inductor, no separate inductor is needed.
Figure 11 Flyback Converter
Figure 12 Circuit with a Transformer model showing the magnetizing inductance L M
When switch S is turned on, magnetic field of the inductor stores energy. When the
switch is in the open state, the energy is emptied into the output voltage circuit. The
Duty cycle in the Flyback converter is determined by the output voltage.
Self-Oscillating Flyback
Self-Oscillating Flyback is said to be the simplest and basic converter which is
based from the flyback principle. Flyback principle applies the same concept with the
buck-boost converter only that an additional transformer is used obtain isolation of
the input and output. A switching transistor, a converter transformer, a fast recovery
rectifier and an output filter capacitor are what makes up the converter
Fig. 9: Diagram Showing Self-Oscillating Flyback Converter
During conduction, a current through the transformer primary starts to ramp up
linearly with the slope Vin/Lp. Due to the voltage induced in the feedback winding
and the secondary winding the fast recovery rectifier starts to operate in reverse
biased and hold the conducting transistor ON. The core begins to saturate once the
current reaches its peak value. The result is a sharp rise in current is not supported
by the fixed base drive supported by feedback windings. Hence, the switching begins
to come out of saturation.
The output capacitor supplies the load current during the ON time of the transistor
when no energy is being transferred from the primary side. It is constant output
power converter. The output voltage reduces as the load increases and vice versa.
Proper care should be taken to ensure that the load is not accidentally taken off the
converter. In that case, the output voltage would rise without limit till any of the
converter components gets damaged. It is suitable for low power output applications
and may be used with advantage up to an output power of 150 W. It has high output
voltage ripple.
Table 1 Switch mode Power Supply Topologies Compared (Retrieved from: www.we-online.com)
Control Of SMPS
Voltage Mode Control
The voltage feedback arrangement is known as the voltage-mode control when
applied to dc-dc converters. This type of control is widely used because it is easy to
design and to implement. Voltage mode control has only a single feedback loop from
the output voltage. Having that single loop in this type of control is used to compare
the output voltage with the reference voltage. The actual output voltage is compared
to the desired output voltage and the difference is used to adjust the PWM duty cycle
to control the voltage across the inductor. Voltage mode control has several
disadvantages.
• Poor reliability of the main switch,
• Degraded reliability, stability, or performance when several converters in
parallel supply one load,
• Complex and often inefficient methods of keeping the main transformer of a
push-pull converter operating in the center of its linear region, and
• A slow system response time, this may be several tens of switching cycles
Figure 13 Voltage Mode Control (retrieved from: ijareeie.com)
Current Mode Control
Current mode control introduces a control for the user in both output voltage and
output current of the SMPS. It features a much faster input and output response time
, as compared to the voltage mode control, and has a built-in over current protection
because of the current in the inductor is controlled instead of the output voltage which
is measured on the output resistor. This type of control is commonly used for forward
mode converters. Unlike the voltage mode control having only a single feedback loop,
current mode control provides an additional inner control loop control. Current-mode
uses the error between the desired and actual output voltages to control the peak
current through the inductor.
Figure 14 Current Mode Control (retrieved from: ijareeie.com)
Hysteretic Mode Control
Hysteric mode power converters are inherently fast response. Their designs are
simple and easy to implement. Hysteric mode control responds well with load change
and that they do not require components for the closed loop compensation network.
This reduces the component count and solution size in implementation and
eliminates the design effort in adjusting component values for the network upon
parameters (like input voltage, inductor, bulk capacitors) change making it a good
solution for power supply. One major thing to consider in using this control is its
stability issue.
Two types:
a.) Hysteretic voltage-mode controller
Hysteretic voltage-mode controller are the simplest control method and
their operation are very simple. When the output voltage falls below the
minimum set value, the switch turns on and turns off when output voltage
is higher than the set value. During transient condition, the converter react
quickly to make the better solution without compensation network.
Figure 15 Hysteretic Voltage Control (retrieved from: ijareeie.com)
b.) Hysteretic Current-Mode Control
The maximum and minimum inductor current can be controlled by using
this type of control. The ability to provide a fast response at transient
condition does not require external oscillator or sawtooth generator.
Figure 16 Hysteretic Current Control (retrieved from ijareeie.com)
Hysteretic current control system (figure 16) has two control loops, current
control loop and voltage control loop. The error voltage can be determined
by the difference of error between the actual output voltage and reference
voltage.
Table 2 Common Control Methods (retrieved from: ijareeie.com)
Table 3 Comparison Between VMC, CMC, and Hysteretic control (retrieved from: ijareeie.com)
method
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