SMPS Full Form

SMPS Full Form is Switched Mode Power Supply. It is a piece of electronic

equipment that combines capacitors, inductors, and other semiconductor
devices such as diodes. It is used to convert electrical power efficiently.
Furthermore, it is used to switch from one DC voltage level to another DC
voltage level. An SMPS Full Form, like other power supplies, converts थे voltage
and current characteristics while transferring power from a DC or AC source to
DC loads, like a personal computer. When more efficiency, a smaller size, or
lighter weight is required, switching regulators are used in place of linear
regulators. Applications for SMPS Full Form typically involve power connectors,
desktop power, or server control.

What is the full form of SMPS?

The full form of SMPS is Switched Mode Power Supply. The various topologies of
circuits, each with their own special features, benefits, and modes of operation,
dictate how input power is transported to the output. A transformer is a key
component in the majority of widely used topologies, including flyback, push-
pull, half-bridge, and full bridge, as it offers isolation, voltage scaling, and
numerous output voltages. In the non-isolated versions, the power conversion is
accomplished through inductive energy transfer rather than a transformer.

Working principles of SMPS

It works by using a semiconductor switch, such as a MOSFET, to turn on and off
the supply voltage at a particular exchange frequency in order to control the
yield voltage. The yield voltage will change as the exchange recurrence is
changed. Like other kinds of power supplies, an SMPS full-form power supply
sends electricity from a source (typically an AC outlet) to a DC device. The ability
of SMPS Full Form to control output voltage is what makes it unique. To maintain
a consistent output regardless of variations in load, it can raise or lower the
output voltage. Compared to linear regulators, which can only regulate the
output down, this dual capability gives it a competitive edge.

Advantages and Disadvantages of SMPS

Advantages of SMPS Full Form include:

 The weight of the switch mode power supply is minimal.

 No matter how the input supply voltage varies, the outputs will be controlled and
reliable in an SMPS.
 SMPS has a higher efficiency of 68% to 90%.
 It has a high power density.
Disadvantages of SMPS include:

 It has a complex circuit design.

 An SMPS generates electromagnetic interference.
 An SMPS Full Form is more expensive than linear supply.
Limitations of SMPS
The SMPS Full Form also has some limitations. In an SMPS, the production
reflection is high and the control is poor. The voltage output in SMPS is just one.

Switch Mode Power Supply (SMPS)

 May 23, 2017

 By Anusha

Switch Mode or Switching Mode Power Supply or simply SMPS is a type of

Power Supply Unit (PSU) that uses some kind of switching devices to

transfer electrical energy from source to load. Usually the source is either

AC or DC and the load is DC.

The most common application of an SMPS is the power supply unit of a

computer. Switching Mode Power Supply (SMPS) has become a standard

type of power supply unit for electronic devices because of their high

efficiency, low cost and high power density.

The following image shows an SMPS unit from an old desktop computer.

This particular SMPS is rated for 90W of power.

Linear Regulator vs. SMPS

A Power Supply Unit is an important part of an electric circuit as it provides

the power to the circuit for a proper operation. Almost all electronic devices

require a constant voltage without any fluctuations. A power supply will

take an unregulated power and converts it into a stable regulated power.

There are basically two categories of Power Supplies: Linear Regulated Power

Supply and Switching Mode Power Supply (SMPS).

Linear Regulated Power Supply is a type of power supply that regulates the

output voltage with the help of a series pass control element. The basic

example of a series pass element is a resistor. But the frequently used series

pass elements are BJT or MOSFET in active or Linear Mode and is

connected in series with the load.

Depending on the changes in either input or load, the current through the

transistor changes in order to keep the output constant. The difference

between the input and output (load) voltages is dropped across the

transistor and this excess power i.e. difference between the input and

output (load) power is dissipated as heat by the transistor.

The following image shows a basic structure of a Linear Regulated Power

Supply.

From the above image, the input AC source is given to a rectifier and filter to

convert it into DC. But this DC Supply is unregulated as it is susceptible to

change with the changes in input. This unregulated DC supply is given as

input to the Linear Regulator.

SMPS is a type of regulated power supply that uses a high frequency

switching regulator to convert the power supply and also regulate the output

in a highly efficient way.

The switching regulator is again a transistor (like a power MOSFET) just like

in Linear Regulator but the difference is that the pass transistor in SMPS

doesn’t continuously stay in saturation or fully ON state but rather switches

between fully ON and fully OFF states at a very high frequency. Hence the

name Switching Mode Power Supply.

Since the average time the switching element i.e. the transistor stays in

active state is less, the amount of power wasted or dissipated as heat is very

less when compared to Linear Regulators. This in turn leads to high

efficiency of SMPS as the voltage drop across the pass transistor (or

switching element) is very less.

The switching action of the transistor is controlled using a technique called

Pulse Width Modulation (PWM) and the output voltage can be regulated by

the duty cycle of the PWM.

The above image shows a basic structure of an SMPS unit. In this, the

unregulated DC supply is given to a Switched Mode DC – to – DC Chopper

Circuit and the output is a regulated DC Supply.

The main difference between the structures of Linear Regulated Power

supply and Switch Mode Power Supply shown here is that in case of Linear

Power Supply, the input AC is stepped down, rectified and filtered to get

unregulated DC and in case of SMPS, the input AC is directly rectified and

filtered and the unregulated high voltage DC is given to a High Frequency

DC – to – DC Converter.

Usually, a high frequency transformer will be a part of this DC – to – DC

converter for scaling and isolation.

Specification Linear SMPS

Efficiency Typical efficiency of 30-40% Typical efficiency of 60-95%
can be achieved with good
design
Output Always less than Input Can be more or less than
Voltage Input
Regulation By dissipating excess power By varying duty cycle of PWM
Method
Circuit Less complex; consists of Very complex; consists of
Complexity regulator and filter as main switching element, high
components frequency transformer,
rectifiers and filters, feedback
circuit
Noise and Less electronic noise at output High interference and noise
Interference and mild high frequency due to frequent switching od
interference current
Size and Bulky because of transformer No transformer at input but
weight and heat sink requires a tiny high
frequency transformer
Applications Low power, simple and low cost High power, complex and
systems stable power requirements

Even though the design of Switch Mode Power Supply (SMPS) is more

complex than a Linear Regulated Power Supply, its high efficiency, high

power capabilities and stability are the main factors in choosing SMPS as

the power supply unit for sensitive electronic devices.

What is the purpose of SMPS?

Majority of Electronic DC Loads like Microprocessors, Microcontrollers,

LEDs, Transistors, ICs, Motors etc. are supplied with standard power

sources like batteries for example. Unfortunately, the prime problem with

batteries is the voltage is either too high or too low. Hence, an SMPS will

provide a regulated DC output.

SMPS is a versatile power supply as we can choose from different topologies

like Step – up (Boost), Step – down (Buck), power supplies with isolation at

input and output depending on the type of application.

Coming to the major factor of why we need SMPS, the efficiency of a good

SMPS design can be as high as 90% or even more. In contrast, the efficiency

of a Linear Regulated Power Supply is dependent on the voltage drop at the

pass transistor.

For example, assume we have a 3V Lithium Cell that must be stepped down

to a 1.8V load drawing a current of 100mA. The power wasted in the

transistor as heat is 0.12W and hence the efficiency of the power supply is

40%.

SMPS ICs come with more or less all the features of a discrete SMPS design

allowing engineers to experiment with design for custom projects.

SMPS Design

The design of Switched Mode Power Supply or SMPS is fairly complex when

compared to linear regulated power supply. But this complexity in design

has an advantage as it will result in stable and regulated DC supply that is

capable of delivering more power in an efficient way for a given physical

specification (size, weight and cost).

A simplified block diagram of an SMPS which converts AC input to a

regulated DC is shown in the following image.

Although there are many number of design types for an SMPS power supply,

all the designs will be more or less similar to the structure shown above. The

main design types in SMPS are:

 AC to DC, where AC mains is given as input and we get a regulated

DC at the output,

 DC to DC Step up converter, where an input DC voltage is stepped up

i.e. output voltage is greater than input and

 DC to DC Step down converter, where the input DC voltage is stepped

down i.e. output voltage is less than or equal to input voltage.

In case of DC to DC SMPS systems, the input DC is usually given from a

battery and hence, both the DC to DC converter circuits (Step up and Step

down) are commonly found in battery operated systems.

Coming back to SMPS design in the above image, it represents a typical AC

to DC converter. We will see the basic working of this SMPS design. The

input AC supply is given to rectifier and filter circuits. This step will convert

the High Voltage AC to High Voltage DC.

This high voltage DC is given to a High Speed Switching Element like a

Power MOSFET. The output of this switch, which is a High Frequency, High

Voltage Pulsating AC, is given to a High Frequency Step down Transformer.

The output of this transformer is a Low Voltage AC signal which is in turn

given to a rectifier and a filter circuit to obtain Low Voltage DC.

Important Points to Note:

 The common feature of any SMPS design is to convert input AC to

High Voltage DC and convert this High Voltage DC to High Voltage,

High Frequency Square Wave (AC). This High Voltage and Frequency

AC is converted to Regulated DC.

 Square Wave Oscillator and High Speed Electronic Switch (like a

MOSFET) are responsible for converting DC to High Frequency AC.

The same principle is also used in Square Wave Inverters.

 By converting the input AC or DC (after rectifying and filtering the AC)

to High Frequency AC, the size and price of the components like

inductors, transformers and capacitors can reduced i.e. they can be

smaller and cheaper.

 As the High Frequency AC signal generated at the switch is a square

wave, the output voltage can be regulated with the help of Pulse Width

Modulation (PWM). There is a voltage feedback through an isolator

circuit to the control circuit (which controls the PWM). With this

feedback, the duty cycle of the PWM from the oscillator can be varied

and hence the output is perfectly regulated without any over voltages.

 A sample current from the High Frequency AC (signal after the switch)

and a reference current are compared and given to the control circuit

and hence provides an over current protection.

  Also note that the output DC is completely isolated from the input

mains and even the feedback signal is isolated with the help of an

Opto coupler.

 Driving the Switching Transistor (MOSFET) with square wave ensures

that the power dissipation is very less when compared to the

Transistor being operated as a series pass transistor in Linear

Regulated Power Supplies.

 Since there is a High Frequency AC Signal in the SMPS, there is a

chance of high frequency harmonics and as a result, SMPS is more

susceptible to RF Interference.

SMPS Topologies

We have seen the basic design of a Switched Mode Power Supply (SMPS) in

the above section. Now we will see the different types or topologies of SMPS.

Switched Mode Power Supplies or SMPS can be classified into two types

based on its circuit topology: Non-isolated Converters and Isolated

Converters.

Non – isolated Converters are a type of SMPS Topology where the switching

circuit and output are not isolated i.e. they have a common terminal. The

three basic and important types in Non – isolated SMPS are:

 Buck Converter or Step – down Converter

 Boost Converter or Step – up Converter

 Buck – Boost Converter

There are other non – isolated SMPS designs like Switched Capacitors, Cuk

Converter and SEPIC Converter but these three types are very important.

They are the simplest of SMPS designs and use a single inductor as an

energy storing element and two switches, out of which one is an active

switch (a Transistor – Power MOSFET) while the other can be a diode.

The output voltage can be higher (Boost or Step – up) or lower (Buck or Step

– down) and can be controlled by the duty cycle of the high frequency square

wave (that is applied to the switch). One main drawback of Non – isolated

Topology is that the efficiency of the switches falls as the duty cycle is

reduced. Isolated Topology will suit better for larger voltage changes.

Isolated Topology in SMPS uses a transformer as an isolator between the

switching element and output. Depending on the transformer’s turns ratio,

the output voltage can be higher or lower than the input. Transformer based

SMPS topologies can be designed to generate multiple output voltage by

using multiple windings at the transformer.

The energy storage element can be transformers secondary winding or a

separate inductor. The two important Isolated Topology based SMPS

converters are:

 Flyback Converter

 Forward Converter

Some of the other commonly used isolated SMPS topologies are Half –
bridge, Full – bridge, Push – Pull, Half – Forward, Isolated Cuk, etc.

Buck Converter or Step – down Converter

Buck Converter is a type of SMPS circuit and DC to DC Converter, where

the output voltage is less than input voltage. Hence, a Buck Converter is

also known as a Step – down Converter.

It is one of the simplest SMPS power converter techniques and is often used

in RAM, CPU, USB etc. The input DC in buck converter can be a rectified AC

or a battery. A simple buck converter using two switches (one transistor and
one diode) and an energy storing element (inductor) is shown in the image

below.

Buck Converter Operation

A simple Buck Converter or Step down Converter is shown in the above

image and it consists of a switching transistor, diode, inductor and

capacitor. The combination of Inductor , Diode and Capacitor is called as

Flywheel Circuit.

The operation of the Buck Converter is explained with respect to square

wave pulse. The following image shows the operation of the Buck Converter

when the input pulse is HIGH i.e. the switching Transistor is ON.
When the pulse input to the Gate terminal of the MOSFET is HIGH, the

Transistor is turned ON. As a result, the transistor will supply current to the

load. During this time, the Diode D is reverse biased and will not be a part

of the circuit during this period.

Initially, the inductor resists the change in current and hence, the current

to the load will increase gradually with expanding magnetic field. Also, the

charge on the capacitor is built up gradually up to the supply voltage. The

next image is for the condition where the pulse becomes LOW i.e. the

Transistor is OFF.
When the pulse becomes LOW, the switching Transistor is turned OFF. The

magnetic field that is built up during the Transistor ON state, starts

collapsing now and releases the energy back in to the circuit. The polarity of

the voltage across the inductor i.e. its back e.m.f is now reversed. The

energy from the inductor starts collapsing and keeps the current flowing in

the circuit through load and the diode, as the diode D is forward biased.

Once the energy from the inductor is completely utilized, the capacitor starts

discharging and acts as the main source of supply until the transistor is

turned ON. When the transistor is turned ON, it will once again supply

current to inductor, capacitor and load and the process continues.

The output voltage is dependent on the ON and OFF time i.e. the Duty Cycle

of the square wave pulse and the formula for output voltage is

VOUT = D x VIN, where D = TON/(TON+TOFF)

With Buck Converters, we can achieve more than 90% efficiency and as a

result, they often employed in computer systems where they convert 12V

supply to typically 1.8V (for RAM, CPU and USB).

Boost Converter or Step – up Converter

In the previous section, we have seen a Buck Converter type SMPS. Now, we

will see about another type of SMPS called Boost Converter or Step – up

converter. A Boost Converter, as the name suggests, is type of switched

mode power supply, which boosts or increases the output voltage with

respect to the input voltage. Boost Converters are also known as Step – up

Converters as the output voltage is higher than the input voltage.

One of the best known application for Boost Converters is in electric cars.

The supply from electric cars batteries won’t be sufficient for its working as

they require voltages that are much higher (typically in region of 500V) than

those supplied by the batteries. Another important application of Boost

Converters is Laptop Chargers in Cars.

Typical Car batteries provide 12V and Laptops require anywhere between 18

to 22V. The following image shows a simple Boost Converter.

Boost Converter Operation

This simple Boost Converter consists of a Switching Transistor (BJT or

MOSFETS can be used), an energy storing element i.e. inductor, another

switch (Diode or another Transistor), capacitor and a high frequency square

wave oscillator with controllable duty cycle.

The input to this Boost converter is unregulated DC, which can be given

from rectified AC, batteries, Solar, DC Generators, etc. We will see the

working operating of this Boost Converter. First we will see for period when

the Transistor is ON for the first time. The following image shows this

condition.
When the pulse is HIGH for the first time, the transistor is turned ON and it

closes a part of the circuit consisting of Inductor, Transistor and input

supply. Current flows from the input through the inductor and transistor.

The inductor, initially resists the change in current but the magnetic field

will increase gradually allowing inductor to store energy. The impedance of

the rest of the circuit i.e. Diode, Capacitor and Load is much higher and

hence, there will be no flow of current in that part of the circuit.

When the square wave pulse goes LOW, the transistor is turned OFF. This

action will cause a drop in the current through the inductor, producing a

back e.m.f in the circuit due to collapsing magnetic field. Also, the polarity of

the voltage across the inductor is now reversed and will be in series with the

input voltage.
The combination of the input voltage and Inductor Back e.m.f cannot pass

through the inductor as it is turned OFF. Hence, the diode is forward biased

and charges the Capacitor and also supplies current to load.

An important point to note here is that the voltage supplied to the capacitor

and load during the Transistor OFF state is a combination of input voltage

and inductors back e.m.f, which is higher than the input voltage.

When the transistor is turned ON again, the current flows again through the

inductor and transistor. As the diode is reverse biased, the capacitor

discharges it potential, which is sum of input voltage and inductor voltage,

through the load acting as its source during this period. The output voltage

is given by the formula

VOUT= VIN x 1/(1-D) where D = TON/(TON+TOFF)

Flyback Converter

Flyback Converter is a type of Switch Mode Power Supply typically used in

low power applications. Flyback Converter is an Isolated Type SMPS where

the input and output are isolated with a transformer. The following is the

circuit of a simple Flyback Converter.

The main components of a Flyback Converter are a Switching Transistor,

Oscillator Circuit, Transformer, switch (like a Diode) and a Capacitor. The

Transformer is different from a normal transformer and is called a Flyback

Transformer. In this transformer, the Primary and Secondary do not

conduct simultaneously.

Flyback Converter Operation

When the Transistor is turned ON, the current flows through the primary of

the transformer with the dot being higher potential. As a result, the polarity

of the voltage induced in the secondary will be reverse to that of primary.

Hence, the diode D gets reverse biased.

If the capacitor got charged in the previous cycle, it will discharge through

the load. The following image shows this period of operation in the flyback

converter.
The operation of the Flyback converter in the other period i.e. Transistor

OFF period is illustrated in the following image. When the pulse becomes

LOW, the transistor is turned OFF and the primary of the transformer do

not conduct.

The energy in the secondary of the transformer will be released into the

circuit and also the polarity in the secondary is reversed i.e. it becomes

positive. Hence, the diode is forward biased allowing the energy stored in the

secondary coil acting as the source. It recharges the capacitor and also

supplies the current to load.

The output voltage in Flyback Converter can be higher or lower than the
input voltage and is dependent on the turns ratio of the primary and

secondary of the transformer.

Forward Converter

Another important switch mode power supply is Forward Converter. It is

another isolated type SMPS and produces controlled and regulated DC from

an unregulated DC supply.

The efficiency of Forward Converter is slightly more than that of Flyback

Converter and is often used in application where the power requirements are

a little higher (typically around 200W). The design of Forward Converters is

slightly complex than Flyback Converters and a simple structure is shown

below.

The simple circuit of Forward Converter consists of a fast switching

transistor, a control circuit to control the duty cycle of the Square Wave, a

normal transformer, two diodes for rectifying the AC, an inductor and a

capacitor for filtering.

Forward Converter Operation

The following image shows the operation of the Forward Converter when the

Transistor is turned ON. When the pulse is HIGH, the transistor is turned
ON and as a result, the primary coil of the transformer starts conducting. As

a result, a voltage is induced in the secondary coil of the transformer.

The polarity of the voltage induced in the secondary is similar to that of the

primary and hence, the diode D1 gets forward biased. The voltage from the

secondary will start to flow through the diode D1, inductor, capacitor and

finally the load. During this period, both the inductor and capacitor store

energy in the form of magnetic field and electric field respectively.

When the pulse becomes LOW, the transistor is turned OFF and as a result,

the primary coil stops conducting. This will in turn stop inducing current in

the secondary. This sudden change (or drop) in current will generate a back

e.m.f of the inductor and polarity of its voltage is reversed.

This period of operation of the Forward Converter is shown in the image

below. The energy in the inductor start collapsing in the circuit through the

load and Diode D2 (as it is forward biased). As soon as the energy in the

inductor finishes, the capacitor starts discharging through the load and acts

as a temporary source to the load. This continues until the transistor is

turned ON again
The output voltage of the Forward Converter is dependent on the

transformer turns ratio as well as the duty cycle of the Pulse Width

Modulator. The output voltage is given by

VOUT = VIN x D x NS/NP

Switched Mode Power Supply: SMPS Design & Applications

In this article we will learn about the Switched Mode Power
Supply a.k.a. SMPS & its design working & applications.
What is Switched Mode Power Supply (SMPS)?
SMPS stands for switched mode power supply. It is known by a wide range
of names like power supply, supply unit, regulator, or switcher in an
electronic power supply. It incorporates a switching regulator to convert
electrical power efficiently. It is mainly used for obtaining a controlled dc
power supply as output.
It is used to convert power (voltage) using switching devices that are turned
on and off alternatively at high frequencies. It uses storage components
like inductors or capacitors to supply power when the switching device is
in its non-conduction state (off-state). SMPS possesses high efficiency and
is widely used in various electronic equipment such as computers, battery
chargers, and other sensitive equipment requiring a stable and efficient
power supply.

Design & Working

The working & design of SMPS is divided into various sections and stages.

1: Input Stage
The AC input supply of frequency (50-60) Hz feds directly to
the rectifier and filter circuit. Its output contains many variations and
the capacitance value of the capacitor should be higher enough to handle
the input fluctuations. Finally, the unregulated dc is given to the central
switching section of SMPS in order to regulate it. This section does not
contain any transformer for the step down in input voltage supply.
2: Switching Section
It consists of fast switching devices like a Power transistor or a MOSFET,
which switches ON and OFF according to the variations in the voltage. The
output obtained is given to the primary of the transformer which is present
in this section.
The transformer used here is a much smaller, lighter, and highly effective
one that steps down voltage. These are much efficient compared to other
step-down methods. Hence, the power conversion ratio is higher.
3: Output Stage
The output that is derived from the switching section is again rectified and
filtered. It uses a rectification and filter circuit to get the desired DC voltage.
The obtained regulated output voltage is then given to the control circuit.
4: Control Unit
This unit is all about feedback, which has many sections contain in it. Lets
see the brief information about this section.
The inner control unit consists of an oscillator, amplifier, sensor, etc. The
sensor senses the output signal and feedback to the control unit. All the
signals are isolated from each other so that, any sudden spikes should not
affect the circuitry. The reference voltage is given as one input along with
the signal to the error amplifier. The amplifier is a comparator that
compares the signal with the required signal level.
The next stage is Controlling the chopping frequency. The final voltage
level is controlled by comparing the inputs given to the error amplifier,
whose output helps to decide whether to increase or to decrease the
chopping frequency. The oscillator produces a standard PWM wave with a
fixed frequency.

The SMPS is mostly used where switching of voltages is not at all a problem,
but where the efficiency of the system really matters. The design and
working of SMPS of based on the same concept.

Types of SMPS
1: Non-isolated
Non-isolated converters are mostly used when the change in the voltage is
comparatively small. The non-isolated SMPS are the ones whose input and
output circuitry are not isolated from each other. The major disadvantage is
that it cannot provide protection from high electrical voltages and it
poses more noise. They are of 3 types.
I: Buck
In a typical non-isolated step-down (buck) converter the output voltage
VOUT depends on the input voltage VIN and the switching duty cycle of the
power switch.
II: Boost
It is used to boost voltage and it uses the same number of passive
components but arranged to step up the input voltage so that the output is
higher than that of the input.
III: Buck-Boost
This converter allows the input voltage to be either stepped-up or stepped-
down, depending on the duty cycle. The output voltage is given by the
relation
VOUT = -VIN *D/ (1-D)

2: Isolated
Isolated SMPS are the ones where there is isolation maintained between the
input and output circuitry. The supplies make use of a transformer to
separate the switching from the output. The secondary winding of the
transformer acts as the energy storing element.
I: Fly-back Converter:
The working of this converter is similar to the buck-boost converter of the
non-isolating category. The only difference is that it uses a transformer to
store energy instead of an inductor in the circuit.
II: Forward Converter
The working of this converter makes use of the transformer to send the
energy, between the input and output in a single step.

Application of Switched Mode power supply (SMPS)

• It is used in servers, power stations, and personal computers.
• It is used in vehicles for charging batteries.
• It is used in factories and industries for power.
• It is used in the railway system, security system.
• It is also used in mobile and also as lighting.

Advantages & Disadvantages of SMPS

Advantages
• Smaller in size and light-weighted.
• Better power efficiency of around 60 to 70 percent.
• Strong anti-interference.
• Wide range of output.
• Produces less heat.

Disadvantages
• The SMPS design & working is more complex.
• Has higher output ripple and its regulation is not satisfactory.
• Mostly limited to the step-down regulator.
• Has high-frequency electrical noise.
• Leads to harmonic distortion.