power electronics 12. application devices

Power Electronics Applications

ER. FARUK BIN POYEN

DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA

[email protected]

Contents:

Energy Storage Elements

Uninterruptible Power Supply (UPS)

Switch Mode Power Supply (SMPS)

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Energy Storage Elements: Inductors & Capacitors

Inductors and Capacitors are the fundamental energy storage elements.

These are the essential basic components to build any power electronics circuit.

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Inductors:

Inductance is the name given to the property of a circuit whereby there is an e.m.f.

induced into the circuit by the change of flux linkages produced by a current change.

When the e.m.f. is induced in the same circuit as that in which the current is changing,

the property is called self-inductance(L) .

When the e.m.f. is induced in a circuit by a change of flux due to current changing in

an adjacent circuit, the property is called mutual inductance(M) .

Inductors store the electrical energy in the form of electro magnetic field.

The unit of inductance is the Henry (H) .

Henry(H): A circuit has an inductance of one Henry when an e.m.f. of one Volt is

induced in it by a current changing at the rate of one Ampere per second.

𝑉 = 𝐿 𝑑𝑖 𝑑𝑡

where V = voltage across Inductor in Volts; L = Inductance in henry; i = Current

through the inductor in Ampere.

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Inductors:

The current through an inductor cannot change instantaneously.

The voltage across the inductor changes instantaneously from positive to negative

when we switch from storing energy in the inductor to removing energy from the

inductor. ( i.e. the di/dt is changing from positive to negative)

The converse of the equation is

𝐼 = 1 𝐿 𝑉𝑑𝑡 + 𝐼𝑖𝑛𝑖𝑡𝑖𝑎𝑙

It is used to find the inductor current when we know the voltage across the inductor.

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Capacitors:

A capacitor is an electrical device that is used to store electrical energy.

Next to the resistor, the capacitor is the most commonly encountered component inelectrical circuits.

For example, capacitors are used to smooth rectified AC outputs, they are used intelecommunication equipment—such as radio receivers—for tuning to the requiredfrequency, they are used in time delay circuits, in electrical filters, in oscillator circuits,and in magnetic resonance imaging (MRI) in medical body scanners, to name but a fewpractical applications.

Capacitors store electrical energy in the form of electro static field.

Unit of capacitance is Farad (F).

The current waveform of the filter capacitor of a switching power supply is typically asaw tooth waveform.

The change in voltage in the output waveform is called as ripple voltage.

The goal of the capacitor is to limit this ripple voltage.

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Capacitors:

𝑄 = 𝐶𝑉

Q = Charge; C = Capacitance in Farad; V = Voltage across Capacitor in volts.

A Capacitor is one farad if storing one coulomb of charge creates one volt.

𝑉 = 1 𝐶 𝑖𝑑𝑡 + 𝑉𝑖𝑛𝑖𝑡𝑖𝑎𝑙 ≡ 𝐼 = 𝐶 𝑑𝑉 𝑑𝑡

We can control the ripple voltage by controlling two variables. i.e. by increasing the

capacitance (C) or by decreasing dt.

One of the major advantage of the Switching Power Supplies is that we can make dt very

small by increasing the switching frequency.

It allows the value of C also be very small.

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Uninterruptable Power Supply (UPS)

A Uninterruptible Power Supply (UPS) is a device that maintains a continuous supply

of electric power to the equipment by supplying power from a separate source when

the main power supply is not available.

The UPS is normally inserted between the commercial utility mains and the critical

loads.

When a power failure occurs, the UPS will immediately switch from utility power to

its own power source.

The Uninterruptible Power Supply (UPS) is used in applications where loss of the

mains supply could be disastrous, like hospital operating theatres or intensive care

units.

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Uninterruptable Power Supply (UPS) 9

Types of UPS

UPS systems are generally classified as

1. Static UPS systems, which use power electronic converters with semiconductor devices.

2. Rotary (or dynamic) UPS systems, which use electromechanical engines such as motors

and generators.

3. The combination of both static and rotary UPS systems is often called Hybrid UPS

systems.

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Static UPS

Static Uninterruptible Power Supply systems are based on power electronic devices.The continuous development of devices like IGBTs allows high frequency operation,which results in a fast transient response and low total harmonic distortion (THD) inthe output voltage.

Solid state (Static) UPS system consists of several major elements like Rectifier &Battery charger, inverter, static transfer switch, battery bank.

All UPS systems use an internal battery that produces AC power via an inverter.

How and when this inverter comes into play largely determines the effectiveness of theUPS.

All UPS systems have at least one large, low-frequency, magnetic component, usuallya transformer. In early days these transformers are large and heavy. Now a days smallersize, less expensive high frequency magnetics are used.

The main advantages of higher frequencies are that acoustic noise can be reduced, andflicker components become smaller.

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Static UPS

According to the international standards, Static Uninterruptible Power Supply systems

can be classified into three main categories:

1. Off-line UPS

2. On-line UPS

3. Line Interactive UPS

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Off line and On line Static UPS

Off Line Static UPS

In the case of the off-line system, in normal operation power is supplied directly fromAC mains.

In the event of mains failure, a transfer switch disconnects the power line and connectsthe inverter to the load.

When the mains power is restored, the load is reconnected to the power line.

On Line Static UPS

In the case of on-line systems, the rectifier-inverter combination supplies the loadpower from the AC mains during normal operation.

In the event of mains failure, the battery automatically supplies the dc link to theinverter and there is no time delay involved.

When the rectifier-inverter system fail, the load could be transferred to ac mains usinga transfer switch.

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Off Line UPS 14

On Line UPS 15

Line Interactive Static UPS

In line interactive UPS system, when the mains supply is present, the static switch is

ON.

The static switch connects load to mains supply through inductor L.

The batteries are charged through the charger block.

When main power supply is turned off, the mains static switch is open.

Consequently the inverter turns ON and provides power to the load.

When the main power is available, the Charger/Inverter block operates as a charger and

when the main supply is not available it works as an Inverter.

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Line Interactive Static UPS 17


SMPS stands for switched mode power supply. It is an electronic device in which

energy conversion and regulation is provided by power semiconductors that are

continuously switching with high frequency between "on" and "off" states. An output

parameter (usually output voltage) is controlled by varying duty cycle, frequency or a

phase shift of these transitions.

D.C. to D.C. converters and D.C. to A.C. Converters belong to the category of

Switched Mode Power Supplies (SMPS).

The input D.C. Supply is chopped at a higher frequency around 15 to 50 kHz using an

active device like the BJT, power MOSFET or SCR and the converter transformer.

The size of the ferrite core reduces inversely with the frequency.

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AC power first passes through fuses and a line filter.

Then it is rectified by a full-wave bridge rectifier.

The rectified voltage is next applied to the power factor correction (PFC) pre-regulator

followed by the downstream DC-DC converter(s).

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If we see as a black box with two input terminals and two output terminals, the SMPS

is identical to Linear power supply.

The linear regulator regulates a continuous flow of current from the input to load in

order to maintain a constant load voltage.

The SMPS regulates the current flow by chopping up the input voltage and controlling

the average current by controlling duty cycle.

The pulse width modulated SMPS are classified into two types based on the basic

principle of operation.

They are

(1) Forward Mode Switching Regulators (or) Forward Mode SMPS

(2) Fly-back Mode Switching Regulators (or) Fly back Mode SMPS

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Switch Mode Power Supply (SMPS) 21

Switch Mode Power Supply (SMPS) 22

DC to DC Converter SMPS 23

DC to DC Converter SMPS Working Principle.

High – voltage DC power is directed obtained from DC power source.

At an extremely high switching speed usually in the range of 15 KHz to 50 KHz.

And then it’s fed to a step-down transformer that is comparable to the weight & size of

a transformer unit of 50Hz.

The output of the step down transformer is then further provided to the rectifier.

This filtered & rectified output DC power is utilized as a source for loads & a sample

of this output power is used as a feedback for controlling the output voltage.

The ON time of the oscillator is controlled with this feedback voltage, & a closed-loop

regulator is formed.


The output of the smps is regulated by means of PWM (Pulse-Width-Modulation).

As given in the circuit, the switch can be driven by the PWM-oscillator, such that the

power delivered to the step-down transformer is controlled indirectly, & hence, the

output is controlled by the pulse-width-modulation, as this pulse-width signal & the

output-voltage are inversely related to each other.

If the duty cycle is 50%, the maximum power is transported through the step-down

transformer & if duty cycle decreases, the power transferred will also decrease by

decreasing the dissipation of power.

AC to DC Converter SMPS 26

There is an AC input in an AC to DC converter SMPS.

It is converted into DC by rectification process using a rectifier & filter.

This unregulated DC voltage is fed to the large-filter capacitor or Power FactorCorrection (PFC) circuits for correction of power factor as it is affected.

This is because around voltage peaks, a short current is drawn through the rectifier,these current pulses have appreciably high-frequency energy which causes the powerfactor to decrease.

Instead of direct DC power supply, here AC input is used.

Conversion of AC into DC & switching is done by using a power ‘MOSFET’ amplifierwith which very high gain can be obtained.

MOSFET transistor has low on-resistance & can withstand high current.

The switching-frequency is chosen such that it must be placed inaudible to normalhuman beings (mostly above 20KHz) & switching action is controlled by a feedbackusing the PWM-oscillator.

AC to DC Converter SMPS 27

The AC voltage is again fed to the output transformer. The output of this transformer is

then rectified & smoothed by using the output rectifier & filter.

In order to control the output voltage a feedback circuit is used by comparing it with

the reference voltage.

Forward Mode Switching Regulators SMPS

Forward Mode SMPS form a large family of switching power supply topologies.

They can be identified by an L-C filter just after the power switch or after the output

rectifier on the secondary of a transformer.

A form of the forward-mode regulator is buck regulator.

In this category, the power switch is placed directly between the input voltage and the

Inductor.

In between the power switch and the filter section (Inductor) there may be a

transformer for stepping up or down the input voltage as in transformer-isolated

forward regulators.

When the switch is turned-on the load current passes from the input source, through the

inductor to the load, and back again through the return lines to the input source.

At this moment the diode is reverse biased.

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Forward Mode Switching Regulators SMPS 29

When the switch is turned-off, the inductor still expects current to flow through it.

The former current path through the input source is open-circuited at this moment.

So the free wheeling diode starts to conduct and maintain a closed current loop through

the load.

When the switch turns on again, the voltage stored in the inductor reverse biases the

freewheeling diode.

In summary, the forward current is always flowing through the inductor and thus these

supplies named as forward mode switching regulators.

The amount of energy being delivered to the load is determined by duty cycle of the

switch.

Duty cycle is defined as 𝐷 = 𝑇𝑜𝑛 𝑇𝑜𝑛 + 𝑇𝑜𝑓𝑓

For forward mode switching regulators the value of duty cycle is in between 5% and

95%

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Fly back Mode Switching Regulators SMPS

Very low output power of less than 100W (watts)

This type of SMPS use the same four basic components (L, D, Switch, C) as that offorward mode switching regulators, but the they are rearranged.

Here the Inductor is placed directly between the input voltage source and the powerswitch.

When the switch is turned On, current is being drawn through the inductor.

It causes energy to be stored in the inductor.

When the switch is turned-off, the current cannot change the direction instantaneouslyand it tries to flow in the same direction as before.

Thus the inductor voltage reverses (or flies back).

Thus the diode turns on and the energy from the inductor stores in the capacitor.

Since the inductor voltage flies back above the input voltage , the voltage that appearson the output capacitor is higher than the input voltage.

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Fly back Mode Switching Regulators SMPS 32

The only storage for the load is the output filter capacitor.

It makes the output ripple voltage of fly back converters worse than their forward-

mode regulators.

Due to the restriction of the time required to empty the inductor’s flux into the output

capacitor, the duty cycle is limited to 0-50% (the forward mode switching regulator

duty cycle is in between 5 - 95%)

Based on the presence of transformer in the circuit, the SMPS is classified as follows:

(1) Non-transformer-Isolated Switching Power Supply Topologies

(2) Transformer-Isolated Switching Power Supply Topologies

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Non-transformer-Isolated Switching Power

Supply Topology SMPS The non-transformer-isolated type of SMPS are easy to understand.

They are used when some external component provides the DC isolation or protectionin place of the switching supply.

These external components are usually 50-60 Hz transformers or isolated bulk powersupplies.

Typically they are used in local board-level voltage regulation.

In these topologies, only the semiconductors provide the DC isolation from the input tothe output. Failure of the switch leads to catastrophic failure.

Some of the non-transformer-isolated topologies are:

1. Buck regulator

2. Boost regulator

3. Buck-boost regulator

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Transformer-Isolated Switching Power Supply

Topology (SMPS)

Power Supplies that are intended to run directly from the AC source (offline power

supplies) require a transformer to isolate the load side from AC lines.

Transformers can also be used in power supplies where isolation is required for other

reasons such as medical equipment use.

Some of the transformer-isolated topologies are

1. Fly back regulator

2. Push-pull regulator

3. Half-Bridge regulator

4. Full-Bridge regulator

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References:

Power Electronics A to Z: ANALOG CIRCUIT DESIGN BASICS

Power Electronics A to Z: UNINTERRUPTIBLE POWER SUPPLY [UPS]

Power Electronics A to Z: SWITCH MODE POWER SUPPLY [SMPS]

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power electronics 12. application devices

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