This chapter introduces the idea of SMPS, UPS and other power electronics application devices.

1. Power Electronics Applications
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
FARUK.POYEN@GMAIL.COM

2. Contents:
 Energy Storage Elements
 Uninterruptible Power Supply (UPS)
 Switch Mode Power Supply (SMPS)
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3. 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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4. 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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5. 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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6. 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 in
electrical circuits.
 For example, capacitors are used to smooth rectified AC outputs, they are used in
telecommunication equipment—such as radio receivers—for tuning to the required
frequency, 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 few
practical 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 a
saw 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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7. 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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8. 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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9. Uninterruptable Power Supply (UPS) 9

10. 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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11. 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) in
the 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 the
UPS.
 All UPS systems have at least one large, low-frequency, magnetic component, usually
a transformer. In early days these transformers are large and heavy. Now a days smaller
size, less expensive high frequency magnetics are used.
 The main advantages of higher frequencies are that acoustic noise can be reduced, and
flicker components become smaller.
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12. 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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13. 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 from
AC mains.
 In the event of mains failure, a transfer switch disconnects the power line and connects
the 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 load
power from the AC mains during normal operation.
 In the event of mains failure, the battery automatically supplies the dc link to the
inverter and there is no time delay involved.
 When the rectifier-inverter system fail, the load could be transferred to ac mains using
a transfer switch.
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14. Off Line UPS 14

15. On Line UPS 15

16. 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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17. Line Interactive Static UPS 17

18. Switch Mode Power Supply (SMPS)
 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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19. Switch Mode Power Supply (SMPS)
 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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20. Switch Mode Power Supply (SMPS)
 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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21. Switch Mode Power Supply (SMPS) 21

22. Switch Mode Power Supply (SMPS) 22

23. 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.

24. DC to DC Converter SMPS 24
 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.

25. DC to DC Converter SMPS 25

26. 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 Factor
Correction (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 power
factor 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’ amplifier
with 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 normal
human beings (mostly above 20KHz) & switching action is controlled by a feedback
using the PWM-oscillator.

27. 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.

28. 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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29. Forward Mode Switching Regulators SMPS 29

30.  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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31. 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 of
forward mode switching regulators, but the they are rearranged.
 Here the Inductor is placed directly between the input voltage source and the power
switch.
 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 instantaneously
and 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 appears
on the output capacitor is higher than the input voltage.
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32. Fly back Mode Switching Regulators SMPS 32

33.  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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34. 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 protection
in place of the switching supply.
 These external components are usually 50-60 Hz transformers or isolated bulk power
supplies.
 Typically they are used in local board-level voltage regulation.
 In these topologies, only the semiconductors provide the DC isolation from the input to
the 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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35. 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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36. 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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