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Static SwitchesThyristors can be turned on and off within a few microseconds may be operated as fast-

acting switches to replace mechanical and electromechanical circuit breakers.

Static switches can be classified into 2 types:1) ac switches 2) dc switches

DC switches

In the case of dc switches, the input voltage is dc and power transistors or fast

switching thyristors can be used. Once a thyristor is turned on, it must be turned off

by forced commutation. If thyristor T3 is fired, the capacitor C is charged through

the supply

Vs T3- L- C. After time t= t0 , the charging current becomes zero, and the

polarity of the inductor reverses ; therefore capacitor is charged to 2Vs.

If thyristor T1 is conducting and supplies power to the load

Vs T1- RLThen thyristor T2 is fired to switch T1 off. T3 is self-commutated.

Firing T2 causes a resonant pulse of current of the capacitor C through capacitor C,

inductor L, and thyristor T2. ( it means Capacitor across 2Vs; will discharge

through C+ - L- T2- T1- supply).

As the resonant current increases, the current through thyristor T1 reduces. When

the resonant current rises to the load current IL, the current of thyristor T1 falls to

zero and thyristor T1 is turned off. (at any instant , load current through T1 and

discharge current becomes equal. At that instant current through T1 becomes zero).

The capacitor discharges its remaining charge through the load resistance RL .

C- L- T2- RL .T2 is self-commutated. A freewheeling diode, Dm , across the load is necessary for

an inductive load. The capacitor must be discharged completely in every switching

action; and a negative voltage on the capacitor can be prevented by connecting a

resistor and a diode. It is not very easy to turn- off dc circuits, and dc static

switches require additional circuits for turning off.

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Dc switches can be applied for control of power flow in very high voltage and high

current applications and these can also be used as fast-acting current breaker.

Switching regulators

1) Buck converter or forward converter or step- down converter (regulator)The buck converter has the output voltage always less than the input

voltage and can be practically anywhere between 10% and 90% of the input

voltage- hence the name buck. The rectangular pulses from the pulse width

modulator (PWM) on the base of the transistor saturate or cut off the transistor

during each cycle. Because of the ON-OFF switching, the average value is always

less than the input voltage. The output of the buck converter is shown in fig. ; is

compared with a reference voltage and the error signal is amplified by an error

amplifier. This amplified error signal is used to generate a pulse width modulated

waveform. This controls the ON-OFF periods of the switching transistor.

Principle of OperationWhen the transistor switch Q is turned on by the positive output pulse of the PWM,

current flows through the inductor L and divides itself into two parts. one

flowing through the output capacitor C and the other through the load RL. As the

transistor starts conducting, the induced voltage in the inductor L bucks (opposes)

the input voltage.

When the o/p voltage Vout exceeds the i/p voltage Vin , the transistor switch Q is

turned off by the negative o/p pulse of the PWM. At this instant, the stored energy

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in the inductor L reverses its polarity, and sends current into the load via diode D

while the voltage is maintained by the capacitor C. As all the stored energy in the

inductor is being used up, the capacitor discharges and the o/p voltage decreases.

The diode D prevents the discharge of capacitor C through it. In this condition the

switch Q is turned on and the process continues so that the o/p voltage is

maintained very near to the i/p voltage Vin .

If the o/p voltage increases becoz of any decrease in load, the error voltage will

increase too. This enhanced error voltage reduces the ON time of the switching

transistor Q, thereby reducing the o/p voltage to the desired value. The o/p voltage

is also maintained at its desired value if it tends to decrease becoz of an increase in

load.

2) Boost converter or Step-up converter (regulator)The boost converter has its o/p voltage always more than the i/p voltage-

hence the name boost. This switching regulator can be used as a dc step-uptransformer. The rectangular pulses from the pulse width modulator (PWM) on the

base of the transistor saturate or cut off the transistor during each cycle. Because of

the ON-OFF switching, the average value is always more than the input voltage.

The output of the boost converter is shown in fig. ; is compared to a reference

voltage and the error signal is amplified by an error amplifier. This amplified error

signal is used to generate a pulse width modulated waveform. This waveform

controls the ON-OFF periods of the switching transistor.

Principle of Operation

When the transistor switch Q is turned on by the positive output pulse of the

PWM, current flows through the inductor L and energy is stored in it. When the

switching transistor Q is turned off by the negative output pulse of the PWM, the

magnetic field in the inductor collapses and induces a large voltage across the coil

of opposite polarity; that adds (boosts) to the i/p voltage. This ensures the flow of

current in the same direction. The diode D is now forward biased. Thus, the

voltage across the inductor and the i/p voltage are in series and together charge the

output capacitor C to a voltage higher than the i/p voltage Vin. The current flows

through the inductor L and divides itself into two parts- one flowing through the

output capacitor C and the other through the load RL . When the transistor Q turns

on again, the diode D prevents capacitor C from discharging itself. The capacitorthus maintains the load voltage Vout when transistor Q is ON. During the

conduction of transistor Q, the stored energy in the capacitor C, therefore

maintains the load current.

If the o/p voltage increases becoz of any decrease in load, the error voltage will

increase too. This enhanced error voltage reduces the ON time of the switching

transistor Q, thereby reducing the o/p voltage to the desired value. The o/p voltage

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is also maintained at its desired value if it tends to decrease becoz of an increase in

load.

3) Buck-Boost converter (regulator)This regulator operates on the principle of both buck and boost. The o/p

voltage may be less than or more than the i/p voltage- hence the name buck-

boost. The o/p voltage polarity is opposite to that of the i/p voltage. This converter

is also known as inverting converter.

Principle of Operation

When the switching transistor Q is turned on, the current flows through

the inductor L and transistor Q. at this moment, the diode D is reverse biased.When the switching transistor Q is turned off , the voltage polarity in the inductor

L changes its sign and the stored energy in L charges the capacitor C through the

diode D, and sends current into the load as well. This takes the o/p voltage to an

opposite polarity compared to that of the i/p voltage. The inductor current during

energy release falls until transistor Q is switched on again in the next cycle. During

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the conduction of transistor Q, the stored energy in capacitor C maintains the load

current.

If the o/p voltage increases becoz of any decrease in load, the error voltage will

increase too. This enhanced error voltage reduces the ON time of the switching

transistor Q, thereby reducing the o/p voltage to the desired value. The o/p voltage

is also regulated to its desired value if it tends to decrease becoz of an increase in

load.

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UNINTERRUPTIBLE POWER SUPPLY (UPS)

There are two types of configurations which are commonly used in UPS.

1) ON-line UPS2) OFF-line UPSi) ON-line (Continuous) UPS

The inverter in this configuration operates continuously and its output is

connected to the load through static switch-2 which is normally ON. Thus there is

no break in the supply in the event of main supply failure. The rectifier/ battery

charger maintains the charge on the standby battery and the battery in turn supplies

dc power to the controlled frequency inverter.

The controlled frequency inverter is usedi) To condition the supply to the loadii)To protect the load from transients in the main supplyiii)To maintain the load frequency at the desired value.

In the case of the inverter failure, the load is switched ON directly to the main

supply without interruption through a fast- acting static transfer switch-1. This

transfer by a solid state switch usually takes 4 to 5 ms. During the emergency

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power flow, back-up time available depends on the ampere-hour capacity of the

battery. This type of UPS is more costly and it is available in ratings above 5KVA.

This configuration is best as it provides

i) Full isolation of the critical load from the unhealthy ac supplyii)Power conditioning

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OFF-line UPS

The load is normally supplied from the ac supply and the rectifier/ battery charger

maintains the full charge of the battery. If the utility supply fails, the load is

switched ON to the output of the inverter through static switch-2 which then takes

over the main supply. This configuration requires breaking the circuit momentarily

and the transfer by a solid state switch usually takes 4 to 5ms. The inverter runs

only during the time when the supply fails. The back-up time available depends on

the ampere-hour capacity of the battery. After restoration of the ac supply, the load

is transferred to the ac supply through static switch-1. Now the battery is put on

charge again and remains on float when fully charged.

Applications of UPS:

Uninterruptible power supplies are used in computers, data processors, data

transmitters, microwave relay stations, digital controllers.