Power handling devices and circuits - full details ~ MONU TUTORIALS ACADEMY

Introduction

since the inventions of semiconductor diode PN juncti Power electronics on there has been worn development in this field. Sofa, a large number of semiconductor devices have been developed for their different uses in various fields. One of the major field of application of solid state devices coma in the recent year coma has been to handle and control large amount of power in system.

Power electronics

the branch of electronic that deals with the control of power handled by a system is called power electronics.

Silicon controlled rectifier (SCR)

a silicon controlled rectifier is a three terminal junction semiconductor device which can be used as a controlled switch to perform various functions such as rectifications, inversion and regulation of power flow.

Constructional analysis of SCR

N SCR is a PN PN semiconductor device consisting of three PN junction J1 J2 and j3 as shown in figure.

it is as if an ordinary diode and transistor are combined in one unit. Three terminal are taken one from the outer P type material called anode second from the outer and type material called cathode and the third from the other P type material placed in between and called gate.

Working of silicon controlled rectifier

the lord RL is connected in series with the anode the working of an SCR can be study under the following two headings..

1. Action of anode voltage :- consider an SCR circuit shown in figure where and node is made positive with respect to cathode and the gate voltage is kept at zero. With the application of this voltage, the junction J1 and j3 are forward bias, where is junction J2 is reverse biased. Hence the flow of current in the circuit is blocked example thyristor is cutoff. However, if anytu cathode voltage v is increased by stage is reached when breakdown of junction jaaye to occur and the thyristor suddenly switches to hai conductive state. In this state the load current is only limited by supply voltage v and load Rl.

2. Action of gate voltage :- consider the thyristor circuit shown in figure where anode is positive with respect to cathode latest suppose that anode voltage v is less than 4 word breakdown voltage so that Mr does not conduct. If now a small positive voltage is applied to the gate, the gate current IG start flowing in the circuit get circuit. The flow of get current helps in making current flow across junction J2.

From the above discussion the following points are worth noting..

An SCR has two stage, namely; on and off state.
The SCR will conduct if forward voltage is more than forward breakdown voltage.
NSE r can be turned on at much lower forward voltage by the application of propagate current.

Important terms

Some of the important terms used while studying LCR are given below:

Breakdown voltage

The maximum forward voltage comic gate been open at which SCR start conducting heavily is called breakdown voltage.

Peak reverse voltage

the maximum reverse voltage which can be applied to an SCR without conducting is called peak reverse voltage.

Holding current

The maximum anode current, gate being open coma at which is turned off from on condition is called holding current.

Forward current rating

the maximum anode current at an SCR can handle without destruction is called forward current rating of an SCR.

V i characteristics of SCR

Agra between anode cathode voltage and anode current at constant get current is called v-i characteristics of SCR.

Forward characteristics :- the curve between v and I went forward voltage is applied is known as forward characteristics of an SCR.

Reverse characteristics :- when anode is connected to negative and cathode to positive of power supply the curve between v and I is called reverse characteristics in this case junction J1 and j3 are reverse bias while J2 is forward biased. it is clear that by increasing the reverse voltage beyond a certain value will result in the breakdown of the junction J1 J2 and thyristor can no longer block the flow of current throw it.

Silicon controlled rectifier as a switch

a silicon controlled rectifier is a device which is operated usually at the forward voltage having magnitude much less than its back over voltage. When the gate is open this device does not conduct at all and behaves as a open switch. However when the required triggering current is supplied to the device through the gate, it is start conducting heavily and remain in the position in definitely even if the gate is opened. This corresponds to on condition. the SCR can only be turned off when the anode current is reduced to the holding current. The value of holding current of an SCR can be found from its data Book.

Thus, it is seen that SCR is a device which has only two States example on state and of state and no state in between, Hence,its behaviour is similar to mechanical switch. Since SCR is an electronic device it is more appropriately called as electronic switch.

Advantages

It does not have any moving path, therefore, it provides noiseless operations.
Its switching speed is very high up to 10 to power 9 operation per second.
There is no question of wear and tear therefore less maintenance.
Its operating efficiency is high.
It is similar in size and gives trouble-free service.
It can handle and control large current as compared to other electronic switches such as transistor.

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