Showing posts with label Power Electronics. Show all posts
Showing posts with label Power Electronics. Show all posts

Saturday, December 22, 2018

December 22, 2018

SCR Triggering / SCR Turn On Methods Full explanation.

SCR Triggering / SCR Turn On Method

SCR Triggering / SCR Turn On Methods Full explanation


SCR is unidirectional, reverse blocking semiconductor device. Today we are going to learn SCR Triggering Methods or you can say SCR Turn On Methods. Because we already know that SCR cannot conduct current in forward condition until we manually turn it on.


What is SCR Triggering?

The process of switching the SCR from OFF state to ON state is called SCR Triggering. That means the Turn ON of SCR is also known as SCR Triggering.
In other words, we can say the process of switching the SCR from forward blocking state to forward conduction state is called SCR Triggering.


Methods of SCR Triggering:

There are many methods of SCR turn on or triggering. The methods of SCR triggering are,


  1. Thermal or Temperature Triggering
  2. Radiation or Light Triggering
  3. Voltage Triggering or Breakover Voltage Turn On
  4. dv/dt triggering
  5. Gate Triggering


What is the Thermal or Temperature Triggering of SCR?

We already know that when the anode of the SCR is positive respect to the cathode the junctions J1 and J2 will be the forward bias and J2 will be the reverse bias. So a small amount of reverse leakage current will flow across the junction J2. 

 As the leakage current depends on the temperature of the junction, in fact, it is directly proportional to the junction temperature. So when the reverse leakage current flow through the junction the temperature will increase which further increase the flow of current. So when the value of the current increased greater than the Latching Current the SCR will be Turn ON.



What is the Radiation or Light Triggering of SCR?

If a beam of light is applied to the junction of the SCR it can produce sufficient energy to break the electron-hole pairs in the semiconductor and the SCR will get Turn ON. 

 So when the intensity of the applied light is increased more than a nominal value the SCR start conducting. It is not an easy process that is why it is used rarely. This triggering mainly used in HVDC Transmission Lines.



Radiation or Light Triggering of SCR, SCR Triggering Methods, SCR Turn On Methods




What is the Voltage Triggering or Breakover Voltage Turn On of SCR?


In forward condition the junction J1 and J3 are forward biased and the junction J2 is reverse biased. So if we add an additional voltage greater than the normal rated forward voltage across the SCR the Junction J2 will go to avalanche breakdown. 

 So the current will start flowing through the junction J2. And also the current will flow through the whole SCR. This additional applied voltage is known as Breakover Voltage. Remember that in this case, the anode current would be limited by only the external load impedance.



Voltage Triggering or Breakover Voltage Turn On of SCR, SCR Triggering Methods, SCR Turn On Methods



What is the dv/dt triggering of SCR?

 We know that in forward condition the junction J1 and J3 are forward biased and the junction J2 is reverse biased. So we can assume that there is a capacitance will produce in the SCR whose junction J2 act as a dielectric medium and the upper portion of the junction J2 is connected to the positive voltage and lower portion to the negative voltage. 

 In this situation, if we increase the rate of change in forward anode-cathode voltage a transient gate current will produce(even the gate terminal is open) which can turn on the SCR.
As in this method, we create change the voltage respect to the time it is called dv/dt triggering.


What is the Gate triggering of SCR?

The process of addition minority carriers into the gate region through the gate electrode to switch on the SCR is known as Gate Triggering.
If we apply a positive voltage to the gate with respect to the cathode a current will flow through the gate terminal and if the gate current is large enough the SCR will switch ON.



Gate triggering of SCR, SCR Triggering Methods, SCR Turn On Methods


Various methods of Gate triggering of SCR:

  1. Triggering by a DC Gate Signal
  2. Triggering by an AC Gate Signal
  3. Triggering by a Pulsed Gate Signal


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Sunday, October 14, 2018

October 14, 2018

Very simple Explanation: What is Silicon Controlled Rectifier, Construction of SCR, symbol of SCR, Working of SCR, Application of SCR

What is Silicon Controlled Rectifier, Construction of SCR, the symbol of SCR, Working of SCR, Application of SCR?


In Power Electronics as well as the whole field of Electronics Silicon Controlled Rectifier(SCR) is a very important and useful device.
In this post, we will discuss only the basics concepts of,
                                     

  • What is a Silicon Controlled Rectifier(SCR)? 
  • Construction of an SCR
  • Symbol of an SCR
  • Working principle of an SCR
  • Application of an SCR


What is a Silicon Controlled Rectifier(SCR)? 

                               
                             A Silicon Controlled Rectifier(SCR) is a semiconductor device made by Silicon (It is used as the intrinsic semiconductor to which the proper impurities are added). SCR is a three terminal device. The terminals of SCR are Anode, Cathode, and Gate. The Gate terminal of SCR is also called the control electrode. SCR also called a unidirectional, Switching Device.

Simple structure and construction of Silicon Controlled Rectifier(SCR):


The basic construction of an SCR is shown below. According to the power rating, there are two types of construction of SCR.
For low-power rating, Planner type construction.
For high-power rating, Mesa type construction.
For understanding the basic construction of SCR only Planner type Construction of SCR is given below.



What is Silicon Controlled Rectifier, Construction of SCR, symbol of SCR, Working of SCR, Application of SCR



                    As you see in the above picture the SCR consists of four different layers pellet of P-type and N-type semiconductor materials. In this construction, all the junctions are diffused.

          The cathode terminal is connected to the most top N-type semiconductor material. Next P-type semiconductor material is connected to the Gate terminal. And the most bottom P-type semiconductor material is connected to the Anode terminal.


What is Silicon Controlled Rectifier, Construction of SCR, symbol of SCR, Working of SCR, Application of SCR




Symbol of Silicon Controlled Rectifier(SCR):


What is Silicon Controlled Rectifier, Construction of SCR, symbol of SCR, Working of SCR, Application of SCR



Working Principle of a Silicon Controlled Rectifier(SCR):

I will discuss only the basic working principle of SCR. Because before reading deeply you must know the basic working of an SCR. 
At first, see the structure of an SCR carefully which is given above.
When we applied a positive voltage to the anode with respect to the cathode, J1 and J3 will be forward bias but  J2 will be reverse bias. For current flow through the SCR, all three junctions must be forward bias. So in this condition, current can not flow through the SCR. This is Known as Forward Blocking.
     But when we apply a positive pulse voltage to the Gate terminal the Junction J2 becomes forward bias and current can flow through the SCR. This is known as Forward Conduction.

              When we applied a negative voltage to the anode with respect to the cathode, J1  and J3  will be reverse bias and  J2 will be forward bias. In this condition, no current can flow through the SCR. This is known as Reverse blocking.

Application of Silicon Controlled Rectifier(SCR):

(1) SCR is used for making controlled AC to DC rectifier.
(2) SCR is used to make DC to AC Inverter.
(3) SCR is used for switching in the electronic circuit.
(4) SCR is used to control AC as well as DC motors.

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Tuesday, March 1, 2016

March 01, 2016

Turn ON methods of SCR/Thyristor Triggering

A thyristor can be switched from a non conducting state to a conducting state in several ways-

Forward Voltage Triggering (High Voltage)

When anode to cathode forward voltage is increased with gate circuit open, the leakage current of the thyristor increases. Due to internal current multiplication taking place inside, this current increases. As soon as the forward voltage reaches the breakover voltage (VBO), the reverse biased junction J2 will have an avalanche breakdown. 
VI_Characteristics of SCR
At this voltage, thyristor changes from OFF state to ON state characterized by a low forward voltage across it with large forward current. This type of turn ON may be destructive and should be avoided.

Thermal Triggering ( Temperature Triggering)

Like any other semiconductor, the width of depletion layer of a thyristor decreases on increasing junction temperature. When the temperature of thyristor is high, there is an increase in the number of electron-hole pairs which increases the leakage current. This increase in leakage current causes increase in current amplification factor ?1 and ?2. Due to the regenerative action, ?1+?2 may tend to be unity and the thyristor may be turned ON. This is called thermal triggering of thyristor. This type of turn ON may cause thermal runaway and is normally avoided.

Light Triggering (Radiation Triggering)


If light of adequate frequency and intensity is allowed to strike the thyristor junction, then the photons will strike the electrons and increase the number of electron-hole pairs. This leads to instantaneous flow of current within the device and the triggering of the device. For light triggering to occur, the device must have high value of rate of change of voltage (dV/dt).

dV/dt Triggering

With forward voltage across the anode and cathode of a device, junction J1 and J3 are forward biased whereas the junction J2 becomes reverse biased. This reverse biased junction J2 has the characteristic of a capacitor due to charge existing across the junction. If a forward voltage is suddenly applied, a charging current will flow tending to turn ON the device. If the voltage impressed across the device is denoted by V, the charge by Q and capacitance by Cj then,
The rate of change of junction capacitance may be negligible as the junction capacitance is almost constant. If the rate of change of voltage across the device is large, the device may turn ON even though the small voltage appearing across the device is small.

Gate Triggering

This is the most commonly used method for SCR triggering. The injection of gate current by applying positive gate voltage between the gate and cathode terminals turn ON the SCR much before the specified breakover voltage. The conduction period of the SCR can be controlled by varying the gate signal within the specified value of maximum and minimum gate current. Three types of signals can be used for triggering the SCR using gate. They are either a.c. signal, d.c. signal or pulse signal.


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March 01, 2016

Silicon Controlled Rectifier (SCR)

Silicon Controlled Rectifier(SCR)
Silicon Controlled Rectifier or SCR is one of the oldest type of solid state power device. It was invented in 1975 by the General Electric Research Laboratory. It has the highest power handling capacity of all the power semiconductor device. It has four layer construction with three user accessible terminals. SCR is a latching type device that can be turned ON by the gate terminal but once turned ON, the Gate loses control on it.

Important Features

  1. It is latching type device.
  2. It can handle a very large power.
  3. It is a current controlled device because the gate current controls the SCR.
  4. It acts as an open or closed switch.
  5. The ON state voltage drop is very low.
  6. It can handle thousands of ampere of current.

Construction

It is a four layer PNPN device with three terminals brought out for the user, namely Anode(A), Cathode(K) and Gate(G). The Gate terminal is used in ON process. It can be split into two sections of NPN and PNP as shown below,
Construction of SCR(Silicon Controlled Rectifier)

It has three junctions J1, J2 and J3. The anode and cathode are connected to the main power circuit. The gate terminal carries a low level gate current in the direction of gate to cathode. Normally, the gate terminal is provided at the P layer near the cathode as shown in above figure. This is known as cathode gate.
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Monday, February 8, 2016

February 08, 2016

Principle of operation of SCR (Silicon Controlled Rectifier)

Silicon Controlled Rectifier
Silicon Controlled Rectifier
When the anode voltage voltage is made positive with respect to the cathode, the junctions J1 and J3 are forward biased but the middle junction J2 is reverse biased and only a small leakage current flows from anode to cathode due to the mobile charges. The junction J2, because of the presence of depletion layer does not allow any current to flow through the device. The leakage current is insufficient to make the device conduct. The depletion layer mostly of immovable charges does not constitute any flow of current. The SCR is then said to be in the forward blocking or OFF sate condition and the leakage current is known as OFF state current ID.
Silicon Controlled Rectifier
Silicon Controlled Rectifier
When the cathode voltage is positive with respect to the anode, the middle junction J2 becomes forward biased but the two outer junctions J1 and J3 becomes reverse biased. This is like two series connected diodes with reverse voltage across them. The junction J1 and J3 do not allow any current to flow through the device. Only a very small leakage current may flow because of the drift the charges. This leakage current is again insufficient to make the device conduct. The SCR is in the reverse blocking state or OFF state and a reverse leakage current known as reverse current IR flows through the device. The width of the depletion layer at the junction J2 decreases with increase in anode to cathode voltage (since the width is inversely proportional to the voltage). If the anode to cathode voltage VAK is kept on increasing sufficiently to a large value, a stage comes when the depletion layer at J2 vanishes. The reverse biased junction J2 will breakdown due to the large voltage gradient across its depletion layer. This is known as avalanche breakdown and the corresponding voltage is called forward breakdown voltage VBO.
Silicon Controlled Rectifier
Silicon Controlled Rectifier
Because the other junctions J1 and J3 are already forward biased, there will be a free carrier movement across all three junctions resulting in a large forward anode to cathode current through the device. Due to the flow of this anode to cathode forward current, the device is said to be in conducting state or ON state. The voltage drop would be due to the ohmic drop in the four layers and is small typically, 1V.
The anode to cathode forward current must be more than latching current IL to maintain the required amount of carrier flow across the junction; otherwise, the device reverts to blocking state as the anode to cathode voltage is reduced.

Latching Current (IL):- 

It is the minimum anode to cathode current that must flow through SCR to maintain the device in the ON state immediately after it has been turned ON and the gate signal has been removed.

Once an SCR conducts, it behaves like a conducting diode and there is no control over the device. The device continues to conduct because there is no depletion layer on the junction J2 due to free movements of carriers. However, if the forward anode current is reduced below a level known as holding current IH, a depletion region develops around junction J2 due to the reduced number of carriers and SCR is in the blocking state.

Holding Current (IH):-

It represents the minimum current that can flow through SCR and still "hold" it in the ON state. The accompanying voltage is termed as VH. If the forward anode current is reduced below holding current, SCR will be turned OFF. The holding current is defined for zero gate current (IG = 0).

Note:- The ON state of SCR is known as firing or triggering.
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