Silicon Controlled Rectifier (Thyristor)

Silicon Controlled Rectifier (SCR) SCR (Silicon Controlled Rectifier) is the first member of thyristor family in Power Electronics. It has high power withstand capacity and least switching frequency among all the Power Electronics devices. SCR is three terminals (Anode, Cathode and Gate) and four layered p-n-p-n power semiconductor switch. These four layers make three junctions J1 J2 and J3. It is semi-controlled switch means gate current can control only turn on process for turn off the SCR we have to use external circuit known as commutation circuit. Since turn on process is controlled by gate current means it is current controlled switch.

SCR Symbol

SCR VI Characteristics

SCR Working SCR works in three modes these are   

Forward Conduction Mode Forward Blocking Mode Reverse Blocking Mode

Forward Conduction Mode

In this mode anode is made positive with respect to cathode hence junctions J1 and J3 is forward biased and junction J2 is reversed biased.

Therefore Depletion layer is formed at junction J 2. At this region electrons and holes are diffused together from N- and P+ layer respectively. So there is no conduction in this region. So we can say initially it is in forward blocking mode. But by applying different triggering method it brought from forward blocking mode to forward conduction mode.

Forward Blocking Mode

When forward voltage applied to SCR anode is made positive with respect to cathode. AS we discussed above when anode is positive and

cathode is negative then junctions J 1 and J3 is forward biased and junction J2 is reversed biased. Therefore Depletion layer is formed at junction J2 and this will increases the reverse voltage blocking capabilities of junction J2 and this will not aloow any conduction. Hence SCR is off in this mode.

Reverse Blocking Mode

When reverse voltage applied to SCR anode is made negative and cathode is made positive. Hence junctions J 1 and J3 are reverse biased and junction J2 is forward biased. Therefore Depletion layer is formed as junctions J1 and J2. And this increases the reverse voltage blocking capabilities of SCR. Hence SCR is turn off in this mode.

Since SCR conduct only forward current so SCR is an unidirectional Semiconductor switch.

LATCHING CURRENT Latching current is related with turn on process of scr. It is the minimum anode current below which SCR fails to turn on after removing the gate pulse. See gate pulse initiate the turn process of SCR once SCR starts conducting gate losses the control on it. Therefore we have to remove the gate signal when SCR starts conducting to avoid the continuous gate power loss.

If we remove the gate pulse when anode current is less than the latching current then SCR fails to turn on. There we have to maintain the gate pulse at-least for a period until the anode current becomes equal to latching current.

SCR TRIGGERING METHODS    

Forward Voltage Triggering dv/dt triggering Light Triggering Gate Triggering

Forward Voltage Triggering In this method we have to apply sufficient amount of forward voltage so that breakdown occurs at junction J2. So by applying forward voltage across SCR, Junction J1 and J3 become forward bias and junction J 2 is in breakdown region. So SCR conducts successfully. But in this method losses increases because of increasing reverse voltage across junction J2. So this method is not preferred. J2

dv/dt triggering By applying high dv/dt charging current increases across the terminals if this charging current become greater than latching current then SCR conducts successfully.

Gate Triggering By applying the gate current initial conduction area in depletion region increases which helps the SCR to turn on. In this method we have to apply signal for certain time period to avoid continuous gate power loss.

It is the most practical method.

Light Triggering When light radiation is incident near the gate junction then the depletion layer absorb the light energy and produce more number of electrons and holes and this will increase the initial turn on process.

Holding Current Since SCR is not a fully controlled device it is a semi-controlled device means gate current can only control the turn on process it cannot control the turn off process. SCR will only turn off if its anode current becomes less than the holding current. So we can say that holding current is that minimum current below which anode current makes SCR turn off. The turn off process of SCR is called commutation. In case of ac supply where supply is sinusoidal anode current becomes less than holding current in every 10 ms hence commutation is possible naturally and this is natural commutation. But in case of dc supply whether natural commutation is not possible we have to use force commutation.

SCR Protection

1 Overcurrent protection

Over current protection in SCR is achieved through the use of fuse. We must connect either fuse or circuit breaker in series with SCR. 2 Over voltage protection Thyristor can be protected from overvoltage by using varistor. We must connect varistor across SCR for over voltage protection.

3 dv/dt protection At high dv/dt SCR may turn on before the gate pulse is given. This is an accidental turn on this unwanted turn on is known as false turn on. dv/dt protection is needed to avoid this false turn on. For protection against this false turn on Snubber circuit is connected across the SCR. Snubber circuit is nothing but a series RC circuit.

4 di/dt protection During turn process if di/dt is greater then the spread velocity of charge carrier then the charge density increases cumulatively and this results in the formation of local hotspots and damage the device. For protect the SCR from high di/dt stress we must connect inductor in series with SCR. 5 Thermal protection Thermal protection is needed to protect the SCR from the heat generated due to its internal power loss. SCR is mounted on the heat sink for thermal protection. Heat sink will extract the heat due to internal power losses and reduce the junction temperature with in safe limit.

SERIES AND PARALLEL COMBINATION OF SCR

String Efficiency Series Connected Thyristor

n = number of series connected thyristor (SCR)

Parallel Connected Thyristor

n

=

number

Derating Factor

of

parallel

connected

thyristor

It is the measure of reliability of string.

Derating Factor

=

1 – String Efficiency