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Rating of SCR or Thyristor

Thyristor ratings or SCR ratings are very much required for operating it in a safe zone. The reliable operation can be done when a thyristor does not violate its voltage and current rating during working period. A thyristor, or SCR may have several ratings, such as voltage, current, power, , , turn on time, turn off time, etc. Generally these ratings are specified in the data sheet given by manufacturer.

Anode Voltage Rating

This rating gives us a brief idea about withstanding power of a thyristor in forward blocking made in the absence of gate current.

Peak Working Forward Blocking or Forward OFF State Voltage (VDWM)

It specifies the maximum forward voltage (positive voltage that applied across anode and cathode) that can be withstand by the SCR at the time of working.

Peak Repetitive Forward Blocking Voltage (VDRM)

It specifies the peak forward transient voltage that a SCR can block repeatedly or periodically in forward blocking mode. This rating is specified at a maximum allowable junction temperature with gate circuit open. During commutation process, due to high decreasing rate of reverse anode current a voltage spike is produced which is the cause of VDRM generation.

Peak Non-Repetitive or Surge Forward Blocking Voltage (VDSM)

It is the peak value of the forward transient voltage that does not appear periodically. This type of over voltage generated at the time of switching operation of circuit breaker. This voltage is 130 % of VDRM), although it lies under the forward break over voltage (VBD).

Peak Working Revere Voltage (VRWM)

It is the maximum reverse voltage (anode is negative with respect to cathode) which can be withstand by the thyristor repeatedly or periodically. It is nothing but peak negative value of the AC sinusoidal voltage.

Peak Repetitive Revere Voltage (VRRM)

It is the value of transient voltage that can be withstand by SCR in reverse bias at maximum allowable temperature. This reason behind the appearance of this voltage is also same as VDRM.

Peak Non Repetitive Revere Voltage (VRSM)

It implies the reverse transient voltage that does not appear repetitively. Though this voltage value is 130% of VRRM, it lies under reverse break over voltage, VBR. Forward ON State voltage Drop (VT)This is the voltage drop across the anode and cathode when rated current flows through the SCR at rated junction temperature. Generally this value is lie between 1 to 1.5 volts.

Forward dv/dt Rating

When we apply a forward voltage to the thyristor Junction J1 and J3 are forward biased whereas junction J2 is reverse biased and hence it acts a capacitor. So due to a leakage current flows through the device. This value of current will increase with the value of . One thing we have to keep in mind that voltage value is not the reason behind flowing of leakage current, the reason is the rate of voltage increasing. The value of capacitance of the junction is constant hence when increases to a suitable value that leakage current occurs an avalanche breakdown across junction J2. This value of in called forward rating which can turn on the SCR without help of gate current. In practice it is not suitable to apply high due to high temperature malfunction of SCR.

Voltage Safety Factor of SCR (VSF)

It is described as the ratio of peak repetitive reverse voltage (VRRM) to the maximum value of input voltage.

Finger Voltage of SCR (VFV)

Minimum value of voltage which must be applied between anode and cathode for turning off the device by gate triggering. Generally this voltage value is little mare than normal ON state voltage drop.

Current Rating of SCR

We all know that a thyristor, hence a SCR is made of semiconductor which is very much thermal sensitive. Even due to short time over current, the temperature of the device may rise to such a high value that it may cross its maximum allowable limit. Hence there will be a high chance of permanent destruction of the device. For this reason, current rating of SCR is very essential part to protect the SCR.

Maximum RMS Current Rating (IRMS)

Generations of heat in the device present where resistive elements are present in the device. Resistive elements such as metallic joints are totally dependent upon rms current as power loss is IRMS2R, which is converts to heat, hence cause of temperature rise of the device. Hence, IRMS rating of the thyristor must be a suitable value so that maximum heat capability of SCR cannot exceed.

Maximum Average Current Rating (IAV)

It is the allowable average current that can be applied safely such that maximum junction temperature and rms current limit cannot be exceeded. Generally manufacturer of SCR, provides a characteristic diagram which shows IAV as a function of the case temperature IC with the current conduction angle φ as a parameter. This characteristic is known as "forward average current de-rating characteristic".

Maximum Surge Current (ISM)

If a thyristor operates under its repetitive voltage and current ratings, its maximum allowable temperature is never exceeded. But a SCR may fall into a abnormal operating condition due to fault in the circuit. To overcome this problem, a maximum allowable surge current rating is also specified by manufacturer. This rating specifies maximum non repetitive surge current, that the device can withstand. This rating is specified dependent upon the number of surge cycle. At the time of manufacturing at least three different surge current ratings for different durations are specified. For example, ISM = 3,000A for 1/2 cycle ISM = 2,100A for 3 cycles ISM = 1,800A for 5 cycles A plot between ISM and cycle numbers are also provided for dealing with the various cycle surge current.

I2R Rating of SCR

This rating is provided to get an idea about over-voltage tackle power of a thyristor. The rating in term of A2S is the measure of energy that can be handled by a thyristor for a short while. An electrical fuse I2R rating must be less than that of thyristor to be used to protect it.

di/dt Rating of SCR

While, SCR is getting turn on, conduction stays in a very small area nearer to the gate. This small area of conduction spreads throughout the whole area of the junctions. But if spreading velocity of the charge carriers will be smaller than the then local hot spot may arise nearer to the gate which may destroy the device. To overcome this problem a maximum rate of rise of current, is also specified during manufacturing of the devices.

Latching Current of Thyristor

This is the rating of current below which the SCR can't be turned on even the gate signal is applied. That means this much anode current must rise to turn on the device. The gate pulse must be continuous until anode current is greater or equal to latching current of thyristor other wise the device will fail to be turned on.

Holding Current of Thyristor

This is the rating of current below, which anode current must fall to turn off the device.

Gate Specification of SCR

Gate Current to Trigger (IGT)

This is the value of gate current below which device cannot be turned on. This value of current specified at a particular forward break down voltage.

Gate Triggering Voltage (VGT)

This is the value of minimum gate voltage that must be a acquired by the gate circuit. for proper turn on of the SCR. This voltage value is also specified at a particular forward breakdown voltage similar to IGT.

Non Triggering Gate Voltage (VNG)

This is the maximum value of gate circuit source voltage below which the device must be in off state. All unwanted noise signals must lie under this voltage to avoid unwanted turn on of the device.

Peak Reverse Gate Voltage (VGRM)

This is the value of maximum reverse voltage which can be applied across the cathode and gate.

Average Gate Power Dissipation (PGAR)

This is the value of average power dissipation which cannot be exceeded by a gate circuit for a gate current pulse wider than 100 microsecond.

Peak Forwarded Gate Current (IGRM)

This is the rating of maximum forward gate current that should not be exceeded to reliable and safe operation.



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