The rating of a Silicon Controlled Rectifier (SCR) or thyristor typically includes the current rating of scr and anode voltage rating of scr.Thyristor ratings indicate voltage, current, power and temperature limits within which a thyristor can be used without damage or malfunction. Ratings and specifications serve as a link between the designer and the user of SCR systems.
For reliable operation of a thyristor, it should be ensured that its current and voltage ratings are not exceeded during its working. One of the major disadvantages of thyristors is that they have low thermal time constant. If a thyristor handles voltage, current and power greater than its specified ratings, the junction temperature may rise above the safe limit and as a result thyristor may get damaged. Therefore, when SCRs are selected, some safety margin must be kept in the form of choosing device ratings somewhat higher than their normal working values. The manufacturers of thyristors make a comprehensive list of the voltage, current, power and temperature ratings after carefully testing the device. If SCR are operated under these specified conditions, no damage will be done to SCRS. The object of this section is to discuss the various SCR ratings.
A thyristor has several ratings such as voltage, current, power, du/dt, di/dt, turn-on time, turn-off time etc. For correct application of the device in thyristor circuits, a knowledge of these ratings is desirable.
Some subscripts are associated with voltage and current ratings for convenience in identifying them.
First subscript letter indicates the direction or the state:
- D-forward-blocking region with gate circuit open;
- T-on-state;
- R-reverse;
- F-forward Except for the gate G,
- VBR-reverse breakover voltage
second subscript letter denotes the operating values:
- W-working value;
- R-repetitive value;
- S-surge or non-repetitive value;
- T-trigger
- VBO-forward breakover voltage
Third subscript letter M indicates the maximum or peak value.
Ratings with less than three subscripts may not follow these rules. Gate ratings involve the subscript G. Subscript A usually stands for anode and subscript AV for average.
Anode Voltage Ratings
A thyristor is made up of four layers and three junctions as shown.The middle junction J2 blocks the forward voltage whereas the two end junctions J1, J3 block the reverse voltage. The anode voltage ratings indicate the values of maximum voltages that a thyristor can withstand without a breakdown of the junction area with gate circuit open.The anode voltage rating refers to the maximum voltage that can be applied across the anode and cathode terminals of the SCR without causing breakdown or damage to the device. This rating is specified in units of volts (V) and is also dependent on the SCR's physical size and other factors.
For ac systems, the supply voltage may not be a smooth sine wave. The different anode voltage ratings are as under:
Peak working forward-blocking voltage(VDWM)
It specifies the maximum forward-blocking voltage that a thyristor can withstand during its working.shows that Pea working forward-blocking voltage is equal to the maximum value of the sine voltage wave.
Peak repetitive forward-blocking voltage(VDRM)
It refers to the peak transient voltage that a thyristor can withstand repeatedly or periodically in its forward-blocking modeThe rating is specified at a maximum allowable junction temperature with gate circuit open or with a specified biasing resistance between gate and cathode. Peak repetitive forward-blocking voltage is encountered when a thyristor is commutated or turned-offIt may be recalled that during turn-off process, an abrupt change in reverse recovery current is accompanied by a Spike voltage L * di/dt ; this is responsible for the appearance of Peak repetitive forward-blocking voltage across thyristor terminals.
Pak surge (or non-repetitive) forward-blocking voltage (VDSM)
It refers to the peak value of the forward surge voltage that does not repeat. Its value is about 130% of Pak surge (or non-repetitive) forward-blocking voltage but Pak surge (or non-repetitive) forward-blocking voltage is less than forward breakover voltage
Peak working reverse voltage(VRWM)
It is the maximum reverse voltage that a thyristor can withstand repeatedly. Actually, it is equal to the peak negative value of a sine voltage wave.
Peak repetitive reverse voltage(VRRM)
It specifies the peak reverse transient voltage that may occur repeatedly in the reverse direction at the allowable maximum junction temperature. The transient lasts for a fraction of the time of one cycle.The reason for the periodic appearance of Peak repetitive reverse voltage is the same as for Peak repetitive forward-blocking voltage.
Peak surge (or non-repetitive) reverse voltage(VRSM)
It represents the peak value of the reverse surge voltage that does not repeat its value is about 130% of Peak repetitive reverse voltage.But Peak surge (or non-repetitive) reverse voltage less than reverse breakover voltage.
On-state voltage drop(VT)
It is the voltage drop between anode and cathode with specified forward un-state current and junction temperature Its value is of the order of 1 to1.5 V.
Forward dv/dt rating
The dv/dt rating of a thyristor indicates the maximum rate of rise of the anode voltage that will not trigger the device without any gate signal.du/dt triggering is never employed as it gives random turn-on of a thyristor.This type of triggering also leads to destruction of the thyristor through high junction temperature.
Current Ratings
The current rating refers to the maximum current that the SCR can handle continuously without getting damaged. This rating is specified in units of amperes (A) and is dependent on the SCR's physical size, heat dissipation capabilities, and other factors.A thyristor is made up of semiconductor material, its thermal capacity is therefore quite small Even for short overcurrents, the junction temperature may exceed the rated value and the device may be damaged As the junction temperature is dependent on the current handled by a thyristor, a correct choice of current ratings is essential for a long working life of the deviceIn this part of the article, current ratings of SCRs are discussed for both repetitive and non-repetitive type of current waveforms.
Average on-state current (ITAV)
The forward voltage drop across conducting SCR is low, therefore power loss in a thyristor depends primarily on forward average on-state current. For the purpose of illustrating the significance of average on-state current, consider a continuous dc current OA flowing through the SCR. After the application of this current at t = 0 junction temperature begins to rise until finally it reaches its rated value Tj = 125°C.As the SCR has low thermal time constant, final temperature of 125°C is reached in a relatively short time. Suppose now that anode current is of rectangular waveshape with conduction angle 180°.
The average on-state power loss Pav in this figure is approximately given by Pav = (forward on-state voltage across a thyristor) * ITAV
It can be obtained more accurately from the relation
- Pav = 1/T ∫(instantaneous voltage across SCR) (instantaneous current through SCR) dt
- T-periodic time of the anode current waveform
The rms current for an SCR is constant whatever the conduction angle may be But average current is given by (Irms /FF) where FF is the form factor of the current waveform.
RMS Current (Irms)
Heating of the resistive elements of a thyristor, such as metallic joints, leads and interfaces depends on the forward rms current.The rms current rating is used as an upper limit for constant as well as pulsed anode current ratings of the thyristor.Its value is equal to Idc. The value of the rms forward current for an SCR remains the same for different conduction angles. Average current, however, is dependent on conduction angle.The derating of the SCR below the dc value depends upon the current waveshape and it is defined as under:
SCR derating below dc value = Idc - Idc/FF =Idc(1- 1/FF )
where,
- FF - The form factor of the waveform
- Its value is always more than one
Surge Current Rating
A surge current rating indicates the maximum possible non-repetitive, or surge, current which the device can withstand. Higher currents caused by non-repetitive faults or short circuits should occur once in a while during the life span of a thyristor to prevent its degradation.
Surge currents are assumed to be sine waves with frequency of 50, or 60Hz depending upon the supply frequency. This rating is specified in terms of the number of surge cycles with corresponding surge current peak. Surge current rating is inversely proportional to the duration of the surge. It is usual to measure the surge duration in terms of the number of cycles of normal power frequency of 50 or 60 Hz.
One cycle surge current rating is the peak value of allowable non-recurrent half-sine wave of 10 msec duration for 50 Hz.For duration less than half-cycle i.e. 10 msec, a subcycle surge current rating is also specified.This rating for 50 or 60 Hz supply is the peak value for a part of the half-sine wave. The subcycle surge current rating I ab can be determined by equating the energies involved in one cycle surge and one subcycle surge as follows:
Isb² * t=I² * T
or Isb = I*√(T/t)
where
- T = time for one half-cycle of supply frequency, see
- I = one-cycle surge current rating, A
- Isb = subcycle surge current rating. A
- t=duration of subcycle surge, sec
For 50Hz supply , T=10msec
I²T Rating
This rating is employed in the choice of a fuse or other protective equipment for thyristors. The rating in terms of amp-sec specifies the energy that the device can absorb for a short time before the fault is cleared. It is usually specified for overloads lasting for less than, or equal to, one-half cycle of 50 or 60 Hz supply. The I²t rating is given by the relation.
(rms value of one-cycle surge current)²* time for one cycle
As an example, I²t rating for 4 A (rms) SCR is 10 amp²-sec and for 35 A SCR is 100 amp²-sec.
In order that a fuse (or other protective equipment) protects a thyristor reliably, the It rating of fuse must be less than the Pt rating of the series-connected thyristor. di/dt rating. This rating of a thyristor indicates the maximum rate of rise of current.
di/dt Rating
This rating of a thyristor indicates the maximum rate of rise of current from anode to cathode without any harm to the device. When a thyristor is turned on, conduction starts at a place near the gate. This small area of conduction spreads to the whole area of junction. If the rate of rise of anode current (di/dt) is large as compared to the spreading velocity of carriers across the cathode junction, local hot spots will be formed near the gate connection on account of high current density. This causes the junction temperature to rise above the safe limit and as a consequence, SCR may be damaged permanently. Therefore, a limit on the value of di/dt at turn-on is specified in amperes per microsecond for all SCRS. Typical values of di/dt are 20 to 500 A/μ sec.
Latching Current Rating
Latching current rating is a specification for thyristors that refers to the minimum current required to maintain the thyristor in its "ON" state even after the gate signal is removed.The latching current rating of a thyristor is the minimum anode-to-cathode current required to maintain the thyristor in its "ON" state even if the gate signal is removed. If the anode-to-cathode current falls below this value, the thyristor may turn off, and a new gate signal would be required to turn it back on.
Holding Current Rating
The minimum current required to maintain conduction once the thyristor has been triggered.When a thyristor is triggered into its "ON" state, it latches into that state and remains conducting until the anode-to-cathode current falls below a certain value called the holding current. However, to trigger the thyristor initially, a gate signal is required, which typically has a higher current than the holding current.
Turn-on and Turn-of Time Rating
Turn-on time refers to the time required for an SCR to switch from the off state to the on state, and it is typically in the range of a few microseconds to a few milliseconds, depending on the SCR's specific characteristics and the external circuit conditions.
Turn-off time refers to the time required for an SCR to switch from the on state to the off state, and it is typically longer than the turn-on time, ranging from a few microseconds to several milliseconds. The turn-off time is affected by the external circuit conditions such as the load, the temperature, and the applied voltage.
Gate Circuit Voltage Rating
The gate circuit voltage rating is an important parameter to consider when designing an SCR circuit, as applying a voltage beyond the rated value can cause permanent damage to the device. It is recommended to select an SCR with a gate circuit voltage rating that exceeds the maximum voltage that will be applied to the device in the circuit.
Voltage Rating
This is the maximum reverse voltage that a thyristor can withstand without breaking down. It is typically given as the peak repetitive voltage and the peak non-repetitive voltage.
Current Rating
This is the maximum continuous current that a thyristor can carry without overheating and failing. It is typically given as the average forward current and the peak surge current.
Power Rating
This is the maximum amount of power that a thyristor can handle without overheating and failing. It is typically given as the average power dissipation and the peak power dissipation.
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