SCR-Ratings and Specifications
The performance of an SCR is affected by the junction temperature, because of change in carrier densities in the four layers and the junction temperature naturally depends upon the internal power losses of the device and the efficiency of heat transfer mechanism. The factors contributing toward rise in junction temperature are
- on-state voltage drop across the device (SCR),
- leakage current in the blocking or off state and
- power dissipation at gate.
There are three types of ratings for an SCR like continuous; repetitive and non-repetitive. Because the SCR has a short thermal time constant, there is no difference between continuous and intermittent ratings above a few second conduction period.
(a) Continuous Current Ratings.
Continuous current ratings of SCRs are normally given in terms of average or rms values depending upon whether the device is unilateral or bilateral. However, rms current rating is more commonly used in the rating procedure for a device. The rms current rating of the device is useful, because it is essentially independent of the conduction angle, whereas the average current rating decreases with the decrease in conduction angle. Also, the rms current rating of the device is larger than the average current rating. This rms current flows through the conductive part of the lead assembly, the device wiring and internal assembly parts and raises the temperature. The rms current rating must be limited to a safe value to prevent excessive heating in the resistive elements of the SCR, such as joints, leads etc. The rms current rating is also important when SCRs are used to supply large peak current to a load with low duty cycle.
(b) Repetitive Overload Current Ratings.
It is very common for rectifier equipment to frequently run on over-load. Though the duration of over-load is short, it is repetitive. It is quite obvious that the peak allowable junction temperature is never to be exceeded under any condition. So, the magnitude, duration and repetitive frequency of the overload current of the device should be such that under no case the peak allowable junction temperature is exceeded. This again depends on the thermal resistance and transient thermal impedances of the device and the type and size of the heat sink. It also depends on the method of cooling (i.e. natural air cooling or forced air cooling), the velocity of cooling air, ambient temperature etc.
(c) Non-repetitive Surge Current Ratings.
Non-repetitive surge current ratings of the SCRs are provided by the manufacturer. Such surge current is assumed to be imposed on the device when it is operating at the maximum rated voltage, current and temperature condition in a half-wave circuit delivering a resistive load. These ratings are such that during the surge current, the maximum repetitive junction temperature of the device may be exceeded. During this brief period, the forward blocking capability of the device is lost until the device is cooled down to or below the maximum rated operating temperature. The surge current is not a regular feature of the device and occurs during severe fault condition. These ratings provide the instantaneous over-load capacity of the device and are used in designing the protective devices for it. Such ratings are usually provided in terms of non-recurring surge current with respect to time duration of occurrence and I2t. The maximum surge current rating is provided for minimum time duration of one half cycle of the supply frequency, i.e. 10 ms; for the supply frequency of 50 Hz. I2t ratings apply for non-repetitive surge overloads shorter than one half cycle. At this condition SCR behaves like a resistor with a fixed thermal capacity. The heat dissipation during the small period is negligible. I2t rating of the device represents the capability of the device to withstand the overload current for the specified time. The current is rms value for the time interval t.