Current ratings of an SCR
The current carrying capability of an SCR is solely determined by the junction temperature. Except in case of surge currents, in no other case the junction temperature is permitted to exceed the permissible value. Some of the current ratings used in industry to specify the device are given below.
(i) Forward Current Rating.
The maximum value of anode current, that an SCR can handle safely (without any damage), is called the forward current rating. The usual current rating of SCRs is from about 30 A to 100 A. In case the current exceeds the forward current rating, the SCR may get damaged due to intensive heating at the junctions.
(ii) On-state Current.
When the device is in conduction, it carries a load current determined by the supply voltage and the load. On-state current is defined in terms of average and rms values.
ITav is the average value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling. The temperature at which the current is permissible has to be mentioned. It is this current which determines the application of device.
ITrms is the rms value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling.
It is the minimum device current, which must be attained by the device, before the gate drive is removed while turning-on, for maintaining it into conduction.
It is the minimum on-state current required to keep the SCR in conducting state without any gate drive. Its usual value is 5 m A.
(v) Surge Current.
It is the maximum admissible peak value of a sinusoidal half cycle of 10 ms duration at a frequency of 50 Hz. The value is specified at a given junction temperature.
During maximum surge on-state current the junction temperature is exceeded though temporarily and forward blocking capabilities are lost for a short period. The maximum surge on-state current should only occur occasionally.
(vi) I2t Value.
I2t value is the time integral of the square of the maximum sinusiodal on-state current. This is usually specified for 3 ms and 10 ms, and determines the thermal rating of the device.
(vii) Critical Rate of Rise of Current.
The maximum rate of increase of current during on-state which the SCR can tolerate is called the critical rate of rise of current for the device. This is specified at maximum junction temperature.
During initial period of turning-on, only a small area near the gate conducts the anode current. If the current increases too fast, localised overheating may take place. This is called the hole storage effect. Due to localised heating the device may get permanently damaged. To-day devices are available which can withstand rate of rise of current upto 200-250 A/microsecond, however in application this rate is hardly allowed to exceed beyond 5-10 A/micro second.
Protection against dI/dt is provided by series inductor.
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