Understanding Thyristor SCR Specifications & Parameters

Understand the key thyristor or SCR specifications and parameters found in datasheets so the correct device can be selected or chosen.

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When choosing a thyristor or SCR, there are several datasheet parameters that need to be understood so that the correct device can be selected.

The various SCR / thyristor datasheet specifications and parameters are rather different to the more familiar transistor and FET specifications, but even so they are relatively straightforward.

It is worth noting that many of the thyristor specifications are also applicable for TRIACs and DIACs as well.

Common Thyristor Specifications & Datasheet Parameters
	Spec	Thyristor Specification / Parameter Details
dI/dt	Maximum rise of on-state current	There is a maximum rate of rise for the on-state current during turn on. If this rate is exceeded then the device can be damaged.
I_GM	Peak gate current	This is the maximum level of gate current that should not be exceeded.
I_GT	Gate trigger current	This is the current required in the gate to enables the device to be triggered and to latch in its on-state provided there is sufficient anode cathode current to maintain the current flow.
I²t	Overcurrent protection	The I²t parameter indicates the fuse that is required for protection. It is for the 10ms overcurrent duration.
I_T(AV)	Average on-state current	This parameter is different to the RMS current as it defines the average current rather then the RMS. RMS will give the true heating effect of the current.
I_T(RMS)	RMS on-state current	This thyristor specification is the maximum allowed RMS current through the device. It is specified for a given temperature. Different datasheet specifications may quote ambient temperature, T_a, case temperature, T_c, or even the lead temperature, T_l. The method used for quoting the temperature normally depends upon the type of case used for the thyristor / SCR.
I_TSM	Non-repetitive surge on-state current	As the name implies this datasheet parameter for thyristors defines the maximum peak current in the device under pulse conditions. It is necessary to look at the exact conditions for the manufacturer in question, but it is often defined for a half sine wave. The duration is specified for 50 Hz (10ms duration) and 60Hz (8.3ms duration). It is required because a surge current exceeding the maximum may cause failure of the device.
T_J	Junction temperature	This is the junction temperature and often the maximum junction temperature is specified in the datasheets. Calculating the thermal resistance it is possible to determine the conditions under which the maximum junction temperature is not exceeded.
T_stg	Storage temperature	This is the minimum temperature in which the device can be stored.
V_DRM / V_RRM	Repetitive peak off-state voltage	This parameter is the maximum peak voltage that is allowed across the device. This datasheet specification parameter should not be exceeded otherwise the device may fail. It is also always good to leave sufficient margin to allow for transients. This parameter is specified for conditions up to the maximum junction temperature. Also leakage currents (I_DRM / I_RRM) are also normally defined under this datasheet specification.
V_GT	Gate trigger voltage	This is the voltage that needs to be applied between the gate and cathode to enable the gate trigger current to be reached and the device to fire.
V_RGM	Peak reverse gate voltage	This is the maximum level of gate voltage that can be applied across the gate cathode junction without the possibility of damage resulting. It is wise to operate well below this voltage.

Whilst there are many other thyristor specifications and parameters that are used in their datasheets, these are some of the more widely sued ones which are needed in the deign of circuits and selection of the right components.