Electrical Revolution: Current Rating of the SCR

The thermal capacity of the SCR is very low due it is made of semiconductor materials.
The SCR may damage because of high temperature of junction in spite of over current flows only for short duration time.
The repetitive and non – repetitive current rating of the SCR is given below.

I_TAV – Average on state current

The forward voltage drop in the SCR is very low therefore power loss during on state of the SCR depends upon average current.
The average current depends upon firing angle of the SCR.
The average current changes due to change in conduction angle of the phase controlled SCR.
The instantaneous current increases due to decrease in the conduction angle if the average current is kept same for different conduction angle.
This will increase voltage drop across SCR and junction temperature increases due to increase in power loss. Therefore it must be necessary to decrease average current due to decrease in the conduction angle.
The average on state current is given at specific junction temperature and it is repetitive type.
The RMS current and average current is equal for DC circuit.
The average current depends upon waveform of the supply voltage.
Table 2.1 shows value of form factor for half sinusoidal and rectangular waveform.

Conduction angle	Half wave sinusoidal waveform			Rectangular waveform
Conduction angle	180⁰	90⁰	30⁰	180⁰	90⁰	30⁰
I_AV	I_M / π	I_M / 2π	0.021I_M	I / √2	I / 4	I / 12
I_RMS	I_M / 2	I_M / 2√2	0.084I_M	I / √2	I / 2	I / √12
Form factor	1.57	2.22	3.98	1.41	2	3.464

It is concluded that the value of average current for the same RMS current during on state of the SCR for half wave sinusoidal waveform less than the rectangular waveform.

I_RMS – RMS on state current

When the SCR supplies maximum current for low duty cycle, this rating has significant importance.
The RMS current increases whereas the average current is kept within limit during this period.
The RMS current produces more heat because it passes through metallic joints, lead, device wiring and interfaces.
This rating is repetitive type and given at maximum junction temperature.

I_TSM – Non repetitive surge current rating

There are possibilities of abnormal condition due to short circuit or other fault in the SCR therefore the manufacture provides surge current rating in the datasheet.
The SCR which can withstand against non repetitive surge current is called as non repetitive surge current.
Let us consider that the waveform of surge current is sinusoidal and its frequency is same as that of supply frequency. The sub cycle surge current rating is decided by comparing energy of one surge cycle and one sub surge cycle.

Therefore

I²_SUBt= I²T

I_SUB = I√ ( T / t )

Where I_SUB = Sub cycle surge current rating

I = Surge current rating for one cycle

T = Time for supply frequency half cycle

t = Time for sub cycle surge

I²T rating

This rating enables to know which type of fuse used for the protection of semiconductor device.
Its unit is amp² – second.
The I²T rating of the fuse must be less than the I²T rating of the protected semiconductor device

di/dt rating

The di/dt rating of the SCR is given in the datasheet of the SCR.
The SCR can withstand maximum value of di/dt which is given in the datasheet.
There is formation of hot spot near gate junction due to high current density if the di/dt rating exceeds its specific rating.
This will result in increase in junction temperature beyond safe limit and may damage SCR.
The di/dt rating is given in ampere/μsec.

I_H – Holding current

It is defined as the minimum anode current beyond SCR turns off.
The holding current is associated with turned off process and it is given mili – ampere.

I_L– Latching current

It is defined as the minimum anode current requires keeping turned on SCR after removing gate signal.
The latching current is associated with turned on process.
The value of latching current is approximately 2 to 3 times to that of holding current.

Electrical Revolution