19 May 2017

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.

ITAV – 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 waveforms of the supply voltage. 
  • Table 2.1 shows value of form factor for half sinusoidal and rectangular waveforms.
Conduction  angle
Half wave sinusoidal wave forms
Rectangular wave forms
Im / π
Im / 2π
I / √2
I / 4
I / 12
Im / 2
Im / 2√2
I / √2
I / 2
I / √12
Form factor

  •  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.

IRMS – 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.

ITSM – 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.
        I2SUBt = I2T
        ISUB = I√ ( T / t )
        Where ISUB = 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.

IH – 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.

IL – 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. 

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