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
	1800	900	300	1800	900	300
IAV	IM / π	IM / 2π	0.021IM	I / √2	I / 4	I / 12
IRMS	IM / 2	IM / 2√2	0.084IM	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.

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Voltage Rating of the SCR

• The rating of the SCR does not increase beyond its specific value in order to operate SCR for reliable operation. 
• The specific rating of voltage, current, power and maximum limit of temperature is given the SCR data sheet in order to protect SCR against over voltage, over current or any other malfunction. 

Gate Protection

• There is possibility of false triggering due to over voltage across gate cathode junction of the SCR. 
• The possibility of increase in the temperature of gate – cathode junction due to high current flows through it and this may result in damage of

Power Rating of the SCR

There are following power loss when SCR is in turned on condition.

Forward conduction loss

• The forward conduction loss in the SCR is product of average anode current and forward voltage drop. 
• The average current and average power loss for different conduction angle is shown in the figure A.

Mounting of SCR

• When the current passes through SCR is greater than the rated value, the thermal stress produced in it which generates mechanical force. 
• If this mechanical force does not control, the SCR may damaged. 

Thermal resistance

The following losses take place during on condition of the SCR.

• On state conduction losses
• Gate triggering losses
• Forward and reverse blocking leakage current

Heat Sink

• The function of the heat sink is to absorb heat of the semiconductor device and dissipates it to surrounding medium by thermal conductivity. 
• It transfers heat from high temperature to low temperature. The heat sink plays important role for cooling of semiconductor device.

Selenium Diode

• The metal oxide varistor or selenium diode is used to protect semiconductor device against transient over voltage or high dv/dt. 
• The MOV is called as voltage dependent Non linear resistor. 

Metal Oxide Varistor ( MOV )

Metal oxide Varistor (MOV)

• The MOV is a non linear resistor.  
• There are two metal particles oxide layers are separated by insulation. When voltage across MOV increases beyond its specific value, the

Snubber Circuit

• The SCR may be turned on due to transient or high dv/dt voltage across it in spite of switch S is kept open. 
• The SCR may damage due to this type of surge voltage. 

Over Voltage Protection of Semiconductor

• When the SCR is turned on , its maximum voltage rating must be less than the specific rating of the SCR. 
• There are following main reasons for generation of over voltage or transient voltage.

Current - Time Characteristic of Fuse

Current – Time Characteristics

• The fast acting fuse is used to protect semi conductor device. 
• The fuse is connected in series with semi conductor device and it open when the current exceeds its rated value. 

Function of Freewheeling Diode

• The diode is connected across load in the controlled or uncontrolled converter circuit. This diode is called as freewheeling diode or flywheel diode.