# Characteristics of SCR

In this post, static and dynamic characteristic of SCR is given. The static characteristic is related to voltage – current relation of the SCR during on state whereas the dynamic characteristic is related to current and voltage across SCR during its ON state and OFF state.

## Static Characteristics of SCR

• Figure A shows the circuit diagram to obtain V – I characteristic of the SCR
• Here VAK is voltage between anode to cathode and Vg is gate voltage.
• The static V – I characteristic of the SCR is divided into following mode.

### Reverse Blocking Mode

• When cathode is made positive with respect to anode and gate switch is kept open circuited, junction J1 and J3 are reverse biased whereas the junction J2 becomes forward biased.
• Hence only small leakage current flows through the device.
• If the anode voltage increases to certain level, an avalanche break down occurs at the junction J2 and current flows sharply through all the junctions.
• This current is limited to safe value in order to protect SCR due to high power dissipation.
• The region OA is known as reverse blocking mode and AB is known as reverse avalanche breakdown mode of the SCR.

### It is defined as the time required for the forward off state voltage reduces form 90% to 10% of initial value. The rise time is reduced by applying high and steep gate pulses. However the rise time depends upon the nature of the anode current i.e. The rise time is less for RC circuit and more for RL circuit. The total turn on time is given in the range of micro second. The actual turn on time of the SCR is much higher than the turn on time given in the manufacturer’s data sheet.

#### Once the SCR start to conduct, gate loses control. The SCR can be turned off by reducing the anode current below holding current for sufficient time. This can be achieved by natural commutation or forced commutation. The turn off time of the SCR is defined as the time interval between the instant at which the anode current becomes zero and the instant at which SCR regain forward blocking voltage. The total turn off time can be divided in to two intervals : reverse recovery time ( trr ) and gate recovery time ( tgr )

• The anode current becomes zero at instant t1
• The anode current flows in the opposite direction during reverse recovery time ( t– t1 ).
• The reverse anode voltage developed across the SCR at the instant t2 and reverse current continue to decrease.
• Therefore the reverse recovery time is defined as the time between the current reversal and the instant at reverse current has decayed to 10% of negative peak value.
• The reverse recovery time increases as the forward current increases.
• When the reverse recovery current stops, high transient voltage appears across the SCR which may damage it.
• The middle junction J2 contains charges which must decay only by recombination at the end of reverse recovery time.
• This recombination is possible if a reverse voltage is maintained across the SCR therefore the time for recombination of charges ( t3 – t4 ) is called as gate recovery time tgr
• The recombination is stopped at time t4 and forward voltage can be reapplied at time instant.
• The turn off time of the SCR is given is in the range of 3 to 100 micro second.
• The circuit turn off time must be greater than the SCR turn off time in actual practice.
• Therefore the circuit turned off time tc is defined as the time between the instant anode current becomes zero and at the instant reverse voltage becomes practically zero.
• The circuit turned off time tc must be greater than the SCR turn off time otherwise the SCR may turn on at undesired instant and it is known as commutation failure.
• The turn off time increases with increase in the magnitude of anode current and junction temperature.
• It also depends upon type of commutation circuit i.e. natural commutation or forced commutation