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. 
• If the voltage and current rating of the SCR increases beyond its specific rating, the junction temperature increases and SCR may damages. 
• Therefore some margin should be kept before deciding rating of the SCR.

There are some specific letters used for voltage and current rating of the SCR.

First letter ( Operating condition )

• D : Forward off state,Gate open
• T : On state
• R : Reverse
• F : Forward

Second letter ( Operating value )

• R : Repetitive
• S : Surge or Non – repetitive
• T : Trigger
• W : Working

Third letter ( Maximum value )

• M : Maximum value

Other letters

• G : Gate
• AV : Average

Voltage Ratings

The voltage rating of the SCR should be such that it withstand against transient voltage or peak repetitive voltage. The voltage rating of the SCR is as below.

V_DWM – Peak working off state voltage

• It is defined as the maximum forward voltage withstands capability of the SCR during its off state.

V_DRM – Repetitive peak forward off state voltage

• It is defined as the repetitive maximum transient voltage withstands capability of the SCR when it is in forward blocking state. 
• This rating specified at maximum junction temperature and gate open circuited or biasing resistance is given between gate and cathode.

V_DSM – Non – repetitive peak forward off state voltage

• It is defined as the maximum forward surge voltage across SCR which repeats again and again. 
• The value of voltage V_DSM always less than voltage V_BO.

V_BO – Forward break over voltage

• It is defined as the minimum anode to cathode which turned on SCR from its off condition with gate open circuit.

V_RWM – Peak reverse working voltage

• It is defined as the reverse voltage across SCR which repeat again and again. The SCR must withstand this voltage.

V_RRM – Repetitive peak reverse voltage

• It is defined as the maximum transient voltage across SCR which repeats again and again.

V_RSM – Non repetitive peak reverse voltage

• It is defined as the maximum transient voltage across SCR which does not repeat again and again.

V_T – ON state voltage

• It is defined as the voltage drop across SCR during its on condition with specific forward on state current and specific junction temperature. 
• The value of ON state voltage of the SCR lies is in the range of 1.0 Volt to 1.5 Volt.

V_SF – Voltage safety factor

• The operating voltage of the SCR is less than the V_RSM voltage in order to protect SCR against uncertain conditions. 
• The voltage safety factor of the semiconductor is a product of maximum non – repetitive reverse voltage and maximum input voltage.

       V_SF = V_RSM / √2 × Input RMS voltage

• The value of voltage safety factor lies is in the range of 2.0 to 2.5.

Forward dV/dt rating

• It is defined as the maximum dV/dt anode to cathode voltage which does not turn on SCR without gate signal. 
• When the forward dV/dt rating of the SCR increases beyond its specific rating, it turns on.
• The dV/dt rating of the SCR depends upon junction temperature. 
• The dV/dt rating decreases as the junction temperature increases and vice versa. 
• The Snubber circuit is used to limit the dV/dt rating of the SCR.

Finger voltage

• It is defined as the minimum voltage requires between anode to cathode with gate open in order to turn on the SCR.

VOLTAGE RATING OF THE SCR.PDF

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