RATINGS OF SR

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POWER ELECTRONICS
RATINGS OF SCR

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ANODE
CATHODE
GATE

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Ratings of SCR
• Voltage Rating
• Current Rating
• Power Rating
• Thermal Rating
• dv/dt rating
• di/dt rating

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For reliable
operation SCR or
any other
device, it must
operate with in
specified ratings
Ratings are
generally
specified in data
sheet specified
by manufacture.

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What is
DATA
SHEET?

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Few Important ratings of SCR
• Gate Trigger Voltage
• It’s the minimum gate voltage required to turn ON SCR
• Forward break over Voltage
• Maximum foreword blocking voltage in absence of gate pulse,
without turning it ON
• On State Voltage
• It is voltage drop across SCR at specified Voltage & temperature

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Peak Repetitive Forward Blocking Voltage (VDRM)
• It is the peak forward transient voltage that
SCR can block repeatedly or periodically in
forward blocking mode.
• Generally it is specified at a maximum
allowable junction temperature with gate
circuit open.

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Peak Working Revere Voltage (VRWM)
•It is the maximum reverse voltage
that can be withstand by the
thyristor repeatedly or periodically
without damaging it.

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Voltage Safety Factor of SCR (VSF)
•It is described as the ratio of
peak repetitive reverse voltage
(VRRM) to the maximum value of
input voltage.

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I2Rt rating
• It’s a measure of power than can be handled by SCR. Fuse
rating must be lower than this rating to protect SCR .
Surge current rating
• Maximum possible non repetitive current that SCR can with
stand. So that, SCR never exceeds temperature range.
Maximum Average Current Rating (IAV)
• Its a maximum current that can flow with in the range of
allowable temperature .

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di/dt rating of SCR
• When SCR is turning ON, conduction starts from
gate area and slowly spreads across junction
• If this spreading velocity is lower, than local hot
spot may occur, which can damage SCR,
• Thus manufacture supplies maximum rate of
rise of current to protect SCR

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Latching Current
Holding Current
Gate triggering current
Gate triggering Voltage

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Forward Conduction loss
Turn On loss
Turn Off loss
Forward & reverse blocking loss
Junction temperature (Tj)
Transient Thermal impedance(Z th)A

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Ton
Toff
Duty Cycle?

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Duty Cycle is the ratio of timings of
Ton to Total time
Duty cycle = Ton/(Ton+Toff)