SCR PROTECTION PART 1

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POWER ELECTRONICS
SCR PROTECTION
PART 1

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• Very delicate device
• Sensitive to Temperature
• Unpredictable Over Voltage
• Unpredictable Over Current
• Sensitivity to Changes in V & I
• Important component in circuitry
• Reliability of full system
WHY
PROTECTION
TO SCR?

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• High dv/dt
• High di/dt
• Over Voltage
• Over Current
• Gate malfunctioning
• Thermal protection
PROTECT
from
whom?

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• Protection against high dv/dt
– Capacitance develops across Jn J2
– Charging current flows as leakage
– Leading to breaking of junction
– Even in absence of Gate pulse
– Thus malfunctioning of SCR

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Prevention

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Damage due to High di/dt
SCR turns
ON
Charge
carrier
spreads
rapidly
If di/dt >
spreading
charge
carriers
then local
hot spots
Damage to
SCR

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Protection for di/dt

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• Over Voltage Protection
– Internal Over voltage
• High reverse recovery voltage results
• Due to V =Ldi/dt, (due to abrupt commutation)
• If this voltage is greater than breakover voltage of SCR,
then SCR gets damaged

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External reasons of Over Voltage:
• Lightening Surge
• Heavy load Switching
• Switching of Transformer
• Inductive line switching

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Over Voltage
Protection
Snubber Circuit
Non Linear
Surge
Suppressors

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What are Non linear Resistance?
These are special devices, which offers very high resistance in
NORMAL condition
But very low resistance in TRANSIENT condition
Thus heavy current flows through it
Thus saves SCR across it

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NON LINEAR
RESISTORS
VARISTOR
SELENIUM
SURGE
SUPPRESSOR
AVALANCHE
DIODE

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VARISTOR
• Also called VDR( Voltage Dependent Resisitor)
• Used for High voltage surge protection
• Automatically varies resistance wrt voltage change
• Thus functionality similar to Zener diode

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Selenium Surge Suppressor
• Operation is similar to Zener diode
• Very little current flows in normal operation
• Beyond operating point heavy current flows
• Thus protects circuit from Over voltage

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Avalanche Diode
• It’s a semiconducting device
• Designed to break down & conduct at a
specified reverse bias voltage
• Characteristic similar to Zener diode

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AVALANCHE
MEANS
WHAT?

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ZENER DIODE AVALANCHE DIODE
Heavy electric field is applied across
the PN- junction.
Breakdown which occurs because
of the collision of the electrons
inside the PN-junction
Occurs in the thin region Occurs in the thick region
Occurs in HIGH doping materialaaaa Occurs in LOW doping material
Zener breakdown voltage is
INVERSLY proportional to the
temperature
Avalanche breakdown voltage is
DIRECTLY proportional to the
temperature

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OVER
CURRENT
PROTECTION