SCR PROTECTION
- 1. EASY ENGINEERING POWER ELECTRONICS SCR PROTECTION PART 1
- 2. EASY ENGINEERING • 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?
- 3. EASY ENGINEERING • High dv/dt • High di/dt • Over Voltage • Over Current • Gate malfunctioning • Thermal protection PROTECT from whom?
- 4. EASY ENGINEERING • 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
- 6. EASY ENGINEERING 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
- 7. EASY ENGINEERING Protection for di/dt
- 9. EASY ENGINEERING • 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
- 10. EASY ENGINEERING External reasons of Over Voltage: • Lightening Surge • Heavy load Switching • Switching of Transformer • Inductive line switching
- 11. EASY ENGINEERING Over Voltage Protection Snubber Circuit Non Linear Surge Suppressors
- 12. EASY ENGINEERING 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
- 14. EASY ENGINEERING 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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- 18. EASY ENGINEERING 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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- 20. EASY ENGINEERING 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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- 23. EASY ENGINEERING 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