This document discusses various techniques for protecting SCRs (silicon controlled rectifiers). It describes how an SCR crowbar circuit uses a zener diode and resistor to protect loads from overvoltage. It then discusses three gate protection techniques that use resistors, zener diodes, and other components to prevent unwanted triggering of the SCR gate due to issues like overvoltage, di/dt, and dv/dt. Finally, it covers attaching heat sinks to SCRs for thermal protection and lists common heat sink mounting techniques like lead and stud mounting.

1. SCR Protection Techniques
Presented By:
Shahneel Siddiqui
BE-VII

2. Agenda
 Protection of Load using SCR Crowbar Circuits
 Gate Protection Techniques
 Thermal Protection of SCR

3. SCR Crowbar Circuit
 It prevents the load from getting damaged
 Simple and effective method
 Widely used for over-voltage protections
 Consists of a Zener Diode, a Resistor, and a SCR
 These components are connected between the Source & the Load

4. SCR Crowbar Circuit (Cont.)

5. SCR Crowbar Circuit (Cont.)
 Rating of Zener Diode is kept slightly greater than normal voltage limit.
 Zener Diode Makes the crowbar circuit act as open switch at normal voltage.
 When the voltage limit increases, the Zener diode starts conducting and voltage appears across
the Resister.
 This, in turn activates the SCR because of which the voltage drop across the load is reduced.
 The gate current determines the amount of voltage to be supplied to the Load.

6. Gate Protection of SCR
 Gate plays an important role in normal triggering of an SCR
 Various reasons can automatically trigger the gate even if it is not desired, the reasons include
• Negative Voltages
• Dv/dt & di/dt
• Over voltages
• Over currents
• Holding & latching current
 There are various techniques to protect the gate of an SCR from these issues.

7. Gate Protection (Technique 1)
 This is the simplest Gate protection Technique
 This technique prevents the gate from being
triggered due to over-voltages
 The Resistance RG will limit the gate current
 The Zener diode will act as close circuit if the
voltage is exceeded than the limit of Zener diode,
preventing from the gate being triggered

8. Gate Protection (Technique 2)
 This technique prevents the gate from being
triggered due to di/dt, dv/dt.
 When there is rapid current variation, inductor
smooths it and protects the thyristor from damage
 The Resistance R would increase dv/dt capability.
 The Zener diode will act as close circuit if the
voltage is exceeded than the limit of Zener diode,
preventing from the gate being triggered

9. Gate Protection (Technique 3)
 This method prevents the Gate from every kind of
unwanted triggering.
 The Diodes are used to protect the gate from
negative voltages by rectification process
 The Resister R1 limits the current
 The Resister R2 provides dv/dt Protection
 The capacitor is removing high frequency
components.

10. SCR Thermal Protection
 With the increase in junction temperature, insulation of SCR may get fail.
 Proper measures are required when working with SCR on high current and temperature levels.
 A heat sink may be mounted on an SCR to prevent it from getting damaged due to heat.
 The heat sinks are made of high thermal conductivity metals, such as
 Aluminium
 Copper
 Maily Aluminum is used due to its low cost.

11. Types of Heat Sinks mounting
 Types of Heat Sinks mounting techniques used
 Lead Mounting
 Stud Mounting
 Bolt Down Mounting
 Press Fit Mounting
 Press Pack Mounting
 Mostly used are
 Lead Mounting
 Stud Mounting

12. Lead Mounting

13. Stud Mounting

14. This was it
Thankyou for your time
Questions are very much welcomed 