SCR protections

SCR Protection Techniques

Presented By:Shahneel SiddiquiBE-VII

Agenda

Protection of Load using SCR Crowbar Circuits Gate Protection Techniques Thermal Protection of SCR

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

SCR Crowbar Circuit (Cont.)

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.

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.

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

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

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.

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.

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

Lead Mounting

Stud Mounting

This was it

Thankyou for your time

Questions are very much welcomed