Thyristor (SCR) - Protection Methods and Technologies

On Aug 11, 2012 by Sunil Saharan At 7:12 AM | , | 4 comments »
Thyristors has changed the electronics life of human. These tiny devices are now the building block of our life. It was early 1956 when Bell Laboratories invented SCR and from then we are moving toward nano technology.
The huge vacuum tubes are now completely replaced by their semiconductor replacement. Diode, Transistors and now SCR and ICs has completely overcome voltage controlled vacuum tubes is now a old time concept today electronics is dependent on current controlled Semiconductor devices.
SCR being a widely used component is needed to be considered first. One thing that is to be considered from the point of view of safety is speed of operation.
No doubt we have changed the shape of devices, reduced their size and increased the speed of operations but these changes also created one challenge toward our scientific approach. This is the protection.
For current controlled devices protection should be fast and effective.
In current article I'll discuss the methods of SCR protection.

SCR Protection Methods

An SCR must be provided proper protection against following things :
  • Over Voltage
  • Over Current
  • Excessive di/dt
  • Excessive dV/dt
These four are the basic things that should be considered while designing a protection circuit for SCR.

High di/dt Protection

Protection provided against high current or high rate of rise of current i.e. di/dt is somewhat similar in case of SCR or thyristor.
Effect of High Current Rise Rate
As we know that when the SCR is forward biased and a proper gate signal is provided it starts conduction i.e. charge carriers start flowing through it. However if rate of rise of current is increased, more charge carrier will enter into SCR and it will generate local hotspot due to heavy current density and if the heat is not dissipated properly it may destroy the SCR. So proper protection must be provided against it.
Protection Method
The protection against high di/dt is very simple. An Inductor is connected in series with the thyristor. As we know inductor opposes the growth or decay of current hence when current growth rate or decay rate is increased it is lowered by the inductor and the SCR is protected against High di/dt.

High dV/dt Protection

Protection against high rate of change of voltage i.e. dV/dt also needed for satisfactory operation of SCR.
Effect of High Voltage Rise Rate
Protection against high rate of voltage rise is necessary because if SCR is not in conduction mode and is forward biased mode then high dV/dt may trigger the SCR, and SCR will not be able to serve its purpose.
Protection Method
As we all know capacitor is a good charge storing option and provide less resistance for high frequency voltage so a capacitor may be connected in parallel to the SCR to protect it from high rate of change of voltage i.e. dV/dt.
These were the methods of protection of SCR against high di/dt and high dV/dt.

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