Necessity of Snubber Circuit- Why it is used in Power Electronics?

Why Snubber Circuit used in Power Electronics?

Introduction:

Due to unexpected changes in the supply and load voltages, fault conditions are happened. Under these conditions, the current or voltage ratings of the power devices are exceeded which leads to a possibility of damage to the power device. Such damages can be avoided with the help of protection circuits.

In this post, we will discuss about the over voltage protection issue. The over current issue is discussed in other posts

Reasons for Over Voltages across the Devices:

1. Any inductance in series with the SCR causes over voltage across the device when the reverse recovery current falls to zero value quickly. This will happen during every turn OFF. So periodically during the operation of SCR.
2. Over voltages may occur due to external circuit conditions such as lightning, travelling waves caused by switching ON of long lines etc.

What is dv/dt?
The high rate of change of voltage across the power devices is called dv/dt. ie increasing or decreasing the applied voltage to the device at a very fast manner. The overvoltage and the dv/dt must be limited to safe values to avoid false triggering of the thyristor.
False Triggering = Turning On of the SCR  at the undesired moments or undefined sequence. The dv/dt is the major reasosn for the false triggering which leads to the device failure. The value of dv/dt can be reduced significantly using snubbers circuit and Metal oxide varistors (MOVs).

The Snubber Circuit is not a fundamental part of the basic power electronic circuits. It is an addition to the basic circuit. Depends on the requirement, the snubber circuit may be used in single or in combination...

Snubber circuit Functions:

The functions of the Snubber circuits are listed below:

1. To protect the SCR from over voltage occurring due to switching ON either on the supply side or load side.
2. To limit voltage applied to thyristor during turn off transients.
3. To protect the thyristor from dv/dt that may occur during reverse recovery. It also protects a thyristor from dv/dt of reapplied voltage.
   [ Note: After the SCR attained its positive blocking capability, reapplied voltage must have a dv/dt less than the permissible value.]
4. To limit SCR current during turn ON transients.
5. To limit the rate of rise of currents (di/dt) through thyristors at the turn ON.
6. To reduce thyristor switching losses.

The following are the broad classes of snubber circuit

1. Unpolarized seris RC snubber circuit
2. Polarised RC snubber circuits
3. Polarised LR snubber cicuits.

The popular RC snubber circuit is discussed in the separate post. How snubber circuit protects the Thyristor?

Read More:

SCR Ratings and Specifications 
SCR Series Operation Tutorial 
Power Electronics Technical Interview Questions: Set-2 
BrushLess DC (BLDC) Motor Tutorial 

Thanks for reading about Why Snubber Circuit used in Power Electronics....

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