Natural vs. Forced Commutation for SCR Thyristors

This article compares natural commutation and forced commutation, highlighting the key differences between these two methods for turning off semiconductor thyristor devices, such as SCRs (Silicon Controlled Rectifiers), in power electronics. Commutation, in essence, is the process of switching a conducting thyristor off. This can be achieved through current or voltage based methods, leading to the two primary types of commutation: natural and forced.

Understanding Commutation

Commutation refers to the process of turning off a conducting thyristor. There are two main types based on the underlying principle:

• Current Commutation: Turning off the thyristor by reducing the current flowing through it below the holding current.
• Voltage Commutation: Turning off the thyristor by applying a reverse voltage across it.

Regardless of the underlying principle, two primary methods exist for achieving commutation: natural commutation and forced commutation.

Natural Commutation (Line Commutation)

Natural commutation, also known as line commutation, is a method where the switching of a semiconductor device from its conducting state to its non-conducting state happens automatically, without needing any external circuits or added components. This process hinges on the inherent properties of the circuit itself.

The switch-off is automatic, dictated by the circuit conditions like voltage or current reversal.

• This occurs in AC circuits. Due to this, the SCR turns off when a negative voltage appears across it.
• Because no special circuits are needed to turn off the SCR, this type is called natural commutation.
• Natural commutation in thyristors is common in phase-controlled rectifiers, AC voltage controllers, and cyclo-converters.

Figure-1 depicts a circuit where natural commutation occurs. It’s also known as “Line Commutation.” Figure-2 shows the associated waveforms. Crucially, $t_c$ must be greater than $t_q$, where:

• $t_c$ = Time offered by the circuit during which the SCR must be completely OFF.
• $t_q$ = Turn-off time of the SCR.

If this condition isn’t met, the SCR will become forward-biased before it’s fully turned off, and start conducting without a gate signal.

Forced Commutation

Forced commutation, unlike its natural counterpart, actively employs external circuitry or components to switch off the semiconductor device (SCR, Diac, Triac, etc.), forcing it from the conducting to the non-conducting state.

In forced commutation, an external circuit controls the switching process, actively triggering the turn-off.

• This is applied to DC circuits.
• Forced commutation is achieved by reverse biasing the SCR device or reducing the SCR current below the holding current value.
• Commutating elements like inductances and capacitances are used.
• Forced commutation is used in choppers and inverters.

Common Forced Commutation Methods:

• Self commutation
• Impulse commutation
• Resonant pulse commutation
• Complementary commutation
• External pulse commutation
• Load Side Commutation
• Line Side Commutation

Difference between Natural and Forced Commutation

Conclusion

In essence, natural commutation relies on the inherent characteristics of the circuit, automating the switching process. Conversely, forced commutation actively uses external circuitry to switch off the semiconductor device. The selection between these methodologies depends on the specific needs and characteristics of the power electronic application.