Understanding Inverter Grade Thyristors and Turn-Off Time

Thyristors, also known as Silicon Controlled Rectifiers (SCRs), are semiconductor devices used as switches in power electronics. They come in different types depending on their application requirements.

Two main types of thyristors are:

• Converter Grade Thyristors: These are used in applications like phase-controlled rectifiers where the turn-off is achieved naturally by the AC supply voltage (line commutation). They typically have longer turn-off times.
• Inverter Grade Thyristors: These are used in applications like inverters, choppers, and forced commutated converters where the thyristor needs to be turned off rapidly by external circuitry (forced commutation). A fast turn-off time is crucial for these applications to work efficiently and at higher frequencies.

The turn-off time (t_q) of a thyristor is the time required for the device to regain its forward blocking capability after the forward current has dropped to zero. A shorter turn-off time means the thyristor can be switched off and on again more quickly, allowing for operation at higher switching frequencies.

Inverter grade thyristors are specifically designed and manufactured to have a very short turn-off time compared to converter grade thyristors. This short turn-off time is necessary to ensure reliable commutation (turning off) in inverter and chopper circuits, which rely on forced commutation techniques.

The classification of a thyristor as "inverter grade" is based on this critical parameter, the turn-off time. Devices with a turn-off time less than a specific threshold are designated as inverter grade.

Based on standard classifications in power electronics, thyristors with a turn-off time less than \(25 \, \mu\text{s}\) are generally considered inverter grade thyristors. This short time allows them to be used in circuits requiring rapid switching.

Looking at the options provided:

• \(15 \, \mu\text{s}\) is less than \(25 \, \mu\text{s}\), so a thyristor with this turn-off time would be inverter grade.
• \(45 \, \mu\text{s}\) is greater than \(25 \, \mu\text{s}\), typically classifying it as converter grade.
• \(25 \, \mu\text{s}\) is the threshold value. Thyristors with turn-off time less than this value are inverter grade.
• \(35 \, \mu\text{s}\) is greater than \(25 \, \mu\text{s}\), typically classifying it as converter grade.

Therefore, the thyristors which have a turn-off time less than \(25 \, \mu\text{s}\) are called inverter grade thyristors.