Asymmetrical Thyristors (ASCR)

Asymmetrical Thyristors (ASCR) with Symbol

Asymmetrical Thyristors (ASCR), also known as asymmetric thyristors or asymmetrical gate turn-off thyristors (AGTO), are a breakthrough in power electronics technology. With their unique characteristics and superior performance, ASCRs have gained significant attention and have become essential components in various applications that require high-power switching.

An asymmetrical thyristors  or asymmetrical scr, also known as an ASCR (Asymmetrical Silicon Controlled Rectifier), is a specially designed thyristor that has limited reverse voltage capability. Unlike conventional thyristors, which are able to block a large reverse voltage, ASCRs are optimized for specific industrial applications where the blocking capability is not required.

In certain applications such as voltage source inverters that convert DC to AC, and some chopper circuits, a freewheeling diode is connected in antiparallel across each thyristor. This diode helps clamp the voltage across the thyristor to a low value (around 1 to 2 volts) under steady-state conditions. However, using a conventional thyristor with high reverse voltage capability in such circuits is unnecessary and can result in longer turn-on and turn-off times, as well as higher on-state voltage drop.

ASCRs are fabricated with a limited reverse voltage capability, typically in the range of 20 to 30 volts, while still having a forward blocking voltage of 400 to 2000 volts. By reducing the reverse voltage capability, ASCRs can achieve faster turn-on and turn-off times compared to similarly rated conventional thyristors. In fact, some ASCRs have been developed with turn-off times that are half of those of comparable conventional thyristors.

The fast turn-off characteristics of ASCRs provide several advantages. They help minimize the size, weight, and cost of commutating components in power electronic circuits. Additionally, ASCRs enable high-frequency operation, typically 20 kHz or more, with improved efficiency.This article provides a comprehensive overview of ASCRs, highlighting their working principle, advantages, and applications.

What is asymmetrical thyristors?

An SCR (Silicon-Controlled Rectifier) that is incapable of blocking reverse voltage is called an asymmetrical SCR or ASCR. It does not have the ability to block reverse voltage, and its reverse breakdown rating is typically in the range of tens of volts.

Asymmetrical thyristor working principle

ASCRs are power semiconductor devices designed to handle high currents and voltages. They belong to the thyristor family, which includes devices like silicon-controlled rectifiers (SCR) and gate turn-off thyristors (GTO). What sets ASCRs apart from their counterparts is their asymmetrical structure, allowing for asymmetric blocking and conducting capabilities.

The ASCR consists of four layers of semiconducting material (p-n-p-n), forming three junctions. The device's asymmetry is achieved by doping the various layers differently, resulting in distinct characteristics for each junction. The asymmetric structure enables the ASCR to provide a high blocking voltage in one direction and a lower blocking voltage in the opposite direction.

When a positive voltage is applied to the anode with respect to the cathode, the ASCR operates as a regular thyristor. However, in reverse bias, the junctions are designed to withstand a lower blocking voltage. This characteristic makes ASCRs ideal for applications that require unilateral power flow, where power is predominantly transferred in one direction.

Asymmetrical characteristics and design 

What is the reverse voltage rating of an asymmetrical SCR? An asymmetrical thyristor (ASCR) is a specialized type of thyristor that is designed for applications where the reverse blocking capability is not necessary. While conventional thyristors have high reverse voltage ratings, an ASCR typically has a reverse voltage rating of about 20-30 V, while its forward voltage rating ranges from 400-2000 V.

ASCRs are optimized for specific characteristics such as switching times and on-state voltage drop. They have faster switching times and lower on-state voltage drops compared to conventional thyristors of the same rating. This fast turn-off capability is important because it reduces the size, weight, and cost of the commutating circuit components and enables operation at higher switching frequencies.

The design of an ASCR involves introducing a highly doped N-layer adjacent to the P⁺ emitter. This N⁺ layer acts as a buffer, preventing the depletion region from extending into the P⁺ layer and allowing a higher average electric field in the lightly doped N-region. As a result, the N-base can be made thinner while maintaining the same forward blocking voltage capability.

In the reverse blocking mode, the N⁺P⁺ junction quickly avalanches at a voltage less than 50 V, limiting the peak repetitive reverse voltage rating (VRRM). However, the reduction in crystal thickness can be used to reduce the on-state voltage drop and/or achieve faster turn-off times.

By adjusting the doping levels, the performance of an ASCR can be further optimized. Increasing the gold doping level can reduce the on-state voltage drop, while keeping the turn-off time unchanged. Conversely, increasing the doping level to match the on-state voltage of a conventional thyristor can substantially reduce the turn-off time. Doping levels can also be adjusted to achieve lesser gains in both on-state voltage and turn-off time.

VI Asymmetrical Characteristics

What are the advantages of using an asymmetrical thyristor?

1. Unidirectional Power Flow: ASCRs are specifically designed to facilitate power flow in one direction, making them highly suitable for applications such as power rectification, motor drives, and high-voltage direct current (HVDC) transmission systems.
2. High Voltage Blocking Capability: ASCRs offer excellent voltage blocking capability, making them capable of handling high-voltage applications efficiently. Their asymmetric structure allows for a high voltage blocking capability in the forward direction and a lower blocking voltage in the reverse direction.
3. High Current Handling: ASCRs are designed to handle high current levels, making them ideal for high-power applications. They are commonly used in industries such as electric vehicles, renewable energy systems, and industrial automation, where high current ratings are required.
4. Fast Switching Speed: ASCRs provide fast switching speeds, allowing for efficient control of power flow and minimizing energy losses. The rapid switching capability makes them suitable for applications that demand precise control and high-frequency switching.

Asymmetrical thyristor applications

ASCRs are used in applications where either a reverse conducting diode is connected in parallel or where reverse voltage is not expected to occur. For example, in voltage-source inverters, a reverse conducting diode can be connected in parallel with the ASCR to handle the reverse voltage. In switching power supplies or DC traction choppers, where reverse voltage would not occur in normal operation, ASCRs can be used without additional protection.

1. Power Electronics Converters: ASCRs are extensively used in power electronic converters, such as rectifiers and inverters, where efficient power flow control is crucial. These converters find applications in renewable energy systems, motor drives, and industrial power supplies.
2. HVDC Transmission Systems: The asymmetric characteristics of ASCRs make them ideal for HVDC transmission systems, where power is transmitted over long distances. ASCRs enable efficient power conversion, reducing losses and improving the overall transmission efficiency.
3. Electric Vehicles (EVs): ASCRs play a vital role in electric vehicle charging infrastructure, where they are employed in high-power charging stations. The fast switching speed and high current handling capabilities of ASCRs allow for efficient charging of EVs, reducing charging times significantly.
4. Industrial Automation: ASCRs find applications in various industrial automation systems, such as motor control and robotics, where precise power flow control and high power handling capabilities are required.

Difference between SCR and asymmetrical thyristor

SCR

1. The SCR has a reverse blocking capacity of thousands of volts: An SCR is a semiconductor device that can handle high voltages in the forward direction while blocking the current in the reverse direction. It is commonly used in power electronics applications. It is true that SCR devices typically have a high reverse voltage blocking capability, often in the range of hundreds or even thousands of volts.
2. The requirement for reverse blocking capacity may vary in different applications, especially in inverters: The need for reverse blocking capacity depends on the specific application and the circuit configuration. In some cases, such as inverter circuits, the need for reverse blocking capability may be reduced or eliminated. Inverters convert DC power to AC power and may not require the SCR to block reverse voltage. However, this depends on the specific design and requirements of the inverter circuit.
3. The flywheel diode is connected in anti-parallel with the load to conduct inductive load: When dealing with inductive loads, such as motors or solenoids, a flywheel diode is often connected in anti-parallel with the load. This diode provides a path for the current to flow when the SCR or ASCR turns off, preventing voltage spikes or transients that can damage the circuit or the device.
4. The peak repetitive voltage rating of the SCR is higher than the ASCR: This statement implies that the SCR can withstand higher peak repetitive voltages compared to an ASCR. However, it's important to note that "ASCR" is not a widely recognized acronym in the field of power electronics. It is possible that you meant "ASCR," which stands for Asymmetrically Controlled Silicon Rectifier. If this is the case, the comparison between the peak repetitive voltage ratings of an SCR and an ASCR would depend on the specific models and datasheets of the devices in question.
5. The on-state voltage drop and turn-off time of the SCR is higher than the ASCR: The on-state voltage drop refers to the voltage across the SCR or ASCR when it is conducting current in the forward direction. Generally, SCR devices have a higher on-state voltage drop compared to some other semiconductor devices like MOSFETs. As for turn-off time, it refers to the time taken for the SCR or ASCR to stop conducting current after the gate signal is removed. In general, the turn-off time of an SCR is longer compared to some other semiconductor devices. However, it is important to note that the specific characteristics, performance, and trade-offs between different devices can vary depending on the specific models and technologies used.

Asymmetrical Thyristor

1. Reverse Blocking Capacity: The ASCR has a low reverse blocking capacity, typically in the range of 20 to 30 volts. This means that it can withstand only a relatively low voltage in the reverse direction.
2. Forward Blocking Capacity: On the other hand, the ASCR has a high forward blocking capacity, typically ranging from 400 to 2000 volts. It is capable of blocking high voltages in the forward direction.
3. Asymmetrical Behavior: The ASCR's forward and reverse blocking capacities are not equal, hence the name "asymmetrical." It is designed to prioritize the forward blocking capability while sacrificing the reverse blocking capacity.
4. N₂⁺ Layer: The ASCR incorporates an additional N₂⁺ layer, which helps narrow the N₂ region. This N₂⁺ layer acts as an electrical field stopper, aiding in faster turn-off times.
5. Usable Peak Repetitive Voltage Rating (VRRM): Due to the presence of the N₂⁺ layer, the P₂N₂⁺ junction of the ASCR can avalanche at very low voltage levels. As a result, the ASCR does not have a usable peak repetitive voltage rating (VRRM) specification.
6. Faster Turn-off Time and ON-State Voltage Reduction: The ASCR achieves a faster turn-off time and a reduction in the ON-state voltage by sacrificing its reverse voltage blocking capacity. This trade-off allows for improved performance in certain applications.
7. Gold Doping: If the gold doping in the ASCR is the same as that of a conventional SCR, it can result in a reduced on-state voltage drop. However, the turn-off time remains unaltered.
8. Doping Level of N₂^-: The on-state voltage drop and turn-off time of the ASCR can be adjusted by appropriately selecting the doping level of the N₂^- region.

Stracture of conventional SCR and ASCR

Asymmetrical thyristor Summary or Conclusion 

An asymmetrical thyristor (ASCR) is designed with limited reverse voltage capability, allowing for faster turn-on and turn-off times and reduced on-state voltage drop. These characteristics make ASCRs suitable for specific industrial applications where blocking a large reverse voltage is unnecessary, leading to improved efficiency and performance in power electronic systems.

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