High Voltage Direct Current (HVDC) transmission has become a cornerstone of modern power infrastructure, enabling efficient long-distance energy transfer, integration of renewable energy sources, and interconnection of asynchronous grids. At the heart of these systems lie high-power semiconductors, which must operate reliably under extreme electrical and environmental conditions.

From classic line-commutated converter (LCC) systems to modern voltage source converter (VSC) topologies, the evolution of HVDC has been closely tied to advancements in thyristors, IGBTs, and associated protection devices.

Semiconductor technologies in HVDC systems

1. Thyristors in LCC HVDC

Traditional HVDC systems rely on phase-controlled thyristors, valued for:

• High voltage blocking capability (several kV per device)
• Exceptional current handling
• Proven long-term reliability in grid environments

These devices remain essential in bulk power transmission where efficiency and robustness outweigh switching flexibility.

2. IGBT modules in VSC HVDC

Modern HVDC systems increasingly use Modular Multilevel Converter (MMC) architectures based on IGBT modules, enabling:

• Independent control of active/reactive power
• Black-start capability
• Easier integration with offshore wind and weak grids

Dynex 6.5 kV trench-gate IGBT modules are now commonly deployed in these systems.

3. Fast Recovery Diodes (FRDs)

Paired with IGBTs, FRDs:

• Provide current commutation paths
• Enable efficient switching at high voltages
• Must withstand significant transient stress during faults

However, FRDs alone are not sufficient for fault protection, particularly under DC fault conditions.

The role of protection semiconductors in HVDC

One of the biggest technical challenges in VSC HVDC systems is managing DC-side faults, where:

• Fault currents rise extremely quickly
• Energy levels exceed the safe operating limits of IGBTs and FRDs

Bypass Thyristors: essential protection elements

As described in the Dynex application note AN6447:

• Bypass thyristors are connected across the antiparallel diode (FRD)
• They act as resettable protection devices, diverting fault current away from sensitive components
• They must handle extreme surge currents while maintaining reliability over long lifetimes

highlights that these devices:

• Divert DC fault currents far beyond FRD capability
• Reduce thermal and electrical stress on the converter

Innovation in HVDC protection: Reverse Asymmetric Bypass Thyristors (RAByT)

A key advancement is the 6.5 kV Reverse Asymmetric Bypass Thyristor (RAByT), developed specifically for VSC HVDC applications.

Design challenges addressed

Conventional thyristors face trade-offs:

• High voltage capability requires thick silicon
• Thick silicon leads to higher on-state voltage drop
• Increased losses and stress on associated devices

RAByT Design Innovations

The RAByT architecture introduces several key improvements:

1. N+ Buffer Layer (Cathode Side)

• Enables thinner silicon structure
• Reduces dynamic forward voltage drop
• Improves conduction efficiency

2. Optimised Gate Structure

• Interdigitated gate design
• Significantly lower pick-up voltage (Vpu)
• Faster and more reliable triggering

3. Junction Termination Optimisation

• Double positive bevel design
• Reduces surface electric field concentration
• Enables full utilisation of silicon capability

4. High-Resistivity N-Base

• Improves resistance to cosmic ray-induced failures (FIT)
• Critical for long-term HVDC operation under constant DC bias

These features collectively deliver:

• Higher surge current capability
• Lower conduction losses
• Improved reliability

Simulation and test data show performance benefits in HVDC systems

• Reduced peak current through FRDs when using RAByT devices
• Lower I²t stress, improving system lifetime
• Significantly lower pick-up voltage compared to standard thyristors
• Enhanced fault current diversion capability

As demonstrated in the application analysis, RAByT devices substantially reduce stress on IGBT diode paths during DC fault events, improving converter robustness.

Advanced HVDC semiconductors also require precise manufacturing control, including:

• Narrow process windows for dopant implantation
• Use of phosphorous ion implantation instead of traditional diffusion
• Tight control over buffer layer profiles

These factors are critical to achieving:

• Target blocking voltages (e.g., 6.5 kV class devices)
• Optimised trade-off between conduction and switching performance

Looking ahead, several trends in HVDC semiconductor technology are shaping the future:

• Higher voltage classes (≥10 kV devices) for increased system efficiency
• Integration of SiC-based devices for faster switching and reduced losses
• Continued innovation in hybrid protection topologies combining thyristors and IGBTs
• Greater emphasis on reliability engineering, particularly for offshore and remote installations

Power semiconductors are the foundation of HVDC technology, enabling efficient, controllable, and reliable transmission of electrical energy at scale.

While IGBTs have enabled the rise of flexible VSC HVDC systems, thyristor-based protection devices remain indispensable, particularly in managing fault conditions. Innovations such as the Reverse Asymmetric Bypass Thyristor (RAByT) represent a significant step forward, offering improved efficiency, robustness, and reliability for next-generation HVDC networks.

Dynex offers expertise in both device physics and application-specific design to support the evolving demands of global power infrastructure.