Power electronic systems are becoming increasingly sophisticated, with higher switching speeds, greater power densities and wider deployment in renewable energy, HVDC transmission, industrial drives, rail traction and energy storage applications. As these systems become more capable, they also become more vulnerable to overvoltage events.

Whether caused by lightning strikes, switching transients, grid disturbances or internal system faults, overvoltages can damage semiconductor devices, degrade insulation systems and reduce equipment lifetime. Selecting the appropriate protection strategy is therefore a fundamental part of reliable power electronics design.

This article explores the principal causes of overvoltage, compares the most common protection technologies, and explains why thyristor crowbar protection is often the preferred solution for high-energy power electronic systems. It is based on Dynex Application Note AN6673.

Why Overvoltage Protection Matters

Modern power semiconductor devices such as IGBTs, thyristors and diodes are designed for high efficiency but have defined voltage limits. Exceeding these limits can result in:

• Semiconductor failure
• Insulation breakdown
• Premature ageing of capacitors and transformers
• Excessive heating
• Damage to connected equipment
• Reduced system reliability
• Increased safety risks

Rather than protecting only the semiconductor device, an effective protection strategy safeguards the entire power conversion system.

Two Fundamental Protection Strategies

Almost every overvoltage protection device operates using one of two approaches - clamping protection or crowbar protection.

Clamping protection

Clamping devices absorb excess energy while limiting the voltage to a safe level.

Typical examples include:

• Metal Oxide Varistors (MOVs)
• Transient Voltage Suppression (TVS) diodes

These devices are ideal for:

• fast transient events
• lightning impulses
• low-energy surges
• signal line protection

The protected circuit continues operating while the voltage is limited.

Crowbar Protection

Crowbar protection works differently.

Instead of absorbing energy, the protection device deliberately creates a controlled low-impedance path across the supply, rapidly collapsing the bus voltage and allowing upstream protection devices such as fuses or circuit breakers to disconnect the fault.

For high-energy systems this approach can provide a much more robust solution than relying solely on clamping devices.

Why thyristor crowbars excel in high-power applications

A thyristor crowbar remains inactive during normal operation. When an overvoltage is detected, the trigger circuit applies a gate pulse to the thyristor. The device immediately switches into conduction, creating a low-impedance path that collapses the protected voltage.

Unlike many other devices, a thyristor latches into conduction after triggering, requiring only a single gate pulse to maintain operation until the current is interrupted by the upstream protection system.

This makes thyristor crowbars particularly effective for applications involving:

• HVDC systems
• Industrial power supplies
• Renewable energy converters
• Scientific power systems
• High-energy capacitor banks
• Medium- and high-power converters

Selecting a thyristor for crowbar protection

Choosing the correct thyristor requires more than selecting a voltage rating. Engineers should evaluate:

• Blocking voltage margin
• Peak surge current
• di/dt capability
• dv/dt capability
• I²t coordination with upstream fuses
• Thermal performance
• Gate drive requirements
• Mechanical assembly and cooling

The application note provides a structured design methodology to ensure the selected device operates reliably during severe fault conditions.

Dynex Thyristors for Crowbar Applications

Dynex manufactures a comprehensive range of high-power thyristors designed for demanding protection applications.

Key characteristics include:

• High surge current capability
• High I²t energy withstand
• Low thermal resistance
• Low on-state voltage
• High mechanical robustness
• Excellent di/dt capability

These characteristics enable Dynex thyristors to withstand severe fault conditions while coordinating effectively with upstream protection devices.

From Device Selection to Complete Assemblies

For many customers, implementing a crowbar involves more than selecting a power semiconductor.

Successful protection also depends on:

• Electrical design
• Thermal modelling
• Mechanical clamping
• Low-inductance layouts
• Protection coordination
• Validation testing

Dynex supports customers with both discrete thyristors and complete crowbar power assemblies, drawing on many years of experience in designing protection solutions for demanding industrial applications.

Conclusion

Overvoltage protection is a critical aspect of power electronic system design. While clamping devices such as MOVs and TVS diodes provide excellent protection against fast transient events, high-energy and sustained overvoltage conditions often require a different approach.

Thyristor crowbars provide a controlled, low-impedance path that rapidly collapses bus voltage, allowing upstream protection devices to safely isolate the fault. When correctly specified and coordinated with the wider system, they deliver a highly reliable solution for protecting valuable power electronic equipment.

Whether designing HVDC converters, renewable energy inverters, industrial drives or capacitor discharge systems, selecting the appropriate protection technology is essential for ensuring long-term system reliability and performance.