Ensuring IGBT Reliability: A Guide to Partial Discharge Testing and Insulation Design

August 25, 2025622 Views 9 min read

The Silent Killer: A Deep Dive into IGBT Partial Discharge Testing and Insulation Design Optimization

In the world of high-power electronics, engineers focus intensely on metrics like conduction losses, switching speed, and thermal resistance. However, a silent and insidious failure mechanism often goes overlooked until it’s too late: Partial Discharge (PD). For systems operating at medium to high voltages—such as wind turbines, railway traction, and industrial motor drives—PD is a leading cause of long-term insulation degradation, culminating in unexpected and often catastrophic device failure. Understanding, testing for, and designing against PD is no longer an academic exercise; it’s a critical requirement for building robust and reliable power systems.

As system voltages push past 1700V and into the 3.3kV, 4.5kV, and 6.5kV classes, the electrical stress on the internal insulation of an IGBT Module becomes immense. This guide provides a practical, engineering-focused look at what partial discharge is, how it’s tested, and most importantly, how to optimize insulation design to ensure the long-term reliability of your power conversion systems.

Understanding Partial Discharge (PD) in the Context of an IGBT Module

What is Partial Discharge?

Imagine partial discharge as tiny, localized lightning strikes occurring within the insulation system of an IGBT module. It’s a localized dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between the two conductors. While a single PD event is low in energy, the cumulative effect of thousands or millions of these events is like a chisel slowly chipping away at the insulation’s integrity. These discharges produce ozone, nitric acid, and other corrosive byproducts that chemically attack the insulating materials (primarily silicone gel), leading to embrittlement, cracking, and eventual complete dielectric failure.

Where Does PD Occur Inside an IGBT Module?

An IGBT module is a complex assembly of different materials, each with a unique dielectric constant. This creates interfaces where electric fields can concentrate and initiate PD. The most common locations for PD inception are:

Voids in the Silicone Gel: The most frequent culprit. Microscopic air bubbles trapped in the silicone gel during manufacturing are prime locations for PD. Since the dielectric strength of air is much lower than that of silicone gel, the electric field concentrates across the void, causing the trapped gas to ionize and discharge.
Gel-to-Substrate Interface: Delamination or poor adhesion between the silicone gel and the Direct Bonded Copper (DBC) or Active Metal Brazed (AMB) ceramic substrate can create thin, flat voids where PD can occur.
Terminal and Housing Interfaces: Gaps between the high-voltage terminals and the plastic housing, or between the gel and the inner walls of the housing, are susceptible to PD if not properly filled and designed.
Surface Contaminants: Any foreign particles or moisture on the surface of the ceramic substrate can act as a PD inception point.

These defects can be introduced during manufacturing or develop over the module’s lifetime due to thermal cycling and mechanical stress, which can cause the materials to expand and contract at different rates, creating new voids or worsening existing ones.

The Essentials of Partial Discharge Testing

Since PD is a precursor to total failure, testing for it is a crucial quality and reliability check. The goal is not just to see if PD happens, but to determine the voltage at which it starts and stops, giving engineers a clear picture of the insulation system’s health and operational margin.

Key PD Metrics: PDIV and PDEV

Two parameters are fundamental in PD testing:

Partial Discharge Inception Voltage (PDIV): This is the minimum applied voltage at which partial discharge activity begins and becomes consistently detectable. A higher PDIV indicates a better, more robust insulation system with fewer defects.
Partial Discharge Extinction Voltage (PDEV): This is the voltage at which PD activity ceases as the applied voltage is decreased from above the PDIV. In a healthy insulation system, the PDEV is very close to the PDIV. A PDEV that is significantly lower than the PDIV is a major red flag, suggesting the presence of significant defects that sustain the discharge even as the voltage stress is reduced.

For a high-reliability system, the specified PDIV must be comfortably above the maximum continuous operating voltage of the application, including any expected overshoots or transient peaks.

Common PD Testing Methods for IGBT Modules

Testing is typically performed according to standards like IEC 61287 for power modules. The two primary methods are AC and DC testing.

Testing Method	Description	Advantages	Disadvantages
AC Partial Discharge Test	The module is subjected to a 50/60 Hz AC voltage ramp. A coupling capacitor and a measuring impedance are used to detect the high-frequency current pulses generated by each PD event.	– Closely simulates real-world operating conditions for AC applications (e.g., drives, grid-tied inverters). – Well-established, standardized method. – Sensitive to detecting voids, which are the most common defect.	– The test itself can be destructive if not properly controlled. – Requires specialized, high-voltage AC test equipment. – May not fully represent the stress in pure DC applications.
DC Partial Discharge Test	A DC voltage is applied and ramped up. PD detection is more challenging as it only occurs during the voltage ramp or at pre-existing charge sites.	– Less stressful on the insulation system compared to AC. – Useful for applications with high DC bias, like HVDC transmission. – Can sometimes identify different types of defects than AC tests.	– Less sensitive to small voids. – PD activity is transient and harder to detect consistently. – Interpretation of results can be more complex.

For most inverter and converter applications, the AC PD test is the industry standard and provides the most relevant data for ensuring long-term reliability.

Engineering Best Practices for PD-Resistant Insulation Design

Preventing partial discharge begins on the drawing board. Robust insulation design is a multi-faceted discipline that involves careful material selection, precise geometric control, and meticulous process management.

Optimizing Creepage and Clearance Distances

These are two fundamental concepts in high-voltage design:

Clearance: The shortest distance through the air between two conductive parts. This is primarily about preventing a direct flashover.
Creepage: The shortest distance along the surface of an insulating material between two conductive parts. This is critical for preventing tracking and surface discharge, especially in the presence of contamination.

Designers must adhere to standards like IEC 60664-1, which define required distances based on the system’s nominal voltage, pollution degree (PD), and the Comparative Tracking Index (CTI) of the insulating materials used. Increasing creepage distances by adding ribs or grooves to the housing surface is a common and effective strategy.

The Critical Role of Encapsulation Materials

The silicone gel that fills most high-power IGBT modules is the primary insulation barrier. Its properties are paramount:

Dielectric Strength: The gel must have a high breakdown voltage.
Viscosity and Flow: It must have a low enough viscosity during potting to flow into all crevices and small gaps, leaving no voids.
Curing Process: The curing process must be precisely controlled to prevent shrinkage, which can create voids or delamination. Vacuum potting is a standard procedure to remove trapped air before and during the filling process.

Leading manufacturers like Infineon and Mitsubishi invest heavily in developing proprietary gel formulations and automated, void-free potting processes to guarantee high PDIV levels.

Geometric Design and Electric Field Control

Sharp corners and edges are enemies of high-voltage insulation. They create points of high electric field concentration, which act as inception sites for PD. A PD-resistant design incorporates:

Smoothed and Rounded Conductors: All internal high-voltage terminals, bond wire heels, and DBC copper traces should have rounded edges to distribute the electric field more uniformly.
Field-Grading Structures: In very high-voltage modules (>3.3kV), designers may use semi-conductive grading layers or optimized geometries to shape the electric field, reducing the peak stress at critical interfaces.
Optimized Layout: The physical layout of the chips on the substrate and the routing of high-voltage tracks are designed to manage E-field distribution and maximize insulation distances.

Practical Application: A Case Study in Wind Power Converters

Problem: A wind farm operator was experiencing premature and seemingly random failures of IGBT modules in their 2MW converters. The failures were more frequent in turbines located at higher altitudes, leading to costly downtime and emergency maintenance.

Investigation: A sample of failed and functioning modules from the field were subjected to a rigorous analysis. While standard electrical tests showed no immediate issues on the functioning units, a partial discharge test revealed the problem. The modules exhibited a low PDIV, barely above their peak operating voltage. Further analysis showed the PDEV was significantly lower, indicating severe internal degradation. Cross-sectioning the modules revealed extensive micro-voids in the silicone gel, particularly around the main power terminals, consistent with long-term PD activity.

Solution: The engineering team specified a replacement power module from a different vendor that explicitly guaranteed a high PDIV rating (e.g., >2500V for a 1700V module). This new module featured a superior, void-free encapsulation process and a design with better E-field control. A strict incoming quality control process, including batch PD testing, was implemented.

Result: After retrofitting a portion of the fleet with the new modules, the mean time between failures (MTBF) for the converters increased by over 60%. The operator could confidently extend maintenance intervals, drastically reducing operational costs and improving the farm’s overall energy production and profitability.

Key Takeaways and Future Outlook

Designing for high voltage is more than just choosing a device with the right voltage rating. It’s about ensuring the long-term integrity of the entire insulation system.

Summary of PD Prevention Strategies

Design Area	Key Considerations
Materials	Select high-dielectric strength, low-shrinkage silicone gel. Ensure high CTI for all insulating plastics and substrates.
Geometry	Maximize creepage and clearance distances. Round all sharp conductive edges. Use field-grading features for very high voltages.
Process	Implement vacuum potting to eliminate voids. Maintain strict cleanliness controls to avoid contamination.
Testing	Specify a clear PDIV and PDEV requirement in your purchasing specifications. Implement incoming PD testing for critical applications.

As the industry continues its push towards electrification and renewable energy, system voltages will only increase. The advent of wide-bandgap semiconductors like SiC, which can operate at higher voltages and frequencies, makes robust insulation design and partial discharge testing more critical than ever. When selecting a high-voltage IGBT module for a demanding application, don’t just look at the datasheet’s front page. Dig deeper into the insulation specifications and PD performance data. For critical applications where reliability is non-negotiable, partnering with a supplier that understands and can guarantee PD-free performance is the surest path to success. If you need help navigating the complexities of insulation systems or selecting a module for your high-voltage project, our team of application engineers is here to help.