Engineering for Endurance and Safety: Understanding TDDB in Isolation Products
By Aditya Ambardar, Business Unit Manager for Switching Power
What is TDDB?
At Diodes Incorporated (Diodes), Time-Dependent Dielectric Breakdown (TDDB) is a foundational reliability metric used to validate the long-term performance of our RobustISO™ isolation portfolio. Whether you're designing for industrial automation, EV systems, or grid infrastructure, TDDB testing ensures that the isolation barrier can withstand decades of electrical stress while maintaining safety and signal integrity.
TDDB refers to the gradual degradation of dielectric materials under sustained voltage and thermal stress, ultimately leading to breakdown. In isolation products, this means the capacitive barrier between high-voltage and low-voltage domains must remain intact for the product’s full operational life.
Diodes’ isolation components are designed to meet or exceed global safety standards, including UL1577, VDE0884-11, and IEC 60950-1.
TDDB reflects the lifetime of isolation under the maximum working isolation voltage (VIOWM). It is derived through voltage acceleration testing combined with mathematical modeling. According to IEC60747, basic insulation requires a 24-year lifetime at a 1000ppm failure rate under VIOWM, while reinforced insulation requires 30 years at a 1ppm failure rate.
TDDB Evaluation Process
The TDDB evaluation process consists of four key steps:
1. Test the voltage acceleration to gather failure data,
2. Calculate the cumulative failure probability and Weibull parameters (Alpha/Beta),
3. Estimate the Weibull percentiles to determine the lifetime values at specific failure rates, and
4. Estimate confidence intervals for the lifetime estimates.
Voltage Acceleration Test Conditions
To conduct accelerated lifetime testing, isolation product samples are stressed at various elevated voltages and temperatures to simulate real-world conditions. This includes:
- AC and DC voltage stressing,
- High-temperature soaks (e.g., 125°C), and
- Multiple sample lots for statistical confidence.
These tests help predict insulation lifetime under VIOTM (Transient Isolation Voltage) conditions.
The following table summarizes the test conditions:
|
Voltage (AC) |
Temperature |
Characteristic Lifetime |
|
Low Voltage |
25℃ |
t_low>116days |
|
125℃ |
t_low>116days |
|
|
Medium Voltage |
25℃ |
t_low<t_m<t_high |
|
125℃ |
t_low<t_m<t_high |
|
|
High Voltage |
25℃ |
t_high<t_low/100 |
|
125℃ |
t_high<t_low/100 |
Figure 1: Voltage Acceleration Test Matrix
Weibull Distribution Analysis
Using the failure data (voltage and time), the cumulative failure probability is modeled using the Weibull distribution. This allows us to estimate:
- Cumulative failure probabilities,
- Alpha/Beta parameters, and
- Expected lifetimes at 1ppm or 1000ppm failure rates.
The data is fitted to a two-parameter Weibull distribution to extract Alpha and Beta values.
Where:
| t | is the measured variable, usually time to break down or the breakdown voltage, |
| F(t) | is the probability of failure at a voltage or time less than or equal to t. For tests with large numbers of specimens, this is approximately the proportion of specimens broken down by time or voltage, t. |
| α | is the scale parameter and is positive, and |
| β | is the shape parameter and is positive. |
Figure 2: Cumulative Failure Probability
Lifetime Estimation at Specific Failure Rates
Weibull percentiles are then used to estimate the lifetime of insulation at 1ppm and 1000ppm failure rates under VIOWM.
Where p is expressed as a percentage.
Figure 3: Failure Probability (Mean Value) Under Different Voltage Stresses
Confidence Intervals for the Lifetime Estimates
Confidence intervals are calculated to provide statistical assurance of the predicted lifetimes.
Figure 4: Failure Probability Under VIOWM=1800VRMS
Conclusion
At Diodes, TDDB is more than a test—it’s a design commitment. By integrating rigorous testing, statistical modeling, and global certification, we deliver isolation products that stand the test of time.
TDDB analysis is applied to our RobustISOTM products, like the API772x digital signal isolators. Voltage- and lifetime-derating strategies are used to optimize reliability. Based on TDDB testing, statistical modeling provides evidence for 40+ years of reliable isolation barrier performance.
The Diodes logo is a registered trademark of Diodes Incorporated in the United States and other countries.
All trademarks are the property of their respective owners.
© 2025 Diodes Incorporated. All Rights Reserved.
