A DC-DC converter is a device that converts one form of energy into another. Several types are available with different levels of performance and reliability. The output DC voltage can be either higher or lower than the DC input voltage. The device operates in a Battery Electric Vehicle (BEV), where a high-voltage battery (400-450V) is transformed into a lower DC voltage (12V) to power other devices.
DC-DC converters are available in different types:
Boost or step-up DC-DC converters: A device for converting to a higher voltage.
Step-down DC-DC converters: A device for converting to a lower voltage.
Boost-Buck Converter: A device capable of converting to a higher or lower voltage.
Negative Voltage or Inverting Converter: Device for supplying a negative voltage
MTBF Estimation
The term MTBF (Mean Time Between Failures) estimates potential device failures over time, expressed in hours. This analysis indicates the time an item operates before failure occurs. The MIL-HDBK-217 F2 is widely used and has two calculation procedures: Part Count Analysis and Part Stress Analysis.
The first analysis is to estimate the MTBF of a device during the early stages of development. The second analysis is once the product has reached production maturity.
The failure rate analysis of the board-mounted DC-DC converter focuses on operating temperature, input voltage, and output power to estimate overall stress. Proper thermal supervision is essential for a reliable device and other critical parts.
Case study
One of our customers has selected a DC-DC converter for an innovative automated vehicle system. He aims to predict the environmental impact of estimating the reliability of a board-mounted DC-DC converter when used in a mobile environment.
The comparative analysis was conducted by gathering specific information from the manufacturer of the DC-DC converter module, which has an MTBF of 1,850,000 hours.
We estimated the Mean Time Between Failures (MTBF) using the MIL-HDBK-217F method in both controlled (GB) and uncontrolled (GM) environments at a temperature of 25ºC.
Elevated temperature, humidity, and vibration are all factors that remain uncontrollable during practical use.
Environment reference
GB: Non-mobile, temperature and humidity-controlled environments readily accessible to maintenance;
GM: Equipment installed on wheeled or tracked vehicles and equipment manually transported.
Product Specifications
Operating Ambient Temperature: -40 ºC to +85 ºC Humidity: 95% RH max. Non-Condensing.
Load: 100%.
Derating, Above 61ºC 3.3V/5V, Linearly to Zero Power at 105ºC.
Derating, Above 65ºC 12V/15V, Linearly to Zero Power at 105℃.
Cooling: Natural Convection.
The study used two different scenarios for analysis
1. Environmental impact estimation from GB to GM
2. Converting MTBF to lifespan
The board-mounted DC-DC converter is considered a high-quality, commercial-grade component. The estimated temperatures for the environmental impact analysis are 40 °C and 60 °C.
The analysis indicates that device failure rates are affected by ambient temperature and environmental conditions. Selecting components appropriate for the installation environment is crucial for proper operation.
| Parameter | Conversion | MTBF @ 25ºC GB | MTBF @ 40ºC GM |
| Temperature | From GB 25ºC to GM 40ºC | 1,850,000 | 349,716 |
| Temperature | From GB 25ºC to GM 60ºC | 1,850,000 | 243,742 |
The environmental impact is based on MIL-HDBK-217F
Converting MTBF to Lifespan (Lx)
There has been confusion in understanding the difference between MTBF and lifespan. A product might have an MTBF of 250,000 hours, but its lifespan expectancy is 22,000 hours. Unlike MTBF calculations, lifespan refers to expected operating hours under normal conditions. That is the period between the initial use of the device and the beginning of its end-of-life. (Wear-out phase).
After performing calculations, the lifespan of the DC-DC converter is as follows:
MTBF (hours) | Enviro. | Temp ºC | Lx (%) | Lifespan (years) | Lifespan operating Hs. | Shape parameter |
1,850,000 | GB | 25 | 10 | 22.2 | 194,917 | 1 |
349,716 | GM | 40 | 10 | 4.21 | 36,846 | 1 |
243,742 | GM | 60 | 10 | 2.93 | 25,681 | 1 |
L10 refers to the time at which 10% of a population fail, and 90% has a probability of survival and is the 10th percentile, which is useful for determining warranty periods of a product.
Conclusion:
The worst-case scenario is 60°C in a GM environment where the lifespan is 25,681 hours. However, the device seems to indicate that it can operate continuously for almost three years with a 24/7 schedule.