Reliability of board-mounted DC-DC converter
A DC-DC converter is a device that converts one form of energy into another. Several types are available with different level of performance and reliability. The output DC voltage can be either higher or lower than the DC input voltage. The device can be used in a BEV, where high voltage battery (400-450V) is converted to a lower DC voltage (12V) which is used to power other devices.
DC-DC converters are available in different types:
Boost or step-up DC-DC converters: Device for converting to a higher voltage
Step-down DC-DC converters: Device for converting to a lower voltage
Boost-Buck Converter: Device capable of converting to a higher or lower voltage
Negative Voltage or Inverting Converter: Device for supplying a negative voltage
The term MTBF (mean time between failures) is used to estimate potential device failures over time expressed in hours. This is a statistic that shows the time it takes for items to function before a failure can be expected. The MIL-HDBK-217 F2 is widely used and has two calculation procedures: Part Count Analysis and Part Stress Analysis.
The first is used to estimate the MTBF of a device during the early stages of development. The second analysis is used once the product has reached production maturity. See our MTBF estimate post.
The reliability of board-mounted DC-DC converter failure rate analysis is focused on the operating temperature, input voltage, and output power to estimate overall stress. Proper thermal supervision is essential for a reliable device including other critical parts.
One of our customers has selected a DC-DC converter for an innovative product that will be installed in an automated vehicle.
He aims to predict the environmental impact in estimating the reliability of boad mounted DC-DC converter when it is used in a mobile environment.
The comparative analysis was conducted by obtaining the specific information from the manufacturer of the DC-DC converter module. According to the technical sheet, the device has an MTBF of 1,850,000 hours. The MTBF estimation has been carried out employing the standard MIL-HDBK-217 F and selecting a controlled environment (GB) at 25ºC.
Two specific scenarios are evaluated for reliability prediction in uncontrolled environments, such as mobile usage (GM). Elevated temperature, humidity, and vibration altogether are out of operational control during such practical usage.
GB: Non-mobile, temperature and humidity controlled environments readily accessible to maintenance;
GM: Equipment installed on wheeled or tracked vehicles and equipment manually transported. More environmental information here.
Operating Ambient Temperature: -40ºC to +85ºC
Humidity: 95% RH max. Non-Condensing
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
Derating curve for natural convection
Two scenarios were used for the study:
1. Environmental impact estimation from GB to GM
2. Converting MTBF to lifespan
The board mounted DC-DC converter is considered to be commercial-grade and of high quality.
Temperatures for the environmental impact analysis are estimated at 40ºC and 60ºC
Environmental impact estimation from GB to GM
The environmental impact is based on MIL-HDBK-217 F
According to the analysis, the device failure rates are influenced by the ambient temperature and the environment. Selecting components that are suitable for the environment where the device is installed, is crucial to ensure proper operation.
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 of 22,000 hours.
Unlike the hours from the MTBF calculations, lifespan indicates operating hours expected under normal conditions. This is the period between the initial use of the device and the beginning of its end-of-life. (Wear-out phase). Click here for reference.
After performing calculations, it has been determined that the lifespan of the DC-DC converter is as follows:
L10 refers to the time at which 10% of a population fail and 90% has a probability of survival.
L10 life is also known as the 10th percentile and it’s useful in establishing warranty periods for a product.
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.