Most automation applications require a DC power supply. If the power supply fails, the whole control panel will not work and valued production time is lost because of just one failing device. The reliability is therefore a critical parameter when choosing a power supply. However, there are uncertainties and irritations about the important keywords like MTBF-time, lifetime and reliability. The following article will try to explain these expressions and help to choose the right power supply.
Mean Time Between Failure
One key to the consideration of reliability is the MTBF-time, which stands for Mean Time Between Failures (not Mean Time Before Failure). The MTBF represents the reciprocal value of the failure rate:
The failure rate of a power supply is similar to the one of electronic components and can be shown with the bathtub life curve (see Figure 1).
The failure of a device can be subdivided into three sections.
(a) Early failures of defective components
(b) Low fault rate during the normal lifetime
(c) Increasing fault rate due to ageing components
Section (a) is usually covered by a run-in testing which however, is not performed by every power supply manufacturer. During the normal lifetime, the reliability of a device is expressed by the MTBF-time and only refers to Section (b) b).
The calculation of the MTBF is performed by a statistical evaluation of operating hours of as many devices as possible.
This calculation takes into consideration every single part of the device, the temperature, and the load. The world-wide accepted standard for such a calculation is IEC 1709.
The reliability of a device is strongly related to the MTBF and is defined as:
In other words, it is the probability of surviving a defined number of operating hours. Therefore a higher MTBF corresponds with a longer lifetime.
If the device is operated for a period equal to its MTBF, the probability of survival is 0,37. If it is only operated for a period of 10 per cent of its MTBF, the probability of survival is 0,9.
Comparing Devices Using MTBF
Therefore, the MTBF value is suitable for comparing devices. However, if you apply the MTBF value for one single device, it can be assumed that the probability of survival to be expected in practice is much better than the actual calculated MTBF values.
In Figure 2 we compared the probability of survival for a device with MTBF 100,000 h and 500,000 h. After 25,000 h (or less than three years) the probability of survival of the device with an MTBF of 100,000 h is less than 78 per cent. That means the likelihood of a failure is over 22 per cent, whereas the power supply with an MTBF of 500,000 h has a survival probability of more than 95 per cent!
Due to the exponential functionality, it gets even worse for devices with a lower MTBF, if we take into consideration a longer operating time.
After 100,000 h for example, the likelihood of a failure of devices with an MTBF of 100,000 h is about 63 per cent, whereas the likelihood of failure for a device with a MTBF of 500,000 h is only 18 per cent.
Critical Power Supply Components
The most critical parts of a power supply are the capacitors. They are usually the first parts to fail, especially at higher temperatures. If a manufacturer fails to use higher quality capacitors, the lifetime will be shorter.
High quality power supplies are therefore using special long-life capacitors that are rated for temperatures of 105 degrees C or higer. At ambient temperatures of 70 degrees the internal temperature may be over 100 degrees. Even if the ambient temperature in the application is not that high, a higher rated capacitor assures a longer lifetime.
An example of a power supply with a MTBF of over 500,000 h is the QUINT series of Phoenix Contact (Figure 3). Large industrial companies in the automotive industry (General Motors, Ford, Daimler-Chrysler, Volkswagen, Audi), process industry (Dupont, Shell, Imperial Oil, Suncor, Fisher Rosemount) or automation industry (ABB, Siemens, GE) are all using these units.
In particular, if installations require availability and long lifetime, an investment in reliable components is especially meaningful from a technical, and economical point of view.
Andreas Sobotta is the Product Marketing Manager for Phoenix Contact Ltd. ET