Power supplies in industrial plants

DIN rail power supplies play a central role in industrial plants but often attract little attention, despite the fact that power supply failures can bring production to a halt, causing huge costs.

Industrial plant owners and operators expect power supplies to meet technical standards and work for several years without causing trouble. As purchasing departments increasingly look to cut costs, manufacturers face the challenge of offering top-quality products at competitive prices. Unfortunately, some manufacturers might resort to cutting corners by compromising on design and the choice of components. Plant operators are therefore advised to choose power supplies that guarantee trouble-free operation over the entire service life of the plant. Opting for cheap modules bears the risk of high costs down the line, should production come to a halt due to a faulty power supply. That is why the general trend in DIN rail power supplies designed for industrial applications is towards greater reliability and extended warranty.

MTBF and design lifetime

A first indication regarding the reliability of a power supply is normally given by the mean time between failures (MTBF) value specified in the datasheet. MTBF is, however, a calculated, and therefore purely theoretical, value. It is determined with reference to official databases such as MIL HDBK 217F and additional MTBF specifications of the individual components contained in the device. The MTBF value therefore gives some indication of the quality of the built-in components but does not tell us a lot about the reliability of the power supply itself.

A much more useful specification is the design lifetime. This value can be determined in high acceleration lifetime tests (HALT) and is generally determined by the service life of the ‘weakest link’ in the overall system. With HALT, devices are exposed to vibration, extreme humidity and fast temperature cycles, at levels that are significantly beyond those expected during normal operation. These tests are generally performed during the prototyping phase in order to identify and eliminate weaknesses in the design.

By testing a large number of samples from a series batch, manufacturers are able to make accurate predictions regarding the average lifetime of the module. Such tests are, however, not yet common practice in the industry, and only a few manufacturers actually publish their results.

Electrolytic capacitors

In many cases, the service life of a module is limited by the lifetime of the electrolytic capacitor, or e-cap. It would, however, be unfair to see e-caps as generally flawed components. When correctly specified and operated within the specifications, they tend to work reliably for long periods of time. One of the primary causes behind the premature failure of e-caps is the evaporation of the electrolyte. This happens obviously most quickly at elevated temperatures.

Designers of power supplies must therefore ensure that the electrolytic capacitors are positioned as far away as possible from heat sinks, transformers and hot semiconductors. In addition, ripple current caused by rapid charging and discharging cycles must be minimised. In conjunction with the equivalent series resistance of the e-cap, such currents cause the part to become hot. The lower the ripple current, the lower the power dissipation inside the electrolytic capacitor. Driven by a desire to produce ever smaller and cheaper power supplies, these clear design rules are often ignored. As a result, the operating temperature in the power supply might be constantly elevated. Given that many control cabinets provide minimum air circulation, the part is bound to fail.

For the design of its new REDIN series, RECOM substituted electrolytic capacitors for standard capacitors wherever possible. Where this was not an option for technical reasons, expensive long-life e-caps approved for operating temperatures of up to +105°C were used.

Optimised thermal design

Air circulation in control cabinets is often very poor. It is therefore important that DIN rail power supplies be optimised for uniform heat distribution inside the device. To locate potential hot spots inside the power supply at the prototype stage, RECOM uses thermal imaging. This is, however, not an easy undertaking, as thermal optimisation of the device can often only be achieved by altering its circuits. To obtain good EMC properties, the distances between switching transistors and filters should be kept as short as possible. Furthermore, each millimetre of additional wiring affects the efficiency of the power supply, which again results in higher self-heating effects inside the module.

With a width of 41 mm, RECOM’s new 45 W (REDIN45) and 60 W (REDIN60) power supplies with outputs of 12 or 24 VDC are compact, despite their curved housing. In control cabinets of limited depth, the devices can be installed at a 90° angle. Optimised for long service life, the power supplies can be operated at full performance level at temperatures between -20 and +50°C. With 87% efficiency and very low no-load consumption (<0.5 W), energy consumption and heat generation are reduced to a minimum. The REDIN power supplies come with a number of safety features as standard and are protected against overcurrent, overtemperature, overvoltage and short circuits. The modules of the REDIN series provide a constant and stable DC output voltage indicated by a DC OK indicator light. The integrated status relay even allows for remote monitoring. The output voltage can be accurately adjusted by means of a potentiometer mounted on the front panel. Operational safety is further enhanced by a mains failure bridging time of 50 ms under full load. As these convection-cooled modules are certified according to a number of international safety standards (EN/UL60950 and UL508) and cater to input voltages from 85 to 264 VAC, they can be installed anywhere in the world.

Image caption: At its new headquarters in Gmunden, Austria, RECOM operates its own reliability lab where prototypes of the REDIN series underwent rigorous testing.