The heart of a system: the design of a reliable power supply

A reliable and failure-free power supply does not just depend on a good power supply unit, when looking to make savings, always bear in mind that the power supply is the 'heart' of every application!
 
In addition to the power supply unit, other components also need to be taken into consideration for a reliable power supply. Depending on requirements, this includes DC-UPS (Uninterrupted Power Supply), battery modules, buffer modules, redundancy modules or also electronic current control systems Even environmental aspects and the efficiency play a huge role. The efficiency and also the integration of the power supply into an energy management system are important factors, too. How intelligent does or should a power supply be?
 
Reliable power supply or just a cheap design? Almost all control cabinets, switchboxes and decentral units have a 24 VDC power supply for measuring, control and regulation equipment (automation equipment). If this power supply fails for whatever reason, this will always lead to production standstills. The result: trouble, expense or delivery problems. This is why failures need to be minimized or avoided completely. In some industries, potential production stoppages are even more critical. For example the glass industry; the process may not stop at all here!
 
Each component must be reviewed critically and selected depending on the application. What is reliability actually? It may be a standard operation, a redundant design or even a fail-safe design that works when the mains power supply fails. Also, steps should be taken to consider how various consumers / consumer groups can be secured with a fuse selectively. So-called electronic current control systems are used here.
 
Almost every modern power supply has a potential relay contact to report the operating status, e.g. to an overriding control unit. Intelligent power supply units, e.g. from the LÜTZE Compact series, can also record and signal the DC power currently flowing, indicate when a current limit has been overstepped or signal an error on the AC input side. This can be acheived by means of a norm signal (4-20mA, 0-10V), relay contact or also by means of software. An additional shutdown input for soft activation and deactivation of the output complete these devices.
 
Mains fluctuations or brief mains outages caused by switching actions in the public grid can lead to system standstills and the error is not directly detected. One remedy here are buffer modules that guarantee e.g. 20A for 300 ms at 24 V. DC UPSs are used in case of longer power outages and/or faults. This is the safest and most reliable solution, but also the most expensive. The bridging time here depends on the selected battery capacity.
 
A redundant design is a popular way of improving reliability. If a power supply unit fails, the entire supply is tapped from another power supply unit. The faulty device is identified and can be replaced. Lots of power supply units have integrated decoupling diodes that can impact on the level of efficiency. Special redundant modules supply information about the status of the power supply units, redundancy errors and show the symmetry of the power supply units.
 
Each application has various consumers, such as sensors, control unit, actuators, HMI or other devices. These should be protected with a fuse either individually or as a group of consumers. This is necessary to shut down individual faulty components within an appropriate time. If this does not happen, the power supply unit that is used may become overloaded and shut down, which in turn would lead to a complete power outage. Modern intelligent current monitoring devices, such as the LOCC-Box, are ideal for this. 5 patented characteristics and 10 current ranges up to 10A allow adapted and safe shutdown. Communication with the control unit can be acheived by means of inputs and outputs or via the field bus system that is used.
 
This means that there are a number of options that need to be taken into account in the planning phase and examined with respect to the costs. It is urgently recommended that attention is paid to ensuring a reliable power supply.

Author: Klaus Wünschirs, Product Manager Control at Friedrich Lütze GmbH