In the course of globalisation, it is often the case that system builders no longer receive any information concerning which networks their systems will later be operated on. For a power supply to be used successfully worldwide, it is necessary to combine wide-range input, an international approval package, and high operational reliability.

Input Voltage Range

Global use requires input voltages within the ranges of 85 to 265 V AC (single-phase) and 320 to 575 V AC (three-phase). If one module can cover these voltage ranges, then this makes logistics easier and saves storage costs. But caution is advised here, because "wide-range input" can be based on very distinct technologies. Frequently, for instance, so-called "auto range circuits" are used in the input circuit. With these devices, an internal circuit attempts to detect the correct input voltage. If transient or lasting voltage drops occur, however, the recurring voltage is often wrongly identified. The result is damage to the power supply and possibly even a system shutdown. Other power supplies are equipped with a continuous wide-range input. These products preclude faulty switching.

Network Stability

To work safely with international networks, two factors are of particular importance, namely a high power failure bridging time and unrestricted operation - even during lasting phase loss. In practice, short voltage drops up to 20 ms occur whenever the local power supplier switches between two networks. A lasting phase loss, on the other hand, is more often caused by the operators, e.g., when they safeguard various loads by using shared circuit breakers in their equipment. A faulty single-phase load alone is sufficient to trigger the shared circuit breaker.

The consequence is that the three-phase power supply is operated with just two phases. In situations such as these, many power supplies switch off or even transmit undefined voltage impulses to the load. Unfortunately, power supplies are only rarely equipped with generously-sized intermediate circuit capacitors, despite the fact that this is a reliable method of buffering against power failure up to 20 ms, even under full capacity (see Figure 1). Moreover, even in cases of lasting phase loss, the 24 V DC level can be maintained without restriction.

International Approval

The issue of approval has to be addressed in due time for systems originally intended for the local market and now foreseen for international use. The consistent use of devices with an international approval package ensures safety. That is why it is particularly useful to have a CB scheme certificate. Country-specific approvals thus just require written confirmation, eliminating time- and cost-intensive inspections on site.

Power Reserves

It is not always possible to know how much power the planned installation will really need in future. Until now, system planners were faced with the choice between an oversized device which would fit the job but was also costly, and an undersized module that had to be replaced with a larger device if necessary. Phoenix Contact power supplies are equipped with a performance reserve called Power Boost (see Figure 2). This enables power supplies to be used to full capacity. Over sizing or defining a utilisation factor for the entire system is thus rendered superfluous. If several 24V DC loads have to be retrofitted during operation, the power supply automatically reports when the output voltage drops more than 10 %. The fault is signalled visually via a flashing LED, and electrically via a transistor switching output and a zero-voltage relay output. Then a power supply of the same type can be connected in parallel at any time to increase performance. Time-intensive adjustment is not required.

Redundancy

Parallel power supplies not only serve to enhance performance, they are also needed to develop redundant systems. This is always necessary whenever system availability takes highest priority, e.g., for safety reasons. In redundancy, power supply output is dimensioned such that all connected loads can also be supplied by a single power supply. If a fault occurs in the no. 1 power supply, then the no. 2 power supply takes over without interruption. The defective power supply emits a signal that can be evaluated via a telecontrol system or a PLC (programmable logic controller). But what happens if the no. 1 power supply fails to the extent that it shorts the output circuit? In this case, the no. 2 power supply would also short circuit and the loads would be without power. Dimensioning the output stage generously can minimize the likelihood of this scenario. Yet this would never occur using a redundancy module, because two power supplies of the same type can be fully decoupled (see Figure 3).

Essentially, a redundancy module consists of two decoupling diodes thermally connected to a heat sink. For industrial applications, the double diode is placed on a common semiconductor substrate. This optimises the symmetry of the load current distribution and the thermal connection to the heat sink. Two power supplies of the same type, with output currents up to 28 A each or with parallel inputs up to 56 A, can be decoupled by using the Quint Diode redundancy module from Phoenix Contact (see Figure 4). Because of the low surface temperature, the device has been approved by the KEMA test centre for use in Zone 2 areas at risk from explosion.

Conclusion

Due to ever-shortening innovation cycles in automation technology, system planners have to focus on key tasks. Universal power supplies must therefore fulfil all essential requirements. Features such as wide-range input and a complete approval package are basic preconditions for use internationally. A power reserve and the suitability for redundant expansion enhance operational safety as well. (Phoenix Contact (India) Pvt Ltd, New Delhi. Tel: 011-30262800, 30262822. Fax: 91-11-26383285. Email: sagrawal@phoenixcontact.co.in