Safety Requirements

Onboard DC/DC converter modules satisfy safety isolation requirements for ICT equipment. The core safety standard that applies to ICT equipment operating from AC-line or battery supplies of up to 600V is IEC/EN 60950-1. Now in its second edition, the standard underpins several regional adoptions worldwide. It encompasses telecommunications infrastructure equipment regardless of its power source, in addition to components and sub-assemblies such as power supplies that have intrinsic safety implications. The standard recognizes that individual parts may not meet all requirements, but demands full compliance from the end product. The standard's guiding principle is to provide two levels of protection from electric shock and energy hazards that may trigger other dangers, such as fire. It creates a hierarchy of protection measures that builds upon five categories of insulation: ‘Functional Insulation' is only necessary for the product to function. It may reduce the likelihood of ignition and fire hazards, but it must satisfy at least one of the sets of the standard's requirements: i - clearance and creepage distances; ii - electrical strength tests; and iii - fault-condition testing. The next level is ‘Basic Insulation', or ‘insulation to provide basic protection against electric shock'. It must satisfy all three parts (i, ii, and iii) that also apply in varying degrees of stringency to subsequent categories of insulation. Clearance and creepage distances are crucial elements of IEC/EN 60950-1. Clearance is the shortest distance between two conductive parts through air, while creepage measurements apply across the surface of an insulator. A key difference is that IEC/EN 60950-1's clearance specifications account for peak voltages including transients on e.g. AC-line supplies, while its creepage requirements consider only average working voltages. As any board-mounting component has conductive terminals, clearance specifications typically dominate. Different minimum distances apply for primary and secondary circuits. A primary circuit connects directly to the AC line supply, while a secondary circuit has no direct connection to a primary circuit and derives power from batteries or a device such as an isolated DC/DC converter. Electrical strength tests ensure that insulation does not break down under stress. Test voltages are a product of primary or secondary circuit use and/or peak working voltages, resulting in a withstand voltage. The standard does not prescribe any specific fault-condition testing for insulation, but includes guidance and example test scenarios that a test house may follow to ensure the end-product's freedom from any identifiable hazards. ‘Supplementary Insulation' adds another layer of basic insulation protection, but can include additional stipulations such as requiring 0.4mm greater distance through insulation for peak working voltages above 71V. ‘Double insulation' results from combining basic and supplementary insulation, while ‘Reinforced Insulation' is a single insulation system that provides the same level of protection as double insulation. This could comprise several layers that are not possible to categorize and test as basic and supplementary insulation. Isolation and SELV Circuits Isolation voltage is the withstand voltage in the standard's electrical strength test. For an isolated DC/DC converter, this parameter depends upon its design and the highest continuous voltage between its input and output, and typically ranges from 1000-1400 VDC. The great majority of commercially-available modules specify 1500 VDC isolation continuously. In rare circumstances such as outdoor Power-over-Ethernet links, the IEEE 802.3af standard for PoE demands 2250 VDC isolation. Safety Extra-Low Voltage (SELV) circuits generally feature safety isolation from primary circuits using reinforced or double insulation. Their output level may not exceed 42.4 V peak or 60 VDC between conductors or from a conductor to protective Earth, and is safe to touch under normal and single-fault conditions. The output of an isolated DC/DC converter module is normally a SELV circuit. Input Power Sources The standard describes many scenarios for interconnecting different classes of circuits and appropriate safety measures, including isolation barriers and protective Earthing strategies. For example, telecoms infrastructure equipment traditionally operates from a 48 VDC battery supply where the service voltage range is 40.5-57.0 VDC, the maximum abnormal voltage is 60 VDC, and the positive rail connects to protective Earth. Typically, an AC/DC ‘front-end' supplies normal operating power with the battery system and an independent generator taking over if the AC supply fails. The reliability of this arrangement explains its widespread adoption in datacomms and increasing popularity within general industrial applications. Systems determine DC/DC converter safety requirements For ICT equipment, designers normally expect that the output side of an isolated DC/DC converter will meet the criteria for a SELV circuit that limits voltages to a safe level under both normal operation and single fault conditions. The isolation requirements that the DC/DC converter must satisfy depend on the isolation provided by the AC/DC front-end, together with the converter's normal input voltage level and the system's arrangements for connection to protective Earth. Functional insulation is permissible for the DC/DC converter if the AC/DC front-end includes reinforced or double insulation, or if the front-end provides basic or supplementary insulation and the DC/DC converter's output connects to protective Earth. The DC/DC converter must withstand electrical strength tests for basic insulation if its normal input voltage exceeds 60 VDC, or if the front-end provides basic or supplementary insulation and the converter's input connects to protective Earth (in this case, the normal input voltage must not exceed 60 VDC.) Basic insulation is required if the supply has functional insulation between the AC line and DC output and the converter's output connects to protective Earth. Supplementary insulation is required if the AC/DC supply has basic insulation between the AC line and DC output (the AC/DC supply's output voltage must not exceed 60 VDC). Reinforced or double insulation is required if the AC/DC supply has no insulation or functional insulation between the AC line and DC output. In practice, this means that functional insulation is adequate for the DC/DC converter in almost all system implementations. System-Level Considerations Because on-board DC/DC converters in ICT equipment are typically part of a ‘two-wire' or ‘three-wire' protective Earthing system, the DC/DC converter almost invariably only requires functional insulation. In the two-wire system (Fig. 1), the input and output grounds connect on the board and then back to the system's main protective Earth point. The three-wire system (Fig. 2) separates the board's input and output grounds and independently returns them to protective Earth. Either arrangement short-circuits the DC/DC converter's isolation barrier.

In either case, the real objective is to secure a stable signal ground while protecting the system from high-energy transients that may be caused by lightning, short-circuits, or other abnormal conditions, such as events on external telecom networks or switching between alternative power sources. For a two-wire system, the low Ohmic connection between the DC/DC converter's input and output prevents any transient voltages developing across the device. In three-wire systems, the resistance and inductance that cabling presents can result in transient voltages between the converter's input and output sides being quite high. Many designers prefer the three-wire alternative as it offers greater flexibility in mitigating EMC, and this consideration makes functional insulation and a 1500 VDC isolation voltage mandatory for standard on-board DC/DC converters. Isolation will always negatively impact efficiency, although techniques such as digital inner-loop control minimize this factor. Basic insulation impacts energy efficiency significantly more severely than functional insulation due to its greater clearance and creepage distance requirements. Increases in the distance between elements in components such as transformers reduce energy coupling efficiency, which also degrades power density and ultimately results in greater cost-of-ownership. Unless there is a proven need for basic insulation, functional insulation is the better choice. www.ericsson.com/powermodules