For the majority of DC-DC converter applications, including telecommunications equipment, the applicable safety standards are UL60950-1 (and its Canadian equivalent C22.2 No. 60950-1) for North America and EN60950-1 for the EU. These are titled “Standards for Safety of Information Technology Equipment (ITE), Including Electrical Business Equipment,” and both are derived from IEC60950-1 and include deviations. Additionally, for EN60950-1, each country in the EU has its own national deviations.
These safety standards address construction issues and are intended to prevent injury to equipment operators and service personnel as well as other forms of damage caused by electric shock or other hazards such as: fire, energy, mechanical, heat, radiation, or chemical. Any product bearing these certification marks has been qualified to the applicable safety standard by a certifying agency accredited by the relevant country(ies). The qualification report explains how the device was tested, what safety levels are met, and more importantly, what its conditions of acceptability are. A number of UL-approved DC-DC converters on the market today, under their conditions of acceptance, have a statement that says the device has OPERATIONAL insulation only. This means there is no protection against electric shock in the event of a breakdown between primary and secondary. The output is considered SELV (Safety Extra Low Voltage, <60Vdc) only when the input is SELV. If the input exceeds 60Vdc, the output will be considered hazardous under a fault condition. For component-level, modular DC-DC converters, the main qualification issues concern protection against electric shock and thermal/heat hazards. The three main areas of investigation are: electrical insulation, electric strength (i.e., isolation voltage), and thermal insulation. Electrical Insulation - For DC-DC converters, insulation requirements consist primarily of minimum spacings (creepage and clearance) between primary and secondary circuits including pcb traces, transformer windings, I/O capacitors, opto-couplers, etc. The minimum-spacing requirements are eliminated or relaxed significantly if an approved “solid insulator” (encapsulate or potting compound) is used. The three distinct grades of electrical insulation are:
OPERATIONAL - The OPERATIONAL level of insulation acknowledges that isolation between primary and secondary circuits is required for proper operation of the device. However, there is no protection against electric shock at the output if there is a breakdown/fault in the primary-to-secondary insulation. The output is SELV only when the input is SELV.
BASIC - BASIC insulation provides for one layer of “basic protection” against shock. BASIC insulation can be used in conjunction with other supporting circuitry to ensure a SELV output, following a fault, under all possible input conditions.
REINFORCED - REINFORCED insulation provides for 2 separate levels of protection. It ensures, without supporting circuitry, a SELV output, following a fault, under all possible input conditions. Electric Strength - Achieving different grades of electrical insulation requires different minimum isolation voltages depending on the peak internal working voltage (usually much higher than the maximum input or output voltages) of the device. The specific voltages are defined by the UL and EN specs. One example is that a device with a peak internal working voltage of 120V must guarantee 1500Vdc isolation to qualify for BASIC insulation.
Thermal Insulation - The temperature at which a manufacturer begins to derate a given DC-DC converter (i.e., its maximum full-power temperature) and the temperature at which UL60950-1/ EN60950-1 begins to derate that same device may be very different.
Transformers are usually the critical area of concern. The assorted materials that make up a transformer (wire, bobbin, tape, cores, silicone, etc.), and the manner in which those materials chemically interact at high temperatures, constitute a “thermal insulating system,” and UL/EN rates such systems. Developing a thermal insulating system and subsequently having a standards agency approve it requires lengthy aging and temperature-cycling processes to determine if any of the constituent materials interact in a manner that forms contaminants.
The majority of today’s DC-DC converters utilize planar transformers constructed within the main printed circuit board. Such transformers are considered safety hazards whenever their temperatures exceed the maximum rating of the printed wiring board under normal conditions. The conditions of acceptability will specify the environment and conditions of use for the converter.
A minority of today’s DC-DC converters utilize transformers constructed with class A (i.e., un-approved) thermal insulating systems. Such transformers are considered safety hazards whenever their internal temperatures exceed +105°C. Most of these DC-DC converters (based on thermal measurements/calculations relating to output power and derating curves) actually require transformers that can officially withstand operating temperatures up to +130°C. This grade of transformer requires a recognized Class B (+130°C) or Class F (+155°C) thermal insulating system. Such transformers must be assembled using an approved system or purchased, fully assembled, as a “system” from a qualified supplier.
Murata Power Solutions both assembles its own transformers and purchases pre-assembled transformers. In house, we have two different approved thermal insulating systems, and our transformer vendors have both approved class B and Class F thermal insulating systems.
I.E.C. The Geneva, Switzerland-based International Electrotechnical Commission (IEC) develops “generic” technical standards for electronic products and components. Designed for use by different industries and governments around the world, IEC standards have no national orientation and no effectivity or compliance dates. The Commission has no testing facilities or enforcement powers, and the adoption of IEC standards is purely voluntary. The USA’s UL60950-1 and Europe’s EN60950-1 are both derived from IEC60950-1.
CENELEC. The 30 member countries of the European Economic Area (EEA) have created CENELEC, the European Committee for Electrotechnical Standardization (Comite Europeen de Normalisation Electrotechnique). Brussels, Belgium-based CENELEC both creates its own standards and adopts standards authored by others (such as IEC), adapting them to the needs of the European Union (EU). CENELEC issues both “European normalizing” documents (with an “EN” prefix) and “harmonizing documents” (with an “HD” prefix). CENELEC also applies effectivity, but not compliance, dates to standards. Once CENELEC issues a standard, all member countries must adopt it in place of any existing national standards, though each country is permitted deviations from the CENELEC spec. For most DC-DC converters, the relevant CENELEC safety specification is EN60950-1.
Underwriters Laboratories, Inc. UL is a USA-based, nongovernmental, for-profit organization that establishes product-safety standards and also conducts product-safety testing. Conforming products are usually designated as UL “Listed” or UL “Recognized.” UL is an EU-accredited testing agency. The relevant UL safety spec for DC-DC converters is UL60950-1, and conforming products can carry the UL logo. Canadian Standards Association CSA is the Canadian equivalent of UL. It is an independent agency that both establishes standards and conducts product-safety testing. Their relevant specification for DC-DC’s, C22.2 No. 60950-1, is similar to UL60950-1.
V.D.E. Verband Deutsher Elektrotechniker (Association of German Electrical Engineers) is a private German organization that, like UL and CSA, establishes product-safety standards, conducts product testing, and issues certificates of approval. VDE tests DC-DC converters to EN60950.
T.U.V. Technisher Uberwachungs-Verein is a testing agency only. They have been authorized by VDE to test and approve products to VDE standards. Unlike VDE, TUV has offices in the USA, and they are often hired by US-based companies to approve products, which can then carry both the VDE and TUV logos.
B.S.I. The British Standards Institution is a quasi-governmental agency that both develops standards and tests products in the United Kingdom.
B.A.B.T. The British Approvals Board for Telecommunications is a private organization that approves telecom equipment sold into the UK market. BABT both conducts its own testing and accredits independent testing laboratories.
Significant differences between UL with its safety standards and EU with its directives include the fact that UL standards often dictate methods of construction. EU directives and their underlying IEC/EN specs rarely dictate construction (EN60950-1) being an exception). UL and its authorized testing labs inspect and test products before approving or recognizing them. EU does not . . . it permits manufacturers to “self certify.” Many manufacturers, however, will often submit their products to EU-accredited testing companies (one of which is UL) as a means of demonstrating full compliance with relevant EU directive.