Transformer design: For which safety standards should you be concerned?

Technology News | July 23, 2014

By eeNews Europe

When designing an offline flyback power supply, integrating the IC with the other components can be overwhelming. Integrating the transformer properly is more than finding the turns-ratio to match with your input and output(s). “I thought a Flyback transformer is just transferring energy across a safety isolation barrier.” “I guess there is more to understand.” Not knowing the safety standards will usually mean more iterations than necessary when working with a transformer manufacturer. Not knowing the safety standards can lead to unwanted surprises of non-conformance at the qualifying labs and sending a product back to design phase rather than qualification before mass production.

Understanding the requirements that govern transformer design and manufacturing will shorten design cycles to finished products using Fairchild parts in Power Supply WebDesigner (PSW). Just knowing the safety standards required for your design will be enough to allow Wurth Electronics Midcom to design the right part the first time over 90% of the time.

Knowing safety standard required during product definition or design definition will ensure unnecessary design iteration and cost. Below you will find the most common standards used in the industry to define the safety requirements of offline transformers. Wurth Electronics Midcom is very familiar with these standards and typically knowing which standard you are trying to meet and the insulation type is enough to get you what you need.

Which transformer safety standard to use is not always so clear for the designer of electronics. Typically the standards to be met for a product are based on the equipment type to be designed and the markets that the equipment is targeting. Product Marketing as well as Safety Engineers with in a company are best equipped to choose which standards apply.

In order to make your search easier, we have listed a number of the more common standards that are referred to by electronics manufacturers that apply to their equipment. We have attempted to take the scope and a brief description of these standards from the standards themselves. While the descriptions have been pulled directly from the standards, they have been para-phrased for readability and are not in any way a substitute for the actual standard. The gain here is to compile the scopes of the most typical standards that are used into one document to let you easily identify which standard might apply. From there you can purchase the standard for further review.

Standards

EN 50470-1 AC Electricity metering equipment

This standard provides the general requirements for class B and C Metering equipment. The scope of this European standard applies to newly manufactured watt-hour meters intended for residential, commercial and light industrial use, for use in 50Hz electrical networks.

The standard applies to electromechanical or static watt-hour meters for both indoor and outdoor application. The standard applies to the active energy metering part and not to other functional elements, like maximum demand indicators, electronic tariff registers, time switches, ripple control receivers, data communication interfaces, etc. that are enclosed in the meter case.

EN 60935 Fixed inductors for electromagnetic interference suppression

This International Standard applies to inductors designed for electromagnetic interference suppression for use with electronic or electrical equipment and machines and is restricted to inductors for which safety tests are appropriate.

Inductors within the scope of this standard may also be used to protect from electrical noise and voltage or current transients coming from either the supply or from other parts of the apparatus.

EN 61347-2-13 LED module lamp control gear for both AC and DC applications

This standard applies to LED control modules that are designed to provide constant voltage or current at SELV or SELV equivalent or higher voltages. As defined the lamp control modules are one or more components between the supply and lamps which may serve to transform the supply voltage, limit the current of the lamps to the required value, provide starting voltage and preheating current, prevent cold starting, correct power factor or reduce radio frequency.

As defined in the standard the lamp control modules shall be designed and constructed that in normal use they operate without danger to the user or surroundings. Where luminaire enclosure for protection against electric shock are not used, they must be sufficiently protected against accidental contact with live parts.

IEC 60335-1 Safety for household and similar electrical appliances

This International Standard deals with the safety of electrical appliances for household and similar purposes with rated voltage not more than 250 V for single-phase appliances and 480 V for other appliances.

This standard also applies to battery-operated appliances and other DC supplied appliances. Other appliances not intended for normal household use but which nevertheless may be a source of danger to the public, such as appliances used by laymen in shops, in light industry and on farms, are within the scope of this standard. Examples of such appliances are catering equipment, cleaning appliances for commercial use, and appliances for hairdressers.

This standard deals with the common hazards presented by appliances that are encountered in and around the home but doesn’t take into account children and those with physical, sensory or mental disabilities; or lack of experience and knowledge prevents them from using the appliance safely without supervision or instruction; children playing with the appliance.

Attention is drawn to the fact that for appliances intended to be used in vehicles or on board ships or aircraft, additional requirements may be necessary; in many countries, additional requirements are specified by the national health authorities, the national authorities responsible for the protection of labor, the national water supply authorities and similar authorities.

This standard does not apply to

appliances intended exclusively for industrial purposes;
appliances intended to be used in locations where special conditions prevail, such as the presence of a corrosive or explosive atmosphere (dust, vapor or gas);
audio, video and similar electronic apparatus (IEC 60065);
appliances for medical purposes (IEC 60601);
hand-held motor-operated electric tools (IEC 60745);
personal computers and similar equipment (IEC 60950-1);
transportable motor-operated electric tools (IEC 61029).

IEC/EN/UL 60601-1 Medical electrical equipment

This standard applies to medical equipment which must have two means of protection to provide operators and patients BASIC SAFETY. This is usually accomplished by providing Basic Insulation (one means of protection) and connecting accessible conductive parts/circuits to earth or by providing Double or Reinforced Insulation (two means of protection).

IEC 61558-2-16 Safety for switch mode power supply units and transformers for switch mode power supply units

This standard applies specifically to the safety of transformers, reactors, power supply units and similar products for supply voltages up to 1,100 V which also includes particular requirements and tests for switch mode power supply units and transformers for switch mode power supply units.

It defines that transformers shall be enclosed and provided with adequate protection against contact with hazardous live parts. Live parts are not hazardous live parts if they are separated from the supply by double or reinforced insulation and the voltage does not exceed 35VAC peak or 60 V ripple free DC; or the touch-current does not exceed 0.7mAAC or 2.0mADC.

Class I transformers must be separated from hazardous live parts accessibility by at least basic insulation.

Class II transformers must be separated from hazardous live parts accessibility by double or reinforced insulation.

IEC 60950-1 Safety for Information technology equipment

This standard is applicable to AC-powered or battery-powered information technology equipment, including telecom, electrical business equipment and associated equipment, with a RATED VOLTAGE not exceeding 600 V. Other standards in both the US and Europe refer to IEC60950-1 as it is a harmonized standard with Underwriters Laboratories.

As defined in the standard, it is also applicable to such information technology equipment:

designed for use as telecommunication terminal equipment and TELECOMMUNICATION NETWORK infrastructure equipment, regardless of the source of power;
designed and intended to be connected directly to, or used as infrastructure equipment in, a CABLE DISTRIBUTION SYSTEM, regardless of the source of power;
designed to use the AC MAINS SUPPLY as a communication transmission medium (see note 4 of clause 6 and note 3 of clause 7).

In particular the standard specifies requirements intended to reduce risks of fire, electric shock or injury for the operator and layman who may come into contact with the equipment and, where specifically stated, for a service person. The standard is also intended to reduce such risks with respect to installed equipment, whether it consists of a system of interconnected units or independent units, subject to installing, operating and maintaining the equipment in the manner prescribed by the manufacturer.

Examples of equipment listed in this standard which are within the scope are:

UL1310 Standard for Class 2 Power Units

This standard applies to indoor and outdoor use Class 2 power supplies and battery chargers. These supplies use an isolating transformer and may incorporate components to provide an alternating or direct-current output. Each output provides Class 2 power levels according to the National Electrical Code, ANSI/NFPA 70. Maximum output voltage must not exceed 42.4 V peak for alternating current or 60 V for continuous direct current. These products are intended primarily to provide power to low voltage, electrically operated devices. The power level of these products are not to exceed 660W under any possible condition of loading.

Products either covered by this standard in part only or not at all include, but are not necessarily limited to those listed below.

Battery chargers for starter motors used to start engines. These types are covered by the Standard for Battery Chargers for Charging Engine-Starter Batteries, UL1236.
While these requirements cover Class 2 products, as defined in this standard, intended for use with toys. Toy products shall also comply with the Standard for Toy, UL697.
These requirements do not cover products battery chargers intended to charge batteries used in wheel chairs or similar mobility aids which are covered by the Standard for Power Units Other Than Class 2, UL1012.
Products without a rectifier may be covered by the Standard for Low Voltage Transformers – Part 1: General Requirements, UL 5085-1, and the Standard for Low Voltage Transformers – Part 3: Class 2 and Class 3 Transformers, UL5085-3.
Products powered solely by a DC source which are covered by the Standard for Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Craft, UL 458, or by other requirements appropriate for the intended application.
Products intended for supplying low voltage landscape lighting are covered by the Standard for Low Voltage Landscape Lighting Systems, UL 1838.

Insulation Types

From Wurth Electonics Midcom you will be asked to define which safety standard you are using. Depending on your application, grounding method, as well as power supply housing your level of insulation can vary. Typical insulation types defined in the standards include, but are not limited to those listed below.

Functional: Ensures operation of the device only and no level of protection
Basic: Provides a single layer of protection and a second is usually required elsewhere
Reinforced: The most common for offline and provide the need two layers of protection

While not all standards define an insulation level, i.e. UL1310 does not, all standards do require meeting a certain dielectric withstand voltage. While dielectric withstand voltage is sufficient for functional insulation designs, it is not sufficient for Basic or Reinforced – other transformer design consideration must be made.

The working voltage is used in defining how the transformer needs to be built to meet the standards. Typically this is the input voltage on the power supply for an offline power supply. Other power supplies such as cell balancing applications can have high (cell) voltages that include the input. This will affect your working voltage for safety considerations.

Once determining the standard that you wish to comply with, including insulation type and clearly knowing your application- you are half way to a compliant design. The other half involves the design of the hardware and especially the isolation transformer to the standard. This is where Wurth Electronics Midcom’s experience in the industry is of great value. If the standard is one listed in this writing, Wurth Electronics Midcom has already been designing to the standard for years and understands just how to design and construct compliant magnetics.

If in the end you determine that a different standard is applicable, Wurth Electronics Midcom can still help. Our knowledgeable staff of design and safety engineers will certainly be able to determine how the standard applies to the magnetics at the isolation barrier, even if they have not worked with that particular standard in the past. Either way contacting Wurth Electronics Midcom for you magnetics needs when isolated products are needed is a good place to start.

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