What Safety Standards Should Be Considered in a Transformer Design

Por Dean Huumala, Richard Chung

Colaboración de Digi-Key Corporation


Offline flyback pulse width modulation (PWM) with integrated MOSFETs is an integral part of a power supply system design. The semiconductor devices will not meet design requirements unless all other parts that complete the system also do their job. The transformer is an important component that is often seen as someone else’s problem. This may be true in companies that can afford a dedicated magnetic specialist, but this position usually does not exist and the work is simply part of another engineer’s responsibility. Designing the right transformer requires consideration in these six areas:
  • Safety standards
  • Design requirements
  • IC selection
  • How to make it for manufacturability and allowing flexibility to the manufacturer
  • How to verify if they have a good design
  • Ensuring a good size-to-cost to performance balance
This article will summarize the top six safety standards and avenues to help with power supply and transformer design.

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 application input and output(s). Many customers might think, “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). Knowing the safety standards required during product definition or design definition will ensure unnecessary design iteration and cost. Discussed below are the most common standards used in the industry to define the safety requirements of offline transformers. Having a company that is very familiar with these standards and typically knowing which standard a design is trying to meet and the insulation type is enough to get the required parts the first time.

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 within a company are best equipped to choose which standards apply.

In order to make the search easier, a number of the more common standards that are referred to by electronics manufacturers that apply to their equipment are listed below. This list is an attempt 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 easily identify which standard might apply. From there, the standard can be purchased for further review.

Standards

EN 50470-1 AC Electricity metering equipment
This standard provides the general requirements for class A, 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 50 Hz electrical networks.

The standard applies to electromechanical or static watt-hour meters for both indoor and outdoor application. This standard also 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, and more 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 such that in normal use they operate without danger to the user or surroundings. Where luminaire enclosures 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.

Additionally, this standard deals with the common hazards presented by appliances that are encountered in and around the home but does not take into account children and those with physical, sensory or mental disabilities; or lack of experience and knowledge which prevents them from using the appliance safely without supervision or instruction; such as 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)
  • Handheld 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 35 VAC peak or 60 V ripple free DC; or the touch-current does not exceed 0.7 mAAC or 2.0 mADC.

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- 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 to 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:

Table of UL1310 Standard for Class 2 Power Units

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 is not to exceed 660 W 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 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
In addition to defining which safety standard the application will be using, the application’s grounding method, as well as power supply housing level of insulation needs to be determined. 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 satisfies the need for two layers of protection
While not all standards define an insulation level, i.e. UL1310 does not, all standards are required to meet 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 considerations 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 the working voltage for safety considerations.

Determining the standard to comply with and insulation type is half of what is needed for 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 it is determined that a different standard is applicable, Wurth Electronics Midcom can still help. The company’s 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 any magnetics needs when isolated products are needed is a good place to start.

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