Prepare yourself for the updated standards about to take hold in power equipment that includes external adapters.
Ron Stull • CUI Inc.
A power adapter is a barrier between lethal ac mains voltage and the dc power line for end equipment. Different environments, however, demand different adapter construction to meet safety levels.
For example, domestic, IT, industrial, and medical end-uses all have their own safety requirements. The primary concerns are the amount of separation through air and across surfaces between hazardous voltages and outputs or casings. However, such criteria are application-dependent and also vary with the over-voltage category of the ac supply, degree of environmental pollution, the type of user who has access, and even operating altitude.
Other parameters also can be critical such as the fusing arrangement and earth leakage current, particularly in medical environments. Safety rating requirements also depend on the point of geographical use with different countries adopting local variants of international standards or their own national versions.
Of course, the selection of an ac-to-dc power adapter involves evaluating the product’s overall voltage and maximum-current requirements. Besides looking at the obvious parameters, you should also confirm the adapter conforms to the applicable safety standards. These can vary depending on the geographic regions of the world where the product is distributed.
There are several internationally recognized safety standards that apply to power adapters. Some of these are specific to the product’s application. To complicate matters, not all countries have adopted “international” safety standards; some have written their own, and others base their standards on the general framework of the international standard.
One facet of safety, as it applies to a power adapter and the equipment it powers, is to ensure a user cannot make contact with lethal ac line voltages. Also, safety goes beyond the potential for electrical shock, to require that the adapter body uses low flammability materials that can enclose any fire that might arise.
For the majority of commercial and domestic equipment, standards IEC 60950-1 and IEC 60065 apply, although these are both intended to be withdrawn in Dec. 2020. Thereafter, IEC 62368-1 applies. IEC 60950-1 covers the broad category of IT and office equipment, and IEC 60065 covers an equally broad range of audio and video equipment from video projectors and media players.
At this time, IEC 62368-1 coexists with the other two, and prudent power adapter manufacturers are certifying their products against IEC 62368-1 and IEC 60950-1. When selecting an external power adapter, check the manufacturer’s datasheet to find out with which safety standard the adapter complies.
A new approach to safety standards
The IEC 62368-1 standard covers all aspects of audio, video, IT, office, and communications equipment. The USA, Canada, and Europe have all adopted it. IEC 62368-1 not only unifies the two previous standards, it also marks a significant change in the approach to electrical safety.
In the past, IEC 60950-1 was quite prescriptive about how safety features should be implemented within a power adapter and the associated end-equipment. The new standard is more flexible. It introduces the concept of hazard-based safety engineering (HBSE).
The underlying idea for HBSE is that the manufacturer’s engineering team is responsible for thoroughly reviewing and identifying any potentially hazardous energy sources and has implemented appropriate measures to prevent user harm. Harm could include not only electrical shock but burns and injury from a resulting fire. In this regard, designers must consider all potential use cases and also all possible fault conditions. The standard does not cover medical equipment; the relevant standard IEC 60601-1 is still in force.
The IEC 62368-1 standard provides categories for both the energy source and likely effects on the body and on the product–in this context, the power adapter– materials and enclosure. Within an ac-to-dc power adapter, the ac line power is a Class 3 energy source. It’s energy is lethal if it passes directly through the adapter circuitry to the dc output and into the end-product. The energy source classifications help the adapter engineering team decide on safety measures to be incorporated into the adapter. Different types of insulation are used to isolate lethal Class 3 energy sources from the end-product user.
Insulation regimes in use are divided between Class I and Class II types of power supplies and adapters. In a Class I adapter, a protective ground(earth) connection is used to transfer any hazardous voltages that have managed to bypass any basic insulation methods. A Class II product does not require a ground connection, relying instead on the use of double or reinforced insulation techniques to protect users.
Functional insulation, also termed operational insulation, purely serves the purpose of preventing faults and allowing the equipment to operate normally. An example of functional insulation is the separation built-in between high-voltage PCB tracks. Basic insulation techniques include the use of insulation-covered wires within the adapter. An additional outer layer of insulating material is called supplementary insulation. An example of basic insulation is the plastic cover between the metal ac line connectors inside the adapter enclosure and the adapter’s PCB.
Double insulation refers to an adapter that incorporates both basic and supplementary insulation. Reinforced insulation provides the same level of protection as a double-insulated adapter but with only a single layer of insulation.
Any form of insulating material used to isolate one part of a circuit from another must be able to continue to insulate against high voltages present within the circuit. Note that an air gap between conductors is considered a type of insulation. Within IEC 60950-1, the minimum clearance is specified as 4 mm for reinforced or double insulated methods, or 2 mm for basic and supplementary insulation.
An air gap – creepage – specification is typically quoted at a given working voltage, usually 264 Vac. The use of an air gap also introduces the need to consider the environmental conditions, because pollution and humidity significantly alter the ability of air to be an insulator. Typically, most insulators will withstand high voltages but may breakdown above a given voltage. After a breakdown condition, there is no longer a safety barrier in place, and lethal voltages may pass through.
The point at which an insulator breaks down will depend on the magnitude of the voltage, the material qualities, and the duration of the applied voltage. During safety testing, a power supply adapter is tested for three different breakdown voltage conditions: input to output, input to ground, and output to ground.
The datasheet will list power adapter isolation characteristics. It should indicate the test voltage used and the duration for which it was supplied. For example, the CUI series SDI200G is rated with an isolation voltage of 3,000 Vac for one minute.
Another important datasheet parameter is the leakage current. Leakage involves any current flowing from the adapter to the ground connection. A Class I power adapter uses the protective ground to shield users from harm. On a Class II adapter, where no protective ground connection is in place, the user may form a path to ground for stray currents. IEC 60950-1 stipulates that leakage from a Class II adapter must not exceed 0.25 mA.
Where the power adapter is powering an item of healthcare monitoring equipment, it is highly likely the patient will have monitoring electrodes or sensors attached to their body. The consequences of excessive leakage current or an insulation failure may prove fatal. The medical safety standard IEC 60601-1 applies to all medical equipment. It has undergone several iterations since its inception in 1977 and is currently in its fourth edition.
IEC 60601-1 approaches safety in the manner resembling that of IEC 60950. It further emphasizes insulation, leakage currents, and air-gap clearances. The earlier second edition of the standard put the potential use cases for medical equipment in three categories: in the vicinity of the patient but without touching the patient’s body, equipment physically connected to the patient’s body (a blood pressure monitor, for example), or equipment such as an implantable defibrillator that has physical contact with the patient’s heart.
Each of these types has specific isolation voltages, creepage, and insulation limits stipulated. Edition three, ratified in 2005, took the initial concepts further and introduced the idea of a means of protection (MOP). Recognizing that hazards for operators differ from those for patients, the third edition also introduced a means of patient protection (MOPP), and a means of operator protection (MOOP). For a patient attached to cardiac equipment, the power adapter must meet MOPP criteria. For example, the CUI SDM65-U is a 65-W ac/dc external desktop power adapter that is designed for 2 x MOPP applications.
The fourth edition of IEC 60601-1 introduces the aspect of electromagnetic interference as a potential safety risk. With the growing use of smartphones, Bluetooth headsets, and other wireless peripherals, there is a potential risk that their emissions could disturb medical equipment. The introduction of electromagnetic immunity levels (EMI) for medical equipment, and the possibility that the machine itself could disrupt the operation of other equipment in the vicinity, has led to the definition of three intended-use environments: professional healthcare facilities, home healthcare, and special environments. The latter category includes places where high levels of electromagnetic interference may arise, as from radiotherapy equipment.
Thus external power adapters maintain a safety barrier between lethal ac line voltage and the dc output. The manufacturer’s datasheet lists all relevant safety certifications and should also take into account both the geographical regions in which the product may be used as well as the application the adapter will power.