The release of the new IEC 60335 safety standard in response to the expanding use of smart appliances and Internet of Things (IoT) connected devices within the home has brought new power supply challenges for designers. The recently released standard has strict requirements for isolation voltages, creepage and clearance distances, and leakage currents in AC-DC power supplies. Designing compact and cost-effective AC-DC power supplies that meet the numerous requirements is difficult and going through the needed testing and approvals process adds more cost and slows time to market.
Adding to the design challenges, many household appliances are expected to be used in environments where moisture or water is present. AC-DC power supply circuits include internal high-voltage power rails, making it difficult to design packaging that is suitable for use in damp or wet environments.
To address these challenges while still meeting tight deadlines and budgets, designers can use encapsulated AC-DC power supplies that are already IEC/EN/UL 62368-1 certified and are designed to meet IEC/EN/UL 61558/60335 requirements for household applications.
This article reviews the basic requirements IEC 60335-1, introduces the concept of testing for multiple simultaneous failures as required by IEC 60335, and briefly considers Part 2 of IEC 60335. It then introduces several AC-DC power supplies from CUI that designers can use to speed the design of IEC 60335 qualified smart appliances and IoT connected devices, as well as commercial information technology equipment (ITE).
What are the basic requirements of IEC 60335?
IEC 60335 covers the “safety of household and similar electrical appliances,” with rated voltages up to 250 volts for single-phase and up to 480 volts for multi-phase. IEC 60335-1 includes the basic requirements placed on all household appliances. Among the challenges faced by designers is understanding how IEC 60335-1 compares with the previously established IEC 60950-1 safety standard for ITE. There are differences and similarities related to maximum leakage current levels, isolation voltages, and creepage and clearance distances.
Under normal operation, when there is a ground connection, leakage current flows in the chassis or protective earth conductor. If the ground connection is broken for any reason, the leakage current can flow through the body of any person operating the equipment, presenting a potential hazard. IEC 60335-1 recognizes two categories of equipment: portable and stationary. IEC 60950-1 includes three equipment categories, hand-held, movable, and stationary. Portable devices in IEC 60335 are limited to 0.75 milliamperes (mA) of leakage current, the same as hand-held devices in IEC 60950-1. Movable and stationary devices are limited to 3.5 mA of leakage current in IEC 60950-1, the same level stipulated for stationary appliances in IEC 60335-1.
Isolation voltage requirements are also subject to different definitions between the two standards. The required isolation level depends upon the location within the circuit: input to output, output to ground, or input to ground. IEC 60950-1 simply includes fixed values such as 3 kilovolts (kV) isolation between the input to output. IEC 60335-1 varies the input-to-output isolation requirement based on the working voltage: It is specified as 2.4 kV plus 2.4 times the working voltage. In the case of output-to-ground isolation, IEC 60335-1 has no requirement, while IEC 60950-1 specifies 500-volt isolation.
The variations are also evident in how the two standards treat creepage and clearance distances. While both standards rely on the working voltage and the insulation type (basic or reinforced) to define creepage and clearance, the requirements may be the same, more stringent, or laxer when comparing IEC 60950-1 and IEC 60335-1.
The shortest distance between two conductive parts along a surface is defined as creepage (Figure 1). When the working voltage is between 250 and 300 volts, IEC 60335-1 is more restrictive and requires 8.0 millimeters (mm) of creepage for reinforced insulation, while IEC 60950-1 requires 6.4 mm of creepage. If the working voltage is between 200 and 250 volts, both standards mandate 5.0 mm of creepage.
Figure 1: The creepage distance is measured on the surface of the insulation. (Image source: CUI)
The distance between two conductive parts through air is the clearance distance (Figure 2). The clearance requirement in IEC 60335-1 is only 3.5 mm, while IEC 60950-1 is more restrictive, requiring 4.0 mm when considering reinforced insulation and a working voltage between 150 and 300 volts.
Figure 2: The clearance distance is measured through the air. (Image source: CUI)
IEC 60335 also requires appliances to meet the ingress protection (IP) rating as defined in IEC 60529. The IP rating is based on the environment where the appliance is used. Many household appliances are expected to operate safely in the presence of humidity or water. IEC 60529 defines specific levels of protection needed depending on the classification of the appliance.
Beyond the basics
The smart appliances and IoT-connected devices that comprise today’s smart home are far more sophisticated than traditional appliances. They often include touch screen displays, software interfaces, digital controls, wireless and/or wired Internet protocol (IP) connectivity, and other capabilities (Figure 3). Due to this added complexity, IEC 60335 covers the possibility of two faults occurring simultaneously, not just single point faults. That contrasts with the IEC 60950-1 safety standard which looks for safe operation only after single faults.
displays ” alt=”How to Meet IEC 60335 Power Supply Requirements for Home Appliances and IoT Devices”>Figure 3: Examples of smart appliances include refrigerators with high-definition displays and IP connectivity (left) and toasters with LCD touch screen controls (right). (Image source: CUI)
IEC 60335-1 considers combinations of two hardware faults or a combination of hardware and software faults. Those tests can be especially important for power electronics devices that often include some form of digital control or monitoring. Many designs include what IEC 60335-1 refers to as “protective electronics circuits” (PECs). The concept of a PEC in IEC 60335 extends beyond hardware and includes various software features such as fault detection software. The standard requires that the equipment maintain safe operation when a PEC fault occurs after another fault such as a failure of basic insulation, as well as when a PEC fault occurs before another fault. The system must remain safe.
The multiple failure requirement also includes electromagnetic compatibility (EMC) specifications. IEC 60335 requires that EMC testing be performed after the PEC is caused to fail. For example, surge arrestors on the AC input are disconnected. This test includes the internal power supply to ensure it does not enter an unsafe operating condition in response to electromagnetic interference (EMI) following the failure of the PEC.
IEC 60355 requires that firmware or software controls operate safely with EMI applied under single fault conditions, such as a PEC failure. In addition to the system controls, this requirement applies to individual AC-DC power supplies, DC-DC converters, and motor drivers with digital controls. These devices must be tested in the system to meet this requirement.
The second part of IEC 60355
Unlike IEC 60950, IEC 60335 has two parts. Part 2 (IEC 60335-2) includes appliance-specific requirements, covering over 100 different appliance types ranging from toasters to air conditioning systems. Designers should familiarize themselves with Part 2 as it applies to the design of specific appliances. When specified, the Part 2 requirements take precedence over the basic requirements in Part 1.
Parts 1 and 2 are treated differently in the U.S. and Europe. UL 60335-1 in the U.S. is harmonized to IEC 60335-1, but the UL standard does not recognize all the Part 2 standards. In Europe, EN 60335-1 has also been harmonized to IEC 60335-1, and, unlike the UL standard, the EN standard recognizes nearly all of the Part 2 standards for specific products.
Designing to meet 60335
To simplify the design of the power supply section while meeting 60335 requirements, designers of smart appliances, IoT-connected devices, and commercial ITE can use pre-packaged modules. For example, the PSK series of encapsulated AC/DC power supplies from CUI are IEC/EN/UL 62368-1 certified and designed to meet IEC/EN/UL 61558/60335 for household applications. These power supplies are offered in power levels from 2 to 60 watts with up to 90% efficiency and come in a variety of mounting styles including board mount, chassis mount, or DIN rail (Figure 4).
Figure 4: CUI’s PSK series encapsulated AC-DC power supplies are available in board (lower right), chassis (lower left), and DIN rail (top) mounting styles. (Image source: CUI)
Examples of PSK series power supplies include:
- The PSK-10D-12-T which operates over a wide input range of 85 to 305 volts AC or 100 to 430 volts DC, and outputs 12 volts DC at up to 10 watts in a chassis mount package.
- The PSK-S2C-24 that has an input range of 85 to 305 volts AC or 120 to 430 volts DC, and delivers up to 2 watts at 24 volts DC in a board mount package.
- The PSK-20D-12-DIN that delivers 20 watts at 12 volts DC, and has an input range of 85 to 305 volts AC or 100 to 430 volts DC in a DIN rail package.
PSK series AC-DC power supplies have 4 kV AC input-to-output isolation, feature wide input voltage ranges, and a wide operating temperature range from -40 up to +70°C, with some models rated up to 85°C. The series also offers single output voltages of 3.3, 5, 9, 12, 15, and 24 volts DC, along with overcurrent, overvoltage, and continuous short-circuit protections.
When working with the modules there are some things to keep in mind. Some external components are required for protection and filtering, as well as to help meet electromagnetic compatibility (EMC) requirements. Much of this information is supplied in the accompanying datasheets.
For example, with CUI’s PSK-10D-12-T application design reference, a 2 A/300 volt slow-blow fuse is provided upfront, along with a metal oxide varistor (MOV) (Figure 5).
Figure 5: A reference design for the PSK-10D-12-T shows input protection and output filtering component placement (top) and their respective values (bottom). (Image source: CUI)
Output filtering is accomplished using a high-frequency electrolytic capacitor (C2) and a ceramic capacitor (C1). It’s important that C2 has a low equivalent series resistance (ESR), and has at least a 20% margin on the rated output voltage. Placing a transient voltage suppression (TVS) diode just before the load will help protect downstream electronics in the (unlikely) event of converter failure.
For EMC compliance, CUI suggests adding a 6.8 ohm (Ω), 3-watt resistor (R1) right before the AC input to the module (Figure 6).
Figure 6: For EMC protection, R1 should be added at the AC input line as shown. (Image source: CUI)
As the number of smart home devices and IoT-connected devices continues to increase, designers need to understand the implications of the IEC 60335 safety standard, as well as its relationship to IEC 60950. The standard directly affects how power supplies are designed and qualified for these applications, creating certain design constraints and layers of complexity.
To address these complexities, designers can turn to encapsulated AC-DC power supplies that support IEC 60335 compliant solutions. These high-efficiency, high-power-density devices are available in a variety of packaging styles including chassis mount, board mount, and DIN rail. As shown, by following some basic, good design practices, these devices can greatly reduce development costs and time to market.
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