“In response to the growing number of smart appliances and Internet of Things (IoT)-connected devices in the home, the industry has released a new IEC 60335 safety standard, creating new power challenges for designers. Recently published standards have stringent requirements for isolation voltages, creepage and clearance distances, and leakage currents for AC and DC power supplies. Designing a compact and cost-effective AC/DC power supply that meets many requirements can be difficult, and going through the required testing and approval process adds even more cost and slows time to market.
Author: Jeff Shepard
The new IEC 60335 safety standard has been published in response to the growing number of smart appliances and Internet of Things (IoT) connected devices in the home, creating new power challenges for designers. Recently published standards have stringent requirements for isolation voltages, creepage and clearance distances, and leakage currents for AC and DC power supplies. Designing a compact and cost-effective AC/DC power supply that meets many requirements can be difficult, and going through the required testing and approval process adds even more cost and slows time to market.
In addition to design challenges, many appliances are used in humid or watery environments. AC and DC power circuits contain high voltage power rails, making it difficult to design packages suitable for use in wet environments.
To meet these challenges, while still meeting tight deadlines and budgets, designers can use packaged AC and DC that are IEC/EN/UL 62368-1 certified and designed to meet the requirements of IEC/EN/UL 61558/60335 household applications power supply.
This article reviews the basic requirements of IEC 60335-1, introduces the concept of multiple simultaneous faults testing required by IEC 60335, and briefly describes IEC 60335 Part 2. This article then describes several of CUI’s AC and DC power supplies that designers can use to accelerate the design of IEC 60335-compliant 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 “Safety of Household and Similar Electrical Appliances” with rated voltages up to 250 V for single-phase and 480 V for multi-phase. IEC 60335-1 includes basic requirements for all household appliances. One of the challenges for designers is to understand how IEC 60335-1 differs from the previous ITE safety standard, IEC 60950-1. There are differences and similarities between the two in terms of maximum leakage current levels, isolation voltages, creepage and clearance distances.
During normal operation, if there is a ground connection, leakage current will flow into the chassis or the protective ground conductor. If the ground connection is disconnected for any reason, leakage current may flow through the body of the equipment operator, creating a potential hazard. IEC 60335-1 recognizes two types of equipment: portable and stationary. IEC 60950-1 includes three equipment categories: hand-held, mobile and stationary. The leakage current limit for portable equipment in IEC 60335 is 0.75 mA, the same as for handheld equipment in IEC 60950-1. IEC 60950-1 specifies a leakage current limit of 3.5 mA for movable and fixed equipment, which is the same level specified for stationary appliances in IEC 60335-1.
The two standards also have different definitions for isolation voltage requirements. The level of isolation required depends on the location in the circuit: input to output, output to ground, or input to ground. IEC 60950-1 includes only fixed values such as 3 kV isolation between input and output. The input-to-output isolation requirements of IEC 60335-1 vary according to the working voltage and are specified as 2.4 kV plus 2.4 times the working voltage. In terms of isolation from output to ground, IEC 60335-1 has no requirements, while IEC 60950-1 specifies 500 V isolation.
The two standards also have significantly different provisions for creepage distances and clearance distances. While both standards define creepage and clearance distances in terms of working voltage and insulation type (basic or reinforced), IEC 60950-1 and IEC 60335-1 may have the same, stricter or more relaxed requirements when comparing .
The shortest distance along a surface between two conductive parts is defined as the creepage distance (Figure 1). For working voltages between 250 V and 300 V, IEC 60335-1 is more stringent, requiring a creepage distance of 8.0 mm for reinforced insulation, while IEC 60950-1 requires a creepage distance of 6.4 mm. Both standards specify a creepage distance of 5.0 mm if the operating voltage is between 200 V and 250 V.
Figure 1: Measurement of creepage distances on the surface of an insulator. (Image credit: CUI)
The air distance between two conductive parts is the gap distance (Figure 2). When reinforced insulation is considered and operating voltages between 150 V and 300 V are considered, IEC 60335-1 requires a clearance distance of only 3.5 mm, while IEC 60950-1 is more stringent and requires 4.0 mm.
Figure 2: Aerial measurement of gap distances. (Image credit: CUI)
IEC 60335 also requires appliances to meet the ingress protection (IP) rating defined in IEC 60529. The IP rating depends on the environment in which the appliance is used. Many appliances need to be able to operate safely in the presence of moisture or water. IEC 60529 specifies the specific level of protection required according to the electrical classification.
Beyond the basics
The smart appliances and IoT-connected devices that make up today’s smart homes are far more complex than traditional appliances, and often include touchscreen displays, software interfaces, digital controls, wireless and/or wired Internet Protocol (IP) connectivity, and other capabilities (Figure 3). Due to the increased complexity, IEC 60335 covers the possibility of two failures occurring simultaneously, not just a single point of failure. This is in stark contrast to the IEC 60950-1 safety standard, which only pursues safe operation after a single fault.
Figure 3: Examples of smart appliances include refrigerators with high-definition displays and IP connectivity (left) and toasters with LCD touchscreen controls (right). (Image credit: CUI)
IEC 60335-1 considers a combination of two hardware faults or a combination of hardware and software faults. These tests can be particularly important for power Electronic equipment that often includes some form of digital control or monitoring. Many designs include “Protective Electronic Circuits (PEC)” as specified in IEC 60335-1. The concept of PEC in IEC 60335 goes beyond hardware to include various software functions such as fault detection software. This standard requires that equipment remain in safe operation when a PEC failure occurs after another failure (eg basic insulation failure) and when a PEC failure occurs before another failure. The system must remain secure.
Multiple fault requirements also include Electromagnetic Compatibility (EMC) specifications. IEC 60335 requires EMC testing after a PEC failure has occurred. For example, the surge arrester at the AC input is disconnected. This test includes the internal power supply to ensure that it does not enter an unsafe operating state due to electromagnetic interference (EMI) following a PEC failure.
Under single fault conditions (eg PEC failure), IEC 60355 requires firmware or software controls to operate safely with EMI applied. In addition to system control, this requirement also applies to separate AC and DC power supplies, DC-DC converters, and motor drives with digital control. To comply with this requirement, these devices must be tested in the system.
IEC 60355 part 2
Unlike IEC 60950, IEC 60335 has two parts. Part 2 (IEC 60335-2) includes specific electrical requirements and covers more than 100 types of electrical appliances, from toasters to air conditioning systems. Designers should be familiar with Part 2 as it applies to the design of specific appliances. If specified, the requirements of Part 2 take precedence over the essential requirements of Part 1.
The US and Europe treat Part 1 and Part 2 differently. UL 60335-1 in the United States is consistent with IEC 60335-1, but the UL standard does not recognize all of the standards in Part 2. In Europe, EN 60335-1 is also harmonized with IEC 60335-1, but unlike the UL standard, the EN standard recognizes almost all of the product-specific standards of Part 2.
Designed to meet 60335 standards
To simplify the design of the power section while complying with 60335 requirements, designers of smart appliances, IoT connected devices and commercial ITE can use prepackaged modules. For example, CUI’s PSK series of encapsulated AC/DC power supplies are IEC/EN/UL 62368-1 certified and designed to meet IEC/EN/UL 61558/60335 requirements for domestic applications. Available in power levels from 2 W to 60 W, these power supplies are up to 90% efficient, and are available in a variety of mounting styles, including board mount, base mount, or DIN rail (Figure 4).
Figure 4: CUI’s PSK series packaged AC and DC power supplies have three mounting methods, panel (bottom right), base (bottom left) and DIN rail (top). (Image credit: CUI)
Examples of PSK series power supplies include:
The PSK-10D-12-T operates over a wide input range of 85 V to 305 V AC or 100 V to 430 V DC and outputs 12 V DC and up to 10 W in a chassis mount package.
The PSK-S2C-24 has an input range of 85 V to 305 V AC or 120 V to 430 V DC and delivers up to 2 W at 24 V DC in a board mount package.
The PSK-20D-12-DIN provides 20 W at 12 V DC, an input range of 85 V to 305 V AC or 100 V to 430 V DC, and is housed in a DIN rail package.
The PSK series AC and DC power supplies feature 4 kV AC input to output isolation, a wide input voltage range, and a wide operating temperature range from -40°C 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 V DC, as well as overcurrent, overvoltage and continuous short circuit protection.
When using these modules, keep the following points in mind. For protection and filtering purposes, and to help comply with electromagnetic compatibility (EMC) requirements, some external components are required. Much of this information is provided in the accompanying datasheet.
For example, in CUI’s PSK-10D-12-T Application Design Reference, a 2 A/300 V slow blow fuse is pre-supplied, along with a metal oxide varistor (MOV) (Figure 5).
Figure 5: The PSK-10D-12-T reference design shows the placement of input protection and output filtering components (top) and their corresponding values (bottom). (Image credit: CUI)
Output filtering is achieved with high frequency electrolytic capacitors (C2) and ceramic capacitors (C1). C2 must have a low equivalent series resistance (ESR) with at least 20% margin over the rated output voltage. Placing a Transient Voltage Suppression (TVS) diode in front of the load helps protect downstream electronics in the event of (unlikely) converter failure.
To comply with EMC requirements, CUI recommends adding a 6.8 Ω, 3 W resistor (R1) in front of the module’s AC input (Figure 6).
Figure 6: For EMC protection, R1 should be added to the AC input line as shown. (Image credit: CUI)
As the number of smart home devices and IoT-connected devices continues to grow, designers need to understand what the IEC 60335 security standard means and how it relates to IEC 60950. The standard directly affects power supply design and certification for these applications, introducing some design constraints and complexities.
To address these complexities, designers can choose packaged AC and DC power supplies that support IEC 60335 compliant solutions. These high-efficiency, high-power density devices are available in a variety of packages, including chassis mount, board mount, and DIN rail. As shown in this article, these devices follow some basic good design practices that can greatly reduce development costs and time to market.