How to Design Protection Circuits Compliant for the New AV/ICT Standard IEC 62368-1
Contributed By Digi-Key's North American Editors
Over time, the lines between audiovisual (AV) and information and communications technology (ICT) have become increasingly blurred (home multimedia products such as smart TVs are one example). In addition, engineers have moved towards a Hazards Based Safety Engineering (HBSE) approach when designing protection for electrical products. These trends impacted the standards designed to protect people who install, maintain, and use such equipment, rendering them obsolete—along with much of the engineering hardware used to ensure AV and ICT products complied.
Anticipating this eventuality, the IEC developed a single new standard, IEC 62368-1 (Information And Communication Technology Equipment - Part 1: Safety Requirements). This new standard replaces two older standards (IEC 60950-1 and IEC 60065) with one which covers both ICT and AV equipment, as well as products such as Internet of Things (IoT) devices and battery-powered electronic appliances, operating up to 600 volts. The standard was implemented in December of 2020 and adopts an HBSE approach.
This article introduces IEC 62368-1 and shows that while it can appear more complex than the previous separate standards, it does simplify matters and enables higher levels of safety and design flexibility. The article will also introduce and describe the use of commercially available electrical protection products from Littelfuse that can be used to make it easier to design products and subsystems that meet the overvoltage and surge requirements for each category covered in IEC 62368-1.
What is IEC 62368-1?
IEC 62368-1 was adopted to replace older standards with one which defines circuit protection for the safety of electrical and electronic ICT, AV, and IoT equipment with a rated voltage not exceeding 600 volts (Figure 1). Designed to protect people who install, maintain, and use such equipment, the standard also reflects the HBSE approach engineers now take to safety engineering. HBSE replaces the previous prescriptive engineering approach—which laid out a set of rules to which protection circuits should adhere—with one that considers the hazards to which a product is likely to be exposed. The result is safety circuits that protect the user even if the product fails when it is subjected to one of the identified hazards.
Figure 1: IEC 62368-1 replaces the older IEC 60951-1 and IEC 60065 safety standards with one that covers ICT, AV, and other products such as IoT and battery-powered electronic devices. (Image source: Littelfuse)
IEC 62368-1 applies to not only the end-user product but also to components and subsystems (such as power supplies) from which it is constructed. For an unspecified period, the new standard temporarily allows for reuse of designs and sub-assemblies that were compliant with the older standards. Engineers are expected to adopt the new standard for key markets such as North America, the U.K., Japan, and Australia/New Zealand.
Circuit protection for people
IEC 62383-1 compliance requires an engineer to employ an HBSE methodology. This means:
- Identifying the energy sources (ESs) used by the product
- Measuring the energy levels produced by those sources
- Determining whether the energy from the sources is hazardous
- Classifying the level of hazard
- Identifying whether the hazard could cause injury or fire
- Determining appropriate safeguard schemes to:
- Protect persons against pain and injury from the classified hazards
- Reduce the likelihood of injury or property damage due to fire originating from a fault within the equipment
- Measuring the effectiveness of those safeguards
The standard details three classes of ESs. A Class 1 ES (ES1) remains under Class 1 limits during normal operating conditions, abnormal conditions, or in the presence of a single fault. The energy present could be detected by a person but would not be painful and would be insufficient to cause ignition. No safeguards are required to protect ordinary users from Class 1 ESs.
Class 2 ES (ES2) energy levels exceed Class 1 limits but remain below Class 2 limits during normal, abnormal, or single-fault product operating conditions. The energy present may be sufficient to cause pain but is unlikely to cause injury. The energy present could be enough to cause ignition under some conditions. At least one safeguard is required to protect ordinary users from Class 2 energy sources.
A Class 3 ES (ES3) is the most hazardous. Its energy exceeds the Class 2 maximum limit under normal, abnormal or single-fault conditions, and can cause injury, or ignition and spread of fire. The type of injury caused by an ES3 could extend to fibrillation, cardiac/respiratory arrest, or skin and/or internal organ burns. A double or reinforced safeguard is required to protect ordinary users from an ES3.
In particular, the new standard determines overvoltage withstand thresholds and surge protection requirements for the different categories, covering different product types and where they are used.
It’s important for the designer to understand that the actual current and voltage limits applicable to ES1, ES2, and ES3 vary. For example, the voltage limit requirements are influenced by power supply operating frequency. For voltages from a supply operating below 1 kilohertz (kHz), the ES1 limit is 30 voltsrms, 42.4 voltspeak, and 60 volts DC. The ES2 limit is 50 voltsrms, 70.7 voltspeak, and 120 volts DC.
Equipment must comply with either the voltage limit or the current limit specified in the applicable energy class but does not have to comply with both. The limits also vary according to normal or abnormal operation, or a single fault condition. These limits are detailed in clause 5 of the standard. There are also sub-clauses, covering things such as limits for pulse waveforms, according to off-time.
Circuit protection for equipment
While protecting people is the primary concern of any equipment maker, protecting the end product against damage from voltage and current spikes is also a major concern. IEC 62368-1 builds on the two older standards and specifies the minimum withstand ratings for equipment to ensure immunity from transient overvoltages and overcurrents.
The standard defines three “Overvoltage Categories” (I, II, and III) for equipment on the domestic side of the electricity meter. Equipment on the distribution side of the meter is in Overvoltage Category IV.
Specifically, Category I is for equipment not connected to the mains (such as battery-powered portable devices), while Category II is for pluggable ICT and AV equipment connected to building wiring. Category III is for systems forming part of building infrastructure such as distribution boards, circuit breakers, wiring, junction boxes, switches, socket outlets, and equipment for industries.
Category II generally covers equipment designs based on 120 or 230 volt AC mains, or for a range like 100 to 250 volt AC power supplies. The standard defines that such equipment must have a minimum transient peak voltage withstand levels of 1.5 kilovolts (kV) for a 120-volt AC supply, and 2.5 kV for a 230-volt AC supply (Figure 2).
Figure 2: IEC 62368-1 specifies different overvoltage categories depending on where the end product is used. Categories I, II, and III are for products used on the domestic side of the electric meter, while Category IV covers products used on the distribution side. (Image source: Littelfuse)
Circuit design to meet IEC 62368-1 surge protection requirements
Designing circuits that comply with the standard’s requirements for protection against transient overvoltage and overcurrent events is not overly difficult. The key is to divert the transient spike away from the sensitive equipment by providing an alternative conduction path. There are two recommended techniques depending on whether the power supply uses a differential mode, or differential and common mode scheme (Figure 3A and B).
Figure 3: Transient voltage and current protection for IEC 62368-1 Category II comprises differential mode (A, top), or differential and common mode schemes (B, bottom). (Image source: Littelfuse)
In the differential mode scheme (3A), protection is achieved by a fuse (I) to protect against overcurrent events, along with a thermally protected metal oxide varistor (TMOV) (II). The TMOV consists of two elements, a thermally activated device designed to open in the event of overheating due to the abnormal overvoltage and an MOV. Under normal operation, the MOV has a very high resistance, allowing normal operational voltages to flow through the circuit. At higher voltages, such as a transient spike, the MOV exhibits low resistance, shorting the current from flowing through to the end product.
The differential and common mode scheme also makes use of the fuse and TMOV across the live and neutral lines but adds two more MOVs and a gas discharge tube (GDT). As shown in Figure 3B, the MOVs are added across the live and ground line, and neutral and ground line, in series with the GDT. Under normal operation, GDTs feature high insulation resistance plus low capacitance and leakage. However, when exposed to high voltage transients, the enclosed gas turns to plasma and dissipates the voltage away from the end product.
While the TMOV option is recommended (because it features thermal protection, and low energy let-through and clamping voltage), other forms of differential mode protection can be considered while remaining compliant with the standard. Examples include an MOV, a protection thyristor plus an MOV (particularly for products such as modems), or a TVS diode. For common mode protection, the MOVs plus GDT protection is the only permitted solution.
Where things get a little trickier for the engineer is during component selection. Devices must meet the protection criteria defined in IEC 62368-1 in order for the end product to comply with the standard.
The fuse (I) is used to prevent damage to sensitive circuits during overcurrent events (and to help the end product pass fault testing). When considering the fuse, the designer needs to consider a component that:
- Avoids nuisance trips
- For example, it must not open during normal operation or open during surge pulse testing
- Has a voltage rating above that of the system’s normal operational voltage
- Safely interrupts the maximum fault current
- Fits the available space
- Meets the required third-party certifications (for example, IEC and UL)
Good options for a 240 volt AC Category II product are the 0215008.MRET1SPP, an 8 amp (A) device, or the 0215012.MRET1P, a 12 A model, both from Littelfuse’s 215 Series. The 215 Series is a 20 by 5 millimeter (mm) time-lag, surge-withstand, ceramic body cartridge fuse designed to comply with the IEC specifications while providing individual protection for components or internal circuits.
A key requirement for a fuse in this application is that its interrupting rating must meet or exceed the maximum fault current of the circuit. Otherwise, the device will not work properly, and there’s a risk that damaging current will continue to flow in the circuit when the fuse should have opened. The 215 Series fuses have a high interrupting rating of 1.5 kV at 250 volts AC.
When selecting the TMOV (II) (shown in the circuits illustrated in Figures 3A and B), the designer should consider the following guidelines:
- The TMOV should comply with a varistor component standard such as IEC 61051-1 or IEC 61643-331
- The maximum continuous operating voltage (MCOV) is ≥ 1.25 x equipment rated voltage
- For example, for a 240-volt AC power supply, the component MCOV must be a minimum of 300 volts
- The TMOV should withstand multiple strikes (as defined by 2.3.6 of IEC 61051-2 or 8.1.1 of IEC 61643-331)
- For example, for a 240-volt AC power supply, the TMOV should withstand 10 pulses of 2.5 kV/1.25 kiloamps (kA) combination wave of 1.2/50 microsecond (μs) voltage and 8/20 μs current
- The component must pass the standard’s varistor overload test
- For example, for a 240-volt AC power supply, testing should apply 2 x rated voltage (480 volts) with an in-series resistor (R) of 3.84 kilohms (kΩ) (for subsequent tests, the R-value is halved until the circuit opens) (Figure 4)).
Figure 4: Overload test schematic. The protection component must be subject to 2 x rated voltage overload, and the test repeated with incrementally halved values of R1 until the circuit opens. (Image source: Littelfuse)
Littelfuse’s TMOV14RP300EL2T7 device is a good candidate for this application. The device has an MCOV of 300 volts (meeting the component standard requirement for a 240-volt AC supply) with a diameter of 14 millimeters (mm), a sufficient body size to meet the multiple strike requirement. Moreover, because the TMOV14RP300EL2T7 is thermally protected, its MCOV of 300 volts is sufficient to pass the varistor overload test. For an additional factor of safety, a non-thermally protected MOV should have an MCOV of 420 volts or higher. The TMOV can withstand a single-event peak surge current (<20 µs) of up to 6 kA. Figure 5 illustrates surge capability for repeated surges and surge duration.
Figure 5: Repetitive surge capability for Littelfuse’s 14 mm MOV. The device can withstand a single-event peak surge current (<20 µs) of up to 6 kA. (Image source: Littelfuse)
The requirements for the MOVs and GDTs used for common-mode protection are also dictated by the IEC 61051-1 or IEC 61643-331 component standard. Adherence to this standard enables sub-assemblies built from compliant components to in turn be compliant with IEC 62368-1. In this case, the MOV needs to meet the same MCOV and surge requirements as noted for the TMOV above, but because the two devices are used in conjunction with a GDT, the overload tests are performed on the combined protection circuit rather than the MOV alone.
Littelfuse’s V10E300P MOV fits the bill. This component has an MCOV of 300 volts and a diameter of 10 mm, making it robust enough to meet the standard’s multiple strike requirement. It can withstand a peak surge current of up to 3.5 kA. To meet the requirements of the standard, the GDT must pass an electric strength test of a 2.5 kV withstand voltage, and meet clearance and creepage compliance.
Littelfuse’s CG33.0LTR GDT is one option for this application. This is a two-electrode, high voltage device designed for surge protection and high isolation applications. The GDT has an insulation resistance of 10 gigaohms (GΩ) at 100 volts, and a capacitance of <1.5 picofarads (pf). It has a breakdown voltage of 4.6 kV and can withstand a maximum surge current of 10 kA.
The combination of two V10E300P MOVs and a single CG33.0LTR GDT are capable of meeting the overload test outlined when describing the TMOV protection circuit above.
IEC 62368-1 introduces a single standard for circuit protection of products operating from up to a 600-volt supply, where previously separate standards for ICT and AV applied. It also formalizes circuit protection for products not covered by the old standard, such as IoT and battery-powered devices. Although engineers familiar with the old standards will need to change their design approach, IEEE 62368-1 simplifies circuit protection engineering and enables higher levels of safety and design flexibility. In addition, protection component manufacturers such as Littelfuse offer devices and advice that make it simpler to design circuits that comply with the new standard.
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