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–By Todd Phillips, Global Market Manager, Electronic Business Unit, Littelfuse, Inc. and Prasad Tawade, Strategic Marketing Manager, Littelfuse, Inc. Plug In Disc Metal Oxide Varistor
Design development and user safety are critical in the regulation of power-based products and components and the International Electrotechnical Commission (IEC) has led the way with standards for designing information technology equipment and audio/video products. However, the traditional approach to monitoring equipment safety that was product-dependent and incident-based, has led the previous standards, IEC 60950-1(information and communication technology equipment) and IEC 60065 (audio and video equipment), to over time become more reactive and less flexible for addressing new emerging technologies.
As a new regulation, IEC 62368-1, became the single default standard on December 20, 2020, designers no longer can choose to comply with either the information/communication technology standard or the audio/video equipment standard. The boundaries between information/communications and audio/video technology have blurred. Thus, the IEC 62368-1 hazards-based safety engineering standard applies to a broad scope of applications.
Focusing on the energy within the equipment and the intended environments, the new standard is less product-specific. This approach, which aims to encourage manufacturers to address known hazards in the design and intended use of the product, is intended to be future-proof. Its use is appliable, no matter for industrial or residential applications.
The new standard has a relatively broad scope. It includes all previously covered applications under different standards, or not covered at all. It addresses electronic equipment up to 600 volts, including point-of-sale, banking, other telecommunication and office equipment, speakers, surveillance cameras, smart home devices, and other audio/video equipment. Internet of things (IoT) devices, laptops, mobile devices, gaming systems, and other battery-powered electronic devices are also covered by this new standard.
IEC 62368-1 has been used for a few years. However, designers could choose whether to comply with IEC 60950 or IEC 60065 over IEC 62368-1. It all depends on the application. Now the designer does no longer have a choice. The hazard-based approach in IEC 62368-1 has included the factor of a device’s design and its use to determine testing and evaluation criteria. The standard also defines which protection components to consider using within the device. As the power-based standards may seem to be evolved in a complicated way, this new approach allows for improved safety and design flexibility.
Helping increase product reliability, AC line protection components must comply with specific tests required by IEC 62368-1. The step of deciding on the overvoltage category is necessary to determine the parameters of some of the tests.
The location of the device connecting to the electrical grid helps define the overvoltage category. If the proximity to the grid is close, the category and the hazard will be higher. We can, for example, consider an electric meter on the outside of a house connected by a service wire to a transformer as Overvoltage Category IV. A lower overvoltage category will be advised for the electric breaker panel inside a home. Including PCs, routers, notebooks, tablets, and related power supplies, the personal devices will fall within Overvoltage Category II.
Engineers can use the Overvoltage Category along with the line voltage to determine the voltage withstand rating. They can connect power adapters to 120-V outlets to have a withstand voltage rating of 1500 V. Adapters connecting to 240-V outlets will have the withstand voltage rating increased to 2500 V. This rating is an important basis for component selection and applicable tests.
While the older standards did not have the tests related to varistors and gas discharge tubes (GDTs) for surge protection, the new standard includes these key tests. Exposed to surge events, varistors can wear out over time. Eventually, they become a hazard themselves. Now, additional tests are required as IEC 62368-1 refers to a varistor as a possible ignition source.
A stress test called “the varistor overload test (the Annex G.8 portion of the standard) progressively steps up the power in the varistor. Using an AC source, the test applies twice the equipment’s voltage rating through a resistor to the varistor-under-test. Until either the varistor fails or a fuse, thermal disconnect, GDT, or other components safely open the circuit, the test starts with high resistance and lowers the resistance.
The temporary overvoltage test is similar; however, it defines specific current values and the test duration. Damage cannot be caused by the varistor in both cases. By increasing the varistor rating as defined in Table G.10 of IEC 62368-1, engineers can bypass both overvoltage tests. However, provided that a design engineer sizes the varistor to avoid the test, downstream components must be selected by the engineer with higher ratings adding to the product’s cost. The third test is the basic insulation test that evaluates the electric strength of equipment with an unreliable ground bond, most non-industrial plugs. This test is not necessary if the ground meets the defined criteria of a reliable ground.
Commonly used in IT equipment, universal power adapters accept a wide range of voltage inputs, for example, 90 to 240 VAC. With a common set of electronics, this voltage range allows the product to be used worldwide. Dictated by IEC 62368-1, safety requirements require both overcurrent and surge protection.
To prevent damage from overcurrent events and to fault testing I passed, engineers should select the correct fuse and consider the following points when choosing the right fuse:
Using these requirements identifies the best fuse for the application. To cite an example, Littelfuse recommends its 3.15-A fuse in the 215 Series because of a high breaking capacity of 1500 A at 250 VAC.
There are several surge protection technologies available. Varistors, TVS diodes, protection thyristors, and GDTs are included in the category of safety components.
Engineers should first consider whether the ground is deemed to be reliable in order to find the best solution for the application. Unreliable ground connectors are found in many places, for example, home, office, and commercial spaces. Wall sockets with a loose earth connector or a damaged ground terminal in the plug are also examples. Ground connectors that typically exist in industrial applications are reliable. In such a case, the ground of industrial applications is hardwired, or the equipment does not function without a good ground connection.
For unreliable ground applications, when using varistors in the common mode, connections between High and protective earth, or between Neutral and protective earth, IEC 62368-1 states that you should consider using varistors with a GDT. However, you should ensure that they comply with the Annex G.8 varistor overload test. Varistors can be used in the differential mode, High-to-Neutral. In this case, the varistors must meet all the criteria described in Annex G.8.
How to choose the right varistor? Engineers should note that the minimum continuous operating voltage should be at least 1.25 times the maximum voltage rating of the equipment. The varistor’s diameter will be determined by selecting the varistor’s required surge rating. To withstand the necessary voltage for compliance, a GDT should also pass the electric strength test. The pair must pass the overload and temporary overvoltage tests after choosing a GDT and varistor.
To cite the universal power adapter as an example, the line-to-line and line-to-neutral connections can be protected from voltage transients and lightning by a 300-V thermally-protected varistor that meets minimum surge requirements. Using a 3000-V GDT combined in series with a 300-V varistor in both line-to-ground and neutral-to-ground connections is a good option.
Figure 4, reveals two recommended options for overload and surge protection of universal power adapters. While employing an unreliable ground connection, you should use a fuse and a varistor for differential mode protection. With a product connecting to a reliable ground, you should use two items – one is the fuse-varistor series combination for differential mode line protection; another one is a combination of two varistors with a GDT for common-mode protection.
Including the TMOV and UltraMOV series varistors as well as the CG3 series GDTs, the components listed below the schematics are recommended by Littelfuse.
Withstanding a 6 kA pulse and pulse energy of 250 J, the TMOV series TMOV14RP300E is a thermally-protected, 14-mm disc diameter varistor rated for 300 VAC. The UltaMov series V10E300P has a 10-mm diameter disc and absorbs a peak current pulse of 3.5 kA. We recommended it for common-mode protection with a GDT.
Littelfuse recommends the CG3 3.3, which is accompanied by the V10E300P varistor in common mode connections, for use. High breakdown voltage is combined with a surge current rating of 10 kA by CG3 series gas discharge tubes.
For many electronic applications, this is the most common surge protection solution. At the same time, designers can also consider other solutions. When comparing technologies, engineers should consider various elements of components:
Under the premise that engineers should consider known hazards and use environments when designing a product, IEC 62369-1 introduces a new way to approach electronics product testing. This hazards-based approach aims to keep pace with technological advances. At the same time, product designers are given more flexibility within the framework.
As the products and components that are certified to IEC 62369-1 are ensured by manufacturers, an approach using new, innovative design and construction methods can be taken. Designers can find the right solutions for safe and effective products by partnering with manufacturers including Littelfuse or a distributor experienced with the new standard.
Surge Withstand Disc Mov If you want to learn more, download the Circuit Protection Products Selection Guide, courtesy of Littelfuse, Inc.