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Does every step-down switch mode power supply need to be subjected to Energy Source Classification?

More specifically you asked: If the product has one or more on-board step-down switch-mode power supplies, should these power supplies be classed for Energy Sources? E.g., 5V input to 3V or 1V output.


The complete answer to this topic will depend on the specific construction of the system, and there appears to be a specific, detailed construction that needs review / analysis. Therefore, you are encouraged to contact UL for an in-depth consultation, either via your local account executive or via https://62368-ul-solutions.com/contact-ul.html .


However below are a few key facts to consider:


Your question is related to the basic principle of IEC 62368-1 and its Hazard Based Safety Engineering approach based on three-block model of safety: energy source class - safeguard - person (or combustible material).


Identification and classification of Energy Sources is the first step to evaluate safeguards required in product. Therefore, all energy sources need to be classified in the product, either based on declaration (e.g., PS3) or evaluation and / or test of the product connect to a representative supply.


When DC-DC converters are involved, the evaluation and classification is not limited to the voltage levels of the circuit supplying the converter; the working voltages and frequency generated within the converter and supplied by the converter also need to be considered.  For example, when circuits are intended to be ES1 due to some accessibility requirements in the final application, the entire system and its circuits must be analyzed by construction review and, when required, by measurement & tests such as Working Voltage Measurement and Single Fault Condition (SFC), to demonstrate that the ES1 circuits are not compromised by higher ES2 or ES3 voltages generated within the converter. Investigation and test may show that additional Safeguards are required.


With reference to clause 4.2.3 and 4.2.4, the standard allows manufactures to declare higher class than there is in the product. (For example, if an energy source class 3 is declared by the manufacturer, there is no need to conduct any measurements.)


Also, related to power supplies:


Electrical Energy Source is based on voltage and/or touch current - it is worth noting that switch-mode voltages may be generated that exceed the ES1 limit even if the input to the supply is ES1. Therefore, more detailed evaluation and tests may be needed even for DC-DC converters. However, for most converters such as provided in the example (5V input to 3.3V or 1 Vdc output), typical switching voltages are not expected to exceed ES1 limits under normal, anormal and single fault condition. 


Also, it also is worth noting, per Clause 6, Power Energy Source classification is irrelevant of voltages and depends on available power. Additionally, components may need classification as Potential Ignition Sources.

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