in doing UL certification at the same time, the creepage distance of the LED drive power supply circuit and the distance of the electrical gap sometimes bring troubles and problems to many circuit design staff, especially the design of the electrical gap between the primary and secondary of the transformer. Ancojet testing and certification from UL 8750 from the perspective of this standard, we can analyze the distance requirements of UL's LED drive power supply standard for the primary and secondary of transformers, so that the LED drivers designed by customers can better meet the standard requirements of UL certification.
Common transformers:
LED power supply in UL certification, according to the requirements of UL 8750, we can put the transformer primary secondary electrical clearance requirements are divided into three parts:
distance requirements between the primary and secondary of the transformer on the circuit board
physical insulation distance requirements between the primary and secondary of the transformer itself
physical insulation distance requirements between primary and secondary inside the transformer
01 Requirements for the distance between the primary and secondary of the transformer on the circuit board
requirements- measure the voltage RMS value between the primary and secondary, then find the maximum value, and query according to the following table to find the required value of UL standard.
For example- when the measuring voltage is 360V, the minimum distance on the circuit board is 2.25/4.5mm according to UL certification requirements of UL 8750.
02. Physical insulation distance requirements between the primary and secondary of the transformer itself
requirements- measure the voltage RMS value between the primary and secondary, then find the maximum value, and query according to the following table to find the standard required value.
For example- when the measured voltage is 360V, the physical insulation distance between the primary and secondary of the transformer itself is required to be 9.5/12.7mm according to UL certification requirements of UL 8750.
03. Physical insulation distance requirements between the primary and secondary of the transformer
LED power supply in the submission of UL certification, according UL 8750 the standard requirements, it is generally recommended that the customer secondary with triple insulated wire or primary, secondary between the design of Margin Tape structure to exempt from this requirement.
Customer friends will have questions: since the two sides of the transformer are isolated, can the voltage be measured directly? Is the value of this test accurate?
Answer: generally, when measuring the voltage of the primary and secondary of the transformer, the laboratory will use a differential probe oscilloscope to measure it.
Slightly popularize the application theory of differential probe: differential probe is to transform the floating signal between any two points into a relative signal through differential probe, which is mainly used to observe differential signal. Differential signals are signals that refer to each other rather than ground as a reference point, so there is no problem with using a differential probe oscilloscope to measure the relative voltage difference between the primary and secondary of the transformer.
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