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JANUARY 2022 I DESIGN007 MAGAZINE 25 tivity to prevent arcing, and a low power factor that reduces heating effects and the possibility of thermal breakdown. Along with those speci- fications, design teams should also consider tensile strength, hardness, surface breakdown strength, thermal expansion, chemical resistance, and stability against aging and oxidation. While circuits require functional insulation to operate, other types of insulation prevent high-voltage problems in PCBs. Insulating materi- als may include encapsulating resins applied to high-voltage cavities, con- formal coatings, or solid insulation that surrounds conductors. Regulatory stan- dards require additional layers of insulation if the potential for human contact with the sys- tem exists. When selecting dielectrics and insulators for a PCB, use the comparative tracking index (CTI) to determine which material type works best for the specific application. e CTI is the maximum voltage measured in volts at which a material withstands 50 drops of contami- nated water without forming conductive paths because of electrical stress, contamination, or humidity. Manufacturers use the CTI to com- pare the performance of insulating materials under wet or contaminated conditions. Mate- rials that have a high CTI value have a lower required minimum creepage distance and allow a shorter distance between two conduc- tive parts. e shorter distance allows the use of high-density circuits in a high-voltage envi- ronment. UL standards divide the CTI levels for mate- rials into the four groups shown in Table 1. When looking at the table, materials classified within Material Group One have the highest CTI rating. Along with requiring different types of insu- lating materials, high-voltage circuits also require board materials that protect from volt- age breakdown and offer the physical proper- ties that match application needs. Although FR-4 laminates have a high breakdown voltage, the weaker structure and porosity of FR-4 can allow the material to become prone to contam- ination and a gradual lessening of the dielectric value. Because of the FR-4 limitations, high- voltage laminates that have a non-conductive base layer and prevent arcing serve as the gold standard for high-voltage circuit design. High- voltage laminates have higher levels of resin and glass than standard board materials. Using the correct copper thickness and weight also assists with good high-voltage circuit design. Thicker copper pathways withstand high currents and add physical strength to the board. Design teams should work with fabricators to ensure that the copper Figure 2: Working within high-voltage circuit boxes becomes easier when you design them properly. Table 1: The CTI levels for materials are divided into four groups by Underwriters Laboratories.

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