Design007 Magazine


Issue link:

Contents of this Issue


Page 23 of 131

24 DESIGN007 MAGAZINE I JANUARY 2022 Commission (IEC) and the Under- writers Laboratories (UL) also pro- duced IEC/UL 60950-1, the "Safety of Information Technology Equip- ment" standard, that describes safety requirements for products and details minimum allowed PCB spac- ing requirements. As a combination, the standards also set guidelines for PCB layouts that include two impor- tant parameters called clearance and creepage. Using the IEC 60950 definition, clearance equals the shortest dis- tance between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured through air. A small clearance value between two conductors establishes the environment for a high-voltage flashover or arc. Clearance values vary according to the type of PCB mate- rial used for the circuit, the voltages, and oper- ating environment conditions such as humidity and dust. ose environmental factors—and others—decrease the breakdown voltage of air and increase the opportunities for a high- voltage flashover and a short circuit. We can address clearance issues through ECAD/MCAD design principles. Since the bounding surface described in the IEC defini- tion is the outer surface of an electrical enclo- sure, we can use 3D design tools and design rules to establish the clearance between enclo- sures and components for rigid and rigid-flex circuits. We can also apply good PCB design principles by isolating high-voltage circuits from low-voltage circuits. Fabricators oen recommend placing the high-voltage compo- nents on the top side of a multilayer board and the low voltage circuits on the bottom side of the PCB. Other methods involve placing the appropriate insulating materials between high- voltage nodes and over any exposed high-volt- age leads. Again referring to the IEC 60950 definition, creepage represents the shortest path between two conductive parts, or between a conduc- tive part and the bounding surface of the equipment, measured along the surface of the insulation. Let's pause at this point and ensure that the difference between the two definitions is clear. While clearance refers to the spac- ing through conductive elements through air, creepage considers the space between conduc- tive elements over an insulating surface. e design rules that we establish for trace spacing, pad-to-pad spacing, and pad-to-trace spacing for high-voltage PCB designs must address creepage. e IPC 2221A standard pro- vides clearance and creepage tables that assist with setting design rules and with performing design rule checks and electrical rule checks for minimum requirements. Along with apply- ing design rules, PCB layouts can also include slots or vertical insulation barriers between traces. Because any metallic print pattern or printed circuit trace that has sharp edges can cause a high electric field across insulators and a flashover, the trace layout for a high-voltage power supply must avoid sharp corners and acute angles. High-Voltage Design: Material Selection NASA's report about high-voltage supply problems speaks about the need for insulation that has high dielectric strength, high resis- Figure 1: While a high-voltage design doesn't always have to be a power grid, be wary when dealing with any electrification designs.

Articles in this issue

Archives of this issue

view archives of Design007 Magazine - Design007-Jan2022