High Voltage Circuit Design Guidelines and Materials
The Hubble telescope, the Cassini-Huygens mission, and other exploratory spacecraft utilize high voltage dc power supplies for everything from vidicon camera tubes and mass spectrometers to radar and laser technologies. NASA has experienced performance problems with the 1.5 kV supplies because--as a 2006 report stated--“designers did not take the high voltage problems seriously in the initial design.” The report cited a very narrow parts parameters, electrical insulation problems in dielectrics, ceramics, bad geometries, small spacing, the use of the wrong insulating materials, and thermal expansion as causes for the power supply failures.
Designing a circuit that includes high voltages requires a different--and much more rigorous--approach than seen with other PCB designs. And...the need for more attention increases for high-density designs. Along with that approach, design teams also must become familiar with terminology that covers insulation, board materials, clearance, creepage, and altitude. Designers also should have an overall knowledge of regulations that can impact the circuit.
High Voltage Design Problem Solving Begins with the PCB Layout
All of us know that proper trace spacing in a PCB design maintains signal integrity and helps with preventing the propagation of electromagnetic interference. In high voltage PCB design, trace spacing becomes even more important. If we rightfully consider the board as a series of conductive elements, the possibility of differences in potential creating high voltage flashover with narrow trace spacing becomes a certainty.
Along with the Association of Connecting Electronics Industries (IPC) IPC-2221 Generic Standard on Printed Board Design standard that establishes the design principles for interconnections on PCBs, the International Electrotechnical Commission (IEC) and the Underwriters Laboratories (UL) also produced the IEC/UL 60950-1 “Safety of Information Technology Equipment” standard that describes safety requirements for products and details minimum allowed PCB spacing requirements. As a combination, the standards also set guidelines for PCB layouts that include two important parameters called clearance and creepage.
Using the IEC 60950 definition, clearance equals the shortest distance 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 material used for the circuit, the voltages, and operating environment conditions such as humidity and dust. Those 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 definition is the outer surface of an electrical enclosure, we can use 3D design tools and design rules to establish the clearance between enclosures 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 often recommend placing the high voltage components 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 voltage leads.
While a high-voltage design doesn’t always have to be a power grid, you’ll want to be wary when dealing with any electrification designs.
Again referring to the IEC 60950 definition, creepage represents the shortest path between two conductive parts, or between a conductive 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 spacing through conductive elements through air, creepage considers the space between conductive elements over an insulating surface.
The design rules that we establish for trace spacing, pad-to-pad spacing, and pad-to-trace spacing for PCB designs that incorporate high voltages address creepage. The IPC2221A standard provides 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 applying 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 Problem-solving Continues with Material Selection
NASA’s report about high voltage supply problems speaks about the need for insulation that has high dielectric strength, high resistivity to prevent arcing, and a low power factor that reduces heating effects and the possibility of thermal breakdown. Along with those specifications, 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 materials may include encapsulating resins applied to high voltage cavities, conformal coatings, or solid insulation that surrounds conductors. Regulatory standards require additional layers of insulation if the potential for human contact with the system 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. The CTI is the maximum voltage measured in volts at which a material withstands 50 drops of contaminated water without forming conductive paths because of electrical stress, contamination, or humidity. Manufacturers uses the CTI to compare the performance of insulating materials under wet or contaminated conditions. Materials that have a high CTI value have a lower required minimum creepage distance and allow a shorter distance between two conductive parts. The shorter distance allows the use of high density circuits in a high voltage environment.
UL standards divide the CTI levels for materials into the four groups shown in table one. When looking at the table, materials classified within Material Group One have the highest CTI rating.
Along with requiring different types of insulating materials, high voltage circuits also require board materials that protect from voltage breakdown and offer the physical properties that match application needs. Although FR4 laminates have a high breakdown voltage, the weaker structure and porosity of FR4 can allow the material to become prone to contamination and a gradual lessening of the dielectric value. Because of the FR4 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 that 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 has a smooth, unblemished surface to prevent arcing.
High Voltage Design Problem-solving Includes Component Selection
High voltages rely on passive as well as active components. While design specifications for high voltage resistors may require low inductance and low temperature coefficients, ceramic capacitors must have high resistance, high temperature coatings and dielectrics that withstand high voltages. Capacitors used in high voltage circuits also should exhibit stable electrical parameters with a wide range of applied dc voltages and under different environmental conditions.
Working within high-voltage circuit boxes becomes easier when you design them properly.
High-voltage semiconductor devices used for motor control circuits and power supplies include MOSFETs, Insulated Gate Bipolar Transistors, MOS-controlled Thyristors, Power FETs, and Silicon-Controlled Rectifiers. PCB design rules must follow manufacturer guidelines for not exceeding values that can destroy the devices. For example, high voltage circuits may require components that have higher breakdown voltage ratings and the capability to handle higher currents.
Circuit optimization can protect those devices from inductive loads or any large stray inductance that can cause reverse voltages that damage the components. Good circuit design also routes cables and shielding to prevent any large voltage or current transients that can induce instantaneous voltages on control lines.
With a suite of design tools and analysis products, Cadence has the capability to work through any voltage requirement in your electronic designs. Don’t sweat layout and component placement with OrCAD PCB Designer’s intuitive and integrative capabilities.
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