High Voltage PCB Design Guidelines

Key Takeaways

• Proper trace spacing and creepage are vital to avoid high voltage flashover, arcing, and insulation failures.
• Choose dielectric materials with high CTI ratings to minimize creepage distances and ensure durability in high voltage environments.
• Isolate high voltage components and follow IPC-2221 and IEC 60950 standards to maintain safe clearance and prevent arcing.

NASA experienced performance issues with 1.5 kV power supplies in spacecrafts like the Hubble and Cassini Telescopes due to initial design flaws, including narrow part parameters, insulation issues, and incorrect materials. As a 2006 report stated, “Designers did not take the high voltage problems seriously in the initial design.” 

For today's high-density layouts and demanding applications, a rigorous approach to high voltage PCB design guidelines is crucial. This involves understanding insulation, board materials, clearance, creepage, and the impact of environmental factors like altitude while adhering to relevant industry standards.

High Voltage Design Begins with the PCB Layout

In high voltage PCB design, proper trace spacing maintains signal integrity and helps prevent the propagation of electromagnetic interference. 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.

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.

High Voltage PCB Standards

In addition to the IPC-2221 standard by the Association of Connecting Electronics Industries (IPC), which sets the design principles for interconnections on printed circuit boards (PCBs), there are also the IEC/UL 60950-1 standards by the International Electrotechnical Commission (IEC) and Underwriters Laboratories (UL). These standards focus on the "safety of information technology equipment" and outline the minimum spacing requirements for PCBs. Together, these standards provide guidelines for high voltage PCB design, emphasizing two key parameters: clearance and creepage.

High Voltage Fundamental: Clearance and Creepage

Two of the most critical parameters in high voltage PCB design are clearance and creepage, as defined by standards like IEC/UL 60950-1 and IPC-2221.

• Clearance: This is the shortest distance through air between two conductive parts or between a conductive part and the bounding surface of the equipment. Insufficient clearance can lead to arcing or flashover, especially as voltage increases or environmental conditions (humidity, dust, altitude) decrease the breakdown voltage of air.
• Creepage: This is the shortest path along the surfaceof an insulating material between two conductive parts. Contamination or moisture on the PCB surface can create conductive paths, and creepage distance is designed to mitigate this risk.

Clearance and Creepage Guidelines

High Voltage Design Through Material Selection

Materials used in high voltage PCBs must offer high dielectric strength, high resistivity, and a low power factor to minimize arcing, heating, and thermal breakdown. Additional key properties include tensile strength, hardness, surface breakdown strength, thermal expansion, chemical resistance, and long-term stability against aging and oxidation.

High voltage insulation may involve functional insulation for circuit operation and protective insulation to prevent breakdown or electric shock. Common materials include encapsulating resins, conformal coatings, and solid insulating barriers. Where human contact is possible, regulatory standards mandate additional insulation layers for safety.

Material & Layout Considerations

Selecting Dielectric Materials

When selecting dielectrics and insulators for a PCB, use the comparative tracking index (CTI) to determine which material is best. 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 use 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.

CTI Standards 

The Comparative Tracking Index (CTI) is a critical property used to classify insulating materials based on their resistance to electrical tracking, which can occur due to moisture and contamination. A higher CTI value indicates greater resistance to tracking and allows for shorter creepage distances in high voltage designs.

According to UL 746A, materials are divided into four material groups based on their CTI ratings. These groups help determine the minimum required creepage distances for safety and compliance with design standards like IEC 60664 or IPC-2221. When referencing the table below, keep in mind the materials in Material Group I have the highest tracking resistance, making them preferable for high voltage or high-humidity applications.

Material Group and Associated CTI Rating

Design Guidelines for Dielectrics

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 lessening of the dielectric value. Because of  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 than standard board materials.

Working within high voltage circuit boxes becomes easier when you design them properly.

High Voltage Design Guidelines For Component Selection

High voltage circuits rely on both passive and active components:

• Resistors in high voltage circuits require low inductance and low-temperature coefficients.
• Ceramic capacitors must have high resistance, high-temperature coatings, and dielectrics that can withstand high voltages. These capacitors should maintain stable electrical parameters across a wide range of applied DC voltages and under various environmental conditions.
• Semiconductor devices for high voltage circuits include MOSFETs, IGBTs, MOS-controlled Thyristors, Power FETs, and SCRs, which 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. 

Component-Level Considerations

Following High Voltage PCB Design Guidelines with OrCAD X

Successfully navigating high voltage PCB design guidelines demands meticulous attention to detail, from material selection and component ratings to precise clearance and creepage calculations. OrCAD X provides an integrated environment with tools like the Constraint Manager for defining specific high voltage rules, DRC for verification against standards, and a robust layout editor for implementing isolation techniques. Try OrCAD X for free, or explore the full OrCAD X platform to leverage its comprehensive features for your design challenges.