PCB Design Guidelines for High-Rel Power Electronics

Power electronics face a different set of challenges compared to high speed digital electronic systems or RF systems, yet the drivers of these trends are common to both types of electronics. Miniaturization, higher levels of integration in components, and faster switching speeds create noise challenges in power electronics that have to be solved with similar techniques as in digital systems. Power electronics carry an additional set of design challenges in terms of heat dissipation, material selection to ensure reliability, and ESD protection.

If you're planning a new power electronics design that requires high reliability, we've prepared a set of PCB design guidelines for power electronics that can help you plan your design from the system level. Once you've gathered the initial requirements for the design, you'll be ready to go a step further to prepare a PCB layout and eventually take the design into production. If you need to design a new power system, or you need to integrate unique power systems into other boards, then consider these guidelines for power electronics design.

Power Electronics PCB Design Guidelines

Before you get started determining a power regulator topology, floorplanning the PCB layout, and selecting materials, you'll have to determine a few functional requirements that will drive the remaining portions of the system design:

1. Power requirements, specifically voltage and current
2. Input and output power comparison, which will drive topology
3. Operating environment characteristics, specifically temperature, ESD, and vibration
4. Acceptable noise levels for your application beyond standard EMC requirements
5. Relevant standards governing the product and application area

For standard power supply designs (Class I or maybe Class II), your power system will most likely not be driven by the final point other than the standard set of IPC-2221 creepage/clearance standards and/or material-driven design standards (IPC-6012, 6018, etc.). For more advanced systems that must have higher reliability and will be deployed in a specialized application area (automotive, aerospace, etc.), there will almost always be some reliability, noise/EMC, and thermal requirements that come from an application standard. Finally, the voltage/current involved will determine how the board is laid out, and even how it will be thermally regulated.

Topology Selection

Input and output power requirements will drive the power topology of individual power regulator circuits, as well as the topology of the entire power distribution system in the design. A typical power tree is shown below. Within this power tree are multiple regulator circuits, each with their own power levels and topology.

Stepping up or down over large voltage ranges is best done with a switching regulator. These circuits use reactive elements to store and release power at controlled intervals, rather than by dropping power over a resistive element where it is converted to heat. Small steps down in voltage can be accomplished with an LDO, even if moderate currents approaching multiple Amps are required. Finally, if you're going to operate with high current at standard voltages, an isolated power converter with a high power transformer is the best option to balance power conversion efficiency, safety, and noise.

The best topology choices for various power electronics applications are shown below.

• Small step down, moderate current:?LDO
• Large step up or step down, moderate current:?Non-isolated buck or boost converter
• High power output and high efficiency:?isolated switching converter (LLC resonant, flyback,bridge, etc.)
• Broad power range with low ripple current:multiphase converter

Power Electronics Material Selection

Novice designers will generally default to an FR4 material system or possibly an aluminum-back PCB without considering the reliability of the material system itself. The material resin content, curing agent, and glass weave style/content will determine the level of reliability that can be expected in the design. Some of the common reliability problems to be avoided are conductive anodic filamentation, voiding, and electromigration.

In general, higher voltage designs will prefer a finer glass weave balanced with a higher resin content. The two go against each other obviously. At higher current, where there will be more DC losses, a preferred material would have higher thermal conductivity and glass transition temperature. Finally, for high reliability designs, we would prefer phenolic curing agent instead of DICY. Make sure you check your laminate datasheets and clearly specify your requirements for your manufacturer.

The main performance metric used to summarize material performance in power systems, particularly in high voltage systems, is the comparative tracking index (CTI). These values are further classified into performance level categories (PLC) based on the material's degradation voltage.

PCB Stackup and Layout

Once topology and materials are selected, It's time to capture schematics and begin the PCB layout. The layout portion of the design can be challenging when low noise is required, but many of the same EMC guidelines used in high speed digital design will be effective in power systems, especially when laying out switching converters. Some of these points include:

• Grounding strategy:?This is probably the most important portion of the stackup. Having a grounding strategy with plane placement and floorplanning is the best strategy to help reduce noise in the PCB layout.
• Routing for low inductance:?If you create the stackup correctly, it will be much easier to route and ensure low inductance throughout your routing paths. This is very important if you plan to route on two layers.
• Separation from data lines:?I often see people ask about this after they already have a crosstalk problem. If you understand crosstalk, then you will know that you should not route data lines near a fast switching regulator. The resulting crosstalk can corrupt data and lead to channel non-compliance.
• Filter selection and placement:?Filters often make an appearance in power systems, typically on the input to remove noise from mains power. They can also be used on the output or between stages to remove conducted noise around the layout.

If you can successfully implement these guidelines, you can rest assured your designs will operate with relatively low noise. PCB design guidelines for power electronics have multiple power, thermal, and test challenges associated with them, but an experienced design team can help you make the transition into production successfully.