There is no doubt that the applications of power electronics are drastically increasing. Switching power supplies are no exception. Switching power supplies, and switching power converters in general, are being implemented in almost every modern electrical and electronic device we use nowadays.
Without switching power supplies, we wouldn’t have durable, small-sized, less-heating chargers for our mobiles and laptops. Modern day hybrid vehicles, as another example, use power converters extensively. The function of power converters is altering power in different current and voltage ratings for several sub systems. Inverters for photovoltaic (PV) systems applications, motor controllers, battery chargers, DIY applications, and many other examples are just endless.
Having worked in this field for several years, in both professional and academic positions, doing several PCB applications to do several power conversion operations, I am writing this article to help juniors in this field getting their switching circuits done. Whether you are a junior engineer in this field, a student, or just a hobbyist, this article is just for you.
What is a Switching Power Supply?
In brief, and in simple words, switching power supplies are a special category of electronic power converters, in which power is transformered from one form to another, and supplied to a load. From an input-output point of view, a Switching Power Supply (SPS, or SMPS, for Switching-Mode Power Supply), does exactly what a regular physical transformer-based adaptor does. An SMPS essentially alters the voltage and current ratings across the device. However, from the inside, a switching power supply is a totally different thing, compared to the ordinary transformer.
As the name suggests, an SMPS implements higher (relative to the input frequency) frequency switching to transform power. There are several standard topologies of an SMPS, or a power converter in general. All of topolgies implement the same theory of “controlled switching” of the input current through an inductor or a transformer. Of course, to achieve the desired outputs. An SMPS prevails over their classic transformer supplies with their durability, lower and more flexible size, reduced heat signature, and much more controllability.
The detailed study of power converters is a huge and quick-evolving matter. It is an interesting research and application topic nowadays. However, it is not the article’s goal to go into these many complex details. Rather, it aims to focus on giving the reader an idea on where to start and how to finish a PCB for an SMPS. This is a general base of knowledge that can be easily digested and extended to other types of power converters and general applications.
The General Characteristics of a Switching Power Converter
Being said above, power conversion circuits come in different topologies and types. Commonly, expect to work with the simple buck, boost, or buck-boost circuits as a beginner, before you start working on the more complex topologies. SEPIC (Single-Ended Primary-Inductor Converter), or Flyback circuits are sort of a more complex form of topologies.
Additionally, what can be learned about those mentioned DC/DC topologies, can be easily extended and generalized to DC/AC (Inverter) applications, and other circuits. Most forms of power converters share similar characteristics. All of these different topologies and implementations share one thing in common: converting power, using switching. Simple as that!
Having said that, let’s discuss the general topology of any converter. First of all, let’s always remember that the resulting PCB will have to deliver a fairly high amount of current through it, from the input to the output point. Let’s keep this in mind at all times, while planning our PCB.
The power that we will be working with is going pass through one or several inductive elements on its path to output. Inductive elements can be a single inductor, a transformer, chokes, or others. In addition of the many other protection and filtering components we expect to find in a power circuit, such as capacitors, the current will also pass through some semiconductor elements that act as switches.
Through the high-current path across the circuit, we expect to see the switching elements. In terms of semiconductors, switching elements are typically Schottky diodes, FETs (Field Effect Transistors), or IGBTs (Insulated-Gate Bipolar Transistors). The switching elements have to driven by some controlling (switching) signal, which is often governed by a dedicated controller.
Between power delivery and switching control, there are typically feedback signal, auxiliary subsystems. In addition, there are some other minor details to worry about, but remember: the main two characteristics to worry about are those two: power delivery, and switching signals.
PCB Considerations
If you are a PCB expert, you’d easily tell why we had to go through the characteristics of the average switching power converter by now. We have mentioned two things: delivering power, and the controlled switching signals.
For the part of delivering power, we will have to consider common techniques for the category of applications: such as taking care of handling the high current, taking care of heat, and so on. On the other hand, for the switching part, what applies to a high frequency circuit, applies to the switching signal of an SMPS: signal integrity, noise, parasitic impedances, and crosstalk to some extent.
For this article, I will just assume that the reader has managed to do his functional design somehow, acquire it, or design it by any other means; we will just skip this part. As well, we will skip all the other basic tips we have posted in another article, and other PCB basics that should be known before attempting to make a switching power layout. I'll just assume that you also know how to use protective elements, such as TVS diodes, varistors, Zeners, etc, so let’s just dive into it!
Proper Placement
Understanding how to plan your components placement in the first place, saves you a lot of confusions and indecisiveness later. This sentence doesn’t only apply to power circuits, but to all other circuits as well, including high-speed designs. Let me put it as simple as my greatest PCB instructor, Robert Feranec, did: “If your placement is incorrect, your PCB will not work”. It is as simple as that.
When planning your layout, make sure to first refer to all the datasheets you have for relevant instructions. Most of the times, especially for the big names in the industry, you will find a “Layout Considerations” section in your circuit controller datasheet. This section might have an illustration of how your layout should look like, or clear instructions about it, or both.
If you are unlucky enough to never find these in your datasheets, you can still refer to other designs, similar datasheets, or just refer to seniors in the field. They key point is to understand your topology, and imagine the paths of the power delivery (high-current) circuit, the approximate placement of your switching elements, and the controller. Generally speaking, start with the big components, and then collect the smaller ones around. Consider any mechanical product constraints and user interface constraints before starting as well.
Copper Fills
Copper fills, zones, islands, polygons, or whatever EDA suites will prefer to name them, are just large areas of copper on layout. Not to be confused with internal power planes, copper fills cover limited areas of the board, connecting specific nets together. We use copper zones to make a sufficient low-impedance, heat-dissipating path for high currents; in our case, the power delivery path. Additionally, we use copper fills for grounds, if we don’t have internal power planes. Having understood this part, is almost one third of your design.
Remember: you cannot use thin signal-rated traces to deliver power. Such traces will present high impedance to the current path, possibly burning out in the process, if working in the first place. There are several calculators to do the math of trace or fill width, but the general rule, for 1oz copper boards, is that you need 1mm of copper width per 1A of current, and about 8mm for 1A. When planning your fills, make sure they go as straight-forward as possible; this will make a better path for the current, as well as making your job much easier. Generally, the bigger, the better, especially for the ground fills.
Switching Signals and Feedback Signals
The switching frequency in a power conversion circuit is often in the range of few hundred Hertz. This means the signal lines in a power circuit is even more sensitive to parasitic effects than audio signals. Having said that, you should avoid sharp corners, loops, long paths, and crossing traces. Remember to place the switching element and the controller close to each other, and give priority to routing the driving signals over others. Feedback lines are less vulnerable, but careful when routing them as well. Always keep grounding, signal termination concepts, and return paths in consideration.
Implementing Net Labels and Net Classes
Use net naming, and net classes to classify your nets in the circuit. Naming your nets will help you make the copper fills for them seamlessly, and classifying them helps you set different rules for different nets. Classify your nets by the type of current (AC or DC), the current rating, and the voltage rating. Be sure to understand electrical clearance and track width to make full use of the net classes.
Heat Dissipation and EMI Radiation
Switching elements, and inductors as well, often heat up during operation, due to their internal resistances. Make sure to implement heatsinks correctly, and address the heat issue in the placement stage as well. In addition, to heat, power circuits tend to both be affected and product EMI, that is Electromagnetic Interference. Make sure to have a compliant circuit, and consider proper measures to seperate the circuit from its surroudning systems, in terms of radiation.
The Big Picture
Each topology is different, and each design is different. Each design has its special considerations, especially if it combines several converters and additional circuits. Once you get the hands on experience on one design, you will easily get into the other, just get started! Power layouts are just like any other type of layouts: an art and a science. The only way to master them is to acquire the proper knowledge about this science, and practice the part where art is needed. In all fields of PCB, experience is the best teacher.
