Whether performing AC-DC or DC-DC conversion on
PCB board, switching power supply layouts are common in high-voltage designs and must be constructed carefully. Although this system is very common, it is prone to radiated EMI due to the rapid changes in voltage and current during switching. Designers are rarely able to adapt existing designs to new systems because small changes in one area can create EMI problems that are difficult to diagnose. With proper placement selection and routing, noise can be prevented from becoming a significant problem on the SMPS output. Low voltage converters can be purchased as ICs with different form factors, but high voltage converters will need to be produced from discrete components on dedicated boards. Here are some important SMPS PCB board layout tips to help you keep components cool and prevent noise issues in your system.
Noise and Thermal Issues in SMPS PCB Board Layout
There is no fix: any SMPS will generate moderate high frequency noise due to the switching action of the transistor driver. In effect, you are converting low frequency ripple (i.e. from a full wave rectifier during AC-DC conversion) into high frequency switching noise. Although this conversion produces a more stable DC output, there are still two significant sources of noise: direct switching noise from the switching elements. Transient noise elsewhere in the system. Noise can appear at the output of the SMPS unit in the form of conducted and radiated noise. Although the cause of each problem is difficult to diagnose, two types of noise can be easily distinguished. Another design challenge in SMPS PCB board layout is the heat generated on the board. While this can be influenced by choosing the correct PWM frequency, duty cycle and rise time, you still need to use the correct thermal management strategy on the board. With these two challenges in mind, let's take a look at some details to pay attention to in SMPS PCB board layout.
thermal management
An ideal SMPS would dissipate zero power, although in practice this does not happen. Your switching transistors (and the input transformer for AC-DC conversion) will dissipate most of the heat. Even though efficiency can reach 90% in switching power supply topologies, power MOSFETs can still dissipate a lot of heat during switching. A common practice here is to place heat sinks on critical switch components. Make sure to reconnect them to the ground plane to prevent new EMI. In high voltage/high current power supplies, these heat sinks can be quite large. You can enhance the cooling capacity of your system by installing fans on the chassis. Again, make sure to follow good practices for powering this fan to prevent new EMI issues.
Some SMPS PCB board layout tips
Your layout will help with thermal management to some extent, but it's a bigger determinant of EMI susceptibility. Typically, conducted noise is handled by using EMI filters on the input and output circuits. Like many EMI problems in high speed/high frequency systems, your stackup will be the main determinant of resistance to radiated EMI. The relevant frequency range for SMPS operation is ~10kHz to ~1MHz, so radiated EMI will induce induced noise. Therefore, you want to place the ground plane in the stackup directly below the surface layer, along with all power components. This will ensure low loop inductance of the surface circuit. Any induced noise signal propagating to the output is usually eliminated by filtering at the output.
transient ringing
Transients are a more difficult problem to solve because they are related to your stackup, routing, presence of vias, and excessive decoupling/impedance. As is the case in high-speed designs, do not route any copper carrying switching signals onto the gaps in the ground plane, as this creates some type of antenna structure that radiates strongly during transients. These transients tend to be high frequency (anywhere from 10 to 100 MHz). The problem with transient ringing is an impedance management problem. High impedance results in strong voltage ripple. Components should be placed with the correct land pattern to minimize impedance in the board PDN. Examples of good and bad pads for your component are shown below.
The quality of the components in the layout
In general, don't keep any isolated islands in your layout. Use decoupling capacitors to connect any power islands that may contain control circuitry or passive components back to the ground/ground plane. In this case, be careful placing any vias on
PCB board, as you don't want to create unintended gaps or sockets in the ground plane.
