“Whether performing AC-DC conversion or DC-DC conversion, switching power supply layouts are common in high-voltage designs and must be carefully constructed. Although this system is very common, it can easily radiate EMI due to the rapid changes in voltage and current during switching. Designers are rarely able to apply existing designs to new systems, because small changes in an area can produce EMI problems that are difficult to diagnose.
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Whether performing AC-DC conversion or DC-DC conversion, switching power supply layouts are common in high-voltage designs and must be carefully constructed. Although this system is very common, it can easily radiate EMI due to the rapid changes in voltage and current during switching. Designers are rarely able to apply existing designs to new systems, because small changes in an area can produce EMI problems that are difficult to diagnose.
Through correct layout selection and routing, noise can be prevented from becoming an important issue on 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. The following are some important SMPSPCB layout tips that can help you keep your components cool and prevent noise issues in your system.
Noise and heat issues in SMPSPCB layout
There is no solution: 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 (that is, generated from a full-wave rectifier during the AC-DC conversion process) into high-frequency switching noise. Although this conversion produces a more stable DC output, there are still two important noise sources:
Direct switching noise from switching elements.
Transient noise elsewhere in the system.
Noise will appear on the output of the SMPS unit in the form of conducted noise 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 the SMPSPCB layout is the heat generated on the board. Although this can be affected by choosing the correct PWM frequency, duty cycle and rise time, you still need to use the correct thermal management strategy on the board. Considering these two challenges, let’s look at some details to pay attention to in the SMPSPCB layout.
Thermal management
An ideal SMPS will dissipate zero power, although this does not actually happen. Your switching transistor (and the input transformer for AC-DC conversion) will dissipate most of the heat. Even if the efficiency can reach 90% in the switching power supply topology, the power MOSFET can still dissipate a lot of heat during the switching process. The common practice here is to place the heat sink on the critical switch assembly. Make sure to reconnect them to the ground plane to prevent new EMI.
In high-voltage/high-current power supplies, these radiators can be very large. You can increase the heat dissipation capacity of the system by installing a fan on the chassis. Also, make sure to follow good practices regarding powering the fan to prevent new EMI problems.
Some SMPSPCB layout skills
Your stack
Your layout will help thermal management to some extent, but this is a greater determinant of EMI sensitivity. Generally, conduction noise is dealt with by using EMI filters on the input and output circuits. Like many EMI issues in high-speed/high-frequency systems, your stack will be the main determinant of resistance to radiated EMI.
The relevant frequency range for SMPS operation is from ?10kHz to ?1MHz, so the radiated EMI will induce induced noise. Therefore, you want to place the ground plane with all power components in the stack directly below the surface. This will ensure low loop inductance of the surface circuit. Usually filtering at the output end is used to eliminate any induced noise signal that propagates to the output end.
Transient ringing
Transients are a more difficult problem to solve because they are related to your stackup, wiring, the presence of vias, and excessive decoupling/impedance. As in the case of high-speed designs, do not route any copper wires that carry switching signals to the gaps in the ground plane, because this will form a certain type of antenna structure that will radiate strongly during transients. These transients are often high frequency (anywhere from 10 to 100 MHz).
The problem of transient ringing is impedance management. High impedance will cause strong voltage ripple. The components should be placed with the correct land pattern to optimize the impedance in the PDN of the circuit board. Examples of good and bad pads for your component are shown below.
The quality of the components in the layout
Generally do not keep any isolated islands in the layout. Use decoupling capacitors to connect any power islands that may contain control circuits or passive components back to the ground/ground plane. In this case, place any vias carefully, because you don’t want to create accidental notches or slots in the ground plane.
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