How to quickly create a PCB layout design for a switching power supply?

Today’s switching regulators and power supplies are getting more compact and more powerful, and higher and higher switching frequencies are one of the main issues facing designers, making PCB design more and more difficult. In fact, the PCB layout has become a watershed in distinguishing good from bad switching power supply designs. This article provides some suggestions on how to create a great PCB layout in one go.

Consider a 3A switching regulator that steps down 24V to 3.3V. Designing such a 10W regulator may not seem too difficult at first, and designers may soon be able to move to the implementation stage. However, let’s see what problems are actually encountered after adopting design software such as Webench. If we enter the above requirements, Webench selects the LM25576 (a 42V input device including a 3A FET) in the “Simpler Switcher” family from several ICs. The chip is housed in a TSSOP-20 package with thermal pad.

Design optimizations for volume or efficiency are included in the Webench menu. The design requires a large amount of inductors and capacitors, which requires a large PCB space. Webench provides options as shown in Table 1.

How to quickly create a PCB layout design for a switching power supply?

It is worth noting that the highest efficiency is 84%, and this highest efficiency is achieved when the input-output pressure difference is very low. In this example, the input/output ratio is greater than 7. In general, a two-stage circuit can be used to reduce the stage-to-stage ratio, but the efficiency achieved with two regulators will not be better.

How to quickly create a PCB layout design for a switching power supply?

Figure 1: Efficiency achieved with two regulators would not be better

Next, we choose the highest switching frequency with the smallest PCB area. High switching frequencies are most likely to cause layout problems. Webench can generate circuit diagrams with all active and passive components.

How to quickly create a PCB layout design for a switching power supply?

Figure 2: Simplified switching power supply circuit diagram

The simplified circuit diagram shown in Figure 2 is very helpful in understanding the basics. Take a look at the current paths: mark the loop of the FET in the on state in red; mark the loop of the FET in the off state in green. We can observe two different cases: regions with two colors and regions with only one color. We must pay special attention to the latter case, so the current alternates between zero and full scale. These are regions with high di/dt.

A high di/dt alternating current will generate a significant magnetic field around the PCB traces, which will be a major source of interference to other devices within the circuit and even to other circuits on the same or adjacent PCB. Assuming this is not an alternating current, the common current path is not too important and the di/dt effect is much smaller. On the other hand, these areas will carry a greater load over time. In this example, the common paths are from the diode cathode to the output and from the output ground to the diode anode. As the output capacitor charges and discharges, this capacitor produces a high di/dt. All segments connecting the output capacitors must satisfy two conditions: they must be wide because of the high current flow, and they must be as short as possible to minimize the di/dt effect.

PCB layout design points

In fact, designers should not implement so-called traditional layouts by routing wires from Vout and ground to capacitors. These wires will carry large alternating currents, so connecting the output and ground directly to the capacitor terminals is a better approach. Such alternating currents appear only on the capacitors. The other wires connecting the capacitors now carry almost constant current, so any problems with di/dt are well resolved. Earth is another often misunderstood conundrum. Simply placing a ground plane on “Layer 2” and connecting all grounds to this layer will not work very well.

How to quickly create a PCB layout design for a switching power supply?

Figure 3: Connecting the output and ground directly to the capacitor terminals is a better approach

Let’s see why. Our design example has up to 3A of current that must flow from ground back to the source (a 24V car battery or a 24V power supply). The ground connection of the diode, COUT, CIN, and the load will have a larger current flow, while the ground connection of the switching regulator will have a small current flow. The same applies to the ground reference of the resistor divider. If all the above ground pins are connected to one ground plane, ground bounce will occur. Although small, sensitive points in the circuit (such as the resistive divider through which the feedback voltage is obtained) will not have a stable ground reference. In this way, the entire voltage regulation accuracy will be greatly affected. In fact, sources hidden in the second-level ground plane also produce “ringing” and are very difficult to locate.

Additionally, high-current connections must use vias to ground planes, which are another source of interference and noise. A better solution is to connect the CIN ground as a star node for all high current ground conductors on the input and output sides of the circuit. This star node connects the ground plane and two small current ground connections (IC and voltage divider).

Figure 4:

Author: Yoyokuo