Simple switcher PCB layout guide

One of the difficulties in writing notes about PCB layout applications is that the engineer reading the article is not the one who intends to use it. Even designers have made great efforts in electromagnetic field, EMC, EMI, circuit board parasitic effect, line effect, grounding, etc. He is likely to focus on the main design tasks, leaving the task of layout to the dedicated CAD / layout staff. Unfortunately, especially for switching regulators, it is not enough to consider only the basic layout, connectivity and mechanical issues. Designers and CAD personnel should pay attention to that the design of switching power converter is closely related to its layout. This can explain why a large number of guest room calls are related to the application of switching power supply, and they all come down to the poor actual layout. In fact, these problems should and can be avoided in the initial prototype circuit board design, which can save time and money in all aspects.

The whole topic of PCB design is a rather broad topic, including some testing, mechanical, and production issues, as well as some compatibility and control issues. If we can understand it more clearly and deeply, there are a certain number of physical and electromagnetic problems. However, the purpose of this application note is for those readers who are most likely to use it. Although experienced designers / CAD personnel already have enough design information, this application note contains a series of clear and clear basic rules for quick setting. Following these specifications carefully can avoid a lot of problems. In particular, we provide the recommended initial layout for lm267x, lm259x and lm257x series. The focus is on National Semiconductor’s buck simple switcher chip, but the same principle is suitable for any topology and switching power supply applications.

Most of the problems discussed in this application note only focus on ensuring that the basic electrical functions meet the required performance indicators. Fortunately, as vexed switching power supply designers will be happy to learn, generally all the electrical performances involved are related, and the influence trend has the same “direction”. For example, an “ideal” layout not only helps to achieve chip functions correctly, but also reduces electromagnetic radiation, and vice versa. For example, reducing the switching current loop area will help to improve the performance of EMI. However, designers should note that there are some exceptions to this common “trend”. This will focus on the “copper clad” process, which will help reduce parasitic inductance and noise induced IC problems, but it will also increase EMI.

(2) quick setting of PCB layout rules of simple switcher:

1. Place clamping diode and input capacitor, as shown in Figure 2;

2. For high-speed devices (such as lm267x), the input decoupling / bypass ceramic capacitor (0.1 μ F-0.47 μ F) As shown in Figure 2;

3. If possible, connect the via to the ground plane (optional, mark “X” in Figure 2)

4. If the through holes fall under the surface mount (SMT) power supply pad, they can be called heat dissipation holes. Using the correct size can avoid production problems. Or place the through hole close to the pad, but not directly below it;

5. As discussed, the feedback line should be correctly arranged away from noise sources, such as inductors and diodes;

6. It is forbidden to increase the width of copper foil wrongly on the switch node;

7. If the clamping diode requires a very large cooling area (the cooling area requirements have been correctly predicted), isolation is required as shown in the discussion.

8. For higher power SMT applications, 2 ounces of circuit board is required for less copper foil area and less heat management.

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