DC-DC布线详解：关键规则与注意事项
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本篇文章是关于DC-DC转换器的PCB布线指南，由Texas Instruments发布，标题为"DC-DC布线指南"（SNVA054C – June 2002 – Revised April 2013），属于AN-1229系列。这份应用报告旨在提供SIMPLESWITCHER系列产品的PCB布局建议，确保电路设计的高效性和可靠性。
1. **引言**：
开篇部分介绍了布线指南的目的，帮助设计者遵循一套快速的规则，以优化SIMPLESWITCHER DC-DC转换器在PCB上的布局，包括 Buck转换器的设计。
2. **快速布线规则**：
提供了针对SIMPLESWITCHER PCB布局的基本准则，可能涉及到元件布局、电源路径和信号线处理。
3. **AC和DC电流路径**：
部分内容讲述了如何正确规划直流和交流电流的流动路径，以减少电磁干扰（EMI）和提高效率。
4. **放置电容（acap）**：
强调将电容器（如滤波或去耦电容）尽可能靠近电源输入，以减小信号延迟和提高稳定性。
5. **路由电极**：
关注于信号和电源线的走线技巧，可能涉及最小化线长、宽度、弯曲半径等参数，以控制阻抗、减小互感和提升信号完整性。
6. **铜填充：何时停止**：
分析了铜箔填充的使用时机，包括处理电流承载能力、减小导线自感（trace inductance）以及考虑热管理，确保组件不会过热。
7. **地平面**：
地线设计的重要性，可能讨论了地线布局如何影响噪声抑制、信号完整性以及整体系统的电磁兼容性。
8. **信号轨迹：反馈**：
着重于反馈信号的处理，强调了哪些线路可能更容易受到噪声的影响，并提供了相应的布局策略。
本文通过一系列的图表和曲线，如电流密度与温度升高的MIL-STD-275E曲线，来指导读者进行具体的设计实践。最后，还包含了版权信息和提交文档反馈的渠道。
这篇布线指南对于任何从事SIMPLESWITCHER DC-DC转换器PCB设计的专业人员来说，都是一份实用且重要的参考资料，能够帮助他们遵循最佳实践，提高设计质量。
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AC and DC Current Paths
Figure 1. Recommended Layout Starting Points
3 AC and DC Current Paths
Referring to Figure 2a, the bold lines represents the main (power) current flow in the converter during the
time the switch is ON. As the switch turns ON, the edge of the of the current waveform is provided largely
by CBYPASS, the remainder coming mainly from CIN. Some slower current components come from the
input DC power supply (not shown) and also refresh these input caps. Figure 2b represents the situation
when the switch is OFF. We can therefore see that in certain trace sections, current has to start flowing
suddenly during the instant of switch turn-off and in some sections it needs to stop flowing equally
suddenly. Figure 2c represents the ‘difference’, that is, traces shown bold in Figure 2 are those where the
current flow changes suddenly. During the turn-on transition the picture reverses, but the 'difference'
trace sections are the same. Therefore during either switch transition, 'step changes' of current take place
in these difference sections. These traces encounter the harmonic-rich rising or trailing edges of the
current pedestal waveform. The difference traces are considered 'critical' and deserve utmost attention
during PCB layout. It is often stated colloquially, that ‘AC current’ flows in these trace sections, and 'DC
current' in the others. The reason is that the basic switching PWM frequency forms only a fraction of the
total harmonic (Fourier) content of the current waveform in the 'AC' traces. In comparison, where ‘DC
current’ flows, the current does not change in a stepped fashion and so the harmonic content is lower. It is
also no surprise that the DC sections are those in series with the main inductor, because it is known that
inductors have the property of preventing sudden changes in current (this is analogous to a capacitor
which ‘resists’ sudden changes in voltage).
3
SNVA054C–June 2002–Revised April 2013 AN-1229 SIMPLE SWITCHER
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Copyright © 2002–2013, Texas Instruments Incorporated
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