基于该 PCB 电源布局，本文介绍了优化简单开关电源模块性能的最佳PCB布局方法、示例和技术。

在规划电源布局时，首先要考虑的是两个开关电流回路的物理回路面积。 Although these loop regions are largely invisible in the power module, it is important to understand the respective current paths of the two loops because they extend beyond the module. In loop 1 shown in Figure 1, the current self-conducting input bypass capacitor (Cin1) passes through the MOSFET to the internal inductor and output bypass capacitor (CO1) during the continuous conduction time of the high-end MOSFET, and finally returns to the input bypass capacitor.

Schematic diagram of loop in the power module www.elecfans.com

Figure 1 Schematic diagram of loop in power module

Loop 2 is formed during the turn-off time of the internal high-end MOSFEts and the turn-on time of the low-end MOSFEts. 存储在内部电感器中的能量在返回到 GND 之前流经输出旁路电容器和低端 MOSFET（见图 1）。 The region where two loops do not overlap each other (including the boundary between loops) is the region with high DI/DT current. The input bypass capacitor (Cin1) plays a key role in supplying the high frequency current to the converter and returning the high frequency current to its source path.

输出旁路电容器 (Co1) 不承载太多交流电流，但用作开关噪声的高频滤波器。 由于上述原因，输入和输出电容应尽可能靠近模块上各自的 VIN 和 VOUT 引脚放置。 As shown in Figure 2, the inductance generated by these connections can be minimized by making the wiring between the bypass capacitors and their respective VIN and VOUT pins as short and wide as possible.

图 2 SIMPLE SWITCHER 回路

最小化 PCB 布局中的电感有两个主要好处。 首先，通过促进 Cin1 和 CO1 之间的能量转移来提高组件性能。 这确保模块具有良好的高频旁路，从而最大限度地减少由于高 DI/DT 电流引起的感应电压峰值。 它还最大限度地降低了设备噪声和电压应力，以确保正常运行。 其次，尽量减少 EMI。

连接寄生电感较小的电容器对高频表现出低阻抗特性，从而减少传导辐射。 Ceramic capacitors (X7R or X5R) or other low ESR type capacitors are recommended. 只有在 GND 和 VIN 端附近放置额外的电容器时，额外的输入电容器才能发挥作用。 The Power module of the SIMPLE SWITCHER is uniquely designed to have low radiation and conducted EMI. However, follow the PCB layout guidelines described in this article to achieve higher performance.

Circuit current path planning is often neglected, but it plays a key role in optimizing power supply design. In addition, ground wires to Cin1 and CO1 should be shortened and widened as much as possible, and bare pads should be directly connected, which is especially important for input capacitor (Cin1) ground connections with large AC currents.

模块中的接地引脚（包括裸焊盘）、输入和输出电容、软启动电容和反馈电阻都应连接到 PCB 上的环路层。 该环路层可用作具有极低电感电流的返回路径和下文讨论的散热装置。

无花果。 3 模块与PCB作为热阻示意图

反馈电阻也应尽可能靠近模块的 FB（反馈）引脚。 To minimize the potential noise extraction value at this high impedance node, it is critical to keep the line between the FB pin and the feedback resistor’s middle tap as short as possible. Available compensation components or feedforward capacitors should be placed as close to the upper feedback resistor as possible. For an example, see the PCB layout diagram in the relevant module data table.

For AN example layout of LMZ14203, see the application guide document AN-2024 provided at www.naTIonal.com.

Heat Dissipation Design Suggestions

模块的紧凑布局在提供电气优势的同时，对散热设计产生了负面影响，其中较小的空间耗散了等效的功率。 To address this problem, a single large bare pad is designed on the back of the Power module package of the SIMPLE SWITCHER and is electrically grounded. The pad helps to provide extremely low thermal impedance from the internal MOSFEts, which typically generate most of the heat, to the PCB.

这些器件从半导体结到外封装的热阻抗 (θJC) 为 1.9℃/W。 While achieving an industry-leading θJC value is ideal, a low θJC value makes no sense when the thermal impedance (θCA) of the outer package to the air is too great! If no low-impedance heat dissipation path is provided to the surrounding air, the heat will accumulate on the bare pad and cannot be dissipated. So what determines θCA? The thermal resistance from bare pad to air is completely controlled by the PCB design and associated heat sink.

Now for a quick look at how to design a simple PCB without fins, figure 3 illustrates the module and PCB as thermal impedance. Because the thermal impedance between the junction and the top of the outer package is relatively high compared to the thermal impedance from the junction to the bare pad, we can ignore the θJA heat dissipation path during the first estimate of the thermal resistance from the junction to the surrounding air (θJT).

散热设计的第一步是确定要耗散的功率量。 The power consumed by the module (PD) can be easily calculated using the efficiency graph (η) published in the data table.

We then use the temperature constraints of the maximum temperature in the design, TAmbient, and the rated junction temperature, TJuncTIon(125 ° C), to determine the thermal resistance required for the packaged modules on the PCB.

Finally, we used a simplified approximation of the maximum convective heat transfer on the PCB surface (with undamaged 1-ounce copper fins and numerous heat sink holes on both the top and bottom floors) to determine the plate area required for heat dissipation.

The required PCB area approximation does not take into account the role played by heat dissipation holes that transfer heat from the top metal layer (the package is connected to the PCB) to the bottom metal layer. 底层作为第二个表面层，对流可以通过它从板传递热量。 至少应使用 8 到 10 个冷却孔，以使电路板面积近似值有效。 散热器的热阻由下式近似计算。

此近似值适用于直径为 12 密耳且具有 0.5 盎司铜侧壁的典型通孔。 裸焊盘下方的整个区域应设计尽可能多的散热孔，这些散热孔应形成一个阵列，间距为1～1.5mm。

结论

SIMPLE SWITCHER 电源模块为复杂的电源设计和与 DC/DC 转换器相关的典型 PCB 布局提供了替代方案。 虽然已经消除了布局挑战，但仍需要完成一些工程工作，以通过良好的旁路和散热设计来优化模块性能。