Shulou(Shulou.com)06/02 Report--

Source: EETOP TI Community author: Texas Instruments Gavin Wang

Power design engineers usually use some DC/DC step-down converters in automotive systems to support multiple power rails. However, there are several factors to consider when selecting these types of step-down converters. For example, on the one hand, a high switching frequency DC/DC converter (operating at a frequency higher than 2 MHz) needs to be selected for the automotive infotainment system / host unit to avoid interfering with the radio AM band; on the other hand, the solution size needs to be reduced by selecting relatively small inductors. In addition, the high switching frequency DC/DC buck converter can also help reduce the input current ripple, thus optimizing the size of the input electromagnetic interference (EMI) filter.

However, for large automotive original design manufacturers (ODM) who are trying to create the latest automotive systems, compliance with the required EMI standards is critical. These requirements are so stringent that manufacturers must comply with many standards, such as the International Special Committee on Radio interference (CISPR) 25. In many cases, if the manufacturer does not meet the standards, the carmaker will not be able to accept the corresponding design.

Therefore, the PCB layout is very important to improve the EMI performance of the DC/DC buck converter. In order to obtain good EMI performance, it is the key to optimize the high current power loop and reduce the influence of parasitic parameters on the loop.

Take the two-output step-down converter DC/DC buck converter composed of LMR14030-Q1 as an example, such as the two different printed circuit board (PCB) layouts shown in figures 1 and 2. The red line shows how the power circuit flows in the layout. The flow direction of the power circuit in figure 1 is U-shaped, while the flow direction in figure 2 is type I. These two layouts are the most common layouts in automotive and industrial applications. So, which layout is better?

Figure 1: U-shaped layout

Figure 2: type I layout

Conducted EMI is divided into two types: differential mode and common mode. The differential mode noise comes from the current change rate (di/dt), while the common mode noise comes from the voltage change rate (dv/dt). Whether it is di/dt or dv/dt, the key point of EMI performance is how to minimize the parasitic inductance.

Figure 3 is the equivalent circuit of the buck converter. Most designers know how to minimize the parasitic inductance of Lp1, Lp3, Lp4 and Lp5 in high frequency circuits, but ignore Lp2 and Lp6. For two different layouts, the parasitic inductance on the Lp2 and Lp6 of the U-shaped layout is smaller than that of the I-type layout. In the U-shaped layout, reducing the power loop of the switch Q1 will also help to improve the performance of the EMI.

Figure 3: equivalent circuit of buck converter

In order to verify the optimal layout, it is very important to measure EMI data. Figures 4 and 5 compare the conducted EMI of a two-output converter. At the same time, the circuit adopts phase shift control to reduce the input current ripple and optimize the input filter. From the test results, it can be seen that the EMI performance of U-shaped layout is better than that of I-shaped layout, especially in the high frequency part.

Figure 4: U-shaped EMI performance under phase shift control

Figure 5: performance of type I EMI under phase shift control

Adding EMI filter can effectively improve the performance of EMI. Figure 6 shows a simplified version of the EMI filter, which includes a common mode (CM) filter and a differential mode (DM) filter. Generally speaking, the noise of differential mode filter is less than 30MHz, and the noise range of common mode filter is from 30MHz to 100MHz. Both filters affect the entire band that the EMI needs to limit. Figs. 7 and 8 compare the conductive EMI with common-mode filter and differential-mode filter respectively. U-shaped layouts can meet CISPR 25 Class 3 standards, while I-shaped layouts do not.

Figure 6: simplified EMI filter

Figure 7: EMI performance of U-shaped layout with differential-mode and common-mode filters

Figure 8: EMI performance of type I layout with differential mode and common mode filters

In this paper, two different PCB layouts of two output step-down converters under phase shift control are compared. It can be seen that the EMI performance of U-shaped layout is better than that of I-type layout.

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