Power design engineers typically use DC/DC buck converters in automotive systems to power multiple power rails. However, several factors need to be considered when selecting these types of buck converters. For example, on the one hand, high-switching-frequency DC/DC converters (operating frequencies above 2MHz) are needed for automotive infotainment systems/head units to avoid interference with the AM radio band; on the other hand, relatively small inductors are also needed to reduce solution size. Furthermore, high-switching-frequency DC/DC buck converters can help reduce input current ripple, thereby optimizing the size of the input electromagnetic interference (EMI) filter.
However, for large automotive original design manufacturers (ODMs) attempting to create state-of-the-art automotive systems, compliance with required EMI standards is crucial. These requirements are extremely stringent, and manufacturers must adhere to numerous standards, such as the International Special Committee on Radio Interference (CISPR) 25 standard. In many cases, if a manufacturer fails to meet these standards, the automaker will not accept the corresponding design.
Therefore, PCB layout is crucial for improving the EMI performance of DC/DC buck converters. To achieve good EMI performance, optimizing high-current power loops and minimizing the impact of parasitic parameters on the loop are key.
Taking the LMR14030-Q1 dual-output DC/DC buck converter as an example, Figures 1 and 2 show two different printed circuit board (PCB) layouts. The red lines indicate the power loop flow pattern in the layout. In Figure 1, the power loop flow direction is U-shaped, while in Figure 2, it is I-shaped. These two layouts are the most common in automotive and industrial applications. So, which layout is better?
Figure 1: U-shaped layout
Figure 2: Type I layout
Conducted EMI is classified into two types: differential mode and common mode. Differential mode noise originates from the rate of change of current (di/dt), while common mode noise originates from the rate of change of voltage (dv/dt). Regardless of whether it is di/dt or dv/dt, the key to EMI performance lies in minimizing parasitic inductance.
Figure 3 shows the equivalent circuit of the buck converter. Most designers know how to minimize the parasitic inductances of Lp1, Lp3, Lp4, and Lp5 in the high-frequency loop, but neglect Lp2 and Lp6. For the two different layouts, U-type and I-type, the parasitic inductances on Lp2 and Lp6 are smaller in the U-type layout than in the I-type layout. In the U-type layout, reducing the power loop when switch Q1 is on also helps improve EMI performance.
Figure 3: Equivalent circuit of buck converter
To verify the optimal layout, measuring EMI data is crucial. Figures 4 and 5 compare conducted EMI of a two-output converter. This circuit also employs phase-shift control to reduce input current ripple, thereby optimizing the input filter. The test results show that the U-shaped layout outperforms the I-shaped layout in EMI performance, especially in the high-frequency range.
Figure 4: U-shaped EMI performance under phase-shift control
Figure 5: Type I EMI performance under phase-shift control
Adding an EMI filter can effectively improve EMI performance. Figure 6 shows a simplified EMI filter, which includes a common-mode (CM) filter and a differential-mode (DM) filter. Generally, the noise level of the differential-mode filter is less than 30MHz, while the noise level of the common-mode filter ranges from 30MHz to 100MHz. Both filters affect the entire frequency band where EMI needs to be limited. Figures 7 and 8 compare conducted EMI with common-mode and differential-mode filters, respectively. A U-shaped layout conforms to CISPR 253 standards, while an I-shaped layout does not.
Figure 6: Simplified EMI filter
Figure 7: EMI performance of a U-shaped layout using differential-mode and common-mode filters
Figure 8: EMI performance of a Type I layout using differential-mode and common-mode filters.
This article compares two different PCB layouts for dual-output buck converters under phase-shift control. It shows that the U-shaped layout has better EMI performance than the I-shaped layout. For more information, please refer to the TI website application report "How SYNCLogic Affects EMI Performance for Dual-Channel Buck Converters".
