“Power design engineers usually use some DC/DC buck converters in automotive systems to provide support for multiple power rails. However, there are several factors to consider when choosing these types of buck converters. For example, on the one hand, it is necessary to select a high switching frequency DC/DC converter (operating frequency higher than 2 MHz) for the car infotainment system/host unit to avoid interference with the radio AM frequency band; on the other hand, it is also necessary to select a relatively small Inductors to reduce the solution size.
Author: Texas Instruments Gavin Wang
Power design engineers usually use some DC/DC buck converters in automotive systems to provide support for multiple power rails. However, there are several factors to consider when choosing these types of buck converters. For example, on the one hand, it is necessary to select a high switching frequency DC/DC converter (operating frequency higher than 2 MHz) for the car infotainment system/host unit to avoid interference with the radio AM frequency band; on the other hand, it is also necessary to select a relatively small Inductors to reduce the solution size. In addition, the high switching frequency DC/DC buck converter can also help reduce input current ripple, thereby optimizing the size of the input electromagnetic interference (EMI) filter.
However, for large automotive original design manufacturers (ODMs) who are trying to create the latest automotive systems, meeting the required EMI standards is critical. These requirements are very strict, and manufacturers must comply with many standards, such as the International Special Committee on Radio Interference (CISPR) 25 standard. In many cases, if the manufacturer does not meet the standards, the car manufacturer cannot accept the corresponding design.
Therefore, the PCB layout is very important for the improvement of the EMI performance of the DC/DC step-down converter. To obtain good EMI performance, optimizing the high-current power loop and reducing the influence of parasitic parameters on the loop are the key.
Take the two-output step-down converter DC/DC step-down converter composed of LMR14030-Q1 as an example, as shown in Figure 1 and Figure 2 with two different printed circuit board (PCB) layouts. The red line shows how the power loop 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 I-shaped. These two layouts are the most common layouts in automotive and industrial application systems. 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. Differential mode noise comes from the rate of change of current (di/dt), and common mode noise comes from the rate of change of voltage (dv/dt). Regardless of whether it is di/dt or dv/dt, the key point of EMI performance is how to minimize parasitic inductance.
Figure 3 is the equivalent circuit of a buck converter. Most designers know how to minimize the parasitic inductance of Lp1, Lp3, Lp4, and Lp5 in high-frequency loops, but ignore Lp2 and Lp6. For two different layouts, U-type and I-type, the parasitic inductance on Lp2 and Lp6 of U-type layout is smaller than that of I-type layout. In the U-shaped layout, reducing the power loop when the switch Q1 is turned on will also help improve EMI performance.
Figure 3: The equivalent circuit of a buck converter
In order to verify the best layout, measuring EMI data is essential. Figure 4 and Figure 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, thereby optimizing the input filter. It can be seen from the test results that the EMI performance of the U-shaped layout is better than that of the I-shaped layout, especially in the high-frequency part.
Figure 4: U-shaped EMI performance under phase shift control
Figure 5: Type I EMI performance under phase shift control
Adding EMI filters can effectively improve EMI performance. 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 the differential mode filter is less than 30MHz, and the noise range of the common mode filter is 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 filters and differential mode filters, respectively. The U-shaped layout can meet the CISPR 25 Type 3 standard, while the I-shaped layout does 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 I-type layout with differential mode and common mode filters
This paper compares two different PCB layouts of dual-output buck converters under phase-shift control. It can be seen that the EMI performance of the U-type layout is better than that of the I-type layout. For more information, please refer to TI’s official website application report “How SYNC Logic Affects EMI Performance for Dual-Channel Buck Converters”.