For example, at a 400 kHz switching frequency (as opposed to 2 MHz), in addition to increasing the size of the inductor and capacitor, the inductors and capacitors in the EMI filter must also be relatively large to pass the conducted EMI standard required in automotive applications. One reason is that they must not only attenuate the switching fundamental frequency at 400 kHz, but all of its harmonics up to 1.8 MHz. A regulator operating at 2 MHz does not have this problem. Figure 2 shows the size of a 2 MHz solution against a 400 kHz solution.
Shielding might be a last ditch remedy to reduce radiated emissions, but shielding takes space that might not be available in the application and would require additional mechanical design and test iterations.
To avoid the AM frequency bandwidth and maintain a small solution size, a 2 MHz or higher switching frequency is preferred in automotive applications. With the AM band avoided, it is just a matter of ensuring that higher frequency noise—also known as harmonics—and switch ringing are also minimized. Unfortunately, high frequency switching usually results in increased radiated emissions from 30 MHz to 1 GHz.
There are switching regulators that feature fast and clean switching edges, which reduce EMI, such as the Silent Switcher® devices in ADI’s Power by Linear™ line. First, though, let’s look at some other features that can help.
Spread spectrum frequency modulation (SSFM) is a technique that dithers the system clock within a known range, thus distributing the EMI energy over the frequency domain. Although the switching frequency is often chosen to be outside the AM band (530 kHz to 1.8 MHz), unmitigated switching harmonics can still violate stringent automotive EMI requirements within the AM band. Adding SSFM significantly reduces EMI within the AM band and other regions.
Figure 3 shows an ultralow EMI and high efficiency 12 V to 5 V/5 A converter that operates at a switching frequency at 2 MHz using the LT8636 Silent Switcher monolithic buck regulator. Figure 4 shows conducted and radiated EMI performance for a tested demonstration circuit at a 14 V input and an output of 5 A at 5 V. At the front end, a small inductor and ceramic cap helps to filter out the conducted noise, while the ferrite bead and the ceramic capacitor help to reduce the radiated noise. Two small ceramic caps are placed to the input and ground pins to minimize the area of the hot loop, while also splitting the hot loop, which helps to cancel out high frequency noise.