ADI's Silent Switcher has proven to be helpful in EMI management in power design.

On the 14th, e4ds news held an EEWebinar on the topic of 'Optimal power design solution considering EMI'. In this webinar, Diarmuid Carey of ANALOG DEVICES presented on "Maximizing Power Supply Performance While Minimizing EMI."

Diarmuid Carey states that there are low frequency components that appear as output voltage ripple due to the switching frequency cycles, which typically range from hundreds of kHz to several MHz, and high frequency harmonics are caused by very fast switching transitions in the region of 10 MHz to 200 MHz, which produces low rise times of 5 ns, and a lot of energy is generated because the switch node can go from 0 V to several volts, and 12 V, 24 V, or 48 V in just a few nanoseconds.

This also manifests in the time domain, as seen in the blue plot at the top of the screen as switch node ringing, and it is noted that switching frequency is often not as strongly correlated with noise as switching transition times, as switching transitions are the core of the noise generation.

Additionally, the downside of fast switching transitions is mostly the noise effect due to trace inductance as seen in the previous slide, but fast switching transitions can also be a big problem, in which case the power supply may not work at all due to this voltage offset, or in most cases the power supply will work but generate a lot of EMI.

Power supply designers can then determine the switching time speed and make adjustments to improve it, such as selecting a switching regulator optimized for these applications or using resistors in the gate drive to reduce the switching time, he advised.

Another thing that the designer can control is optimizing the board layout to minimize trace length, but this is a very tricky and difficult to achieve task, and keeping traces as short as possible minimizes noise.It is the most important layout rule in terms of sound quality, and it helps significantly reduce EMI and noise, he said.

Along with this, it was mentioned that it is very important to connect the ground of the input capacitor in the buck regulator to the freewheeling diode or low side FET ground with a very short trace.

Traditional methods for reducing EMI include slowing down the switching edges of the switching driver or using a simple RC snubber from the switch to ground to leave a footprint on the board and then slowing down the switching speed.

Additionally, some devices feature spread spectrum frequency modulation, which modulates the switching frequency to help reduce peak radiated emissions. While the energy levels remain the same, the modulated switching flattens the peak energy across a wider frequency range, he said.

Additionally, he noted that there are devices with built-in filters and unique devices with special shielding capabilities, including EMI.

Finally, the company revealed a silent switcher solution that provides low EMI without compromising efficiency or reducing switching frequency.

Silent Switcher cannot make the loop size zero, but it can cancel it out by creating a similar identical loop with opposite poles and magnetic fields. This helps suppress the size of the radiated emissions by creating two low-power current flow paths with opposing magnetic fields that cancel each other out, he said.

He also said that layout is a critical part of any power supply design, whether or not a silent switcher is used, as misplaced components can lead to incorrect layer stackup and parasitics.