3 What Causes EMI in a Switched-Mode DC/DC Regulator?

In battery-powered systems (see Figure 3-1), a switched-mode power supply like a buck converter commutates a switch node between a low voltage (GND) and an input voltage. Filtering the switch-node voltage generates an average DC output voltage, which is between the input voltage and GND in the case of a buck converter. The switching causes input ripple at the fundamental switching frequency, and the square edges result in higher-frequency harmonics. Sharper edges with faster slew rates generate higher-frequency harmonic energy. The trapezoidal input current ripple is discontinuous and exhibits the same high-frequency components in the current ripple.

Parasitic capacitances and inductances are nonideal properties of components in the circuit. These parasitics interact negatively with the voltage and current and generate very-high-frequency spikes and ringing noise. On the other hand, jitter and dithering (spread spectrum) can create low-frequency oscillation and noise. Figure 3-2 shows common frequency ranges where noise occurs from these sources.

Figure 3-1 Simplified buck controller schematic and operating waveforms.

Figure 3-2 EMI frequency ranges for spread spectrum and jitter, switching frequency and harmonics, and switch ringing.

This document discusses advanced power-converter features that improve upon existing methods to further reduce EMI. The five features entail the use of:

• An active EMI filter (AEF) to improve passive EMI filter performance.
• Dual-random spread spectrum (DRSS) to improve triangular and random spread spectrum.
• True slew-rate control to improve the series boot resistor technique.
• HotRod™ package technology to improve standard wire-bond connections.
• Integrated capacitors to improve external decoupling capacitor performance.