DC-to-DC Converter Combats EMI
In Figure 1, the switching regulator IC (U1) has an external clock input. Driving this input with a digital signal of pseudo-random noise (PN) provides the regulator with a spread-spectrum clock that reduces EMI. By spreading interference frequencies over a wide range, this technique lowers the EMI power density that is otherwise concentrated at a single clock frequency.
Figure 1. To reduce EMI, this conventional step-up DC-DC converter employs spread-spectrum pulse-width modulation (SSPWM) produced by the PN clock input.
The PN generator (Figure 2) spreads interference over a wide spectrum. Its key element is two 8-bit shift registers (U2 and U3) connected in series to form a 16-bit shift register, with feedback from the XOR gate U4A. The result is an almost random (pseudo-random) output, consisting of a repeating sequence of ones and zeroes at a nominal frequency of 650kHz. The D flipflop (U5) divides this frequency by two, producing a nominal 325kHz spread-spectrum clock signal to the switching regulator.
Figure 2. This generator of pseudo-random noise (PN) produces a nominal 325kHz clock signal for the Figure 1 circuit.
Bench measurements show a 15dB reduction in peak power-density at about 300kHz. Except for 9mA of extra current drawn by the PN generator, the regulator's efficiency remains unchanged. (The efficiency is 94% while delivering 0.5A with a 3.6V input and 5V output.) Ripple amplitude in the time domain also remains unchanged. Output spectra show that a conventional fixed-frequency clock (Figure 3) produces considerably more noise than does the spread-spectrum technique (Figure 4).
Figure 3. This output-noise spectrum is produced by the Figure 1 circuit operating with a fixed-frequency control scheme.
Figure 4. An SSPWM control scheme produces less output noise in the Figure 1 circuit than does the conventional fixed-frequency approach.
A similar version of this article appeared in the November 5, 2001 issue of Electronic Design magazine.