Novel technique improves dithering, reduces EMI

Typical off-line PFC
In off-line ac-to-dc converter dithering, the pulse width modulator (PWM) frequency has proven to reduce average EMI by changing it from narrowband to broadband. This article covers a novel technique to dither the switching frequency 20 percent of an off- line power factor corrected (PFC) pre-regulator. The circuitry used to dither the frequency is accomplished by taking advantage of the PWM’s internal timing circuitry.

Typical Off-line PFC Pre-Regulator
Figure 1 is a schematic of a 250W ac-to-dc off-line power converter with PFC that was designed for a Universal input voltage. This application is similar to many off-line converters that use a pulse width modulator (PWM) to manage the power converter. Many of these PWMs develop timing through internal circuitry. It is this internal circuitry that can be taken advantage of to dither the converters switching frequency to a lower narrowband EMI.

Figure 1 - 250W ac-to-dc off-line power converter with PFC designed for a Universal input voltage

Frequency Dithering
The frequency dithering technique reduces the narrowband EMI by spreading out the power supply’s noise spectrum. There are limitations on how much the oscillator frequency (fs) can be dithered. Some of the limiting factors are switching losses and magnetic design. To keep the boost inductor as small as possible and to keep the switching losses in-check, the frequency dithering should be no more than 20 to 30 percent of the fundamental frequency. The circuitry featured here was designed to vary the PWM frequency from 80 percent to 100 percent as a function of the input voltage sine wave. At input line crossover, the oscillator frequency will be at 80 percent of its maximum and will increase-and-decrease proportionally with changes in the line voltage. Figure 2 shows how fs varies with input voltage.

Figure 2. - Frequency Dithering with Rectified Line Voltage