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Because of the high level of output power that switching power-supply designs have, systems designers are challenged to develop designs that meet consumer demands for greater functionality and, at the same time, meet regulatory agency requirements for electromagnetic compliance.
Fixed switching frequency is a main component of conducted and radiated noise. A spectrum analyzer reveals a high amplitude peak at the switching frequency of the supply. Designers are often forced to add additional shielding, snubbing and filtering to designs in order to control the noise. While these protections may help systems meet electromagnetic compatibility (EMC) requirements, they require larger circuits and the addition of components, thus increasing overall design costs.
The good news is that, based on the spread-spectrum communications concept, it is possible to vary a power supply's switching frequency over time. This technique, known as “dithering,” reduces the noise at any single frequency by spreading it out over a range of frequencies. As long as the switching frequency changes rapidly, the amplitude of the spectral peaks will decrease.
Rather than using a voltage-controlled oscillator and the randomly fluctuating voltage across a noise diode, a pseudo-random clock generator is easily created using the tunable oscillator of a small, low-cost microcontroller such as Microchip Technology's PIC10F206. This microcontroller has the ability to provide the instruction clock as an output.
Furthermore, the frequency of the microcontroller's internal oscillator is tunable via the oscillator calibration (OSCCAL) register. Loading the OSCCAL register with a new value causes the frequency of Microchip Technology's PIC microcontroller to shift until it reaches the desired value. By continuously updating the calibration register with a new, pseudo-random value, the oscillator frequency continuously shifts between frequencies. This effect is shown in Fig. 1, which displays spectral plots of the clock output for both the dithered and nondithered clocks.
Using the full 7-bit calibration range of the OSCCAL register allows for frequency values as low as 600 kHz and as high as 1.2 MHz. Although these minimum and maximum frequencies are not guaranteed, a significant frequency shift is obtained using this technique. Additionally, by modifying the pseudo-random number-generation routine, it is possible to determine the mean center frequency and frequency deviation.
Besides dithering, microcontrollers can implement useful power-supply functions such as undervoltage protection (using an onboard comparator and reference), shutdown (using a digital input with internal pull-ups), soft-start (using a software pulse-width-modulation routine) and overtemperature protection (using an analog temperature sensor and comparator). An example schematic is shown in Fig. 2.
For more information on clock dithering and example software for pseudo-random number generation, see Microchip Technology Application Note AN544, available at www.microchip.com/AN544.