Design Articles

One of the greatest challenges in designing switching power supplies is limiting its Electromagnetic Interference (EMI) with other electronics. Beyond layout techniques to minimize high di/dt current loop areas and snub high dv/dt nodes, extensive filtering, slowing switching edges, and shielding, one method for limiting such interference and/or becoming certified by regulatory agencies is to spread the frequency spectrum of the main oscillator, also called adding “frequency dither” to the oscillator. This small variation in switching frequency decreases the narrowband emissions by changing the frequency content of the converter from a relatively narrowband spectrum to more wideband. To demonstrate how to design such a dither circuit, this article will showcase a simple example using a TLV3201 comparator to add dither to a UCC28950 phase-shifted full-bridge controller. This article is intended as more of a practical example. For a detailed observation on the theory that lends to this article, please see the provided references.

Background

Though the electromagnetic emissions of a switch-mode power supply are indeed spread across many frequencies by using a square-wave pulse, the greatest energy is at the fundamental frequency. If we were to slightly vary the fundamental frequency , we would see that the peak energy would be shared among multiple fundamental frequencies. The total EM energy remains the same, but narrowband energy has been decreased.

In essence, we are seeking to modulate our fundamental frequency slightly. For the UCC28950, the switching frequency is set by the choice of resistor value at the Rt pin. With the controller set as a master, this resistor is tied from the Rt pin to the Vref pin which supplies a 5V source. For a slave configuration the Rt resistor is tied to ground. The Rt pin itself has an internal 2.5V source and therefore the current into or out of the Rt pin(depending on Master/Slave configuration) determines the switching frequency of the controller. This frequency-setting method is very similar to most power-supply controllers available and thus this dithering circuit can apply to nearly all modern power controllers. Our practical objective will therefore be to apply a time-varying current signal to modulate the dc-current set by the Rt resistor to add a small periodic variation in the main controller frequency.