Power factor corrected switched mode power supply

A power factor corrected switched mode power supply including a phase shifter that senses the mains frequency and phase shifts it to produce a phase shifted signal which modulates the switching signal supplied to the switches based on the phase shifted signal. The rate of change of the frequency of the switching signal may be controlled to be greatest in regions of greatest power transfer.

RELATED APPLICATIONS

The present application is a 35 U.S.C. §371 national phase application of PCT International Application No. PCT/NZ2009/000169, having an international filing date of Aug. 17, 2009, claiming priority to New Zealand Patent Application No. 571084, filed Sep. 5, 2008. The disclosures of each application are incorporated herein by reference in their entireties. The above PCT International Application was published in the English language and has International Publication No. WO 2010/027276 A1.

FIELD OF THE INVENTION

This invention relates to a power factor corrected switched mode power supply having reduced electro-magnetic interference (EMI) noise.

BACKGROUND OF THE INVENTION

When the switches of a power factor corrected switched mode power supply are switched at a constant frequency there is a significant EMI noise signature at certain frequencies. It is known to modulate the switching frequency applied to the switches to spread the noise spectrum over a wider range. When compared to an un-modulated switching frequency, this reduces the average noise energy.

FIG. 1shows a prior art approach to modulating the switching frequency based on the phase of the rectified power supply (as per the application note AN833 for the L4981B integrated circuit produced by STMicroelectronics). Curve1shows the mains waveform and line2the switching frequency modulation. As shown inFIG. 1the switching frequency is reduced to 80% at the point of maximum power transfer and increased to 100% at minimum power transfer. This approach can reduce EMI noise by about 3 dB. The problem with this approach is that the rate of change of the switching frequency is minimized at the point of maximum power transfer and so much of the EMI noise power is confined to a narrow range.

It would be desirable to provide a power factor corrected switched mode power supply having reduced EMI noise or to at least provide the public with a useful choice.

EXEMPLARY EMBODIMENTS

According to one exemplary embodiment there is provided a power factor corrected switched mode power supply including:i. a power rectifier;ii. one or more switch for switching power supplied by the rectifier;iii. a phase shifter for sensing the mains frequency and phase shifting it to produce a phase shifted signal; andiv. a modulator which produces a switching signal supplied to the one or more switches wherein the frequency of the switching signal is modulated based on the phase shifted signal.

According to another exemplary embodiment there is provided a method of controlling the switching frequency of a power factor corrected switched mode power supply comprising:i. determining the mains frequency of power supplied to the converter;ii. phase shifting the mains frequency to produce a phase shifted signal;iii. controlling frequency modulation of a switching signal supplied to switches of the converter according to the phase shifted signal.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2illustrates the region3of 50% power transfer and the region4of 75% power transfer with respect to the mains waveform5and power transfer waveform6. It will be seen that most power is transferred near the peak of the mains waveform where the prior art approach had the lowest rate of frequency modulation.

FIG. 3shows a power factor corrected switched mode power supply according to a first embodiment including a line filter7for conditioning the mains supply, a power rectifier8, an inductor9, a switch10, an output diode11and an output capacitor12. Chip13(a L4981AD in this embodiment) supplies a switching signal to switch10(which may be one or more switches). A sensing rectifier consisting of diodes14and15provides a rectified mains signal to a voltage divider consisting of resistors16to20and a smoothing capacitor21(which could be replaced by a capacitive divider or a combination of capacitive and resistive elements). The voltage from the tap in the voltage divider is supplied to a phase shifting circuit formed by capacitors22and24and resistor23. The output of the phase shifter circuit is supplied to the ROSC terminal of chip13and controls the modulation of the switching signal as will be described below.

FIG. 5shows a mains waveform26and the phase shifted signal25supplied to the ROSC terminal of chip13. It will be seen that the modulation waveform25is twice the frequency of the mains and exhibits its maximum rate of change during periods of maximum power transfer. This spreads the EMI noise spectrum over a wider range. Waveform27illustrates another possible modulation strategy with modulation waveform27at mains frequency and lagging the mains by 90°.

Referring now toFIG. 4an alternative embodiment is shown, which is identical to the power factor corrected switched mode power supply shown inFIG. 3except that the passive phase shifter and chip13have been replaced with a digital controller28that performs the phase shift and/or control functions. The voltage sensor has been simplified to a resistive divider formed by resistors29and30. Otherwise like components have been given the same numbering as inFIG. 3. A digital signal processor may be used to implement the phase shift or more advanced signal processing characteristics such as modulating the switching frequency according to any desirable/arbitrary waveform.

It has been found that by maximizing the change in switching frequency with maximum power transfer that EMI noise can be reduced by an additional 3 dB over the prior art method described (i.e. 6 dB EMI noise reduction over a fixed switching frequency).