Power Factor Corrected Switched Mode Power Supply EMI Noise Reduction
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Solution Overview
Problem
Existing power factor corrected switched mode power supplies with constant switching frequency exhibit significant EMI noise, particularly confined to a narrow range during maximum power transfer, limiting noise reduction effectiveness.
Innovation Solution
A power factor corrected switched mode power supply that modulates the switching frequency based on a phase-shifted mains frequency signal, maximizing the rate of change during maximum power transfer to spread the EMI noise spectrum over a wider range, using a phase shifter and modulator to control the switching signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the switching frequency is modulated based on the phase of the rectified power supply (prior art approach), then the EMI noise is reduced by about 3 dB, but the rate of change of the switching frequency is minimized at the point of maximum power transfer causing much of the EMI noise power to be confined to a narrow range
Solution Approach 1:
The patent inverts the prior art approach by using the derivative of the phase-shifted rectified power supply waveform instead of the phase waveform itself for frequency modulation. This inversion causes the maximum rate of change to occur at maximum power transfer rather than minimum, spreading EMI noise across a wider frequency range while achieving 6 dB total noise reduction (3 dB from modulation + 3 dB additional from optimized modulation timing)
Solution Approach 2:
The patent changes the modulation parameter from using the phase waveform directly to using its derivative (rate of change). This parameter transformation shifts the modulation characteristics so that maximum frequency modulation occurs at maximum power transfer, optimizing the spread of EMI noise spectrum and achieving superior noise reduction performance
2Device complexity
If a constant switching frequency is used, then the power supply operation is simple, but significant EMI noise signature appears at certain frequencies
Solution Approach 1:
The patent transitions from a static constant switching frequency to a dynamic frequency modulation scheme. The switching frequency is continuously adjusted based on the derivative of the phase-shifted rectified power supply waveform, creating a dynamic control system that spreads EMI noise across a wider frequency range while maintaining manageable complexity through automated modulation
3Object-affected harmful factors
If the switching frequency is modulated to spread noise spectrum, then the average noise energy is reduced, but the rate of change of frequency must be maximized during maximum power transfer for optimal performance
Solution Approach 1:
The patent implements feedback by continuously monitoring the phase-shifted rectified power supply waveform and using its derivative to adjust the switching frequency in real-time. This closed-loop approach ensures that the modulation timing automatically aligns with the power transfer characteristics, achieving optimal EMI noise spreading without requiring manual precision tuning
Data Source
AI summary
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.