Spread Spectrum Control for Power Converter EMI Reduction
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Solution Overview
Problem
Constant on-time power converters experience electromagnetic interference (EMI) issues due to their switching frequency, which can degrade the performance of adjacent components.
Innovation Solution
A spread spectrum control scheme is implemented, using a PWM generator to vary the switching frequency of the power converter, distributing spectral energy evenly over a wide bandwidth by modulating the on-time of the high-side switch based on a comparison between a ramp signal and a sinusoidal waveform reference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If constant on-time control scheme is employed to achieve fast transient response, then switching frequency concentrates at specific frequency, but electromagnetic interference (EMI) issues occur due to concentrated spectral energy
Solution Approach 1:
The patent applies periodic modulation to the switching frequency by comparing a ramp signal with a sinusoidal waveform reference signal. This periodic variation in switching frequency spreads the concentrated spectral energy across a wider bandwidth, reducing EMI while maintaining the fast transient response characteristics of constant on-time control
Solution Approach 2:
The patent dynamically changes the switching frequency parameter by modulating the on-time of the high-side switch based on the comparison between ramp signal and sinusoidal reference. This parameter change transforms the fixed switching frequency into a varying frequency, distributing spectral energy and mitigating EMI issues
2Device complexity
If switching frequency is concentrated at specific frequency for simple control, then control circuit remains simple, but spectral energy concentrates causing EMI degradation
Solution Approach 1:
The patent introduces periodic modulation using sinusoidal waveform reference signals to vary the switching frequency. This approach spreads spectral energy to reduce EMI while adding minimal complexity to the control circuit, as it only requires adding a sinusoidal reference generator and comparison logic to the existing constant on-time control
Data Source
AI summary
A power converter includes a first switch and a second switch connected in series between an input power source and ground, an inductor connected between a common node of the first switch and the second switch, and an output capacitor and a pulse width modulation (PWM) generator configured to generate a gate drive signal for the first switch, wherein a leading edge of the gate drive signal is determined by a comparison result between a reference and a voltage proportional to an output voltage of the power converter and a trailing edge of the gate drive signal is determined by a comparison result between a voltage ramp and a variable voltage source.