Time-Varying Capacitance for EMI Reduction in Quasi-Resonant Converters
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Solution Overview
Problem
In quasi-resonant converters, implementing effective frequency jitter to reduce electromagnetic interference (EMI) is challenging due to the limited spread of switching frequencies during discontinuous conduction mode, which restricts the effectiveness of jittering techniques.
Innovation Solution
A power converter with a time-varying capacitance coupled in parallel to the power switch, allowing the switching frequency to be varied by adding capacitance at peak points of the resonant waveform, thereby changing the oscillation period and spreading the switching frequency across a larger range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If frequency jitter is implemented by varying switching frequency in traditional flyback converter, then EMI is reduced, but the effectiveness is limited due to narrow frequency spread in discontinuous conduction mode
Solution Approach 1:
The patent applies dynamics by making the capacitance value time-varying during the discontinuous conduction mode. The capacitance C(t) is dynamically adjusted based on the resonant waveform characteristics, allowing the oscillation period and switching frequency to vary continuously. This dynamic capacitance modulation enables a wider frequency spread compared to static frequency jittering methods, thereby improving EMI reduction effectiveness while adapting to the resonant process in QR converters.
Solution Approach 2:
The patent changes the electrical parameter (capacitance) during operation to achieve frequency variation. By introducing a time-varying capacitance that modulates the resonant frequency during discontinuous conduction mode, the system achieves a broader frequency spectrum. This parameter change approach allows the switching frequency to spread over a larger range, directly addressing the limitation of traditional frequency jittering in QR converters.
2Adaptability or versatility
If a time-varying capacitance is added in parallel to the power switch, then the switching frequency spread is increased, but the device complexity increases
Solution Approach 1:
The patent introduces a time-varying capacitance as an intermediary element between the power switch and ground. This capacitance acts as a mediator that modifies the resonant characteristics during discontinuous conduction mode without fundamentally changing the power converter topology. The capacitance can be implemented using existing circuit elements or simple additional components, minimizing the increase in device complexity while achieving the desired frequency spread expansion.
Solution Approach 2:
The time-varying capacitance serves multiple functions: it extends the frequency spread for EMI reduction, maintains compatibility with existing QR converter topology, and utilizes the natural resonant waveform of the discontinuous conduction mode. By making this single element time-varying, the patent achieves frequency modulation without requiring complex additional circuitry, thus balancing the trade-off between frequency spread and device complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively reduces EMI by introducing frequency jitter in the switching frequency, enhancing the efficiency of EMI reduction in quasi-resonant converters.
Implementation Method 1
Under discontinuous conduction mode (DCM), the flyback converter has an LC resonant waveform during the discontinuous time after the secondary side current is discharged
Data Source
AI summary
A Quasi-Resonant (QR) converter includes a power switch controlling the primary current flow and a time-varying capacitance coupled in parallel to the power switch. The time-varying capacitance is configured to add a frequency jitter to the frequency switch of the converter.