Programmable Clock Dithering Circuit for EMI Reduction
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Solution Overview
Problem
Existing methods for reducing Electromagnetic Interference (EMI) in power converters, such as using bulky filters or precision components, are costly, bulky, and not suitable for integration in portable devices, and there is a need for a more flexible and cost-effective solution that can be integrated with other circuits.
Innovation Solution
A frequency dithering circuit using a digital-to-analog converter (DAC) to modulate the clock frequency, reducing the need for bulky filters and precision components, allowing for integration and a small form factor, while also enabling programmability of the dithering degree.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If traditional filtering components (inductors, capacitors) are used to reduce EMI, then EMI reduction effectiveness is improved, but device size and cost increase
Solution Approach 1:
The patent changes the operating parameters of the power converter by dynamically adjusting the switching frequency around a nominal value. This frequency modulation spreads the EMI energy across a broader spectrum, reducing peak emissions without requiring traditional filtering components. The frequency deviation is controlled to remain within acceptable operational limits while achieving EMI reduction.
Solution Approach 2:
The patent replaces mechanical/physical filtering components (inductors, capacitors, shields) with a digital control mechanism. A frequency dithering circuit generates controlled frequency variations, and a feedback system monitors and adjusts the switching frequency dynamically. This substitution eliminates bulky passive components while maintaining EMI reduction effectiveness through active parameter control.
2Object-affected harmful factors
If precision components are used for EMI filtering, then EMI reduction effectiveness is improved, but manufacturing cost increases
Solution Approach 1:
Instead of using precision-matched passive components with tight tolerances, the patent achieves EMI reduction through dynamic parameter adjustment. The frequency dithering circuit uses standard components with relaxed tolerances, controlling EMI through programmed frequency variation rather than precise component values. This dramatically reduces component cost and simplifies manufacturing.
Solution Approach 2:
The patent replaces expensive precision components with inexpensive standard components. The frequency control is achieved through digital logic and simple analog circuitry rather than precision inductors, capacitors, or resistors. The system uses readily available, low-cost parts that can be easily manufactured and replaced if needed.
3Object-affected harmful factors
If spread spectrum frequency modulation is used to reduce EMI, then EMI peak intensity is reduced, but frequency stability deteriorates
Solution Approach 1:
The patent implements periodic frequency modulation around a nominal switching frequency. The frequency deviates sinusoidally or triangularly from the center frequency in a controlled manner, ensuring that the average frequency remains stable while peak EMI emissions are reduced through spectral spreading. The modulation depth and frequency are carefully selected to maintain both EMI reduction and frequency stability.
Solution Approach 2:
The patent employs a feedback control system that monitors the actual switching frequency and adjusts it to maintain the desired frequency trajectory. The feedback ensures that the frequency modulation remains within specified bounds and that the average frequency stays at the nominal value, preventing drift and maintaining stability while achieving EMI reduction through controlled frequency variation.
Data Source
AI summary
A frequency dithering circuit reduces emissions that cause Electro-Magnetic Interference (EMI) by spreading the spectrum of a clock. The clock sequences a counter that drives a digital count value to a digital-to-analog converter (DAC). The DAC outputs a sawtooth wave with a wide voltage swing. A subtractor scales down the voltage swing to produce a reduced-swing sawtooth wave which is used as an upper limit voltage. Comparators trigger a set-reset latch to toggle the clock when current pumps charge and discharge a capacitor beyond voltage limits. Since the upper limit voltage is the reduced sawtooth wave from the subtractor, the amount of time to charge the capacitor varies, dithering the period of the clock. The degree of dithering can be adjusted by programming the feedback resistance in the subtractor. The subtractor reduces the sensitivity of dithering to errors in the DAC, allowing for an inexpensive, less precise DAC.