Jittering Switching Frequency via Current Limit Modulation
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Solution Overview
Problem
Conventional switched mode power converters often emit excessive Electromagnetic Interference (EMI) that exceeds safety regulation standards due to constant switching frequencies, leading to interference on fundamental and harmonic levels.
Innovation Solution
Introducing frequency jitter by perturbing the current limit in each switching cycle, which varies the switching frequency without using a variable frequency oscillator, thereby spreading energy across different frequencies and reducing audio noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a constant switching frequency is used in a switched mode power converter, then the converter operates efficiently with simple control, but excessive EMI interference is generated that exceeds safety regulation standards
Solution Approach 1:
The patent applies dynamics by modulating the current limit signal to intentionally vary the switching frequency around a nominal value. This creates frequency jitter that spreads EMI energy across a broader spectrum, reducing peak interference levels. The switching frequency becomes dynamic rather than constant, resolving the contradiction between simple constant-frequency operation and EMI reduction.
Solution Approach 2:
The patent changes the switching frequency parameter by modulating the current limit signal. This parameter change transforms the fixed frequency operation into variable frequency operation, which distributes electromagnetic energy across multiple frequencies and reduces the intensity of interference at any single frequency, thereby meeting EMI standards.
2Object-generated harmful factors
If the switching frequency is varied to reduce EMI, then EMI interference is reduced, but additional complexity is introduced through variable frequency oscillators or control circuits
Solution Approach 1:
The patent makes the existing current limit circuit multi-functional by having it perform both its traditional current protection function and the additional function of frequency modulation for EMI reduction. This eliminates the need for separate variable frequency oscillator circuits, reducing overall system complexity while achieving EMI mitigation.
Solution Approach 2:
The current limit circuit serves itself by generating the modulation signal needed for frequency variation. Instead of requiring an external control circuit to generate frequency variations, the current limit circuit inherently produces the modulated signal that varies the switching frequency, thereby reducing EMI without adding external complexity.
Data Source
AI summary
A controller includes a current limit generator that generates a current limit threshold. A switch controller receives the current limit threshold, an enable signal, and a current sense signal representative of a current through a power switch. The switch controller generates a drive signal to control switching of the power switch to control the transfer of energy from an input of the power converter to an output of the power converter in response to the enable signal and the current sense signal. The switch controller outputs the drive signal to turn on the power switch in response to the enable signal and turns off the power switch when the current sense signal reaches the current limit threshold. A jitter generator generates a jitter signal for jittering a switching period of the drive signal. The current limit threshold is modulated by the jitter signal.