Adaptive On-Time Converter Circuit for Spread-Spectrum EMI Reduction
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Solution Overview
Problem
Switching voltage converters generate significant electromagnetic interference (EMI) due to their fixed switching frequency, which can lead to both conducted and radiated EMI, affecting the performance and efficiency of the converter systems.
Innovation Solution
The implementation of a circuit that modulates the switching frequency by varying the discharge current linearly within a fixed range, allowing for a linear dithering of the working frequency, thereby reducing EMI through the use of adaptive on-time modulation and current source configurations that maintain a volt-second balance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If switching voltage converters operate at a fixed switching frequency, then the power conversion efficiency is maintained, but electromagnetic interference (EMI) increases due to harmonic noise peaks
Solution Approach 1:
The patent implements spread spectrum frequency modulation by dynamically varying the switching frequency around a center frequency using a triangle wave modulation signal. The switching frequency is adjusted based on the sum of the triangle wave signal and a dither signal, transforming the fixed frequency operation into dynamic frequency variation. This reduces EMI by distributing energy across a frequency range rather than concentrating it at a single frequency.
Solution Approach 2:
The patent changes the switching frequency parameter by adding a modulation signal to the base switching frequency. The modulated switching frequency is expressed as fsw = fcenter + Δf, where Δf is the frequency deviation determined by the triangle wave amplitude. This parameter change transforms the EMI spectrum from peaked harmonics to a flattened distribution across a broader frequency range.
2Object-affected harmful factors
If spread spectrum frequency modulation is implemented, then EMI is reduced through frequency variation, but the circuit complexity increases due to additional modulation components
Solution Approach 1:
The triangle wave generator circuit serves multiple functions: it generates the frequency modulation signal for spread spectrum operation, provides the dither signal for further frequency variation, and can be integrated with the existing switching control logic. By making this single circuit perform multiple functions, the patent reduces the need for separate dedicated modulation circuits, thereby limiting the increase in overall circuit complexity.
Solution Approach 2:
The patent combines the frequency modulation function with the existing switching control circuitry by integrating the triangle wave generator and modulator into the control path. The modulated switching signal is generated by combining the base switching signal with the frequency modulation signal through addition, merging the spread spectrum function with the standard PWM control architecture rather than requiring completely separate circuitry.
3Object-affected harmful factors
If the switching frequency is varied over a range of frequencies, then EMI is reduced by blending harmonic energy, but the control precision becomes more difficult to maintain
Solution Approach 1:
The patent incorporates feedback mechanisms to maintain control precision during frequency modulation. The switching frequency is modulated based on a controlled triangle wave signal, and the feedback loop adjusts the duty cycle and timing to ensure accurate voltage conversion ratios are maintained despite the frequency variations. This feedback ensures that the spread spectrum operation does not compromise the precision of voltage regulation.
Solution Approach 2:
The patent uses a relatively small frequency deviation (Δf) around the center frequency (fcenter), typically keeping the modulation depth within a limited range such as ±5% to ±10% of the center frequency. This partial action approach provides sufficient EMI reduction by spreading the energy over a moderate bandwidth while maintaining tight control over the switching frequency, avoiding the need for extreme frequency variations that would be difficult to control precisely.
Data Source
AI summary
Described embodiments include a circuit for adapting the on time in a switching voltage converter that includes a first transistor having a current terminal coupled to an output voltage terminal and to its control terminal. A second transistor is coupled between the first transistor and a ground terminal, and has a control terminal coupled to the first transistor. A third transistor is coupled between the output voltage terminal and a capacitor, and has a third control terminal coupled to the first control terminal. A current source is configured to provide a current that varies linearly between a first value and a second value. A fourth transistor is coupled between terminals of the capacitor, and has a fourth control terminal. A comparator has a first comparator input coupled to the capacitor. A logic circuit has an input coupled to the comparator output, and an output coupled to the fourth control terminal.