Spread Spectrum PFM Control for DC-DC Converter Spur Reduction
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Solution Overview
Problem
DC-to-DC voltage converters operating in pulse-frequency modulation (PFM) mode generate significant spur energy at low frequencies, which can disturb other sensitive circuits in systems, particularly at low loads where efficiency drops and battery life is affected.
Innovation Solution
A control circuit monitors the output voltage and provides a random number of additional drive pulses to the switching transistor when it rises above an upper threshold, then ceases pulsing, and resumes pulsing when the voltage drops below a lower threshold, modulating the PFM frequency and reducing spur energy by spreading it.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If PFM mode is used to improve efficiency at low loads, then efficiency is improved, but spur energy is generated that disturbs other circuits
Solution Approach 1:
The patent applies dynamics by making the PFM switching frequency variable rather than fixed. The control circuit dynamically adjusts the switching frequency based on load conditions, preventing the system from operating continuously at a single frequency that generates harmful spurs. This dynamic frequency adjustment maintains efficiency improvements while reducing electromagnetic interference.
Solution Approach 2:
The patent employs periodic action through spread spectrum modulation, where the switching frequency is modulated according to a predetermined periodic function (such as sinusoidal or triangular). This periodic variation distributes the spur energy across multiple frequencies rather than concentrating it at one frequency, thereby maintaining low-load efficiency while reducing disturbances to other circuits.
2Ease of operation
If PWM mode is used for voltage regulation, then voltage regulation is achieved, but efficiency falls off rapidly at low loads
Solution Approach 1:
The patent transitions from static PWM duty cycle control to dynamic PFM frequency control. The system dynamically switches between PWM and PFM modes and adjusts the PFM switching frequency based on real-time load conditions, enabling efficient low-load operation while maintaining adequate voltage regulation through adaptive control.
Solution Approach 2:
The patent changes the control parameter from duty cycle (PWM) to switching frequency (PFM). By varying the switching frequency rather than the duty cycle, the system achieves better efficiency at low loads. The spread spectrum technique further modifies this parameter by introducing controlled frequency variation to reduce electromagnetic interference.
Data Source
AI summary
Methods and apparatus for operating a DC-to-DC voltage converter that has a power stage that includes at least one switching transistor. The output voltage of the DC-to-DC voltage converter is monitored. If the output voltage drops below a lower output voltage threshold, a series of drive pulses is provided to the at least one switching transistor to commence switching of the at least one switching transistor. If the output voltage rises above an upper output voltage threshold, a random number of additional drive pulses is provided to the at least one switching transistor and then the provision of drive pulses to the at least one switching transistor is ceased.