Switching Power Supply Spread Spectrum Ripple Control
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Solution Overview
Problem
Switching power supply apparatuses experience increased output ripple due to frequency fluctuations during spread spectrum operations, which are used as a countermeasure for electromagnetic interference, especially in vehicular applications where high voltage accuracy and reduced noise are critical.
Innovation Solution
A spread spectrum correction circuit generates an offset signal to stabilize the duty ratio of the PWM signal by adjusting the slope signal or control AMP signal in response to switching frequency fluctuations, ensuring consistent output voltage through equations like ΔV=(VOUT/VIN)×(T2−T)×GiV×(S+Se), thereby preventing ripple occurrence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If spread spectrum operation is performed to reduce electromagnetic interference, then EMI is reduced, but output ripple increases
Solution Approach 1:
The control circuit preemptively adjusts the PWM duty ratio in opposite direction to the frequency fluctuation before the ripple can occur. When frequency decreases, the controller increases duty ratio; when frequency increases, it decreases duty ratio. This preliminary counter-action prevents the duty ratio from remaining constant during frequency changes, thereby preventing output ripple before it happens while maintaining the spread spectrum EMI reduction effect
Solution Approach 2:
The control circuit continuously monitors the switching frequency and dynamically adjusts the PWM duty ratio based on frequency fluctuations. By implementing feedback control that detects frequency changes and responds by adjusting duty ratio accordingly, the system maintains stable output voltage despite spread spectrum operation, resolving the contradiction between EMI reduction and ripple suppression
2Object-affected harmful factors
If switching frequency is fluctuated for spread spectrum, then EMI noise peaks are reduced, but voltage accuracy deteriorates
Solution Approach 1:
The control circuit preemptively compensates for voltage deviations caused by frequency fluctuations. By adjusting the PWM duty ratio in opposite direction to frequency changes before significant voltage deviation occurs, the system maintains accurate output voltage while preserving the spread spectrum EMI reduction benefit
Solution Approach 2:
The control circuit uses feedback to continuously monitor output voltage and switching frequency, then dynamically adjusts duty ratio to maintain voltage accuracy despite frequency fluctuations. This feedback mechanism ensures voltage precision is maintained during spread spectrum operation while EMI noise peaks are reduced
Data Source
AI summary
A switching power supply apparatus of each embodiment includes: a switch circuit configured to supply a pulse output to a load; a drive signal control circuit configured to perform ON/OFF driving operation of the switch circuit according to a duty ratio of a PWM signal; an error detection amplifier configured to compare output voltage supplied to the load and reference voltage and generate an error detection signal; a comparator configured to generate the PWM signal by comparison between a slope signal that starts level change in a cycle of the PWM signal and the error detection signal and give the PWM signal to the drive signal control circuit; and a spread spectrum correction circuit configured to generate an offset corresponding to the fluctuation of the cycle of the PWM signal and add the generated offset to the slope signal or the error detection signal.