Switching Power Supply Control for Silent Light-Load Operation
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Solution Overview
Problem
Switching power supplies experience noise generation due to fluctuating switching frequencies during light load modes, which can fall into the human audible range, causing offensive sounds.
Innovation Solution
Implementing a silent light load mode in the control circuit that periodically turns on the synchronous rectification transistor during the off period to maintain a switching frequency above a predetermined lower limit, typically above 20 kHz, thereby suppressing noise generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If switching pulses are thinned out at light load to reduce switching loss, then energy efficiency is improved, but switching frequency drops to human audible range causing noise
Solution Approach 1:
The synchronous rectification transistor is periodically turned on during the off period at light load, creating a periodic action that maintains switching frequency above audible range while still reducing overall switching loss compared to continuous switching
Solution Approach 2:
The switching frequency parameter is dynamically adjusted by controlling the turn-on timing of the synchronous rectification transistor, changing it from a fixed low frequency to a variable frequency that stays above 20 kHz while minimizing switching operations
2Object-generated harmful factors
If load resistor is connected to increase load current and raise switching frequency, then noise is prevented, but power consumption increases
Solution Approach 1:
The synchronous rectification transistor utilizes the existing off-period current flow to maintain switching frequency, serving the dual purpose of noise prevention and efficient power management without requiring additional load resistors or extra power consumption
Solution Approach 2:
The synchronous rectification transistor performs multiple functions: rectification during normal operation and frequency maintenance during light load, eliminating the need for separate noise prevention circuitry and reducing overall power consumption
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively prevents noise generation while maintaining efficiency by ensuring the switching frequency remains above the human audible range, thus reducing switching power supply noise without decreasing efficiency.
Implementation Method 1
a switching frequency of the output transistor becomes lower as a load becomes lighter in an operation mode (so-called light load mode) in which output feedback control is performed
Implementation Method 2
periodically turn on a synchronous rectification transistor during an off period from a time at which both the output transistor and the synchronous rectification transistor are turned off to an on timing of the output transistor
Data Source
AI summary
A power supply control apparatus, includes: a driver configured to respectively drive an output transistor and a synchronous rectification transistor configured to generate an output voltage from an input voltage and supply the output voltage to a load; and a controller configured to, in a light load mode in which output feedback control is performed such that a switching frequency of the output transistor becomes lower as the load becomes lighter, during an off period from a time at which both the output transistor and the synchronous rectification transistor are turned off to an on timing of the output transistor based on the output feedback control, periodically turn on the synchronous rectification transistor within a range in which the switching frequency does not fall below a predetermined lower limit value.