Synchronous Rectifier Control for Silent Light-Load Power Supplies
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Solution Overview
Problem
Switching power supplies generate offensive noise in light load modes due to fluctuating switching frequencies that can drop into the human audible range, causing input and output capacitors to produce sound.
Innovation Solution
A power supply control apparatus with a control circuit that includes a silent light load mode, where the synchronous rectification transistor is periodically turned on during the off period to maintain a switching frequency above a predetermined lower limit, preventing noise generation and maintaining efficiency.
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 into human audible range causing noise
Solution Approach 1:
The patent applies periodic action by implementing a silent light load mode that periodically turns on the synchronous rectification transistor during the off period. This periodic intervention maintains the switching frequency above the human audible range (20 kHz) while still allowing pulse thinning to reduce switching loss, thereby eliminating noise without sacrificing energy efficiency.
2Object-generated harmful factors
If load resistor is connected to increase load current and raise switching frequency, then noise is prevented, but power efficiency decreases
Solution Approach 1:
The patent uses a virtual load resistor function implemented through the synchronous rectification transistor rather than a physical load resistor. The control circuit creates a virtual resistance effect by periodically switching the transistor, which raises the switching frequency to prevent noise without the continuous power dissipation that would occur with an actual load resistor connected to increase load current.
3Object-generated harmful factors
If switching frequency is maintained above lower limit to prevent noise, then noise generation is suppressed, but switching loss increases
Solution Approach 1:
The patent applies dynamics by making the switching frequency adaptive rather than fixed. The control circuit dynamically adjusts the switching frequency based on operating conditions: during normal operation, frequency is maintained at optimal levels for efficiency, while during light load conditions, the silent light load mode periodically activates to boost frequency only when needed to stay above the 20 kHz threshold, minimizing unnecessary switching losses.
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.