Synchronous Rectification Control Circuit for Switching Loss Reduction
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Solution Overview
Problem
Existing synchronous rectification circuits face accuracy degradation and increased switching losses due to input offset voltage in comparators and poor reverse recovery performance of MOSFETs, which affects the operation in discontinuous conduction mode (DCM) and overall efficiency of switching mode power supplies.
Innovation Solution
A synchronous rectification control method that sets a count value when the rectification switch is turned on, generates an off enable signal after a delay, and compares the drain-source voltage against a reference voltage to adjust the delay time, ensuring the rectification switch is turned off when the inductor current is zero, thereby reducing switching losses and improving detection accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the rectification switch is turned off when inductor current is zero (DCM mode), then switching losses are reduced and electromagnetic losses are reduced, but accuracy degradation occurs due to input offset voltage in comparators
Solution Approach 1:
The patent changes the reference voltage parameter from zero to a small positive value (greater than the comparator's input offset voltage). This parameter change ensures that the comparator can accurately detect when the inductor current reaches zero despite the presence of offset voltage, thereby maintaining measurement precision while enabling DCM operation for reduced switching losses
Solution Approach 2:
The patent introduces a reference voltage as an intermediary element in the comparison process. Instead of directly comparing the inductor current to zero, the system compares it to a reference voltage that compensates for the comparator's offset voltage. This intermediary reference voltage acts as a mediator that eliminates the detection error caused by the offset voltage
2Stability of the object's composition
If the rectification switch is turned off when inductor current is zero, then current reverse flow is prevented and stability is improved, but device complexity increases due to additional control circuits
Solution Approach 1:
The patent merges the zero-current detection function with the existing comparator circuit used for PWM modulation. By utilizing the same comparator to perform both PWM generation and zero-current detection (with appropriate reference voltage), the system achieves improved stability through DCM operation without significantly increasing control circuit complexity
Solution Approach 2:
The comparator circuit is designed to serve multiple functions: it generates PWM modulation signals and simultaneously detects zero-current conditions. This multi-functional design allows the system to achieve stable DCM operation while avoiding the need for separate dedicated zero-current detection circuits, thereby limiting the increase in device complexity
3Ease of operation
If a fixed delay time is used to turn off the rectification switch, then control simplicity is maintained, but switching losses increase due to inaccurate zero-current detection
Solution Approach 1:
The patent implements a feedback mechanism where the comparator continuously monitors the inductor current and compares it against a reference voltage. When the current reaches the reference level (indicating zero current), the comparator output changes state, automatically triggering the turn-off of the rectification switch. This feedback-based approach eliminates the need for fixed delay timing while maintaining control simplicity, as the system self-regulates based on actual current conditions
Data Source
AI summary
In one embodiment, a synchronous rectification control method can include: (i) setting or updating a count value when a rectification switch is turned on; (ii) generating an off enable signal after a delay time corresponding to the count value has elapsed; (iii) turning off the rectification switch based on the off enable signal, and comparing a drain-source voltage of the rectification switch against a reference voltage; and (iv) generating a comparison signal for updating the count value based on the drain-source voltage and the reference voltage.