Synchronous Rectifier Control Circuit for Switching Power Supply
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Solution Overview
Problem
In switching power supply circuits with synchronous rectifiers, accurately determining the ON state of the primary switch becomes challenging due to the increasing switching frequency, leading to potential mis-triggering of the synchronous rectifier switch, especially when distinguishing between the slew rate of the drain-source voltage during normal operation and ringing.
Innovation Solution
A control method and circuit that utilize a dynamic reference voltage based on the maximum drain-source voltage across the synchronous rectifier switch, combined with a preset window time period and working mode, to accurately determine when the primary switch is turned ON, thereby preventing mis-triggering by adjusting the timing for turning ON the synchronous rectifier switch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If switching frequency is increased to improve productivity, then the switching power supply circuit can process more cycles per second, but the slew rate of the ring becomes closer to the slew rate of the drain-source voltage when normally turned ON, making accurate detection more difficult
Solution Approach 1:
The patent applies preliminary action by detecting the drain-source voltage of the synchronous rectifier switch in advance to determine the ON state of the primary switch before the actual switching occurs. This early detection mechanism allows the system to prepare for the upcoming switching event, ensuring accurate identification of the primary switch state even at high frequencies where the voltage transitions are rapid and closely spaced.
Solution Approach 2:
The patent uses the drain-source voltage of the synchronous rectifier switch as an intermediary signal to indirectly detect the ON state of the primary switch. Instead of directly measuring the primary switch voltage which has become difficult to distinguish at high frequencies, the system uses the related but more distinguishable drain-source voltage as a mediator to infer the primary switch state, solving the detection difficulty caused by high switching frequencies.
2Loss of energy
If synchronous rectifier switch is used instead of diode to improve energy efficiency, then conversion efficiency is improved, but mis-triggering may occur due to ring of drain-source voltage making accurate control difficult
Solution Approach 1:
The patent implements feedback by continuously monitoring the drain-source voltage of the synchronous rectifier switch and using this information to control the switching timing. The control circuit adjusts the switching of the synchronous rectifier switch based on the detected voltage state, creating a closed-loop system that ensures accurate control while maintaining the high efficiency benefits of synchronous rectification.
Solution Approach 2:
The control circuit performs preliminary detection of the drain-source voltage to determine the appropriate timing for turning on the synchronous rectifier switch. This preliminary action prevents mis-triggering by ensuring the switch is activated only when the voltage conditions are correct, maintaining both efficiency and reliability.
Data Source
AI summary
A switching power supply circuit has an energy storage component, a synchronous rectifier switch and a synchronous rectifier control circuit. The synchronous rectifier switch is coupled to a secondary side of the energy storage component, and the synchronous rectifier control circuit turns ON the synchronous rectifier switch based on a drain-source voltage across the synchronous rectifier switch when a primary switch is judged as turned ON. When the switching power supply circuit is not operating in a preset mode, the primary switch is judged as turned ON when the drain-source voltage remains larger than a dynamic reference voltage during a preset window time period, and when the switching power supply circuit is operating in the preset mode, the primary switch is judged as turned ON once the drain-source voltage is larger than the dynamic reference voltage.