Flyback Synchronous Rectifier Control for CCM Cross-Conduction
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Solution Overview
Problem
Previous generations of fly-back converters operating in continuous conduction mode (CCM) experience cross-conduction between the primary side power switch (PS_FET) and the synchronous rectifier (SR_FET), leading to reduced efficiency and potential damage to the SR_FET.
Innovation Solution
The method involves turning on the SR_FET, causing a rapid drop in SR-drain-voltage, followed by a slow rise. The PS is turned on before the secondary side current is completely discharged, changing the rate of rise of the SR-drain-voltage. The SR_FET is then turned off within tens of nanoseconds of this change, minimizing cross-conduction without relying on turn-on information from the primary side controller.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the PS_FET and SR_FET are both turned on during CCM operation, then the converter can maintain continuous current flow, but cross-conduction occurs reducing efficiency and potentially damaging the SR_FET
Solution Approach 1:
The patent applies preliminary action by detecting the upcoming turn-on event of the primary switch PS_FET before it actually occurs. The controller monitors the drain voltage of the synchronous rectifier SR_FET and detects when it begins to rise, indicating that the primary switch is about to turn on. Based on this early detection, the controller proactively turns off the SR_FET in advance, preventing cross-conduction from occurring in the first place. This preliminary detection and preventive action resolves the contradiction by maintaining continuous current flow while avoiding the harmful cross-conduction period.
2Loss of energy
If the SR_FET is turned off quickly to avoid cross-conduction, then efficiency improves, but voltage spikes may damage the SR_FET
Solution Approach 1:
The patent implements beforehand cushioning by introducing a clamping circuit that limits the maximum voltage that can appear across the SR_FET during its turn-off transition. The clamping circuit is configured to activate when the drain voltage exceeds a predetermined threshold, effectively cushioning or limiting the voltage spike before it can reach damaging levels. This allows the SR_FET to be turned off quickly to prevent cross-conduction while simultaneously protecting the device from voltage-induced damage, thus resolving the contradiction between efficiency improvement and component reliability.
3Device complexity
If traditional zero-crossing detection is used to control SR_FET, then the circuit is simple, but cross-conduction cannot be avoided in CCM mode
Solution Approach 1:
The patent applies feedback by using the drain voltage of the synchronous rectifier SR_FET as a real-time indicator of the transformer's magnetic state and primary switch status. The controller continuously monitors this voltage and uses it as feedback to determine when to turn off the SR_FET. Specifically, when the drain voltage begins to rise (indicating the primary switch is about to turn on), the controller responds by turning off the SR_FET. This feedback mechanism enables the system to avoid cross-conduction in CCM mode while maintaining relatively simple circuitry, resolving the contradiction between control simplicity and cross-conduction prevention.
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
This approach effectively minimizes or eliminates cross-conduction between the primary and secondary switches in CCM, enhancing the efficiency and reliability of the fly-back converter.
Implementation Method 1
a transformer having a primary side electrically coupled to an AC input and a secondary side coupled to a DC output
Implementation Method 2
turning off the synchronous rectifier (SR) in response to a change in a rise of a SR-drain-voltage caused by turning on the PS
Implementation Method 3
a fly-back synchronous rectifier (SR) controller with a circuit for rapidly turning off the SR to avoid cross-conduction
Data Source
AI summary
A fly-back converter and a synchronous-rectifier (SR) controller therefor are provided to eliminate cross-conduction between a power switch on the primary side of a transformer and a SR field effect transistor (FET) on the secondary side of the transformer when operating in continuous conduction mode. Generally, the SR controller comprises a SR sense pin coupled to a drain of the SR FET, and a gate driver coupled to control a gate of the SR FET. A negative-sensing (NSN) comparator is coupled to the SR sense pin and is operable to generate a turn-on signal for the gate driver based on a voltage on the SR sense pin. A zero-crossing detector (ZCD) comparator is coupled to the SR sense pin and is operable to generate a turn-off signal for the gate driver based on the voltage on the SR sense pin. The SR controller is operable to turn off the SR FET without turn-on information of the power switch received from the primary side controller.