Synchronous Rectifier Controller Soft Turn-Off
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Solution Overview
Problem
Conventional switched mode power supplies with primary side sensing experience sampling errors and increased electromagnetic interference due to rapid turn-off of synchronous rectifiers, leading to inefficiencies and inaccurate output voltage regulation.
Innovation Solution
A synchronous rectifier controller that employs a sensor to detect the turn-off of the switch, charges the control terminal beyond the threshold voltage, and uses a linear amplifier to inhibit discharge until the voltage reaches a reference value, allowing for a soft turn-off of the synchronous rectifier, thereby reducing oscillations and electromagnetic interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the synchronous rectifier is turned off rapidly, then the switching speed is improved, but sampling errors and electromagnetic interference increase
Solution Approach 1:
The patent applies preliminary action by detecting the switch turn-off event beforehand and pre-charging the control terminal of the synchronous rectifier beyond the threshold voltage. This ensures the rectifier is already in a controlled state before the actual turn-off occurs, preventing rapid discharge and the associated sampling errors and EMI. The charge source activates in response to detecting the switch turn-off, preparing the system in advance to avoid the harmful effects of hard turn-off.
2Productivity
If the synchronous rectifier is turned off rapidly, then the switching efficiency is improved, but electromagnetic interference increases
Solution Approach 1:
The patent converts the potentially harmful rapid turn-off effect into a beneficial controlled process. By using the linear amplifier to inhibit discharge and maintain voltage above the threshold, the patent transforms what would be a harmful EMI-generating event into a beneficial soft turn-off process. The controlled discharge through the linear amplifier maintains stability while still achieving the necessary switching function, effectively converting the harm of rapid switching into a beneficial controlled transition.
3Loss of time
If the synchronous rectifier is turned off rapidly, then the response time is improved, but output voltage regulation stability deteriorates
Solution Approach 1:
The patent implements feedback by using the linear amplifier to continuously monitor the voltage across the synchronous rectifier and adjust the discharge current accordingly. The amplifier compares the rectifier voltage with a reference and dynamically controls the discharge rate to maintain stability. This feedback mechanism ensures that even though the switch turns off rapidly, the synchronous rectifier's voltage remains controlled, preventing oscillations and maintaining stable output voltage regulation throughout the transition.
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 enables accurate sampling and reduced electromagnetic interference, improving the efficiency and stability of output regulation in switched mode power supplies by avoiding hard turn-off and promoting a gradual decline in secondary current.
Implementation Method 1
a sensor to sense a signal on the secondary side, to thereby detect turn off of the switch
Implementation Method 2
a charge source to, in response to a said detection, charge a control terminal of the synchronous rectifier to a voltage beyond a threshold voltage
Implementation Method 3
a linear amplifier having an output to sink current from the control terminal dependent on a difference between a voltage across the synchronous rectifier and an amplifier reference value
Data Source
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AI summary
This invention generally relates to synchronous rectifier controllers for a switched mode power supply (SMPS) comprising a synchronous rectifier, SMPSs and methods for controlling a synchronous rectifier of an SMPS. For example, a synchronous rectifier controller for a switched mode power supply (SMPS) comprising a synchronous rectifier comprises: a sensor to sense a signal on a secondary side, to thereby detect turn off of the switch; a charge source to, in response to a said detection, charge a control terminal of the synchronous rectifier to a voltage beyond a threshold voltage of the synchronous rectifier to allow the synchronous rectifier to conduct current of the secondary winding; and a linear amplifier having an output to sink current from the control terminal dependent on a difference between a voltage across the synchronous rectifier and an amplifier reference value, said voltage across the synchronous rectifier being a voltage across a controllable conduction path for current of the secondary winding, the linear amplifier thereby configured to inhibit discharge of the control terminal from the voltage beyond the threshold voltage until the voltage across the synchronous rectifier reaches the amplifier reference value.