Synchronous Rectifier Adaptive Blanking for Flyback Converters
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Solution Overview
Problem
Existing synchronous rectification (SR) switch controllers in flyback converters face issues with high switching losses due to early turn-off caused by ringing in the current, leading to inefficiencies and potential reverse current, which increases component stress and costs.
Innovation Solution
Incorporating a blanking circuit that scales the blanking time in proportion to the peak current, preventing early turn-off of the SR switch and minimizing reverse current by regulating the gate voltage and using adaptive blanking to manage ringing, thus reducing switching losses and maintaining efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed blanking time is used to prevent ringing interference, then early turn-off of SR switch is avoided, but switching losses increase and efficiency decreases
Solution Approach 1:
The blanking time is made dynamic by scaling it in proportion to the peak current detected in the secondary winding. The controller adjusts the blanking time based on the magnitude of the peak current, using longer blanking times for higher currents and shorter or zero blanking times for lower currents. This dynamic adjustment prevents early turn-off due to ringing while minimizing unnecessary blanking periods that cause switching losses.
Solution Approach 2:
The blanking time parameter is changed from a fixed value to a variable value that scales with the peak current. The controller modifies the blanking time parameter based on the detected peak current level, optimizing the trade-off between preventing ringing interference and minimizing switching losses for different operating conditions.
2Reliability
If blanking time is extended to prevent ringing interference, then SR switch remains on longer, but reverse current increases and component stress increases
Solution Approach 1:
The blanking time is dynamically adjusted based on the peak current magnitude. For lower peak currents, the scaled blanking time is shorter, allowing the SR switch to turn off earlier and preventing excessive reverse current. For higher peak currents, the longer blanking time is necessary to prevent early turn-off, and the proportional scaling ensures the optimal balance is maintained.
Solution Approach 2:
The controller uses feedback from the detected peak current to determine the appropriate blanking time. The peak current detection provides information about the operating conditions, and this feedback is used to scale the blanking time accordingly, optimizing both reliability and minimizing harmful reverse current effects.
3Reliability
If gate voltage is increased to keep SR switch on during ringing, then switching losses increase, but if gate voltage is decreased, then early turn-off occurs due to ringing
Solution Approach 1:
The gate voltage is dynamically adjusted based on the detected peak current. For higher peak currents, the gate voltage is maintained at a higher level to ensure continuous conduction during the longer blanking period. For lower peak currents, the gate voltage can be reduced, minimizing gate drive energy while still preventing early turn-off. This dynamic gate voltage control optimizes both reliability and energy efficiency.
Solution Approach 2:
The gate voltage parameter is changed from a fixed value to a variable value that scales with the peak current. The controller adjusts the gate voltage parameter based on the detected peak current level, optimizing the balance between maintaining SR switch conduction and minimizing gate drive energy consumption for different operating conditions.
Data Source
Figure 1
Figure 2A~2B
Figure 3A~3B
AI summary
Disclosed is a method to control the synchronous rectification in a power converter including a primary winding and a secondary winding, including detecting a peak current in a secondary winding, determining a blanking threshold based on the peak current, and blanking a turning off of a synchronous rectifier (SR) switch for a blanking time based on the blanking threshold.