Secondary Side Switch-Off Timing Control for Power Converters
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Solution Overview
Problem
Conventional switching power converters face inefficiencies due to high conduction voltage in diodes, especially at low DC output voltages, and struggle to appropriately switch off the secondary side current path in both Continuous Current Mode (CCM) and Discrete Current Mode (DCM), leading to potential system disasters from reverse currents.
Innovation Solution
A novel method and apparatus for adjusting secondary side switch-off timing using comparators, latches, and a turn-off predictor to generate an off-predicting signal, detecting voltage edges and cycle periods to effectively cut off the secondary side current path, setting a threshold voltage between 0 and 2VO to capture discharging end instances in both CCM and DCM.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a diode is used to cut off the secondary side current path, then the current path can be effectively cut off, but the conduction voltage of 0.7V consumes significant energy and degrades conversion efficiency
Solution Approach 1:
The patent changes the electrical parameters of the secondary side by replacing the fixed 0.7V diode with a controllable switch (NMOS transistor) whose on-resistance can be optimized to be much lower than diode forward voltage, and whose conduction state can be dynamically controlled based on operating conditions (CCM/DCM modes), thereby reducing conduction losses while maintaining reliable current path cut-off
Solution Approach 2:
The patent introduces dynamic control of the secondary side switch based on detection of operating modes (CCM or DCM). The control signal adjusts the switch timing and state according to the actual current mode, making the system adaptive to different loading conditions and improving overall efficiency across varying operating points
2Loss of energy
If a comparator and NMOS transistor are used to emulate diode function with lower conduction voltage, then conversion efficiency improves, but the threshold voltage detection becomes complex and unreliable in CCM mode
Solution Approach 1:
The patent introduces an intermediary control mechanism that detects the end of secondary side discharge through voltage threshold comparison and uses this information to generate controlled turn-off signals. This intermediary control layer simplifies the detection logic by using clear voltage thresholds (VTH1, VTH2) rather than requiring precise threshold voltage measurement, and adds hysteresis through the control signal to prevent false triggering in CCM mode
Solution Approach 2:
The patent implements feedback control by monitoring the secondary side voltage and using this information to adjust the turn-off timing of the switch. The control signal is generated based on feedback from voltage detection, creating a closed-loop system that adapts to changing operating conditions and maintains reliable operation in both CCM and DCM modes
3Reliability
If the secondary side current path is not properly switched off in CCM mode, then the system may experience reverse currents leading to system disaster, but implementing proper switch-off control increases system complexity
Solution Approach 1:
The patent applies preliminary action by generating control signals that anticipate and prevent reverse current conditions. The control circuit detects discharge end timing and proactively generates turn-off signals before reverse current can occur, especially in CCM mode where the risk is highest. This preventive approach ensures system safety without requiring complex real-time intervention circuits
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 solution enhances conversion efficiency by preventing reverse currents and ensuring safe operation in both CCM and DCM, with improved ease of implementation and noise resistance.
Implementation Method 1
detecting the falling edge of the voltage across a secondary side transistor according to a first reference voltage to generate a first reset signal
Implementation Method 2
detecting the rising edge of the voltage across the secondary side transistor according to a second reference voltage to generate a first set signal
Implementation Method 3
the main transformer 102 is used to transfer the input DC power VIN to a DC output voltage VCC
Implementation Method 4
releasing the magnetic flux to the capacitor 104 when the NMOS transistor 101 is off
Data Source
AI summary
The present invention discloses a secondary side switch-off timing adjusting method for a switching power conversion, comprising: detecting the falling edge of the voltage across a secondary side transistor according to a first reference voltage to generate a first reset signal; detecting the rising edge of the voltage across the secondary side transistor according to a second reference voltage to generate a first set signal; generating a secondary side discharging end signal from a latch operation in response to the first reset signal and the first set signal; and generating an off-predicting signal according to the cycle period of the secondary side discharging end signal. The present invention also provides a secondary side switch-off timing adjusting apparatus, and a system using the apparatus for a power conversion.