LLC Synchronous Rectifier Timing for Zero-Crossing MOSFET Control
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Solution Overview
Problem
The precise timing of MOSFETs in an active rectification stage of a DC-DC LLC resonant converter is challenging due to capacitive spikes and inductive advance, leading to inefficiencies, current inversions, and potential component failure.
Innovation Solution
A controller is used to measure elapsed times during a calibration cycle to determine delay times for MOSFET turn-on and turn-off, adjusting these timings to avoid capacitive spikes and inductive advance, ensuring accurate synchronization with rectified current zero-crossings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If passive rectification is used, then device complexity is reduced, but power losses increase due to voltage drop across diodes
Solution Approach 1:
The patent replaces passive diode rectification with active MOSFET-based synchronous rectification. The MOSFETs act as electronically controlled switches that substitute for the passive diodes, enabling active control of the rectification process. This substitution reduces conduction losses by minimizing voltage drop across the switching devices while maintaining the rectification function.
2Loss of energy
If synchronous rectification with MOSFETs is implemented, then power losses are reduced, but timing precision requirements increase due to capacitive spikes and inductive advance
Solution Approach 1:
The patent employs a calibration cycle that executes before normal operation to pre-determine the optimal turn-on delay time. During this calibration phase, the system measures the actual timing characteristics including capacitive spikes and inductive advance effects, then stores these measurements for use during subsequent normal operation. This preliminary characterization eliminates the need for complex real-time timing calculations.
Solution Approach 2:
The system uses feedback from the calibration cycle to adjust and optimize the turn-on timing of MOSFETs during normal operation. The measured delay time from calibration is fed back into the control logic, allowing the system to compensate for capacitive spikes and inductive advance effects. This feedback mechanism ensures accurate synchronization without requiring complex real-time sensing.
3Productivity
If MOSFET turn-on timing is advanced to capture rectified current, then efficiency improves, but current inversion risk increases leading to potential component failure
Solution Approach 1:
The calibration cycle performs preliminary measurement of the timing characteristics including the point at which rectified current begins to flow. This pre-characterization allows the system to determine the optimal turn-on delay that maximizes efficiency while preventing current inversion. The measured delay time is then applied during normal operation to ensure MOSFETs are turned on at the correct moment.
Solution Approach 2:
The system applies preliminary anti-action by using the calibration measurements to pre-compensate for timing issues that would cause current inversion. The turn-on delay time determined during calibration inherently accounts for capacitive spikes and inductive advance, preventing these harmful effects before they can occur during normal operation. This proactive approach protects against component failure while maintaining high efficiency.
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 method improves efficiency and reliability by minimizing switching losses and preventing voltage spikes, thus enhancing the performance of the DC-DC LLC resonant converter.
Implementation Method 1
The transformer features dual secondary windings Ls1, Ls2, serving to step the battery voltage VBATT to desired levels
Implementation Method 2
a passive rectification stage comprises diodes Dd1 and Dd2 connected to the secondary windings Ls1, Ls2, with these diodes rectifying the AC voltage induced in the secondary windings Ls1, Ls2
Data Source
AI summary
A DC-DC LLC resonant-converter utilizes a synchronous-rectifier stage with first and second branches. In a calibration-cycle, a first elapsed-time from commutation of a drain-to-source voltage of the first branch's transistor, through inversion of the current in the first branch, to a next zero-crossing of that current is measured, and a second elapsed-time is measured the same way for the second branch. In a normal operating-cycle thereafter, turn-on of the first branch's transistor with respect to commutation of its drain-to-source voltage is delayed by the first elapsed-time, and turn-on of the second branch's transistor with respect to commutation of its drain-to-source voltage is delayed by the second elapsed-time. The first branch's transistor is turned-off in response to change in slope of a drain-to-source voltage of the second branch's transistor, and the second branch's transistor is turned-off in response to change in slope of a drain-to-source voltage of the first branch's transistor.