Rectifier Control Device for LLC Converter Body Diode Conduction
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Solution Overview
Problem
In LLC resonant converters, the maximum efficiency is limited by losses in secondary rectification, and existing solutions like synchronous rectification are hindered by parasitic inductances causing premature turn-off of transistors, leading to increased residual conduction time of body diodes and efficiency loss.
Innovation Solution
A control device for the rectifier that measures the conduction time of the body diode during each switching half-cycle, adjusts the voltage threshold for the next cycle based on the measured conduction time, and uses a comparator and setting circuit to manage the turn-on and turn-off signals of the transistors, thereby compensating for parasitic inductances and preventing current inversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If synchronous rectification is used to replace rectifier diodes with power MOSFETs, then voltage drop and power dissipation are reduced, but parasitic inductances cause premature turn-off of transistors leading to increased residual conduction time of body diodes
Solution Approach 1:
The patent implements a feedback mechanism where the control device monitors the actual turn-off timing of the MOSFET and adjusts the drive signal accordingly. By detecting the drain-source voltage and comparing it with reference values, the system dynamically compensates for parasitic inductance effects, ensuring optimal turn-off timing that minimizes body diode conduction while preventing premature shutdown.
Solution Approach 2:
The patent dynamically adjusts operating parameters including voltage thresholds and timing delays based on detected conditions. The control device modifies the drive signal characteristics (voltage level, pulse width, timing) to compensate for parasitic inductances, thereby optimizing the transistor switching behavior and reducing residual body diode conduction time.
2Reliability
If the voltage threshold is increased to prevent premature turn-off, then transistor conduction time is extended, but efficiency gains from synchronous rectification are reduced
Solution Approach 1:
The patent employs dynamic threshold adjustment where the voltage reference is not fixed but adapts based on operating conditions. The control device dynamically modifies the voltage threshold and timing parameters in response to detected drain-source voltage levels and current conditions, optimizing the balance between maintaining conduction continuity and maximizing efficiency gains.
Solution Approach 2:
The system dynamically changes operating parameters including voltage thresholds and timing delays based on real-time detection of circuit conditions. This allows the system to extend conduction time when necessary while minimizing body diode conduction losses, thereby maintaining both reliability and efficiency.
3Device complexity
If fixed voltage threshold control is used, then control circuit simplicity is maintained, but parasitic inductances cause current inversion and converter malfunctions during fast load transients
Solution Approach 1:
The patent introduces a feedback-based control mechanism that monitors drain-source voltage and compares it with dynamically adjusted reference values. This feedback loop enables the control circuit to detect and compensate for parasitic inductance effects during fast load transients, preventing current inversion while maintaining operational stability without requiring overly complex control logic.
Solution Approach 2:
The control device performs preliminary detection and adjustment of voltage thresholds before current inversion can occur. By anticipating potential issues during fast load transients and pre-adjusting control parameters, the system prevents malfunctions while maintaining relatively simple control circuitry.
Data Source
AI summary
A control device for a rectifier of a switching converter that includes a rectifier with at least one MOS transistor and a control device that is configured to generate a turn on and off signal for the at least one transistor. The control device also includes a measuring circuit to measure the conduction time of the body diode of the at least one transistor during each converter switching half-cycle. The control device is configured to, cycle by cycle: verify if the drain-source voltage of the at least one transistor is greater or less than a voltage threshold, and if the drain-source voltage is greater than the voltage threshold to turn off the at least one transistor, measure the conduction time of the body diode and increase the voltage threshold by a quantity in the next switching cycle.