Adaptive Reference Voltage for Flyback SR MOSFET Timing
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Solution Overview
Problem
In flyback switched mode power supplies with synchronous rectification, the control of SR MOSFET timing becomes challenging with lower drain-source on-resistance (RdsOn) due to reduced drain-source voltages, leading to non-optimal efficiency, as the reference voltage for gate-source regulation cannot be set too low due to offsets.
Innovation Solution
An adaptive reference voltage system is implemented using a timer that starts when the voltage on the conduction terminal of the output transistor reaches a reference voltage and stops when the current reaches zero, sampling the voltage difference between the target and accumulated voltage to adjust the reference voltage dynamically, allowing for efficient regulation of the SR MOSFET gate voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the drain-source on-resistance (RdsOn) of the SR MOSFET is reduced to improve efficiency, then conduction losses are reduced, but the drain-source voltage becomes too low for reliable timing control
Solution Approach 1:
The patent changes the reference parameter from drain-source voltage to gate-source voltage. By regulating Vgs to a reference level and using a timer to control the duration of conduction, the system can use MOSFETs with very low RdsOn while maintaining proper timing control through voltage-time product equivalence.
Solution Approach 2:
The patent introduces a timer circuit as an intermediary between the voltage sensing and current control. The timer converts the voltage information into a time-based control signal, which then regulates the MOSFET gate, enabling indirect control that works effectively with low-voltage devices.
2Loss of energy
If the reference voltage for gate-source regulation is reduced to optimize efficiency for low RdsOn MOSFETs, then efficiency is improved, but offset issues prevent the reference voltage from being set too low
Solution Approach 1:
The patent makes the reference voltage dynamic rather than fixed. The reference voltage is adjusted based on the timer output, allowing the system to adapt the reference level to match the specific characteristics of the MOSFET being used, thereby overcoming fixed offset limitations.
Solution Approach 2:
The patent implements a feedback loop where the timer monitors the gate-source voltage regulation and adjusts the reference voltage accordingly. This closed-loop control ensures that the reference voltage is optimized for each operating condition, eliminating the need for a fixed reference that must account for worst-case offsets.
3Device complexity
If a fixed reference voltage is used for regulating the gate-source voltage, then the control circuit is simple, but efficiency is suboptimal for MOSFETs with very low RdsOn
Solution Approach 1:
The patent designs the control circuit to perform multiple functions: voltage regulation, timing control, and adaptive reference generation. The timer circuit serves both as a timing element and as a dynamic reference voltage generator, reducing the need for separate dedicated components for each function.
Data Source
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AI summary
Switched mode power supply (SMPS) with adaptive reference voltage for controlling an output transistor and method of operating the SMPS are described. The adaptive reference voltage is implemented using a timer that starts when the voltage on a conduction terminal of the output transistor reaches a reference voltage and stops when the current through the output transistor reaches zero. The voltage difference between a target voltage and a voltage accumulated when the timer was active is then sampled to produce a sampled voltage, which defines a new reference voltage if the accumulated voltage is not equal to the target voltage so that the reference voltage is adaptively adjusted in accordance with an operational characteristic of the output transistor.