Flyback Converter Primary Side Sensing Overvoltage Protection
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Solution Overview
Problem
Conventional isolated flyback converters require a minimum load resistor to prevent output voltage from exceeding regulated levels during light load conditions, which reduces efficiency and shortens battery life.
Innovation Solution
The flyback converter employs primary side sensing to detect output voltage and uses the synchronous rectifier to briefly draw a reverse current through the secondary winding, recycling excess energy without needing a minimum load resistor or zener diode, thereby maintaining regulation and improving efficiency during light loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a minimum load resistor is used to maintain output voltage regulation during light load conditions, then the output voltage remains regulated, but the converter efficiency decreases and battery life is shortened
Solution Approach 1:
The patent extracts and removes the minimum load resistor from the circuit by implementing a synchronous rectifier that can operate with zero or near-zero load current. The synchronous rectifier actively controls the secondary side switching to maintain proper voltage regulation without requiring a resistive load, thereby eliminating the continuous power dissipation that previously degraded efficiency during light load conditions.
Solution Approach 2:
The patent changes the operating parameters of the flyback converter by implementing active synchronous rectification control that dynamically adjusts the secondary side switching timing and duty cycle. This allows the converter to maintain stable output voltage regulation across the full load range from full load down to zero load, eliminating the need for a minimum load resistor and the associated energy losses.
2Device complexity
If primary side sensing is used to detect output voltage, then additional circuitry such as optocouplers or third windings is eliminated, but accurate sensing requires a minimum duty cycle and minimum load current
Solution Approach 1:
The patent applies dynamic control to the synchronous rectifier switching timing to ensure that the primary side sensing occurs at the optimal moment during the flyback cycle. By dynamically adjusting the secondary side switch turn-off timing, the system guarantees that sufficient energy has been transferred to the output, ensuring accurate voltage sensing on the primary side without requiring a minimum load condition, while maintaining isolation and eliminating additional circuitry.
3Loss of energy
If the actual load enters standby mode drawing very little current, then power consumption is reduced, but the output voltage exceeds the desired regulated level without a minimum load resistor
Solution Approach 1:
The patent implements a feedback control mechanism through the synchronous rectifier control circuit that continuously monitors the output voltage (via primary side sensing) and dynamically adjusts the secondary side switching parameters. When the load enters standby mode, the feedback loop detects the rising output voltage and adjusts the rectifier timing and duty cycle to maintain precise voltage regulation, allowing the system to operate efficiently at very low power consumption while preventing overvoltage conditions.
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 approach allows efficient regulation of output voltage at zero or very little load current without additional circuitry, enhancing battery life and reducing power wastage by recycling excess energy.
Implementation Method 1
uses the synchronous rectifier to briefly draw a reverse current through the secondary winding, recycling excess energy
Implementation Method 2
sense a voltage at a terminal of the power switch when the power switch is turned off during the discharge (or flyback) cycle of the converter. Such a sensed voltage is substantially proportional to the output voltage
Data Source
AI summary
A flyback converter (20) uses primary side sensing to sense the output voltage for regulating feedback. The flyback converter comprises a transformer with a primary winding (L1) and a secondary winding (L2), a first transistor (M1) coupled to the primary winding for conducting a current through the primary winding when the first transistor is on a second transistor (M2) for conducting a current through the secondary winding when the second transistor is on a regulator (14) coupled to the first transistor for controlling a duty cycle of the first transistor to regulate an output voltage of the converter, the regulator being configured to control the first transistor to have a minimum duty cycle, an output capacitor (C1) coupled to an output terminal of the converter, a synchronous rectifier controller (24) coupled to the second transistor for controlling the second transistor to be on or off, a comparator (42) having one input coupled to receive a voltage corresponding to the output voltage of the converter and having another input connected to a reference voltage representing a threshold voltage exceeding a regulated voltage of the converter, wherein triggering of the comparator signifies an over-voltage condition, an output of the comparator being coupled so as to control the synchronous rectifier controller to turn the second transistor on for an interval to conduct a reverse current through the secondary winding, upon an over-voltage condition being detected, to reduce the output voltage of the converter to mitigate the over-voltage condition, and a diode (D1) coupled to the primary winding to conduct a current through the primary winding after the interval without turning on the first transistor, such that power is transferred from a secondary side of the transformer to the power source while mitigating the over-voltage condition.