Two-Level Switch Driver to Prevent SR Switch Avalanche in Burst Mode
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Solution Overview
Problem
Switched-mode power converters face challenges in limiting excessive voltage levels across synchronous rectification (SR) switches during burst-mode operation, which can lead to avalanche breakdown and damage, especially in battery chargers where the load is inactive, causing repetitive voltage excursions beyond the SR switch's safe threshold.
Innovation Solution
A two-level switch driver is employed, featuring a low-resistance and high-resistance pull-up driver switch, where the high-resistance switch is enabled during burst-mode operation to slow down the rising edge of voltage pulses, thereby reducing the maximum voltage across the SR switch without the need for additional snubber circuits like RCD snubbers, thus preventing avalanche breakdown.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a synchronous rectification (SR) switch is used for efficient power transfer, then power losses and heat are reduced, but the voltage across the SR switch reaches avalanche breakdown levels during burst-mode operation, causing damage
Solution Approach 1:
The driver circuit changes the switching parameters of the power switch based on operating mode. During burst-mode operation, the driver slows down the turn-on transition of the power switch by controlling the gate voltage ramp rate, which reduces the voltage spike and ringing across the SR switch, preventing avalanche breakdown while maintaining efficient rectification during normal operation
Solution Approach 2:
The driver circuit dynamically adjusts its behavior based on the operating mode of the power converter. It detects burst-mode operation and adapts the switching waveform characteristics in real-time, making the system flexible enough to prevent SR switch damage during low-power operation while maintaining high efficiency during normal operation
2Use of energy by stationary object
If burst-mode operation is used to minimize power loss during standby, then power efficiency is improved, but voltage ringing across the SR switch exceeds maximum allowed voltage, causing repetitive avalanche breakdown
Solution Approach 1:
The driver circuit modifies the switching parameters during burst-mode operation by slowing the gate voltage rise time, which reduces the rate of change of current (di/dt) and consequently reduces the voltage ringing across the SR switch, preventing harmful voltage excursions while maintaining burst-mode power savings
Solution Approach 2:
The driver circuit takes preliminary action by pre-slowing the switching transition before the voltage spike occurs during burst-mode operation. This preventive measure reduces the severity of voltage ringing before it can cause avalanche breakdown, addressing the harmful effect before it fully manifests
3Reliability
If snubber circuits like RCD snubbers are added to limit voltage across the SR switch, then SR switch protection is improved, but power efficiency deteriorates due to additional power losses
Solution Approach 1:
The invention extracts and removes the need for external snubber circuits by integrating the voltage spike suppression function directly into the primary-side driver circuit. The driver controls the power switch transition to inherently prevent excessive voltage across the SR switch, eliminating the need for additional protection components and their associated power losses
Solution Approach 2:
The driver circuit provides self-service by incorporating the SR switch protection function within itself through intelligent control of the power switch. Instead of requiring external protection circuits, the driver autonomously adjusts its switching behavior to prevent harmful voltage ringing, making the system self-protecting without additional energy-consuming components
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 effectively limits the voltage across the SR switch to below its avalanche breakdown level, preventing damage and maintaining power efficiency by reducing power losses, even during standby modes, without incurring the inefficiencies of snubber circuits.
Implementation Method 1
A first pull-up driver switch has a first on-state resistance, and a second pull-up driver switch has a second on-state resistance that is greater than the first on-state resistance
Data Source
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AI summary
Techniques are provided for avoiding an avalanche breakdown voltage across a synchronous rectification (SR) switch on the secondary side of an isolated switched-mode power converter operating in a low-power mode, e.g., a burst mode, during which a load of the power converter draws negligible current. This is accomplished via use of a two-level switch driver for controlling a power switch on the primary side of the power converter. The two-level switch driver is configured to source low current levels to a control terminal (e.g., gate) of the power switch during burst-mode operation. This low current reduces the slope of the rising edge of voltage pulses on the primary and secondary sides of the power converter which, in turn, limits the peak of the voltage ringing across the SR switch. By limiting the voltage in this manner, the SR switch avoids entering avalanche breakdown.