Synchronous Rectifier Gate Voltage Control for Switching Loss Reduction
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Solution Overview
Problem
The reverse recovery current caused by the body diode of the synchronous rectifier during the switching process in switching power supplies leads to significant switching losses and reduced system efficiency.
Innovation Solution
A power device driving method and circuit that involves pulling down the gate voltage of the synchronous rectifier below its threshold voltage using a MOS transistor's body effect, followed by timing adjustments to ensure the synchronous rectifier is turned off before the main switching transistor is turned on, thereby minimizing the conduction of the body diode and reducing switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the synchronous rectifier is turned off before the main switching transistor is turned on, then shorting of the main switching transistor is prevented, but reverse recovery current is generated causing switching loss
Solution Approach 1:
The gate voltage of the synchronous rectifier is pulled down to a first voltage (higher than zero voltage) before the main switching transistor is turned on. This preliminary action ensures the synchronous rectifier is fully turned off and its body diode is reverse-biased before the main switching transistor conducts, preventing reverse recovery current while maintaining safe operating conditions.
Solution Approach 2:
The gate voltage is changed from a second voltage (zero voltage or negative voltage) to a first voltage (higher than zero voltage) during the dead time period. This parameter change optimizes the turning-off process of the synchronous rectifier, ensuring complete cutoff of the body diode current before the main switching transistor turns on, thereby eliminating reverse recovery loss while maintaining reliability.
2Loss of energy
If the gate voltage of the synchronous rectifier is pulled down to zero voltage immediately, then the body diode conduction is reduced, but the switching transistor may be shorted
Solution Approach 1:
The gate voltage is first pulled down to a first voltage (higher than zero voltage) as a preliminary step to turn off the synchronous rectifier. Only after this preliminary turning-off is confirmed does the gate voltage get pulled down to zero voltage. This two-stage preliminary action ensures the synchronous rectifier is safely turned off before complete voltage discharge, preventing transistor shorting while minimizing body diode conduction.
Solution Approach 2:
The gate voltage pulling-down process is divided into two distinct time periods: first to a first voltage (higher than zero) and then to zero voltage. This periodic action with different voltage levels allows sequential optimization - first ensuring safety, then minimizing losses - resolving the contradiction between reliability and energy 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 approach decreases the conduction of the body diode from the time the synchronous rectifier is turned off to when the main switching transistor is turned on, resulting in reduced switching losses and improved conversion efficiency.
Implementation Method 1
a gate voltage of the synchronous rectifier is pulled down to be lower than a threshold voltage of the synchronous rectifier and higher than zero voltage by a body effect of a metal-oxide-semiconductor (MOS) transistor
Data Source
AI summary
A power device driving method and a driving circuit for a switching circuit having a main switching transistor, a synchronous rectifier, and an inductive element. When a switching signal indicates that the synchronous rectifier is turned from on to off and the main switching transistor is turned from off to on, a gate voltage of the synchronous rectifier is pulled down to be lower than a threshold voltage of the synchronous rectifier and higher than zero voltage by a body effect of a MOS transistor, and timing is started. When detecting that a gate voltage of the main switching transistor rises to a first voltage or the timing reaches a first time, the gate voltage of the synchronous rectifier is pulled down to the zero voltage.