Synchronous Rectifier Gate Voltage Control for Switching Loss Reduction
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Solution Overview
Problem
In switching power supplies, the reverse recovery current of the body diode in synchronous rectifiers causes significant switching losses when the synchronous rectifier is turned off and the main switching transistor is turned on, leading to reduced system efficiency.
Innovation Solution
A control method and circuit that utilize a resistor-capacitor delay effect to manage the gate voltage of the synchronous rectifier, ensuring it is pulled down below the threshold voltage before the main switching transistor turns on, thereby reducing the conduction of the body diode and minimizing switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dead time is added to prevent shoot-through when synchronous rectifier is turned off and main switching transistor is turned on, then shoot-through is prevented, but reverse recovery current causes large switching loss
Solution Approach 1:
The gate voltage of the synchronous rectifier is pulled down to a voltage between 0V and threshold voltage before the main switching transistor turns on. This preliminary action ensures the body diode is already off before the main transistor conducts, eliminating reverse recovery current and switching loss while maintaining shoot-through prevention.
Solution Approach 2:
The gate voltage parameter is changed from a simple binary on/off state to a three-level state: high voltage (on), intermediate voltage between 0V and threshold voltage (off but not fully discharged), and zero voltage (fully off). This parameter change allows the body diode to be turned off before the main transistor turns on, eliminating reverse recovery effects.
2Loss of energy
If gate voltage of synchronous rectifier is pulled down to zero voltage immediately, then switching loss is reduced, but body diode may not be properly turned off leading to shoot-through
Solution Approach 1:
The gate voltage is pulled down to an intermediate voltage level (between 0V and threshold voltage) as a preliminary action before the main switching transistor turns on. This ensures the body diode is turned off while maintaining the gate voltage at a safe level that prevents shoot-through, and only after this preliminary action is complete is the gate voltage fully discharged to zero.
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 reduces the conduction of the body diode and minimizes switching losses, thereby improving the conversion efficiency of the switching power supply circuit.
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 a zero voltage by using a resistor-capacitor delay effect
Data Source
AI summary
A control method and a control circuit for a switching power supply circuit and the switching power supply circuit. The switching power supply circuit includes 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 a zero voltage by using a resistor-capacitor delay effect and timing is started. When a gate voltage of the main switching transistor is detected to rise 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.