Synchronous Rectification Control Using Body Diode Forward Voltage Detection
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Solution Overview
Problem
Synchronous rectification type switching power supply apparatuses face challenges in minimizing losses due to the need for precise and complex detection circuits to control switching MOSFETs, especially when reducing the resistance value of sense resistors leads to small voltage drops and increased complexity.
Innovation Solution
A control circuit that detects timing using drain side voltages of switching MOSFETs and counter electromotive voltage generated by the transformer, generating on/off control signals to turn the MOSFETs on during forward current flow and off before counter electromotive voltage generation, thereby reducing the time current flows through body diodes and minimizing losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If sense resistor Rs is used to detect on/off timing of MOSFET, then on/off timing can be detected by voltage reduction, but loss is generated by the sense resistor and resistance value must be reduced to minimize loss, which makes voltage drop small and requires precise and complicated detection circuit
Solution Approach 1:
The invention extracts the detection function from the sense resistor voltage drop method and relocates it to the body diode forward voltage detection method. By detecting the forward voltage of the body diode instead of using a separate sense resistor, the system eliminates the need for precise detection of small voltage drops while maintaining accurate timing detection capability.
Solution Approach 2:
The body diode serves as an intermediary element that provides a detectable voltage signal during its conduction period. This intermediary mechanism allows the system to detect on/off timing without relying on small voltage drops across low-value resistors, thereby simplifying the detection circuit while minimizing energy loss.
2Loss of energy
If resistance value of sense resistor is reduced to minimize loss, then loss by sense resistor becomes negligible, but voltage drop becomes small and precise and complicated detection circuit (comparator) is required
Solution Approach 1:
The invention extracts the detection target from the small voltage drop across the sense resistor and shifts it to the forward voltage of the body diode. This extraction allows the system to detect timing signals with adequate voltage levels without requiring high-precision detection circuits or comparators.
Solution Approach 2:
The invention changes the detection parameter from small voltage drop (millivolt range) to body diode forward voltage (volt range). This parameter change significantly improves the signal-to-noise ratio and eliminates the need for high-precision detection circuits while maintaining accurate timing detection.
3Loss of energy
If voltage between source and drain of switching MOSFET is used to produce on/off control signals, then loss by sense resistor is eliminated, but voltage reduction caused by resistance component of MOSFET is extremely small and precise and stable detection circuit (comparator) having small variation is required
Solution Approach 1:
The body diode acts as an intermediary that provides a detectable voltage signal during its conduction period. By monitoring the forward voltage of the body diode instead of the small voltage drop across the MOSFET's on-resistance, the system achieves accurate timing detection without requiring high-precision comparators or stable detection circuits.
Solution Approach 2:
The invention changes the detection parameter from the extremely small voltage reduction across the MOSFET's on-resistance to the body diode's forward voltage, which is significantly larger and easier to detect with standard circuit components. This parameter change eliminates the need for high-precision, low-variation detection circuits.
4Device complexity
If diodes are used for rectifying current, then circuit structure is simple, but rectification loss Vf·I is generated by forward voltage of diodes and current flowing through diodes
Solution Approach 1:
The invention changes the rectification mechanism from passive diode rectification to active MOSFET synchronous rectification. By controlling the MOSFET's on/off timing based on body diode forward voltage detection, the system achieves both simple circuit structure and low rectification loss, as MOSFETs have much lower on-resistance compared to diode forward voltage.
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 for stable and precise control of switching MOSFETs without requiring precise detection of small voltage drops, reducing losses and eliminating the need for complex detection circuits, while maintaining control stability despite variations in circuit elements.
Implementation Method 1
a second timing detection circuit to detect second timing with counter electromotive voltage that is generated at an instant when the body diode is turned off
Implementation Method 2
a first timing detection circuit to detect first timing at which forward current flows through a body diode of the switching element
Data Source
AI summary
Disclosed is a synchronous rectification type switching power supply apparatus, including: a transformer that receives input voltage on a primary side; a switching element that is to be turned on or off for rectifying current of a secondary coil of the transformer; a control circuit to drive the switching element. The control circuit includes a first timing detection circuit to detect first timing at which forward current flows through a body diode of the switching element, and a second timing detection circuit to detect second timing with counter electromotive voltage that is generated at an instant when the body diode is turned off. The control unit is to generate an on/off control signal for the switching element to turn on the switching element at the first timing, to turn off the switching element before the second timing and to bring off-timing of the switching element close to the second timing.