Self-Driven Synchronous Rectifier Circuit Using Body Diode Feedback
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Solution Overview
Problem
Conventional synchronous rectifier controllers are complex, expensive, and not easily adaptable for various applications, leading to inefficiencies and potential shoot-through currents due to improper switching responses in power supply systems using MOSFETs as synchronous rectifiers.
Innovation Solution
A simplified control circuit using a MOSFET, BJT, and diode network that senses current flow to synchronize the active switch's turn-on with the desired current direction, reducing voltage drop and power dissipation by allowing current to flow in one direction through the MOSFET, thereby preventing cross-conduction and shoot-through currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional synchronous rectifier controllers are used, then switching control is provided, but the controller complexity and cost increase
Solution Approach 1:
The MOSFET's body diode automatically provides current direction sensing and gate drive timing without external control circuitry. The diode's natural conduction characteristics during reverse recovery period inherently generate the required gate drive signal, eliminating the need for complex conventional controllers
Solution Approach 2:
The patent extracts and utilizes the inherent body diode functionality from the MOSFET structure itself to provide the control function, rather than adding a separate complex control circuit. This removes the unnecessary controller complexity while maintaining reliable switching control
2Loss of energy
If MOSFET is used as synchronous rectifier, then voltage drop and power dissipation are reduced, but improper switching timing causes shoot-through currents
Solution Approach 1:
The body diode's reverse recovery current serves as automatic feedback that detects the zero-crossing point of the load current. This feedback mechanism precisely times the MOSFET turn-on event to occur exactly when the current transitions, preventing shoot-through currents while maintaining low power dissipation
Solution Approach 2:
The body diode conducts in advance during the reverse recovery period, preparing the current path and generating the gate drive signal before the MOSFET needs to turn on. This preliminary action ensures the MOSFET switches at the correct moment, avoiding harmful shoot-through currents
3Ease of manufacture
If simple controller is used, then manufacturing cost is reduced, but switching response timing may be improper
Solution Approach 1:
The body diode automatically generates the precise timing signal for MOSFET switching through its natural reverse recovery characteristics, eliminating the need for expensive complex controllers while maintaining accurate switching response timing
Solution Approach 2:
The patent exploits the time-dependent parameter of the body diode's reverse recovery process to generate the correct switching timing. By utilizing the diode's inherent electrical characteristics rather than external control, the system achieves both low cost and precise timing
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
The control circuit enhances efficiency and reduces manufacturing costs by providing a self-driven method for MOSFETs in synchronous rectification, ensuring correct timing of gate drive signals and minimizing power losses, making it more versatile and cost-effective compared to conventional designs.
Implementation Method 1
sensing the current flow at the MOSFET's on-state resistance
Implementation Method 2
A cathode side of the diode is connected to the drain of the MOSFET. The method also includes allowing current to flow through the diode but not through the BJT
Implementation Method 3
the on-state resistance of a MOSFET has a very low resistance value that provides a low voltage drop when compared to a Schottky diode alone
Data Source
AI summary
A control circuit that is applicable to power supply systems that use synchronous rectification techniques is described. The control circuit provides a self-driven method of control to an active switch by sensing the current flow over the switch. The control circuit includes a diode, a MOSFET, and a BJT. The control circuit may include a first resistor and a second resistor that are both connected to a voltage source. An anode side of the diode is connected to the first resistor while a cathode side of the diode is connected to a drain of the MOSFET. The second resistor is connected to a collector of the BJT as well as a gate of the MOSFET. A base of the BJT is connected to the first resistor and the anode side of the diode. An emitter of the BJT is coupled to a source of the MOSFET.