Synchronous Rectification Control Circuit for DC-DC Converters
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Solution Overview
Problem
In insulated DC-DC converters with synchronous rectification, the delay in turning off the synchronous rectification element can lead to backflow and potential element destruction, especially during transitions from discontinuous to continuous current modes, and the turn-off threshold adjustment is sensitive to input/output conditions and noise, causing efficiency reductions.
Innovation Solution
A switching power supply device with a secondary side control circuit that includes an off-timing detection circuit and a threshold voltage setting circuit, which adjusts the threshold voltage based on the conduction periods of the secondary and primary coils, and a judgement timing detection circuit to determine the optimal turn-off timing, reducing the response speed requirements and preventing backflow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the turn-off threshold is adjusted based on body diode conduction time, then the turn-off timing can be optimized, but the adjustment is delayed during mode transitions causing backflow risk
Solution Approach 1:
The patent applies preliminary action by detecting the secondary coil conduction period in advance and pre-setting the turn-off threshold voltage before mode transitions occur. The control circuit determines the conduction period of the secondary coil and sets the turn-off threshold voltage based on this detected period, ensuring the threshold is ready before the transition happens, thus avoiding delayed adjustment and backflow risks
2Measurement precision
If a highly accurate comparator is used to detect source-drain voltage, then detection precision improves, but the circuit complexity and cost increase
Solution Approach 1:
The patent changes the detection parameter from source-drain voltage to drain voltage (with source connected to ground). By detecting only the drain voltage and comparing it with a predetermined threshold voltage, the circuit achieves sufficient detection accuracy without requiring highly accurate comparators, thus reducing circuit complexity and cost while maintaining functional effectiveness
3Productivity
If the turn-off threshold is adjusted based on body diode conduction time, then turn-off timing can be optimized, but the feedback adjustment takes too long during mode transitions
Solution Approach 1:
The control circuit detects the conduction period of the secondary coil and determines the turn-off threshold voltage in advance, before the actual turn-off event. This preliminary determination ensures the threshold is set appropriately for upcoming mode transitions, eliminating delayed feedback adjustment and preventing backflow during transitions between discontinuous and continuous current modes
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 solution stabilizes the turn-off timing, reduces the risk of backflow and element destruction, and maintains efficiency by prioritizing the judgement timing over the off-timing detection, even under varying load conditions and noise disturbances.
Implementation Method 1
a transformer for voltage conversion
Implementation Method 2
a switching element for synchronous rectification (MOS transistor) instead of the rectifying diode of the secondary circuit, and a secondary side control circuit to detect the terminal voltage (source-drain voltage) of a secondary switching element and control the turning on of the secondary switching element in synchronization with the off timing of a switching element in the primary circuit
Data Source
AI summary
Disclosed is a switching power supply device including: a transformer for voltage conversion; a main switching element connected in series to a primary coil; a primary side control circuit that performs on/off control of the main switching element; a synchronous rectification MOS transistor connected in series to a secondary coil; and a secondary side control circuit that performs on/off control of the MOS transistor, wherein the secondary side control circuit includes: an off-timing detection circuit that compares a drain voltage of the MOS transistor with a predetermined threshold voltage, and detects a timing to turn off the MOS transistor; and a threshold voltage setting circuit that sets the threshold voltage, and the threshold voltage setting circuit sets the threshold voltage based on a conduction period of the secondary coil or a period obtained by adding the conduction period to a conduction period of the primary coil.