Flyback Converter Secondary Switch Detection Circuit
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Solution Overview
Problem
Flyback converters face issues with battery discharge and potential damage during direct charge mode when disconnected from the AC mains, as they lack effective means to prevent power delivery reversal and excessive current flow to fully charged batteries.
Innovation Solution
A secondary-side switch detection circuit is implemented to indirectly detect the cycling of the power switch, either by monitoring synchronous rectification or output diode voltage, allowing the secondary-side controller to disconnect the voltage bus switch and prevent battery discharge or damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the flyback converter operates in direct charge mode without a battery charging management system, then device profile and cost are reduced, but the risk of battery discharge and damage increases when disconnected from AC mains
Solution Approach 1:
A secondary-side controller is introduced as an intermediary component that monitors the power delivery status and controls a voltage bus switch. This controller acts as a mediator between the flyback converter and the battery, detecting whether the primary power switch is cycling and disconnecting the output voltage lead when power is not being delivered, thereby preventing battery discharge without requiring a full battery charging management system
Solution Approach 2:
The secondary-side controller implements a feedback mechanism by monitoring the switching status of the primary power switch through the transformer's secondary winding. When the controller detects that the power switch is not cycling (indicating the converter is disconnected from AC mains), it provides feedback to open the voltage bus switch, thereby preventing harmful reverse current flow to the battery
2Productivity
If the voltage bus switch remains closed during direct charge mode, then power delivery to the battery is maintained, but battery damage occurs when the converter is disconnected from AC mains
Solution Approach 1:
The voltage bus switch is designed to be dynamic rather than static, changing its state based on real-time monitoring of the power switch cycling status. The switch remains closed during normal power delivery but automatically opens when the secondary-side controller detects that the primary power switch has stopped cycling, thereby adapting the circuit configuration to prevent battery damage while maintaining power delivery capability
3Reliability
If the secondary-side controller monitors the primary power switch cycling, then battery protection is achieved, but the controller must be isolated from the primary side switching
Solution Approach 1:
The control system is segmented into primary-side and secondary-side controllers that are electrically isolated by the transformer. The secondary-side controller U3 is isolated from the primary power switch S1 by the transformer T1, yet can still detect the power switch cycling status through the transformer's magnetic coupling. This segmentation allows the controller to monitor power delivery status without being directly exposed to high-voltage switching transients
Data Source
AI summary
A switch detection circuit is provided that senses the voltage on a rectifying component for rectifying a secondary winding current through a secondary winding of a flyback converter's transformer to determine whether a power switch transistor attached to a primary winding of the transformer has ceased cycling.