Secondary Side Synchronous Rectifier Control in Forward Converters
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Solution Overview
Problem
Existing forward converters require a separate transformer for primary side-to-secondary side communication to control secondary side synchronous rectifiers, increasing cost, size, and complexity.
Innovation Solution
A forward converter design that controls secondary side MOSFETs solely based on secondary side signals, using a delay locked loop to predict primary side MOSFET switching and prevent simultaneous operation of secondary side MOSFETs with the primary side MOSFET, eliminating the need for a transformer for communication.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a separate transformer is used for primary side-to-secondary side communication to control secondary side synchronous rectifiers, then reliable control of secondary side MOSFETs is achieved, but converter size, cost, and complexity increase
Solution Approach 1:
The patent extracts the communication function from the traditional transformer-based approach and implements it using secondary side voltage detection. The control signals are derived directly from the secondary side voltage waveform, eliminating the need for a separate communication transformer and reducing system complexity while maintaining reliable control of secondary side MOSFETs
Solution Approach 2:
The secondary side circuit serves itself by detecting its own voltage waveform to generate control signals for the synchronous rectifier MOSFETs. The voltage across the secondary winding naturally provides the timing information needed to coordinate primary and secondary side switching, making the system self-regulating without external communication infrastructure
2Measurement precision
If a separate transformer is used for primary side-to-secondary side communication, then accurate timing control is achieved, but converter size and cost increase
Solution Approach 1:
The patent removes the communication transformer from the system and extracts timing information directly from the secondary side voltage waveform. The zero-crossing detection of this waveform provides precise timing signals for MOSFET switching without requiring additional heavy magnetic components
Solution Approach 2:
The secondary side voltage waveform serves multiple functions simultaneously: it provides the timing reference for synchronous rectification, indicates the presence of primary side switching events, and enables coordination between primary and secondary side MOSFETs. This multi-functionality eliminates the need for dedicated communication hardware
3Device complexity
If secondary side MOSFETs are controlled without explicit communication from primary side, then converter size and cost are reduced, but risk of simultaneous MOSFET conduction increases
Solution Approach 1:
The patent implements preliminary action by detecting the voltage waveform on the secondary side and using it to predict and prepare for primary side MOSFET switching events. The control circuit turns off the secondary side MOSFETs in advance based on the detected voltage transitions, preventing simultaneous conduction before it can occur
Solution Approach 2:
The system uses feedback from the secondary side voltage waveform to continuously adjust the timing of secondary side MOSFET switching. The detected voltage transitions provide real-time information about primary side switching events, enabling the control circuit to synchronize and coordinate MOSFET operations across both sides of the converter
Data Source
AI summary
A forward converter has a primary side containing a PWM controller for controlling switching of a power switch and has a secondary side coupled to the primary side via a transformer. The secondary side includes a forward transistor and a catch transistor. A secondary side switch controller controls switching of the forward transistor and the catch transistor without communication from the primary side. The secondary side switch controller detects the rising and falling of the voltages at the ends of the secondary winding to control the switching of the forward and catch transistors. A delay locked loop (DLL) is provided in the secondary side switch controller that turns on the catch transistor when the power switch is turned off and turns off the catch transistor at a predetermined time before the power switch is turned on. A separate circuit controls the catch transistor during a discontinuous mode.