Secondary Side Current Mode Control for Isolated Converters
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Solution Overview
Problem
In electric power conversion systems, converters with galvanic isolation face challenges in feedback control due to parameters residing in two isolated power domains, making it difficult to effectively control the converter from the primary side based on secondary side feedback.
Innovation Solution
A flyback converter with a transformer having a primary and secondary winding, a secondary side control stage that generates a control signal based on output voltage and reference voltage, and an isolation stage that converts this signal to a primary control signal using an optocoupler, allowing the converter to be driven from the primary side based on current sense signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If galvanic isolation is used between primary and secondary sides, then safety and electrical isolation are improved, but feedback control complexity increases due to parameters residing in two isolated power domains
Solution Approach 1:
An optocoupler is introduced as an intermediary device to transfer control signals from the secondary side to the primary side while maintaining galvanic isolation. The optocoupler converts electrical signals to optical signals and back, enabling feedback control without direct electrical connection between the isolated power domains, thus resolving the contradiction between maintaining isolation and simplifying control.
2Measurement precision
If secondary side control is implemented, then control accuracy based on output voltage feedback is improved, but signal transmission across isolation boundary becomes more challenging
Solution Approach 1:
The optocoupler serves as a mediator to transmit the secondary control signal (generated based on accurate output voltage feedback) to the primary side. This allows the system to maintain high measurement precision for output voltage while managing the signal transmission challenge across the isolation boundary through optical coupling.
3Speed
If primary side switching control is used, then switching speed and power conversion efficiency are improved, but control based on secondary side feedback becomes difficult
Solution Approach 1:
A feedback loop is established where the secondary side monitors the output voltage and generates a control signal, which is then transmitted via optocoupler to the primary side. The primary side switching element (MOSFET) is controlled based on this feedback signal, enabling fast switching while maintaining ease of operation through closed-loop control that responds to actual output conditions.
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
Enables efficient feedback control of the converter without breaching the power domain isolation boundary, allowing for precise control of current conduction through the primary winding and maintaining the same circuit footprint, suitable for applications like USB power delivery.
Implementation Method 1
an isolation stage configured to generate a primary control signal on the primary side based on the secondary control signal
Data Source
AI summary
A method and apparatus for secondary side current mode control of a converter are provided. In the method and apparatus, an output voltage of the converter is detected, where the converter has primary and secondary windings that are galvanically isolated in respective primary and secondary sides. A secondary control signal is generated in the secondary side based at least in part on the output voltage and a reference voltage. The secondary control signal is converted to a primary control signal provided in the primary side. The converter is driven in the primary side based at least in part on the primary control signal and a current sense signal indicative of a current flowing through the primary winding.