Optimization of a Shunt Reference for Flyback Converters
Abstract
Flyback converters are widely used in AC/DC power adapters and provide key advantages compared to other conventional AC/DC converters, such as isolation and competitive cost. Shunt references enable high-precision SSR (Secondary Side Regulation) in flyback converters and, when implemented properly, can make sure low standby power dissipation, high output accuracy, and quick transient responses to the flyback controller. When incorporating a shunt reference into a flyback feedback loop, designers must consider biasing, power dissipation, and transient response, and select their components accordingly. This application note outlines the key specs of shunt references in flyback design and explains how to effectively use these shunt references in the flyback control loop for SSR. Furthermore, these explanations are demonstrated with the implementation and comparison of the TL431, TLVH432, ATL431, TL431LI, and ATL431LI in the feedback loop of a flyback converter, which are some of the popular designs for SSR. This application note does not discuss the impacts of different types of compensation on the flyback feedback network.
Trademarks
All trademarks are the property of their respective owners.
1 Introduction
Flyback converters are widely used in power adapters for personal electronics. These AC/DC or DC/DC converters provide the advantage of electrical isolation due to an isolation transformer, which protects end equipment from voltage surges and ground loops. The key elements of a flyback converter include a feedback network, PWM controller, gate driver IC, and switching MOSFETs. A flyback converter uses either primary-side regulation (PSR) or secondary-side regulation (SSR) to provide feedback to the PWM controller to set the output voltage. SSR provides the advantages of a quick transient response, reduced noise coupling between the primary and secondary sides, and improved accuracy across all load conditions with the disadvantage of an increased component count. SSR implementation requires an optocoupler and a shunt reference acting as an error amplifier in the feedback network of the flyback converter. When selecting this shunt reference for the feedback network, designers must balance accuracy, power dissipation, and transient response requirements. Once this shunt reference is selected, the flyback converter must be configured as an error amplifier to set the flyback converter's output voltage properly. This shunt references performance, and proper biasing can optimize power dissipation. Figure 1-1 shows a list of popular shunt references for designer needs.
Figure 1-1 Shunt Reference Chart