2 Designing for SSR With a Shunt Reference

The design of a SSR feedback loop utilizes a shunt reference and optocoupler to send a continuous feedback signal from the secondary side of the flyback converter to a PWM controller on the primary side. The optocoupler allows for this feedback signal to be transferred while maintaining electrical isolation between the primary and secondary sides. The flyback converters output voltage is sampled through a resistor divider, which is compared to the internal reference voltage of a shunt reference. This reference voltage controls the current being shunted to ground through the shunt references cathode. This current being shunted to ground is pulled through the optocoupler LED, which activates the phototransistor on the primary side of the feedback loop. This phototransistor is connected to the flyback PWM controller, which allows a feedback signal from the secondary side to be received on the primary side. This feedback signal being received by the PWM controller increases and decrease proportionally with the current being shunted to ground through the shunt reference, meaning that this feedback signal changes with the output voltage. The PWM controller responds to these feedback signals by regulating the output voltage to the programmed value.

Figure 2-1 Typical SSR Feedback Loop (Without Compensation)

When designing a SSR feedback loop of a flyback converter the implementation of the chosen shunt reference is critical to the performance of the flyback converter. The performance of this chosen shunt reference can be optimized by properly setting the output voltage of the flyback converter, meeting the shunt references biasing requirements, and selecting a shunt reference that can meet transient requirements. When selecting a shunt reference for SSR, a designer also needs to balance the requirements of power dissipation, transient response, and accuracy.