Cutting the coupler

Normally, an optocoupler is used to close the feedback loop on low power (10 to 60W) isolated flyback power supplies. In this technique, a secondary side voltage reference and error amplifier drive an optocoupler, which sends a control signal back to the primary side for regulation and transient response. In addition to being parts intensive, placing an optocoupler in the feedback loop introduces design problems. If an optocoupler is to be used here, it must maintain tight tolerance of its current transfer ratio (CTR). A typical optocoupler’s CTR will vary 100% over a temperature range from 0 to 70°C, making it difficult to maintain proper gain and phase margins. Optocouplers are subject to degradation over time and it’s important not to drive excessive current through the emitting diode, which will cause premature aging. Changes to the CTR can cause oscillation or failure of the power supply. In addition, the closed loop power supply system’s response is dependent upon the optocoupler’s response time. Under the best of conditions, an optocoupler has a propagation delay of several microseconds. With typical operating frequencies in excess of 100kHz, the optocoupler could be the slowest responding part of the closed loop system – and this means excessive output voltage deviation from nominal during a transient.