In the design of an isolated power supply, one of the first decisions to be made is the use of an open loop or closed loop configuration. Open loop designs are simpler because loop dynamics such as settling time, bandwidth and stability do not need to be considered or are much easier to analyze. Closed loop designs offer better performance in terms of regulation, temperature performance, and accuracy at the expense of a more complex design and potentially a higher bill of materials cost. A traditional closed loop architecture uses a primary-side controller with an optocoupler for isolated feedback from secondary to primary. The input voltage connects to the primary side of the transformer, which is switched on and off by a primary controller. The secondary of the transformer connects to a rectifying diode and then to the output filter capacitor. The output voltage is sampled by the shunt regulator and fed back to the primary side controller by the optocoupler. The shunt regulator has an internal reference and error amplifier, requiring a compensation network to stabilize the loop. Type 2 or type 3 multi-order R/C networks are typically needed, requiring complex analysis of loop dynamics and margining of component tolerances. The optocoupler’s current transfer ratio (or CTR) must be considered in this analysis since it affects the open loop gain. The CTR degrades over time and is a function of both the diode’s forward current and the operating temperature. These factors must be taken into account in the compensation network design, further complicating the analysis.