No need for a dedicated isolated feedback loop, this is how a simple flyback controller works~

Figure 1. Traditional flyback controller with optocoupler feedback path.

This feedback path adds cost, takes up space on the board, and together with the isolation voltage of the transformer determines the maximum isolation voltage of the circuit. Optocouplers typically age, changing their characteristics over time, and are generally not rated for temperatures above 85°C.

In addition to the optocoupler, a third transformer winding can be used to provide information about the state of the output voltage. The output voltage can be regulated based on this information. However, this additional transformer winding makes the transformer more expensive, and the regulation of the output voltage is not particularly precise.

A better alternative is to replace the optocoupler and the optocoupler's secondary-side control block with an alternative device. The ADuM3190 is designed to transmit the feedback signal across the galvanic isolation using iCoupler® ^isolation technology by inductive coupling (i.e., without the need for an optocoupler).

However, there is an alternative. A particularly elegant solution is to eliminate the discrete feedback path altogether. Figure 2 shows a flyback converter without a discrete feedback loop. A suitable converter IC, the LT8300 from ADI, shown in Figure 2, can recognize if and how to adjust the duty cycle produced by the PWM generator by the voltage that is fed back from the secondary side to the primary side. The advantage of this solution is that no optocoupler or other feedback circuitry is required. This saves cost and space. And there are no limitations associated with the maximum isolation voltage of the feedback path. As long as the transformer used is designed for a specific isolation voltage, the entire circuit can operate at that maximum isolation voltage.

This concept is based on boundary mode regulation. Here, the secondary current drops to zero amperes during each cycle. The output voltage that is flybacked back to the primary winding of the transformer can then be measured and used for primary side regulation.

Figure 2. A flyback controller does not require a discrete feedback path but instead provides regulation via the primary transformer winding.

Whether such a circuit can be used without a discrete feedback path in a given application depends largely on the required output voltage regulation accuracy. This accuracy can be better than ±1%, but the deviation can also be greater, depending on the application.

The output voltage can be calculated as follows:

Rfb is shown in Figure 2. It can be used to adjust the output voltage. Nps is the turns ratio of the transformer used and Vf is the voltage drop across the secondary flyback diode. It is usually very temperature-dependent. For output voltages set to higher values, such as 12 V or 24 V, the absolute effect of temperature on Vf is small. For output voltages set to 3.3 V or lower, the effect of temperature on the output voltage is very large. Some series without optocouplers have built-in temperature correction to compensate for the voltage drop of different rectifier diodes at different temperatures.

For the voltage regulation function to work properly, there is usually also a minimum load required at the output. In the LT8300, it is about 0.5% of the maximum possible load.