Flyback converters require stable output voltage.
Consider an alternative device to the optocoupler control module.
Eliminating discrete feedback paths simplifies the solution.


Isolated flyback converters are commonly used for power conversion of approximately 60 W or less. They convert the power supply voltage into an output voltage by using a transformer with an adjustable turns ratio and a primary switch.

To maintain the output voltage as stable as possible, information about the output voltage is transmitted to the primary PWM generator through a feedback path. If the output voltage is too high or too low, the duty cycle of the PWM generator is adjusted accordingly.

These feedback paths add cost, take up board space, and, along with the transformer's isolation voltage, determine the circuit's maximum isolation voltage. Optocouplers age over time, changing their characteristics, and are typically not designed to operate above 85°C.

Besides optocouplers, there are also methods to provide information about the output voltage status using transformer auxiliary windings. Here, output voltage regulation can be performed accordingly.

However, using these additional transformer windings increases the transformer cost and does not make the output voltage regulation particularly more accurate.

A better approach is to use a device that replaces the optocoupler and its secondary control module. Analog Devices' (ADI) ADuM3190 integrates iCoupler isolation technology, enabling the feedback signal to be transmitted inductively via galvanic isolation, eliminating the need for an optocoupler.

There's another way. If there's a way to completely eliminate the discrete feedback path, this would simplify the solution. Figure 2 shows a flyback converter without the discrete feedback path, such as ADI's LT8300.

This device allows you to measure the reflected voltage from the secondary to the primary, and thus determine whether and how to adjust the duty cycle produced by the PWM generator.

The advantage of this solution is that it eliminates the need for optocouplers or other feedback circuits, saving cost and space. Furthermore, it avoids the limitations of the feedback path's maximum isolation voltage. The transformer used can operate at the maximum isolation voltage within the range designed for that specific voltage.

This solution is based on the concept of boundary mode regulation, which means that the secondary current drops to 0A on each cycle. Next, the output voltage is reflected to the transformer primary winding, which is measured and used for primary side regulation.

Whether this circuit, which eliminates the discrete feedback path, is feasible will depend on the output voltage regulation accuracy required by the application. While accuracy within ±1% can be achieved, variations may occur depending on the application.

The output voltage can be calculated using the following formula:
Rfb is shown in Figure 2, and can be used to control the output voltage. Nps is the turns ratio of the transformer used, and Vf is the voltage drop across the secondary flyback diode. This is largely affected by temperature.

When the output voltage is high, such as 12 V or 24 V, the effect of Vf is small. When the output voltage is below 3.3 V, the effect of temperature on the output voltage becomes quite large. Some products that do not use optocouplers include a temperature compensation feature to compensate for different rectifier diode voltage drops at different temperatures.

Additionally, to maintain proper regulation, a minimal load is required at the output. For the LT8300, this is approximately 0.5% of the maximum load.

By eliminating the discrete feedback path and using a flyback controller that controls through the primary transformer winding, the design can be simplified by eliminating the error-prone optocoupler.



This article is a summary of an article titled "The Elegance of a Flyback Controller Without a Dedicated Isolated Feedback Path" by Frederik Dostal, FAE, Power Management, ADI Munich, Germany.