Is it possible to solve design challenges related to isolated DC-DC conversion without using optocouplers? The short answer is yes. Fortunately, we can now utilize non-opto flyback DC-DC converter solutions that do not require optocouplers, their feedback circuits, or third-order transformer windings. This new solution sets a new standard for output voltage accuracy.

■ Using a solution that does not require an optocoupler simplifies design and can reduce solution size

Isolated DC-DC solutions are often required for safety reasons or to ensure proper operation in complex systems. Conventional isolated solutions utilize optocouplers and associated circuitry to form feedback loops through isolation barriers to control output voltage, or employ complex transformer designs. These additional components complicate the design and increase the size of the solution. Furthermore, optocouplers can degrade over time, potentially compromising system reliability. In addition, the trend of continuously shrinking final system form factors is reducing the space available for power supplies, which exacerbates thermal management issues. System engineers must address all these challenges when designing isolated DC-DC devices. This requires a solution that is small, low-cost, highly reliable, and easy to design. By using a solution that does not require optocouplers to meet these requirements, design can be simplified and solution size reduced.

■ Reasons for using an isolated DC-DC converter

Many power supplies used in factory automation, building automation, e-mobility, vehicles, avionics, medical, commercial, and various other industries use isolated DC-DC converters for the following three reasons.

○ Safety

Isolated DC-DC converters can be used to prevent current surges from damaging equipment and to protect workers from the main power source. Figure 1 illustrates a scenario where a worker may come into physical contact with a power system in which the main power source is isolated from the secondary side. If proper safety insulation is not provided, in the event of a lightning strike, a very high voltage surge can be transmitted to the ground through the equipment and the worker. This would result in fatal consequences. In such cases, the insulation barrier prevents the high voltage surge from being transmitted to the worker by diverting the dangerous surge energy to the primary ground.

○ Prevention of grounding loops

In large or complex systems, ground potential differences exist at various points. In such cases, using electrical insulation can avoid destructive ground loops and block digital noise in precision analog systems.
○ Level Shifting

Sometimes, in systems using multiple power rails, an isolated DC-DC converter can be used to generate multiple isolated positive and negative output voltages.
■ Fundamentals of Isolated DC-DC Converters

Figure 4 shows a conventional isolated DC-DC converter. This solution forms a feedback loop through an isolation barrier by using an optocoupler, an error amplifier, and a voltage reference. In this method, the error amplifier detects the output voltage and then compares it with the voltage reference. When this information is transmitted to the primary side through the optocoupler and the insulation barrier, the control circuit modulates the power stage to regulate the output voltage.
This solution will faithfully perform its function until the equipment size is gradually reduced to the point where there is no longer any space left for the power supply. However, since the optocoupler, error amplifier, and voltage reference circuit consist of 12 components, they significantly increase the total number of components in the power supply design and occupy a large amount of board space (Figure 5). Therefore, it is a natural step to seek a way to eliminate this circuit.
Optocouplers have another significant problem: their performance varies with temperature and degrades over time. This can lead to reliability issues depending on the application. Figure 6 shows that the Current Transfer Ratio (CTR) of a typical optocoupler varies by up to 270% over a temperature range of -60 to 120°C. In addition, this CTR decreases by 30 to 40% over time2,3,4.
■ Remove optocoupler

○ Primary control topology

One way to eliminate the optocoupler is to use a primary-side control technique. This method uses a tertiary winding on a power isolation transformer to indirectly measure the output voltage during the off-cycle. Figure 7 shows this circuit. The reflected voltage (VW) is proportional to the output voltage according to the following formula.
In this formula, VO is the output voltage, VF is the output rectifier diode voltage drop, Na is the number of turns in the tertiary winding, and NS is the number of turns in the secondary winding.
Although this method effectively eliminates optocouplers, it introduces the following new problems.

a) Since a third winding must be added, the transformer design and structure become more complex and the cost increases.
b) The reflected voltage is related to the output rectifier diode voltage (VF), which varies with load and temperature. This causes an error in the detected output voltage.
c) Due to the leakage inductance ringing phenomenon on the VW, the reading error of the detected output voltage is further exacerbated.

This primary-side control technique is not very practical in many applications due to poor output voltage regulation. Therefore, designers must use a post-regulator, which increases costs and expands the overall solution size.

○ Flyback topology that does not require an optocoupler

Flyback DC-DC, which does not require an optocoupler, is a modified primary side control technique. This method eliminates the need for a tertiary winding in the power transformer because it solves the aforementioned problem (a) by directly detecting the primary side voltage. This significantly reduces complexity in transformer design and structure, as well as PCB layout. Figure 8 illustrates this topology.
The reflected voltage (VP) is proportional to the output voltage according to the following formula.
In this formula, VO is the output voltage, VF is the output rectifier diode voltage drop, NP is the number of turns in the primary winding, and NS is the number of turns in the secondary winding.

Flyback topologies that do not require optocouplers are not new, but they are still causing trouble due to the other two problems mentioned earlier, (b) and (c). In this case, (c) leakage inductance ringing occurs at VP rather than VW. Poor output voltage regulation in flyback circuits that do not require optocouplers remains a significant technical challenge.

Fortunately, this problem has been significantly improved with the recent introduction of enhanced circuit techniques.

○ Resolving poor output voltage regulation issues

Figure 9 shows the MAX17690, a flyback isolated DC-DC converter solution that does not require an optocoupler, and the MAX17690 achieves 5% output voltage regulation.
To overcome errors in detected output voltage readings, the MAX17690 samples the reflected voltage when the secondary current (ISEC) is low. This mitigates variations in diode voltage caused by the output load. Additionally, this IC is designed to compensate for diode voltage and temperature-dependent variability. The IC also filters out leakage inductance ringing using advanced techniques. Combining all these characteristics, the MAX17690 sets a new standard for output voltage regulation in flyback topologies that do not require optocouplers.

Figure 10 shows the MAX17691, another model in the same product family. The MAX17691 integrates a power FET and a current detection element, so the entire circuit can be completed by adding only a very small number of external components. This product provides a true high-performance isolated DC-DC converter solution in a highly simplified form.
The MAX17690 and MAX17691 achieve excellent output voltage regulation. Figure 11 shows the performance of these devices under temperature, line, and load variations.
■ Conclusion

As equipment size and board space continue to shrink, conventional isolated DC-DC converters that require optocouplers for feedback loops are no longer effective in terms of size. Furthermore, optocouplers have the additional problem of performance varying with temperature and degrading over time. Flyback topologies, which do not require optocouplers, are naturally a better choice as they simplify the design and require fewer external components. Recently, advancements in design techniques have made it possible to significantly improve output voltage regulation. Thanks to this, flyback DC-DC converters that do not require optocouplers have become suitable for practical use in isolated power supply applications.

source
1 Vishay VOS618A Data Sheet
2 Vishay Optocoupler Application Note, Document Number: 80059
3 Toshiba Photocoupler Application Note
4 CTR Degradation and Aging Problem of Optocouplers, Prof. Eng. Titu I. BAJENESCO, M. Sc.

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※ Author Introduction
Thong Anthony Huynh was a member of the technical staff and a Senior Application Engineering MTS at Analog Devices. He has over 20 years of experience in the design and development of isolated and non-isolated switching power supplies and power management products. He participated in the development of over 100 power management products adopted by major global customers, including DC-DC converters, hot-swap controllers, Power over Ethernet (PoE), and various system protection ICs.
He holds four U.S. patents related to power electronics and has authored numerous articles and application notes. He earned a bachelor's degree in electrical engineering from Oregon State University and completed a master's program in electrical engineering at Portland State University. He also taught power electronics as an instructor at Portland State University.