Designing high-power applications using flyback converters

Abstract

This article discusses the possibility of increasing the power level of flyback converters by using multi-phase operation (i.e., multiple transformers connected in parallel). In addition, this configuration reduces conducted emissions on the input side of flyback switch mode power supply topologies.

Introduction

The flyback converter can break through the maximum power limit, has the advantage of being easy to design, and can effectively reduce conducted interference.

The flyback converter is an ideal choice for generating regulated and electrically isolated voltages. With its simple circuit architecture and mature technology system, this voltage conversion technology has been widely applied in many fields. Figure 1 shows a simplified schematic diagram of the flyback converter.

However, the application of flyback technology has some limitations. Its maximum transmissible power is limited. This is because, during the conduction period of switch Q1, current flows through the primary side of the transformer, as shown in Figure 1. During this period, energy is stored in the transformer coil T1. During the off period of Q1, no current flows through the primary side, but a current is formed on the secondary side of the transformer. The previously stored energy is released through the secondary winding.

The maximum amount of energy that can be stored in the transformer is limited. Therefore, the maximum power of the flyback converter is also restricted. Although special transformer designs can enable output power exceeding 100 W, generally speaking, the flyback structure is more suitable for applications with output power of approximately 60 W or less.

Figure 1. A simple flyback converter using optocoupler technology.

To enable the flyback topology to operate efficiently in higher power applications, there is an ingenious yet highly sophisticated solution. By using multiple channels, the flyback converter can be equipped with two or more transformers to distribute the output power across these transformers. Such transformers are widely available in the market, offer diverse selection options, and support parallel operation.

Figure 2 illustrates a dual-channel flyback converter circuit. It is controlled by a special controller integrated circuit, the MAX15159. This integrated circuit is a dual-channel flyback controller that operates using phase shift technology to ensure uniform current distribution across the two parallel power paths. Notably, by using two MAX15159 flyback controllers, it is also possible to drive a four-transformer flyback circuit. As a result, such a circuit can achieve high-power output exceeding 100 W even when using small transformers.

Figure 2. The MAX15159 can control multiple flyback circuits.

Similar to single-channel flyback circuits, multi-channel flyback circuits can also operate without relying on optocouplers in the feedback path. The MAX15159 is equipped with optocoupler-free technology. This technology regulates the output voltage by evaluating the voltage across the primary-side winding during the turn-off time.

A unique advantage of the multi-flyback (converter) is its ability to effectively reduce conducted interference. On the input side, the flyback (converter) operates similarly to a switch-mode buck converter; that is, like a regulator using a buck topology. Both topologies generate pulsed input current. To minimise interference on the input side, phase-shifting technology is employed in the channels of the multi-channel flyback converter; that is, each channel is activated at different times. This not only improves electromagnetic interference (EMI) performance but also reduces the size and number of capacitors required on the input side. Figure 3 illustrates the input-side current characteristics of a dual-channel flyback converter.

Figure 3. Current flow on the input side of a multi-phase converter.

The multi-opposite-excitation converter has a unique advantage in that it can replace a single large transformer with multiple simple, low-cost, and compact transformers in many applications.

Conclusion

When developing power supplies with electrical isolation functionality, in addition to common solutions (i.e., using flyback converters for power ratings below 60 W and forward converters for power ratings above 60 W), there are other options available. Multi-stage flyback converters are also capable of handling applications with power ratings above 60 W. Solutions represented by the MAX15159 controller integrated circuit innovatively employ optocoupler-free technology. Such solutions eliminate the need for optocouplers and utilise phase-shift control strategies to suppress conducted interference to extremely low levels.

About the author

Frederik Dostal is a power management expert with over 20 years of industry experience. He studied microelectronics at the University of Erlangen in Germany and joined National Semiconductor in 2001 as a field application engineer, helping customers implement power management solutions in their projects and gaining valuable experience in the process. During this time, he also worked in Phoenix, Arizona, USA, for four years as an application engineer, focusing on switch-mode power supply products. He joined ADI in 2009, holding various roles across multiple product lines and European technical support positions, gaining extensive design and application knowledge. He currently serves as a power management expert. Frederik is based at ADI's Munich office in Germany.