Designing with Flyback Converters for High Power Applications
摘要
This article describes the possibility of increasing the power level of a flyback converter by running multiple phases, with multiple transformers in parallel. This setup also lowers the conducted emissions on the input side of a flyback switch-mode power supply topology.
Introduction
A multiphase flyback converter pushes the maximum possible power limit, is easy to design, and generates less conducted interference.
Flyback converters are a good way to generate a regulated, galvanically isolated voltage. The range of applications of this voltage conversion technology is wide, as the circuit is quite simple, and the technology is mature. Figure 1 shows a simplified representation of a flyback converter.
Figure 1. A simple flyback converter with no-opto technology.
However, there are some limitations to the use of the flyback technology. The maximum transferable power is limited. This is because during the on-time of switch Q1 in Figure 1, a current flows on the primary side of the transformer. During this time, energy is stored in the transformer core T1. During the off-time of Q1, there is no current flow on the primary side, but a current is formed on the secondary side of the transformer. The previously stored energy is released via the secondary winding.
There is a limit to the maximum energy that can be stored in a transformer. Thus, the maximum power of a flyback converter is limited. While it is possible to achieve an output power of over 100 W with special transformers, a flyback configuration is typically employed for output powers up to approximately 60 W.
To effectively operate a flyback topology at higher power levels, there is an unusual but clever method. By using multiple channels, a flyback converter can be run with two or more transformers, dividing the output power among them. These transformers are readily available in a wide range of options and can be used in parallel.
Figure 2 shows a 2-channel flyback converter circuit. This is controlled by a special controller integrated circuit, the MAX15159. This IC is a 2-channel flyback controller that works with phase shift and ensures that the currents are evenly distributed through the two parallel power paths. It is even possible to operate a 4-phase flyback with four transformers using two MAX15159 flyback controllers. As a result, such a circuit can generate very high power of over 100 W while using small transformers.
Figure 2. A MAX15159 can control multiphase flyback circuits.
Similar to single-channel flybacks, a multiphase flyback can also be operated without an optocoupler in the feedback path. The MAX15159 is equipped with the no-opto technology. This technique regulates the output voltage by evaluating the voltage across the primary-side winding during the off-time.
One unique advantage of the multiphase flyback is the fact that conducted interference can be reduced. On the input side, a flyback behaves like a switch-mode step-down converter; that is, like a regulator using the buck topology. In both topologies, pulsed input currents are being generated. To minimize interference on the input side, the individual channels in the multiphase reverse converter are phase-shifted; that is, activated at different times. This not only improves the electromagnetic interference (EMI) behavior but also reduces the size and number of necessary input-side capacitors. Figure 3 shows the input-side currents of a 2-channel flyback converter.
Figure 3. The current flow on the input side of a multiphase flyback converter.
An interesting advantage of multiphase flyback converters is that they can be used in many applications with simple, inexpensive, and small transformers instead of one large one.
Conclusion
To develop a galvanically isolated power supply, there are other options beyond the usual solutions with a flyback converter at a low power level below 60 W and a forward converter above 60 W. It is also possible to operate a multiphase flyback converter at power levels above 60 W. Solutions such as the MAX15159 controller integrated circuit are available with no-opto technology. No optocouplers are required, and the generated conducted interference is minimized by the phase-shifted control scheme.
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