Technical Article

Isolated Flying Capacitor Multilevel Converter Basics

April 19, 2023 by Rakesh Kumar

Learn about the basics, design, and characteristics of isolated flying capacitor multilevel converters in this first article in a four-part series.

Modern society relies on direct current (DC). DC is also the preferred energy source for renewable energy sources, with photovoltaic energy being one of the most prominent examples. Power electronics are used to process the power, converting one DC voltage to another and ultimately inverting it into usable AC power. A sizeable percentage of that AC electricity will be converted back to DC and used in consumer gadgets like LED lighting, gaming consoles, televisions, and cell phones.


What is the Relevance of DC-DC Converters?

High-frequency flying capacitor multilevel converters (FCMC) have been developed to increase power density and efficiency. The use of DC-DC converters for power conversion is a growing industry that constantly calls for smaller and more effective solutions. The integration of renewable energy, microgrids, industrial power sources, and transportation are all included in the applications.

The capabilities of fundamental power electronic topologies, including the multilayer boost converter, have been improved using flying capacitors. The multilevel structure provides better efficiency, less device stress, and higher power density by distributing voltage gain over individual switching cells that house the FC. The buck and buck-boost converters have also been given multilevel architectures with similarly high efficiencies. This method has not yet been tested and paired with the flyback converter.


Why Do We Need Isolated Flyback Converter Topologies?

Flyback converter topologies are frequently used in power conversion because they are inexpensive, highly versatile, and operate over a wide range of input and output voltages. The flyback converter is an excellent choice for high step-up voltage conversion because it also offers electrical isolation. The flyback converter is a well-liked isolated converter for renewable applications. However, the switch and diode stress it can withstand restrict its use.

To achieve sophisticated control approaches, such as zero-voltage/current switching, active clamp switching schemes are used to reduce the stress on the primary switch. These active circuits are effective at decreasing primary FET stress. However, the flying capacitor multilevel flyback converter (FCMFC) already performs this by using secondary flying capacitor voltage cells to lower the primary reflected voltage. The power potential of this converter is raised by using the suggested multilevel flying capacitor variation due to the reduced stress and improved efficiency.


Figure 1. Flying capacitor multilevel flyback converter general form. Image used courtesy of IEEE Open Journal of Power Electronics


In Figure 1, a generalized FCMFC with a single diode capacitor (SDC) stage is depicted. This is a flyback converter rather than a typical voltage multiplier. This general form can be used to create multiple converters for N levels, where (N-1) is the number of capacitors in a particular converter. A larger turn ratio can be employed for additional gain at the expense of higher voltage and magnetic strains.


Design Aspects of Isolated Flyback Converters

This develops an alternative method of boosting gain such that equation (1) will always be added to the gain already obtained from the double wound inductor, equation (2) minimizes the use of custom magnetic designs, and equation (3) allows for the secondary stage to experience efficiency improvement rather than the magnetics. FCMFC can use an off-the-shelf transformer made for a flyback design but can be used for higher voltage gain at the cost of more semiconductors and more complicated control. Yet, the FCMFC can attain larger output voltages and high efficiencies because of the considerable gain boost offered by flying capacitors.




\[\Delta i_{Lm}=\frac{V_{tin}D}{2f_{s}L_{m}}\,\,\,\,\,(3)\]


Designers use a high turn ratio to produce high voltage gain for new applications using flyback converters. The primary FET and secondary output diode still achieve high stress ratings despite the elaborate custom magnetic design needed. Transformers are frequently constructed to order for a specific application, although numerous commercially available choices are available and can be used in an FCMFC framework to cover a wider application range than a simple flyback could. The proposed topology's higher voltage gain and lower primary FET stress make this practicable.


Flyback Converters and Other DC-DC Converter Topologies

Boost converters and other voltage multipliers are frequently used in this field to modify flyback converters. Modified boost converters are also used, but their gain is constrained. The control system complexity will increase since all the modified flyback converters will employ a moderate turn ratio and obtain the remaining gain from a different converter stage. The SDC secondary side stages of the FCMFC can utilize a phase-shifted pulse width modulation (PSPWM) method to obtain high voltage gain. FCMFCs have the advantage that the voltage gain goes up with more stages, no matter what the transformer turn ratio is.

Some FC topologies do not directly relate gains to the number of stages. Few FC converter topologies function by adding one flying capacitor stage, which increases the available voltage gain of the standard flyback converter by two; adding two will increase the gain by three, and so on for higher-order FCMFCs. This demonstrates that the main input switch can operate with a lower duty cycle and stress than a flyback converter. The suggested topology will always have double, triple, etc., the gain of any flyback converter for a given transformer and duty cycle, allowing for greater capability to be drawn from existing flyback topologies.


Isolated Flyback Converters with Multilevel Converter Features

Multilevel converters present difficulties, notably in balancing the FC voltages. Voltage imbalance is a problem that is made worse with higher-level converters because it results from variations in component values and gate-triggering time delays. Despite the natural balance that some converters encounter, any variance immediately interferes with the lower component stress requirements. Control methods have been created to resolve this problem and stop a stage from failing too soon.

By changing the switching scheme, the natural balance has been achieved. Value valley current detection and a constant effective duty cycle were used to achieve active balancing while considering the light load situation. Previous research has introduced the FCMFC and demonstrated how open-loop PSPWM provides natural flying capacitor voltage balancing. This is essential to the decreased voltage stress observed by semiconductor devices.


Takeaways of Isolated Flying Capacitor Multilevel Converters

  • DC-DC converters are necessary for modern society, powering everything from renewable energy sources to consumer electronics.
  • High-frequency flying capacitor multilevel converters have been developed to increase power density and efficiency, improving the performance of DC-DC converters.
  • Multilevel architectures, such as the multilayer boost converter, provide better efficiency, less device stress, and higher power density than traditional topologies.
  • The latest advancements in converter technology continue to improve their efficiency and effectiveness, with researchers exploring new materials and designs to enhance their capabilities further.
  • The future of DC-DC converters looks promising as they become even smaller and more effective, enabling new applications in medical devices, transportation, and microgrids.

This post is based on an IEEE Open Journal of Power Electronics research article.

Featured image used courtesy of Adobe Stock