Fly-Buck vs Flyback Converters

By Tomáš Zedníček
1 year Ago

This article provides a detailed technical explanation of the Fly-Buck converter and compares it with the traditional Flyback topology.

Key Takeaways

The Fly-Buck converter is an isolated DC/DC converter that simplifies design compared to the traditional Flyback converter.
Fly-Buck uses direct coupling for energy transfer, while Flyback relies on energy storage in a transformer.
Fly-Buck is more efficient, has a simpler control loop, and produces smaller components, making it ideal for low-power applications.
Flyback is versatile and effective for higher power needs, but it requires more complex regulation and larger components.
Choosing between Fly-Buck and Flyback depends on application requirements, efficiency, and design constraints.

Video from prof. Sam Ben-Yaakov introduces and explains Fly-Buck converter topology and its comparison to flyback converter design.

Fundamentals Background

Modern power electronics rely heavily on isolated DC/DC converters for bias supplies, gate drivers, and auxiliary rails. Two widely discussed topologies are the Fly-Buck converter and the Flyback converter. While both provide isolation and voltage conversion, their operating principles, design complexity, and performance differ significantly.

Fly-Buck Converter Fundamentals

The Fly-Buck is essentially an isolated buck converter. It uses a synchronous buck stage with a coupled inductor winding to generate isolated outputs. Unlike the Flyback, energy transfer is not based on transformer storage but on direct coupling between primary and secondary windings.

Primary stage: operates as a standard buck regulator.
Secondary stage: derives isolated voltage via coupled winding.
Feedback: typically taken from the primary side, simplifying control loop design.

Flyback Converter Fundamentals

The Flyback converter is a transformer-based isolated topology. It stores energy in the magnetizing inductance during the switch-on period and releases it to the secondary during switch-off. This makes the transformer act as a coupled inductor.

Energy storage: magnetic core stores energy during on-time.
Energy release: energy “flies back” to secondary during off-time.
Feedback: often requires optocoupler or auxiliary winding for regulation.

Historical Context and Motivation

The Flyback converter has been a cornerstone of isolated power supply design for decades, particularly in low-to-medium power ranges. Its simplicity and ability to provide multiple isolated outputs made it the default choice for bias supplies and auxiliary rails. However, with the rise of synchronous buck controllers and the need for compact, cost-effective isolated supplies, the Fly-Buck topology emerged as an alternative.

Texas Instruments popularized the term “Fly-Buck” around 2015, describing it as an isolated buck converter where the secondary winding is piggybacked onto the buck inductor. This approach reduces magnetic size and simplifies regulation, especially for gate driver supplies.

Operating Principle Comparison

The fundamental difference lies in how energy is transferred:

Fly-Buck: Energy is continuously delivered to the load through the buck inductor. The secondary winding couples directly to the primary current, producing isolated outputs without storing energy in the magnetic core.
Flyback: Energy is first stored in the transformer’s magnetizing inductance during switch-on, then released to the secondary during switch-off. This two-step process defines the Flyback’s characteristic discontinuous energy transfer.

Aspect	Fly-Buck Converter	Flyback Converter
Energy Transfer	Direct coupling via inductor winding	Stored in transformer core, released later
Feedback	Primary-side regulation	Requires optocoupler or auxiliary winding
Magnetics	Coupled inductor, smaller size	Transformer, larger core
Design Complexity	Simpler control loop	More complex compensation
Efficiency	High, due to synchronous buck core	Moderate, losses in transformer

Control and Feedback Mechanisms

Regulation is a critical design aspect:

Fly-Buck: Feedback is taken from the primary buck output. Since the secondary voltage is proportional to the turns ratio, regulation is indirect but sufficient for bias supplies. This eliminates the need for optocouplers.
Flyback: Feedback often requires optocouplers or auxiliary windings to sense secondary voltage. This adds complexity but allows precise regulation of isolated outputs.

Magnetic Design Considerations

Magnetics are central to both topologies:

Parameter	Fly-Buck	Flyback
Core Size	Smaller, due to continuous conduction	Larger, due to energy storage requirements
Flux Density	Lower, since energy is not fully stored	Higher, magnetizing inductance stores energy
Leakage Sensitivity	Moderate, affects coupling efficiency	High, impacts energy transfer and ringing

Mathematical Representation

For Fly-Buck, the secondary output voltage is directly tied to the buck duty cycle and turns ratio:

V_sec = D ⋅ V_in ⋅ N_s N_p, , where D is the duty cycle, Vin is the input voltage, and Ns/Np is the turns ratio of the coupled winding.

In contrast, Flyback output voltage depends on stored energy and release timing:

V_out = D (1-D) ⋅ V_in ⋅ N_s N_p

Design Trade-offs

– Fly-Buck advantages: simpler regulation, reduced component count, smaller magnetics, cost-effective for low-power isolated supplies.
– Flyback advantages: flexible voltage ratios, well-established design practices, suitable for higher power levels.

Efficiency and Noise Characteristics

– Fly-Buck: Higher efficiency due to synchronous buck operation and reduced switching losses. Noise is lower, but cross-regulation between multiple outputs can be challenging.
– Flyback: Efficiency is lower, especially at light loads, due to transformer losses. Noise is higher due to leakage inductance and ringing, requiring snubbers or clamp circuits.

Application Domains

– Fly-Buck: Ideal for isolated bias supplies in gate drivers, auxiliary rails in industrial systems, and low-power isolated outputs where simplicity and cost are critical.
– Flyback: Preferred for higher power adapters, chargers, and multi-output supplies where precise regulation and flexibility outweigh complexity.

Conclusion

The Fly-Buck converter represents a modern evolution of isolated bias supply design, offering simplicity, compact magnetics, and primary-side regulation. It is particularly well-suited for low-to-medium power applications such as gate driver supplies, auxiliary rails, and industrial bias circuits. In contrast, the Flyback converter remains a robust and flexible choice for higher power levels, multi-output systems, and applications requiring precise secondary regulation.

Ultimately, the choice between Fly-Buck and Flyback should be guided by application requirements, efficiency targets, and design constraints. While Fly-Buck simplifies control and reduces cost, Flyback provides proven reliability and scalability across a wide range of power levels.

Design Guidelines

1. When to Choose Fly-Buck

Bias supplies for IGBT/MOSFET gate drivers requiring isolated rails.
Auxiliary rails in industrial and automotive systems where compact size is critical.
Applications with moderate isolation requirements and predictable load conditions.
Designs seeking reduced BOM cost and simplified control loop tuning.

2. When to Choose Flyback

Power adapters and chargers requiring wide input/output voltage ranges.
Multi-output supplies where independent regulation is necessary.
Higher power levels (>20 W) where transformer-based energy storage is advantageous.
Applications sensitive to cross-regulation or requiring tight secondary voltage control.

3. Practical Design Considerations

Design Aspect	Fly-Buck Recommendation	Flyback Recommendation
Magnetics	Use coupled inductors with optimized leakage control.	Select transformer cores sized for energy storage and flux density.
Feedback	Primary-side sensing; avoid optocouplers.	Optocoupler or auxiliary winding for precise secondary regulation.
Efficiency	Leverage synchronous buck controllers for high efficiency.	Optimize snubber/clamp circuits to reduce switching losses.
Noise	Minimize cross-regulation with careful layout and decoupling.	Mitigate ringing with RC snubbers or active clamps.

4. Rule of Thumb

– For low-power isolated bias supplies (typically under 15 W), the Fly-Buck is often the most efficient and cost-effective choice.
– For higher power or multi-output systems, the Flyback remains the more versatile and reliable topology.

Fly-Buck vs Flyback Converter – FAQ

What is a Fly-Buck converter?

A Fly-Buck converter is an isolated buck topology that uses a coupled inductor winding to generate isolated outputs. It simplifies regulation by using primary-side feedback and reduces magnetic size compared to traditional transformer-based designs.

How does a Flyback converter differ from Fly-Buck?

The Flyback converter stores energy in the transformer’s magnetizing inductance during switch-on and releases it to the secondary during switch-off. In contrast, the Fly-Buck transfers energy continuously through coupled windings without relying on magnetic storage.

Which applications are best suited for Fly-Buck?

Fly-Buck converters are ideal for low-to-medium power isolated bias supplies, gate driver rails, and auxiliary outputs where compact size and cost efficiency are critical.

When should I use a Flyback converter?

Flyback converters are recommended for higher power adapters, chargers, and multi-output supplies requiring precise secondary regulation and flexibility across wide voltage ranges.

How to Choose Between Fly-Buck and Flyback Converter

Step 1: Define Power Requirements
For low-power isolated bias supplies (under ~15 W), consider Fly-Buck. For higher power levels, Flyback is more suitable.
Step 2: Evaluate Regulation Needs
If primary-side regulation is sufficient, Fly-Buck simplifies design. If tight secondary regulation is required, Flyback provides better control.
Step 3: Consider Magnetic Design
Fly-Buck uses smaller coupled inductors, while Flyback requires larger transformer cores to store energy. Choose based on space and efficiency targets.
Step 4: Assess Application Domain
Use Fly-Buck for gate drivers, auxiliary rails, and compact bias supplies. Use Flyback for chargers, adapters, and multi-output systems.
Step 5: Optimize for Efficiency and Noise
Fly-Buck offers higher efficiency and lower noise but may face cross-regulation challenges. Flyback requires snubbers or clamps to mitigate ringing and transformer losses.