Introduction
A flyback converter is a type of switched-mode power supply (SMPS) commonly used in AC-DC power adapters and chargers for consumer electronics. The flyback topology provides an efficient, cost-effective, and compact method for converting AC mains voltage to a lower DC voltage required by many electronic devices.
This article provides an overview of how a flyback converter works along with its key applications and capabilities. Read on for an understanding of the operating principles, transformer action, design considerations, pros and cons, and typical uses of flyback power supplies.
Flyback Converter Overview
A flyback converter provides DC-DC conversion by storing energy in the magnetic field of a coupled inductor then releasing it to the output. Key characteristics include:
- Provides galvanic isolation between input and output via a transformer
- Steps down DC voltage from high to low level
- Can output multiple regulated secondary rails
- Wide input voltage range capability
- Requires a relatively small transformer for power level
- Cost effective topology using fewer components
- Used extensively in lower power AC-DC power adapters
The flyback design is an isolated variant of the classic buck-boost converter using a coupled inductor rather than discrete inductor and capacitor storage elements. Flyback topology is popular due to its simplicity, efficiency, small form factor, and wide input/output range capabilities.
How Does a Flyback Converter Work?
The flyback converter operation consists of two repetitive phases:
Phase 1: Charge
During the primary switch ON time, input voltage is applied across the transformer primary winding storing energy in its magnetic field. The primary current and magnetic field ramp up linearly. No power is transferred to the secondary side.
Phase 2: Discharge
During the switch OFF time, the stored magnetic field collapses inducing a voltage spike across the transformer windings. This forwarded voltage spike allows current to flow from ground to the secondary side output, thus replenishing charge to the output capacitor and powering the load.
Cyclically charging the magnetic field when the switch closes, then discharging to the secondary side when it opens, allows efficient power transfer isolated through the transformer. Varying the duty cycle regulates the outputs.
The alternating charge and discharge modes give rise to the terminology “flyback converter” since energy is intermittently “flying back” from the transformer primary to secondary in pulses.
Flyback Converter Waveforms
Analyzing the voltage and current waveforms provides further insight into the flyback operation:
Vsw: The switching waveform applied to the primary side transistor. This chops input DC voltage on and off at a high frequency, typically 25 kHz to 1 MHz.
Ip: The primary winding current. Ramps up when the switch turns on as energy is stored in the magnetic field. Drops to zero and may reverse when the switch turns off.
Vs: The secondary winding voltage. The reflected output voltage plus the induced flyback spike used to deliver power to the load.
Is: The secondary winding current delivered to the load. Pulses each flyback interval to replenish charge on the output capacitor.
Vout: The output voltage across the load capacitor. Smoothed DC output relative to secondary GND despite pulsed power transfer.
Careful timing of the magnetization and discharge phases regulates energy transfer to achieve the desired steady DC output voltage.
Transformer Isolation and Windings
The coupled inductor transformer provides critical functionality:
- Isolation – The lack of direct conductive paths across the transformer provides electrical isolation between primary high voltage input and secondary outputs. This is both a functional and safety requirement for many power supplies.
- Voltage Transformation – Per the transformer equations, the secondary to primary voltage ratio is defined by the number of respective wire turns. This transformer action enables stepping the primary voltage up or down to the desired level.
- Energy Transfer – The inductive coupling passes energy from primary to secondary through the common magnetic field. This enables transmitting power despite a lack of direct connections.
- DC Blocking – The high frequency transformer only passes pulsing AC from the switched input. This blocks DC voltage from reaching the secondary and prevents core saturation.
Careful transformer design is crucial to optimize the voltage transformations, flux levels, coupling, and isolation performance. Multiple secondary windings provide different isolated output voltages from the same converter.
Flyback Converter Design Considerations
Engineers must consider numerous factors when designing a flyback power supply:
- Desired output voltage(s) and power levels
- Required voltage and current regulation limits
- Efficiency and heat dissipation requirements
- Safety isolation and surge withstand needs
- Size constraints and transformer core selection
- Input voltage range specification
- Environmental factors like temperature and humidity
- Output ripple and noise requirements
- Cost targets and component selection
- Control loop compensation and stability
- Driver circuitry and isolated feedback
- Protections from overcurrent, overvoltage, thermal events
Extensive electrical simulations, transformer design analysis, prototyping, and testing are used to iterate and optimize the flyback converter design to meet all product requirements.
Flyback Converter Pros and Cons
The flyback topology provides some key advantages but also has downsides to consider:
Advantages
- Simple, low component count design
- Provides galvanic isolation
- Wide input and output voltage range capability
- Multiple outputs readily supported
- High efficiency with advanced control ICs
- Compact, small transformer size for power level
- Lower EMI compared to forward converter
- Readily available reference design kits
Disadvantages
- Discontinuous output current pulses
- Requires complex transformer design
- Associated EMI challenges require filter circuits
- Output ripple voltage typically higher
- High voltage stresses on primary switch
- Aux power winding required for controller
- Audible noise possible without optimizations
- Slope compensation needed for stability
Overall, the flyback design represents an excellent balance of size, performance and cost for lower power AC-DC applications.
Typical Flyback Converter Applications
Some common uses for flyback power supplies include:
- AC Adaptors – Compact plug-in power bricks for laptops, mobile devices, gaming consoles, monitors, TVs, routers, and modems. Output power from 5W to 200W.
- Chargers – Standalone battery chargers for consumer devices such as phones, tablets, power tools, and electric toothbrushes.
- Appliances – Providing DC supplies for appliances like microwaves, printers, scanners, coffeemakers.
- Medical Equipment – Isolated DC power for portable medical devices requiring regulatory safety compliance.
- Industrial Systems – Providing auxiliary DC power within industrial equipment like PLCs, transmitters, sensors, indicators, and controls.
- Consumer Electronics – Any device needing an internal AC-DC power supply such as routers, set top boxes, media streamers.
The wide input range, flexible output capabilities, small size, and low cost make flyback converters a workhorse DC-DC conversion technology for consumer and industrial applications under 200W.
Flyback Controller ICs
Dedicated flyback controller ICs simplify implementation and improve performance. Common controllers include:
- ON Semiconductor NCP101x, NCP102x, NCP103x Series
- Power Integrations TOPSwitch Series
- Infineon OPTIGA Series
- STMicro STPSxx Series
- TI UCC287xx Series
These ICs provide functionality like:
- Adjustable output voltage regulation
- Auto-restart overcurrent protection
- Slope compensation and loop stability
- Primary switch drivers or integrated FETs
- Fault protections and diagnostics
- Sync rectification drivers
- Multi-output secondary controllers
- Flux density limiting and optimization
- Frequency jittering for EMI reduction
- Thermal shutdown protection
Advanced controllers handle control loop tuning, protection features, driving the primary FET, and secondary regulation – drastically reducing system complexity compared to discrete flyback designs.
Conclusion
In summary, a flyback converter provides an efficient method for converting an AC mains voltage to a lower DC level required by electronics devices. Storing energy in a magnetic field which is cyclically discharged to a secondary winding enables isolated power transfer across a small transformer. The pulsed flyback architecture is well suited for compact, cost sensitive, lower power adapters making it prevalent in consumer products. When carefully designed and controlled, the versatile flyback topology will continue providing safe, reliable, and efficient AC-DC conversion.
Flyback Converter Frequently Asked Questions
How does a flyback converter compare to a buck converter?
Unlike a buck converter, the flyback provides electrical isolation across the transformer but requires a more complex winding arrangement. The pulsating output current also leads to higher ripple.
What determines the turns ratio of the transformer windings?
The turns ratio between the primary and secondary windings defines the voltage step down/step up ratio according to the transformer volt-second relationship.
What is the difference between DCM and CCM flyback operation?
In discontinuous conduction mode (DCM) the secondary current falls to zero during each cycle. Continuous conduction mode (CCM) has the current always flowing. DCM is more common for lower power flyback designs.
Why are snubbers used in flyback converters?
Snubbers absorb energy from transformer leakage inductance to limit voltage spikes on the switch during turn-off transitions improving reliability.
What causes audible noise issues?
The magnetostriction effect can cause the transformer core material to vibrate and generate audible buzzing. Potting, gluing, lowest switching frequency, and other optimizations reduce audible noise.
