Flyback Transformer Design Calculation | High Frequency SMPS Transformer Design

Introduction to the Flyback Transformer Design Tool

This guide accompanies our Flyback Transformer Design Tool, an online calculator designed to help engineers calculate wire gauge, inductance, and the number of turns for discontinuous mode flyback converters.

In a Flyback Converter, the main winding of the transformer is energized when the switching transistor is turned on. Crucially, no energy is delivered to the secondary windings during this "on" phase. Instead, energy is stored in the magnetic field. When the transistor is switched off, the field collapses, and the energy is then transferred to the secondary windings.

This operation is the opposite of a forward converter topology, where energy is transferred immediately when the switch is on. You can visually distinguish the two topologies by the dot convention on the transformer symbol:

• Flyback Converter: The dots on the primary and secondary windings are inverted (opposite polarity).
• Forward Converter: The dots are aligned (same polarity).

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Flyback Controller Overview

How Does a Flyback Transformer Work?

During the first half of the switching cycle, energy is stored in the transformer's magnetic field. During the second half, this energy is released to the secondary winding(s) and the load.

Unlike standard transformers, flyback transformers utilize a gapped-core structure. This gap allows the core to store significant amounts of energy without saturating. Because of this energy storage capability, flyback transformers are technically classified as coupled inductors.

What is a Flyback Transformer?

A flyback transformer acts as both an isolation transformer and a storage inductor.

1. Storage: When input voltage is applied to the primary winding, energy accumulates in the core gap.
2. Transfer: When the input switches off, that stored energy is "flown back" to the secondary winding to power the load.

Why use them? Flyback transformers are the preferred choice for cost-effective, high-efficiency isolated power supplies (typically up to 120 Watts).

• Versatility: They can output multiple voltages (positive or negative) from a single core.
• Wide Range: They operate effectively over broad input voltage and load ranges.
• Simplicity: Unlike other isolated topologies, the flyback design does not require a separate output filter inductor, reducing component count and cost.

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Design & Operation

How is Flyback Voltage Calculated?

The flyback voltage depends on the input voltage, the turns ratio, and the duty cycle of the switch.

To understand the current flow and operation in Continuous Conduction Mode (CCM) versus Discontinuous Conduction Mode (DCM), referring to standard voltage equations is essential.

Basic Duty Cycle Relationship:

$V_{out} = V_{in} \times \left(\frac{N_s}{N_p}\right) \times \left(\frac{D}{1-D}\right)$

Where:

• $V_{out}$: Output Voltage
• $V_{in}$: Input Voltage
• $N_s/N_p$: Secondary to Primary Turns Ratio
• $D$: Duty Cycle (typically < 0.5)

Circuit Construction

The flyback architecture is based on a Buck-Boost topology but with isolation provided by the transformer.

• Switching Component: A MOSFET is the most common switch, though Silicon Carbide (SiC) or Gallium Nitride (GaN) are used in high-performance designs.
• Controller: The controller opens and closes the switch to maintain the required output voltage.

Example Flyback Circuit

Consider an SMPS circuit designed to boost voltage from 20V to 40V.

• Frequency Control: Using a controller like the LM3479, you can adjust resistor $R1$ to set the operating frequency (e.g., 39kΩ sets it to ~500kHz).
• Power Separation: A unique feature of many designs is separating the regulator power (e.g., +5V) from the main coil supply ($V_{PIN}$). This allows the main input to vary widely while the controller remains stable.
• Snubber Circuit: A non-dissipative circuit is typically placed on the MOSFET drain to clamp dangerous voltage spikes that occur when the transistor switches off.

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Applications of Flyback Transformers

Flyback transformers are ubiquitous in low-to-medium power electronics. Standard off-the-shelf units (like those from Coilcraft) generally range from a few Watts up to 120 Watts and 10 Amps.

Common Applications:

• AC-DC Power Supplies: Wall adapters and chargers.
• DC-DC Converters: Telecom power supplies (37–72Vdc range).
• LED Drivers: Lighting ballasts.
• PoE: Power over Ethernet devices.
• Solar Microinverters: Energy harvesting.
• Battery Charging: Isolated charging circuits.

Note: For higher power requirements (>120W), topologies like Forward, Push-Pull, or Half-Bridge are generally more efficient.

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Introduction to Flyback Converters

The flyback converter is a power supply architecture that uses a mutually coupled inductor to store energy.

• Phase 1 (Switch ON): Current flows through the primary winding; energy is stored in the core. The secondary diode is reverse-biased, blocking current flow to the output.
• Phase 2 (Switch OFF): The primary field collapses. The voltage polarity on windings reverses. The secondary diode becomes forward-biased, and energy is released to the output capacitor and load.

In terms of construction, it is similar to a Boost Converter, but with the inductor replaced by a transformer to provide galvanic isolation and voltage scaling.