Flyback Transformer Design and Calculator

What is Flyback Transformer Design

When the switch transistor in a flyback converter is activated, the primary winding of the transformer starts humming with energy, but the secondary windings remain in a waiting game. As soon as the transistor turns off, the stored energy in the magnetic field rushes over to the secondary side. This dance of energy is quite different from a forward converter, where power flows directly to the secondary windings while the transistor is still switched on. How can you spot the difference between the two? Just take a look at the dot markings—the dots are aligned in forward converters but are flipped in flyback configurations.

If you’re working on a discontinuous mode flyback converter, don’t sweat the math. Use our flyback transformer design tool to easily calculate wire gauges, inductance, and the number of turns needed for each winding. This handy tool will save you from crunching numbers manually and ensure you’ve got the perfect specs for your project.

Flyback Voltage: How to Calculate It

Want to know how to calculate flyback voltage without pulling your hair out? There's a great YouTube video that walks you through the operation of a flyback converter, focusing on continuous conduction mode (CCM) with a switch turning on and off. It breaks down the current flow and operation so that you can compute the flyback voltage like a pro. Or, just save time by using our online flyback transformer calculator—no need to grab a calculator.

Understanding Flyback Operation: Energy Storage and Transfer

Here’s how the flyback transformer does its magic: in the first part of each switching cycle, energy is stored in the transformer’s magnetic field. As soon as the switch flips, this energy zips across to the secondary winding, powering whatever load is connected. Flyback transformers feature a gapped-core design, which allows them to store significant energy without saturating. This separates them from forward converters, where energy quickly transfers between the primary and secondary windings without much storage. Flyback transformers, due to their gapped cores, are also categorized as coupled inductors.

Exactly is a Flyback Transformer?

Think of a flyback transformer as a specialized, gapped-core coupled inductor. When voltage hits the primary winding, energy gets packed into the gap of the core. Once it’s full, the energy is released to the secondary winding to power your load. Flyback transformers shine in isolated power supplies, offering cost-effective solutions up to 120 watts. Not only do they provide circuit isolation, but they also support multiple outputs and can deliver both positive and negative voltages. Unlike other isolated topologies, the flyback design stores energy directly in the transformer, simplifying your design by cutting down the number of components.

Creating a Flyback Transformer Circuit

So, you’re ready to build your flyback transformer circuit? Check out the video below, which offers a step-by-step guide to designing one from scratch using an AI-powered online tool. With this guide, designing a flyback transformer becomes straightforward.

The Working Mechanism of Flyback Transformers

In a flyback converter, the transformer takes on dual roles—providing isolation and voltage transformation. The turns ratio dictates the voltage transformation, while a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) usually handles the switching. That said, sometimes you’ll see silicon carbide (SiC), bipolar transistors, or gallium nitride (GaN) in the mix. The controller toggles the switch to hit the desired output voltage, generally keeping the duty cycle below 50%. You can play around with different turns ratios and duty cycles to achieve the right voltage output for your application.

Example: Building a Flyback SMPS Circuit

Here’s a practical example of a flyback Switch Mode Power Supply (SMPS) circuit that bumps up the voltage from around 20V to 40V. By tweaking the feedback resistors, you can fine-tune the output voltage. The LM3479 controller is particularly handy here, as it lets you adjust the frequency by altering the R1 resistor. A distinctive feature of this circuit is that the regulator is powered by a 5V input, which is lower than VPIN. This means that VPIN can be just about any value, making the circuit flexible. There’s also a non-dissipative circuit on the MOSFET’s drain, which helps mitigate voltage spikes when the transistor switches off.

Applications of Flyback Transformers

transformers are incredibly versatile and find their way into a wide range of applications. Think AC-DC and DC-DC power supplies, battery chargers, LED lighting, Power over Ethernet (PoE), solar microinverters, and telecom systems. These transformers are particularly favored in designs that require high efficiency, compact size, and cost-effectiveness.

When dealing with lower power needs—typically under 10 amps and 100 watts—flyback transformers are the go-to. For larger loads, other topologies like forward-mode or push-pull may be more suitable. But for most small-scale applications, flyback transformers are king. They work seamlessly across the telecom voltage range of 37-72V DC and are compatible with the universal AC line input range of 85-265V AC. If you need more power, Coilcraft offers standard flyback transformers that can handle up to 120 watts.

Meet the Flyback Converter

A flyback converter is all about clever energy storage. During each switching cycle, it uses a coupled inductor to store energy when the current is flowing and release it once the power cuts out. Functionally, it's similar to a boost converter, but with the transformer’s primary winding doubling as the inductor, and the secondary delivering the output. Essentially, the primary and secondary windings act as two independent inductors in this design.