A flyback converter is a type of power supply circuit that converts one DC voltage to another by storing energy in a magnetic component and then releasing it. It’s one of the most common designs in low-to-mid power electronics, typically used in the 2W to 100W range. You’ll find flyback converters inside phone chargers, laptop adapters, LED drivers, and countless other devices where the input voltage needs to be stepped up or down while keeping the user electrically isolated from the power source.
How a Flyback Converter Works
The flyback converter operates in two distinct phases, controlled by a switch (usually a transistor) that flips on and off thousands of times per second.
In the first phase, the switch turns on and current flows through the primary winding of a special transformer. During this time, energy builds up in the magnetic field of the transformer’s core. A diode on the secondary side is oriented so that no current flows to the output yet. The output capacitor alone keeps the load powered during this phase.
In the second phase, the switch turns off. The magnetic field collapses, and the stored energy transfers through the secondary winding to the output. The diode on the secondary side now conducts, delivering current to the load and recharging the output capacitor. Then the switch turns on again, and the cycle repeats.
This store-then-release approach is what makes the flyback topology unique. In other converter designs (called “forward” converters), energy passes from input to output continuously through the transformer. The flyback instead uses its transformer as a temporary energy reservoir, which eliminates the need for a separate output filter inductor. That saves a bulky, expensive component.
The Transformer Is Really a Coupled Inductor
The “transformer” in a flyback converter doesn’t work like a traditional transformer. A normal transformer transfers energy from primary to secondary simultaneously. A flyback transformer stores energy first, then delivers it. For this reason, engineers often call it a coupled inductor rather than a true transformer.
The key difference is in the core. A flyback transformer has a deliberate air gap in its magnetic core. This gap is where the energy actually gets stored during the switch-on phase. Without the gap, the core would saturate (become magnetically “full”) almost immediately and the converter wouldn’t function. The gapped core serves double duty as both the isolation transformer and the energy storage element, which is a big reason flyback converters can be so compact.
Voltage Conversion and the Duty Cycle
The output voltage of a flyback converter depends on two things: the turns ratio of the transformer (how many wire loops on the primary versus the secondary) and the duty cycle (what fraction of each switching cycle the switch stays on). The relationship looks like this: the output voltage equals the input voltage multiplied by the turns ratio and by the duty cycle divided by one minus the duty cycle.
This means a flyback converter can produce an output voltage that’s higher or lower than the input, just by adjusting the duty cycle or choosing the right turns ratio. It’s similar to a buck-boost converter in that flexibility, but with the added benefit of electrical isolation between input and output. A control circuit continuously adjusts the duty cycle to keep the output voltage stable as the input voltage or load changes.
Why Galvanic Isolation Matters
One of the biggest reasons flyback converters are so widely used is galvanic isolation. The primary side (connected to mains power) and the secondary side (connected to your device) have no direct electrical connection. Energy transfers purely through the magnetic field. This means if something fails, dangerous high voltage from the wall outlet can’t reach the user.
Safety standards like IEC 60950-1 require specific isolation distances and insulation ratings between the high-voltage and low-voltage sides of a power supply. Flyback converters meet these requirements naturally because the transformer provides a physical barrier between the two circuits. This is why nearly every USB charger and small power adapter uses a flyback topology. The same isolation principle also allows a single flyback transformer to generate multiple independent output voltages with minimal extra circuitry, which is useful in devices that need several different supply rails.
Essential Circuit Components
A basic flyback converter uses surprisingly few parts:
- Coupled inductor (flyback transformer): The gapped-core component that stores and transfers energy.
- Switch: A transistor on the primary side that controls when current flows into the transformer. It switches on and off at frequencies typically ranging from tens of kilohertz to hundreds of kilohertz.
- Diode: Positioned on the secondary side, it blocks current during the switch-on phase and conducts during the switch-off phase, directing stored energy to the output.
- Output capacitor: Smooths the pulsing energy delivery into a steady DC voltage for the load.
- Control circuit: Monitors the output voltage and adjusts the duty cycle to keep it constant.
This low component count is one of the flyback converter’s strongest advantages. Fewer parts means lower cost, smaller size, and simpler circuit board layout compared to other isolated converter topologies.
The Snubber Circuit
When the switch turns off, the sudden interruption of current through the transformer creates a voltage spike on the primary side. This spike can be severe enough to destroy the switching transistor. Every practical flyback converter includes a snubber circuit to absorb this energy and clamp the voltage to a safe level.
The snubber typically consists of a resistor, capacitor, and diode arranged to catch the spike and dissipate it as heat. Without it, the voltage across the switch can ring (oscillate) and exceed the transistor’s maximum rating. The snubber keeps the switching trajectory within the component’s safe operating area. Designing the snubber properly is one of the trickier parts of building a flyback converter, since it directly affects both reliability and efficiency. Energy burned in the snubber is wasted as heat.
Efficiency and Power Limits
Flyback converters are most competitive in the 2W to 100W power range. A typical low-power flyback achieves around 80% efficiency, meaning 20% of the input power is lost as heat. That’s lower than some other topologies, but the simplicity and low cost often make it the best overall choice at these power levels.
At higher power levels, the flyback topology runs into problems. The energy stored in the transformer gap scales with the square of the current, so the transformer and its core losses grow quickly. Voltage spikes on the switch become harder to manage, and electromagnetic interference (EMI) from the sharp current pulses becomes more difficult to filter. For power supplies above roughly 100 to 150W, forward converters or other topologies generally take over.
Active Clamp and GaN Improvements
Newer flyback designs use a technique called active clamping, which replaces the passive snubber with a second switch and capacitor. Instead of wasting the voltage spike energy as heat, the active clamp recycles it back into the circuit. This also enables “soft switching,” where the main transistor turns on at zero voltage rather than slamming on against a full voltage difference. Soft switching dramatically reduces switching losses and EMI.
Pairing active clamp designs with gallium nitride (GaN) transistors pushes performance even further. GaN switches faster and with lower losses than traditional silicon transistors. Prototypes using GaN active clamp flyback designs have demonstrated operation at 500 kHz and 200W, well above the traditional power ceiling for flyback converters. This combination is behind the compact, high-power USB-C chargers that have become common in recent years, delivering 60W or more from an adapter smaller than a deck of cards.
Common Applications
Flyback converters appear anywhere you need an isolated, regulated DC voltage from an AC or DC source at relatively low power. Phone and laptop chargers are the most visible examples. LED drivers use them to convert AC mains power to the low DC voltage that LEDs need. Standby power supplies in televisions, set-top boxes, and appliances are almost always flyback circuits, since standby loads are small and the topology is cheap to implement.
Industrial and scientific equipment also relies on flyback converters for specialized tasks. High-voltage flyback designs can step up low input voltages to hundreds or even thousands of volts for safety beacons, sensors, and test equipment in electrical laboratories. The topology’s ability to generate multiple isolated outputs from a single transformer makes it especially useful in systems where several subsystems need electrically independent power rails.