Table of Contents
What Is a Flyback?
A flyback converter is an isolated SMPS topology storing energy in a transformer's magnetizing inductance during switch-on and transferring it to the output during switch-off. It is the most common topology for low-power (<150W) applications: phone chargers, laptop adapters, and standby supplies. It provides isolation, multiple outputs, and uses few components.
The name "flyback" comes from the transformer's energy transfer mechanism. Unlike a forward converter where energy transfers while the switch is on, the flyback stores energy magnetically and releases it when the switch turns off, causing the secondary voltage to "fly back" in polarity.
Equations
Design Tips
| Parameter | Range | Impact |
|---|---|---|
| Duty cycle | 30-50% | Efficiency |
| Frequency | 50-200 kHz | Size vs losses |
| Turns ratio | 5-20 | Duty, voltage stress |
| Leakage | 1-5% | Spikes, EMI |
FAQ
What limits power?
Above ~150W, core size and leakage losses become problematic. High peak currents stress switches. For higher power, forward, half-bridge, or full-bridge topologies are preferred.
Why limit duty to 50%?
Exceeding 50% increases peak flux and switch stress dramatically. Most designs target 30-45% for optimal transformer size and efficiency balance.
How does frequency affect size?
Higher frequency = smaller transformer and caps (less energy/cycle). But switching losses increase linearly and core losses rise. Optimal is typically 65-150 kHz for most designs.