Flyback Transformers
Part 2 – How does a Flyback Transformer work?
The flyback converter is derived from the buck-boost topology but includes a transformer that provides galvanic isolation and allows voltage scaling via the turns ratio. A typical flyback circuit is shown in Figure 1.
Switch and Control
The primary switching device (SW) in a flyback converter is usually a MOSFET, although bipolar transistors, GaN, and SiC devices are also used depending on power level and switching speed requirements.
A flyback controller modulates the duty cycle of the switch to regulate the output voltage. The duty cycle D is defined as:
For ideal operation in continuous conduction mode (CCM), the output voltage is given by:
Where:
-
: Input voltage
-
Vᵒᵘᵗ: Output voltage
-
Nₚ, : Primary and secondary turns
-
D: Duty cycle
Operating Principle
The flyback cycle has two primary phases:
- Switch ON (Energy Storage)
When the switch is closed, current flows through the primary winding, storing energy in the transformer core. Due to the winding polarity, the secondary diode is reverse-biased and no energy is delivered to the load. The primary current ramps up, and the stored energy is:
- Switch OFF (Energy Transfer)
Opening the switch causes the magnetic field to collapse, reversing the winding polarities and forward-biasing the output diode. Energy stored in the core is then delivered to the load via the secondary winding.
Conduction Modes
Flyback converters operate in either:
- Continuous Conduction Mode (CCM):
Energy is still flowing to the secondary when the next cycle begins. Secondary current never reaches zero. See Figure 2:
- Discontinuous Conduction Mode (DCM):
The core is fully demagnetized before the switch turns ON again, creating a zero-current interval in the secondary. See Figure 3:
Designs may target either mode or allow mode transitions depending on line/load conditions.
Transformer Selection and Saturation Considerations
The worst-case design point is maximum load at minimum input voltage, which results in the highest peak primary current. The selected transformer must have sufficient Ipk or saturation current (Isat) margin above this level.
If the core saturates, the effective inductance drops sharply, reducing energy storage capacity and potentially causing regulation loss. For this reason, ensuring sufficient magnetic headroom is critical.
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