Flyback Converter

The flyback converter is the most widely used isolated power supply topology for low to medium power applications below 150 W. It is derived from the buck-boost converter but uses a coupled inductor (often loosely called a transformer) to provide galvanic isolation and multiple output voltages from a single switching stage.

Flyback converters are found in mobile phone chargers, set-top boxes, standby power supplies, and LED drivers. Their minimal component count, ability to produce multiple isolated outputs, and simple control circuitry make them the preferred topology when isolation is needed at low cost.

Core Concept Explanation

The flyback converter operates on the same principle as a buck-boost converter, but the inductor is replaced by a coupled inductor with two windings. During the switch ON period, the primary winding stores energy in the magnetic core. The secondary winding is wound in the opposite polarity (shown by dot notation), so the output diode is reverse biased and no current flows in the secondary.

When the switch turns OFF, the magnetic flux in the core reverses its tendency to change, causing the secondary voltage to reverse polarity. This forward biases the output diode and the stored energy is transferred to the output capacitor and load. This is the flyback action that gives the topology its name: energy is delivered to the output during the switch OFF time, opposite to a forward converter.

The turns ratio Np:Ns of the coupled inductor provides voltage transformation and isolation. By choosing the turns ratio, the designer can set the output voltage level. Multiple secondary windings allow multiple isolated output voltages from the same topology, which is a major advantage for cost sensitive designs.

Continuous and Discontinuous Conduction Mode

A flyback converter can operate in Continuous Conduction Mode (CCM) where the transformer core retains some flux at the start of each switching cycle, or in Discontinuous Conduction Mode (DCM) where the core flux returns to zero before the next cycle begins. DCM is simpler to control and avoids right-half-plane zero problems, but has higher peak currents. CCM is used in higher power designs for lower ripple.

Most low power chargers use DCM because the simpler transfer function makes feedback loop design straightforward. The boundary between CCM and DCM is determined by the inductance value, switching frequency, load current, and duty cycle.

Mathematical Expression

The voltage conversion ratio of the flyback converter in CCM is derived from volt-second balance on the transformer:

Vout = Vin x (Ns/Np) x D / (1 - D), where D is the duty cycle of the primary switch, Np is primary turns, and Ns is secondary turns. This is similar to the buck-boost formula but scaled by the turns ratio Ns/Np.

In DCM, the conversion ratio also depends on load current and switching frequency, making the output voltage more load-dependent. The volt-second balance principle states that the integral of voltage across the magnetizing inductance over one full switching period must equal zero in steady state: Vin x D x T = (Vout x Np/Ns) x (1-D) x T.

Practical Understanding

The leakage inductance of the flyback transformer is a critical practical concern. When the primary switch opens, the leakage inductance resonates with switch parasitic capacitance, creating a voltage spike that can exceed device breakdown voltage. A snubber circuit (RCD or TVS based) is placed across the primary switch to clamp this spike and protect the MOSFET.

Transformer design in a flyback converter is more complex than a simple coupled inductor because the core must store and release energy efficiently. The core material, gap length, and winding arrangement must be carefully chosen to avoid core saturation while maintaining low leakage inductance.

Modern flyback controllers include features like valley switching, quasi-resonant operation, and active clamp circuits to improve efficiency. The primary side regulation (PSR) technique eliminates the optocoupler by sensing output voltage from the primary side waveform, reducing component count and cost.

Given:
Flyback converter in CCM
Vin = 48 V, Np:Ns = 3:1, D = 0.4

Why this formula applies:
Flyback CCM voltage conversion:
Vout = Vin x (Ns/Np) x D/(1-D)

Formula:
Vout = Vin x (Ns/Np) x D / (1-D)

Substitution:
Vout = 48 x (1/3) x 0.4 / (1 - 0.4)
Vout = 48 x 0.333 x 0.4 / 0.6
Vout = 48 x 0.333 x 0.667

Calculation:
Vout = 48 x 0.222 = 10.67 V

Final Answer:
Vout = 10.67 V for Vin = 48V, turns ratio 3:1, D = 0.4

Exam Tip: The flyback converter formula Vout = Vin x (Ns/Np) x D/(1-D) is often tested. Remember that the turns ratio multiplies the buck-boost conversion factor. A very common GATE trap is applying the buck-boost formula without the turns ratio, giving wrong output. Also note that flyback delivers energy during switch OFF (unlike forward converter which delivers during switch ON).

Flyback Operating Phases

• During switch ON: primary current ramps up, energy stored in transformer magnetizing inductance. Secondary diode blocks.
• During switch OFF: primary current drops to zero, transformer polarity reverses, secondary diode conducts and charges output capacitor.
• Output voltage regulation is achieved by varying duty cycle through the PWM feedback loop.
• Optocoupler or primary side sensing provides isolated feedback from secondary to primary control IC.
• Snubber circuit across MOSFET clamps leakage inductance voltage spike during turn-off.
• Multiple secondary windings allow multiple isolated output rails without additional switches.

Quick Revision

• Flyback is a buck-boost derived isolated topology: energy stored during switch ON, delivered during switch OFF.
• Voltage ratio (CCM): Vout = Vin x (Ns/Np) x D/(1-D).
• Dot notation on secondary winding is opposite to forward converter convention, so diode is reverse biased during ON.
• CCM vs DCM: DCM simpler control, higher peak current. CCM lower ripple, used at higher power.
• Leakage inductance spike at turn-off: always use snubber circuit across primary switch.
• Multiple isolated outputs possible with multiple secondary windings, key advantage for charger designs.
• Used below 150 W typically. Above this, forward, push-pull, or bridge topologies are preferred.