A flyback transformer is a coupled inductor with a gapped core. During each cycle, when the input voltage is applied to the primary winding, energy is stored in the gape of the core. It is then transferred to the secondary winding to provide energy to the load. Flyback transformers are used to provide voltage transformation and circuit isolation in the flyback converters.
Flyback transformers are the most popular choice for cost-effective, high-efficiency isolated power supply designs. They provide circuit isolation, the potential for multiple outputs and the possibility of positive or negative output voltages. They can also be regulated over a wide range of input voltage and load conditions. Because energy is stored in the transformer, the flyback topology does not require a separate output filter inductor like the other isolated topologies. This reduces the component count and simplifies the circuit requirements.
In the flyback topology, energy is stored in the magnetic field of the transformer during the first half of the switching cycle and then released to the secondary winding(s) connected ot the load in the second half of the cycle. Flyback transformers feature a gapped-core construction, which allows high energy storage without saturating the core. This energy storage aspect distinguishes flybacks from other topologies such as forward-mode where energy transfers immediately from primary to secondary. Flyback transformers are also known as coupled inductors, because they have a gapped core construction and store energy in the core.
Flyback circuits repeat a cycle of two or three stages; a charging stage, a discharging stage, and in some applications idle time following a complete discharge. Charging creates a magnetic field. Discharging action results from the collapse of the magnetic field. The typical flyback transformer application is a unipolar application. The magnetic field flux density varies up in down in value ( 0 or larger ) but keeps the same ( hence unipolar ) direction.
The basic flyback cycle includes the following portions:
If the SWITCH is turned back on before all of the flyback energy is transferred to the secondary, the secondary current never reaches zero. This is referred to as continuous conduction mode (CCM). If the stored flyback energy is completely emptied to the secondary before the SWITCH is turned back on, the secondary current reaches zero before the end of the period, creating an “idle time” during the cycle. This is called discontinuous conduction mode (DCM). Transformers may be designed from CCM, DCM or both. Flyback transformers may operate in both CCM and DCM modes, depending on the input voltage and load conditions.
The flyback transformer ( or inductor ) draws current from the power source. The current increases over time. The current flow creates a magnetic field flux that also increases over time. Energy is stored within the magnetic field. The associated positive flux change over time induces a voltage in the flyback transformer (or inductor) which opposes the source voltage. Typically, a diode and a capacitor are series connected across a flyback transformer winding (or inductor). A load resistor is then connected across the capacitor. The diode is oriented to block current flow from the flyback transformer (or source) to the capacitor and the load resistor during the charging stage. Controlling the charging time duration (known as duty cycle) in a cycle can control the amount of energy stored during each cycle. Stored energy value, E = ( I x I x L ) / 2, where E is in joules, I = current in amps, L = inductance in Henries. Current is defined by the differential equation V(t) = L x di/dt. Applying this equation to applications with constant source voltage and constant inductance value one obtains the following equation; I = Io + V x t / L , where I = currents in amps, Io = starting current in amps, V = voltage in volts across the flyback transformer winding (or inductor), L = inductance in Henries, and t = elapsed time in seconds. Note that increasing L will decrease the current. Stored energy will consequently decrease because effects of the current squared decrease will more than offset the effects of the inductance increase. Also be aware that the flyback transformer (or inductor) input voltage is less than the source voltage due to switching and resistive voltage drops in the circuit.
The current (which creates the magnetic field) from the source is then interrupted by opening a switch, thereby causing the magnetic field to collapse or decrease, hence a reversal in the direction of the magnetic field flux change (negative flux change over time). The negative flux change induces a voltage in the opposite direction from that induced during the charging stage. The terms flyback or kickback originate from the induced voltage reversal that occurs when the supply current is interrupted. The reversed induced voltage(s) tries to create (induce) a current flow. The open switch prevents current from flowing through the power supply. With the voltage reversed, the diode now permits current flow through it, hence current flows into the capacitor and the load across the capacitor. If current can flow, then the resulting flow of current is in the direction, which tries to maintain the existing magnetic field. The induced current cannot maintain this field but does slow down the decline of the magnetic field. A slower decline translates to a lower induced flyback voltage. If current cannot flow, the magnetic field will decline very rapidly and consequently create a much higher induced voltage. In effect, the flyback action will create the necessary voltage needed to discharge the energy stored in the flyback transformer or inductor. This principle, along with controlling the duration of the charging stage, allows a flyback inductor to increase or decrease the voltage without the use of a step-up or step-down turns ratio. In the typical flyback circuit, the output capacitor clamps the flyback voltage to the capacitor voltage plus the diode and resistive voltage drops. For a sufficiently large & fully charged capacitor, the clamping capacitor voltage can be treated as a constant value. The equations V(t) = L x di/dt, and I = Io + V x t / L can also be applied to the discharge stage. Use the inductance value of the discharging winding and the time duration of the discharging stage. The time will either be the cycle time minus the charging time (no idle time), or the time it takes to fully discharge the magnetic field thereby reaching zero current. The cycle time equals the period which equals 1 / frequency.
This stage occurs whenever the flyback transformer (or inductor) has completely discharged its stored energy. Input and output current (of the transformer or inductor) is at zero value.
To find the right off-the-shelf flyback transformer, use the following criteria: