What is a Flyback Converter : Design & Its Working

The flyback converter is designed like the switch mode power supply from the past 70 years to perform any type of conversion like AC to DC and DC to DC. The design of flyback gave the advantage to develop the television for communication in the earliest 1930s to 1940s. It uses a non-linear switching supply concept. The flyback transformer stores magnetic energy and acts as an inductor when compared to a non-flyback design. This article is all about the flyback converter working and its topology.

What is a Flyback Converter?

Flyback converters are defined as power converters, which convert AC to DC with galvanic isolation between the inputs and outputs. It stores the energy when the current flowing through the circuit and releases the energy when the power is removed. It used a mutually coupled inductor and acts as an isolated switching converter for step down or step-up voltage transformers.

It can control and regulate the multiple output voltages with a wide range of input voltages. The components required to design a flyback converter is a few when compared to other switching mode power supply circuits. The word flyback is referred to as the on/off action of the switch used in the design.

Flyback Converter Design

The flyback converter design is very simple and contains electrical components like a flyback transformer, switch, rectifier, filter, and a control device to drive the switch and achieve regulation.

The switch is used to ON and OFF the primary circuit, which can magnetize or demagnetize the transformer. The PWM signal from the controller controls the operation of the switch. In most of the flyback transformer designs, FET or MOSFET or a basic transistor is used as the switch.

Rectifier rectifies the voltage of the secondary winding to get pulsating DC output and disconnects the load from the secondary winding of the transformer. The capacitor filters the rectifier output voltage and increases the DC output level as per the desired application.

The flyback transformer is used as an inductor to store the magnetic energy. It is designed as a two coupled inductor, which acts as the primary and secondary winding. It operates at high frequencies of nearly 50KHz.

Design Calculations

It is necessary to consider the flyback converter design calculations of the turns ratio, duty cycle, and the currents of primary and secondary windings. Because the turns ratio might affect the current flowing through the primary and secondary winding and also the duty cycle. When the turn ratio is high, then the duty cycle also becomes high, and the current passing through the primary and secondary winding decreases.

As the transformer used in the circuit is a custom type, it’s not possible to get a perfect transformer with a turns ratio these days. Hence by choosing the transformer with the desired ratings and closer to the required ratings might compensate for the difference in the voltage and output.

The other parameters like core material, the effect of the air gap, and polarization should be considered by the engineers.

The flyback converter design calculations by considering the switch position are discussed below.

When the Switch is ON

Vin – VL – Vs = 0

At ideal condition, Vs = 0 ( voltage drop)

Then Vin – VL = 0

VL = Lp di/dt

di = (VL / Lp) x dt

Since VL = Vin

di = (Vin / Lp) x dt

By applying integration on both sides we get,

Current at the primary winding is

Ipri =( Vin. /Lp )Ton

The total energy stored in the primary winding is,

Epri = ½ Ipri² X Lp

Where Vin = input voltage

Lp = inductance of the primary winding or primary inductance.

Ton = period when the switch is turned ON

When the Switch is OFF

VL (secondary) – VD – Vout = 0

The diode voltage drop will be zero at an ideal condition

VL ( secondary) – Vout = 0

VL (secondary) = Vout

VL = Ls di/dt

di = (VL secondary / Ls) / dt

Since VL secondary = Vout

Hence,

di = Vout / Ls ) X dt

By applying integration, we get

Isec = (Vsec/Ls) (T – Ton)

The total energy transferred is expressed as

Esec = ½[(Vsec/Ls) . (T – Ton) ]². Ls

Where Vsec = voltage in the secondary winding = total output voltage at the load

Ls = inductance of the secondary winding

T = pwm signal period

Ton = switch ON time

Operation of Flyback Converter/Working Principle

The operation of the flyback converter can be understood from the above diagram. The working principle is based on the switch mode power supply (SMPS) mode.

When the switch is in the ON position, there is no energy transfer between the input and the load. The total energy will be stored in the primary winding of the circuit. Here drain voltage Vd = 0 and the current Ip passes through the primary winding. The energy is stored in the form of the magnetic inductance of the transformer and the current increases with time linearly. Then the diode becomes reverse biased and no current flows to the secondary winding of the transformer and the total energy are stored in the capacitor used at the output.

When the switch is in the OFF position, the energy is transferred to the load by changing the polarity of the transformer windings due to the magnetic field and the rectifier circuit starts rectifying the voltage. The total energy in the core will be transferred to the load will be rectified and the process would be continued till the energy in the core is depleted or until the switch is turned ON.

Flyback Converter Topology

The flyback converter topology is adaptable, flexible, simple mostly used SMPS (switch mode power supply) design with good performance characteristics that gives an advantage to many applications.
The performance characteristics of the flyback converter topology are shown below.

The above waveforms show the sudden transitions and reversal currents of the primary and secondary winding of the flyback transformer. The output voltage will be regulated by adjusting the on/off actions of the duty cycle of the primary winding. We can isolate the input and output by using the feedback, or by using an additional winding on the transformer

Flyback Topology SMPS

The flyback topology SMPS diagrams is shown below.

The flyback topology SMPS design requires less no. Of components for a given power range when compared to other SMPS topologies. It can operate for a given AC or DC source. If the input is taken from the AC source, then the output voltage would be fully rectified. Here MOSFET is used as an SMPS.

The operation of SMPS flyback topology is completely based on the position of the switch ie, MOSFET.

It can operate in a continuous or discontinued mode based on the position of the switch or FET. In the discontinued model, the current in the secondary winding becomes zero before the switch is turned ON. In the continuous mode, the current in the secondary doesn’t become zero.

When the switch is turned OFF, the energy stored in the leakage inductance of the transformer flows through the primary winding and is absorbed by the input clamp circuit or snubber circuit. The role of the snubber circuit is to protect the switch from high inductive voltages. There will be power dissipation during the ON and OFF transitions of the switch.

SMPS Flyback Transformer Design

SMPS flyback transformer design is more popular than normal power supply designs because of its low cost, efficiency, and simple design. It isolates the primary and secondary winding of the transformer for given multiple inputs and provide multiple output voltages, which may be positive or negative.

The basic SMPS flyback transformer design when the switch is turned ON and OFF is shown below. It is also used as an isolated power converter. The flyback transformer used in the design contains primary and secondary winding, separated electrically to avoid transient coupling, ground loops, and provides flexibility.

The use of SMPS flyback transformer design has an advantage over conventional transformer design. Here the current doesn’t flow through the primary and secondary winding at the same time because the phase of the winding gets reversed as shown in the above figure.

It stores the energy in the form of the magnetic field in the primary winding for a certain amount of time and transfers to the primary winding. The maximum output load voltage, operating ranges, input and output voltage ranges, power delivery capability, and the characteristics of flyback cycles are the important parameters in the SMPS flyback transformer design.

Applications

The flyback converter applications are,

• Used in television sets, and pcs with low power of up to 250W
• Used in Stand by power supplies in electronic pcs (low power switch mode)
• Used in mobile phones and mobile chargers
• Used in high voltage supplies like television, CRTs, Lasers, flashlights, and copy devices, etc.
• Used in multiple input-output power supplies
• Used in isolated gate drive circuits.

Thus, this is all about an overview of the flyback converter – design, working principle, operation, topology, SMPS flyback transformer design, topology, SMPS topology design, and applications. Here is a question for you, ” What are the advantages of flyback converter? “