# Flyback Converter Working Principle

### Fly Back Converter - Basic Concept:

This is the popular isolated converter (input and outputs are isolated) used in low and moderate power levels.
The high frequency transformer provides the isolation between control switch and load.
The basic circuit is shown below.
The transformer T plays the role of transformer(electrical isolation) and energy storage element.

The simplified working principle of Flyback Converter is given below:
[1] During the switch(SW) is in ON position, the current builds up in the transformer primary (and thus storing energy).

[2] When the switch turned OFF, the polarity of primary and secondary coil voltages reversed.
[3] Now the diode D is in forward biased and the energy stored in the transformer is transferred to Capacitor (C), sequentially to the load(RL).

#### Flyback Converter - Circuit Diagram:

The figure shows the circuit diagram of flyback converter.

• It has a transformer of high frequency and also have the turns ratio of NP/NS.
• Observe the dot conventions on the transformer primary and secondary.

#### Flyback Converter - Operation and Waveforms:

The flyback converter operation can be explained with the help of following two modes:

Mode 1: Switch (T1) ON Condition:

• In this mode the transistor T1 is turned ON.
• So current starts flowing in the primary coil of the transformer.
• Equivalent circuit for Mode 1 operation is given below:
As shown in the figure, observe that the tranformer's secondary voltage reverse biases diode D1 when switch T1 is conducting.
• So, the load current is supplied by capacitor C1.
• Refer the Flyback waveform given below.
Here, note down that the current through transistor T1 increases from 0 to t1.
• At t1, the switch T1 is turned off.

Mode 2: Switch (T1) OFF Condition & Diode (D1) Conducts:

• Refer the below circuit to understand this mode.
• This mode will start when the switch is turned OFF at t1.
• As shown in the equivalent circuit, the voltage polarity of the tranformer's secondary coil is inverted.
• This phenomena forward biases diode D1 and the diode starts conducting.
• Thus the transfomer's secondary coil supplies energy to the capacitor C1 and load.
• Remember that this energy was stored in the transfomer when T1 was conducting in Mode 1.