Flyback Converter

Flyback converter is one of the most popular converters in the commercial products for its least components count, used in various lower and medium power applications.

It is a buck boost derived converter with isolation. The input and output of the converter is electrically isolated.

It is very useful when designing a DC to DC converter for the operation off the power line.

The mathematical expression used in this converter is

Vout = (N2/N1* D/1-D)* VIN  

Where

Vout is the output voltage,

 N2 is the secondary winding, N1 is the primary turns,

VIN is the input voltage source.

The main component of the Flyback converter is the couple inductor.

The couple inductor looks like a transformer, but it is not a transformer. It is a ferrite core high frequency transformer. It acts as energy storage of the transformer.

In a transformer if you have current coming in one winding, it will come out from the other windings without having stored energy in the core of the transformer.

Now in the case of a coupled inductor, when there is a flow of current, the energy is stored in the core and then at a later stage, this energy can be back through to another winding.

The ratios of currents and voltages, the ratio of the inductances seen from each winding is the ratio of the number of turns. Once you know the inductance in one side, you can calculate the value of inductance used for the transformer.

Applications of the Flyback converter: –

  • In Low supply television sets of 250W, the converter is used.
  • For Standby power supplies in computers.
  • In Cell phone and mobile device chargers, the converter is present.
  • High-voltage supplies in TV and Monitor CRTs, Lasers, Xenon flashlights, copiers, etc., the use of converter has been seen.

How Flyback converter works?

Below Figure 1 is showing the circuit of the Flyback converter.

Circuit diagram of Flyback converter

Fig. 1 Circuit diagram of Flyback converter

The Flyback converter used here is in the discontinuous mode. The discontinuous mode means diode is not connected in series with any inductance at the secondary side of the output.

The working principle of the converter: –

During the switch (T) is in ON, the current builds up in the transformer primary (and thus storing energy).

When the switch turned OFF, the polarity of primary and secondary coil voltage reversed.

Now the diode D is in forward biased and the energy stored in the transformer is transferred to capacitor (C), and sequentially to the load (RL).

 Mode 1:

Switch is in ON condition

On State of Flyback Converter

Fig.2 Mode 1

In mode 1 current flows from positive terminal towards the primary winding.  The switch is in the ON condition. It is forming a close path for the charge flow. The switch can be a MOSFET, IGBT etc. So, energy is getting charge in the primary winding. Capacitor is already charged in this condition. Diode is in reverse biased condition. Blue line shows the on state operation of Flyback converter.

Mode 2:

Switch T is turned OFF

Fig.3 Mode 2

 

In mode 2 switch is turned OFF. The secondary winding will now conduct the primary winding energy. The diode is in forward biased. According to the property of wiring of the winding the secondary side will now become positive. The top side of the secondary winding will become positive and the down side will become negative. The charge will now flow to the load through the capacitor. Blue line shows the off state operation of Flyback converter.

Waveform:

The operation of the converter can be explained with waveforms:

The waveform is explained in two modes, Mode 1 and Mode 2. Fig. 4 shows the discontinuous mode Flyback converter waveform.

Mode 1:

During mode 1 the switch T is ON by Fig.1. No voltage drop across the switch T. i.e. VT from the waveform. The current flows through the switch T given as I. The current LT is the inductance current due to mutual inductance characteristics of the transformer. During the mode 1 there is no current flow through the diode, as diode is in reverse biased condition. Vp is the primary winding voltage of the transformer. During T ON voltage across winding is shown in the waveform. By the transformer property flux is induced in both the windings.

Mode 2:

When Mode 1 is OFF then mode 2 the switch T is OFF from Fig.1. Voltage drop across the switch T. i.e. VT is shown in the waveform. No current flows through the switch T, in mode 2 is shown as I. The current LT is the inductance current due to mutual inductance characteristics of the transformer which is nil. During the mode 2 current flows through the diode, as diode is in forward biased condition. Vp is the secondary winding voltage of the transformer. During T OFF, secondary voltage (VS) across winding is present in the waveform. By the transformer property flux is induced in both the windings. The energy stored in the transformer will move to the capacitor and finally to the load (RL).

Fig.4 Discontinuous mode Flyback converter

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