Flyback Converter Circuit

How Flyback Converter Works – Operation and Principle

A flyback converter is composed of a flyback transformer, switch, rectifier and filter and the control mechanism to drive the switch and provide regulation. It is a low part count switching converter and relatively easy to make or design. A flyback converter is an isolated switching converter that can be a step down or step up configuration. Most cell phone, tablets and laptop chargers are made from flyback converter. Following topics will discuss how flyback converter works.

Flyback Converter Basic Components

Flyback Converter Circuit

Flyback Transformer:

The flyback transformer is not actually a transformer. A transformer will transfer energy from the primary to secondary ideally real time and perfectly. Flyback transformer on the other hand will store energy on the primary magnetic field and after a certain amount of time it delivers the energy to the secondary.

Switch:

The role of the switch is to turn ON and OFF the primary circuit which able to magnetize and demagnetize the transformer. The switch is being controlled by the PWM signal which is coming from the selected controller.

Rectifier and Filter:

The rectifier will rectify the voltage on the secondary winding to become a pulsating DC. Another role of the rectifier or the diode is to cut and connect the load from the secondary winding. The rectified voltage is then filter out by the capacitor to increase the DC level and can be usable by the intended application.

Not present in the above schematic is a snubber circuit. Most of the times, a flyback converter will need a snubber to combat the voltage spike across the switch or the diode.

How Flyback Converter Works when The Switch is ON

Flyback Converter When Switch is ON

The switch really plays an important role on how flyback converter works. When the switch is ON, the current will flow from Vin down to the primary ground. This will charge the primary winding and store energy. During this time, the secondary winding has no current flow as the diode is reverse bias. The load demand at this time is supplied by the output capacitance (Cout).

Flyback Converter Current When Switch is ON

How Flyback Converter Works when The Switch is OFF

Flyback Converter When Switch is OFF

When the primary switch is cut off, the primary winding will resist to sudden change in current and reverses the polarity of the winding. This will result to the forward bias of the output diode. The stored energy in the primary will be transferred to the secondary and to the load via the diode. During this time, the output capacitor will replenish its charge.

Flyback Converter Current When Switch is OFF

Flyback Converter Operation and Principle When the Switch is ON

In short description, storing and transferring energy is how flyback converter works. When the switch is ON, the primary will charge and there is a current flow. When doing KVL, it will be:

Vin – VL – Vs = 0

Assuming ideal condition; the switch voltage drop (Vs) is zero,

Vin – VL = 0

VL = Vin

VL = Lp di / dt

di = ( VL / Lp ) X dt

VL = Vin, so

di = ( Vin / Lp ) X dt

by applying integration,

Expressing the stored energy,

Where;

Vin – is the input voltage applied to the primary winding

Lp – primary inductance of the transformer

Ton – this is the time the switch is ON

Flyback Converter Operation and Principle When the Switch is OFF

Flyback converter operation when switch if OFF will focus on the secondary. When the switch is OFF, there is secondary current flow. When doing KLV, it will be:

VL_secondary – VD – Vout = 0

Considering ideal; the voltage drop of the secondary diode is zero.

VL_secondary – Vout = 0

VL_secondary = Vout

VL = Ls di / dt

di = ( VL_secondary / Ls ) X dt

VL_secondary = Vout, so

di = ( Vout / Ls ) X dt by doing integration,

Expressing the transferred energy,

Where;

Vsec – this is the voltage on the secondary winding. This is just ideally equal to the output voltage.

Ls – secondary inductance of the transformer

T – this is the period of the PWM signal (1/Fsw)

Ton – this is the time the switch is ON

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