Flyback Converter Basic Components
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
How Flyback Converter Works when The 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,
Where;Vin – is the input voltage applied to the primary windingLp – primary inductance of the transformerTon – 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,
Where;Vsec – this is the voltage on the secondary winding. This is just ideally equal to the output voltage.Ls – secondary inductance of the transformerT – this is the period of the PWM signal (1/Fsw)Ton – this is the time the switch is ON