Expert talk: how to design a Flyback Converter?

The Flyback topology is the isolated version (transformer) of the buck-boost converter covering output

voltage ranges which are both higher and lower than the input voltage. It is very popular for offline adapters

up to 150 Watts. Due to the reflected output voltage the MOSFET switch has a higher voltage class rating.

Overview

Output voltage (V_out): can be higher or lower than the input voltage (non-inverting or inverting depending on transformer)

Power rating P_out: up to 150 W

Isolation: yes

Continuous Conduction Mode (CCM)

V_out= ((N2/N1) * D * V_in) / (1 - D); Duty cycle D= T_on/T with conducting pulse width T_on, switching period T

N2/N1 is the transformer ratio with N1 windings on the primary and N2 windings on the secondary side

Component Ratings

MOSFET voltage class V_DS= 1.5 * V_in

MOSFET Current I_D= (2 * P_out)/V_in(min)

Diode voltage class V_R= 5 * V_out

Other Remarks

the Buck-Boost converter is the non-isolated version of the Flyback converter

Example Converter

set your CCM converter spec in the Interpreter window on the left, click on Run to calculate the components and simulate it!

V_in= 220 V

V_out= 24 V (lower than V_in)

I_out= 2 A

Important Note (Disclaimer)

This is an idealized circuit for learning and experimental use. Test carefully when using calculated values for real hardware.

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{1. click here to set application parameters}

{2. click on "Run" to calculate components}

{3. click on "OK" and Simulate Transient }

{Input voltage [V]}

V_in:= 220 {use 100... 300}

{Output voltage [V]}

V_out:= 24 {use lower than V_in}

{Output current [A]}

I_out:= 2 {use 1 .. 4}

R_load:= V_out/I_out

R_load=[12]

{Transformer}

N1:= 10 {Primary winding}

N2:= 1 {Secondary winding}

Ratio:= -N2/N1 {voltage Ratio}

Ratio=[-100m]

{=== Control settings: change with care ! ===}

{Switching frequency [Hz]}

fs:= 100k {use 100k .. 300k}

{Duty cycle}

Duty:= 1/(1+(V_in/V_out*N2/N1))

Duty=[521.7391m]

L_INIT:= (V_out*I_out)/V_in { DC value}

C_INIT:= V_out {capacitor DC value}

T:=1/fs

T_on:= Duty*T

T_off:=T-T_on

ControlT2:=T_on

ControlT4:=T_off

{Calculate components for CCM}

L_crcm:= ((1-Duty)^2*R_load)/(2*fs*(N2/N1)^2)

L_crcm=[1.3724m]

L:= 2*L_crcm

L=[2.7448m]

R_crcm:= (2*L*fs*(N2/N1)^2)/(1-Duty)^2

R_crcm=[24]

\a(Power Selection Guide,https://edit.infineon.com/dgdl?fileId=5546d4625607bd13015621522aa012cb)

\a(55 W Flyback with IRS2982S

Technical Assistance

Other circuits

Flyback startup

Flyback steady-state