Expert talk: how to design a SEPIC Converter?

Use the SEPIC topology (Single-Ended Primary-Inductor Converter) if the converter needs to cover output

voltage ranges which are both higher and lower than the input voltage. Similar to the boost topology the

input current is continuous and smooth while the ripple of the output current is large.

Overview

Output voltage (V_out): can be higher or lower than the input voltage

Power rating P_out: up to 150 W

Isolation: not available for the basic version (an isolated version is also possible)

Continuous Conduction Mode (CCM)

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

Current Ripple I_LR= (V_in * D * T) / (L1 + L2)

Component Ratings

MOSFET voltage class V_DS= V_in + V_out

MOSFET Current I_D= I_in

Diode voltage class V_R= V_in + V_out

Other Remarks

Use the Cuk converter if you need a small ripple at the output however the polarity will be negative

The SEPIC is difficult to control due to the fourth order nature; use only for slow voltage varying applications

A big capacitor with high current capability is required for the energy transfer

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= 12 V

V_out= 24 V (V_out higher or lower than V_in)

I_out= 0.5 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:= 12 {use 12 ... 40}

{Output voltage [V]}

V_out:= 24 {lower or higher V_in}

{Output current [A]}

I_out:= 0.5 {use 0.5 ..1}

R_load:= V_out/I_out

R_load=[48]

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

{Switching frequency [Hz]}

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

{Duty cycle}

Duty:= 1/(1+(V_in/V_out))

Duty=[666.6667m]

L1_INIT:= 0 {inductor DC value}

L2_INIT:= 0 {capacitor DC value}

C1_INIT:= 0

C2_INIT:= V_out

T:=1/fs

T_on:= Duty*T

T_off:=T-T_on

ControlT2:=T_on

ControlT4:=T_off

{Calculate components for CCM}

C1:= 30u

C2:= 4m

I_lr:= (0.3*I_out*V_out)/(V_in)

I_lr=[300m]

L1:= (V_in*Duty)/(2*I_lr*fs)

L1=[133.3333u]

L2:=L1

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

\a(LITIX™ LED Controller

Technical Assistance

Other circuits

SEPIC steady-state

SEPIC startup