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Expert Talk: Design a 24V SEPIC Converter in CCM mode

Open online SPICE simulator circuit link: learn_power_sepic_24V_ccm_startup.tsc

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.

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.


{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

{=== Control settings: change with care ! ===}
{Switching frequency [Hz]}
fs:= 100k {use 100k .. 300k}
{Duty cycle}
Duty:= 1/(1+(V_in/V_out))
L1_INIT:= 0 {inductor DC value}
L2_INIT:= 0 {capacitor DC value}
C1_INIT:= 0
C2_INIT:= V_out
T_on:= Duty*T
{Calculate components for CCM}
C1:= 30u
C2:= 4m
I_lr:= (0.3*I_out*V_out)/(V_in)
L1:= (V_in*Duty)/(2*I_lr*fs)

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Technical Assistance
Other circuits

SEPIC steady-state

SEPIC startup