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Expert Talk: Design a 24V Buck-Boost Converter in CCM mode

Open online SPICE simulator circuit link: learn_power_buckboost_24V_ccm_startup.tsc

{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:= 24 {use 12... 40}
{Output voltage [V]}
V_out:= -5 {negative of V_in}

{Output current [A]}
I_out:= 2 {use 1 .. 2}
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))
L_INIT:= I_out/(1-Duty) {inductor DC value}
C_INIT:= V_out {capacitor DC value}
T_on:= Duty*T
{Calculate components for CCM}
L_crcm:= ((1-Duty)^2*R_load)/(2*fs)
L:= 2*L_crcm
R_crcm:= (2*L*fs)/(1-Duty)^2


Use the buck-boost topology if you need an inverted output voltage with opposite polarity of the input voltage.
The absolute output voltage can be lower or higher than the input voltage. If you need isolation use the Flyback

Output voltage V_out: inverted, absolute can be higher or lower than input voltage
Power rating P_out: up to 150 W or higher
Isolation: not available (if isolation needed use Flyback converter)

Continuous Conduction Mode (CCM)
Output voltage 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_out * T) * (1 -D) / L

Component Ratings
MOSFET voltage class V_DS= V_in - V_out
MOSFET current I_D= (2 * P_out)/V_in(min)
Diode voltage class V_R= V_in - V_out

Other Remarks
design for CCM for lower peak currents

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= 24 V
V_out= -5 V (inverted, absolute of V_out higher or 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.

Expert talk: how to design a Buck-Boost (inverting) Converter?

Buck-Boost steady-state
Buck-Boost startup

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