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

Open online SPICE simulator circuit link: learn_power_buck_24V_ccm_startup.tsc

Expert talk: how to design a Buck (step-down) Converter?

Use the buck (step-down) topology if the output voltage has to be lower than the input voltage. In case of a
very low output voltage requirement the diode can be replaced by a MOSFET switch for highest efficiency
(i.e. synchronous rectification). In a Multiphase topology multiple buck converter circuits are placed in parallel
for very high peak current supply of CPU or FPGA loads.

Overview
Output voltage V_out: lower than input voltage
Power rating P_out: up to 150 W or higher
Isolation: not available (if human accessible, V_in must be smaller than 42.5 V)

Continuous Conduction Mode (CCM)
V_out= D * V_in; Duty cycle D= T_on/T with conducting pulse width T_on, switching period T
Current Ripple I_LR= (V_out * (1 - D) * T)/L

Component Ratings
MOSFET voltage class V_DS= V_in
MOSFET current I_D= I_out
Diode voltage class V_R= V_in

Other Remarks
Continuous Conduction Mode (CCM): Inductor current I_L always positive
Discontinuous Conduction Mode (DCM): coil current reaches zero

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= 12 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:= 24 {use 12... 40}
{Output voltage [V]}
V_out:= 12 {use lower than V_in}

{Output current [A]}
I_out:= 2 {use 1 .. 2}
R_load:= V_out/I_out
R_load=[6]

{=== Control settings: change with care ! ===}
{Switching frequency [Hz]}
fs:= 100k {use 100k .. 300k}
{Duty cycle}
Duty:= V_out/V_in
Duty=[500m]
L_INIT:= I_out {inductor 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)*R_load)/(2*fs)
L_crcm=[15u]
L:= 2*L_crcm
L=[30u]
R_crcm:= (2*L*fs)/(1-Duty)
R_crcm=[12]