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]

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

\a(LED Buck Converter design

Technical Assistance

Other circuits

Buck startup

Buck steady-state