벅 컨버터의 파워스테이지 계산, Basic Calculation of a Buck Converter's Power stage

출처 : Basic Calculation of a Buck Converter's Power Stage, TI

http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=slva477b&docCategoryId=1&familyId=64&keyMatch=slva477b&tisearch=Search-EN-Everything

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1 Basic Configuration of a Buck Converter

어떤 회로들은 Converter 회로에 있는 Diode를 2nd SW로 바꿔서 사용하기도 함

1.1 Necessary Parameters of the Power Stage

계산하기 위해 필요한 파라메터들... (사실 Buck converter Datasheet 한 장 있으면 끝남)

1. Input voltage range: VIN(min) and VIN(max)

2. Nominal output voltage: VOUT

3. Maximum output current: IOUT(max)

4. Integrated circuit used to build the buck converter. This is necessary because some parameters for the calculations must be derived from the data sheet.

2 Calculate the Maximum Switch Current

2.1 Duty 계산

Maximum Duty Cycle: D = Vout / ( VIN(max) x η )

η = efficiency of the converter, e.g., estimated 90%

2.2 max switching current 계산

IMAXOUT = ILIM(min) - (ΔIL/2)

ΔIL = inductor ripple current = {  ( VIN(max) - VOUT ) * D  }/ ( fs * L )

Application specific maximum switch current: ISW(max) = ( ΔIL/2 ) + IOUT(max)

ILIM(min) = minimum value of the current limit of the integrated switch (given in the data sheet)

This is the peak current, the inductor, the integrated switch(es), and the external diode have to withstand.

3 Inductor Selection

The higher the inductor value, the higher is the maximum output current because of the reduced ripple current.

인덕터 value 높음 --> max output current 높음

인덕터 value 낮음 --> 패키지 작음

L= VOUT * (VIN-VOUT) / (ΔIL * fs * VIN)

대략적으로 계산해보면

ΔIL = (0.2 ~ 0.4) * IOUT(max)

4 Rectifier Diode Selection

IF = IOUT(max) * (1-D)

PD = IF * VF

IF = average forward current of the rectifier diode

IOUT(max) = maximum output current necessary in the application

VF = forward voltage of the rectified diode

5 Output Voltage Setting

With the given feedback voltage, VFB, and feedback bias current, IFB, the voltage divider can be calculated.

The current through the resistive divider needs to be at least 100 times as big as the feedback bias current:

IR1/2 ≥ 100 * IFB

R2 = VFB / IR1/2

R1 = R2 * (  (VOUT / VFB) -1  )

IR1/2 = current through the resistive divider to GND

IFB = feedback bias current from data sheet

R1,R2 = resistive divider, see Figure 2.

VFB = feedback voltage from the data sheet

VOUT = desired output voltage

6 Input Capacitor Selection

Datasheet 따라서 해라.

Input Capacitor 사용 목적 : stabilize the input voltage due to the peak current requirement of a switching power supply

7 Output Capacitor Selection

case 1. external compensation : min recommended value 이상 사용하며 capacitance에 맞춰서 compensation 한다.

COUT(min) = ΔIL / (8 * fs * ΔVOUT)

COUT(min) = minimum output capacitance

ΔIL = estimated inductor ripple current

ΔVOUT = desired output voltage ripple

case 2. internal compensation : recommended value 꼭 사용, 또는 L * C의 비율을 꼭 맞춰라.