デザインキット・マイクロコントローラ(U pc494)による降圧回路の解説書

All Rights Reserved Copyright (C) Bee Technologies Corporation 2009 1

Design KitBuck Converter Using PC494

Contents

Slide #

1. Buck Converter using PC494: VIN=12V, [email protected].....................................................................

1.1 Output voltage...................................................................................................................................

1.2 Output current...................................................................................................................................

1.3 Output ripple voltage ........................................................................................................................

1.4 Efficiency……....................................................................................................................................

1.5 Step-load response...........................................................................................................................

2. Basic Operation.......................................................................................................................................

2.1 Output voltage equations..................................................................................................................

2.2 Basic operation waveforms...............................................................................................................

3. Switching Transistor Q1..........................................................................................................................

3.1 Voltage and current stresses............................................................................................................

3.2 Transistor characteristics..................................................................................................................

3.3 Transistor Q1 losses ........................................................................................................................

4. Freewheeling Diode D1 (SBD)…………………………….......................................................................

4.1 Voltage and current stresses............................................................................................................

4.2 Schottky barrier diode (SBD) characteristics....................................................................................

4.3 Freewheeling diode D1 (SBD) losses...............................................................................................

4.4 Schottky barrier diode Standard model…….....................................................................................

5. Output Inductor Value..............................................................................................................................

6. Output Capacitor.....................................................................................................................................

7. Voltage Control Feedback Loop..............................................................................................................

Simulations index..........................................................................................................................................

3

4

5

6

7

8

9

10

11

12

13

14

15-16

17

18

19

20-21

22

23-26

27-29

30-31

32

2All Rights Reserved Copyright (C) Bee Technologies Corporation 2009

1. Buck Converter using PC494: VIN=12V, [email protected]

• VIN = 12V

• VOUT = 5V

• IOUT = 0.15~0.5A , IOUT below 0.15A DCM

• Efficiency = >74% with VIN = 12V, load 0.5A

• Output voltage ripple < 1% VOUT


Schottky barrier diode (SBD) D1 is simulated using a Professional model.

Lo

8RHB

Vcc

0

Vcc12Vdc

OUT+

Q1Q2SA1680

RB

390 RL10

Rsense

0.25

0

OUT-

0

R14500

U1UPC494 1 2 3 4 5 6 7 8

910

11

12

13

14

15

16

R13.9k

R25.1k

R35.1k

R45.1k

R5240k

R62.4k

R77.5k

0

0

VR11.45k

VR3

2k

0

0

C2

0.01uF

C3100nF

C410u

RV2-1{2k-R1}

0

RV2-2{R1}

0 0

R15100

R16100k

R13

7.5k R11500

C51000pFIC = 0

R815k

R171k

C7100n

PARAMETERS:

R1 = 0.1

C1

+

CoRJJ-35V221MG5-T20

IC = 0

D1

XBS104V14R_P

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms

V(OUT+,OUT-)

0V

1.0V

2.0V

3.0V

4.0V

5.0V

6.0V

1.1 Output voltage

• The output voltage is regulated at 5V (RL=10) ,voltage overshoot at startup is 0.6V.

Steady state is reached within 4ms.


Overshoot voltage

Time to reach steady state

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms

I(RL)

0A

100mA

200mA

300mA

400mA

500mA

600mA

1.2 Output current


Overshoot current

• The output current is 0.5A (RL=10) ,current overshoot at startup is 60mA. Steady

state is reached within 4ms.

Time to reach steady state

Time

7.81ms 7.82ms 7.83ms 7.84ms 7.85ms 7.86ms 7.87ms 7.88ms 7.89ms 7.90ms 7.91ms

V(OUT+,OUT-)

4.80V

4.84V

4.88V

4.92V

4.96V

5.00V

5.04V

5.08V

5.12V

1.3 Output ripple voltage

• The output ripple voltage is less than 1% of the output voltage ( < 50 mVP-P ).


Output voltage ripple

Time

5ms 6ms 7ms 8ms 9ms 10ms

100*AVG(V(RL:1)*I(RL))/AVG(-I(V1)*V(V1:+))

0

10

20

30

40

50

60

70

80

90

100

1.4 Efficiency

• The efficiency of the converter at VIN=12V and load RL=10 is 74% or more.

(Efficiency = > 74%)


Efficiency = > 74%

Time

6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms5ms

V(OUT+,OUT-)

4.5V

5.0V

5.5V

SEL>>

I(IL)

0A

250mA

500mA

750mA

1.5 Step-load response

• Simulation results shows the transient responses, when load current steps up and

steps down.


250mA / 500mA step-load

-250mV

250mV

Vcc

Q1

D1

L11 2

C1

OUT-

Rload

0

R1

R2

PWM Control

Circuit

+

-

OUT+

Vref2.5V

0

2. Basic Operation

The basic operation of how the output voltage is regulated at the desired voltage level.

• VO is sensed by sampling resistors R1, R2 and compare to a reference voltage Vref in

the Error Amp.

• The error voltage is modulated with a sawtooth waveform.

• Pulse-width-modulator (PWM) output pulse width TON is proportional to the Error Amp

output DC voltage level.

• VOUT ,which is proportional to TON ,is regulated at the desired voltage level.


-IC(Q1)

I(D1)

I(L1)

V in Vcc VO

Error Amp

PWM output pulse

2.1 Output voltage equations

• R1 and R2 are calculated by follow equation. In practical design a variable resistor

,that is higher than the calculated value ,is chosen for R1.


VinT

TV

ONOUT

VrefR

RRVOUT

2

21

• If the circuit works in continuous conduction mode (CCM) , output voltage and TON

follow the equation below.

(1)

(2)

Time

7.82ms 7.84ms 7.86ms 7.88ms 7.90ms7.81ms 7.91ms

I(Lo:1) I(RL)

0.3A

0.7A

I(D1)

0A

500mA

IE(Q1)

0A

500mA

SEL>>

V(Q1:c)

0V

14V

2.2 Basic operation waveforms


TON

Vin

-IC(Q1)

I(D1)

I(L1)

/ I(RL)

T

ΟΝΟCC

ΤL

VVΔΙ

1ΟFF

ΟΤ

L

VΔΙ

1I(RL)=VO/Rload

• Simulation result shows the waveforms and magnitude of the current throughout the

circuit.

Lo

8RHB

0

Vcc

Vcc12Vdc

OUT+

Q1Q2SA1680

RB

390 RL10

Rsense

0.25

0

OUT-

0

R14500

U1UPC494 1 2 3 4 5 6 7 8

910

11

12

13

14

15

16

R13.9k

R25.1k

R35.1k

R45.1k

R5240k

R62.4k

R77.5k

0

0

VR11.45k

VR3

2k

0

0

C2

0.01uF

C3100nF

C410u

RV2-1{2k-R1}

0

RV2-2{R1}

0 0

R15100

R16100k

R13

7.5k R11500

C51000pFIC = 0

R815k

R171k

C7100n

PARAMETERS:

R1 = 0.1

C1

+

CoRJJ-35V221MG5-T20

IC = 0

D1

XBS104V14R_P

3. Switching Transistor Q1


• VIN = 12V

• VOUT = 5V

• RL = 10

• Switching transistor Q1 is Toshiba 2SA1680 (IC,MAX = 2A, VCE,MAX = 50V )

• RB=390

3.1 Voltage and current stresses

• Simulation result shows the collector peak current at start-up transient. This peak

current is limited by the resistor RB. The current should not exceed the maximum

current rating of transistor Q1 (2SA1680 IC,MAX = 2A, VCE,MAX = 50V ).


Time

0s 2ms 4ms 6ms 8ms 10ms

1 V(Q1:e,Q1:c) 2 IE(Q1)

0V

10V

20V1

SEL>>

0A

1.0A

2.0A2

SEL>>

V(OUT+,OUT-)

0V

2.5V

5.0V

-VCE(t)

-IC(t)

Peak current

at start-up

VO(t)

Time

20.6us 21.0us 21.4us 21.8us 22.2us

1 -IB(Q1) 2 -IC(Q1)

0A

200mA

-100mA

1

>>

0A

2.0A

-1.0A

2

3.2 Transistor characteristics

VCE(sat)-IC characteristics Turn-off characteristics

• Losses in Q1 depend on the transistor characteristics. Conduction loss is depends on VCE (sat) – ICand switching loss is depends on Switching time characteristics ( turn-on and turn-off )


0.01

0.1

1

10

100

0.001 0.01 0.1 1

- IC(mA)

- V

CE

(SA

T)

(V)

Measurement

Simulation

tf

_90%

_10%

-IB(t)

-IC(t)

Time

9.954ms 9.958ms 9.962ms 9.966ms 9.970ms 9.974ms 9.978ms 9.982ms

1 V(Q1:e,Q1:c) 2 IE(Q1)

0V

4V

8V

12V

16V

20V1

0A

0.2A

0.4A

0.6A

0.8A

1.0A2

SEL>>SEL>>

W(Q1) AVG(W(Q1))

0W

1.0W

2.0W

3.0W

4.0W

5.0W

6.0W

7.0W

8.0W

3.3 Transistor Q1 losses

• Simulation results shows waveforms of IC and VCE of transistor Q1. Switching power

loss and average power loss are also shown.

Q1 Power Loss

Average Loss

-VCE(t)-IC(t)

Peak

magnitude

current


D1(SBD) trr

contribute

Switching

loss (turn-on)

Switching

loss (turn-off)

Conduction loss

(VCE(SAT) x IC)

Time

9.960ms 9.962ms 9.964ms

1 V(Q1:e,Q1:c) 2 IE(Q1)

0V

4V

8V

12V

16V

20V1

0A

200mA

400mA

2

SEL>>SEL>>

W(Q1) AVG(W(Q1))

0W

1.0W

2.0W

3.0W

4.0W

5.0W

6.0W

7.0W

8.0W

3.3 Transistor Q1 losses (zoomed)

• Simulation results shows the contribute of D1(SBD) recovery time trr ,that elevate

losses in the switch transistor Q1.

Q1 Power Loss

-VCE(t)

-IC(t)


Switching

loss (turn-on)

Average Loss

D1(SBD) trr

contribute

Lo

8RHB

0

Vcc

Vcc12Vdc

OUT+

Q1Q2SA1680

RB

390 RL10

Rsense

0.25

0

OUT-

0

R14500

U1UPC494 1 2 3 4 5 6 7 8

910

11

12

13

14

15

16

R13.9k

R25.1k

R35.1k

R45.1k

R5240k

R62.4k

R77.5k

0

0

VR11.45k

VR3

2k

0

0

C2

0.01uF

C3100nF

C410u

RV2-1{2k-R1}

0

RV2-2{R1}

0 0

R15100

R16100k

R13

7.5k R11500

C51000pFIC = 0

R815k

R171k

C7100n

PARAMETERS:

R1 = 0.1

C1

+

CoRJJ-35V221MG5-T20

IC = 0

D1

XBS104V14R_P

4. Freewheeling Diode D1 (SBD)


• VIN = 12V

• VOUT = 5V

• RL = 10

• Freewheeling diode D1 is Schottky Barrier Diode (SBD), XBS104V14R (IFSM=20A ,

VRM=40V) from Torex Semiconductor.

Time

0s 2ms 4ms 6ms 8ms 10ms

1 V(D1:2,D1:1) 2 I(D1)

10V

20V

-1V

1

SEL>>0A

1.0A

2.0A2

SEL>>

V(OUT+,OUT-)

0V

2.5V

5.0V

4.1 Voltage and current stresses

• Simulation result shows the forward peak current at start-up transient. The peak

current should not exceed the maximum current rating of shottky barrier diode D1

(XBS104V14R : IFSM=20A , VRM=40V).


VR(t)

IF(t)

Peak current

at start-up

VO(t)

4.2 Schottky barrier diode (SBD) characteristics

VR - IR characteristics CT - VR characteristics

• Losses in D1 depend on the SBD characteristics. Reverse leakage loss is depends on VR – IRcharacteristics and reverse recovery loss is depends on junction capacitance characteristics.

• Graphs show agreement between the simulations and measurements of SBD Professional model.


0.001

0.010

0.100

1.000

10.000

0 10 20 30 40

Re

ve

rse

Cu

rre

nt:

IR (

mA

)

Reverse Voltage:VR (V)

Measurement

Simulation

0

100

200

300

400

500

5 15 25 35

Inte

r-T

erm

ina

l Ca

pa

cit

y:C

t (p

F)

Reverse Voltage:VR(V)

Measurement

Simulation

Time

9.955ms 9.960ms 9.965ms 9.970ms 9.975ms 9.980ms

1 V(D1:A,D1:C) 2 I(D1:A)

0V

-14V

14V1

0A

-0.6A

0.6A2

>>

W(D1) AVG(W(D1))

0W

200mW

400mW

SEL>>

4.3 Freewheeling diode D1 (SBD) losses

• Simulation results shows waveforms of IF and VAK of freewheel diode D1 ,which is a

Schottky type. Switching power loss and average power loss are also shown.

VAK(t)

IF(t)


Reverse recovery

characteristic

Conduction

loss

Reverse

recovery loss

D1 (SBD) Power Loss

Average Loss

Reverse

leakage loss

Time

9.9602ms 9.9604ms 9.9606ms 9.9608ms 9.9610ms 9.9612ms 9.9614ms 9.9616ms 9.9618ms 9.9620ms

1 V(D1:A,D1:K) 2 I(D1:A)

0V

-12V

12V1

0A

400mA

-200mA

2

>>

W(D1) AVG(W(D1))

0W

200mW

400mW

SEL>>

4.3 Freewheeling diode D1 (SBD) losses (Zoomed)

• Simulation results shows the reverse recovery time - trr of D1.

• Average loss in diode is 72.75mW

• Reverse leakage loss is approximately 907uW.


Average Loss = 72.75mW

Reverse Leakage Loss = 907uW

Reverse recovery

Reverse

recovery loss

Reverse

leakage loss

Time


1 V(D1:A,D1:C) 2 I(D1:A)

0V

-12V

12V1

SEL>>

0A

-200mA

400mA2

SEL>>

W(D1) AVG(W(D1))

0W

200mW

400mW

4.4 Schottky barrier diode Standard model

• VR – IR characteristics is not included in the Standard model.

• Average loss in diode is 72.4mW ( the Professional model result is 72.75mW )

• Reverse leakage loss is approximately 187uW (the Professional model result is

907uW).


Reverse recovery

characteristic

Reverse

recovery loss

Reverse

leakage loss Average Loss = 72.4mW

Reverse Leakage Loss = 187uW

0

Vcc

Vcc12Vdc

OUT+

Q1Q2SA1680

RB

390 RL34

Rsense

0.25

0

OUT-

0

R14500

U1UPC494 1 2 3 4 5 6 7 8

910

11

12

13

14

15

16

R13.9k

R25.1k

R35.1k

R45.1k

R5240k

R62.4k

R77.5k

0

0

VR11.45k

VR3

2k

0

0

C2

0.01uF

C3100nF

C410u

RV2-1{2k-R1}

0

RV2-2{R1}

0 0

R15100

R16100k

R13

7.5k R11500

C51000pFIC = 0

R815k

R171k

C7100n

PARAMETERS:

R1 = 0.1

C1

+

CoRJJ-35V221MG5-T20

IC = 0

D1

XBS104V14R_P

Lo

{Lo}

1 2Rlo

0.63

PARAMETERS:

Lo = 300u

5. Output Inductor Value


• VIN = 12V

• VOUT = 5V

• RL=34 (IOUT 0.15A)

• Output inductor value Lo are 150uH and 300uH (Parametric sweep)

Time


I(Lo:1) I(RL)

0A

500mA

600mA


• Simulation results shows the inductor current compare to minimum load current. If

0.5*IL,RIPPLE is less than IO,min, the inductor will operate in the “continuous” operating

mode (CCM)


Inductor current at

L=330uF, CCM,

f=38kHz

IL,RIPPLE

IO,min = 150mA


The inductor value is calculate by.


Where

• LCCM is output inductance that converter at load RL still working in CCM.

• RL is load resistance at the minimum output current, RL = VO / IO,min

• f is switching frequency

I

LOICCM

fV

RVVL

2

Which the IO,min is 150mA and VO is 5V, RL is approximately 34. At VIN=12V and f=38kHz ,this

equation calls for L 261uH.

The output inductor value is selected to set the ripple current. The too small value leads to

larger ripple current that will lead to more output ripple voltage due to the output capacitor ESR.

(3)

Time

9.83ms 9.84ms 9.85ms 9.86ms 9.87ms 9.88ms 9.89ms 9.90ms 9.91ms 9.92ms 9.93ms

I(Lo) I(RL)

0A

500mA

600mA


• Simulation results shows that the inductor will operates in the “discontinuous”

operating mode (DCM), If reduce the inductance value to 150uH.


I(LO) @ 300uH,

CCM

I(LO) @ 150uH,

DCM

_IO,min = 150mA

Lo

8RHB

Vcc

0

Vcc12Vdc

OUT+

Q1Q2SA1680

RB

390 RL10

Rsense

0.25

0

OUT-

0

R14500

U1UPC494 1 2 3 4 5 6 7 8

910

11

12

13

14

15

16

R13.9k

R25.1k

R35.1k

R45.1k

R5240k

R62.4k

R77.5k

0

0

VR11.45k

VR3

2k

0

0

C2

0.01uF

C3100nF

C410u

RV2-1{2k-R1}

0

RV2-2{R1}

0 0

R15100

R16100k

R13

7.5k R11500

C51000pFIC = 0

R815k

R171k

C7100n

PARAMETERS:

R1 = 0.1

C1

Co

{Co}

RESR

{ESR}

D1

DXBS104V14R

PARAMETERS:Co = 200uESR = 200m

6. Output Capacitor


• VIN = 12V

• VOUT = 5V

• RL=10

• Output capacitor value Co are 100uF and 200uF (Parametric sweep)

6. Output Capacitor


In addition, the voltage component due to the capacitor ESR must be considered, as

shown in equation (5).

Where

• IL,RIPPLE is an inductor ripple current.

• VO,RIPPLE is an output ripple voltage.

• f is switching frequency

RIIPLEO

RIPPLELO

Vf

IC

,

,min,

8

RIPPLEL

RIIPLEOESR

I

VR

,

,

• RESR VRIPPLE / IL,RIPPLE , At VRIPPLE =50mV and IL,RIPPLE =250mA , this equation calls for

RESR value less than 200m.

The minimum output capacitor is determined by the amount of inductor ripple current, and can

be calculated by the equation:

(4)

(5)

Time

0s 1.0ms 2.0ms 3.0ms 4.0ms 5.0ms

V(OUT+,OUT-)

0V

2.0V

3.0V

4.0V

5.0V

6.0V

SEL>>

Time

4.90ms 4.92ms 4.94ms 4.96ms 4.98ms 5.00ms

V(OUT+,OUT-)

5.0V

4.9V

6. Output Capacitor

• Overshoot voltage transient is also considered for output capacitor selection,


RJJ-35V221MG5-T20

(220uF electrolytic capacitor

with ESR=200m)

VO,RIPPLE (Zoomed)

100uF with ESR=200m

capacitor, overshoot voltage

is too big

Overshoot

CO=100uFCO=200uF

PARAMETERS:

C3 = 0.1u

Lo

8RHB

0

Vcc

Vcc12Vdc

OUT+

Q1Q2SA1680

RB

390 RL10

Rsense

0.25

0

OUT-

0

R14500

U1UPC494 1 2 3 4 5 6 7 8

910

11

12

13

14

15

16

R13.9k

R25.1k

R35.1k

R45.1k

R5240k

R62.4k

R77.5k

0

0

VR11.45k

VR3

2k

0

0

C2

0.01uF

C3{C3}

C410u

RV2-1{2k-R1}

0

RV2-2{R1}

0 0

R15100

R16100k

R13

7.5k R11500

C51000pFIC = 0

R815k

R171k

C7100n

PARAMETERS:

R1 = 0.1

C1

+

CoRJJ-35V221MG5-T20

IC = 0

D1

XBS104V14R_P

7. Voltage Control Feedback Loop


• VIN = 12V

• VOUT = 5V

• RL=10

• Capacitor C3 value are 0.01uF and 0.1uF (Parametric sweep)

Time

2.5ms 3.0ms 3.5ms 4.0ms 4.5ms 5.0ms

V(OUT+,OUT-)

4.5V

5.0V

5.5V

V(U1:3)

0V

1.0V

2.0V

SEL>>

7. Voltage Control Feedback Loop

• Simulation result shows the oscillation ripple that given from the feed back loop with

C3=0.01uF. This circuit calls for C3 more than 0.01uF to avoid oscillation, C3=0.1uF is

selected.


Oscillation

ripple!

C3 = 0.01uF

Feedback voltage:

V(IC pin3)

C3 = 0.1uF

C3 = 0.01uF

C3 = 0.1uF

Output voltage:

VO

Simulation Index


Simulations Folder name

1. Transient simulation (@ VIN=12V, RL=10)......................................

2. Efficiency (@ VIN=12V, RL=10).........................................................

3. Step-load response (@ VIN=12V, IOUT=250mA / 500mA )...................

4. Power switch devices losses (Q1 and D1)..........................................

5. Output inductor....................................................................................

6. Output capacitor..................................................................................

7. Voltage control feedback loop.............................................................

Transient

Efficiency

Step-load

Losses

Lout

Cout

Feedback

Technology

デザインキット・マイクロコントローラ(U pc494)による降圧回路の解説書