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

Design Kit
Buck Converter Using PC494

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

Contents

Slide #

1. Buck Converter using PC494: VIN=12V, VOUT=5V@0.5A..................................................................... 3
1.1 Output voltage................................................................................................................................... 4
1.2 Output current................................................................................................................................... 5
1.3 Output ripple voltage ........................................................................................................................ 6
1.4 Efficiency…….................................................................................................................................... 7
1.5 Step-load response........................................................................................................................... 8
2. Basic Operation....................................................................................................................................... 9
2.1 Output voltage equations.................................................................................................................. 10
2.2 Basic operation waveforms............................................................................................................... 11
3. Switching Transistor Q1.......................................................................................................................... 12
3.1 Voltage and current stresses............................................................................................................ 13
3.2 Transistor characteristics.................................................................................................................. 14
3.3 Transistor Q1 losses ........................................................................................................................ 15-16
4. Freewheeling Diode D1 (SBD)……………………………....................................................................... 17
4.1 Voltage and current stresses............................................................................................................ 18
4.2 Schottky barrier diode (SBD) characteristics.................................................................................... 19
4.3 Freewheeling diode D1 (SBD) losses............................................................................................... 20-21
4.4 Schottky barrier diode Standard model……..................................................................................... 22
5. Output Inductor Value.............................................................................................................................. 23-26
6. Output Capacitor..................................................................................................................................... 27-29
7. Voltage Control Feedback Loop.............................................................................................................. 30-31
Simulations index.......................................................................................................................................... 32


1. Buck Converter using PC494: VIN=12V, VOUT=5V@0.5A

Vcc Lo
OUT+
8RHB
Vcc R13 VR3 Q1
12Vdc Q2SA1680 D1 +
500 XBS104V14R_P
0 R14 7.5k 2k R11 RB IC = 0
500 390 Co RL
R15 RJJ-35V221MG5-T20 10
100 C1
0.01uF

C2 Rsense
OUT-
R17

16
15
14
13
12
11
10
9
0 0.25
R1 1k R16
3.9k R3 100k 0
5.1k
C7 U1
0

1
2
3
4
5
6
7
8
VR1 100n UPC494
1.45k

R5 0
240k

RV2-2
R2 R4 C3 R6 {R1} C5 R8
5.1k 5.1k 100nF 2.4k 1000pF 15k
IC = 0

RV2-1 0 0 0
0 0 {2k-R1}
C4 PARAMETERS:
10u R1 = 0.1
R7
7.5k

• 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.

1.1 Output voltage

6.0V

Overshoot voltage

5.0V

4.0V

3.0V

2.0V

1.0V

Time to reach steady state

0V
0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms
V(OUT+,OUT-)
Time

• The output voltage is regulated at 5V (RL=10) ,voltage overshoot at startup is 0.6V.
Steady state is reached within 4ms.


1.2 Output current

600mA
Overshoot current

500mA

400mA

300mA

200mA

100mA

Time to reach steady state

0A
0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10ms
I(RL)
Time

• The output current is 0.5A (RL=10) ,current overshoot at startup is 60mA. Steady
state is reached within 4ms.


1.3 Output ripple voltage

5.12V

5.08V

5.04V

5.00V

4.96V

Output voltage ripple
4.92V

4.88V

4.84V

4.80V
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-)
Time

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


1.4 Efficiency

100

90

80
Efficiency = > 74%
70

60

50

40

30

20

10

0
5ms 6ms 7ms 8ms 9ms 10ms
100*AVG(V(RL:1)*I(RL))/AVG(-I(V1)*V(V1:+))
Time

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


1.5 Step-load response

750mA

500mA

250mA / 500mA step-load

250mA

0A
I(IL)
5.5V

250mV

5.0V

-250mV
SEL>>
4.5V
5ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
V(OUT+,OUT-)
Time

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


2. Basic Operation
V in Vcc VO
-IC(Q1) L1 I(L1)
Q1 1 2 OUT+
I(D1)
D1 C1 R1 Rload
Vcc

R2
OUT-
Error Amp
0 +
PWM Control
PWM output pulse Circuit -
Vref
2.5V

0
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.


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.

 R1  R 2 
VOUT     Vref (1)
 R2 
• If the circuit works in continuous conduction mode (CCM) , output voltage and TON
follow the equation below.

 TON 
VOUT     Vin (2)
 T 


2.2 Basic operation waveforms
14V

Vin
TON
0V

V(Q1:c) T
-IC(Q1)
500mA

SEL>>
0A
IE(Q1)

500mA
I(D1)
0A

I(D1)
 VCC  VΟ   VΟ 
0.7A ΔΙ     ΤΟΝ ΔΙ     ΤΟFF
I(L1)  L1   L1 
I(RL)=VO/Rload
/ I(RL)
0.3A
7.81ms 7.82ms 7.84ms 7.86ms 7.88ms 7.90ms 7.91ms
I(Lo:1) I(RL)
Time

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


3. Switching Transistor Q1

Vcc Lo
OUT+
8RHB
Vcc R13 VR3 Q1
12Vdc Q2SA1680 D1 +
500 XBS104V14R_P
0 R14 7.5k 2k R11 RB IC = 0
500 390 Co RL
R15 RJJ-35V221MG5-T20 10
100 C1
0.01uF

C2 Rsense
OUT-
R17

16
15
14
13
12
11
10
9
0 0.25
R1 1k R16
3.9k R3 100k 0
5.1k
C7 U1
0

1
2
3
4
5
6
7
8
VR1 100n UPC494
1.45k

R5 0
240k

RV2-2
R2 R4 C3 R6 {R1} C5 R8
5.1k 5.1k 100nF 2.4k 1000pF 15k
IC = 0

RV2-1 0 0 0
0 0 {2k-R1}
C4 PARAMETERS:
10u R1 = 0.1
R7
7.5k

• 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

5.0V
VO(t)

2.5V

0V
V(OUT+,OUT-)
20V 2.0A Peak current
1 2
at start-up

10V 1.0A -VCE(t)

SEL>>
-IC(t)
0V 0A
0s 2ms 4ms 6ms 8ms 10ms
1 V(Q1:e,Q1:c) 2 IE(Q1)
Time

• 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 ).


3.2 Transistor characteristics

VCE(sat)-IC characteristics Turn-off characteristics

100 200mA 2.0A
Measurement 1 2

Simulation

10
-IC(t)
_90%
- VCE(SAT) (V)

-IB(t)
1 tf
_10%
0A 0A

0.1

>>
0.01 -100mA -1.0A
20.6us 21.0us 21.4us 21.8us 22.2us
0.001 0.01 0.1 1
1 -IB(Q1) 2 -IC(Q1)
- IC(mA)
Time

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


3.3 Transistor Q1 losses

8.0W
7.0W Q1 Power Loss
6.0W Switching
loss (turn-on)
5.0W
Switching
4.0W loss (turn-off)
3.0W
Conduction loss
2.0W Average Loss
(VCE(SAT) x IC)
1.0W
0W
W(Q1) AVG(W(Q1))
20V 1.0A
1 2
Peak
16V 0.8A D1(SBD) trr magnitude
-VCE(t) contribute current
12V 0.6A
-IC(t)

8V 0.4A

4V 0.2A
SEL>>
0V 0A
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)
Time

• Simulation results shows waveforms of IC and VCE of transistor Q1. Switching power
loss and average power loss are also shown.


3.3 Transistor Q1 losses (zoomed)

8.0W
7.0W Q1 Power Loss
6.0W
5.0W Switching
4.0W loss (turn-on)
3.0W
2.0W
Average Loss
1.0W
0W
W(Q1) AVG(W(Q1))
20V
1 2
-IC(t)
16V 400mA
-VCE(t)
12V D1(SBD) trr
contribute
8V 200mA

4V
SEL>>
0V 0A
9.960ms 9.962ms 9.964ms
1 V(Q1:e,Q1:c) 2 IE(Q1)
Time

• Simulation results shows the contribute of D1(SBD) recovery time trr ,that elevate
losses in the switch transistor Q1.


4. Freewheeling Diode D1 (SBD)

Vcc Lo
OUT+
8RHB
Vcc R13 VR3 Q1
12Vdc Q2SA1680 D1 +
500 XBS104V14R_P
0 R14 7.5k 2k R11 RB IC = 0
500 390 Co RL
R15 RJJ-35V221MG5-T20 10
100 C1
0.01uF

C2 Rsense
OUT-
R17

16
15
14
13
12
11
10
9
0 0.25
R1 1k R16
3.9k R3 100k 0
5.1k
C7 U1
0

1
2
3
4
5
6
7
8
VR1 100n UPC494
1.45k

R5 0
240k

RV2-2
R2 R4 C3 R6 {R1} C5 R8
5.1k 5.1k 100nF 2.4k 1000pF 15k
IC = 0

RV2-1 0 0 0
0 0 {2k-R1}
C4 PARAMETERS:
10u R1 = 0.1
R7
7.5k

• VIN = 12V
• VOUT = 5V
• RL = 10
• Freewheeling diode D1 is Schottky Barrier Diode (SBD), XBS104V14R (IFSM=20A ,
VRM=40V) from Torex Semiconductor.


4.1 Voltage and current stresses

5.0V
VO(t)

2.5V

0V
V(OUT+,OUT-)
20V 2.0A Peak current
1 2
at start-up

10V 1.0A
VR(t)

SEL>>
IF(t)
0A
-1V
0s 2ms 4ms 6ms 8ms 10ms
1 V(D1:2,D1:1) 2 I(D1)
Time

• 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).


4.2 Schottky barrier diode (SBD) characteristics

VR - IR characteristics CT - VR characteristics

10.000 500
Measurement Measurement
Simulation
Simulation

Inter-Terminal Capacity:Ct (pF)
400
1.000
Reverse Current:IR (mA)

300

0.100

200

0.010
100

0.001 0
0 10 20 30 40 5 15 25 35
Reverse Voltage:VR (V) Reverse Voltage:VR(V)

• Losses in D1 depend on the SBD characteristics. Reverse leakage loss is depends on VR – IR
characteristics and reverse recovery loss is depends on junction capacitance characteristics.
• Graphs show agreement between the simulations and measurements of SBD Professional model.


4.3 Freewheeling diode D1 (SBD) losses

400mW

Reverse Conduction
recovery loss loss
200mW
D1 (SBD) Power Loss Reverse
leakage loss

0W
SEL>>
Average Loss

W(D1) AVG(W(D1))
14V 0.6A
1 2
IF(t)
Reverse recovery
characteristic
VAK(t)
0V 0A

>>
-14V -0.6A
9.955ms 9.960ms 9.965ms 9.970ms 9.975ms 9.980ms
1 V(D1:A,D1:C) 2 I(D1:A)
Time

• 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.


4.3 Freewheeling diode D1 (SBD) losses (Zoomed)

400mW

Reverse
recovery loss
200mW Reverse
leakage loss
Average Loss = 72.75mW

0W
Reverse Leakage Loss = 907uW
SEL>>

W(D1) AVG(W(D1))
12V 400mA
1 2

Reverse recovery
0V

0A

>>
-12V -200mA
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)
Time

• Simulation results shows the reverse recovery time - trr of D1.
• Average loss in diode is 72.75mW
• Reverse leakage loss is approximately 907uW.


4.4 Schottky barrier diode Standard model

400mW

Reverse
recovery loss
200mW Reverse
leakage loss
Average Loss = 72.4mW

0W
Reverse Leakage Loss = 187uW

W(D1) AVG(W(D1))
12V 400mA
1 2

Reverse recovery
characteristic
0V

0A

SEL>>
-12V -200mA
1 V(D1:A,D1:C) 2 I(D1:A)
Time

• 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).


5. Output Inductor Value

PARAMETERS:
Lo = 300u
Vcc Lo Rlo
1 2 OUT+
{Lo} 0.63
Vcc R13 VR3 Q1
12Vdc Q2SA1680 D1 +
500 XBS104V14R_P
0 R14 7.5k 2k R11 RB IC = 0
500 390 Co RL
R15 RJJ-35V221MG5-T20 34
100 C1
0.01uF

C2 Rsense
OUT-
R17

16
15
14
13
12
11
10
9
0 0.25
R1 1k R16
3.9k R3 100k 0
5.1k
C7 U1
0

1
2
3
4
5
6
7
8
VR1 100n UPC494
1.45k

R5 0
240k

RV2-2
R2 R4 C3 R6 {R1} C5 R8
5.1k 5.1k 100nF 2.4k 1000pF 15k
IC = 0

RV2-1 0 0 0
0 0 {2k-R1}
C4 PARAMETERS:
10u R1 = 0.1
R7
7.5k

• VIN = 12V
• VOUT = 5V
• RL=34 (IOUT  0.15A)
• Output inductor value Lo are 150uH and 300uH (Parametric sweep)



600mA

500mA

Inductor current at
L=330uF, CCM,
f=38kHz

IO,min = 150mA

IL,RIPPLE

0A
I(Lo:1) I(RL)
Time

• 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)



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.

The inductor value is calculate by.

LCCM 
VI  VO  RL (3)
2 fVI
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

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.



600mA

500mA

I(LO) @ 300uH,
CCM

_IO,min = 150mA

0A
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)
Time I(LO) @ 150uH,
DCM

• Simulation results shows that the inductor will operates in the “discontinuous”
operating mode (DCM), If reduce the inductance value to 150uH.


6. Output Capacitor

PARAMETERS:
Co = 200u
Vcc Lo ESR = 200m
OUT+
8RHB
Vcc R13 VR3 Q1
12Vdc Q2SA1680 D1 Co
500 DXBS104V14R {Co}
0 R14 7.5k 2k R11 RB
500 390 RL
R15 RESR 10
100 C1 {ESR}
0.01uF

C2 Rsense
OUT-
R17

9
16
15
14
13
12
11
10
0 0.25
R1 1k R16
3.9k R3 100k 0
5.1k
C7 U1
0

1
2
3
4
5
6
7
8
VR1 100n UPC494
1.45k

R5 0
240k

RV2-2
R2 R4 C3 R6 {R1} C5 R8
5.1k 5.1k 100nF 2.4k 1000pF 15k
IC = 0

RV2-1 0 0 0
0 0 {2k-R1}
C4 PARAMETERS:
10u R1 = 0.1
R7
7.5k

• VIN = 12V
• VOUT = 5V
• RL=10
• Output capacitor value Co are 100uF and 200uF (Parametric sweep)


6. Output Capacitor

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

IL , RIPPLE
CO , min  (4)
8  f  VO , RIIPLE
Where
• IL,RIPPLE is an inductor ripple current.
• VO,RIPPLE is an output ripple voltage.
• f is switching frequency

In addition, the voltage component due to the capacitor ESR must be considered, as
shown in equation (5).

VO, RIIPLE
RESR  (5)
IL, RIPPLE

• RESR  VRIPPLE / IL,RIPPLE , At VRIPPLE =50mV and IL,RIPPLE =250mA , this equation calls for
RESR value less than 200m.


6. Output Capacitor

5.0V

CO=100uF
CO=200uF

VO,RIPPLE (Zoomed)
4.9V
100uF with 4.90ms
ESR=200m RJJ-35V221MG5-T20
4.92ms 4.94ms 4.96ms 4.98ms 5.00ms
capacitor, overshoot V(OUT+,OUT-)
voltage (220uF electrolytic capacitor
is too big with ESR=200m) Time

6.0V
Overshoot
5.0V

4.0V

3.0V

2.0V

SEL>>
0V
0s 1.0ms 2.0ms 3.0ms 4.0ms 5.0ms
V(OUT+,OUT-)
Time

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


7. Voltage Control Feedback Loop

Vcc Lo
OUT+
8RHB
Vcc R13 VR3 Q1
12Vdc Q2SA1680 D1 +
500 XBS104V14R_P
0 R14 7.5k 2k R11 RB IC = 0
500 390 Co RL
R15 RJJ-35V221MG5-T20 10
100 C1
0.01uF

C2 Rsense
OUT-
R17

16
15
14
13
12
11
10
9
0 0.25
R1 1k R16
3.9k R3 100k 0
5.1k
C7 U1
0

1
2
3
4
5
6
7
8
VR1 100n UPC494
1.45k

R5 0
240k

RV2-2
R2 R4 C3 R6 {R1} C5 R8
5.1k 5.1k {C3} 2.4k 1000pF 15k
IC = 0

PARAMETERS: RV2-1 0 0 0
0 0 C3 = 0.1u {2k-R1}
C4 PARAMETERS:
10u R1 = 0.1
R7
7.5k

• VIN = 12V
• VOUT = 5V
• RL=10
• Capacitor C3 value are 0.01uF and 0.1uF (Parametric sweep)


7. Voltage Control Feedback Loop

2.0V

Feedback voltage:
V(IC pin3) C3 = 0.01uF
C3 = 0.1uF
1.0V

SEL>>
0V
V(U1:3)
5.5V

Output voltage:
VO C3 = 0.01uF
C3 = 0.1uF

5.0V

Oscillation
ripple!
4.5V
2.5ms 3.0ms 3.5ms 4.0ms 4.5ms 5.0ms
V(OUT+,OUT-)
Time

• 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.


Simulation Index

Simulations Folder name

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

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

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

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

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

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

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


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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