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Design a Forward CONVERTER
Gorge Hsieh
Technical Marketing
Tel :02-8226-7698 ext.3001
Cellular phone : 0935041907
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•Forward converter v.s Buck converter vs. Flyback converter
•Features of Forward converter
•Operation of Forward CoverterOperation
Current waveforms
Magnetics of forward converter
Reset schemes
Synchronous forward converter
•Design ProceduresDesign a transformer
Measure the magnetic inductance & leakage inductance of transformer
Mosfet
Secondary rectifier
Output inductor
AGENDA
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•Design Procedures --- cont.Output Capacitor
Loop compensation
Active Clamp Forward converter --- LM5025ZVS --- zero voltage switching
Operation
Decide the Cr
•Other TopologiesDouble end Forward
Half bridge
Full bridge
Phase-shift Full bridge
Current double forward
AGENDA --- cont.
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Buck vs. Forward vs. Flyback
NsNpn =
DVinVo
=
nD
VinVo
=
Q
Vin Vo )1(N DD
VinVo
−=
Buck Converter
Forward Converter
FlybackConverter
12Vin 3.3V, duty=27.5% 48Vin 3.3V, duty=6.87%
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The features of Forward Converter
A. For larger Vin to Vo step ex. Vin=48, Vo=3.3V
B. Isolation
C. Lower ripple & noise --- compare to Flyback converter
D. Smaller transformer --- compare to Flyback converter
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D1T1
Q1
D2
Mode 1: Q 1 on , D1(Forward diode) on, D2(free wheel diode) off
D1
Q1
D2
T1
Mode 2: Q 1 off , D1 off, D2 on --- Free wheel
L
L
Note : Work like buck converter, L maintain the output current continuous. T1, transformer, step down the input voltage by Np/Ns turn ratio.
Operation
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Vg
Vs
Operation of Forward converter
Q1
D2
Vg
Vs
1DI
D1
LI
1DI
2DI
LI
(1)
Vin2DI
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Vg
Vs
Operation of Forward converter
Q1
D2Vs
Vg
1DI
D1
LI
1DI
2DI
LI
(2)
VinNpNsVin •
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Operation of Forward converter
Vg
Vs
1DI
2DI
LI
NpNsVin •
Q1
D2
Vg
Vs
1DI
D1
LI
2DI
(3)
Vin
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Operation of Forward converter
Vg
Vs
1DI
2DI
LI
NpNsVin •
(4)
Q1
D2Vs
Vg
1DI
D1
LI
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Operation of Forward converter
Vg
Vs
1DI
2DI
LI
NpNsVin •
(5)
Q1
D2
Vg
Vs
1DI
D1
LI
2DI
Io
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Model of Forward Transformer
Q1
D2Lm
Np : Ns
Overhead of forward transformer
Im
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Current waveforms
IΔ
: ripple current
Io
Magnetizing current
Q1
D2Lm Im
IΔ
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Magnetics of Forward converter
Q1
D2Lm
Np : Ns
Overhead of forward transformer
Im
lINH m•
=
VdtdN =Φ
Vdt
AdBN e =•
dtAN
VBT
e∫ •
=0
dtAN
VBe
∫ •=
Flux Density
Magnetic Force
(Gauss)
(A/m)
V1
+
-
t1
V1
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Q1
D2Lm
Np : Ns
Overhead of forward transformer
Im
lINH m•
=
VdtdN =Φ
Vdt
AdBN e =•
dtAN
VBT
e∫ •
=0
dtAN
VBe
∫ •=
Flux Density
Magnetic Force
(Gauss)
(A/m)
V1
+
-
Flux need be reset in every cycle
0t1
t2V1
Magnetics of Forward converter
Saturated
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Need a nagative voltage in the rest of time to reset the flux
Q1
D2Lm
Np : Ns
Overhead of forward transformer
Im
lINH m•
=
VdtdN =Φ
Vdt
AdBN e =•
dtAN
VBT
e∫ •
=0
dtAN
VBe
∫ •=
Flux Density
Magnetic Force
(Gauss)
(A/m)
V2
+
-
0
T
t1
t2V1
V2
V1*t1=V2*t2VT balance
Magnetics of Forward converter
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Then the transformer can work again
Q1
D2Lm
Np : Ns
Overhead of forward transformer
Im
lINH m•
=
VdtdN =Φ
Vdt
AdBN e =•
dtAN
VBT
e∫ •
=0
dtAN
VBe
∫ •=
Flux Density
Magnetic Force
(Gauss)
(A/m)
V+
-
0
T
t1
t2V1
V2
V1*t1=V2*t2VT balance
Magnetics of Forward converter
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D1T1
Q1
D2
Mode 1: Q 1 on , D1(Forward diode) on, D2(free wheel diode) off
D1
Q1
D2
T1
Mode 2: Q 1 off , D1 off, D2 on --- Free wheel
L
L
Unfortunately, Forward converter inherent no rest path
In this case, there is no negative current path
+-
+-
+-
+-
In order to make the magnetic current continuous, transformer will induce a very high voltage thus destroy the Mosfet, Q1
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Flyback has inherent rest path
D
TQ
+
-
-+
RL
TQ
--+
+RL
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Reset schemes
Reset winding RCD clamp Active Clamp
+
-+
- -
+
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Vg
Vds
CsLf
m •=
π21 Larger Cs
smaller Cs
Vin
Vg VdsCs
Cost effective solution for magnetic resetDisadvantage --- higher Vds, but may be lower switching loss while Mosfet turned on
Vin
Lower switching loss
higher switching loss
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Q1
Q2
Q3
+
-
+
-
-
+
-
+
Synchronous Forward Converter
Q1 on, Q2, On, Q3 off
Q1 off, Q2, Off, Q3 on
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DESIGN Procedure
Vi: Input voltage
Ii : Input current
Vo : output voltage
Ro : Output load resistance
Vr : Output voltage ripple
D : duty cycle
Fs : Switching frequency
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A. Design a transformer
a. Select a magnetic core
100W
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b. Pick Ae --- effective core area & lA
%25//tan: 2 −+TnHturnsperceinducAl
lANL •= 2
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c. Calculate Np --- Primary tunns
10max 10•••Δ
•=
sep FAB
DViNHzFmmAgaussBvoltVi
s
e
::
::
2
volatgeinputMaximumVi :
cycledutyMaximumD :max
areacoreeffectAe :
densityfluxdeltaB:Δ gaussNote 2000: ≈
FrequencySwitchingFs :
Lpm ANL •= 2Lm : the larger is the better
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d. Calculate Turn ratio:
VdVDV
NN
no
i
s
p
+
•≤= max(min)
e. Calculate wire diameter:
Criterion : cross area/current
AmilcirculAformmDI /400105.04 2 ≈⇒=⇒= φ
AmilcirculAformmDI /256141.06 2 ≈⇒=⇒= φ
AmilcirculAformmDI /190135.08 2 ≈⇒=⇒= φ
You must decide maximum duty cycle(<50%)
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Create a specification for the transformer
ψ= 0.16mm, 8Ts
ψ=2* 0.6mm, 12TsPin 9~6 / ψ= 0.6mm x 4, 2TsPin 10~7 / ψ= 0.6mm x 4, 2Ts
1
5
42
6,7
9,10
Np
Ns
Nb
Schematic
Electrical SpecificationConverter Type : ForwardVin,min : 32V, Vin,max : 60VDuty,max : 0.45Frequency : 250KHzVo : 3.3VIo : 10A
. 1. Magnetic Core: EFD30
. 2. Leakage Inductance: Pin 1-2 0.5uH +/- 10%Np: Pin1~2 / ψ= 0.6mm x2 Ns: Pin9,10~6,7 / ψ= 0.6mm x 8Nb: Pin4~5 / ψ= 0.16mm
3. Wire diameterI. Np 6 Turns, 加TAPEII. Ns 2 Turns, 加TAPEIII. Nb 8 Turns, 加TAPEIV. Np 6 Turns, 加TAPE
4. Winding sequence
1500V 1,4 6,71500V 1,4,6,7 CORE
5. High pot
Np2Ns
Np1Insulate tape
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Planar transformer
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Core Losses
Core loss = hssteresis loss+eddy current lossnm fBK )(Δ=
m = 2-2.5n = 1.1-1.5
Ferrite : <2MHz
Powder core : <1MHz
Alloy : <200KHz
Amorphous : <300KHz
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How to measure Lm & leakage inductance?
a. Measure Lm
LCR meter
LCR meter
b. Measure Leakage inductance lL
lLmL
lLmL Secondary
short
Secondary open
lm LL >>
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Power Stage 1. Components
A. Power Mosfet
Vds : >2*Vin, max, it is dependent on the reset capacitor Cs (refer to page )
Ids :
Rds(on)
Ciss
Coss
Tfall (fall time)
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B. Output Diode
Vr : reverse voltage
If(average) : Forward current
If(peak) :
Vd :
Cj :
[⎥⎥⎦
⎤•+•>
p
S
p
S
NNVresetVo
NNVinVr ,max
IIo Δ+21
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Output inductor
VdtdiL =
)( do VVdidtL +=
TsDdt vin •−= )1( max@
di=0.2*Io,max(A)Fs
Ts 1=
Q1
VdVs
Vg
LV
+
- +
-Vo
Vs
LIΔ
Maximum ripple happen when Vin@maxoffQLI 1@Δ
max,max@ *
in
oDvin V
VVnD +=
s
p
NN
n =
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Output capacitance
CVLI 22 )(21)(
21
Δ=Δ
1. Consider transient response
loadtransientI :Δ overshootacceptedV :Δ
2. Consider ripple voltage
LIVrESRΔ
≤
Vr : Ripple voltage
currentrippleinductorIL :Δ
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Comparison with high value capacitors – variety of capacitors
OS-CON (SANYO) SP Cap (Panasonic) POS CAP (SANYO)
Ta electrolytic capacitorAl electrolytic capacitorMultilayer ceramic capacitorNi based electrodes(TAIYO)
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Polarity Derating Ripple CurrentLimit
Solder Heat Resistance
Anti-solvent
Loading Test
MLCC No ◎ ◎ ◎ ◎ ◎
Ta cap Yes × △ × △ ×
AE cap Yes × × △ × △
* Layout issue
* Mounting issue
* Reverse voltage
* Require 70 to 50% of ratedVoltage.
* Must consider allowable ripple current when determine cap value.
* Heat generationwill lower the reliability.
* Limited condition when reflow is used.
*Solvent penetration into capacitor body.Problem
◎ is Excellent △ is Limited × is Bad
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Comparison of Capacitor’s Self Heat Generation
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Impedance vs. Frequency
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R 22uF OS-CON Z
R 22uF MLCC Z
Frequency (Hz)
0.001
0.01
0.1
1
10
100
Impe
danc
e, E
SR
(oh
ms)
1K 10K 100K 1M 10M
Impedance and ESR versusFrequency characteristics
22uF MLCC
22uF OS-CON
MLCC vs. OS-CON
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The ripple current of 47uF MLCC is about 2.5A
Ripple Current (A rms)
Temperature Rise Versus Ripple CurrentTe
mpe
ratu
re R
ise
(°C
)
47uF MLCC47uF Ta Cap100uF POSCAP
0 0.5 1 1.5 2 2.5 3 3.5 4
100
10
1
0.1
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DC Bias Characteristic
-100
-80
-60
-40
-20
0
0 2 4 6 8 10DC Bias Voltage [V]
ΔC
/C
[%]
Temperature Characteristic
-100
-80
-60
-40
-20
0
20
-50 -25 0 25 50 75 100Temperature [degrees C]
ΔC
/C
[%]
DC Bias characteristic
-50
-40
-30
-20
-10
0
10
0 1 2 3 4 5 6 7DC Bias voltage [V]
DC
/C
[%]
Temperature Characteristic
-50-40
-30
-20
-10
0
10
20
-75 -50 -25 0 25 50 75 100 125 150Temperature [degree C]
DC
/C
[%]
NPO vs. X7R vs. Y5V
-55~125
+/-30 ppm
-55~125
+/- 15%
10~85
+22%~-56%
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Sense resistor
Q1
Vg
1DI LI
Rs2K
680pF
CS
1DI
0
Np:Ns
PI
CSV+
-
IoLO II Δ+
21
n
II LO Δ+21
n=Np/Ns
n
II
VRsLo
CS
Δ+=
21 VVCS 8.0~5.0=
s
CS
RVLoss
2
=Ω
=1.08.04.6
2
W
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Current transformer DCRILoss •= 2
10AΩ1.01V+
-WVloss 10
1.01 2
=Ω
=
10A
0.1A
Ω10
+
-1V
WVloss 1.0101 2
=Ω
=
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Reset capacitor --- Cs
Vg
Vds
Vin
Vg Cs
Vin
f1
21•
CsLf
m •=
π21
sFD
fmax11
21 −
≤•
)1(221
maxDF
CLs
sm −•≥
•π
,for reference)1(2 maxD
Ff s
−•≥
m
SS L
FD
C
2max )1(•
−
≤π pFpFCs 1000~100≈
mL
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Vin
Vg Cs
jlr CL
f•
=π2
1
R C
Output diode snubber
rf
ceinducleakageLl tan:
cecapacijunctiondiodeC j tan:
jrj
l
CfCLR
••==
π21
≅C 2~5 times of Cj
lL
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RL
_
+
Vref
_R
S
+
Q
Vg
Comparator
Vin Vo
5V Vo Id
↓↓↓↑↑↑↑ VoIpcompVeIcIdFBVo )(
FB
Anode
cathode
_
Vref
FBCOMP
OP AMP
+Ve
Rs
Current Mode Controller
TL431
R4R5
Control Loop
C
esrR
Ic
Ip
Open loop
sn
L
Cs
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Photo Coupler --- For isolation
1. CTR : Current Transfer Ratio
80%-----150%
2. Bandwidth --- must > cross over frequency of flyback converter (1/10 of switching frequency)
3. Popular solution: PC817 --- Sharp
TLP521 --- Toshiba
))(*4
(*5*5IdIcCTR
RVeVoRVccIcRVccVcomp −
−=−=
CTRRRVeVcomp *
45*Δ=Δ
Photo Coupler provide signal transfer, the Idea is a DC gain
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Gain
f33KHz142Hz9.9Hz
_
+
10K 47nF
1nF
HzwZ 338≅ KHzwP 79≅
p
z
ws
ws
RR
VeVo
+
+=
1
1
21 *
910910
10 ..
=K
K
0.91K
Close loop Gain( + )
0db
0db
Close over frequency
With Photo Coupler
Vcc Vo
Ic
R5 R4
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Gain
f33KHz142Hz9.9Hz
Close loop Gain( + )
0db
0db
Close over frequency
Vcc Vo
Ic
R2
_+
Vref
ref
Anode
cathode
TL431
With Photo Coupler
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TL431--- Shunt regulator
_
+
Vref
ref
Anode
cathode
TL431
RL
Vin
VoL
KA RVoVinI −
=
KAo II =max,
Shunt Current
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Q1
Vin
R1
R
R2
R3
Q2 Q3
Ref
32
213 RRVVVV QBEQBEQBEREF )( )()()( −+=
I1 I2
2121 10IIVV QBEQBE =≠ Q)()(
21 10II =
321
2 RVV
I QBEQBE )()( −=
21
21 II
qKTVV QBEQBE ln)()( =−
)()()( ToTV
ToTVV BEOgoQBE +−= 13
gapbandVgo −=
121
32 I
qK
RR
ToVV
ToV
dtdV BEO
BEOgoREF ln++−=
KToqVVI
RR
QBEOgo )(ln )( 3121
32
−=If,
0=dt
dVREF VVgo 221.=
Band gap reference of 431
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Control Loopa. Open loop --- Similar to current mode Buck converter
A. DC gain : Ro: load resistanceRsRn o•
B. Low frequency Pole :
C. ESR zero :
D. Double Pole :
LRC ••π21
esrRC ••π21
2sF
0db
f
Gain
2sFFp Fz
n : main transformer turn ratio, Ns/Np
iS
op
s
sin
oo
e
oo
FNRN
nRI
RIV
RIDCgain•
•=
•
•=
•=
Note:
Note
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0db
f
Gain
2sFFp Fz
_
+
Vref
FB
cathode
Anode
R4R5
VoVo5V
comp
C1
Loop Compensation
R1
R2
R3
C2
213RR
R
1321
CRfz
π=
2321
CRfp
π=
1. Lower more phase margin, stable but slow response 2. Lower less phase margin, but noise immunity 3. DC gain & crossover frequency is limited by 431 & photo couple
431
K
Kd
IVVVoR −−
=4.4
fzfp
mAIK 63−≅ VVK 24.1≅)1(
1
23121
13
++
CSRCRSRCSR
Crossover frequency
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Vcc Bias
Vcc
Np Ns
Nb High power dissipation
Long start up time
Simple
elRVin
arg
Vin
Vin
Vcc
Np
Ns
Nb
R,small
Vin
Vin
R,large
R,large
zener
High power dissipation
short start up time
Complex
elRVin
arg
Np
Ns
Nb
Vin
Vcc
LDO
Internal Vcc
Vin Lower power dissipation
short start up time
Simple
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Design Example Vin,min 36
Vin,max 78
Duty, max 0.55
Vo 3.3
Vs =Vo+Vd 3.4
Io 30
eff 0.85
In Vo*Io/Vin*eff 3.33
Fs 240000
Vbias 12
n(Np/Ns) 5.824
Trnsformer sectionMagnetic Core EFD 30Delta B 2000
Ae (mm^2) 69
Al nH 2150
Np 13.0
Lm 3.61E-04
Ns Np/n 2.2
Nb 7.9
WindingSkin depth(mm) 0.15
Maximum Diameter of wire 2*s 0.29
Dim, pri,total sqrt(Iin/8), unit: mm 0.65
wire # (primary) 4
Dim,pri, per wire 0.32
Dim, sec sqrt(Io/8), unit: mm 1.94
wire # (secondary) 8
Dim,sec, per wire unit: mm 0.68
D
in
VVo
DVn
+
×=
maxmin,
10max 10×××Δ
×=
Se FABDVin
Np
)(mmFs
s 72=
Do
biasS VV
VN
+×
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Secondary sectionOutput InductorDeltaIL 0.2* Io, max 6.0
D@vin,max (Vo+Vd)*n/Vin,max 0.3
L 1.8E-06
IL 33
Output capacitorVripple accepted ripple voltage 0.05
I,transient transinet current 10
V,overshot accepted overshoot in transinet response 0.1
C 1.8E-02
ESR 0.0083
Sense resistorVs,max Maximum current sensing voltage 0.6
Rs 0.106
Reset capacitorLm 0.00036Cs 9.9E-10
IFDVVo
LS
vinD
Δ
−+=
*)1(*)( max,@
LIIo Δ+21
2
2
VILC =
L
ripple
IVΔ
n
II
VRsLo
CS
Δ+=
21
m
SS L
FD
C
2max )1(•
−
≤π
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Loop compensationRL Vo/Io 0.11
C 0.0009ESR 0.0040 17mOHM for 47uF ceramicDC gain 31
Cross over frequency 10000
First pole 1675
ESR zero 46075
open loopGain at cross over DC gain - loss 16
R3/R* 0.16
R* 2000
R3 319
C1 2.98E-07
C2 1.08E-08
LRC ••π21
esrRC ••π21
RsRN o•
ZfRC
3211
π=
PfRC
321
2 π=
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SchematicC2
1nF
R1
470
1 2
C9
2.2uF
1
2
C10
2.2uF
1
2
C19
470pF
1
2
R12
1K1 2
R51K
12
C18
680pF
1
2
C152.2nF
1
2
C14 0.1uF
1 2
Q1MMBTA42
C
B
E
R468K
12
D312V
R61K
12
L147uH D1
BAV70
13
2
R310
12
VCC
T2
CT
6
4
78
C16
0.1uF
1
2
U1
AZ3843
COMP1
VFB2
Vc7
PGND5
VREF8
RT/CT4
Output6
I Sense3
L35.6uH
D4 1N4148K A
R88.2
12
Q4
IRF640
12
3
Vin = 48V
U2
TLP521
12
43
D2
32CTQ030
1 3
2
L22uF 10A
C21 10nF
R9
0
C172.2uF
1
2
C1
1uF
C22
3300pF
+
C5
330u
F
+
C4
330u
F
T1
T
11
66
1010
77
33
44
8899
1111
1212
C7
68uF
C20
330nF
C131uF1
2
C31nF
VCC3V3
R2470
12
Q3
MM
BT29
07
C
B
E
R15330OHM
R7
10K
12
C6
68uF
U3AZ432
32
1
C8
68uF
R13 10
R14
6.34K
C11
2.2uF
C12
2.2uF
R16
3.09K
R10
1K
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ThanksGorge Hsieh
Technical Marketing
Tel :02-8226-7698 ext.3001
Cellular phone : 0935041907