Design a Forward CONVERTER - Techmosa a Forward... · A Reliable business Partner •Forward converter v.s Buck converter vs. Flyback converter •Features of Forward converter •Operation

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Design a Forward CONVERTER

Gorge Hsieh

Technical Marketing

George_hsieh@techmosa.com.tw

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

George_hsieh@techmosa.com.tw

Tel :02-8226-7698 ext.3001

Cellular phone : 0935041907