slua653b 5w usb flyback design review

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    Application Report

    SLUA653B Revised July 2013

    1

    5W USB Flyback Design Review/App l ication Repo rt

    Michael OLoughlin Senior Power Applications Engineer

    Introduction:In USB and isolated low power converter designs-quasi resonant and discontinuous conduction mode

    flyback converter topologies are a popular choice, due to their low parts count and relatively low cost.

    To reduce the cost even further, TI has developed a quasi-resonant/discontinuous current mode flybackcontroller with primary-side control. This removes the need for optocoupler and TL431 feedback

    circuitry reducing the cost of these low power designs even more. To achieve relatively low no load

    input power and regulate the output voltage and output current this device uses a control methodology

    known as control law. This control methodology uses a combination of primary peak current amplitudemodulation (AM) and frequency modulation (FM) to regulate the output current and voltage please refer

    to the data sheet [1] for details. This application report reviews the design of the 5W adapter,

    UCC28700EVM-068, evaluation module [2] using the UCC28700 power supply controller. The designcalculations are based on typical values. In a production design the values need to be modified forworst case conditions. Also note there is a MathCAD design tool [3] that goes along with this

    application note to make the power supply design process easier using this device.

    Design Specifications:

    Description Minimum Typical Maximum Units

    RMS Input Voltage 90 (VINMIN) 115/230 265 (VINMAX) V

    No Load Input Power 30 (PINL) mW

    Output Voltage 4.75 5 (VOUT) 5.25 VOutput Voltage Ripple 100 (VRIPPLE) mVpp

    Output Load Step(0.1 to 0.6A), (0.6 to 0.1A)

    4.1 (VOTRM) 6.0 V

    Output Current 1(IOUT) A

    Switching Frequency 105 (fMAX) kHz

    Full Load Efficiency(230/115V RMS input)

    73() %

    Table 1, Design Specifications

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    Functional Schematic:

    VIN

    DA

    DB

    DC

    DD

    2

    6

    5

    3

    4

    1

    UCC28700

    CS

    VDD DRV

    GNDCBC/

    NTC

    T1

    T1

    VS

    VOUT+

    VOUT-

    CA CB

    LA

    RS1

    RS2

    RCBC

    RLC

    RF1

    CDD

    DZ RS

    CS

    COUT

    RCSRG2

    RG1

    RZ

    RT

    3.01k

    DE

    DF

    DG

    10

    10k

    330nF

    22

    NP NS

    NA

    RL

    10

    470uH

    QA1nF

    Figure 1, UCC28700 5W Offline Flyback Functional Schematic

    Selecting RCB/NTC Resistor:In this design cable compensation was not used and resistor R8 was not populated.

    Please refer to the data sheet on how to setup cable compensation [1].

    Initial Power Budget:To meet the efficiency () goal an initial powerloss budget (PBUDGET) needs to be set.

    WAVIVP OUTOUTOUT 515

    WPP

    P OUTOUT

    BUDGET 85.174.0

    Bridge Rectifier Selection (DA ..DD):

    For this design a 600V, 0.8A, bridge rectifier from Diodes Incorporated was chosen for the bridgerectifier diode (DA.. DD), part number HD06.

    VVFDA 1 , forward voltage drop of bridge rectifier diode (VFDA)

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    5W USB FlybackDesign Review/Application Report 3

    mA

    V

    W

    V

    P

    I

    INMIN

    OUT

    DA85

    22

    90

    74.0

    5

    22

    , bridge rectifier average diode current (IDA)

    mWIVP DAFDADA 85 , estimate power dissipated in bridge rectifier diode (PDA)

    Estimate remaining power budget based on bridge rectifier loss.

    WPPP DABUDGETBUDGET 68.12

    Transformer Calculations (T1):

    Transformer demagnetizing duty cycle (DMAG) is fixed to 42.5% based on the UCC28700 control lawmethodology [1].

    425.0MAGD

    TRis the estimated period of the LC resonant frequency at the switch node.

    usTR 2

    Calculate maximum duty cycle (DMAX):

    47.02

    2105425.01

    21

    skHz

    TfDD R

    MAXMAGMAX

    Calculate transformer primary peak current (IPPK) based on a minimum flyback input voltage. This

    calculation includes a factor of 0.6 to account for the reduction in flyback input voltage caused by the

    ripple voltage across the input capacitors (CA and CB).

    mAV

    W

    DV

    PI

    MAXINMIN

    OUTPPK

    38247.06.029074.0

    52

    6.02

    2

    Selected primary magnetizing inductance (LPM) based on minimum flyback input voltage, transformer,primary peak current, efficiency and maximum switching frequency (fMAX).

    uH

    kHzmA

    W

    fI

    P

    LMAXPPK

    OUT

    PM 896105376

    74.0

    522

    22

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    VVQAON 2 , estimated voltage drop across FET during conduction

    VVRCS 75.0 , voltage drop across current sense resistor

    VVDG 6.0 , estimated forward voltage drop across output diode

    Calculate transformer turns ratio primary to secondary (a1) based on volt-second balance.Note in the following equation LSM is secondary magnetizing inductance.

    5.146.02

    1

    DGOUTMAG

    RCSAONINMINMAX

    SM

    PM

    S

    P

    VVD

    VVVD

    L

    L

    N

    Na

    VVDDMIN 8 , UCC28700 minimum VDD voltage before UVLO turnoff.

    VVDE 3.0 , estimated auxiliary diode forward voltage drop

    VV INITOUT 2_ , Minimum voltage on the output when adapter is connected to a device with a depleted

    battery.

    Calculate transformer auxiliary to secondary turns ratio (a2)

    2.3_

    2

    DGINITOUT

    DEDDMIN

    S

    A

    VV

    VV

    N

    Na

    Transformer primary RMS current (IPRMS)

    mAD

    II MAXPPKPRMS 1513

    Transformer secondary peak current RMS current (ISPK)

    ADV

    PI

    MAGOUT

    OUTSPK 7.4

    2

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    Transformer secondary RMS current (ISRMS)

    AD

    II MAGSPKSRMS 8.13

    , transformer secondary RMS current

    For this design we estimated the power dissipated by the UCC28700 (PIC) would be 50mW maximum.

    Note this will vary in the design based on the FET that is being driven and the maximum frequency it is

    being driven at.

    mWPIC 50

    Calculate auxiliary winding peak current (IAPK)

    mADaVV

    PI

    MAGDEOUT

    IC

    APK13

    )(

    2

    2

    Calculate auxiliary winding RMS current (IARMS)

    mAD

    II MAGAPKARMS 0.53

    For this transformer we allow for 3% efficiency loss from the transformer (PT1)

    mWPP OUTT 15003.01

    Recalculate remaining power budget

    WPPP TBUDGETBUDGET 53.11

    A Wurth Electronik transformer was designed for this application, part number 750312723, which hasthe following specifications:

    a1 = 15.33

    a2 = 3.83

    uHLPM 925

    uHLLK 16 , primary leakage inductance

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    Input Capacitor Selection (CIN = CA + CB):

    Calculate input capacitor charge time (tCH) based on 40% input capacitor ripple voltage.

    msHz

    V

    VV

    tINMIN

    INMININMIN

    CH 4.3474

    2

    6.022sin1

    1

    Calculate flyback average primary current (IPT1) during input capacitor discharge.

    mAV

    P

    V

    P

    I INMIN

    OUT

    INMIN

    OUT

    PT 722

    6.0221

    Calculate total input capacitance (CIN) based on minimum flyback input voltage and 40% ripple voltage

    across the input capacitor.

    msHz

    TRL 11472

    1

    , longest period of the rectified line voltage

    VVV INMININRIPPLE 9.504.02 , input ripple to the flyback converter

    uF

    V

    tTIC

    INRIPPLE

    CHRLPT

    IN 101

    uFC

    CC INBA 52

    Calculate input capacitor (CA) RMS current (IC_ARMS) based on 40% input capacitor ripple voltage.

    mAt

    VCCI

    CH

    INMINBACINP 311

    4.02)(2

    , peak input capacitor charge current (ICINP)

    mAI

    T

    tTI

    T

    tII PT

    RL

    CHRLCINP

    RL

    CHCINPRMSCA 82

    23232

    2

    1

    22

    _

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    Estimate of capacitor CBs low frequency (1/TRL) RMS current ( LFRMSCBI _ )

    RMSCALFRMSCB II __

    Estimate of CBs high frequency RMS current (ICB_HFRMS)

    mAD

    ID

    II MAXPPKMAX

    PPKHFRMSCB 12223

    22

    _

    Estimate of CBs total RMS current (ICB_RMS)

    mAIII HFRMSCBLFRMSCBRMSCB 1472

    _

    2

    __

    For this design 4.7uF, 400V electrolytic capacitors, from Nichicon part number UVR2G4R7MPD were

    chosen for the design.

    uFCC BA 7.4

    These capacitors had a measured ESR of 6 ohms at 105 kHz

    ESRCA = ESRCB = 6

    Recalculate remaining power budget based on power dissipation by the ESRs in the input capacitors.

    WESRIESRIPP CBRMSCBCARMSCABUDGETBUDGET 36.12

    _

    2

    _

    Filter Inductor (LA):Filter inductor (LA) is used for EMI filtering. In this design it is just a place holder and the design has

    not been optimized for EMI. 470uH inductor from Bourns was chosen, part number RLB0608-471KL.

    This inductor has a DCR of 6.5 ohms.

    5.6DCR

    Recalculate power budget based on DCR losses

    WDCRIPP PRMSBUDGETBUDGET 212.12

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    Fusible Resistor (RL):

    To limit the inrush current during power and for safety a 10 ohm, 3W fusible resistor from Bourn, partnumber PWR4522AS10R0JA was placed at the input of this design.

    10LR

    Recalculate power budget based on estimated RL losses

    WRIPP LPRMSBUDGETBUDGET 984.02

    Trickle Charge Resistor (RT):

    To reduce no load power losses RT and to keep no load power to a minimum, three 5.11M are used inseries for RT

    MMRT 33.15311.5

    mW

    R

    VP

    T

    INMAXRT 2.9

    22

    , Total trickle charge resistor power dissipation

    Recalculate power budget

    WPPP RTBUDGETBUDGET 974.0

    VDD Capacitor Selection (CDD):

    The CDD is selected with the following equation based on the desired startup time (dtCDDS) of the

    UCC28700 controller and knowing the start current (ISTART), as well as, the UCC28700 device startupthreshold (VVDD(on)). For this design a 330nF capacitor was selected.

    sdtCDDS 1

    uAISTART 5.1

    VV onVDD 21)(

    nFnFV

    dtIR

    V

    ConVDD

    CDDSSTART

    T

    INMIN

    DD 330324

    2

    )(

    Note after CDD has been charged up to the device turn on threshold (VVDD(on)), the UCC28700 willinitiate three small gate drive pulses (DRV) and start sensing current and voltage. (Please refer to figure

    2) If a fault is detected such as an input under voltage or any other fault, the UCC28700 will terminate

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    5W USB FlybackDesign Review/Application Report 9

    the gate drive pulses and discharge CDD to initiate an under voltage lockout. This capacitor will be

    discharged with the run current of the UCC28700 (IRUN) intil the VDD turnoff (VVDD(off)) threshold isreached. Note the CDD discharge time (tCDDD) from this forced soft start can be calculated knowing the

    controller run current (IRUN) without out gate driver switching and the controllers VDD turnoff

    threshold (VVDD(off)) and the following equations. If no fault is detected, the UCC28700 will continue

    driving QA and controlling the input and output currents [1] and a soft start will not be initiated.

    mAIRUN 1.2

    VV offVDD 8)(

    ms

    IR

    V

    VVCdt

    RUN

    T

    INMAX

    offVDDonVDD

    DDCDD71

    2

    )()(

    VDD

    0VDRV

    VVDD(on) =21V

    VVDD(off) =8V3 Initial DRV Pulses After VDD(ON)

    Figure 2, VDD and DRV at Startup with Fault

    Current Sense Resistor (RCS):

    For this design 2.05 ohm resistor was selected based on a nominal maximum current sense signal of0.75V.

    ohmI

    VR

    PPK

    CS 05.2965.175.0

    WRIP CSPRMSRCS 046.02

    , nominal current sense resistor power dissipation

    Recalculate power budget

    WPPP RCSBUDGETBUDGET 928.0

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    Select Output Diode (DG):

    Calculate diode reverse voltage (VRDG)

    VaVVV INMAXOUTRDG 45.29

    1

    21

    Calculate peak output diode (IDGPK)

    AII SPKDGPK 7.4

    For this design we selected a 3A, 40V schottky rectifier with a forward voltage drop (VFDG) of 0.31V.

    VVFDG 31.0

    Estimated diode power loss (PDG)

    OUT

    FDGOUTDG

    V

    VPP 0.31W

    Recalculate power budget

    WPPP DGBUDGETBUDGET 618.0

    Select Output Capacitors (COUT):

    Select output ESR based on 90% of the allowable output ripple voltage

    mA

    mV

    I

    VESR

    SPK

    RIPPLECOUT 19

    7.4

    9.01009.0

    For this design the output capacitor (COUT) was selected to prevent VOUT from dropping below the

    minimum output voltage during transients (VOTRM).

    VVOTRM 1.4

    mFVV

    V

    Pms

    COTRMOUT

    OUT

    OUT

    OUT 1.12

    2

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    For this design two 560uF capacitors were used in parallel on the output, with an ESR of 13m each.

    mFuFCOUT 12.15602

    mm

    ESRCOUT 5.62

    13

    Estimate total output capacitor RMS current (ICOUT_RMS)

    AV

    PDII

    OUT

    OUTMAGSPK

    RMSCOUT 46.13

    22

    _

    Estimate total output capacitor loss (PCOUT)

    mWESRIP COUTRMSCOUTCOUT 142_

    Recalculate power budget

    WPPP COUTBUDGETBUDGET 604.0

    Select FET QA:

    For this design we had chosen a 600V rated MOSFET with the following characteristics:

    ohmRDSON 5.4 , FET QA on resistance

    pFCOSS 5.8 , average FET drain to source capacitance

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    Figure 3, Gate Charge vs Vgs

    Estimate FET losses (PQA)

    AIDRIVE 35.0 , maximum FET gate drive current

    nCQg 9 , gate charge just above the miller plateau

    ns

    I

    QtDRIVE

    gr 522

    , estimated FET Vds rise and fall time

    Estimate FET power loss by driving the FETs gate (Pg)

    nCQg 121 , Gate charge at 12V drive clamp

    VVg 12

    mWfQVP MAXgg 1512 1

    Calculate the average input voltage to the flyback at the maximum input voltage (VINMAX).

    VVV

    VV FDAINRIPPLE

    INMAXFLY 34722

    2

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    Estimate FET average switching loss (PSW)

    mWftI

    aVVVP MAXrPPKVDGOUTFLYSW 2692

    1

    Estimated FET Coss power dissipation (PCOSS)

    pFCOSS 5.8 , average FET drain to source capacitance

    mWfVC

    P MAXFLYOSS

    COSS 542

    2

    Calculate power loss from Rdson (PRDSON)

    WRIP DSONPRMSRDSON 1.0)(2

    Estimate total FET losses (PQA)

    mWPPPPP COSSgSWRDSONQA 441

    Recalculate the power budget

    mWPPP QABUDGETBUDGET 163

    Setup Zener Clamp to Protect FET QA:

    VVZ 82 , Zener Clamp Voltage (DZ)

    VV MAXds 600_ , FET maximum drain to source voltage

    VVVV INMAXMAXdsCLAMP 2.16529.0_ , Available Clamp Voltage to Protect FET QA

    5.2166.0

    PPK

    ZCLAMPS

    I

    VVR

    Select a standard resistor for the design.

    215SR

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    Estimate Zener Clamp/LLKpower dissipation (PLK)

    mW

    fILP MAXPPKLPKLLK 122

    2

    2

    Recalculate power budget

    mWPPP LLKBUDGETBUDGET 40

    Select VS voltage divider (RS1, RS2):

    uAI runVSL 220)( VS Line-sense run current

    Note RS1 so the converter will go into under voltage lockout when the input is below 80% of the

    minimum specified input voltage.

    kI

    Va

    a

    RrunVSL

    INMIN

    S 6.115

    8.02

    )(

    1

    2

    1

    Select a standard resistor for the design

    kRS 1211

    k

    R

    VaVV

    VR

    S

    DGOUT

    S 7.274

    4

    1

    2

    2

    Calculated RS2 is a starting point and will need to be adjusted in circuit. To have a 5V regulated output

    this resistor was adjusted to 30.1k

    kRS 1.302

    Calculate VS divider power dissipation (PVS)

    mWRR

    aVVDP

    SS

    DGOUTMAX

    VS 3.121

    2

    2

    Select auxiliary diode (DE) for this design that had a forward voltage drop (VDE) of 0.6V.

    VVDE 6.0

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    VVaVVV DEDGOUTDD 8.202 , UCC28700 supply voltage at VDD

    Va

    aVVV INMAXDDRDE 1152

    1

    2 , maximum reverse voltage across VDE

    mAIRUN 1.2 , UCC28700 bias current when gate drive = 0V

    mAV

    VIPI

    DD

    VDDRUNg

    DD 8.2

    , Estimated UCC28700 VDD current.

    Calculate DE power dissipation (PDE)

    mWVIP DEDDDE 7.1

    Recalculated power budget

    mWPPPP DEVSBUDGETBUDGET 037

    Preload Resistor Selection (RZ):

    To keep the output voltage from climbing at no load a pre-load resistor is required. This is generally a

    trial an error process. For this designs the preload resistor that kept the output regulated under no loadconditions was 3.01 k.

    KRZ 01.3

    Calculate RZ power dissipation (PRZ)

    mW

    R

    VP

    Z

    OUTRZ 3.8

    2

    Recalculated power budget and there is 29 mW of margin left in the power budget to meet the efficiencyrequirements of the design. Note in production designs, more margin might be required. Also note

    these calculations are estimations and the final design may need to be adjusted to hit efficiency and

    regulation requirements.

    mWPPP RZBUDGETBUDGET 29

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    Internal BlankingThe UCC2870X controller regulates the output voltage by sensing the auxiliary (Aux) winding. This

    removes the need for opto isolator feedback scheme reducing the cost of the design. However, this

    voltage control feedback scheme is susceptible to leakage spikes at the switch node that occur in most

    flyback converters. This signal is coupled through the turns ratio of the transformer (T1) and shows upon the Aux winding during tLK_RESET , please refer to figure 4 for details.

    To help insure the leakage spike on the Aux winding does not cause a control issues, the UCC2870X

    blanks (tB) the Aux signal to the controller for 500 ns to 1.5 us depending on loading. Please see the

    data sheet details [1]. Note the ringing on the auxiliary winding needs to be less than 100mV peak

    to peak after tB. Snubbing circuitry on the secondary and/or auxiliary winding may be required to

    reduce ringing.

    Aux/

    DE Cathode

    RCS

    DRV

    0V

    tB

    0V

    tLK_RESET

    VS Samples Aux Voltage for Control

    QA on

    QA off

    Aux Ringing after tB < 100mV

    Figure 4, Auxiliary Winding VS Blanking

    To ensure the leakage spike does not cause control issues it needs to be dissipated before the Aux

    blanking (tB) has terminated. The tank frequency (fLC) between the switch node capacitance (CSWN) and

    the transformer leakage inductance (LLK) should be greater than 1MHz.

    SWNLK

    LC

    CL

    f

    2

    1

    MHzns

    fLC 15002

    1

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    Select line compensation resistor RLC:Resistor RLCprovides offset to the peak current comparator input (CS). RLC is adjusted to terminate the

    gate drive signal (B) early to prevent primary current (A) from over shooting [1]. Please refer to figures

    5 and 6 for details.

    Figure 5, Peak Current Limit Comparator Figure 6, CS(A), QAg(B), VQADS Signals

    25LCK , Line Compensating Ratio [1]

    Calculate RLC initial resistor setting based on QADS rise and fall time (tr)

    kL

    atRRKR

    PM

    rCSSLCLC 29.5

    11 , Starting Point for RLC

    In circuit adjust RLC so the maximum output current is (IOUT). For this design RLC was set to 4.64 k.

    kRLC 64.4

    Estimate no load input power (PNL):

    kHzfMIN

    1 , Minimum operating frequency

    uWfQVP MINggg 144 , gate drive power dissipation at fMIN

    uAIWAIT 85 , VDD input current at 1 kHz operating frequency [1]

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    mWVIPP DDWAITgVDD 9.1 , Estimated UCC28700 power dissipation at fMIN

    Estimate switching losses (PSWFM) at high line at fMIN

    uWft

    I

    aVVVPMINrPPK

    VDGOUTFLYSWFM 9452

    31

    mWfVC

    P MINFLYOSS

    COSS 5.02

    2 , estimated COSS losses at fMIN

    uW

    fI

    L

    P

    MINPPK

    LPK

    LLK 1292

    3

    2

    , estimate of leakage power dissipation at no load

    The estimated no load input power (PNL) is roughly 22 mW. In the actual 5W design the no load inputpower was roughly 20 mW at 230V RMS input voltage.

    mWPRZ 3.8

    mWPRT 2.9

    mWPPPPPPP LLKRTRZCOSSSWFMVDDNL 22

    5W EVM Schematic:

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    Figure 7, Schematic

    Efficiency:

    Efficiency

    64%

    66%

    68%

    70%

    72%

    74%

    76%

    78%

    25% 50% 75% 100%

    Output Power

    %E

    fficiency

    Efficiency @ 115V RMS

    Efficiency @ 230V RMS

    Figure 8, Efficiency

    Load Transient at 115V RMS

    CH1 = IOUT, CH4 = VOUT with a 5V offset

    Figure 9, 0.1 to 0.6A load step Figure 10, 0.6 to 0.1A load step

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    Load Transient at 230V RMS

    a. CH1 = IOUT, CH4 = VOUT with a 5V offset

    Figure 11, 0.1 to 0.6A load step Figure 12, 0.6 to 0.1A load step

    REFERENCES

    [1] UCC28700/1/2/3 Data Sheet, Constant-Voltage, Constant-Current Controller with Primary Side

    Regulation, SLUSB41, July 2012, http://www.ti.com/lit/gpn/ucc28700[2]Using the UCC28700EVM-068, UCC28700EVM-068 5W USB Adapter, SLUU968, July 2012,

    http://www.ti.com/litv/pdf/sluu968

    [3] UCC28700 MathCAD design tool,http://www.ti.com/litv/zip/sluc381

    http://www.ti.com/lit/gpn/ucc28700http://www.ti.com/lit/gpn/ucc28700http://www.ti.com/litv/pdf/sluu968http://www.ti.com/litv/pdf/sluu968http://www.ti.com/litv/zip/sluc381http://www.ti.com/litv/zip/sluc381http://www.ti.com/litv/zip/sluc381http://www.ti.com/litv/zip/sluc381http://www.ti.com/litv/pdf/sluu968http://www.ti.com/lit/gpn/ucc28700
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    IMPORTANT NOTICE

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