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AN-35 参数 Datasheet PDF下载

AN-35图片预览
型号: AN-35
PDF下载: 下载PDF文件 查看货源
内容描述: LinkSwitch的设计指南\n [LinkSwitch Design Guide ]
分类和应用:
文件页数/大小: 16 页 / 504 K
品牌: ETC [ ETC ]
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AN-35  
During CC operation, duty cycle is controlled by the peak drain  
current limit (ILIM). The device current limit is designed to be a  
function of reflected voltage such that the load current remains  
approximately constant as the load impedance is reduced. When  
the output voltage falls to approximately 30% of nominal value  
(normally associated with a failed battery), LinkSwitch enters the  
auto-restartmodeofoperationtosafelylimitaveragefaultcurrent  
(typically 8% of IO).  
QUICK START  
Figure 1 shows the key parameters and components  
needed to generate an initial LinkSwitch design.  
Where initial estimates can be used, they are shown  
below the parameter they refer to.  
1) Let VOR equal 50 V.  
With discontinuous mode design, maximum output power is  
independentofinputvoltageandisasimplefunctionofprimary  
inductance, peak primary current squared and switching  
frequency (Equation 6). LinkSwitch controls and cancels out  
variationsnormallyassociatedwithfrequencyandpeakcurrent  
by specifying a device I2f term. This allows users to easily  
design for a specific corner point where CV mode transitions to  
CC mode.  
2) Define the transformer turns ratio according to  
Equation 5. If no better estimates or measure-  
ments are available, then let VDOUT equal 0.7 V  
foraSchottkyor1.1VforaPNdiode,RCABLE equal  
0.3 , RSEC equal 0.15 , ISEC(RMS) equal 2x IO, and  
I
SEC(PEAK) equal 4 x IO, where IO is the desired CC  
output current and VO is the desired output  
voltage at the CV/CC transition point.  
Scope  
3) Calculate PO(EFF) according to Equation 13. As an  
initial estimate for PCORE use 0.1 W.  
This application note is for engineers designing an AC-DC  
powersupplyusingtheLinkSwitchLNK500orLNK501devices  
in a discontinuous mode flyback converter. Since LinkSwitch is  
designed to replace linear transformer based power supplies,  
the output characteristic provides an approximate CV  
characteristic, offering much better line and load regulation  
thananequivalentlineartransformer. Theverysimplenatureof  
the LinkSwitch circuit allows an initial paper design to be  
completed quickly using simple design equations. It is then  
recommended that the circuit performance be tuned with a  
prototypepowersupplytofinalizeexternalcomponentchoices.  
This document therefore highlights the key design parameters  
and provides expressions to calculate the transformer turns  
ratio,primaryinductanceandclamp/feedbackcomponentvalues.  
This enables designers to build an operating prototype and  
iterate to reach the final design.  
4) Calculate LP according to Equation 14 and  
other transformer parameters from Equations  
15, 16, 17, 18 and 19.  
5) Calculate value for feedback resistor RFB accord-  
ing to Equations 20, 21, 22, 23 and 24.  
This should be a 1/4 W, 1% part.  
6) Set clamp capacitor CCLAMP as a 0.1 µF, 100 V  
metalized plastic film type.  
7) Set clamp resistor RLF as 100 , 1/4 W.  
8) Set CONTROL pin capacitor CCP to be 0.22 µF,  
10 V for battery loads or 1 µF, 10 V for resistive  
loads.  
9) Select input and output components. See  
Figure 3 and relevant sections.  
For readers who want to generate a design as quickly as  
possible, the Quick Start section provides enough information  
to generate an initial prototype.  
10) Construct prototype.  
11) Iterate design (see Hints and Tips section).  
This document does not address transformer construction.  
Please see LinkSwitch DAK Engineering Prototype Report for  
examplesshowingtypicaltransformerconstructiontechniques.  
Further details of support tools and updates to this document  
can be found at www.powerint.com.  
nominalpeakpowerpointoutputvoltageVO,whiletransformer  
primary inductance is calculated from the total output power.  
Few components require computations, while the balance are  
selected from the included recommendations.  
CV/CC Circuit Design  
The LinkSwitch circuit shown in Figure 3 serves as a CV/CC  
chargerexampletoillustratedesigntechniques.Nominaloutput  
voltage is 5.5 V and nominal CC output current is 500 mA.  
Design and selection criteria for each component are described  
startingwiththetransformer. Onceset, transformerparameters  
and behavior are used to design clamp, bias and feedback  
componentsforpropersupplyoperation. Outputcapacitorsand  
the input circuitry can then be determined.  
LinkSwitch design methodology is very simple. Transformer  
turns ratios and bias component values are selected at the  
B
4/03  
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