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LNK520 参数 Datasheet PDF下载

LNK520图片预览
型号: LNK520
PDF下载: 下载PDF文件 查看货源
内容描述: 高效节能, CV或CV / CC切换为非常低的成本适配器和充电器 [Energy Efficient, CV or CV/CC Switcher for Very Low Cost Adapters and Chargers]
分类和应用:
文件页数/大小: 20 页 / 844 K
品牌: POWERINT [ Power Integrations ]
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LNK520  
The output during CV operation is equal to the primary-side  
bias voltage multiplied by the turns ratio. The bias voltage, in  
turn, is the sum of the CONTROL pin voltage (approximately  
5.7 V), the voltage across the bias feedback resistor R4 and  
the forward voltage of D6B. Resistor R3 can be neglected as  
proportionallythevoltagedropacrossthisresistanceissmall. In  
CV operation, the voltage across R4 is equal to the CONTROL  
pin current, IDCT (2.15 mA) multiplied by the value of R4.  
2. Design is a discontinuous mode flyback converter with  
nominal primary inductance value and a VOR in the range  
40 V to 80 V. Continuous mode designs can result in loop  
instability and are therefore not recommended.  
3. A secondary output of 5 V with a Schottky rectifier diode.  
4. Assumed efficiency of 65%.  
5. The part is board mounted with SOURCE pins soldered to  
sufficient area of copper to keep the die temperature at or  
below 100 °C.  
As the output load is decreased, the output and therefore  
bias voltage increase resulting in increased current into the  
CONTROLpin. AsthecurrentintotheCONTROLpinexceeds  
IDCS (~2 mA), the duty cycle begins to reduce, maintaining  
regulation of the output, reaching 30% at a CONTROL pin  
current of 2.15 mA.  
6. An output cable with a total resistance of 0.2 .  
In addition to the thermal environment (sealed enclosure,  
ventilated, open frame, etc.), the maximum power capability  
of LinkSwitch in a given application depends on transformer  
core size, efficiency, primary inductance tolerance, minimum  
specifiedinputvoltage,inputstoragecapacitance,outputvoltage,  
output diode forward drop, etc., and can be different from the  
values shown in Table 1.  
Under light or no-load conditions, when the duty cycle reaches  
approximately 4%, the switching frequency is reduced from  
44 kHz to 29 kHz to lower light and no-load input power.  
Transformer Design  
Astheoutputloadisincreased,thepeakpowerpoint(definedby  
2
0.5 LP ILIM f) is exceeded. The output voltage and therefore  
primary-side bias voltage reduce. The reduction in the bias  
voltage results in a proportional reduction of CONTROL pin  
current, which lowers the internal LinkSwitch current limit  
(current limit control).  
To provide an approximately CV/CC output, the transformer  
should be designed to be discontinuous; all the energy stored  
in the transformer is transferred to the secondary during the  
MOSFET off time. Energy transfer in discontinuous mode is  
independent of line voltage.  
Constantcurrent(CC)operationcontrolssecondary-sideoutput  
current by reducing the primary-side current limit. The current  
limit reduction characteristic has been optimized to maintain  
an approximate constant output current as the output voltage  
and therefore, bias voltage is reduced.  
The peak power point prior to entering constant current  
operation is defined by the maximum power transferred by the  
transformer. The power transferred is given by the expression  
P = 0.5 LP I2 f, where LP is the primary inductance, I2  
is the primary peak current squared and f is the switching  
frequency.  
If the load is increased further and the CONTROL pin current  
fallsbelowapproximately0.8mA,theCONTROLpincapacitor  
C5 will discharge and LinkSwitch will enter auto-restart  
operation.  
To simplify analysis, the data sheet parameter table specifies an  
I2f coefficient. This is the product of current limit squared and  
switchingfrequencynormalizedtothefeedbackparameterIDCT  
.
This provides a single term that specifies the variation of the  
Current limit control removes the need for any secondary-  
side current sensing components (sense resistor, transistor,  
optocoupler and associated components). Removing the  
secondarysensecircuitdramaticallyimprovesefficiency,giving  
the associated benefit of reduced enclosure size.  
peak power point in the power supply due to LinkSwitch.  
As primary inductance tolerance is part of the expression  
that determines the peak output power point (start of the CC  
characteristic) this parameter should be well controlled. For  
an estimated overall constant current tolerance of ±24%, the  
primary inductance tolerance should be ±7.5% or better. This  
is achievable using standard low cost, center leg gapping  
techniques where the gap size is typically 0.08 mm or larger.  
Smaller gap sizes are possible but require non-standard, tighter  
ferrite AL tolerances.  
Key Application Considerations  
Design Output Power  
Table 1 (front page) shows the maximum continuous output  
power that can be obtained under the following conditions:  
Other gapping techniques such as film gapping allow tighter  
tolerances (±7% or better) with associated improvements in  
the tolerance of the peak power point. Please consult your  
transformer vendor for guidance.  
1. The minimum DC input bus voltage is 90 V or higher. This  
correspondstoaltercapacitorof3µF/Wforuniversalinput  
and 1 µF/W for 230 VAC or 115 VAC input with doubler  
input stage.  
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