光强可调大功率LED开关电源的设计与研究

卢 忠,朱 伟,梅进光,阮世平

(1.浙江工业大学理学院,杭州 310023;2.浙江科技学院理学院,杭州 310023)

1 Introduction

As a solid light source,light emitting diode(LED)is becoming the focus of attention for many present scholars and businessmen because of its particular advantages,such as energy conservation,environmental protection,firm structure,long life,short start-up time,small volume,low operating voltage and so forth[1].The United States,Japan,the European Union,China Taiwan and other countries and regions have developed the corresponding semiconductor lighting program[2].In China,LED lighting products and engineering has also been included in the key projects through the National Program for Medium-to long-term scientific and technological development and‘Eleventh Five-year'863 Plan.

The many advantages of LED lay the foundation for the extensive use,however,the light efficiency,cost,the elimination of heating,especially the design of driving power need to be further resolved,if the LED wants to be widely used.At the present time,there are three main methods to drive the LED[3]:current-limiting type driving,using resistor;constant voltage type driving and constant current type driving.Current-limiting type drive because of its low efficiency and the constant voltage drive because of its larger damage are increasingly replaced by the constant current drive.Although there are some chips currently used to produce the constant current drive,the PWM frequencies of these drive powers are too high to be expensive,moreover,the complex designs of their peripheral circuit lead to large volume[4-6].This article designs a switching power supply of intensity adjustable powerful LED based on LM2575 DC/DC step-down chip that overcomes the shortcomings of the above driving circuits.And it can be applied to the electric motor car toimproveits power efficiency.

2 The design of switching power supply

2.1 diagram

This research based on LM2575 DC/DC step-down chip designs a driving power which can convert the 24～36 V input voltage into output constant current.It can drive multiple serial powerful light emitting diodes,and by adjusting the external potentiometer Rx,it alsocan vary the output current in the rating of 20%—100%to achieve the purpose of adjustable light inten-sity.Its diagram is shown in Fig.1.

The circuit mainly consists of two parts.The first part which is made up of U 1,L2,C4,D1,DS1 as well as Rx and R1 is a step-down,steady flow and the light intensity adjustable circuit.As the reference voltage of LM-2575T-ADJ equal 1.23 V,if the value of Rxremains unchanged,the current through Rx and R1 is also unchanged,namely,the current through the powerful LED is constant,playing a role in constant current.If Rxis changed,the current through Rxwill be changed because the V FB should be 1.23 V.The luminous flux of the powerful LED is determined by the current through it,and that change the value of Rxcan adjust the luminous flux of LED that achieves the purpose of adjustable light intensity.The second part which is made up of the capacitors C1,C2,C3 and inductor L1 constitutes the filter circuit.The selection of the value of R1:the rated current equal 300 mA and V FB equal 1.23 V,so the sum of Rx and R1 is 4Ω.But actually Rx is difficult to adjust to zero,so R1 is taken 3Ω.When the Rx is 22Ω,thecurrent through LEDis 50 mA.When thevalue of Rx changes from 1 to 22Ω,thecurrent of LEDalso changes from 50 mA to 300 mA.

Fig.1 The diagram of switching power supply

2.2 Hardware production

It is greatly important to consider the PCB layout in the switching power supply,because the fast switch current as well as wiring inductance,parasitic capacitance,and spurious inductance will generate transient voltage.And the transient voltage will yield electromagnetic interference.So the input capacitors,such as C1 and C2 and C3,output capacitor like C4,diode D1 and the inductors L 1,L2 should be as much as possible close to the LM2575T-ADJ chip.Moreover,use the short pins to weld to shorten the current circuit and thecircuit common port ispreferably at thesame point.In order to minimizethesensitive circuit,the output pin 2 of the chip should be welded as small as possibleand the reactive circuit of feedback pin 4 should be as short as possible.Last but not least,we must pay attention to the order of each pin.

3 Test and analysis

Because whether the driving LED current is constant has greatly influence on the LED life,the constant level of the switching power supply is an important index.Of course,the efficiency of the switching power supply is also an important index.As this switching power supply will be used in the electric motor car,it is an index that whether theintensity of LED is adjustable and the scope of regulation of the luminous intensity.This study analyzes the steady flow performance,intensity adjustable performance and the power efficiency of the switching power supply by using HP860LED intensity distribution tester,HP860LED 300 mm integrating sphere,HY3003F-3 DCpower supplies and VC9805A+DMM.

3.1 Steady flow analysis of single LED

The relationships between input voltage V i and output current Io,reference voltage V FB,relative flux of LEDΦras well as the voltage across LED VLEDare shown in Fig.2 and Fig.3.

From Fig.2 and Fig.3 when the input voltage changes from 11 V to 38 V,the output current of the circuit does not change.The constant flow effect is very good when the input voltage is above 11 V.When it is used in the 36 V electric motor car and the voltage of the electric motor car's storage battery falls from 36 V to 11 V,LED brightness can still keep constant.Not only does it guarantee the LED life,but also increases the users'safety.

3.2 Theanalysis of single LED intensity adjustable

The relationships between different resistance of Rx and output current Io,reference voltage V FB,relative flux of LEDΦr as well as the voltage across LED V LED are shown in Fig.4 and Fig.5.

From Fig.4 and Fig.5 when the external regulation resistancechanges from 1Ωto 18Ω,the output current changes from 300 mA to 50 mA and the LED relative flux changes from 3.5 to 1.It achieves the intensity adjustable purpose.

3.3 Therelationship between the number of LEDand the power efficiency

When different numbers of LED connect in series,the relationship between the number of LED and the switching power supply efficiency is shown in Fig.6.

From Fig.6 when driving single LED,becauseof the relatively large consumption of the resistor that the LM2575T feedback sample,the power efficiency is low.With theincrement of the LED quantity,the circuit conversion efficiency is also greatly improved.

4 Conclusions

The switching power supply which designed in this study possesses simple structure,steady performance and small volume.By the test data and analytical curves it can be seen that when the input voltage of the switching power supply changes from 11 V to 38 V,its output current and the LED relative flux remain constant.Single LED luminous efficiency is between 50%and 60%.And from the data and curve of the relationship between number of LED and luminous efficiency can be seen that with the increment of the LED quantity,the luminous efficiency is continuously increased.It can reach above 80%when the number of LED is more than five.Also from the intensity adjustable curve it can be learned that the alteration of the value of the regulation resistance can achieve the intensity adjustable purpose.The actual measurements prove that the power supply has a high performance cost ratio and promotional value.As can be seen from the test parameter,the switching power supply meets the requirements of theelectric motor car and can be applied to the electric motor car.So it has utility value.

Fig.6 The relationship between the number of LED and power efficiency

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