AlthoughHigh power LEDIt is not yet possible to replace traditional incandescent lamps on a large scale, but they are more and more widely used in indoor and outdoor decoration and special lighting, so mastering high-power LED constant currentdriveThe design technology of the device is very important for developing new applications of high-power LEDs.ledaccording topowerand glowbrightnessCan be divided into high-power LED,High Brightness LEDand ordinary LEDs. Generally speaking, the power of high-power LED is at least 1W or more, and the more common ones are 1W, 3W, 5W, 8W and 10W.
constant current drive and boost LED’sopticsEfficiency is two key issues in LED application design. This article first introduces the application of high-power LEDs and the selection guide for constant current drive solutions, and then takes the US nationalsemiconductor(NS) products as an example, it focuses on how to skillfully apply the sampling resistor of the LED constant current drive circuit to improve the efficiency of high-power LEDs, and gives the precautions for high-power LED driver design and heat dissipation design.
Selection of driver chips
LED driveronly occupyLED lightingA small fraction of the system cost, but it is related to the reliability of the overall system performance. Currently,National SemiconductorThe LED driver solutions are mainly positioned in the mid-to-high-end LED lighting and lighting markets.Lighting is divided into indoor and outdoor.LED lightsappliedpower supplyThere are two ways of AC/DC and DC/DC converter in the environment, so the choice of driver chip should also be considered from these two aspects.
Figure 1: Conversion from constant voltage drive (left) to constant current drive (right) using the DC/DC regulator FB feedback.
1. AC/DC converter
AC/DC is divided into 220V AC input and 12V AC input. 12V AC is the power source for halogen lamps widely used in hotels, and existing LEDs can be designed under the condition that the existing AC 12V is retained. For the design of replacing halogen lamps, the main advantages of National Semiconductor LM2734 are small size, high reliability, and output current up to 1A, which is just suitable for the small diameter of the halogen lamp socket.
AlthoughHigh power LEDIt is not yet possible to replace traditional incandescent lamps on a large scale, but they are more and more widely used in indoor and outdoor decoration and special lighting, so mastering high-power LED constant currentdriveThe design technology of the device is very important for developing new applications of high-power LEDs.ledaccording topowerand glowbrightnessCan be divided into high-power LED,High Brightness LEDand ordinary LEDs. Generally speaking, the power of high-power LED is at least 1W or more, and the more common ones are 1W, 3W, 5W, 8W and 10W.
constant current drive and boost LED’sopticsEfficiency is two key issues in LED application design. This article first introduces the application of high-power LEDs and the selection guide for constant current drive solutions, and then takes the US nationalsemiconductor(NS) products as an example, it focuses on how to skillfully apply the sampling resistor of the LED constant current drive circuit to improve the efficiency of high-power LEDs, and gives the precautions for high-power LED driver design and heat dissipation design.
Selection of driver chips
LED driveronly occupyLED lightingA small fraction of the system cost, but it is related to the reliability of the overall system performance. Currently,National SemiconductorThe LED driver solutions are mainly positioned in the mid-to-high-end LED lighting and lighting markets.Lighting is divided into indoor and outdoor.LED lightsappliedpower supplyThere are two ways of AC/DC and DC/DC converter in the environment, so the choice of driver chip should also be considered from these two aspects.
Figure 1: Conversion from constant voltage drive (left) to constant current drive (right) using the DC/DC regulator FB feedback.
1. AC/DC converter
AC/DC is divided into 220V AC input and 12V AC input. 12V AC is the power source for halogen lamps widely used in hotels, and existing LEDs can be designed under the condition that the existing AC 12V is retained. For the design of replacing halogen lamps, the main advantages of National Semiconductor LM2734 are small size, high reliability, and output current up to 1A, which is just suitable for the small diameter of the halogen lamp socket.
After replacing halogen lamps, LED lamps are generally made into 1W or 3W. Compared with halogen lamps, LED lamps have two major advantages: (1) The light source is relatively concentrated, and the brightness obtained by 1W lighting is equivalent to the brightness of more than ten watts of halogen lamps, so it is more power-saving; (2) The life of LED lamps is longer than that of halogen lamps. long.
The main weakness of LED lights is that the angle of incidence of the light is too narrow and the cost is relatively high. But in the long run, LED lamps still have a very large cost advantage due to their long lifespan. 220V AC/DC converters (such as LM5021) mainly target the stage lighting and street lighting market.
Figure 2: On the FB feedback side andRFPlace an op-amp between B to reduce power consumption.
2. DC/DC converter
Currently, LED flashlights account for most of the demand for DC/DC converters. The LED power used by the flashlight is basically 1W, and the power supply methods include lithium batteries, nickel-zinc batteries, and alkaline batteries. There are still some difficulties in the application of 3W flashlights, because the 3W LED lamp itself needs heat dissipation, and the volume of the heat dissipation device is large, which weakens the advantage of the small size of the LED lamp to a certain extent. In addition, due to the current up to 700mA of the 3W LED light, the battery life after a single charge is shortened. Nonetheless, for the above-mentioned applications, National Semiconductor offers solutions such as the LM3475, LM2623A and LM3485.
Miner’s lamp is also one of the main application fields of LED lamp. It belongs to the special lighting industry and requires professional certification standards. China has always attached great importance to the application of LED in the field of miner’s lamp. Currently,LED DesignThe industry has the problem of insufficient understanding of the needs of special industries, and some unrealistic and novel design schemes are often used in the design. For example, the LED lamp and the battery are embedded in the helmet, but the various needs of the special use environment of the miner’s lamp are not considered, which may be an important reason why the application of LED in the miner’s lamp market has not been opened.
For miner’s lamp LED applications, National Semiconductor offers a wide range of DC/DC regulator products, including the LM3485, LM3478 and LM5010. The user has used a 1W LED light and placed 6 ordinary high-brightness LED lights around to form a miner’s lamp with special flickering function.
All in all, LED lamps have broad development prospects in the lighting and special lighting industries, and National Semiconductor provides a complete new LED driver solution for this purpose.
Figure 3: Constant current drive circuit based on LM2734.
Efficient constant current drive circuit
The basic circuit of constant voltage power supply (left in Figure 1) uses feedback resistors RFB1 and RFB2. When the load current changes, VFB also changes. The DC/DC regulator senses the change of VFB to maintain the output voltage at a fixed value. Level:
V0=(VFB*(RFB1+RFB2))/RFB1 (1)
In the circuit on the right of Figure 1, the FB of the DC/DC regulator is a high-impedance input, and the current IF flowing through the LED is:
IF=VFB/RFB (2)
To keep IF constant, the DC/DC regulator senses VFB and adjusts the LED positive terminal voltage so that the current through the LED remains constant. This is the principle of using the FB feedback terminal of the DC/DC regulator to achieve constant voltage to constant current conversion.
Generally speaking, the DC/DC regulator has a range of perception for the change of VFB. Once the LED is selected, the size of its operating current IF is also determined. The selected resistor must ensure that VFB falls within the DC/DC regulator. within the allowable range of the device.
Taking VFB equal to 1.25V as an example, assuming that the IFs are 15mA, 350mA and 700mA respectively, the power consumption of the sampling resistor will be less than 20mW, 400mW and 800mW respectively. For a 1W LED, the power consumption of the sampling resistor accounts for 2%, 40% and 80% of the total power consumption, respectively. Therefore, the design of the sampling resistor is very important to improve the efficacy of the LED, and it should be selected as small as possible.
Figure 4: The design of taking the feedback voltage directly from the sampling resistor.
Because directly connecting RFB to FB will cause excessive power consumption of RFB, an operational amplifier is placed between FB and RFB to amplify the voltage VTAP collected by RFB (Figure 2).
IF=VTAP/RFB=(VFB/RFB)*(1+RF/RI) (3)
Generally, the typical operating current of a 1W high-power LED is 350mA. If RFB is selected to be equal to 1 ohm, the power consumption of RFB is:
PRFB=I2*R=0.352*1=0.12W (4)
Considering the power consumption of the op amp itself, the power consumption of the RFB and its auxiliary circuits is about 12% of the 1W LED power. In this way, the power consumption of the RFB can be reduced to an acceptable level while ensuring that the LED is powered by a constant current, so that the voltage across the LED is as large as possible and the current flowing through it is also as large as possible. National Semiconductor’s regulators that work according to this principle include the LM2736 and LM2734.
The LM2734 is a 1A step-down regulator. The constant current drive circuit based on LM2734 (Figure 3) uses the LM321 operational amplifier to obtain the voltage on the sampling resistor Rset, combined with other resistors and capacitors to form a complete, high-efficiency high-power LED constant current drive circuit. In actual use, some LED constant current drive circuits can directly obtain the feedback voltage from the sampling resistor, as shown in Figure 4.
The sampling resistor Rset in Figure 3 determines the design of the constant-current drive circuit and has an important impact on the efficiency of the entire system, so careful design of Rset is critical to saving energy. Detailed design files for Figures 3 and 4 are available from your local authorized National Semiconductor distributor.
In general, if the LED drive current is required to vary by no more than 5% to 10% of the nominal value, then a 2% accurate resistor is sufficient. The typical fluctuation range of LED drive current is plus or minus 10%. Due to the large power consumption of the sampling resistor, the use of chip resistors with low power should be avoided. In addition, the LM3478 solution is suitable for the constant current drive of multiple high-power LEDs, while the constant current drive design solution based on the LM5021 is aimed at the application of 220V AC/DC converters.
Constant current drive and heat dissipation considerations
As far as Electronic system design is concerned, engineers must first understand the constant current parameters of LEDs when designing LED constant current drive circuits.CurrentlyLED chipThere are many manufacturers of LEDs at home and abroad. The difference between domestic and foreign LEDs is mainly in the case of the same electrical parameters.lumenThe number may vary, so it is important for design engineers to realize that LED power is not alight effectThe only parameter for the rate. For example, for the same 1W LED, some LEDs can reach a brightness of 40 lumens, while others can only reach a brightness of 20 lumens, because the optical efficiency of LEDs also depends on many aspects such as materials and manufacturing processes.
Some design engineers take the method of increasing the driving current in order to improve the luminous efficiency. For example, for the same 1W LED, after increasing the driving current, the brightness can be increased from 20 lumens to 40 lumens, but the operating temperature of the LED also increases accordingly. . Once the temperature exceeds the temperature limit of the LED, it will affect the life and reliability of the LED, which is an important issue that needs to be paid attention to in the process of designing constant current drive.
In addition, the optical efficiency of an LED lighting system is not only determined by the LED constant current drive scheme, but also closely related to the heat dissipation design of the entire system.In order to reduce the size, some LED constant current drive systems willLED driver circuitDesign close to the heat dissipation part, which can easily affect the reliability.
Generally speaking, the heat source of the LED lighting system is basically the heat source of the LED lamp itself. If the heat source is too concentrated, heat loss will occur, so the LED drive circuit cannot be closely attached to the heat dissipation system. It is recommended to take the following heat dissipation measures: use aluminum substrates for heat dissipation of LED lamps; evenly distribute power devices; avoid designing LED driver circuits close to the heat dissipation part as much as possible; suppress the thermal impedance from the package to the printed circuit substrate; Low thermal impedance.
Table 1: High-power LEDs have great advantages in lifetime.
New application requirements for drives
High-power LEDs are called “green light sources”, and they will develop in the direction of high LED current (300mA to 1.4A), high efficiency (60 to 120 lumens/watt), and adjustable brightness.
Since high-power LEDs have great advantages in life (Table 1), the development prospects are very broad, and the most promising lighting applications are automotive,medical equipmentAnd instrumentation and other special lighting environments.But these applications also put forward new requirements for LED drive system design, including: input voltage range is generally required to be 6V to 24V; surge load protection, reverse phase and overvoltage protection; standby power consumption is very low; reduces current sensing losses and hasPWMThe ability to adjust the brightness, etc.
In response to these needs, National Semiconductor has provided a full range of LED driver design solutions (see Table 2), which can provide users with a comprehensive LED driver solution.
The LED lighting system needs to rely on constant current power supply. The current mainstream constant current drive design scheme is to use linear orswitchType DC/DC regulators combine with a specific feedback circuit to provide constant current power supply for LEDs.inductancetype LED driver and switched capacitor type LED driver. The advantages of the inductive boost driver scheme are that the driving current is higher, the terminal voltage of the LED is lower, the power consumption is lower, and the efficiency remains unchanged, which is especially suitable for driving multiple LEDs. In the design of high-power LED driver, the switched capacitor LED driver scheme is mainly used. Advanced switched capacitor technology can also improve efficiency, so it is widely used in high-power LED drivers.
Table 2: List of National Semiconductor’s LED driver solutions.
Summary of this article
High-power LED lighting technology has broad development prospects, so it has been widely concerned and sought after by investors. At this stage, due to the limitations of many factors such as LED chip design and manufacturing technology and materials, it cannot completely replace traditional incandescent lamps for the time being, so people pay more attention to the application of high-power LEDs in special lighting.
This paper first introduces the application environment of special lighting, and then elaborates the basic principle and practical case of using DC/DC regulator to realize constant voltage to constant current design, and explains that the design of high-power LED driver and heat dissipation part should pay attention to Finally, it pointed out the new requirements for driver design for new high-power LED applications, and gave a guide to National Semiconductor’s complete solution, which helps electronic design engineers engaged in the LED lighting industry to fully grasp the latest LED drivers. system design techniques.
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