LED Driver refers to the power adjustment electronic device that drives LED light or LED module components work normally. Due to the conduction characteristics of the LED PN junction, the voltage and current range of the power supply it can adapt to is very narrow, and a slight deviation may not be able to light the LED, or the luminous efficiency will be seriously reduced, or the service life may be shortened or the chip may be burned.

I What is an LED Driver?

LED Driver refers to the power adjustment electronic device that drives LED light or LED module components work normally. Due to the conduction characteristics of the LED PN junction, the voltage and current range of the power supply it can adapt to is very narrow, and a slight deviation may not be able to light the LED, or the luminous efficiency will be seriously reduced, or the service life may be shortened or the chip may be burned.

II What Does an Led Driver Do?

The current industrial frequency power supply and common battery power supply are not suitable for directly supplying LEDs. LED drivers are such electronic components that can drive LEDs to work under the best voltage or current state.

Since LED applications are almost universal in various fields of electronics applications, the changes in its luminous intensity, light color, and on-off control are almost unpredictable. So LED drivers become almost one-to-one servo devices, making this device family members become diverse.

The simplest LED driver (if you can call it that way) may be one or several series-parallel resistor-capacitor components that divide the current and divide the voltage in the loop. It is not an independent product at all.

For more general commercial applications requiring stable constant current and constant voltage output, a series of system solutions with precise power adjustment capabilities have been formed. Realizing these solutions usually requires a more complex LED driver circuit design, the core of which is the integrated application of LED driver chips.

By setting up different support circuits on the periphery of the LED driver IC, we can build solutions for different LED applications, ranging from mobile phone display backlight and button lighting drivers to high-power LED street lights and large outdoor LED displays.

Figure 1. LED Driver

The design and supply of common high-power LED driver boards are generally undertaken by professional companies. These companies repackage them into modules and supply them to LED terminal application product manufacturers. The LED driver design of the less general LED terminal application products may need to be designed by yourself.

The LED driver has become an important part of the unique technical content of this LED terminal application product. Because the LED as a packaged product is upstream, its technical performance has been solidified in the LED product, and to create a unique terminal LED application product, for the light source, there are not many places we can work with except for the LED driving function.

Due to the importance of the LED driver in LED application products and the wide user needs, the LED driver IC, which is the heart of the LED driver, has become a key element in the entire technical link. Thus, many manufacturers, or even listed companies, make LED drivers as their main products to supply large quantities of LED driver ICs to downstream industries. There are many leading American LED driver manufacturers, such as National Semiconductor, Maxim, Texas Instruments, ON Semiconductor, Linear Technology, Fairchild Semiconductor, and so on.

The LED drive power supply is a voltage converter that converts the power supply into a specific voltage and current to drive the LED to emit light. Under normal circumstances, the input of the LED drive power includes high-voltage power frequency AC (ie mains), low-voltage DC, high-voltage DC, low-voltage high-frequency AC (such as the output of an electronic transformer), etc. The output of the LED light power supply is mostly a constant current source that can change the voltage with the change of the forward voltage drop of the LED.

III Features

According to the power rules of the power grid and the characteristic requirements of the LED drive power supply, the following points should be considered when we select and design the LED drive power supply:  

1. High Reliability

Especially like the driving power of LED street lights, it is inconvenient and expensive to maintain when it's installed at high altitude.

2. High Efficiency

LEDs are energy-saving products, so the efficiency of the drive power supply should be high. For power supply installation, the structure of the LED drive power supply in the lamp is particularly important. Also, because the luminous efficiency of the LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. If the efficiency of the power supply is high, its power consumption is small, and the heat generated in the lamp is small, which reduces the temperature rise of the lamp, beneficial to delay the light decay of LED.

3. High Power Factor

The power factor is the requirement of the power grid on the load. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although a lower power factor of a single electrical appliance with low power has little effect on the power grid, if everyone lights up at night, the similar loads are too concentrated, which will cause serious pollution to the power grid.

It is said that in the near future, there may be certain index requirements for power factors of LED driving power supplies of 30 watts to 40 watts.

4. Driving Methods

There are two common driving methods:

(1) A constant voltage source for multiple constant current sources, and each constant current source individually supplies power to each LED. In this way, the combination is flexible, and the failure of one LED will not affect the work of other LEDs, but the cost will be slightly higher.

(2) Direct constant current power supply, with LED in series or parallel. Its advantage is that the cost is a little lower, but the flexibility is poor, and it must solve the problem of a certain LED failure without affecting the operation of other LEDs.

Figure 2. Constant Voltage LED Driver

These two forms coexist for a period of time. The power supply mode of multi-channel constant current output will be better in terms of cost and performance, which may become the mainstream direction in the future.

5. Surge Protection

The ability of LEDs to resist surges is relatively poor, especially the ability to resist reverse voltage. So, it is important to strengthen protection in this area.

Some LED lights are installed outdoors, such as LED street lights. Due to the onset of the grid load and the induction of lightning strikes, various surges will invade into the grid system, and some surges will cause damage to the LED. Therefore, the LED drive power supply must have the ability to suppress the intrusion of surges and protect the LED from damage.

6. Protection Function

In addition to the common protection function of the LED power supply, it is better to add negative feedback of LED temperature to the constant current output to prevent the LED temperature from being too high.

7. In terms of protection, the lamp is externally mounted, the power supply structure must be waterproof and moisture-proof, and the outer shell must be lightfast.

8. The life of the driving power supply should be matched with the life of the LED.

9. Requirements of safety regulations and electromagnetic compatibility

With the increasing application of LEDs, the performance of LED driving power will be more and more suitable for LED requirements.

IV LED Driver Types

1. According to the Drive Mode

(1) Constant Current Led Driver

1) The output current of the constant current LED drive circuit is constant, but the output DC voltage varies within a certain range with the change of the load resistance. The smaller the load resistance, the lower the output voltage. The larger the load resistance, the higher the output voltage;  

2) The load short is allowed to occur in the constant current circuit, but the load can not be open completely.

3) It is ideal for a constant current LED driver circuit to drive LEDs, but it is relatively expensive.  

4) Pay attention to the maximum withstand current and voltage values used, which limit the number of LEDs used;  

(2) Regulated LED Driver  

1) When the various parameters in the voltage regulator circuit are determined, the output voltage is fixed, but the output current changes with the increase or decrease of the load;  

2) In the voltage regulator circuit, load opening is allowed, but it is strictly forbidden to short-circuit the load completely.  

3) The LED is driven by a voltage-stabilizing LED driving circuit, and each string needs to be added with a suitable resistor to make the brightness of each LED string average;  

4) Brightness will be affected by voltage changes from rectification.

2. According to the Circuit Structure

(1) Capacitor Step-down Method

When the capacitor is used to reduce the voltage, due to the effect of charging and discharging, the instantaneous current through the LED is extremely large, and the chip is easily damaged. The method is easily affected by the fluctuation of the grid voltage, and the power supply has low efficiency and low reliability.

(2) Resistor Step-down Method

The resistor step-down is greatly affected by the change of the grid voltage, and it is not easy to make a regulated power supply. The step-down resistor consumes a large part of the energy, so this power supply method has low efficiency, and the system has low reliability.

(3) Conventional Transformer step-down Method

The power supply is small, heavy, and has low power supply efficiency, generally only 45% to 60%, so it generally has low reliability and is rarely used.

(4) Electronic Transformer Step-down Method

The power supply efficiency is low, and the voltage range is not wide, generally 180-240V, with large ripple interference.

(5) RCC Step-down Switching Power Supply

The voltage regulation range is relatively wide, the power supply efficiency is relatively high, generally achieving 70% to 80%, and the application is also wide. Because the oscillation frequency of this control method is not continuous, the switching frequency is not easy to control. The load voltage ripple coefficient is also relatively large, and the adaptability to abnormal loads is poor.

(6) PWM Control Mode Switching Power Supply

PWM LED driver is mainly composed of four parts, input rectification filtering part, output rectification filtering part, PWM voltage stabilization control part, switching energy conversion part.

Figure 3. 1W High Power LED Driver with PWM

The basic working principle of the PWM switching regulator is that when the input voltage, internal parameters, and external load change, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal to adjust the pulse width of the main circuit switching device. This makes the output voltage or current of the switching power supply stable (that is, the corresponding regulated power supply or constant current power supply).

The power supply efficiency is extremely high, generally achieving 80% to 90%, and the output voltage and current are stable. Generally, this kind of circuit has perfect protection measures and is a high-reliability power supply.

3. According to the Installation Location   

According to the installation location, the driving power can be divided into the external power supply and the built-in power supply.

(1) External power supply

As the name implies, the external power supply is to the power supply installed outside. Generally, for the circuits with relatively high voltage, which is dangerous to human safety, an external power supply is required. The difference between it and the built-in power supply is that the power supply has a shell, and street lights are common ones.  

(2) Built-in power supply

It is to install the power supply in the lamp. Generally, the voltage is relatively low, 12v to 24v, and there is no safety hazard to people. This common one is the bulb light.

V Application

Let's see from the main application market of high-power LEDs－landscape lights, to analyze the high-power LED market.

Figure 4. Landscape Lighting

In China, as the largest application market of LED lighting, the landscape lighting market accounts for about 43%. Its new round of rapid growth will surely drive the rapid development of the overall LED lighting industry.

The landscape lighting of some large and medium-sized cities in such as Shanghai, Xiamen, Beijing, Dalian, Nanchang, etc. has already achieved a considerable scale, and a considerable part of the LED lighting demonstration projects have been completed. The successful completion of these demonstration projects in large and medium cities and the successful use in the Olympic venues indicates that the technology of LED lighting in landscape lighting has gradually matured.

Whether at home or abroad, the city's landscape is the landmark building product of a city. And neon lights, as landscape lighting, are gradually being replaced by LED landscape lights all over the world due to energy, energy saving, environmental protection, and other issues.

There are about 700 thousand cities around the world and assume a city has 5,000 lamps and a lamp is about 1,000 yuan, the huge economic benefits produced by these alone are unpredictable.

The driver is the core component of LED lighting. With the maturity of LED chip technology, the quality of the LED light source has been very reliable. In many cases, the failure of LED lamps comes from the driver. High-power LED driving power is an emerging industry and has not yet formed a concentrated industry cluster, so the regionality is not obvious. Moreover, there are not many companies entering the industry, so the competition is relatively low.

VI Disadvantages

1. The technical personnel of the company that produces LED lighting and related products does not have enough understanding of the switching power supply. The power supply can work normally, but some key evaluations and electromagnetic compatibility are not considered insufficiently, producing certain hidden dangers;  

2. Most of the LED power supply manufacturers are transforming from ordinary switching power supplies to LED power supplies, and they have insufficient knowledge of the characteristics and use of LEDs;  

3. There are almost no standards for LEDs, most of them refer to the standards of switching power supplies and electronic rectifiers;  

4. Most LED power supplies are not unified, so the number of them is relatively small. Since the purchase amount is small, the price is high, and the component suppliers are not very cooperative;  

5. In terms of the stability of the LED power supply, wide voltage input, high and low-temperature operation, over-temperature, over-voltage protection, and other issues have not been solved.

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