Knowledge of LED drive power

Dec 2021

Knowledge of LED drive power

At present, LEDs are driven by DC, so a power adapter, that is, LED driving power, needs to be added between the mains and the LED. Its function is to convert the AC mains into a suitable DC power for the LED. According to the power rules of the grid and the drive of the LED Characteristic requirements, the following points should be considered when selecting and designing LED drive power supplies:
1. High reliability
Especially like the driving power supply of outdoor LED street lights, it is installed at high altitude, which is inconvenient to maintain and costly to repair.
2. high efficiency
LED is an energy-saving product, and the efficiency of the drive power supply should be high. It is especially important for the structure of the power supply installed in the lamp. Because the luminous efficiency of the LED decreases with the increase of the LED temperature, the heat dissipation of the LED is very important. 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. It is beneficial to delay the light decay of the LED.
3. High power factor
The power factor is the requirement of the power grid on the load. Generally, there is no mandatory indicator for electrical appliances below 70 watts. Although the power factor of a single electrical appliance with low power is lower, the impact on the power grid is not large, but everyone lights up at night, and similar loads are too concentrated , It will cause serious pollution to the power grid. For 30 watts to 40 watts of LED drive power, it is said that in the near future, there may be certain indicator requirements for power factors.
4. Drive method
There are two types currently in use: one is a constant voltage source for multiple constant current sources, and each constant current source individually powers each LED. In this way, the combination is flexible, and one LED fails, and it does not affect the work of other LEDs. , But the cost will be slightly higher. The other is direct constant current power supply, LEDs run in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and it must solve the problem of a certain LED failure without affecting the operation of other LEDs. These two forms will coexist for a period of time. The multi-channel constant current output power supply method will be better in terms of cost and performance. It is also 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. It is also important to strengthen protection in this area. Some LED lights are installed outdoors, such as LED street lights. Due to the start-up of the grid load and the induction of lightning strikes, Various surges will invade from 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 the surge and protect the LED from being damaged.
6. Protective function
In addition to the conventional protection functions of the power supply, it is best to increase the negative feedback of the LED temperature in the constant current output to prevent the LED temperature from being too high.
7. Protection
For the external installation type of the lamp, the power supply structure should be waterproof and moisture-proof, and the shell should be resistant to sunlight.
8. The life of the driving power supply should be matched with the life of the LED.
9. To meet the requirements of safety regulations and electromagnetic compatibility.
With the increasing application of LEDs, the performance of LED drive power will become more and more suitable for the requirements of LEDs.
Due to the many advantages of LED, such as environmental protection, long life, high photoelectric efficiency, etc., in recent years, the application of LED has been rapidly developed in various industries. Among them, due to the improper design of the driving power supply and the improper selection of the driving method, the LED is extremely easy to damage.
When we design the LED drive power supply, it is necessary to know the LED current and voltage characteristics. Due to the different LED Lights manufacturers and LED Lights specifications, the current and voltage characteristics are different. Now take the typical specifications of white LEDs as an example, according to the current and voltage changes of the LED According to the law, the general application forward voltage is about 3.0-3.4V, the typical value voltage is 3.2V, and the current is 60mA. When the forward voltage applied to both ends of the LED exceeds 3.6V, the forward voltage will increase slightly, and the positive The current may increase exponentially, causing the temperature of the LED luminous body to rise too fast, thereby accelerating the light attenuation of the LED, shortening the life of the LED, and even burning the LED in severe cases. According to the voltage and current change characteristics of the LED, the impact of the driving power supply The design puts forward strict requirements.
At present, the LED lamp products produced by many manufacturers (such as guardrails, lamp cups, and projection lamps) use resistance and capacitance to step down, and then add a zener diode to stabilize the LED to supply power to the LED. This way of driving the LED has great defects. First of all, the efficiency is low. It consumes a lot of power on the step-down resistor, which may even exceed the power consumed by the LED, and cannot provide high-current drive. Because the larger the current, the greater the power consumed on the step-down resistor, so The LEDs of many products are afraid to use parallel connection, and use series connection to reduce current. Second, the ability to stabilize voltage is extremely poor, and there is no guarantee that the current through the LED will not exceed its normal working requirements. When designing products, the voltage across the LED is reduced to supply power. Drive, this is at the cost of reducing the brightness of the LED. Using the resistance and capacitance step-down method to drive the LED, the brightness of the LED cannot be stable. When the power supply voltage is low, the brightness of the LED becomes dim, and when the power supply voltage is high, the brightness of the LED Brighter. The biggest advantage of driving LEDs with resistance and capacitance step-down methods is low cost.
According to the characteristics of LED current and voltage changes, it is feasible to drive the LED with constant voltage. Although the commonly used voltage stabilization circuit has the disadvantages of insufficient voltage stabilization accuracy and poor current stabilization ability, it may be over-precisely designed in the application of some products. , Its advantage is still not replaced by other driving methods.
The constant current driving method is an ideal LED driving method. It can avoid the current change caused by the change of the forward voltage of the LED, and at the same time, the constant current makes the brightness of the LED stable. Therefore, many manufacturers choose the constant current method to drive the LED.
Another LED driving method is feasible, it is neither constant voltage nor constant current, but through the design of the circuit, when the forward voltage of the LED increases, the driving current is reduced, which ensures the safety of LED products. Of course. The increase in forward voltage can only be within the tolerance range of the LED, and too high will damage the LED.
The ideal LED driving method is to use constant voltage and constant current. But the cost of the driver increases. In fact, each driving method has its advantages and disadvantages. According to the requirements and application of LED products, the LED driving method is reasonably selected, and the precise design of the driving power supply becomes The key. Although LED is more efficient than ordinary light sources in terms of energy saving, LED light sources cannot directly use the public grid voltage like ordinary light sources. It must be equipped with special voltage conversion equipment to provide voltage and current that can meet the LED's rating. In order to make the LED work normally, it is the so-called LED dedicated power supply.
However, because the performance and conversion efficiency of LED power supplies of various specifications are different, choosing a suitable and efficient LED power supply can truly reveal the high-efficiency characteristics of LED light sources. Because low-efficiency LED power supplies themselves need to consume a lot of Electricity, so it is impossible to highlight the energy-saving characteristics of LEDs in the process of powering LEDs. In short, the stability, energy saving, and lifespan of LED power supply in LED work have an important role.
What are the classifications of LED power supplies?
1. The LED power supply can be divided into two categories according to the driving mode:
A. Constant current type:
1.The constant current drive circuit is ideal for driving LEDs, but the disadvantage is that the price is higher.
2.Although the constant current circuit is not afraid of the load short circuit, it is strictly forbidden to open the load completely.
3.The output current of the constant current drive circuit is constant, but the output DC voltage varies within a certain range with the size of the load resistance.
4. Limit the number of LEDs used because it has the maximum withstand current and voltage values.
B. Regulated:
1.After the voltage regulator circuit determines the various parameters, the output is a fixed voltage, but the output current changes with the increase or decrease of the load.
2. Although the voltage regulator circuit is not afraid of the load being opened, it is strictly forbidden to short-circuit the load completely.
3.The voltage change after rectification will affect the brightness of the LED.
4. To make each string of LEDs driven by a voltage stabilizing circuit display uniform brightness, you need to add a suitable resistance.
2. LED power supplies can be divided into six categories according to the circuit structure:
1.Conventional transformer step-down:
The advantage of this kind of power supply is its small size, but its disadvantage is that the weight is heavy and the power supply efficiency is also very low, generally 45% to 60%. Because of the low reliability, it is generally rarely used.
2.Electronic transformer step-down:
The shortcomings of this power supply structure are low conversion efficiency, narrow voltage range, generally 180~240V, and large ripple interference.
3. Capacitor step-down:
The LED power supply in this way is easily affected by the fluctuation of the grid voltage, and the power supply efficiency is low. It is not suitable for the LED to be used when flashing, because the circuit is used to reduce the voltage through the capacitor. Very large, easy to damage the chip.
4. Resistance step-down:
The power supply efficiency of this power supply method is very low, and the reliability of the system is also low. Because the circuit is reduced by resistors, it is greatly interfered by changes in the grid voltage, and it is not easy to make a stabilized power supply, and the step-down resistor itself consumes a lot of energy. Most of the energy.
5. RCC step-down switching power supply:
The advantages of this method of LED power supply are that the voltage regulation range is relatively wide, the power supply efficiency is relatively high, generally 70% to 80%, and it is widely used. The main disadvantages are that the switching frequency is not easy to control, the load voltage ripple coefficient is large, and the load is abnormal. Poor adaptability.
6. PWM control switching power supply:
At present, the LED power supply designed by the PWM control method is ideal, because the output voltage or current of this switching power supply is very stable. The power conversion efficiency is very high, generally up to 80% to 90%, and the output voltage, The current is very stable. This type of LED power supply is mainly composed of four parts: input rectification and filtering part, output rectification and filtering part, PWM voltage stabilization control part, switching energy conversion part. And this circuit has perfect protection measures , It is a high-reliability power supply.
How to judge the quality of LED driver
First, the driver chip-IC
The core of the driving power supply is the IC, and the quality of the IC directly affects the entire power supply. Driver ICs from major manufacturers are all looking for
Large packaging factories come to encapsulate; while the driver IC technology of small factories is to directly copy the drive design schemes of large factories and find small packaging factories.
Packaging, the consistency and stability of the entire batch of ICs cannot be guaranteed normally, which causes the driving power supply to be inexplicable after a period of use
’S failure. Therefore, the IC on the LED power supply refuses to be polished, so that the lamp manufacturer can understand the IC solution and calculate the cost of the driver.
Procurement of power supply products at reasonable prices.
Second, the transformer
The control chip can be regarded as the brain center of the power supply, and the transformer that determines the power and temperature resistance. The transformer is responsible for completing the “AC
-Magnetic energy-direct current", the energy overload will saturate the bomber. The core of the transformer is the magnetic core and the wire package. The quality of the magnetic core is the core of the transformer, but like porcelain, it is extremely difficult to distinguish. The simple appearance identification is: the appearance is brittle ,dense,
Bright, at the same time the back polished pores are top grade. At present, the core used by Shanghai Nuoyi is the open-molded PC44 core, which guarantees the high power supply.
The wire package is made of copper wire winding, and the quality of the copper wire used is the key to the life of the transformer. Copper clad aluminum wire of the same length
The material is 1/4 of the price of pure copper wire. Due to cost pressure, transformer manufacturers often mix copper-clad aluminum wire packages.
The transformer is inside. As a result, when the temperature of the transformer rises, it burns out and fails, causing the power supply and the entire lamp to fail. So many
Lamps, especially lamps with a built-in power supply, often explode in about 6 months after shipment. And how to tell if this copper wire is
Is it pure copper wire or copper clad aluminum? Use a lighter to ignite it, and it will be copper-clad aluminum if it burns quickly. You can also measure the resistance of the coil to identify
Third, electrolytic capacitors and chip ceramic capacitors
Everyone knows the quality and life requirements of input electrolytic capacitors, and everyone attaches great importance to them. But people tend to ignore
The quality requirements of the output capacitor, in fact, the life of the output capacitor has a great impact on the life of the power supply. Up to 60,000 times per second at the output
The high switching frequency causes the parasitic resistance of the capacitor to heat up and produce a substance similar to scale. Finally, the electrolyte heats up and bursts. push
Recommended output electrolytic capacitor: use LED special electrolysis, the general model starts with L. At present, our output electrolysis is Aihua Gaoshou
Life-saving electrolytic capacitors.
Ceramic capacitors: The materials are divided into X7R, X5R and Y5V, and the actual capacitance value of Y5V can only reach 1/10 of the actual value, the nominal
The capacitance value only refers to when working at 0 volts. Therefore, this tiny chip resistor and poor options will also lead to a price difference in cost and greatly shorten the life of the power supply.
Fourth, the circuit design and welding process of power products
Judgment of the pros and cons of the design: Regardless of the professional point of view, it can be distinguished by some intuitive methods, such as the neat and large layout of the components.
Square, orderly, bright and straight welding spot. A good engineer will not make a messy design. As for flying leads, handmade Canadian dollars
Pieces, it is the performance of a serious lack of technical strength.
Welding process: manual welding and wave soldering process. As we all know, the quality of mechanized production of wave soldering process is definitely better than hand soldering.
Worker welding. Identification method: whether there is red glue on the back. (Solder paste technology + soldering fixture can also achieve wave soldering, but the fixture cost is high).
Solder joint inspection instrument for patch: AOI. The equipment can detect the phenomenon of false soldering, false soldering and missing soldering in the patching process.
At present, the flashing phenomenon of lamps after a period of use is basically caused by the power supply or the weak welding of the lamp beads. And this
It is extremely difficult to detect the false soldering of a product through aging, so it is necessary to rely on AOI to detect the quality of the power supply
Fifth, batch inspection of aging racks and high temperature aging rooms for power products
No matter how good the power supply products are controlled by materials and production processes, they still need to be tested for aging. Because the electronic components and transformers
Incoming inspection is difficult to control. Only through the aging of the entire batch of power supplies and the high temperature sampling inspection of the high temperature room, this batch can be tested
The quality stability of the power supply and whether the materials have safety hazards.
The effect of large-scale high-temperature sampling inspection: The current failure of the power supply is between 1% and 1%, and only a few thousand high
This type of failure will be discovered only by temperature aging.
The high-temperature room can simulate the harsh environment of the power supply, and random inspections under strict conditions can find batch problems, such as design inconsistencies.
Reasons, poor raw materials, deduction of failures in lamps, high-voltage switch shocks, etc.
Long-term aging at room temperature: screening out random failures such as false welding, missing welding, collisions, etc., filtering out early failures of components, and effectively reducing costs
Product failure rate (one percent down to one thousandth).
Aging at room temperature is very expensive for aging equipment and personnel. For a factory with a daily output of 100,000 power supplies, the aging equipment occupies at least 500 square meters.
There are more than ten thousand aging bits, and the realization of pipeline aging is relatively small in the industry.

Electrolytic capacitor brand:
Japanese Nikon, Black King Kong, Sanyo, Ruby-based, Hitachi’s expensive, Panasonic’s market share is not high, but the Japanese are all expensive. Belonging to high-end products, domestically produced products include: Jianghai, Warwick, Wanyu Sanxin, Zhijiang, Tosun, U-CON, Aihua, and Taiwanese products: Lilong, Guankun, Kaimei, Zippo, Fengbin. South Korea has Samsung
The brands of MOS tubes are: ST, ON (On Semiconductor), Infineon (Infineon), Fairchildsemi (Fairchild Semiconductor), Taiwan Fuding Advanced, Taiwan Maoda, the mainstream MOS tubes used in the market

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