What is LED Drive Power?

Ⅰ.  What is LED  drive power?

LED drive power supply is a voltage converter that converts the power supply to a specific voltage and current to drive LED  luminescence. Generally, the input of LED  drive power supply includes high-voltage power-frequency AC (i.e. mains), low-voltage DC, high-voltage DC, low-voltage high-frequency AC (such as the output of electronic transformers), etc.

The output of the LED driving power supply is mostly a constant current source that can change voltage with the change of LED  forward voltage drop.

The core components of LED power supply include switching controller, inductor, switching components (MOSFET), feedback resistance, input filter, output filter and so on. According to the requirements of different occasions, but also input overvoltage protection circuit, input undervoltage protection circuit, LED  open circuit protection, overcurrent protection circuit.

Ⅱ. Classification by driving mode

(1) Constant flow

The output current of constant current drive circuit is constant, and the output DC voltage varies with the size of load resistance  in a certain range, the load resistance  is small, the output voltage is low, the load resistance  is larger, the output voltage is higher;

Constant current circuit is not afraid of load short circuit, but it is strictly prohibited to completely open load;

Constant current drive circuit driving LED  is more ideal, but relatively high price;

Note the maximum current and voltage values used, which limit the number of LEDs used.

(2) Stabilized type

When the parameters of the voltage regulator circuit are determined, the output voltage is fixed, and the output current is changed with the increase and decrease of the load;

Voltage regulator circuit is not afraid of load open circuit, but it is strictly prohibited to load completely short circuit;

To drive the  LED  voltage regulator circuit, each string needs to add the appropriate resistance  to make each string of  LED display brightness average;

Brightness is affected by voltage changes resulting from rectification.

 Ⅲ.  Advantages and disadvantages of the overall constant current and constant current working mode

Compared with the whole constant current, constant current by road although more shortcomings, the cost is relatively high. But it can really protect LED and prolong the life of LED.  so constant current by road is the future trend.

Ⅳ.  Shortage of LED driver power 

Reasons for the current shortage of LED drive power:

The technical personnel of the company producing LED lighting and related products does not know enough about the switching power supply, and the power supply can work normally, but some key assessments and electromagnetic compatibility are not enough, or there are certain hidden dangers;

Most LED power supply manufacturers are from the ordinary switching power supply transformation to do LED power supply, the characteristics of LED and use of understanding is not enough;

At present, there are almost no standards for LED.  most of which refer to switching power supply and electronic rectifier standards;

At present, most LED power supplies are not unified, so the quantity is mostly small. Small purchase volume, high price, and component suppliers are not very cooperative;

LED power supply stability: wide voltage input, high temperature and low temperature work, over temperature, over voltage protection and other problems have not been solved one by one;

First is the overall life of the drive circuit, especially key components such as: capacitor  life at high temperature directly affects the life of the power supply;

LED drivers should challenge higher conversion efficiency, especially when driving high-power LED.  because all the power not as light output is dissipated as heat, the power conversion efficiency is too low, affecting the LED energy saving effect;

At present, in the application of small power (1-5W), the proportion of constant current drive power supply cost has been close to 1/3, has been close to the cost of light source, to a certain extent, affect the market promotion.

Ⅴ. Cause of LED drive power invalid

Compared with LED light source, THE structure of LED drive power source is more complex, and there will be more trade-offs so that LED drive power source often fails before LED light source. According to statistics, the whole lamp failure in more than 80% of the cause is the failure of the power supply. There are many reasons leading to the failure of LED drive power supply, which can be summarized into the following categories.

1. Aging of electronic components

Including resistor, capacitor, diode, transistor, LED.  connector, IC and other devices open circuit, short circuit, burn, leakage, functional failure, electrical parameters unqualified, unstable failure and other failure problems.

2.  PCB  quality problem

Including PCB, PCBA poor wetting, board explosion, stratification, CAF, open circuit, short circuit and other failure problems.

3. LED power dissipation is poor

The drive circuit consists of electronic components, a few of which are very sensitive to temperature. For example, the electrolytic capacitor, the common life estimation formula of the electrolytic capacitor is "every 10 degrees of temperature reduction, life is doubled", poor heat dissipation is likely to lead to its life greatly shortened, failure in advance, resulting in LED voltage failure, failure of lamps and lanterns. Especially for built-in power supply (put in the whole lamp power supply), the heat source will increase the heat conduction of the whole lamp, heat dissipation pressure, LED temperature will rise, its light efficiency and life will be greatly reduced. So in the design of LED power supply, we should pay attention to its own heat dissipation problem. Therefore, the above problems can be solved by evaluating the lamp design at the beginning and synchronizing the design of the power supply. In the design, the heat dissipation of LED and power supply should be considered comprehensively, and the temperature rise of lamps and lanterns should be controlled as a whole, so as to design better lamps and lanterns.

4. Problems in power supply design

(1) Power design. Although LED light efficiency is high, but there are 80% ~ 85% of heat loss, resulting in 20 ~ 30K temperature rise inside the lamp, if the room temperature is 25℃, the lamp interior is 45 ~ 55℃, power supply for a long time in high temperature environment, to ensure the life must increase the power margin, generally retained 1.5 ~ 2 times the margin.

(2) Component selection. When the internal temperature of the lamp is 45 ~ 55℃, the internal temperature rise of the power supply is about 20℃, and the temperature of the component accessories should reach 65 ~ 75℃. Some components in high temperature parameters will drift, and even the life will be shortened, so the device to choose can be used for a long time at higher temperature, but also pay special attention to electrolytic capacitor and wire.

(3) Electrical performance design. Switching power supply for LED parameter design, mainly constant current parameters, the size of the current determines the brightness of LED, if the batch current error is large, the brightness of the whole batch of lights is not uniform. And temperature changes can also cause the power supply output current to shift. The general batch error is controlled within ±5% to ensure the consistent brightness of the lamp, and the forward voltage drop of THE LED has a deviation. The constant current voltage range of the power supply design should include the voltage range of the LED,  Multiple LED series use, the minimum voltage drop multiplied by the number of series for the lower limit voltage, maximum voltage drop multiplied by the number of series for the upper limit voltage, the constant current voltage range of the power supply is slightly wider than this range, the general upper limit each leave 1 ~ 2V margin.

(4)  board design. LED lamps leave a smaller size for the power supply (unless the power supply is external), so there are higher requirements and more factors to consider in PCB design. Safe distance should be left enough, input and output isolated power supply, primary side circuit and secondary side circuit should withstand voltage 1500 ~ 2500VAC, at least enough distance of 3mm on PCB. If it is a metal shell of the lamp, the entire power supply of the cloth board also considers the safety distance between the high voltage part and the shell. If there is no space to ensure a safe distance, other measures should be taken to ensure insulation, such as punching holes in the PCB, adding insulating paper, sealing insulation glue, etc. In addition, the board should also consider heat balance, heating elements should be evenly distributed, can not be placed centrally, to avoid local temperature rise. Electrolytic capacitor away from heat source, slow down aging, prolong service life.

5. Lightning damage

Lightning strikes are a common natural phenomenon, especially in the rainy season. The damage and losses are estimated at hundreds of billions of dollars a year worldwide. Lightning is classified into direct lightning and indirect lightning. Indirect lightning mainly includes conduction lightning and induction lighting. Because the energy impact brought by direct lightning is very large, extremely destructive, the general power supply is unable to withstand it, so the main discussion here is indirect lightning.

The surge shock caused by lightning is a transient wave and belongs to transient interference. It can be surge voltage or surge current. Along a power line or other path (conducting lightning) or through an electromagnetic field (inductive lightning) to a power line. The waveform is characterized by a rapid rise and then a slow fall. This phenomenon can have a fatal impact on the power supply, the instantaneous surge impact far beyond the electrical stress of ordinary electronic devices, resulting in the direct result of electronic components damage.

6. Power grid voltage exceeds power supply load

When the distribution of the same transformer power grid branch is too long and there is large power equipment in the branch, the power grid voltage will fluctuate violently when the large equipment starts and stops, and even lead to the instability of the power grid. It is possible to damage the driver when the grid transient voltage exceeds 310 VAC (not even with lightning protection, since lightning protection deals with pulse spikes of tens of microseconds, while grid fluctuations can be tens or even hundreds of milliseconds). Therefore, special attention should be paid to the large power machinery on the street lighting branch power grid. It is best to monitor the fluctuation of the power grid or power supply by a separate power grid transformer.

7. Solder joint failure

Power supply packaging mainly involves the connection process between PCB board and components, among which solder joint plays an important role. The main function of solder joints is to realize the mechanical connection and electrical connection of electronic components and substrates (FOR PCB board in LED power supply), and the quality of solder joints seriously affects the reliability of devices. On the one hand, solder joint failure comes from welding failure in production and assembly, such as solder bridge, virtual welding, void, Manhattan phenomenon. On the other hand, in the service process, when the ambient temperature changes, due to the thermal expansion coefficient difference between the components and PCB board, thermal stress will be generated in the solder joint, and the periodic change of stress will cause fatigue damage to the solder joint, and eventually lead to fatigue failure.