LED Light Drive Technology_Part I

Current Status of LED Lights

Science and technology are advancing with each passing day, and science and technology create a new life for mankind. In recent years, the global human consensus: the future of industries related to green, environmental protection and energy saving is bright and brilliant. The LED lighting industry is the first to bear the brunt, and it will show infinite vitality and an infinite market. In 2018, the LED lighting industry was not at all affected by the global economic crisis. Under the vigorous promotion of governments of various countries, the LED lighting industry is rapidly gaining popularity. 2018 is the first year of LED lighting, and the LED lighting industry chain is growing and improving rapidly in this economic crisis. The rapid development of the new generation of LED lighting technology has driven the prosperity of millions of companies in the LED lighting industry chain.

The LED light source is a dazzling star. Based on LEDs, it can be used for various and ever-changing lighting and lighting fixtures, LED advertising large screens, LED information screens, mobile phone and other consumer electronics display backlights, notebook computers and LCD-TV display backlights.

Basic Architecture of LED Lighting Fixtures

The basic structure of LED lights is: LED light source + drive power + lamp metal/plastic structural parts. LED light source relates to the brightness, color and life of LED lighting products. The driving power supply relates to the electrical performance and stability, reliability and life of LED lighting products. Metal/plastic structural parts are related to the heat dissipation performance and fashionable style of LED lighting products, as well as different applications.

LED Light source Is Highly Energy-efficient and Has o Carbon Emissions

LED light source is a new generation of solid-state light source. As a fourth-generation lighting fixture with green, energy-saving, power-saving, and long-life LED light sources, LED light sources have sprung up, attracting wide attention and are developing rapidly.

When the fourth generation of new energy-saving light sources were born, LED light sources were used as light sources for various fixtures. As the light source, the luminous efficiency of the incandescent lamp is only 5%, while the luminous efficiency of the LED light source is almost close to 90%.

LED lighting has become the focus of widespread attention due to its high energy saving, long life, and environmental protection characteristics. In recent years, high-brightness LED light source manufacturing technology has advanced by leaps and bounds, and its production cost has been steadily decreasing. Nowadays, almost all fixtures can use LED light source. Because LED is a high-brightness, high-efficiency, power-saving, and carbon-free light source.

LED Light Source Needs Driver IC to Light up

The current DC LED light source is a low voltage (VF=2→3.6V), high current (IF=200→1500mA) working semiconductor device. The LED must be provided with a suitable DC constant current to normally emit light.

The technology of direct current (DC) driving LED light source to emit light has become more and more mature. Since the power supply we use for daily lighting is high-voltage alternating current (AC100-220V), we must use step-down technology to obtain a lower voltage.

Commonly used is the switching power supply step-down, and then the alternating current (AC) is converted into direct current (DC), and then converted into a DC constant current source by the driver IC to promote the LED light source to emit light. Therefore, the system application scheme of DC driving LED light source must be: switching power supply + constant current source.

There must be a certain amount of space in the LED fixtures to house this power module. But for the E27 standard screw fittings, the space is very limited and it is difficult to install. Whether it is a switching power supply or other methods to reduce the voltage, the system will have a certain amount of loss. When the DC LED lamps are converted between AC and DC power, about 8-25% of the power is lost, and the system efficiency is difficult to achieve more than 90%. Therefore, improving conversion efficiency has always been a goal pursued by power chip and power supply manufacturers.

Common LED Light Source Driver IC

At present, common LED light source driver ICs are borrowed from general-purpose power supply ICs. There are still very few driver ICs designed for LED light sources. Many companies are in the process of designing, and there are already many available in the market.

At present, the low-cost and unsafe scheme of LED light source driving is realized by the method of resistance-capacitance step-down, such as the previous Christmas lights. CCR and LDO can also be used for low-cost drive, and their drive current is not large. CCR can only drive LEDs below 0.5W, such as a straw hat lamp of 0.06W. LDO can drive 1W LED with voltage stabilization method, but it is detrimental to the life of LED light source. DC/DC→constant current source may be the most ideal solution currently known.

Characteristics of DC/DC constant current drive IC

The DC/DC constant current source driver IC directly provides constant DC power for the LED light source. The most commonly used is to use DC/DC-Buck as a step-down constant current source. The step-down constant current source is like taking water from a large reservoir, so the constant current characteristics are stable and the supply capacity is strong. Using DC/DC-Boost as a step-up constant current source is like pumping water up from a channel, and the output voltage can be higher than the input voltage. If the output current is required to be large, the current needs to be expanded.

LED light sources are often used in series or parallel to improve the brightness of the lamps. Due to the discreteness of LED light sources, multiple series applications are more reasonable than parallel applications. Because the series connection can reduce the uniformity requirements of the LED light source.

Using DC/DC-Buck as a step-down constant current source, the total voltage required after multiple LED light sources are connected in series cannot be higher than the input voltage of the DC/DC-Buck. If it can lower 2V is the most ideal, then the efficiency of the constant current source can be greater than 90%.

For this reason, some main indicators of the new generation of DC/DC-Buck constant current source IC will be qualitatively improved, Vin=5-70V, or even Vin=5-100V. Such chips can reach the basic conditions for automotive electronics applications. The output voltage can also meet the requirements of a string of close to 20-30 LED light sources. The key to increasing the voltage of Vin is the manufacturing process of the integrated circuit.

Accuracy of output constant current

The output constant current accuracy is also an important indicator worthy of attention. It is related to the consistency of the constant current drive power supply in mass industrial production. For example, the inconsistency of the output current of the constant current drive power supply due to the different constant current accuracy of the DC/DC-Buck constant current source IC output will affect the difference in the brightness of the mass-produced LED lamps.

Therefore, the constant current output accuracy of the new generation of DC/DC-Buck constant current source IC is required to be 2%-1%. Over-current, over-voltage, over-temperature, and open-circuit protection are necessary for this type of IC. Switching frequency jitter technology can effectively reduce the EMI of the application circuit, so it will be used more and more. The built-in MOSFET should not be too large. One is to take into account the cost of the chip, and the other is to consider the heat dissipation of the chip itself when it is working. Generally, the output current of a single chip is below 2A.

In addition, since 750mA to 1500mA current flows during operation, such as on a silicon die similar to 1.4X1.1mm, if these currents are passed for a long time, power consumption will inevitably occur, and heat will inevitably be generated.

Physical heat dissipation structure

The physical heat dissipation structure of the chip itself is also very important. The package of the driver chip should be conducive to the rapid heat dissipation of the driver chip die. For example, the die (Die) is directly bound to the copper board, and a Pin is directly extended to the outside of the package, which is convenient for direct soldering on the copper foil of the PCB board to quickly dissipate heat.

The highly integrated function of DC/DC constant current source driver IC is the key to simplifying the application circuit. For example, PT4115 only needs four components around in application, which greatly saves application costs.

(To Be Continued)