LED driver characteristics

Because it can provide high efficiency, durability, long working life, environmental protection and superior visual effects, LEDs have increasingly appeared in industrial applications, electronic signs and signals, home and mobile devices, automobiles and consumer electronics. Therefore, the driver design of these LEDs has also received attention. In theory, the LED's average working life exceeds 100,000 hours. But the premise is the need to choose the appropriate driver and circuit design, which will not only affect the life of the LED, but also affect their performance.

The use of a current-limiting resistor to drive a standard 20-mA LED is a universally accepted common method that is widely used in LED indicator functions.

Simple LED Driver circuits are generally safer if they only contain conventional LEDs and the supply voltage does not fluctuate significantly. However, with the advent of high-power (HP) LED or high-brightness (HB) LED technology, these simple drivers are no longer suitable. HB or HP LEDs operate at higher current levels and require greater heat sinking capabilities, while standard resistors cannot meet such high requirements. The electrical characteristics of the LED may also vary depending on the type. In general, when the forward voltage (Vf) of the LED exceeds the forward voltage drop, even a slight increase in Vf leads to a large increase in the forward current (If). The rapidly increasing If will cause the LED to be brighter and hotter, thereby accelerating the LED loss and shortening the lifetime of the LED.

Based on the voltage-to-current rate-of-change characteristics, the characteristics of the LED driver. 络 络 全球 全球 全球 全球 全球 全球 LED LED LED LED LED LED LED LED LED drivers need a design that meets the requirements, so it is important to understand their characteristics and select the appropriate driver circuit based on the specific application. This dedicated drive circuit can provide the rated voltage and current for these LEDs, creating a good condition for their normal operation.

To achieve the purpose of lighting, the LED needs a forward voltage to allow the current to flow. Therefore, the LED driver provides forward bias to the LED to make it shine. The LED's luminous level or brightness is usually proportional to the magnitude of the forward current. In addition, the current through the LED should not exceed the rated current of the device as it may cause permanent damage.

The constant current drive circuit is thus an ideal solution to control the current at the correct level for driving the LED.

In other words, the LED drive circuit is a power conversion circuit that provides a constant current instead of a constant voltage.

The LED driving circuit should at least include a voltage detection circuit and a current switching circuit.

When the voltage detection circuit detects a different voltage level of the power supply, it sends a signal to the current switching circuit, and then the current switching circuit is automatically activated, and the electrical settings of the LED are re-adjusted using the predetermined current value, thereby effectively lighting As many LEDs as possible.

Linear Driver Linear regulators provide a simple way to generate a constant current by connecting a current sense resistor between the regulator output and the ground node. The constant output voltage of this regulator generates a constant current through the feedback resistor. The power supply reference voltage and current sampling resistance determine the LED current. Linear regulators are commonly used to drive low-power LEDs, such as backlights for portable devices such as PDAs. The typical current for these LEDs is between 15 mA and 25 mA and Vf is between 3.0 V and 3.4 V. If a linear driver is used to power multiple LEDs, these LEDs should be connected in series to ensure that the current through all LEDs is the same so that the amount of light is approximately equal.

The advantage of a linear driver is that the solution has lower cost and electromagnetic interference because the linear regulator only needs to place several resistors around the driver IC and no switching elements are used. Since the linear driver needs to output a very high voltage in order to provide the LED current, the disadvantage of this solution is that the efficiency is low, that is, the ratio of the LED voltage to the power supply voltage is low. The main limitation of the linear regulator is that the power supply voltage is always higher than the LED voltage, so the linear voltage source cannot increase the output voltage but only reduces the voltage to a certain extent. This inefficiency can cause fever problems.

Switch Drivers For high current applications with a wide input range, simple driver schemes such as those mentioned above produce higher heat output and lower efficiency. The switch driver with a constant current output is the first choice for driving high-power LEDs. This driver is typically used for on-off control of the series inductance and LED load or parallel capacitor and LED supply voltage. The inductor or capacitor is used to conserve power when the switch is turned on; then the switch is turned off to provide current to the LED. Unlike a linear driver, a switch driver can be configured to perform voltage bucking, boosting or a coexistence of both. It is therefore clear that the switch driver allows the LED to operate over a wide input voltage range. In addition to the current adjustment function with a constant amount of light, they also minimize power loss. There is no doubt that switching regulators are more efficient than linear regulators. However, compared to linear regulators, switch drivers are more costly and require careful design for EMI issues. In order to drive the LED in an appropriate way, it is necessary to find a most satisfactory performance-cost ratio.

PWM Dimming Many LED applications require dimming features such as LED backlighting or architectural lighting dimming. Dimming can be achieved by adjusting the brightness and contrast of the LEDs. Simply lowering the device's current may be able to adjust the LED's light emission. However, letting LEDs work below the rated current can cause many undesirable effects, such as chromatic aberration.

Instead of a simple current adjustment, a pulse width modulation (PWM) controller is integrated in the LED driver. The PWM signal is not directly used to control the LED but instead controls a switch, such as a MOSFET, to provide the LED with the required current. The PWM controller typically operates at a fixed frequency and adjusts the pulse width to match the desired duty cycle. Most LED chips currently use PWM to control LED lighting. To ensure that people do not feel obvious flicker, the frequency of the PWM pulse must be greater than 100 Hz.

The main advantage of PWM control is that the PWM dimming current is more accurate, which minimizes the color difference when the LED emits light.

Other features of LED drivers High-brightness LEDs occupy a place in many lighting applications because they emit more light than traditional lighting sources. But these LEDs produce more heat than traditional LEDs. Therefore, the LED driver needs overheating protection function to avoid being damaged by the heat emitted during continuous operation.

A thermistor can be used to implement an overheat protection circuit that shuts off the power of the LED when the temperature reaches a preset value. In addition to overheat protection, there are other safety issues that need to be considered, such as short circuit protection and open circuit protection.

Therefore, the ultimate choice of an appropriate LED driver not only depends on the performance of the LED, but also takes into account the use of the final product, taking into account dimming and heat dissipation issues. As with the high-power LED market, it is likely that the Argentinian LED driver IC market will maintain its current growth momentum for many years to come. With more and more diversification of high-power LED applications, manufacturers will provide more different LED driver solutions.