LED drive characteristic analysis

29 May

Based on the current rate of voltage characteristics, high density led needs a to meet the requirements of design, so get their characteristics and application based on particular choice of an appropriate driver circuit is very important. This special driving circuit for these LED to provide the rated voltage and current, for their normal work to create a good condition.

To achieve the purpose of the light, LED to a positive voltage to let the current flow. So LED to provide a positive drive LED to make the light bias. The LED light grade or brightness is usually the size of the current is priced. In addition, through the current LED should not be more than equipment rated current of the provisions. Otherwise may cause permanent damage. Therefore constant current driver circuit will drive current control is in the right level of LED an ideal scheme. In other words, the LED drive circuit is a kind of providing a constant current and voltage of the power supply, the unsteady transform circuit. LED drive circuit at least should contain a voltage detection circuit and a current switch circuit.

When the voltage of power to detect detection circuit different voltage grade, dedication to the current circuit sends a signal, then current switch circuit was automatic startup, use the predetermined value to the electrical set of LED adjustment, thus effectively lit led as much as possible.

Linear driver

Linear regulators through the output voltage stabilizer and grounding in between nodes connected to a current sampling resistance, can provide a simple method of constant current generated. The constant voltage stabilizer output voltage through the feedback resistance to generate the constant current. Power supplies reference voltage and current to the sampling resistance LED current. Linear regulators are usually used to drive the low power LED, such as PDA this portable equipment a backlight. The typical current value in these LED 15 mA to 25 between mA, Vf in 3.0 V to 3.4 V between. If line driver used for multiple LED the power supply, then these LED should be series, to ensure that all of the current Of by the same, so that the shine amount roughly equal.

The advantages of the linear drive are plan cost and electromagnetic interference is low, because linear regulators only need to drive around in IC on several resistance, will not use switch components. Because linear drive needs high voltage output to provide LED current, so the scheme are the shortcomings of low efficiency, which LED to the ratio of the voltage and low voltage power supply. Linear regulators major limitation of the power supply voltage is always higher than LED voltage, and linear voltage source can’t improve output voltage, and will only lower voltage to a certain extent. This kind of low efficiency can cause fever problem.
Switch drive

a wide range of input for the high current application for, such as mentioned above will produce simple drive scheme of higher heat and low efficiency. And with constant current output switch drive is driving high power LED the first choice. The driver is usually used to the series inductance load or in parallel and LED the capacitance and led the power supply voltage for switch control. The inductor or capacitor is used when the switch when open save electric energy; and then in the switch is closed for LED provide current. And different line driver, switch drive can be configured to realize voltage decreasing (buck), increasing (boost) or both the function of coexistence. So obviously switch drives allow LED in a wider range of input voltage. In addition to the amount of light with constant current adjustment function, and they can be energy loss to a minimum. Beyond doubt are switching voltage regulators than linear regulators more efficient. But compared with linear regulators, switch drive cost and need to be higher for EMI problem of careful design. In order to pass the appropriate way to drive LED, need to find the most satisfactory performance to price.

PWM dimmer

Many LED applications need dimmer functions, such as LED back lighting or building lighting dimmer. LED by adjusting the brightness and contrast of can realize the function of light. Simply reduce device of current may be able to adjust to LED. But let LED in less than rated current situation of work has LED to many bad results, such as color difference problem.

Instead of a simple method of adjusting current is led strip on the drive of integrated pulse width modulation (PWM) controller. The PWM signal is not used to control LED directly, but controls a switch, for example a MOSFET, LED to the current to provide. PWM controller usually have a fixed frequency to work and pulse width adjustment, to match the required v. The current most LED chips are used to control PWM LED. To ensure that people won’t be apparent to shine, PWM pulse frequency must be more than 100 Hz.

The main advantage of the PWM control through the optical the PWM is more accurate, so that the greatest degree of reduced when LED the brightness of the photo.

The other characteristic LED drive

Because it is more than traditional lighting of the light, high brightness Flexible LED Strip Lights applications in many occupied a place. But these LED will produce more heat than traditional LED. Therefore, the LED drive needs to avoid overheating protection function in continuous work out in the quantity of heat of the damage.

Use a thermal resistor can realize an overheating protection circuit, used in when the temperature reaches a default value of the power cut off LED. In addition to overheat protection, and other security issues to consider, for example, short circuit protection and open circuit protection.

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