LED drive characteristics analysis

by:MOSO     2020-03-07
Based on the rate of change of voltage - current characteristics of the LED driver needs a conform to the requirements of the design, so to understand their characteristics, and select the appropriate driver circuit based on specific application is crucial. In this way can a dedicated driver circuit for these leds to provide the rated voltage and current, as they work to create a good condition. To achieve the purpose of the light, LED to a forward voltage to current flow. So the LED driver for the LED forward bias to make it shine. Rank or brightness LED light-emitting generally proportional to the size of the forward current. In addition, the current through the LED should not exceed rated current of the equipment, otherwise may cause permanent damage. Thus constant current drive circuit is to control the current drive LED the right level of ideal solution. In other words, the LED driver circuit is to supply constant current instead of a constant voltage power conversion circuit. The LED driver circuit should contain at least one voltage detection circuit and a current switch circuit. When the voltage detection circuit to detect the different voltage levels, to send a signal to current medallion circuit, current switch circuit are then automatically start, using a predetermined current value to readjust LED electric set, thereby effectively LED light as much as possible. Linear drive linear voltage regulator through the connection between the voltage regulator output terminal and earthing point a current sampling resistance, and can provide a simple way to generate constant current. The voltage regulator of constant output voltage through the feedback resistor to generate the constant current. Reference voltage and current sampling resistor determines the LED current. Linear voltage regulator is usually used to drive low power LED, such as portable devices such as PDA backlight. Typical of these LED current value between 15 mA to 25 mA, Vf in 3. 0 V to 3. 4 v. If the linear drive for more than one LED power supply, the LED should be in series, in order to ensure that through all the LED current is the same, so that the count was roughly equal. Linear actuator is the cost of the scheme and the advantages of low electromagnetic interference, because linear regulators need only in driver IC placed around several resistance, will not use the switch components. Due to the linear actuator output high voltage is needed to provide the LED current, so the scheme of defect is low efficiency, which LED the ratio of voltage and power supply voltage is low. Linear regulator is the main limitation of the power supply voltage is always higher than that of the LED voltage, thus linear voltage source can't increase the output voltage, voltage and can only be reduced to a certain extent. The low efficiency problem will cause fever. Switch drive for high current application wide input range, such as a simple drive method mentioned above can produce high calorific value and low efficiency. And with a constant current output switch drive is driving the first selection of high power leds. The drive is usually used in the series or parallel capacitance and inductance and the LED load of the power supply voltage on the LED switch control. The inductor or capacitor is used when the switch opens to save electrical energy; Then in the switch is closed for LED current. Unlike linear actuator, switch drive can be configured to achieve voltage decreasing ( 巴克) Increasing, ( 提高) Or both coexistence of function. So obviously switch drive allows the LED to work on a wide range of input voltage. Besides having a constant amount of luminous current adjustment function, they can minimize power loss. There's little doubt that the switch voltage regulator more efficient than linear regulators. However, compared with the linear voltage regulator, switch drive cost higher and need a careful design for EMI problems. In order to pass the appropriate way to drive the LED, need to find a most satisfying performance/cost ratio. PWM dimming LED many applications require the dimming function, such as the LED backlight, or building lighting dimming. By adjusting the brightness and contrast can realize adjustable LED light function. Simple reduction device of the current may be able to adjust LED. But let the LED in the case of lower than the rated current work will cause many adverse consequences, such as color difference problem. Replace the simple current adjustment method is integrated in the LED driver pulse width modulation ( 脉宽调制) The controller. PWM signal is directly used to control the LED, but to control a switch, a MOSFET, for example, to provide the required for the LED current. PWM controller usually work on a fixed frequency and the pulse width adjustment, to match the required duty cycle. Currently most of the LED chip using PWM to control the LED. In order to ensure that people won't feel obvious scintillation, PWM pulse frequency must be greater than 100 Hz. Main advantage of PWM control is more accurate by PWM adjustable photocurrent, thus reduces the LED to a great extent when the brightness of the photo. LED dimming drives other features because it is more than the traditional lighting light source of light, high brightness LED occupied a place in many lighting applications. But these leds will generate more heat than conventional leds. Therefore, LED drive to overheating protection function to avoid in the continuous work was damaged by heat. With a thermistor can realize an overheat protection circuit, for when the temperature reaches the default value to cut off the LED power supply. In addition to overheating protection, and other security issues to consider, such as short circuit protection and circuit protection.
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