The LED driver design requires a compliant, so is crucial to understand their properties and select the appropriate drive circuit based on the specific application. Such a dedicated drive circuit to rated voltage and current for these LEDs to create a favorable condition for their normal work.
To achieve the purpose of light, LEDs require a forward voltage to allow current flow. LED driver for LED forward bias in order to make it luminous. Level or brightness of the LED light is usually proportional to the size of the forward current. In addition, through the LED current should not exceed the rated current of equipment requirements, or it may cause permanent damage. Thus a constant current drive circuit to control the current drive LEDs in the correct level of the ideal solution. In other words, the LED 400W circuit is a constant current rather than constant voltage power conversion circuits. LED driver circuit should contain at least a voltage detection circuit and a current switching circuit.
Open light circuit sends a signal when the voltage detection circuit to detect when the power supply voltage level to the current, then the current switching circuit automatically starts to use a predetermined current value to set the LED electrical re-adjust in order to effectively light as much as possible the LED.
Linear regulator between the regulator output and ground nodes connected to a current sampling resistor, can provide a simple way to produce a constant current. A constant output voltage of the regulator to produce a constant current through the feedback resistor. The power supply reference voltage and current sampling resistor determine the LED Power Supply current. The linear regulator typically used to drive low-power LEDs, such as portable devices such as PDA backlight. Typical LED current between 15mA to 25mA, Vf between 3.0V to 3.4V. If the linear drive for the various LED power supply, these LEDs should be connected in series in order to ensure the same through all the LED current, so that the light output is roughly equal.
The advantages of the linear drive is the program cost and low electromagnetic interference, linear regulators need only a few resistors placed around the driver IC, and will not use the switching elements. Due to the linear drive output high voltage in order to provide the LED current, so the disadvantage of this program is less efficient, that is, the lower the ratio of the LED voltage and supply voltage. The major limitation of the linear regulator is the supply voltage is always higher than the LED Blue 150W voltage linear voltage source can not increase the output voltage, but only lowers the voltage to a certain extent. This inefficiency will cause the heat problem.
For wide input range of high-current applications, such as mentioned above, a simple drive to produce high heat and low efficiency. The switch driver with constant current output is the first choice of driving high-power LED. The drive is usually used to switch control of the series inductor and LED load or supply voltage on the parallel capacitor and LED. The inductance or capacitance is used to save energy; When the switch is turned on and then switch off the high power led current. Linear drive, the switch driver can be configured to achieve the voltage decreasing (buck), incremental (boost) or both co-exist. Therefore, it is clear switch drive to allow the LED to work in a wide input voltage range. In addition to the current adjustment function with a constant amount of light emitted, they can be energy losses to a minimum. Needless to say, the switching regulator is more efficient than linear regulators. However, compared to linear regulators, switching drive costs higher and the need for careful design of EMI problems. Appropriate way to drive the LED, you need to find a satisfactory performance and low cost.
Many LED applications require dimming functions, such as LED 200W backlight or architectural lighting dimmer. Can be achieved by adjusting the brightness and contrast of the LED dimming function. The lower device current may be able to adjust the LED illumination. But let the LED to less than the rated current may cause many adverse consequences, such as the problem of chromatic aberration.
Method to replace the simple current regulator is integrated in the LED drive pulse width modulation (PWM) controller. The PWM signal is not directly used to control the LEDs, but to control a switch, for example, a MOSFETs to provide the necessary current to the LED. The PWM controller is usually a fixed frequency and pulse width to be adjusted to match the required duty cycle. The most current of the LED chip to use PWM to control LED light. In order to ensure that people will not feel the flicker, the frequency of the PWM pulse must be greater than 100Hz.
The main advantage of PWM control is more accurate, so that the maximum degree of reduction of chromatic aberration in the light of this LED PWM dimming current.
Other features of the LED driver
Can emit more light than traditional lighting, high brightness LED occupied a place in many lighting applications. However, these LEDs will produce more heat than traditional LED. Therefore, LED drivers need overheat protection function to avoid the heat issue in the continuing work of the damage.
With a thermistor can achieve a thermal protection circuit for the LED power supply cut off when the temperature reaches the default value. In addition to thermal protection, there are other security issues need to be considered such as short circuit and open circuit protection.