LED driver characteristics analysis
Based on the voltage-current rate-of-change characteristics, LED drivers require a design that meets the requirements, so it is important to understand their characteristics and select the appropriate drive circuit based on the specific application. Only in this way can the dedicated driving circuit provide rated voltage and current for these LEDs, creating a good condition for their normal operation.
To achieve the purpose of emitting light, the LED needs a forward voltage to let the current flow. Therefore, the LED driver must provide forward bias to the LED in order to make it emit light. The luminous level or brightness of the LED is usually proportional to the magnitude of the forward current. In addition, the current through the LED should not exceed the rated current specified by the device, otherwise it may cause permanent injury.
Therefore, the constant current drive circuit is an ideal solution to control the current at the correct level to drive 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 drive circuit should at least include a voltage detection circuit and a current switching circuit.
When the voltage detection circuit detects different voltage levels of the power supply, it sends a signal to the current opening circuit, and then the current switching circuit is automatically activated, using a predetermined current value to readjust the electrical settings of the LED, thereby effectively lighting As many LEDs as possible.
Linear drive
Linear regulators can provide a simple method of generating a constant current by connecting a current sampling resistor between the regulator output and the ground node. The constant output voltage of the regulator generates a constant current through a feedback resistor. The power supply reference voltage and current sampling resistor determine the LED current. Linear regulators are commonly used to drive low-power LEDs, such as the backlight of portable devices like PDAs. Typical current values ​​for these LEDs are between 15 mA and 25 mA, and Vf is between 3.0 V and 3.4V. 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 emitted is approximately equal.
The advantage of the linear driver is that the cost of the solution and the electromagnetic interference are lower, because the linear regulator only needs to place a few resistors around the driver IC, and does not use switching elements. Since the linear driver needs to output a very high voltage in order to provide LED current, the disadvantage of this scheme is lower efficiency, that is, the ratio of the LED voltage to the power supply voltage is lower. 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 can only reduce the voltage to a certain degree. This inefficiency can cause heat problems.
Switch driver
For high current applications with a wide input range, simple driver solutions such as those mentioned above will produce higher heat generation and lower efficiency. The switch driver with constant current output is the first choice for driving high-power LEDs. The driver is usually used to switch the series inductance and LED load or the parallel capacitor and the supply voltage on the LED. The inductance or capacitor is used to save power when the switch is turned on; then to provide current for the LED when the switch is turned off. Unlike linear drivers, switch drivers can be configured to implement voltage buck, boost or coexistence. It is therefore obvious that the switch driver allows the LED to work over a wide input voltage range. In addition to the current adjustment function with constant luminescence, they can 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 expensive and require careful design for EMI issues. In order to drive LEDs in an appropriate way, it is necessary to find a most satisfactory price-performance ratio.
PWM dimming
Many LED applications require dimming functions, such as LED backlighting or architectural lighting dimming. The dimming function can be achieved by adjusting the brightness and contrast of the LED. Simply reducing the current of the device may be able to adjust the LED light emission. But let the LED work under the rated current will cause many undesirable consequences, such as color difference.
The method to replace the simple current adjustment is to integrate a pulse width modulation (PWM) controller in the LED driver. The PWM signal is not directly used to control the LED, but to control a switch, such as a MOSFET, to provide the required current to the LED. The PWM controller usually works on a fixed frequency and adjusts the pulse width to match the required duty cycle. Most current LED chips use PWM to control LED lighting. To ensure that people do not feel noticeable flicker, the frequency of the PWM pulse must be greater than 100 Hz.
The main advantage of PWM control is that the dimming current through PWM is more accurate, which minimizes the color difference when the LED emits light.
Other characteristics of LED drivers
Because it can emit more light than traditional lighting sources, high-brightness LEDs have occupied a place in many lighting applications. But these LEDs will generate more heat than traditional LEDs. Therefore, the LED driver needs an overheat protection function to avoid being damaged by the heat emitted during continuous operation.
A thermistor can be used to implement an overheat protection circuit, which is used to cut off the LED power when the temperature reaches a preset value. In addition to overheating protection, there are other safety issues that need to be considered, such as short circuit protection and open circuit protection.
A 220V inverter, often referred to as a power inverter, is a device designed to convert direct current (DC) power into 220V alternating current (AC) power. It is commonly used in situations where AC power is not readily available or as a backup power source during power outages. The 220V output voltage is suitable for operating a wide range of electronic devices and appliances, such as laptops, televisions, refrigerators, and power tools.
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