1. Background of high power LED temperature compensation scheme
1. Temperature compensation circuit for protecting high-power LED lighting
High-power LED lights have a theoretical life span of tens of thousands of hours, but the actual life span may be only a few thousand hours. The main reason is that the LED lamp is overheated and damaged without using temperature compensation technology. High-power LED lamps generally adopt constant current driver, whose output current does not change with the change of input voltage, load and ambient temperature. However, when the ambient temperature of the LED is higher than the safe working point temperature, the forward current of the LED will exceed the safe area, so that the life of the LED will be greatly reduced or even damaged. This is where constant current drives fall short, because keeping the output current constant only causes the LED's temperature to rise further. The solution is to use the temperature compensation circuit to continuously reduce the forward current value of LED, avoid LED damage due to high temperature. Therefore, temperature compensation for the output current of the high-power LED driver is an effective measure to greatly extend the service life of the high-power LED lamp.
2. Basic principle of temperature compensation
High power LED temperature compensation is the basic principle of the abnormal situation once LED, when temperature is too high, the LED driver can be detected according to the thermistor temperature, reduce the output current value automatically, make sure the LED work within the safety area, it is fundamentally solves the LED overheating damage or reduce the problem of service life, thus greatly improving the reliability of the LED lamps and lanterns and security; When the temperature drops to a safe area, the LED driver's output current will automatically return to normal. This LED driver with temperature compensation function represents the development direction of high-end LED driver and has important practical value.
The technical scheme of nonlinear temperature compensation includes the following contents: first, only linearize NTCR within the normal operating temperature range, which covers the lower limit temperature TL and the upper limit temperature TH(take TH as Tk); Second, when TA>TH, the output current Io of the high-power LED driver is rapidly reduced exponentially through the external control circuit, forcing the LED brightness to decrease.
It is usually unnecessary to doubt whether a decrease in the forward current of the LED will lead to a significant decrease in the brightness of the LED. According to weber-fechner's law, there is a logarithmic relationship between the subjective perception of human eyes and the change of objective brightness (which can be equivalent to the change of illumination on a white paper surface), and the relationship curve between them is shown in figure 9-8-1. As can be seen from the figure, when the illumination on the paper decreased from 1000kx to 100x, that is, reduced to 10% of the original, the human eye perceived that the brightness only decreased by 50%(the subjective brightness perception of the human eye decreased from 8 to 4). For example, even if the LED drive current decreases from 350mA to 175mA, that is, 50% of the original, this will reduce the objective brightness. However, the changes in brightness detected by the eyes after taking a logarithm are not so obvious.