Constant current drive of the hottest white LED

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Constant current drive of white LED

when using white LED as backlight for display or other lighting equipment, it is necessary to drive it with constant current. The main reasons are:

1 Avoid driving current exceeding the maximum rated value and affecting its reliability

2. Obtain the expected brightness requirements, and ensure the consistency of brightness and chromaticity of each LED

this paper describes the parameter range and constant current drive circuit of typical led

Figure 1 shows the relationship between the forward current and the forward voltage of six randomly selected white LEDs (three of which are from two top manufacturers). In this case, if the six LEDs are driven by 3.4V, the corresponding positive 4) turn on the power supply, and the current difference is large: 10mA to 44ma

Figure 1 The corresponding relationship curve between the forward current and the forward on voltage of six randomly selected white LEDs (three of which are from two top manufacturers). Note that for any given voltage, the forward current varies widely from -10ma to 44ma (at 3.4V)

in order to ensure reliability, the current of driving LED must be lower than the requirements of LED rated value, and the typical maximum value is generally 30mA. However, it can be seen from Figure 2 that the allowable rated current will drop when the ambient temperature rises, which is expected to be helpful to everyone. Generally, when the temperature reaches 50 ° C, the current needs to be limited within 20mA. By observing figures 1 and 2, it is not difficult to draw a conclusion that the scheme of driving white led by constant voltage is less reliable

Figure 2 In general, the maximum absolute value of the forward current of white LED decreases with the increase of ambient temperature (courtesy Nichia Corporation) because the density of titanium alloy is much smaller than that of steel

in addition, the consistency of brightness and chromaticity can be achieved by driving white LED with constant current. Figure 3 shows several general white LED drive circuits

Figure 3 For a typical white LED, its electrical characteristics are usually tested at if = 20mA. Therefore, in order to obtain predicted and matched brightness and chroma, constant current drive (courtesy Nichia Corporation) is recommended

Figure 4 shows four commonly used power circuits for driving LEDs. Figure 5 shows the current adjustment accuracy obtained when the above six LEDs are adjusted accordingly. In Figure 5, the output load line of the regulator is drawn on the VF curve of the LED, and the intersection of the two curves is the adjustment point of each LED

Figure 4 White LEDs usually have four different driving circuits: (a) voltage source and ballast resistance, (b) current source and ballast resistance, (c) multiple current sources, and (d) one current source drives the series LED

Figure 5 The forward voltage (VF) of each white LED has different effects on the accuracy of regulating current, depending on the structure of the regulating circuit: (a) voltage source and ballast resistance, (b) current source and ballast resistance, (c) multiple current sources or one current source drives the series LED. The VF curve of six LEDs (three from manufacturer a and manufacturer b) is shown in the figure. The intersection of the output load curve of the regulator and the LED VF curve is the stable adjustment working point

the circuit shown in Figure 4A uses a voltage stabilizing source in conjunction with a ballast resistor to control the current of the LED. The advantage of this structure is that there is a lot of room to choose a voltage source, and only one connection end point is required between the regulator and the LED; The disadvantage is low efficiency, which is mainly caused by the loss of ballast resistance. In addition, its control of led forward current is not very accurate. From the test curve in Figure 5a, we can see that the current variation range of the six different LEDs is 14.2ma to 18.4ma. The average brightness of the LED provided by manufacturer a is higher than that of manufacturer B, and the working current is 2mA higher

the circuit shown in Figure 4b is used to adjust the total current of the LED, and the ballast resistance is used to realize the matching between the LEDs. Max1910 adopts this structure. This circuit can achieve better results when driving the same batch of products provided by the same manufacturer. Under the condition of providing the same current as the above circuit, the small current resistance can be reduced and the power consumption can be reduced by half. Figure 5B shows the variation range of six different LED driving currents: 15.4ma to 19.6ma. The LED current provided by manufacturer a changes less. The average control current of LEDs from manufacturer a and manufacturer B is the same: 17.5ma. The defect of this structure with declining production cost is that the power consumption of ballast resistance is still large, and the current matching of each LED is not very good. However, this circuit compromise takes into account the performance and simplicity of the circuit

figure 4C can adjust the current of each LED separately without ballast resistance. The accuracy and matching degree of current regulation depend on each independent current regulator. Max1570 adopts this current source structure, with current accuracy of 2% and matching degree of 0.3%. Because the current regulator allows a low differential pressure, higher efficiency can be obtained. Figure 5C shows that the current of all six white LEDs tested is kept at a stable 17.5ma. Due to the elimination of ballast resistance, the area of circuit board can be effectively saved, but four connection terminals are required between the regulator and the LED. This kind of circuit can provide higher performance index and is a competitive scheme based on inductance structure

figure 4D is a boost circuit based on inductance, which is configured as a current regulator with high conversion efficiency. The lower feedback threshold further reduces the power consumption of the current detection resistance. In addition, because the LED is connected in series, the brightness of the LED can be kept consistent under any working conditions. The current accuracy depends on the accuracy of the feedback threshold of the regulator and is not affected by the change of the led forward conduction voltage. Max1848 and max1561 are two typical examples of this current regulation circuit. The conversion efficiency (pled/pin) can reach 87% (3 LEDs in series) or 84% (6 LEDs in series). Another advantage of this circuit is that only two connection terminals are needed between the regulator and the LED, which provides a certain degree of flexibility for users' design. However, due to the inductance used in the circuit, compared with the above scheme, the size is larger, the cost is higher, and the EMI radiation is larger. (end)

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