introduction
As production costs decrease, LEDs are becoming more widely used, including handheld devices, automotive, and architectural lighting. High reliability, excellent efficiency and transient responsiveness make them a good source of light. Although the cost of incandescent bulbs is low, replacing incandescent bulbs multiple times will be a significant expense. A good example of a street light is that a team of workers and a truck need to replace the faulty light bulb. Therefore, in such applications, the use of LEDs can greatly reduce costs. Although LEDs and incandescent bulbs are nearly identical in efficiency, in streetlight applications, LEDs are sometimes used instead of incandescent bulbs for some reason, which not only improves reliability, but also saves energy.
Incandescent bulbs can emit a wide variety of light, but in specific applications, only green, red, and yellow light are usually required—such as traffic lights. To use an incandescent bulb, you need a filter that wastes 60% of the light, and the LED produces the light of the desired color directly, and at power-up, the LED is almost instantaneous, while the incandescent A response time of 200ms is required. Therefore, LEDs are used in the design of brake lights. In addition, LEDs will be used as light sources in DLP video applications to replace mechanical assembly, which enables high frequency switching.
LED IV characteristics
Figure 1 shows the forward voltage characteristics of a typical InGaAlP LED (yellow and amber red). It is also possible to model the LED as a voltage source in series with the resistor and to see a good correlation between the model and the actual measurement. The voltage source has a negative temperature coefficient. When the junction temperature rises, the forward voltage of the voltage source changes negatively. The InGaAlP LED has a coefficient between -3.0mV/K and -5.2mV/K, while the InGaN LED (blue, green, and white) has a coefficient between -3.6mV/K and -5.2mV/K. This is why it is not possible to directly connect the LEDs in parallel. Devices that generate the most heat require more current, and larger currents generate more heat, which causes the heat to run out of control.
Figure 1: LED is modeled in series as a resistor and voltage source
Figure 2 shows the relative light output (light flux) as a function of operating current. Obviously, the light output is closely related to the diode current, so dimming can be done by changing the forward current. Also, when the current is small, the curve is almost a straight line, but as the current increases, the slope becomes smaller. That is to say, when the current is low, if the diode current is doubled, the light output will be doubled; but when the current is high, this is not the case: the current rises by 100% only makes the light The output is increased by 80%. This is important because the LEDs are driven by a switching power supply, which causes considerable ripple current in the LEDs. In fact, the cost of the power supply is determined to some extent by the amount of current allowed. The higher the ripple current, the lower the power supply cost, but the light output is affected.
Figure 2: LED current is reduced when the current exceeds 1A
Figure 3 quantifies the reduction in light output caused by the triangular ripple current superimposed on the DC output current. In most cases, the ripple current has a frequency higher than 80 Hz that can be seen by the naked eye. Moreover, the naked eye's response to light is exponential and it is not noticeable that less than 20% of the light is diminished. Therefore, even if a considerable ripple current appears in the LED, the reduction in light output is not perceived.
Figure 3: Slight ripple effect on LED light output
Ripple current also affects LED performance by increasing power consumption, which can cause junction temperatures to rise and have a significant impact on LED lifetime.
Figure 4 quantifies the increase in LED power consumption due to ripple current. Compared to the LED's heat dissipation time constant, the high ripple current (and high peak power dissipation) does not affect the peak junction temperature due to the higher ripple frequency, which is determined by the average power dissipation. The high voltage drop of the LED is like a voltage source, so the current waveform has no effect on power consumption. However, the voltage drop has a resistive component and the power dissipation is determined by the resistance multiplied by the square of the root mean square (RMS) current.
Figure 4: Ripple current increases the power consumption of the LED
Figure 4 also illustrates that there is no significant impact on power consumption even when the ripple current is large. For example, 50% ripple current only increases power loss by less than 5%. When this level is greatly exceeded, it is necessary to reduce the DC current of the power supply to keep the junction temperature constant, thereby maintaining the lifetime of the semiconductor. The rule of thumb shows that for every 10% reduction in junction temperature, the lifetime of the semiconductor is doubled. Also, many designs tend to have smaller ripple currents due to inductor limitations. Most inductors are designed to handle less than 20% of the Ipk/Iout ripple current ratio.
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