As LED production costs decline, their use is becoming more common, covering applications ranging from handheld devices to in-vehicles to architectural lighting. The high reliability of LEDs (lifetime over 50,000 hours), high efficiency (>120 lumens/watt) and near-instantaneous responsiveness make it an attractive source of light. Compared to the response time of an incandescent bulb of 200 mS, the LED will illuminate in a short 5 ns response time. Therefore, they are currently widely used in brake lights in the automotive industry.
Drive LED
Driving LEDs is not without challenges. The adjustable brightness requires a constant current to drive the LED and it must be kept constant regardless of the input voltage. This is more challenging than simply connecting an incandescent bulb to a battery to power it.
The LED has a forward VI characteristic similar to a diode. Below the LED turn-on threshold (the white LED's turn-on voltage threshold is approximately 3.5V), the current through the LED is very small. Above this threshold, the current will increase exponentially in the form of a forward voltage. This allows the LED to be shaped as a voltage source with a series resistor with a warning that the model is only valid at a single operating DC current. If the DC current in the LED changes, the resistance of the model should also change to reflect the new operating current. At large forward currents, power dissipation in the LEDs can cause the device to heat up, which will change the forward voltage drop and dynamic impedance. It is very important to fully consider the heat dissipation environment when determining the LED impedance.
When driving LEDs through a buck regulator, the LEDs often conduct the AC ripple current and DC current of the inductor based on the selected output filter arrangement. This not only increases the RMS amplitude of the current in the LED, but also increases its power consumption. This will increase the junction temperature and have a significant impact on the life of the LED. If we set a 70% light output limit as the life of the LED, then the life of the LED will be extended from 15,000 hours at 74 degrees Celsius to 40,000 hours at 63 degrees Celsius. The power loss of the LED is determined by multiplying the LED resistance by the square of the RMS current plus the average current multiplied by the forward voltage drop. Since the junction temperature can be determined by the average power consumption, even a large ripple current has little effect on power consumption.
For example, in a buck converter, the peak-to-peak ripple current equal to the DC output current (Ipk- pk = Iout) increases the total power loss by no more than 10%. If you exceed the above loss level, you need to reduce the AC ripple current from the Power Supply to keep the junction temperature and operating life constant. A very useful rule of thumb is that for every 10 degrees Celsius drop in junction temperature, semiconductor lifetime will be doubled. In fact, most designs tend to have lower ripple currents due to the suppression of the inductor. In addition, the peak current in the LED should not exceed the maximum safe operating current rating specified by the manufacturer.
Topology selection
The information shown in Table 1 helps to select the best switching topology for the LED driver . In addition to these topologies, you can use simple current limiting resistors or linear regulators to drive LEDs, but such methods typically waste too much power. All relevant design parameters include input voltage range, number of LEDs driven, LED current, isolation, EMI suppression, and efficiency. Most LED driver circuits fall into the following topological types: buck, boost, buck-boost, SEPIC, and flyback.
Table 1: Alternative LED Power Topology
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