LED lighting power topology

As the cost of LED production has fallen, its use has become more common. The range of applications covered ranges from hand-held terminal equipment to automotive and architectural lighting. The high reliability of the LED (with a lifetime of more than 50,000 hours), high efficiency (>120 lm/Watt) and near instantaneous response make it an attractive light source. Compared with the response time of an incandescent light bulb of 200 mS, LEDs emit light in response time of just 5 ns. As a result, they are currently widely used in brake lights in the automotive industry.

Drive LEDs

Driving LEDs is not without challenges. Adjustable brightness requires a constant current to drive the LED, and the current must be kept constant regardless of the input voltage. This is more challenging than simply connecting an incandescent bulb to the battery to power it.

The LED has a forward VI characteristic similar to that of a diode. Below the LED turn-on threshold (the turn-on voltage threshold of a white LED is approximately 3.5V), the current through the LED is very low. Above this threshold, the current increases exponentially in the form of forward voltage. This allows the LED to be shaped as a voltage source with a series resistor with a caveat that this model is only valid at a single operating DC current. If the DC current in the LED changes, then the model's resistance 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 the LED is driven by a buck regulator, the LED often conducts the inductive AC ripple current and DC current according to the selected output filter arrangement. This will not only increase the RMS amplitude of the current in the LED, but it will also increase its power consumption. This increases the junction temperature and has a significant effect on the lifetime of the LED. If we set a 70% light output limit as the lifetime of the LED, the lifetime 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 an 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 it goes far beyond the above loss levels, then the AC ripple current from the power supply needs to be reduced in order to keep the junction temperature and operating life constant. A very useful rule of thumb is that for every 10 degrees Celsius decrease in junction temperature, the lifetime of the semiconductor will increase by a factor of two. In fact, most designs tend to have lower ripple current due to the inductor's suppression. In addition, the peak current in the LED should not exceed the manufacturer's maximum safe operating current rating.