The research firm IHS just released a 2014 forecast for the LED general lighting market, which indicated that mid-power LEDs would represent >80 percent of the LED units shipped and roughly 48 percent of the revenue. This is quite a change from a few years ago where revenue of high power LEDs dominated the general LED lighting market. There are numerous factors that have driven this shift including improved LED efficacy, adoption of lower cost plastic packaging, and a shift from magnetic transformers to mechanical approaches to achieve safety isolation. This is especially true in integral bulbs and as a result the LED string voltage is no longer limited to 30 to 60 V depending on the regional safety requirements.
In some cases mid-power LEDs are a combination of several low power LEDs in series in a single package. As a result, high voltage mid-power low current LEDs are available with typical forward voltages ranging from 9 to 100 V. One impact of using these LEDs is that alternate driver topologies instead of the classical isolated flyback can be used that result in lower electronics cost and/or higher power conversion efficiency.
Figure 1. Single LED string direct AC drive operation
One of the simplest direct AC approaches involves a bridge rectifier, constant current regulator (CCR) and a string of LEDs as seen in Figure 1. If the LED string voltage is sufficiently high compared to the input AC voltage, the losses in the CCR can be kept to a reasonable level while achieving > 0.9 power factor. This approach does have lower LED utilization since for a portion of the AC cycle, no current flows through the LEDs as seen by the red line. In this example there is no electrolytic capacitor in the circuit for energy storage so this type of driver results in 100 percent optical flicker at 100 / 120 Hz (2x the AC line frequency) but the driver circuit is simple, small and can have a long operating life time. As it is not always practical to match the LED string voltage to the AC line voltage a switching topology is needed.
To better understand how to determine the right switching topology based on the string voltage, we wanted to compare several mainstream topologies and provide some general guidelines to help designers understand how the selection of the LED string voltage impacted the topology choice. To do this a figure of merit (FOM) was created that combined the maximum voltage stress and peak current through the power switch as a function of the LED forward voltage and the input line voltage. This FOM is a good proxy for both efficiency and cost in a switching LED driver as the lower the peak current, the lower the losses in the switch, inductor and diodes.
Two general use scenarios were considered. First, applications where high power factor and low THD (<20 percent) were required and the second where high power factor is not required. As an example for ENERGY STAR LED lamps, there is no power factor requirement for lamps < 5 W and in the EU there is a special input line harmonic exception in EN61000-3-2 for lighting products < 25 W so high power factor is not required. An added benefit of not needing high power factor is that it is easy to achieve low optical flicker.
The buck, buck-boost, boost and isolated flyback with a turn’s ratio of 3 were all compared based on the FOM (the lower the better) as a function of the VF to Vin peak ratio. In the case of the high power factor case, the total harmonic distortion was limited to < 20 percent for analysis and we assumed a 600 V MOSFET (80 percent derating). For VF/Vin ratios from 20 to 40 percent, the power factor corrected buck is the best topology. The upper limitation of the buck is related to THD and power factor and not stress on the power switch. Interestingly it turns out the best topology from the FOM analysis is the boost, which is not the first topology to come to mind for many designers. An example of a complete high PF 10 W boost design is shown in this design note where a 220 V LED string was driven at 30 mA across an input voltage of 90 to 135 Vac.
Figure 2. Topology comparison for high power factor >0.9
Figure 3. Topology comparison for low power factor (EN61000-3-2 Class C Exception)
When looking at applications were it is not critical to achieve high power factor, the clear winner for strings of higher voltage LEDs is the buck topology as seen in Figure 3 as it has the best FOM across a wide range of forward voltages. An added benefit of this approach is that it is one of the simplest to implement and can have low optical flicker. The reason that the curve ends below 70 percent is due to meeting the requirements of the EN61000-3-2 Class C exception where the input capacitor is undersized to control the input current shape. For applications in the US market where compliance to EN61000-3-2 Class C is not required, this range can be extended. An example of a complete 3.8W buck design is shown in this design note where a 150 V LED string was driven at 25 mA across an input voltage of 200 to 265 Vac and achieved 85 percent efficiency.
This FOM comparison is a good reference tool at the beginning of the LED selection and architecture definition. In some cases, especially at more narrow VF/Vin, there are several options that should be considered since the differences between different topologies is relatively small but in many cases this can give clear guidance in what topology is best for lowest system cost and better efficiency.