Direct AC LED driver or AC-AC driver is getting very popular in the solid-state-lighting world. Starting with Seoul Semiconductor with its AC LED and then apply the same technology with conventional LED but better efficiency. Now Seoul Semiconductor even have the solution that works very well with Triac dimmer.
AC drive for LED is actually quite simple but care must be taken into consideration when configure the LED strings. It is not quite straight forward as compared to the SSL with AC-DC power supply. You need to make sure the power loss at the switching IC as little as possible else the IC will shut down due to over temperature. Some IC has the feature of lowering the regulated current output but that will affect the brightness output as well, which is not good. Figure 1 shows the AC drive block diagram.
Figure 1. AC Drive Block Diagram
From the diagram, each channel current is regulated using the current sense, some IC only requires one resistor to set the current. The control circuit will turn on the channel in stages by tracking the incoming AC voltage input. Figure 2 shows when the channel been turned on.
Figure 2. Channel On-Off Sequence
From figure 2, as the AC voltage increases the control circuit will turn on the respective channel when the AC voltage is higher than the total VF LED string. First it will turn on channel #1, then off channel #1 and turn on channel #2, turn off channel #2 and turn on channel #3, and so on. By doing this the Current phase and Voltage phase are almost the same phase that the Power Factor obtain will be close to 1. You don't have to worry about the Total Harmonic Distortion (THD) as well since there is no high frequency switching. No worry on the EMC test on the SSL if you decided to perform the test.
Disadvantage is that you may need more LED compared to the conventional AC-DC power supply. The Lumen-per-Watt (Lm/W) also will be lower as well but still in an acceptable range looking at this less complexity design. Another disadvantage is that the complete SSL PCB is non-isolated, all the conducted area is non-isolated. If the SSL product is designed with plastic casing then it shouldn't be an issue. In other words, if you decide to use this driving technique you need to isolate the design mechanically.
