To combat these difficulties, there are several other types of buck-boost converters that mitigate some of these problems at the cost of extra components. The SEPIC converter shown on the previous page uses two inductors, or one coupled inductor and an extra capacitor, to create a buck-boost topology that provides continuous input current and discontinuous output current, as shown below. The SEPIC yields advantages similar to the boost while allowing for buck-boost operation. An additional benefit provided is capacitive isolation between input and output. This configuration allows for a shorted output condition that would destroy a boost converter and some buck-boost implementations. The major difficulty with a SEPIC is that the control dynamics are much more difficult, as it is a fourth-order control system (two inductors). Good PWM dimming relies on well-behaved dynamics, and at times the SEPIC can be a problem.
Ideal input and output current waveforms of the single-stage topologies.
One other interesting non-isolated buck-boost topology is the Cuk converter shown on the previous page. This is almost identical to the SEPIC, except that the output diode and inductor are in opposite positions. This configuration creates an inverted LED load, which isn't ideal given the available control ICs. However, it yields the benefit of continuous output current and continuous input current, as shown above, which is optimal considering the benefits we described previously. The dynamics are again difficult as in the SEPIC, which may or may not be desirable, depending on the performance specifications. In general, the SEPIC and the Cuk are more expensive than the floating buck-boost topology, but both offer clear benefits over the standard topology. As a note, all of the buck-boost options can be designed to have fairly similar system efficiency.
Looking at all of these single-stage topologies, one can quickly see the benefits of the two-stage topology. For instance, the two-stage design has the buck-boost capability, since the first stage can boost as high as necessary to always ensure that the second stage is bucking. Moreover, the boost and the buck topologies are far easier to work with and far more efficient than any of the buck-boost configurations. Of course, the system's input and output current are both continuous (the optimal case for EMC and dimming performance), and the buck converter can handle any fault conditions very easily. Finally, the two-stage system allows for a much better level of scalability.
Moving forward, not every design will necessitate a two-stage implementation, as the performance upgrade may not outweigh the need for a low-cost design. However, it seems that many manufacturers likely will move toward two-stage solutions more often to leverage platform developments and economy of scale. Ultimately, this helps drive prices of higher-end front lighting systems down, allowing the next-generation headlight to be available over a broader range of automobiles in the future.
For more information about LED lighting, go to: www.ti.com/led-ca.
James Patterson is the systems manager with TI's Lighting Power Products group.