Part 1 of this article gave a brief history of automotive forward lighting, explaining the move from traditional incandescent sources to HID and on to LED-based lighting sources. As we discussed in part 1, the non-incandescent solutions require power electronics to regulate light output. Ultimately, the article predicted a move toward a completely LED-based two-stage electronics design. Through this evolution, the next-generation headlights could combine all the lighting functionality necessary in the front of the automobile: daytime running lights, high beams, low beams, cornering/bending lights, position lights, and turn signals.
This two-stage converter (a single boost stage followed by multiple independent buck stages) was shown to have the most flexibility and scalability while providing the best performance. Though it is fairly easy to describe the benefits of LED-based headlights versus incandescent or HID, the nuances of one LED driver topology versus another are less obvious. To better understand the merits of a two-stage design, the single-stage solutions must be examined in more detail.
Five common single-stage LED driver topologies.
In general, a two-stage topology always performs better than its single-stage counterpart, because there are two layers of power processing. The first stage attempts to provide a stable input for the second stage by attenuating the line disturbances at its input. The second stage takes a much more stabilized input, usually with significant energy storage, and provides a regulated output -- in this case, a current to drive the LEDs. The two-stage method allows each stage to have its own control loop and focus on its own regulation, yielding a more robust and higher-performing solution. The drawback, obviously, is cost and size in most cases. Efficiency can be lower, as well, although that is heavily dependent upon the operating points and has to be evaluated on a case-by-case basis.
So what single-stage topologies are available to the designer? After all, these are the building blocks for any design, and there are basic design specifications that ultimately restrict the choices. The choice of topology depends upon the desired operating points, including input voltage range, output voltage range, regulated output current, dimming method, and fault-handling capabilities.
In cases where the system's lowest input voltage is above the highest output voltage, it is advantageous to use a buck regulator, as shown above. The buck topology is ideal for driving LEDs, as it closely mirrors an ideal current source, due to its high impedance output. The output current is continuous, as indicated in the photo on the next page, due to the direct connection to the inductor in the switching converter. In fact, no output capacitance is necessary, providing the inductor is large enough to attenuate the switching ripple sufficiently. The minimized output capacitance facilitates high-resolution pulse-width modulation (PWM) dimming, which is frequently desirable in automotive systems, due to the benefits of minimal color shift in the LED, as compared to an analog dimming approach. The efficiency of a buck converter is also very high compared to some of the other topologies, reducing the demands on mechanical heat sinking. The main downside to a single-stage buck topology is the discontinuous input current shown on the next page. Capacitance is required at the input to store energy when the MOSFET is off. This makes conducted electromagnetic compatibility (EMC) more difficult to achieve than a solution with continuous input current.
In cases where the maximum input voltage is less than the minimum output voltage, the boost topology, shown above, is preferred. Like the buck topology, it has fairly high efficiency. However, it does not have continuous output current. Therefore, energy storage capacitance is required at the output. This limits the resolution possible using PWM dimming, depending on the size of the output capacitor. To limit this effect, the design can employ a MOSFET in series with the LEDs to open the string during dimming. However, this increases the cost and size while reducing efficiency.
The nice thing about the boost topology is that the input current is continuous, as shown on the next page, due to the direct connection of the input to the boost inductor. This reduces the complexity of the input filtering needed for EMC compliance.
Unfortunately, most automotive systems lie in between these two topologies, because there is a need to boost at times and buck at others. In this case, some style of buck-boost converter is required. The basic buck-boost topology has an inverted output, which can be difficult to implement, so a floating buck-boost, shown above, is more common where the output is referenced to the input. The buck-boost topology, naturally, has discontinuous current at both the input and the output, as shown on the next page. As we mentioned before, that is not advantageous in this system. Furthermore, the peak voltage stresses on the switches are worse, making the components more costly and sometimes bigger. Finally, the efficiency is always worse with a buck-boost compared to a boost or a buck converter.
One other note is that the floating buck-boost needs additional circuitry to level-shift the PWM drive signal above the input voltage.