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.
Single-stage topologies
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.
Very good point about the previous predictions of higher voltages, Thinking J. Ten years ago, we were sick of hearing about the forthcoming arrival of 42V architectures. And where are they now? I would add, though, that hybrids an EVs have used higher voltages (upwards of 300V in many cases). But I agree with you that convntional IC engine-based vehicles will have 12V (14V) for some time to come.
Headlights use about 30-60 watts, regardless of whether they are tungsten, halogen, HID, or LED. There really isn't that much difference in power consumption between the best and the worst. There are of course significant differences in appearance, light levels, and cost; but not in power usage.
A car uses about 15 KW to move down the road at constant speed. That means the headlight's power usage is about half a percent of the total. The difference between the most and least efficient headlight is only going to be one quarter of one percent difference. That's a mighty slim improvement for lights that cost hundreds of dollars more.
My point was that going from a 90% efficient electronic DC/DC converter to a 70% resistor is even less signficant. It seems like a considerable increase in cost and complexity for an insignificant benefit.
On the fuel efficiency impact of daytime running lights: I *challenge* anyone to be able to measure the MPG impact on a statistically significant basis. It's "lost in the noise" and too low to detect!
If you want to save electrical energy in a car's electrical system, there is far more low-hanging fruit that would provide a lot more benefit for the buck. Use schottky diodes in the alternator. Use brushless DC motors instead of the ancient brushed motors. Design the electronics to draw micramps instead of milliamps.
I wrote NHTSA in the spring of 2011 asking them why they do not mandate constant headlight illumination on all North American Vehicles instead of DRL's. NHTSA responded with a 116 page document that basically said it was not "fuel efficient" to use full lighting 24/7 because of the load it pulled on the electrical system. So for you to say, "The power used by headlights is insignificant compared to the other loads in a car" would be a false statement if used in that manner. If NHTSA and the EPA both agree this load is enough to cause excessive loss of fuel mileage, then, in their minds, it IS significant!
My primary concern is safety and safety only. I could care less about the loss of fuel mileage for the use of constant headlights because the safety advantage greatly outweighs the difference in fuel savings. I would gladly give up one mile per gallon to be safe and for my children to be safe.....it's really not that big of a deal. In fact, both of my children turn on their lights every time they start their cars. It has become a natural reflex from day one, just like buckling their seatbelts.
Another concern I voiced with NHTSA, is that there is no taillight illumination with DRL's and some are actually so bright, that the vehicle operator thinks their headlamps are on. I see way too many people driving during the wee early morning hours and late afternoons with no headlights, and it isn't safe! One possible cause of this and another factor is the IP Cluster. On older vehicles, the cluster was never illuminated until the lights were turned on. The darkness inside the vehicle gave the driver an indication his lights weren't on, thus, triggering the reflex to pull out the switch. With newer vehicles, almost every IP cluster is being illuminated as soon as the vehicle is turned on. With this lighting inside the vehicle and the DRL brightness, most drivers think their lights are on, when in fact, they are driving down the road with or without DRL's and no taillights! Another factor is DRL brightness and the DRL being combined with the headlight bulb. Some DRL's are so bright that the vehicle operator cannot distinguish the difference between the headlight and the DRL's, thus thinking their lights are on. A good example of this would be the 1998 and 1999 Chevrolet Trucks, Tahoe and Suburban's. The DRL's are integrated into the headlight bulb and the brightness difference is barely distinguishable. DRL's should never be integrated into the headlight bulb. They should be a dedicated bulb placed in close proximity to the headlight, but not within, and the brightness should be at least, 50% dimmer.
With the new technology of LED's, these will draw less current on the electrical system and result in better fuel efficiency. My hope is that the auto industry, the EPA and NHTSA, will all agree that safety is a greater concern than fuel efficiency and LED's will help change our way of thinking for overall safety. Canada has already implemented full time DRL's and with this, anytime you turn on your windshield wipers, you lights come on. I say go ahead and mandate full time DRL's in North America and add taillight illumination. Remove the option to turn off your DRL's and make them automatic as soon as the vehicle is either started or is taken out of park.
James, I know you work for a semiconductor company, so ICs are a natural choice. But we're driving LEDs here. Why not just use groups of (say) 3 LEDs in series with a series resistor? The auto companies are fanatics about cost, and nothing could be cheaper than a simple resistor. No one has cared about headlight brightness variations with battery voltage in the past. The power used by headlights is insignificant compared to the other loads in a car.
While the general observations and comments are valid... Others are predicting the market in question (automotive) is going to be going though some changes. Changes that would impact assumptions being made.
- Change from 12(14)v system to higher voltages 36(42-48)v. reason: to get more power without impacting size / weight of system. Predicted before without happening. This time may happen because of expanded use of electrical systems in new vehicles has already taken place (hybrids, elect. based power steering, etc..)
- higher voltages with feedback system based on actual light output would negate much of the need for boosting (pre-regulation) on the lighting system. This would bypass the most of the effects of temperature, battery charge status and time on performance of the lighting system. Yea , this is complex also, but with complexity involving much lower power components.
Sounds like a really perfect application for those Digital Power microcontrollers, not only do you get the ability and flexibility of a uC for doing lighting/control tasks that have never been thought of, you can also implement the whole boost-buck, buck-boost, buck, boost, SEPIC, CUK, Flyback, Forward, etc... with just one chip. Microchip and Atmel are good places to start, and have many application notes.
James, this is a good overview of the desing choices for LED lighting. The ability to combine functions in one lighting unit are a great improvement. It should also offer new options that have not been thought of before.
I assume that although the control funcitons may be more complex, the available microcontroller circuitry should make it possible to implement.
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