By Barry Manz, Contributing Editor
[Sponsored by Coilcraft]
The LED has become fundamental—and important—element in automotive design. That’s why it’s more critical than ever to specify the right inductor for the driver.
Although light-emitting diodes (LEDs) first appeared in cars and trucks for interior or exterior lighting, they’re rapidly replacing every light source in the vehicle including headlights—even on models lower in a vehicle manufacturer’s lineup. However, rather than simply a bulb, LED lights are systems. In addition to the LED itself, its driver circuit typically uses a dc-dc converter in which the inductor provides energy storage, and thus is one of the most important components in any design.
The wide acceptance of LEDs is due in large measure to their high efficiency, small size and weight, and ability to deliver much higher brightness levels using high-brightness types. They’re also “green” devices, consuming an estimated 200 mL of fuel per 100 km and 4 g less CO2 emissions, far less than their incandescent and halogen counterparts. Furthermore, the use of LEDs in electric and hybrid vehicles can significantly reduce power consumption from the battery.
Selecting Inductors to Drive LEDs
LED lighting is an exciting and fast growing application. LEDs can provide low cost, reliable lighting for a wide variety of applications ranging from architectural and automotive to signage and handheld devices. The designer faces the challenge of choosing from a vast array of LED manufacturers and device styles. Equally challenging can be the selection of components for the LED driver circuit. Fortunately powerful tools are available that greatly facilitate the power inductor selection.
Although other types of converters can be used for LEDs, switch-mode converters are well-suited for automotive applications in addition to many others. The most common examples are:
Buck converter: This simple type of step-down converter is very common and reduces the dc output voltage to a value lower than the input voltage and generally requires a single inductor.
Boost converter: These circuits step-up a dc input voltage to a higher level at the output while reducing current. A single inductor is used as the storage element. The boost converter is one of the most common types used in LED lighting applications.
Buck-boost converter: This circuit combines the two types above to provide an output voltage either higher or lower than the input voltage with the output voltage inverted. This type also requires inductors.
SEPIC: A single-ended primary inductance converter (SEPIC) is essentially a boost converter followed by a buck-boost converter. However, it allows the output voltage to be the same as, lower, or higher than the input voltage, while output polarity of both remains the same. These circuits typically require two identical inductors, with the dual-winding type preferred because of their smaller footprint, lower leakage inductance, and ability to increase overall circuit efficiency.
Flyback converter: This type of converter provides isolation, energy storage, and voltage scaling, and can deliver more than one output voltage in different polarities by using tapped windings. It electrically functions as two inductors with a common core, but opposite polarity windings.
Forward converter: This type uses a transformer to either increase or decrease the output voltage while providing isolation for the load, and can provide multiple voltages simultaneously with different polarities. It doesn’t use an inductor for energy storage, and schematically looks much like a flyback converter but is more efficient.
Choosing the Topology
The dc-dc switch-mode converters used in LED drivers require energy to be stored during their operation, and because an inductor inherently provides this function, it’s a critical component. For example, the converters need to operate over as wide a voltage range as possible. That’s complicated by the fact that when the battery is charged or while it’s charging, cell voltage is greater than the output voltage.
This means that the cell voltage will be less than the desired output voltage at the end of the discharge cycle, effectively eliminating the possibility of using a buck or boost converter. A more desirable choice is the SEPIC topology, as it allows voltage to step up or down in voltage. The storage characteristics of coupled inductors help achieve this, which is why this approach is becoming very popular in both automotive and general LED lighting applications.
It’s logical to think that a passive component like an inductor should be easy to specify in any application. Just the opposite is true, though, especially for LED driver applications, because its contribution to overall performance is significant.
Specifying the most desirable inductor for an LED driver can obviously be done by the usual, time-consuming process. However, Coilcraft’s dc-dc converter inductor selector tool, which can be found on the company’s website, makes it possible to choose the correct inductors by simply adding a variety of values. More information is available about this tool in the company’s “Selecting Inductors to Drive LEDs” application note, which illustrates how it can be used.