Application Note Automotive LED Lighting

Automotive LED Lighting
As the electrification of automobiles advances, power consumption control is becoming an increasingly important factor. LED Lighting Technology design reduces power consumption, increases lifecycle, offers design freedom and overall control. It is now used in Automotive functions including headlights and interior lighting. The TDK Group offers an extensive lineup of power inductors for use in LED drivers optimized for different systems including step-up, step-down, and step-up/step-down types.

Main circuit Types used in LED drivers

Constant-current circuits are needed to operate LEDs. DC-DC converters are essential for supplying stable electric power from variable battery power. The DC-DC converters (LED drivers) commonly used are categorized into step-up type, step-down type, and step-up/step-down types depending upon the number of LEDs and system selected. Suppose the battery voltage decreases to about 6 V, the typical forward voltage for one general white LED is only 3.5 V, a step-down type converter would be used. If two to four LEDs are connected in series a step-up/step-down type is used. When five or more LEDs are connected in series, a step-up type is used. In a system with multiple functions including headlights, daytime running lights, and turn signals these circuits are then used in combination.

Table 1. Types of LED Drivers
Circuit Type Circuit Diagram
Step-down type
Step-up type
Step-up/step-down type

Circuit Types for high-performance LED headlights

Typically, high-performance LED headlights use variable light distribution functions to change the range of illumination and vary the brightness.
In order to vary the brightness and range of illumination, multiple LEDs are used. The luminance of each LED is then adjusted to achieve the brightness and range of illumination.
Figure 1 shows a basic circuit structure. The battery voltage is increased to an approximate 40 - 60V and a buck converter supplies current to the LED.

Figure 1. LED Light Circuit Structure for Headlights
Figure 1-1
Figure 1-2
Figure 1-3

Multiple LEDs are connected in series and parallel behind the buck converter (Figure 2). The current that flows to each LED is individually adjusted to change the brightness, and in extreme cases, only a single LED is illuminated and voltage close to 60 V is applied to the output inductor of the buck converter. As a result, when used in this application, a higher comparative inductance (e.g., 100 μH) is selected. The smoothing capacitor used with the boost converter in the front end needs to maintain the current against sudden transitions in order to change the illumination range instantaneously. Typically the required range of capacitance for this smoothing capacitor is 1µF to 10µF at a withstanding voltage of 100 V.

Figure 2. Example of a Circuit in the Case Where Multiple LEDs Are Connected in Series and Parallel
Figure 2. Example of a Circuit in the Case Where Multiple LEDs Are Connected in Series and Parallel

Drive circuits for LED headlights and TDK inductors

Single lamp types use a single converter and a step-up/step-down or step-up type driver. They are used for single-function vehicle exterior lighting, interior lighting, infotainment systems, etc.
Multi-beam types use two converters, a step-up circuit in the front end and a step-down circuit or constant current circuit in the back end. For vehicle exterior illumination, in the case of headlights, this structure is used to switch between high beams and low beams. Also, daytime running lights (DRL) can be added.
Adaptive front lighting systems also use two converters, a step-up circuit in the front end and a step-down circuit or constant current circuit in the back end. They are used for LED headlight systems including main headlights, DRL, turn signal lights, and so on.

Table 2. LED Headlight Drive Circuits
Function Single lamp type Multi-beam type Adaptive front-lighting system
Filter 12V Inductor 1pc Inductor 1pc Inductor 1pc
Boost 24V→60V Inductor 1pcs Inductor 2pcs Inductor 2pcs
Back 60V→○V   Inductor 2pcs Inductor n pcs
Basic Circuit
Table 3. Lineup of Recommended TDK Inductors
  Filtering BOOST (Vbat to ~40-60V) BUCK (40/60V TO 3-60V)
Series SPMseries SPMseries CLFseries SPMseries CLFseries
  • Low inductance: 1µH-4.7µH.
  • Low DCR. High Isat
  • π type filter
  • Inductance range: 4.7µH-47µH.
  • Due to working frequency SPM and CLF could be used.
  • Mid Isat – Irated but high ripple (up to 70-75% of Isat)
  • Inductance range: 47µH-100µH.
  • Due to high inductance level SPM could cover up to ~100µH but RDC have to be checked.

Example of a boost converter circuit

Single lamp type LED headlight functions are simple and are used to switch between high and low beams.
In this case, a step-up (boost) converter of about 15 W to 30 W is used. A representative circuit structure is shown below.
The output voltage is determined according to the number of LEDs in series and is in the range of about 20 V to 30 V. In the case of a step-up circuit, switching losses are high compared to a step-down circuit, and increasing the frequency is difficult. The most common frequency range is between 200 to 400 khz.

Figure 3. Boost converter circuit examples
Figure 3. Example of a Boost Converter Circuit

LEDs for DRL and compact lighting applications (low power)

Systems with relatively low power including DRLs, interior illumination, back lighting, and turn signal lights use a constant current circuit or step-up/step-down circuit. The step-up/step-down circuit used in LED circuits commonly make use of SEPIC circuits.

Figure 4. SEPIC Circuit Example

>Figure 4-1

>Figure 4-2

>Figure 4-3

Figure 4 shows a SEPIC circuit example. Two inductors (L1 and L2) a DC cut capacitor (C1) are used. The output is controlled in the same manner as a conventional step-up/step-down converter and is determined using the following formula. In the case of a conventional step-up/step-down converter (Circuit B), the input and output polarities are reversed, but in the case of a SEPIC circuit, the polarities are the same. The fundamental relationship formula is as shown below, and the output voltage is the same as the capacitor (C1) input voltage.

𝑉𝑜=𝑉𝑖𝑛 𝐷/(1−𝐷)  (D:duty)

Based on the above relationship formula, theoretically the voltages applied to L1 and L2 (VL1 and VL2) are the same and a combination dual coil with a 1:1 winding ratio (Circuit C) can be used. In cases where a dual coil is used, the load on the core is double that compared to when a single coil is used, and an inductor with low core loss and high DC superposition is needed. The B82477D* series of TDK inductor products is suitable. These products use low-loss materials in the DR core and control for thermal generation area.

Table 4. Lineup of Dual Coil(2in1 inductor)
  B82477D Series
  • Inductance range: 2µH - 100µH
  • High rated current: 2.2A - 16.1A
  • Operating temperature range: -55℃ - 155℃
  • Adoption of Shield with a ring core and low-loss materials
  • Wide lineup