“Automotive LED lighting systems can be driven using switching regulators in a number of different ways. Depending on the application, lighting designers can design complete subsystems for different lighting requirements throughout the vehicle by selecting switching topologies and configurations. Selecting the correct power conversion switch topology and configuration for a system optimizes requirements such as complexity, efficiency, EMI, and safety.
In many systems, including the numerous regulators deployed in automotive PTO systems, the design of power conversion controllers is a difficult and complex task. This article explains the advantages, trade-offs, and applications of different switching topologies used in LED drivers to simplify the selection process.
LEDs differ from conventional electric lamps with filament or gas components. Using specific semiconductor junctions, LED manufacturers can generate specific colors of light across the entire visible range, as well as infrared and ultraviolet. In automotive applications, LEDs can improve daytime and nighttime driving safety. Efficiency improvements can extend EV battery life, while using multiple LEDs in a single system can avoid single-component failure issues.
Due to their versatility, LEDs can be driven in many different ways. The LED load is different from the traditional load of the power system because the output of the LED has good lighting control. LEDs emit light only by precisely controlled current flow through the semiconductor junction, regardless of the relative voltage of the ports to system ground (or chassis in automotive systems). Therefore, LED systems can take advantage of different topologies offered by switching technology.
How to choose the right switching topology for an automotive LED system
The choice of a specific switching topology in an automotive system is related to the overall system design; minimum input voltage, maximum string voltage, chassis loop capability, short-circuit output capability, maximum input current, output/LED current, and PWM dimming should be considered.
▶ Buck Converter
The buck LED driver regulates the current in the LED string with a voltage higher than the total voltage of the LED string. Buck LED drivers can be safely shorted to system ground, making them intrinsically safe. They have chassis reflow capability (one wire for power) and can be easily adapted for matrix or animation applications. The example schematics in Figures 1 and 2 show a basic system diagram of the controller regulating the high-side switch for current control.
Figure 1. Buck Converter
Buck LED drivers require several key features: fixed frequency operation, high efficiency through excellent switch control and low resistance switching, high accuracy over the entire analog dimming range, and proper spread spectrum design for excellent EMI FM.
▶ Boost (BOOST) converter
The boost LED driver regulates the current in the LED string with a voltage lower than the total voltage of the LED string. This is useful in many automotive systems where many LEDs need to be turned on in a single string. A typical 12 V automotive system operates from 6 V to 18 V, which requires dropping the LED driver down to 6 V, giving the LED a large boost ratio to keep it lit. The example schematics in Figures 3 and 4 show the basic system diagram of the controller regulating the low-side switch for current control.
Figure 2. Example of a Buck Converter: LT3932
Table 1 Advantages and trade-offs of using a buck converter as an LED driver
Figure 3. Boost Converter
Figure 4. Example Boost Converter: LT8356-1
Table 2 Advantages and trade-offs of using a boost converter as an LED driver
▶ Boost-buck using a boost converter
Some boost LED drivers can be configured to return the LED cathode to the power supply. This configuration is called a buck-boost. The total output voltage is VIN (VBATTERY), which is added to the total LED string voltage. The advantage of this topology is the ability to drive an LED string above, below or equal to the supply voltage. The limitation of this topology is that it is limited only by the converter – the low side is limited by the controller IC’s minimum supply voltage, and the high side is limited by the controller IC’s maximum output voltage.
Figure 5. Boost-buck converter
Figure 6. Boost-Buck Converter LT8386
Table 3 Advantages and trade-offs of using a boost-buck converter as an LED driver
▶ Buck mode using boost converter
Some boost LED drivers can be configured to step down from the power supply (rather than ground referenced in standard buck mode), resulting in a buck mode configuration. This configuration has the same limitation as buck mode, that is, the total voltage of the LED string must be lower than the input supply voltage.
Figure 7. Buck Mode Converter
Figure 8. Buck Mode Example: LT3756-2
Table 4 Advantages and trade-offs of using a buck-mode converter as an LED driver
▶ Buck-Boost Converters
A buck-boost LED driver regulates the LED current from a supply above or below the total voltage of the LED string. The converter regulates the high-side switch connected to the input voltage in buck mode and the low-side switch on the output side in boost mode. This topology is the most complex, but also the most flexible. The range of VIN and VOUT is limited only by the controller IC. This is a good choice for matrix-type applications.
Figure 9. Buck-boost converter.
Figure 10. Buck-Boost Example: LT8391
Table 5 Advantages and trade-offs of using a buck-boost converter as an LED driver
Automotive LED lighting systems can be driven using switching regulators in a number of different ways. Depending on the application, lighting designers can design complete subsystems for different lighting requirements throughout the vehicle by selecting switching topologies and configurations. Selecting the correct power conversion switch topology and configuration for a system optimizes requirements such as complexity, efficiency, EMI, and safety.