Improving automotive lighting designs with high-performance LED drivers

Extreme loading
There are two major design hurdles to make the stop-start capability invisible to the driver. The first is a quick restart time. By using an enhanced starter design some manufacturers have reduced the restart time to under 0.5 seconds, making it truly invisible. The second design challenge is to keep all of the vehicle electronics, including air conditioning system and lighting powered directly from the battery while the engine is turned off; all-the-while maintaining enough reserve to quickly restart the engine when it’s time to accelerate.

In order to incorporate a stop-start feature, the drive train requires some design modifications. Namely, what was once the alternator may also double as an enhanced motor starter to ensure a quick re-start. Additionally, a stop-start electronic control unit (ECU) must be added to control when and how the engine starts and stops.

The battery must be capable of powering the vehicles lights, environmental control and other electronics, while the engine/alternator is turned off. Additionally, it must be capable of powering the starter when the engine is once again needed. This extreme loading of the battery introduces yet another design challenge, an electrical one, as the large draw of current required to restart the engine can temporarily pull the battery voltage as low as 6V. The challenge for the LED driver is to continually deliver a well regulated output voltage and LED current when the battery bus voltage briefly drops to 6V, then returns to a nominal 13.8V when the charger returns to steady state conditions.

A cold crank condition occurs when a car’s engine is subjected to cold or freezing temperatures for a period of time. The engine oil becomes extremely viscous and requires the starter motor to deliver more torque, which in turn, draws more current from the battery.  This large load current can pull the battery/primary bus voltage below 6V upon ignition, after which it typically returns to a nominal 13.8V.

A load-dump condition occurs when the battery cables are accidentally disconnected while the alternator is still charging the battery.  This can occur when a battery cable is loose while the car is operating, or when a battery cable breaks while the car is running. Such an abrupt disconnection of the battery cable can produce transient voltage spikes up to 60V as the alternator is attempting to fully charge an absent battery. Transorbs on the alternator usually clamp the bus voltage somewhere between 30V and 34V and absorb the majority of the surge; however DC/DC converters and LED drivers downstream of the alternator are subjected to transient voltage spikes as high as 36V. These LED drivers are not only expected to survive, but must also continually regulate output voltage and LED current through this transient event.

Short-Circuit Protection
For both DRLs and headlamps, the number of HB LEDs in a single string ranges from 6 to as many as 20. As the nominal input voltage is 13.8V and even lower in some transient conditions, a boost-based LED driver architecture is generally preferred as it is more efficient, simpler and more cost effective than a SEPIC or buck-boost design. However, until recently boost architectures have been difficult to protection against short circuits. This is particularly important in automotive applications as the LEDs are susceptible to damage in a front end collision, and any electrical arcing can ignite any spilled gasoline. For this reason, in the past, most front lighting LED applications used a more costly and complex SEPIC solution which has inherent short-circuit protection. However, with the emergence of new boost LED drivers with very robust short-circuit protection, future applications will use this design to offer a more efficient and cost-effective solution.

EMI Concerns
Reducing any electromagnetic interference (EMI) of LED drivers is beneficial to the overall power buss design. As LED drivers are usually based on switching regulators lowering the level of switching noise is desirable. This can be achieved by incorporating spread spectrum frequency modulation. As can be seen in Figure 2, this modulation scheme lowers the output switching noise by 20db by spreading over a wider frequency range dramatically reducing EMI concerns.