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LED Based Rear Combination Lamps and their Practical Applications

Mon, 05/24/2010 - 5:02am
Brian Blackburn, Field Application Engineer, ON Semiconductor
Brian BlackburnLEDs have recently garnered favor in the automotive community for their inherent long life as well as styling and design ques. Unlike the single point incandescent bulb, several LEDs acting in conjunction provide the required intensity for the combined Stop and Tail lamp functions. The Tail lamp requires a lower intensity and therefore must be regulated at a lower average power in comparison to the Stop lamp function. These LED arrays are either electrically connected in a parallel-series arrangement, or in several series strings of (typical three in series) each with their own power resistor and diode bias connected in parallel to construct the overall RCL. The number of strings could be as low as two per RCL up to or exceeding eight strings per RCL. Resistor-based current sources pose some manufacturing and cost constraints when dealing with high volume LEDs and their inherent intensity distribution. Many values of resistors need to be on hand in order to normalize the RCL intensity to meet the customer and government light output requirements.

An opportunity exists for a silicon-based Application Specific Standard Product (ASSP) to provide current regulation for series string LED based RCLs. An example of ASSP will be discussed in this article - as well as how to specifically apply to RCL design. In addition to an explanation of the device capability, an application circuit will show how to provide automatic latch off of the LED array in the case of a single LED open. This latch-off circuit will form the basis for the “one out all out” operation that is still required in order to mimic an incandescent bulb failure.

Background Information and Application
The promise of high reliability, as well as thin design, make LED based lamps the technology of choice. The major tradeoff of incandescent lamps versus LED lamps is the initial and replacement costs.

For this reason, LED based RCLs require current regulation to provide a regulated and or limited power across the LED array over the normal automotive continuous voltage of 9.0 V to 16 V. In addition, an open bulb condition poses a much different problem to the Body Control Module (BCM) for incandescent versus LED technology.

Figure 1 represents a vehicle’s BCM and the RCL interface. The BCM contains multiple channels of high side drivers (HSD) to provide switched power to various grounded loads throughout the vehicle. Each HSD typically has a current limit, and at least a subset of the canonical failure modes as a diagnostic signal such as short to ground, short to battery, open load device on/off. If these HSDs are sized to supply an incandescent bulb, then current limit must accommodate the high in-rush current, as well as the high steady state current, which forces current limit thresholds for the HSD very high. However, when an incandescent bulb burns out, the high side driver can easily determine an open circuit since the incandescent bulb fails open and zero current is easily detected by the BCM’s HSD. This open circuit condition will apply differently in the case of an LED based RCL, and needs special considerations placed on the BCM HSD and the RCL’s current regulator.

Figure 1 Vehicle BCM and RCL interface.

String Theory
In LED based RCLs, the individual LEDs are typically arranged in two popular topologies, cross coupled or series strings. Figures 2a and 2b detail these topologies. The current sourced to the LED array is limited through the use of power resistors. Most LEDs are intolerant of parallel connections due to miss matched LED forward voltages and can result in uneven current sharing. This current sharing is compounded in Tail lamp mode due to lower current requirement. Since LEDs are characterized at their normal operating current and (Stop current), and is usually 10 times greater than the current required for the Tail lamp function, the preferred LED topology is shown in figure 2b. The single power resistor in the Figure 2a, is broken into separate power resistor dedicated to each LED string. In the Tail mode, the total current is evenly distributed to all strings in conjunction with the diode blocking in the Stop power feed. 

Figure 2a. Cross coupled topology and the N-1 condition for and Figure 2b Series Strings

To compound the electrical design issues, the RCL manufacturer must contend with government requirements FMVSS 108. The regulation calls for maintaining a required light output with one LED going open circuit. This is the N-1 rule and may force the RCL to have additional LEDs. In the Figure 2, removing at least one LED from the array does not reduce the supply current; instead, the total current is diverted to the remaining LEDs and this added current reduces the remaining LEDs lifetime. To determine that any individual LED would go open circuit would require a vast amount of circuitry and wiring to each LED and would, consequently, not be cost effective. 

Figure 3. LED arranged in series strings of three allow for individual string bias. Open Circuit Measurement can be easily made on each string.

The N-1 rule, which is applied to figure 2b, removes three LEDs from the array, and the total current does reduce slightly, yet this change in the current cannot be detected by the BCM. However, open LED detection circuitry is reduced to the number of current sources required or the number of strings. Although three LEDs are lost in Figure 3 compared to one LED in Figure 2, if a diagnostic signal is warranted then string topology is the proper choice. This would directly apply to an LED based turn Indicator in which a diagnostic indication is used to latch-off the entire array such that the BCM can make a true open load detection and the driver can be warned. This latch-off behavior is mandated in some markets for the Stop lamp function as well.

The opportunity exist for an Application Specific Standard Product (ASSP) linear integrated circuit that provides matched constant current sinks to power multiple strings of LEDs. The device should provide a diagnostic signal that can act as a flag if any of the strings were subjected to the N-1 rule. This warning flag can be used to latch the entire RCL off and to force the BCM to detect an open circuit condition. In addition, items such as over temperature shut down and power fold-back can be added to the device to make it more robust to the various short circuit and transient electrical conditions that an automotive load is required to withstand.

The SCV7680 from ON Semiconductor has been developed to fulfill all of the requirements that an RCL engineer needs to routinely implement various LED arrays with any topology. The device was specifically designed as a constant current linear regulator and not a switched mode power supply specifically for cost and EMI issues.

Figure 3a shows a block diagram for the SCV7680. Current regulated in each output is set with a single resistor at the Rstop pin. This resistor provides the specific small current to be replicated on each output. The outputs can be programmed from less than 10 ma per output up to 100 ma per string, making a total regulated current of 800 ma. The outputs can be shorted together for higher string currents. The outputs are matched to less than 5 percent over the entire automotive ambient temperature range (-40C to 125C).

When the Stop input is toggled, all outputs turn on and regulate at the programmed Rstop value. The outputs remain on until the Stop input is cycled off. If the Stop pin is cycled low and power is still fed to the array from the BCM with a diode Or (see Figure 3b), the SCV7680 reverts to an internal PWM mode, and the outputs/strings are dimmed with an average current value of intensity. This is the Tail mode, and the duty cycle can be chosen with the Rtail value. This resistor to ground can set the Tail PWM duty cycle between zero percent and 80 percent. The internal oscillator frequency is internally set to 1 kHz and most importantly the current slew rate is limited to a low level of 6 ma/us. This assures that radiated emissions issues should not occur due to the PWM current waveform. The diagnostic signal is an internal diode or from the eight current outputs. Each time the Stop input is brought high, the diagnostic is pulled low. If any LED string goes open circuit, and the Stop function is activated, it releases the diag and a pull up resistor completes the diagnostic function. 

Figure 3a. SCV7680 Block Diagram with all of the functions needed to implement an LED based RCL

Figure 3b. An application circuit with Automatic Latch-Off to accommodate the BCM diagnostic

The SCV7680 is a linear regulator. Thermal management quickly becomes a large issue. Although the device has an exposed thermal pad integrated into the SOIC 16 package, in some cases requires some extra help to mange the Automotive power drops. The device is equipped to drive an external gate of high side P- Channel MOSFET or the base of a PNP bipolar transistor. A feedback pin with controlled by an internal 1 V reference. This feature does not have to be used, however thermal issues always exist with LEDs and with an external transistor, a more direct power path can be designed with the Printed circuit board copper or other thermal arrangement such as TO-220.

Figure 3b is the application circuit for the SCV7680. The RCL is modulated at a Stop level current, and in the Tail mode is precisely dimmed to 15 percent of the Rstop current value. The design incorporates the ballast transistor Q1, and with R11 and R12 for a linear voltage regulator to limit the power drop across the SCV7680. If any string has the N-1 rule applied to it will cause the entire array to latch off with the action of Q 2 and Q3 and the diagnostic output. Once the array latches off, the Stop line needs to be pulled low and then high again. This insures chatter-free behavior, and the reduction in current reduces to less than 6 ma, and can be easily sensed by the BCM.

Conclusion
LED based RCL need special attention if they are to be implemented on a global basis. Regulation force some RCL applications to either provide a diagnostic or latch off when subjected to the N-1 rule. The SCV7680 can be used to implement and control any RCL configuration including Stop, Turn, Tail, and Position lamp functions. With an integrated circuit approach allows for built in protection against overvoltage, overpower, and over temperature shut down.

Brian Blackburn is a Senior Field Application Engineer for ON Semiconductor. He has been awarded 50 patents ranging in diverse area as Vehicle crash detection algorithms to electrified fishing lures. He has a BSEE from the University of Illinois, and spends most of his free time trying to get out of yard work and studying up on fantasy football. He can be reached at brian.blackburn@onsemi.com.
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