WO2012077013A2

WO2012077013A2 - Control circuit for led lamps in automobile applications

Info

Publication number: WO2012077013A2
Application number: PCT/IB2011/055303
Authority: WO
Inventors: Ahmad Sameh Jwania
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-12-08
Filing date: 2011-11-25
Publication date: 2012-06-14
Also published as: WO2012077013A3

Abstract

An automobile rear light LED assembly, such as a tail light, a brake light, a turn signal,or a stop light, uses a plurality of strings of LEDs connected in parallel so that the LEDs may be directly driven by the automobile's 12.8-13.5 battery voltage. Upon failure of an LED in one of the strings, the small current drop would not be detected by the conventional automobile bulb burn-out detection circuitry. To trigger the conventional bulb burn-out detection circuitry upon failure of a single LED string, control circuitry in the LED assembly causes all LEDs in the assembly to be simultaneously disabled. Such a large current drop is then detected by the conventional bulb burn-out detection circuitry, and the conventional lamp failure alarm is triggered. The LED assembly is typically formed as a single replaceable unit separate from the conventional bulb burn-out detection circuitry.

Description

CONTROL CIRCUIT FOR LED LAMPS IN AUTOMOBILE APPLICATIONS

Ahmad Sameh Jwania FIELD OF THE INVENTION

This invention relates to automobile lights, and in particular to a control circuit for detecting failures in a light emitting diode (LED) light assembly used as a tail light or other automobile light.

BACKGROUND Automobile tail lights, such as brake lights, turn signals, reverse lights, and nighttime lights are typically incandescent light bulbs. The automobile's power supply is nominally 12.8 to 13.5 volts. To light up the bulb, the voltage is applied across the bulb terminals, and a current flows through the bulb to light it. To detect whether the bulb is burned out (i.e., an open circuit), a voltage detector detects whether the voltage drop across a low value series resistor is above a threshold. If the voltage drop is substantially zero, the bulb is burned out, and the driver is notified. One type of notification is by either a displayed notice on the dash or a doubling of the turn signal click rate. The series resistor, voltage detector, and failure alarm system are typically mounted near the front of the automobile, remote from the tail light assembly. LEDs are rapidly replacing incandescent bulbs and other types of light sources due to their efficiency, small size, high reliability, and selectable color emission. It is known to replace the light bulbs in automobiles with an array of LEDs so that, upon application of the 12.8-13.5 volts to the LED assembly, the brightness level is approximately that of the bulb, where the brightness range is set by applicable regulations. A typical forward voltage drop for an LED is 2.5-3.5 volts. When multiple LEDs are used to create a beam pattern for a specific function, the LEDs are arranged in a series and parallel configuration. The maximum number of LEDs that can be connected in series depends on the nominal battery voltage. For an automobile, the voltage can range between about 9-16 volts. Therefore, the number of LEDs connected in series cannot be greater than three. The number of parallel strings is selected based on the required brightness level, or on the particular optical pattern desired. There may be four or more strings of LEDs in the tail light. The current through each string is controlled so that each LED emits a predetermined brightness. A tail light using LEDs will draw much less current than a tail light using a bulb, for the same brightness level. For cost, simplicity, and reliability, design engineers prefer to use passive

components in the control circuits for tail lights and want to use proven circuitry when ever possible.

A tail lamp bulb may be 20 watts or greater, so the current drawn is on the order of one amp. An entire LED assembly may draw less than one -half amp for the same brightness. For an LED assembly used as a tail light, if one LED fails and becomes an open circuit, only a single string will fail. This may only drop the current by about 50- 100mA, which would typically not be enough to cause the traditional failure detection circuit to trigger, since the traditional failure detection circuit is designed to detect total failure of a bulb. Therefore, the failure detection circuit would not detect any failure if only one or a few of the LED strings failed. This may result in a safety problem, and the driver will be unaware of it.

It is known to use LEDs in an automobile lamp assembly, but the open circuit detection circuits for such LED assemblies are complete redesigns of the traditional failure detection circuit for bulbs. Such redesigns are prone to reliability problems and are costly. Examples of such LED assemblies and circuits are described in U.S. patent nos. 5,896,084 and 6,963,177, incorporated herein by reference.

What is needed is an LED assembly and control circuitry for use in an automobile that uses conventional failure detection circuitry to detect the failure of a single string of LEDs in the assembly.

SUMMARY In one embodiment of the present invention, an automobile light assembly, such as a tail light, uses a plurality of strings of LEDs connected in parallel so that the LEDs may be directly driven by a typical automobile battery generating about 12.8-13.5 volts. When there is an open circuit failure of one of the strings, this small drop in current through the string is detected by an active circuit within the LED assembly module. This small drop in current would not be sufficient to trigger the traditional failure detection circuit for a bulb, since such traditional failure detection circuits only detect a failure when the current drops to near zero. The traditional failure detection circuit, mounted near the front of the automobile, would have to be triggered in order for the driver's failure notification system to be activated. To achieve the large drop in current necessary to trip the traditional failure detection circuit, the detection of a failure of a single string of LEDs opens one or switches in the LED assembly to cause all the LEDs in the assembly to be turned off simultaneously. This results in a large current drop, which is then detected by the conventional failure detection circuitry. The driver is then notified of the failure by the traditional circuits and alarms. Although embodiments of the invention have been summarized above, the scope of the invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates an LED assembly, including failure detection circuits, used as a rear lamp for an automobile, and also illustrates the automobile's conventional failure detection circuitry that senses a large drop in current and, in response, alerts the driver to the failure.

Fig. 2 illustrates the circuitry in the LED Failure Detection Circuit and the LED Power Control Circuit in Fig. 1.

Fig. 3 illustrates an active current source that may be used to set an LED current in each string of LEDs instead of a current setting resistor. Fig. 4 is a flow chart identifying basic steps carried out by the circuits of Figs. 1 and

2 in accordance with one embodiment of the invention.

Elements that are the same or equivalent in the figures are identified with the same numeral.

DETAILED DESCRIPTION Fig. 1 illustrates an LED rear lamp assembly 10 for a vehicle. The assembly may be a tail light, a stop light, a brake light, a reverse light, a nighttime light or other light in a vehicle. The assembly 10 may even be used as a turn signal or other light source for the front or sides of the vehicle. The assembly 10 is preferably a single replaceable module that has a two-prong input for the nominal 12 volt power and ground. The LED portion is typically mounted in a reflective housing that directs light toward other drivers.

Also shown in Fig. 1 is the conventional failure detection circuit 12 that has been used by automobiles for many years to detect the failure of light bulbs in a tail light assembly. The present invention allows the existing failure detection circuit 12 to be used with the LED assembly 10, thus avoiding any redesign of the automobile's failure detection and

notification circuitry.

When switched on, the LEDs 14 are electrically connected in series and parallel across the 12.8-13.5 battery 16 voltage. Since each LED has a forward voltage of about 3.0 volts, each string 18-20 typically has a maximum of three LEDs 14. There may be any number of parallel strings for achieving the desired brightness and light design pattern. Only three strings are shown for simplicity.

The brightness of the LEDs is controlled by the current through each string. Since automobile manufacturers want to obtain maximum reliability by relying on passive components when feasible, the currents are set by current limiting resistors 22-24. If the supply voltage is 12.8 volts, the combined forward voltages for a string is 9 volts, and the desired current through a string is 50 mA, the value of each resistor 22-24 is chosen to be approximately 62 ohms ([12.8 volts - 0.7 diode drop - 9 volts LED drop]/ 0.05 amp). A switch 26 connects the battery voltage to the LED assembly 10. The switch 26 may be a turn signal switch, a brake switch, a headlight switch, or any other type of switch. The failure detection circuitry described herein is activated only when the switch 26 is closed.

A fuse 28 blows when there is a short circuit in the LED assembly path.

A low value resistor 30 forms part of the conventional failure detection circuit 12. This conventional failure detection circuit 12 is typically remote from the LED assembly 10, such as in the dashboard area or under the automobile hood. A differential amplifier 32, such as an op amp, detects the voltage drop across the resistor 30. If it is below a threshold, determined by a failure threshold detector 34, this indicates that the rear lamp has failed by becoming an open circuit. As a result, a failure alarm 36 is activated, such as by lighting a warning light, or doubling the audible clicking of a turn signal.

A diode 38 between the battery 16 and the LED assembly 10 prevents any voltage spikes from affecting other circuits connected to the battery 16. The voltage drops across each current limiting resistor 22-24 are detected by an LED failure detection circuit 42, shown in more detail in Fig. 2. An LED power control circuit 44, shown in more detail in Fig. 2, connects the strings to ground to complete the current path.

Fig. 2 illustrates the portion of the LED failure detection circuit 42 and LED power control circuit 44 associated with only strings 18 and 19. Other strings are associated with identical circuitry.

Each string is connected to ground via a diode 46/48 and a MOSFET 49/50. The diodes 46/48 prevent any leakage current from flowing in the reverse direction. Instead of multiple, low current MOSFETs 49/50, all strings may be connected to ground via a single high power MOSFET. A bipolar transistor or other switch may be used instead of

MOSFETs.

Since the failure detection circuit 42 for each string is identical, the circuitry for only string 18 will be described in detail. A comparator 52 has a non-inverting input terminal connected to the resistor 22 via a resistor divider 54. The inverting input terminal is connected to a reference voltage Vref established by another resistor divider 56. An input into the resistor divider 56 may be the 12.8-13.5 battery voltage V. A weak pull-up circuit 58 is connected to the output of the comparator 52. If the current through the resistor 22 is sufficiently high, the non-inverting input of the comparator 52 will be greater than the reference voltage and the output terminal of the comparator 52 will be pulled up by the pull- up circuit 58. This high signal is applied to the gate of the MOSFET 49 to keep the

MOSFET 49 on. Therefore, current continues to run through the string 18. Instead of using comparators to detect a failure of a string, other suitable circuitry may be used to detect a failure by detecting drop in current through a string or an increased voltage drop of a string. If one of the LEDs 14 in the string 18 becomes an open circuit, the voltage at the non-inverting input of the comparator 52 will be zero, causing the comparator 52 to trigger and shunt the weak pull-up circuit 58 to ground. This shuts off the MOSFET 49.

Even though the failure of the string 18 reduced the current through the current sense resistor 30 (Fig. 1), such a reduction is only about 50mA, where the entire LED assembly 10 may be drawing one -half amp. Such a small reduction in current would not be sensed as a failure by the failure threshold detector 34 in Fig. 1.

In order to trigger the failure alarm 36 of Fig. 1 , the outputs of all the comparators in the LED failure detection circuit 42 are tied together and connected to the gates of all the MOSFETs so that, even if only one comparator (e.g., comparator 52) output goes low, all the MOSFETs are turned off. The low output of a single comparator overrides the weak pull-up voltages at the output terminals of the other comparators. The connector 59 leads to the gates of the other MOSFETs.

In another embodiment, only one large MOSFET conducts current for all of the strings, and the outputs of all the comparators are connected to the gate of the single

MOSFET.

Since all the LEDs in the assembly 10 are turned off due to the failure of even one LED, there is a large current drop that is sensed by the conventional failure threshold circuit 34, which then initiates the alarm 36.

The LED assembly 10 will typically be provided as a single replaceable unit, with all or most of the electronics being part of an integrated circuit. The current limiting resistors may be extemal to the IC so that their values may be customized depending on the particular LEDs used. The conventional failure detection circuit 12 will typically be located near the front of the vehicle and may be the same as the failure detection circuit used for conventional tail light bulbs.

There are many ways to implement an LED assembly where the failure of a single string triggers a complete disabling of all LEDs to create a large current drop that can be sensed by the conventional failure detection circuitry previously used to detect a failure in a light bulb. Although a passive current setting resistor was used in the embodiment of Fig. 1 , the current may be set by an active circuit, such as shown in Fig. 3. Such a circuit provides a stable current through the LEDs despite variations in the supply voltage or the forward voltage drops. In Fig. 3, there is no current limiting resistor. Instead, a low value current sense resistor 74 drops a voltage proportional to the current through the serial LEDs 14. An amplified difference between the dropped voltage and a reference voltage Vref is output by an op amp 76. The output of the op amp 76 controls the conductivity of a MOSFET 78 in series with the LEDs 14. The feedback path controls the current through the LEDs so that the voltage drop across the resistor 74 approximately equals Vref. The value of the resistor 75 is selected to obtain the desired current.

Fig. 4 is a flow chart identifying the various basic steps used in one embodiment of the invention. In step 62, the LEDs in the LED assembly 10, used as an automobile light, are energized by, for example, the driver turning on the headlights, or turning on the turn signal, or stepping on the brake pedal. In step 64, an open circuit in any one of the LED strings is detected by a detector forming part of the LED assembly module, such as by detecting a zero voltage drop across a resistor in series with the LEDs.

In step 66, upon the detection of an open circuit in any one string, a switching circuit in the LED assembly module then disables all strings of LEDs to create a large current drop. In another embodiment, one or a few strings of LEDs are intentionally not disabled to avoid the tail lamp being totally dark for safety reasons. This may be accomplished by not tying one or more of the MOSFETs to all the comparator outputs. The remaining strings that have been disabled drop the current sufficiently low to trigger the failure threshold detector 34 in Fig. 1. In step 68, the conventional failure detection circuit 12, typically remote from the

LED assembly 10, detects the large current drop.

In step 70, the conventional failure detection circuit triggers an alarm to alert the driver to the rear light failure. Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.

Claims

CLAIMS What is claimed is:

1. A light emitting diode (LED) system for use in a vehicle comprising: a plurality of strings of LEDs connected in parallel for being connected to a power supply of a vehicle; a plurality of string fault detector circuits, each string fault detector circuit being coupled to an associated string of LEDs for detecting that the associated string of LEDs has become an open circuit due to a failure of at least one LED in the string; and at least one switch in series with the plurality of strings of LEDs, the at least one switch having at least one control terminal coupled to outputs of the plurality of string fault detector circuits, an opening of the at least one switch simultaneously disabling the plurality of strings of LEDs, wherein, upon detection by any one of the string fault detector circuits that a single string of LEDs has become an open circuit, the at least one switch is controlled to be open to simultaneously disable the plurality of strings of LEDs.

2. The system of Claim 1 further comprising a vehicle light failure detection circuit connected to the plurality of strings of LEDs, the vehicle light failure detection circuit detecting a total current drawn by the plurality of strings of LEDs, wherein the vehicle light failure detection circuit is configured to not indicate a fault when only a single string of LEDs becomes an open circuit but only indicates a fault when multiple ones of the strings of LEDs become an open circuit.

3. The system of Claim 2 wherein the vehicle light failure detection circuit only indicates a fault when the current drawn by the plurality of strings of LEDs drops to approximately zero current.

4. The system of Claim 1 further comprising at least one additional string of LEDs connected in parallel with the plurality of strings of LEDs, wherein the at least one additional string of LEDs is not connected to the at least one switch so is not disabled when one of the strings in the plurality of strings of LEDs becomes an open circuit.

5. The system of Claim 1 further comprising a current limiting resistor in series with the LEDs in each string of LEDs.

6. The system of Claim 1 wherein each string fault detector circuit comprises a comparator having a first input terminal connected to a resistor in series with LEDs in an associated string of LEDs, and having a second terminal connected to a reference voltage, wherein an output of the comparator is electrically coupled to the at least one control terminal of the at least one switch for simultaneously disabling the plurality of the strings of LEDs when the comparator detects an open circuit fault in the associated string of LEDs.

7. The system of Claim 1 wherein the at least one switch comprises a different transistor switch in series with each one of the strings of LEDs.

8. The system of Claim 1 wherein outputs of each of the string fault detector circuits are connected together and to the at least one control terminal of the at least one switch such that a detection of an open circuit in any one string of LEDs in the plurality of strings of LEDs controls the at least one switch to be open and disable all of the plurality of strings of LEDs.

9. The system of Claim 1 wherein the plurality of strings of LEDs connected in parallel, the plurality of string fault detector circuits, and the at least one switch are located in a rear lamp assembly of a vehicle, the system further comprising: a vehicle light failure detection circuit connected to the plurality of strings of LEDs connected in parallel, the vehicle light failure detection circuit detecting a total current drawn by the plurality of strings of LEDs, wherein the vehicle light failure detection circuit is configured to not indicate a fault when only a single string of LEDs becomes an open circuit but only indicates a fault when multiple ones of the strings of LEDs become an open circuit, the vehicle light failure detection circuit being located in the vehicle remote from the rear lamp assembly.

10. The system of Claim 1 wherein the vehicle light failure detection circuit comprises a series resistor, a difference amplifier having inputs connected across the resistor for measuring a voltage drop across the resistor, and a threshold detection circuit detecting when the voltage drop is below an open circuit failure threshold.

1 1. The system of Claim 1 wherein there are three or more LEDs in each string in the plurality of strings of LEDs and there are at least three strings of LEDs connected in parallel.

12. The system of Claim 1 further comprising a current source connected to each string of LEDs in the plurality of strings of LEDs.

13. A method performed by a light emitting diode (LED) system in a vehicle comprising: energizing, by a power supply of the vehicle, a plurality of strings of LEDs connected in parallel; monitoring an open circuit fault in each of the strings of LEDs by a plurality of string fault detector circuits, each string fault detector circuit being connected to an associated string of LEDs for detecting that the associated string of LEDs has become an open circuit due to a failure of at least one LED in the string; simultaneously disabling the plurality of strings of LEDs by at least one switch connected in series with the plurality of strings of LEDs upon detection by any one of the string fault detector circuits that a single string of LEDs has become an open circuit; and detecting, by a vehicle light failure detection circuit connected to the plurality of strings of LEDs, that there is a sufficient current drop by the plurality of strings of LEDs to indicate an open circuit failure of the plurality of strings of LEDs, wherein the vehicle light failure detection circuit is configured to not indicate a fault when only a single string of LEDs becomes an open circuit but only indicates a fault when multiple ones of the strings of LEDs become an open circuit.

14. The method of Claim 13 further comprising triggering a failure notice to a driver of the vehicle upon the vehicle lamp failure circuit detecting the open circuit failure.

15. The method of Claim 13 wherein the vehicle light failure detection circuit only indicates a fault when the current drawn by the plurality of strings of LEDs drops to approximately zero current.