Failure detection for series of electrical loads

#The invention relates to the field of failure detection to detect failures, such as short circuits or open circuits, of electrical loads, especially to detect failures of light emitting diodes (LEDs) in a chain of LEDs connected in series.

#Illumination devices (e.g., lamps) that comprise light emitting diodes (LEDs) as luminescent components usually can not simply be connected to a voltage supply but have to be driven by special driver circuits (or control circuits) providing a defined load current to the LEDs in order to provide a desired radiant power (radiant flux). Since a single LED exhibits only small forward voltages (from about 1.5 V for infrared GaAs LEDs ranging up to 4 V for violet and ultraviolet InGaN LEDs) compared to commonly used supply voltages (for example, 12 V, 24 V and 42 V in automotive applications) several LEDs are connected in series to form so-called LED chains.

#In many applications it is desirable to have a fault detection included in the driver circuits (or control circuits) that allows for detecting defective LEDS in the LED chains connected to the driver circuit. An LED can be regarded as a two-terminal network. A defective LED becomes manifest in either an open circuit or a short circuit between the two terminals. If one LED of a LED chain fails as an open circuit this is easy to detect since the defective LED interrupts the current for the whole LED chain. If one LED of a LED chain fails as a short circuit only the defective LED stops radiating which in some applications might not be a problem. However, other applications require the radiant power to stay within a narrow range.

#The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, instead emphasis being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts. In the drawings:

#FIG. 1 illustrates a first example of the invention comprising a voltage divider for providing the nominal value;

#FIG. 2 illustrates a second example of the invention comprising a voltage divider having a plurality of intermediate taps and a multiplexer for selecting an appropriate intermediate tap for providing the nominal value; and

#The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

#In many applications it is desirable to have a fault detection included in the driver circuits (or control circuits) that allows for detecting defective LEDS in the LED chains connected to the driver circuit. A defective LED becomes manifest in either an open circuit or a short circuit between the two terminals of the defective LED. If one LED of a LED chain fails as an open circuit the defective LED interrupts the current for the whole LED chain which is easy to detect, for example, by monitoring the load current of the LED chain. If one LED of a LED chain fails as a short circuit only the defective LED stops radiating and the overall voltage drop across the LED chain decreases by the forward voltage of one LED. A short circuit defect may therefore be detected by monitoring the overall voltage drop across the LED chain. If this overall voltage drop falls below a constant threshold voltage, a defective LED (which has failed as a short circuit) is detected.

#A problem inherent with such a concept of short circuit fault detection is that the voltage drop across a LED chain does not only decrease due to a short circuit defect of one LED but may also vary due to variations of temperature as well as due to aging effects. As a result, it is possible that a fault can be detected although all LEDs are good or that a defective LED will not be detected. This may be the case especially in applications with wide temperature ranges, for example in automotive applications where incandescent lamps are increasingly substituted by illumination devices based on LEDs.