A bipolar LED drive technique for high performance, stability and power in the nanosecond time scale
Introduction
Light sources are commonly used for calibrations of detectors such as photomultiplier tubes (PMT) and for monitoring their stability over time. An absolute reference (primary standard) can be provided by weak long-lived radioactive sources coupled to scintillation detectors. However, in cases when one wishes to distribute the same signal over an array of detectors, a more powerful secondary standard is required. Light emitting diodes (LEDs) [1] represent a valid choice for this purpose due to their high light yield per unit surface and the relatively simple electronic drivers they require. The main limitation in using LEDs comes from the minimum pulse duration attainable which is due to the inherent properties of the junction of the LED. Obtaining pulses as short as a few ns represents a technical challenge especially if high output power is required. Simple drivers often make use of a single transistor stage to inject current directly into the LED anode [2]. This approach yields good results but is not generally suitable for the nanosecond time scale due to the parasitic capacitance of the diode. In these conditions the light signal was found to extend significantly beyond the current pulse; this is here referred to as afterglow. Fig. 1 shows an example of afterglow of a high-power LED of 7 mm diameter [3] in response to a 10 ns current pulse produced through a simple transistor stage. A fast PMT suitable for light pulses as short as few ns was used for detection. It can be seen that the tail extends several μs after the peak.
Section snippets
Design of a bipolar LED driver
A bipolar technique has been developed to limit the afterglow and achieve nanosecond performance as shown in Fig. 2. This is composed of two distinct traces. The upper one (black) is applied to the LED anode, and the other (gray) to the cathode. It can be seen that the LED is subjected to a weak forward bias (FB) prior to the pulse, intended to optimize the speed of the off-to-on transition (phase I). The current pulse is generated in a differential fashion. This enhances the light yield from
Characterization of performance and results
The bipolar driver has been tested with several LEDs on detectors of relevance for the MPRu and TOFOR fusion neutron spectrometers [9], [10], [11]. The detectors used are PMT of type MPRu, TOFOR/S1 and TOFOR/S2 [12]. The determination of the shortest attainable light pulse from a given diode is of interest to assess the performance of the bipolar approach in the nanosecond time scale. A MPRu PMT was used here due to its fast response and a small 1.8 mm surface-mounted LED [13] was chosen for
Discussion
The bipolar approach does not have a theoretical limit in the width of the current pulse produced. The current is drawn into the diode when the difference of the two traces exceeds a threshold and this region can be made arbitrarily small by controlling the time offset between the two signals, increasing SL and reducing PW or changing FB. This means that the main limiting factor lies in the physical characteristics of the LED substrate. This statement was confirmed when the driver was tested on
Conclusion
The bipolar approach has proven to be effective for driving LEDs of a variety of sizes and power. It exploits the LED characteristics to their full extent and allows operations on short time scales not generally accessible with simpler implementations. This technique also provides the possibility to shape the trailing edge of the light pulse, although with some limitations. This is of interest in applications of pulse shape analysis and scintillation detector tests, where the light pulse can be
Acknowledgments
This work has been performed under the European Fusion Development Agreement (EFDA) and the Association EURATOM-VR with support from Swedish Research Council (VR), Uppsala University and JET-EFDA. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
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- 1
See the Appendix of M.L. Watkins et al., Fusion Energy 2006 (Proc. 21st Int. Conf. Chengdu, 2006) IAEA, (2006).
- 2
EURATOM-VR Association.