Diode drive current source

Abstract: A current source (20) that uses a regulated constant current power source (21) to supply current to drive a load (13), and the load current is controlled by shunt switches (34). If a plurality of loads (13) utilize less than 50% duty factor, then one current source (20) can drive N multiple dissimilar impedance loads (13), each at 100%/N duty factor. The current source (20) includes a power converter (21) coupled between the power source (11) and the load(s) (13) for providing pulsed current thereto. A current sensor (32) is provided for sensing current flowing through the loads (13). A controller (26) is coupled between the sensor (32) and the power converter (21) for regulating the amplitude of the output current supplied to the loads (13). A shunt switch (34) is coupled across the loads (13), and a duty factor controller (35) is coupled to the shunt switch (34) for setting the duty factor of the shunt switch (34). A laser drive circuit (10), or driving light emitting diode arrays (13) is also disclosed that include a plurality of the current sources (20). Alternatively, if the duty factor is sufficiently low, one current source (20) may be used to drive a plurality of arrays (13).

Description: BACKGROUND The present invention relates to current sources, and more particularly, to a diode drive current source for use with diode pumped lasers, and the like. Diode pumping has become the technique of choice for use as pump sources employed in solid-state laser systems due to their relatively high electrical-to-optical efficiency. Prior to the use of diode pumping, flashlamps were used as pump sources. Typical system efficiencies were in the 1% to 2% range. The low efficiency was due mainly to the low electrical-to-optical efficiency. The use of diode pumping, with its higher electrical-to-optical efficiency, can result in a laser system efficiency of 10%, to 15%. Thus, a tenfold reduction in required input power can be achieved. Diode pumping requires high power, pulsed, regulated current sources to drive the pump diodes. Conventional current sources utilize either a series dissipative regulator or a pulse-width-modulated (PWM) converter to control output current. When used at high output currents, as is required by diode pumped lasers, for example, both of these techniques suffer from high power losses, and are very inefficient. The series dissipative regulator dissipates the power dropped across a series pass transistor element, where the power is given by P = (Vin-Vout)*Iout. At high output currents, this power loss becomes very high. The PWM converter suffers from high switching losses in a switch transistor, particularly due to reverse recovery of a catch diode, and from switching losses in the catch diode. At high output currents, the reverse recovery currents become very large, and the resulting power losses become very high. The assignee of the present invention has previously developed a quasi-resonant diode drive current source that is disclosed in U.S. Patent No. 5,287,372, entitled "Quasi-Resonant Diode Drive Current Source" that overcomes many of the problems associated with conventional diode drive current sources. However such quasi-resonant diode drive current sources have comparatively long (10 µsec) rise and fall times. In addition, the quasi-resonant diode drive current source can drive only one load. Therefore, it is an objective of the present invention to provide for an improved diode drive current source. It is a further objective of the present invention to provide for a diode drive current source for use with diode pumped lasers. It is a further objective of the present invention to provide for a diode drive current source that is capable of driving a plurality of loads. SUMMARY OF THE INVENTION Diode pumping is currently used in solid-state laser systems because it provides higher electrical-to-optical efficiency. The present invention is a critical development in the field of solid-state diode pumped lasers, which require a high current, regulated, current source. Without an efficient power source, diode pumped lasers would not be practical. In order to meet the above and other objectives, the present invention is a diode drive current source, as claimed in claim 1 hereinafter, that uses a regulated constant current power source to supply current. The load current is then controlled by shunt and series switches. The use of a quasi-resonant converter as a current source, such as the one described in U.S. Patent No. 5,287,372, provides relatively high conversion efficiency. This higher efficiency results in less input power drawn from the power source, cooler operation, and therefore higher reliability. This improved efficiency is of great benefit at high output current levels because it represents a very large savings in dissipated power. However, the present diode drive current source provides much faster output pulse rise and fall times compared to conventional current sources, including the source disclosed in U.S. Patent No. 5,287,372, which further improves efficiency. The present invention may be employed with high power solid-state diode pumped lasers such as airborne and ground based laser systems, materials processing lasers, earth observation sensors, undersea surveillance (coastal mine detection and ASW), infrared countermeasures and others.. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which Fig. 1 illustrates a block diagram of a laser diode drive circuit that employs a quasi-resonant diode drive current source of the present invention; Fig. 2 shows a simplified schematic of a diode drive current source; Fig. 3 shows a simplified schematic of a diode drive current source in accordance with the principles of the present invention that is used to drive multiple loads; and Figs. 4 and 5 show waveforms derived from simulation results that illustrate the functions and advantages of the present invention. DETAILED DESCRIPTION Referring to the drawing figures, Fig. 1 illustrates a block diagram of a laser diode drive circuit 10 that employs diode drive current sources 20, 20a in accordance with the present invention to drive one or a plurality of loads 13, such as laser diode arrays 13 that have varying numbers of light emitting diodes 16 therein. Each of the individual laser diode arrays 13 are adapted to pump laser crystals (not shown) that are part of two amplifiers and an oscillator (not shown). Each of the individual laser diode arrays 13 contains separate pluralities of laser diodes 16 that are coupled in series between a power source 11 and one of the diode drive current sources 20, 20a. A protection diode 17 is coupled around each of the pluralities of light emitting diodes 13 and is used to protect the plurality of light emitting diodes 16 in case of a reverse voltage situation. Capacitive isolation is provided between respective ones of the pluralities of laser diodes current sources 20, 20a using charge storage means 12 such as capacitors 12. The capacitors 12 are used to store charge that is eventually controlled by the diode drive current sources 20, 20a to energize the laser diode arrays 13. Fig. 2 shows a simplified schematic of a diode drive current source 20. The diode drive current source 20 comprises a constant current source 21 that provides output current to one load 13, such as the light emitting diode array 13. The diode drive current source 20 comprises a zero-current-switched full-wave quasi-resonant buck converter 21, a duty factor controller 35, and a shunt switch 34. However it is to be understood that the diode drive current source 20 may be readily configured in accordance with other converter topologies. The quasi-resonant converter 21 makes use of component parasitics, or at a minimum, masks component parasitics, such that their effect is negligible. The zero-current-switched quasi-resonant converter 21 is comprised of a switch transistor 24, a resonant inductor 27, a filter inductor 28, a resonant capacitor 25, a catch diode 22, an output current sensor 32, and a quasi-resonant controller 26 interconnected as shown. The current sensor 32 senses the output current flowing in the filter inductor 28 and is coupled by way of a sense line 33 to the quasi-resonant controller 26 that regulates the amount of average current flowing through the filter inductor 28, by varying the switching frequency. The quasi-resonant converter 21 provides regulated current to the output. The resonant inductor 27 provides a high impedance for the switch transistor 24 during the switching time, and thus permits lossless switching of the switch transistor 24. The resonant capacitor 25 masks the capacitance and reverse recovery of the catch diode 22, and thus negates the switching losses of the catch diode 22. This configuration provides essentially lossless switching. Detailed circuit descriptions of resonant converters are available in the published literature, and may be found in "Linear Integrated Circuits Data and Applications Handbook," publication number IC600, published April 1990 by Unitrode Integrated Circuits Corporation, for example. Output power from the constant current source 21 is controlled by switching a shunt switch 34. The shunt switch 34 is switched using a duty factor controller 35 whose input signal comprises a duty factor that determines or sets the duty factor of the shunt switch 34. With the shunt switch 34 closed, no power is delivered to the load 13, and essentially no power is dissipated in the switch 34. With the shunt switch 34 open, regulated current is delivered to the load 13, and no power is dissipated in the switch 34. Thus, the efficiency of the diode drive current source 20 is very high, approaching 100%, and load current rise and fall times are very fast, typically less than 1 µsec. The rise and fall times of the load current are shown in Figs. 4 and 5. Fig. 3 shows a simplified schematic of a diode drive current source 20a in accordance with the principles of the present invention that is used to drive multiple loads 13. This diode drive current source 20a includes the shunt switch 34 coupled across two loads 13 that may each comprise light emitting diode arrays 13. Each of the loads 13 are coupled through respective first and second series switches 34a, 34b to the negative output of the power source 11. Each of the first and second series switches 34a, 34b are controlled by the duty factor controller 35 in the manner described above with regard to the shunt switch 34. In operation, with the shunt switch 34 closed, no power is delivered to the respective loads 13. If the first series switch 34a is closed and the shunt switch 34 opened, regulated current is delivered to a first load 13. If the second series switch 34b is closed and the first series switch 34a is opened, regulated current is delivered to a second load 13. If the shunt switch 34 is closed, no power is delivered to the either of the loads 13. A zero-current-switched full wave quasi-resonant buck converter has been described for use in with the present invention and which may be used as the constant current source 21. However, it is to be understood that the concepts of the present invention applies to other current source topologies as well. A detailed description of the quasi-resonant current source employed in the present invention and described herein is provided in U.S. Patent No. 5,287,372, entitled "Quasi-Resonant Diode Drive Current Source". The diode drive current source 20 has been simulated using spice-based computer analysis. Figs. 4 and 5 show load current waveforms derived from simulation results that illustrate the functions and advantages of the present invention. The spice-based computer analysis was used to generate the waveforms of Figs. 4 and 5. The simulation results illustrate the functions and advantages of the diode drive current source 20 of the present invention. Predicted efficiency is approximately 93%. A prototype has been built and tested. Performance of the prototype closely matches the predicted performance illustrated in Figs. 4 and 5. The efficiency of the prototype substantially matches the predicted efficiency of 93%. Thus there has been described new and improved diode drive current sources, and specifically diode drive current sources for use with diode pumped lasers, and that are capable of driving a plurality of loads. It is to be understood that the above-described embodiment is merely illustrative of some of the many specific embodiments which represent applications of the principles of the present invention.

Claims: A current source (20a) for driving a plurality of multiple dissimilar impedance loads (13) comprising: a power source (11); a constant current source (21) coupled between the power source (11) and the plurality of loads (13); a plurality of series switches (34a, 34b) respectively coupled in series with the plurality of loads (13); a shunt switch (34) coupled in parallel with the plurality of loads (13) and series switches (34a,34b); and a duty factor controller (35) coupled to the shunt switch (34) and to the plurality of series switches (34a, 34b) for providing signals to the shunt switch and to the plurality of series switches that control the respective duty factors of the pulsed current supplied to the plurality of loads. The current source (20a) of Claim 1 wherein the plurality of loads (13) comprise a plurality of light emitting diode arrays (13). The current source (20a) of Claim 1 wherein the constant current source (21) comprises a buck converter (21). The current source (20a) of Claim 3 wherein the buck converter (21) comprises: a switch transistor (24) coupled in series with the power source (11); a resonant inductor (27) coupled in series with the switch transistor (24); a filter inductor (28) coupled in series with the resonant inductor (27); a resonant capacitor (25) and a catch diode (22) coupled in parallel across the filter inductor (28) and the plurality of loads (13); current sensing means (32) for sensing the regulated current provided to the plurality of loads (13); and control means (26) coupled between the current sensing means (32) and the switch transistor (24) for regulating the current supplied to the plurality of loads (13). The current source (20a) of Claim 4 wherein the control means (26) comprises a quasi-resonant controller (26). The current source (20a) of Claim 1 wherein the plurality of loads comprise light emitting diode arrays (13). The current source (20a) of Claim 3 wherein the buck converter (21) comprises: switching means (24) coupled between the charge storage means (12) and the selected diode array (13); a resonant inductor (27) and a filter inductor (28) serially coupled between the switching means (24) and the selected diode array (13); and a catch diode (22) and a resonant capacitor (25) coupled across the filter inductor (28) and the selected diode array (13); diode current sensing means (32) for sensing current flowing through the selected diode array (13); and control means (26) coupled to the diode current sensing means (32) for regulating the current supplied to the selected diode array (13).

Inventors: ORTIZ JOE A, ORTIZ, JOE A.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1995
Main classification: H01S5
IPC codes: H01S3, H05B33, H01S5, H01L33