Abstract:
Optimal power supply topologies that do not use output bulk capacitors as well as freewheeling diodes in output section to therefore provide efficiency, cost, volume, and weight advantage over the existing solutions. The present invention applies a pulse-width modulated switched current at higher frequency (in order of 10 to 100 kHz) to LEDs without degrading optical performance. LEDs&#39; average current control is attained using a feedback loop that senses the average value of LEDs&#39; current and controls the current by varying the pulse width of applied current.

Description:
BACKGROUND OF THE INVENTION 
     Recently light emitting diode (LED) based lighting solutions are gaining popularity over traditional light sources such as incandescent lamps, Fluorescent Lamp, Halogen lamps, and high intensity discharge (HID) lights. LED offers highly reliable and energy efficient lighting solutions. LED light output is controlled by forward current flowing through it, hence LED-based lighting solutions need current control electronic circuits. Cost, volume, weight and efficiency of electronics driver circuit&#39;s components play a major role in order to make LED lighting solutions successful for retrofit applications as well as new products developments. 
     The constant current sources for LEDs are derived using standard DC-DC converter topologies. These may include non-isolated DC-DC converters such as buck, boost, buck-boost, Sepic/cuk converters, or isolated topologies like fly back, forward converters etc. The control logic for these power supplies is modified in such a way to maintain constant LED current. This is achieved by sensing LED current and comparing it with a reference value. The list of these converters can be huge but all these solutions use free-wheeling diode(s) and a bulk capacitor in an output section. All these solutions suffer in terms of efficiency, cost, weight and volume because of the output filter capacitor and freewheeling diodes. 
     Some recent solution shows an optimized LED driver topology without an output bulk filter capacitor to reduce cost, weight and volume of LED drivers. Although this solution suffers in terms of efficiency, weight, cost and volume due to use of freewheeling diodes. Freewheeling diodes create switching and conduction losses which reduces efficiency and require bulkier heat-sinks for larger heat dissipation. In order to improve the efficiency and reduce heat-sink requirements, fast recovery and/or Schottky silicon carbide diodes are used; however this increases the cost of LED drivers. 
     SUMMARY OF THE INVENTION 
     Optimal power supply topologies are proposed that do not use output bulk capacitors as well as freewheeling diodes in output section to therefore provide efficiency, cost, volume, and weight advantage over the existing solutions. The present invention applies a pulse-width modulated switched current at higher frequency (in order of 10 to 100 kHz) to LEDs without degrading optical performance. LEDs&#39; average current control is done using a feedback loop that senses the average value of LEDs&#39; current and controls the current by varying the pulse width of applied current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  is a schematic diagram of an exemplary circuit formed in accordance with an embodiment of the present invention; 
         FIG. 2  shows waveforms of signals within the circuit shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a second exemplary circuit formed in accordance with an embodiment of the present invention; 
         FIGS. 4 and 5  show interleaved circuits that correspond to the circuits shown in  FIGS. 1 and 2 , respectively; and 
         FIG. 6  is a schematic diagram of another exemplary circuit similar to the circuit shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows exemplary power and control circuit  20  according to an embodiment of present invention. The power and control circuit  20  includes a direct current (DC) voltage source  24 , which supplies required power to drive sufficient current to an LED string  26  through a switching network formed by an input inductor  30  and an actively controlled power semiconductor switch  32 . The LED string  26  includes any number of LEDs in series depending upon the application requirement. This topology is recommended for applications in which voltage requirements of the LED string are higher than the input voltage source  24 . A first end of the inductor  30  is connected to the positive lead of the DC voltage source  24 . A second end of the inductor  30  is connected to a drain (or source depending upon the type of switch) of the semiconductor switch  32  and an anode end of the LED string  26 . The cathode end of the LED string  26  is connected to the source (or drain depending upon the type of switch) of the semiconductor switch  32  and the negative lead of the DC voltage source  24 . 
     Those skilled in art may appreciate that there are wide varieties of controlled power semiconductor switches that may be used for this application, such as power transistors, MOSFET, IGBTs, GTOs, etc., which can be used as the power switch  32  in the switching network. 
     In a first mode of operation, when the semiconductor switch  32  is on, current is denied to the LED string  26 . In this mode, current is increased through the inductor  30  storing energy. 
     In a second mode of operation, the switch  32  is turned off thereby causing the inductor  30  to discharge through the LED string  26 . These modes repeat at a predefined frequency thereby providing a desired current to the LED string  26 . 
       FIG. 2  shows the current waveform through the inductor  30 , the current waveform through the LED string  26  and a control signal with appropriate turn-on and turn-off times that is provided to the semiconductor switch  32 . The LED string current becomes equal to the inductor current during the off period of the switch  32  and remains zero during the on period of the switch  32 . The average value of LED string current is proportional to switch on time therefore average LED string current control is possible by modulating switch on time at a given frequency. Switch on time modulation is referred to as switch duty cycle control where duty cycle is defined as ratio of switch on time with respect to total switching period. 
     In order to control the average value of LED string current accurately, a negative feedback system increases the switch duty cycle whenever a higher value of LED string current is desired and reduces the switch duty cycle whenever a lower value of LED string current is desired. A current sense signal is obtained from an LED current sensor  40 . Then the sensed current is applied to a low pass filter  42  to get an average value. An input current reference (Iref)  44  is subtracted from the average value obtained from the low pass filter  42  at a combiner  48  to form part of the negative feedback control system. Iref  44  is the desired average value of the LED current. The subtraction error (i.e., output of the combiner  48 ) is fed to a proportional-integral-derivative (PID) controller  50 , which provides a peak current reference for the switch current waveform. A comparator  54  compares a peak switch current sensed at the source (or drain depending upon the type of switch) of the switch  32  by a current sensor  56  to the peak current reference generated by the PID controller  50 . The current sensor  56  senses the instantaneous value of switch current. 
     Thus when the sensed switch peak current becomes equal to the peak current reference, the comparator  54  gives a pulse to reset an R-S latch  60 . When reset input of the R-S latch  60  goes high, the R-S latch  60  send a low/off signal to the gate of the switch  32  thus turning off the switch  32 . Output of R-S latch  60  is set by an independent clock  62  to provide turn-on pulse for switch  32 . Frequency of the clock  62  decides the switching frequency for the switch  32 . A gate driver buffer circuit  66  amplifies the output of RS-latch  60 . 
     Those who are skilled in art may appreciate that the components of the negative feedback control system mentioned above are readily available as commercial off the shelf (COTS) Integrated Circuit (IC) components, from various manufacturers such as Texas Instrument, Linear Technology, National Semiconductor, etc., generally referred to as current mode controller ICs. 
       FIG. 3  shows a power circuit and control circuit  100  of another embodiment of the present invention. A drain (or source depending upon the type of switch) of a power semiconductor switch  104  is connected to the positive terminal of a DC power source  106 . The source (or drain depending upon the type of switch) of the switch is connected to a first end of an inductor  108  and to a cathode of an LED string  112 . The polarity of the LED string  112  is reversed as compared to the string  26  shown in  FIG. 1 . This circuit  100  can be used for applications in which the LED string  112  requires voltage magnitude that is either lower or higher than supplied by the input voltage source  106 . 
     A feedback control system of the circuit  100  is identical to the feedback control system of the circuit  20  except the LED current sensor  40  is connected to the anode of the LED string  112  and the current sensor  56  is connected to the second end of the inductor  108 . Those skilled in art would appreciate that there exist different type of control methodologies other than explained previously. Some of the control methodologies are mentioned below:
         Constant frequency discontinuous conduction peak current mode control. This is another type of current mode control wherein the inductor current drops down to zero value before the next switching cycle starts;   Constant frequency continuous/discontinuous conduction voltage mode control;   Variable frequency constant ON time/constant OFF time voltage/current mode control; and   Variable frequency Hysteresis current mode control.       

       FIGS. 4 and 5  show interleaved versions of first and second embodiments of  FIGS. 1 and 2  respectively, applying a pulse-width modulated current to the LED string  26 . Interleaved variants gives a major benefit of smoothening the LED string current. If there are “N” interleaved circuits then, LED current frequency will be “N” times the individual circuit switching frequency. 
     In absence of a diode, the moment a switch is turned ON, current in all inductors of the circuit will start ramping up via this turned ON switch. The diodes  34 ,  110  ensure the ramping of current in only one inductor  30 ,  108  at a time. Without the diode the interleaved operation will not work. 
       FIG. 6  shows a circuit that is a slight variation of the circuit  100  shown in  FIG. 3 . The circuit of  FIG. 6  includes a diode  120  located between the LED string  112  and the switch  104 . The diode  120  provides reverse blocking capabilities. 
     The circuit  100  shown in  FIG. 3  can be used for applications in which LED string required voltage magnitude is either lower or higher than input voltage source provided LEDs with appropriate reverse blocking capabilities are available. The circuit  100  works as—is for the applications in which LED string required voltage magnitude is higher than input voltage source. For the operation of circuit  100  in which input voltage is higher than the reverse blocking capability of LED string, the additional diode  120  with appropriate reverse blocking capability will ensure the safe operation of circuit  100 . 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.