Abstract:
A sensor circuit detects a current supplied to a set of light-emitting diodes and produces a current reading dependent on the temperature of .operation of these light-emitting diodes. The sensor circuit comprises first and second serially interconnected resistors also connected in series with the set of light-emitting diodes. The sensor circuit also comprises a temperature-dependent impedance connected in parallel with one of the first and second resistors. At least a portion of the current through the set of light-emitting diodes flows through the sensor circuit to enable the first and second serially interconnected resistors and the temperature-dependent impedance to produce a variable voltage signal representative of the current through the set of light-emitting diodes, this variable voltage signal being dependent upon temperature. The above sensor circuit finds application in a substantially constant intensity light source.

Description:
RELATED PATENT APPLICATION 
     This patent application is a divisional application of application Ser. No. 09/471,372, filed Dec. 23, 1999, for an invention titled “NON-LINEAR LIGHT-EMITTING LOAD CURRENT CONTROL”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a temperature-dependent current sensor circuit and a substantially constant intensity light source and corresponding method using this sensor circuit. 
     2. Brief Description of the Prior Art 
     Insertion of an integrated power factor controller circuit such as controller MC33262 from MOTOROLA in an electric power supply system enables easy and efficient control of the power factor and the level of current harmonics. 
     To obtain a power factor equal to unity, controller MC33262 draws current from the ac source in proportion to the sinusoidal voltage. This concept automatically causes the current waveform to be sinusoidal and in phase with the voltage waveform. 
     Also, operation of power factor controller MC33262 requires that the output supply voltage be higher than the peak amplitude of the input sinusoidal voltage in order to draw current from the ac source throughout every cycle of the sinusoid. Accordingly, the output supply voltage must have an amplitude higher than the peak amplitude of the sinusoidal voltage of the ac source. 
     In certain circumstances, an output supply voltage with an amplitude lower than the peak amplitude of the input ac voltage is required. In such cases, power factor controller MC33262 is used as a power-factor-correcting pre-converter; a second power converter is also required to reduce the level of the supply voltage to the desired amplitude. 
     Necessarily, providing a second power converter involves additional costs and requires more space. 
     Furthermore, the voltage/current characteristic of a light-emitting diode is sensitive to temperature causing the current through a light-emitting diode to change very rapidly and non-linearly with the voltage across the light-emitting diode. 
     For example, for a given type of light-emitting diode widely used in the fabrication of traffic signal lights, a constant voltage of 1.8 volts will produce in the light-emitting diode a current of about 7.5 mA at a temperature of −25° C., a current of about 20.5 mA at a temperature of +25° C., and a current of about 30 mA at a temperature of +60° C. The magnitude of the current through the light-emitting diode at a temperature of +60° C. is therefore, for a constant voltage of 1.8 volt, about 1.6 times higher than the magnitude of the current at a temperature of +25° C. Voltage feedback control would therefore be very detrimental to the service life of such a light-emitting diode. 
     Since voltage feedback control of the supply of a light-emitting diode is not desirable, current feedback control is required to ensure durability of the light-emitting diode. 
     Also, a fixed LED output current presents the following drawbacks: 
     at higher temperature the output LED power decrease; and 
     at lower temperature the output LED power increases. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is therefore to eliminate the above discussed drawbacks of the prior art. 
     Another object of the present invention is to regulate the output power, hence the light intensity, of a non-linear light-emitting load. 
     SUMMARY OF THE INVENTION 
     More specifically, in accordance with the present invention, there is provided a sensor circuit for detecting a current supplied to a non-linear load and for producing a current reading dependent on a condition of operation of the non-linear load. The sensor circuit comprises first and second serially interconnected resistors also connected in series with the non-linear load, and a variable impedance connected in parallel with one of the first and second resistors, the impedance varying with the condition of operation of the non-linear load. At least a portion of the current through the non-linear load flows through the sensor circuit to enable the first and second serially interconnected resistors and the variable impedance to produce a variable voltage signal representative of the current through the non-linear load and dependent on the condition of operation. In a preferred embodiment of the invention, the non-linear load is a light-emitting diode (LED) or a plurality of LEDs, and the condition of operation of the LED is temperature. 
     The invention described above therefore procures the advantage of providing a current-representative signal that may be used for feedback control of a non-linear load. Current feedback control is difficult with current sensor circuits which do not provide an output that varies with the condition of operation of the non-linear load. The invention described herein provides this feature in a simple low-cost circuit. 
     The present invention also relates to a substantially constant intensity light source comprising: 
     a) a non-linear light-emitting load; 
     b) a controllable dc voltage and current source for supplying the non-linear light-emitting load with dc voltage and current; 
     c) a current sensor circuit connected in series with the non-linear light-emitting load and the controllable dc voltage and current source, the current sensor circuit having an impedance varying with a condition of operation of the light-emitting load and being supplied with at least a portion of the current through the non-linear light-emitting load, whereby the variable impedance produces a variable current-representative signal; and 
     d) a voltage and current control feedback circuit connected between the current sensor circuit and said controllable dc voltage and current source for controlling the dc voltage and current source in relation to the variable current-representative signal to thereby adjust the dc voltage and current to amplitudes that keep the light intensity produced by the light source substantially constant. 
     Further in accordance with the present invention, there is provided a substantially constant intensity light source comprising: 
     a) a controllable dc voltage and current source having first and second terminals; 
     b) a non-linear light-emitting load connected between the first and second terminals and supplied with dc voltage and current from the controllable dc voltage and current source; 
     c) a current sensor circuit connected in series with the non-linear light-emitting load between the first and second terminals, the current sensor circuit having an impedance varying with a condition of operation of the light-emitting load and being supplied with at least a portion of the current through the non-linear light-emitting load, whereby the variable impedance produces a variable current-representative signal, and 
     d) a voltage and current control feedback circuit connected between the current sensor circuit and the controllable dc voltage and current source and through which the dc voltage and current source is controlled in relation to the variable current-representative signal to adjust the do voltage and current to amplitudes that keep the light intensity produced by the light source substantially constant. 
     The present invention still further relates to a method for keeping the intensity of a light source substantially constant, comprising: 
     a) supplying from a controllable dc voltage and current source a dc voltage and current to a non-linear light-emitting load: 
     b) supplying at least a portion of the current through the non-linear light-emitting load to a current sensor circuit having an impedance varying with a condition of operation of the light-emitting load, whereby the variable impedance produces a variable current-representative signal, and 
     c) feedback controlling the dc voltage and current in relation to the variable current-representative signal to adjust the dc voltage and current to amplitudes that keep the light intensity produced by the light source substantially constant. 
     The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the appended drawings: 
     FIG. 1 is a schematic block diagram of the electronic circuit of a light-emitting-diode lamp Incorporating the current sensor circuit and a power supply system according to the invention; 
     FIG. 2 is a graph showing a LED current as a function of LED voltage at different temperatures without load current control; 
     FIG. 3 is a graph showing LED output power as a function of temperature without load current control; 
     FIG. 4 is a block diagram of the load current sensor circuit according to the invention; and 
     FIG. 5 is a graph showing LED current, LED voltage, equivalent impedance and LED output power as a function of temperature with load current control according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Although the preferred embodiment of the present invention will be described hereinafter with reference to an application of the current sensor circuit according to the invention to a light-emitting-diode lamp, it should be understood that this example is not intended to limit the range of applications of the present invention. 
     Referring to FIG. 1 of the appended drawings, the LED lamp is generally identified by the reference  1 . Lamp  1  comprises a set  2  of light-emitting diodes such as  3 . The set  2  is formed of a plurality of subsets such as  4  of serially Interconnected light-emitting diodes  3 . The subsets  4  of serially interconnected light-emitting diodes  3  are connected in parallel to form the set  2 . 
     The cathode  7  of the last light-emitting diode  3  of each subset  4  is connected to a first terminal  9  of the current sensor circuit  10 . Current sensor circuit  10  has a terminal  11  connected to ground. 
     The set  2  of light-emitting diodes  3  is supplied by an ac source  14 . The alternating voltage and current from the ac source  14  is rectified by a full-wave rectifier bridge  15  and supplied to the anode  16  of the first diode  3  of each subset  4  through a power converter  17 . To further smoothen the current waveform, an EMI (ElectroMagnetic Interference) filter and inrush current limiter  44  can be added between the ac source  14  and the full-wave rectifier bridge  15 . 
     The current flowing through each subset  4  of light-emitting diodes  3  has a value limited by the impedance of current sensor circuit  10 . Also, the current flowing in all the subsets  4  of light-emitting diodes  3  flows through impedances  5  and  6  of the current sensor circuit  10  serially interconnected between the terminals  9  and  11  to convert the total current flowing through the set  2  of light-emitting diodes  3  to a corresponding current-representative voltage signal present on an output  18  of current sensor circuit  10 . 
     In the illustrated example, the controller  19  is the power factor controller manufactured and commercialized by Motorola and identified by the reference MC33262. 
     To enable the controller  19  to perform variable current feedback control on the set  2  of non-linear light-emitting diodes  3 , the current sensor circuit  10  is connected to the input  24  of the power factor controller  19  through the filter circuit  20 . The function of the current sensor circuit  10  is to transform the non-linear relation (LED current/voltage relation with temperature) between the LED supply dc voltage at the output  26  of the power converter  17  and the dc current supplied to the set  2  of light-emitting diodes  3  with temperature into a linear relation. 
     Referring to FIG. 2, LED current (I LED ) measurements at various temperatures are shown with respect to LED voltage when no current sensor circuit according to the present invention is used. 
     In FIG. 2, temperature θ 1  is smaller than temperature θ 2 , which is itself smaller than temperature θ 3 . Note that at a reference LED current (I LEDref ), LED voltage V F1  is greater than LED voltage V F2 , which is itself greater than LED voltage V F3 . 
     At a fixed current (I LEDref ), the output power (P LED ) as a function of temperature θ is given in FIG.  3 . 
     The output LED power P LED  is defined by: 
      P LED =V F ×I LEDref . 
     FIG. 3 shows that, without the current sensor circuit of this invention, at a lower temperature (θ 1 ), the LED output power P LED1  is higher and, at the higher temperature (θ 3 ), the LED output power P LED3  is lower 
     That is: 
     
       
         P LED1 &gt;P LED2 &gt;P LED3 . 
       
     
     In order to avoid variations in the LED output power P LED  with temperature θ at a fixed current, current sensor circuit  10  of FIG. 4 is introduced. 
     As shown in FIG. 4, the current sensor circuit  10  comprises the temperature dependent variable equivalent impedance Z eq , which includes two impedances in series Z 6  and Z 6 . Z 5  comprises a fixed resistor R 12 , ( 12 ) in parallel with thermistor R TH  ( 8 ). Both R 12  and R TH  are in series with impedance Z 6  which can be implemented as a fixed resistor R 13  ( 13 ). The temperature dependent variable equivalent impedance Z eq  is given by:            Z   eq          (   θ   )       =         Z   5     +     Z   6       =           R   12     *       R   TH          (   θ   )             R   12     +       R   TH          (   θ   )           +     R   13                                
     The current-representative voltage signal I mes  is given by the product of LED current I LED  ( 9 ) and a variable equivalent impedance Z eq  (θ) ( 10 ); where Z eq  is formed of passive elements and is a non-linear impedance dependent on the casing of the LED lamp, the power supply, the LEDs and surrounding temperature θ. 
     
       
         I mes =Z eq  (θ) *I LED   
       
     
     The current-representative voltage signal I mes  has an amplitude that is proportional to the magnitude of the current flowing through current sensor circuit  10  (Z eq ). This circuit enables regulation of the dc current supplied to the LEDs as a function of temperature θ. 
     When the temperature θ is constant, the current sensor circuit  10  impedance value Z eq  is constant and the LEDs are driven by a constant current. 
     Referring to FIG. 5, when the temperature θ rises to the maximum temperature θ max , the value of the thermistor R TH  decreases such that: 
     
       
         Z 5 ≅R TH     min     
       
     
     The equivalent sensor impedance value Z eq  (θ) diminishes until it reaches Z eqmin , 
     where 
     
       
         Z eq     min   ≅R TH     min   +R 13   
       
     
     and the maximum current on the LEDs is:          I     LED     m                 a                 x         ≃       I   ref       Z     eq     m                 i                 n           ≃       I   ref         R     TH     m                 i                 n         +     R   13                                
     where I ref  is a voltage providing a fixed LED current reference. 
     As a result I mes  diminishes and the difference E between fixed reference current I ref  ( 47 ) and filtered LEDs current measure I mes  ( 24 ) increases, so that the LED current is increased by the power supply until the difference E=I ref −I mes  equals zero. 
     The maximum current on the LEDs is therefore limited by the choice of R 13  ( 13 ) of current sensor circuit ( 10 ). This in turn maintains a roughly constant power output from the LEDs. 
     Conversely, if the temperature drops until the minimum temperature θmin, the value of resistor R TM  increases such that: 
     
       
         Z 5 ≅R 12   
       
     
     and the equivalent sensor impedance value Z eq  (θ) rises until: 
     
       
         Z eq     max   ≅R 12 +R 13   
       
     
     and the minimum current on the LEDs is:          I     LED     m                 i                 n         ≃       I   ref       Z     eq     m                 a                 x           ≃       I   ref         R   12     +     R   13                                
     As a result I mes  increases and the difference E decreases so that the power supply decreases the current in the LEDs until the difference E is again equal to zero. 
     Hence, the upper limit for current to the LEDs is limited by R 13 , (i.e., R TH  minimum at higher temperature), while the lower current limit is determined by (R 12 +R 13 ), (i.e., R TH  maximum at lower temperature). 
     As explained above this LED lamp output regulation is based on variation of current measurement with temperature as shown in FIG.  5 . 
     Referring back to FIG. 1, the filter circuit  20  comprises a resistor  21  connected between output  18  of the current sensor circuit  10  and input  24  of the controller  19 , and a capacitor  25  connected between terminal  23  of the resistor  21  and the ground. 
     The values of the resistor  21  and capacitor  25  also contribute to transform the non linear relation between the LED supply dc voltage at the output  26  of the power converter  17  and the dc current supplied to the set  2  of light-emitting diodes  3  into a linear relation. The values of the resistor  21  and capacitor  25  must therefore be precisely and carefully adjusted in relation to the current characteristic of the voltage/current characteristic of the type of diodes  3  being used. 
     To current feedback control the supply of the set  2  of light-emitting diodes  3 , the controller  19  requires on its input  24  a current-representative voltage feedback signal which varies linearly with the LED supply dc voltage at the output  26  of the power converter  17 . The current-representative voltage feedback signal on input  24  will be compared to I ref ( 47 ) in comparator  48 . The output of comparator  48  is a high/low difference-representative signal fed to multiplier  49 . Multiplier  49  also has as an input a reference control voltage from output  52  of an input reference current sensor  51 . Multiplier  49  adjusts its gain according to its inputs and produces a corresponding current reference waveform signal  50 . The multiplier output  50  controls the inductor current sensor  35  threshold as the ac voltage traverses sinusoidally from zero to peak line voltage. This has the effect of forcing the MOSFET  33  “on time” to track the input line voltage, resulting in a fixed drive output “on time”, thus making the preconverter load ( 17  plus  4 ) appear to be resistive to the ac line. Controller  19  also receives on input  38  (zero current detector input) the current-representative voltage appearing across additional coil  37  (to be described later) through resistor  39 . Input  38  is compared with, in a preferred embodiment, a 1.6V reference  56  in comparator  55 . The output of comparator  55  is a high/low difference-representative signal  54  fed to multiplier latch  53 . The multiplier latch  53  also receives a voltage signal input  36  from the inductor current sensor  35 . The multiplier latch  53  ensures that a single pulse appears at the drive output during a given cycle. Multiplier latch  53  will therefore produce the high or low logic level drive output for controlling MOSFET transistor  33  an or off thereby effectively controlling output  28  of flyback power converter  17 . 
     Still referring to FIG. 1, the power converter  17  comprises an inductor device  30  having a core  29 , and a coil  27  supplied with full-wave rectified voltage and current from the rectifier bridge  1   5 . A second multi-tap coil  28  is wound onto the core  29  of the inductor device  30 , The coils  27  and  28  act as primary and secondary windings, respectively, of a transformer. Rectified voltage and current applied to the coil  27  will induce in the coil  28  rectified voltage and current transmitted to a capacitor  31  through a diode  32 . Electrical energy is stored in the capacitor  31  to convert the full-wave rectified voltage and current induced in the coil  28  to dc voltage and current supplied to the output  26  of the power converter  17  and therefore to the set  2  of light-emitting diodes  3 . Diode  32  prevents return of the electrical energy stored in the capacitor  31  toward the coil  28 . The level of the dc voltage across the capacitor  31  and therefore the level of the LED supply dc voltage on the output  25  is adjusted by selecting the appropriate number of LEDs in series on subset  4  and varies with the type of LEDs as well as with temperature. 
     Supply of coil  27  of the inductor device  30  is controlled by an output  34  of the controller  19  through the above mentioned MOSFET power transistor  33 . The current supplying the coil  27  is converted to a voltage signal by the inductor current sensor  35  connected between MOSFET transistor  33  and the ground. The inductor current sensor  35  comprises an output  55  for supplying the voltage signal to an input  36  of the controller  19 , and therefore to the multipler latch  53 . 
     The current through the coil  27  is also measured through the additional coil  37  also wound on the core  29  of the inductor  30 . The current-reprerentative voltage appearing across the additional coil  37  is supplied, as mentioned hereinabove, to the input  38  of the controller  19  through the resistor  39  and therefore to the comparator  55 . 
     The additional coil  37  is also connected to an accumulator  42 . formed by a capacitor  40 , through a diode  41 . The function of the accumulator  42  is to supply an input  43  of the controller  19  with a dc voltage amplitude higher than a minimum voltage reference to enable operation of the controller  19 . The capacitor  40  is charged through a branch switching device  45  and a resistor  46 . 
     Input reference current sensor  51  is responsive to the full-wave rectified voltage at the output of the rectifier bridge  15  to supply on its output  52  the reference control voltage supplied to the multiplier  49  of the controller  19 . 
     Upon switching the LED lamp  1  on, the capacitor  40  is discharged. In response to the full-wave rectified voltage which then appears at the output of the rectifier bridge  15 , the branch switching device  45  closes to allow the full-wave rectified voltage from the rectifier bridge  15  to charge the capacitor  40  through the resistor  46  until the voltage across the capacitor  40  exceeds the minimum voltage reference required to operate the controller  19 . 
     Conduction of the MOSFET transistor  33  causes a current to flow through the sensor  35  which then produces on its output  55  a current signal applied to the multiplier latch  53 . Conduction of the MOSFET transistor  33  also causes current supply to the act  2  of light-emitting diodes  3  as described in the foregoing description, and to the current sensor circuit  10  to produce an input current feedback signal  24  supplied to controller  19  through the filter circuit  20 . 
     It should be mentioned that since the reference control voltage is supplied to the multiplier  49  by the input reference current sensor  51  in response to the full-wave rectified signal from the rectifier bridge  15 , the amplitude of this reference control voltage and therefore the gain of the multiplier  49  varies with the amplitude of the full-wave rectified voltage. 
     It should also be understood that every time the voltage signal from the inductor current sensor  35 , supplied to the multiplier latch  53 , exceeds the amplitude of the signal  50  from the multiplier  49 , the output of multiplier latch  53  (drive output) then passes from a high logic level to a low logic level to turn the MOSFET transistor  33  off, to thereby prevent that the dc current through the set  3  of light-emitting diodes  3  exceeds a safe level. 
     Those of ordinary skill in the art will appreciate that the current flowing though the MOSFET transistor  33  is proportional to the full-wave rectified voltage at the output of the rectifier bridge  15 . The current waveform is sinusoidal and in phase with the voltage waveform so that the power factor is, if not equal to, close to unity. To further smoothen the current waveform and withdraw the MOSFET switching high frequencies therefrom, an EMI filter  44  can be added, as mentioned in the foregoing description, between the ac source  14  and the full-wave rectifier bridge  15 . 
     To draw current from the ac source  14  throughout every cycle of the sinusoid, the supply voltage at the output  26  of the power converter  17 , i.e., the dc voltage across the capacitor  31 , must have an amplitude higher than the peak amplitude of the sinusoidal voltage of the ac source  14 . To enable reduction of the amplitude of the dc voltage across capacitor  31  to a value lower than the peak amplitude of the sinusoidal voltage of the ac source  14 , the key element of the “Boost” type topology of FIG. 1, i.e., the inductor  30 , has been modified. More specifically, the second multi-tap coil  28  has been wound onto the core  29 . The coils  27  and  28  act as the primary and secondary windings, respectively, of a transformer, and each tap  100  corresponds to a given level of the de voltage on the output  26  of the power converter  17 , each given level being of course lower in amplitude than the peak sinusoidal voltage of the ac source. Also, the number of turns associated to the different taps  100  of the coil  28  has been evaluated in relation to the number of turns of the coil  27  of the inductor  30  in order to produce transformation ratios as accurate as possible such that, irrespective of which tap  100  is used to obtain a given output voltage level, the controller  19  will behave in the same manner as when the do voltage at the output  26  of the power converter  17  is fixed and higher than the peak amplitude of the ac input voltage. 
     Operation of the power factor controller  19  manufactured and commercialized by Motorola under the reference MC33262 is believed to be otherwise well know to those of ordinary skill in the art and, accordingly, Will not be further described in the present specification. Of course, it is within the scope of the present invention to use another type of feedback controller. 
     Although the present invention has been described hereinabove by way of a preferred embodiment thereof, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.