Patent Abstract:
The capacitor-less LED drive is an LED drive circuit having a design based on the utilization of the internal capacitance of the LED to replace the smoothing capacitor in a conventional buck converter in a power supply. LED lighting systems usually have many LEDs for better illumination that can reach multiple tens of LEDs. Such a configuration can be utilized to enlarge the total internal capacitance, and hence minimize the output ripple. Also, the switching frequency of the buck converter is selected such that minimum ripple appears at the output. The functionality of the present design is confirmed experimentally, and the efficiency of the drive is 85% at full load.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power supply circuits, and particularly to a capacitor-less LED drive. 
     2. Description of the Related Art 
     Light emitting diodes (LEDs) are beginning to experience widespread use in many lighting applications. LED lighting is replacing the florescent lighting because of its advantages, mainly low power consumption and long life expectancy. However, commercial LED drive circuits limit the life expectancy of the LED lighting system by around one-fifth of the lifetime of the LED itself. The main source of shortening the lifetime of the drive is the smoothing capacitor. This is due to the leakage in this capacitor and, hence, degradation in the drive circuit with time. Several works on electrolytic capacitor-less LED drives have been presented to maximize the overall lifetime of the LED system. However, most of the works presented require relatively complicated power circuit or current-controlled technique to reduce the size of the energy storage capacitor. 
     Thus, a capacitor-less LED drive solving the aforementioned problems is desired. 
     SUMMARY OF THE INVENTION 
     The capacitor-less LED drive circuit is based on a buck converter circuit where an LED replaces the smoothing capacitor. The internal capacitance of the LED (or an LED array) will act as smoothing capacitor when a proper switching frequency and duty cycle are chosen. 
     These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the capacitor-less LED drive according to the present invention. 
         FIG. 2  is a schematic diagram showing a DC mode model of a light emitting diode (LED). 
         FIG. 3  is a schematic diagram showing an AC mode model of a light emitting diode (LED). 
         FIG. 4  is a plot showing V-I characteristics curve of a single white LED. 
         FIG. 5  is a plot showing effective capacitance vs load current (LED current) at different frequencies. 
         FIG. 6  is a plot showing output voltage and ripple voltage as a function of duty cycle at different frequencies in a capacitor-less LED drive according to the present invention. 
         FIG. 7  is a plot showing error percent (deviation of experimental output voltage from theoretical calculations) as a function of duty cycle at different frequencies in a capacitor-less LED drive according to the present invention. 
         FIG. 8  is a plot showing ripple voltage as a function of time of the capacitor-less LED drive according to the present invention at 100 kHz with a duty cycle of 18%. 
         FIG. 9  is a plot showing ripple voltage as a function of time of the capacitor-less LED drive according to the present invention at 100 kHz with a duty cycle of 40%. 
         FIG. 10  is a plot showing DC output voltage and ripple voltage as a function of frequency of the capacitor-less LED drive according to the present invention with a duty cycle of 40%. 
         FIG. 11  is a plot showing efficiency as a function of duty cycle at selected frequencies for the capacitor-less LED drive according to the present invention. 
         FIG. 12  is a plot showing efficiency as a function of frequency for the capacitor-less LED drive according to the present invention. 
     
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The capacitor-less LED drive circuit is based on a buck converter circuit where an LED replaces the smoothing capacitor. The internal capacitance of the LED (or an LED array) will act as smoothing capacitor when a proper switching frequency and duty cycle are chosen. 
     As shown in  FIG. 1 , the capacitor-less LED drive circuit  100  rectifies an AC source VAC using an AC-DC rectifier circuit  101 . The negative terminal of the rectifier circuit  101  is connected to the source of a switching transistor Q 1  in parallel with a diode, as shown at  104 . A gate of the transistor Q 1  is connected to a pulse source, Vpulse, which switches the transistor Q 1  on and off at a selected duty cycle. The drain of Q 1  is connected to the anode of diode D 1  and to a first lead of inductor L, The cathode of diode D 1  is connected to the positive terminal of the AC-DC rectifier circuit  101 . An LED  102  (or an array of LEDs connected in parallel to each other) is connected between the cathode of diode D 1  and a second lead of inductor L (the anode of the LED  102  being connected to cathode of diode D 1 , and hence the positive terminal of the rectifier circuit  102 , and the cathode of LED  102  being connected to the second lead of the inductor L). 
     An LED in conduction mode can be modeled using a resistor and an ideal diode for DC mode  200  and a capacitor and a resistor in parallel for AC mode  300  as shown in  FIGS. 2 and 3 , i.e., an LED inherently exhibits capacitance, which enables substitution of an LED for an electrolytic capacitor in buck converter circuits in power supplies. We have carried out many experimental tests to come up with a new mathematical model that represents the DC output voltage across the LEDs. The LED equivalent circuits shown in  FIGS. 2 and 3  are used. The DC output voltage is given by: 
                       V     O   ⁡     (     D   ⁢           ⁢   C     )         =         D   ⁡     (     Vin   -     V   ds       )       -       D   ′     ⁢     V   d           1   +       R   L       R   LED             ,           (   1   )               
where RLED is the LED&#39;s internal resistance. The value of RLED depends on the current passing through the LED, and it can be deduced from the I-V characteristics curve of the LED shown in graph  400  of  FIG. 4 . It is clear from plot  400  that as the current increases, the value of RLED will decrease. In the AC model  300  of  FIG. 3 , r s  represents the constant series contact resistance and quasi-neutral region resistance of the LED, r d  represents the small signal resistance of the LED at certain DC current, and C d  represents the diffusion capacitance at a certain DC current. In conduction mode, r d  is the reciprocal of the conductance, which is equal to the DC current divided by the thermal voltage. This indicates that as the DC current increases, the value of the resistance r d  will decrease. Moreover, the value of C d  also is a function of the conductance, and its value will increase as the current increases. The behavior of r d  and C d  gives an indication that as the DC current increases, the ripple voltage will decrease, which is another parameter that can control and affect the ripple voltage. This fact is supported by experimental results.
 
     It is important to point out that the value of C d  is linearly changing with the DC current only in strong conduction mode. However, during the OFF period in the switching Buck converter pulse, the LED internal resistance will draw the stored charge, and the output voltage will decrease. If the OFF period is long enough, the value of the diffusion capacitor will be very small, causing a dramatic drop in the output voltage that might cause flicker in the LED light. Consequently, this will limit the OFF period, therefore limiting the frequency and duty cycle to certain ranges. The effective capacitance of the LED is found as follows: 
                       I   pp     =           V     i   ⁢           ⁢   n       -     (       V   o     +     V     d   ⁢           ⁢   s       +     V     r   L         )         Lf   s       ⁢   D       ,           (   2   )               
where I pp  is the ripple current through the inductor L. From circuit  100  and model  300 , assume no diffusion capacitance, C d . Then:
 
 V   r   =I   pp   R   LOAD   =I   pp ( r   d   +r   s )  (3)
 
If we assume a capacitance C d  and an infinite parallel resistance r d , then:
 
                     V   r     =         I   pp     ⁡     (       1     8   ⁢     fC   d         +     r   s       )       .             (   4   )               
From equation 4, the effective impedance of the capacitor is 1/(8f C d ). Equations 3 and 4 can be written as:
 
 V   r   =αI   pp   r   d   +I   pp   r   s ,  (5)
 
and
 
                       V   r     =       β   ⁢           ⁢       I   pp     ⁡     (     1     8   ⁢     fC   d         )         +       I   pp     ⁢     r   s           ,           (   6   )               
where α+β=1 and from the current divider rule,
 
                   α   =         1     1   +     8   ⁢     fC   d     ⁢     r   d           ⁢   and   ⁢           ⁢   β     =         8   ⁢     fC   d     ⁢     r   d         1   +     8   ⁢     fC   d     ⁢     r   d           .               (   7   )               
Using the small model approximation for the pn junction diode, the DC current is related to the value of the dynamic resistance and the diffusion capacitor by:
 
                       r   d     =         1     g   d       ⁢   and   ⁢           ⁢     C   d       =     τ   ⁢           ⁢     g   d           ,           (   8   )               
where τ is the diffusion time constant and g d  is the known transconductance, defined as g d =I DC /ηV t , and V t  is the thermal voltage. By incorporating the definitions of equation (8) in the values of α and β then:
 
     
       
         
           
             
               
                 
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                             τ 
                           
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   9 
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     From equation (9), if the value of 8fτ&gt;&gt;1, then the impedance of the capacitor is very small compared to the resistance r d , leading to α=0 and β=1. This case will satisfy the ideal situation with a negligible load effect on the ripple voltage. In other words, all the current I pp  will flow through the capacitor. Substituting the values of α and β from equation (9) in equations (5) and (6) leads to the ripple equation, which is given by: 
                     V   r     =         I   pp     ⁡     (       1       g   d     ⁡     (     1   +     8   ⁢   f   ⁢           ⁢   τ       )         +     r   s       )       =         I   pp     ⁡     (       1       g   d     +     8   ⁢           ⁢     fC   d           +     r   s       )       .               (   10   )               
Rewriting equation (10) to find the effective capacitance C d  using the experimental data yields:
 
                     C   d     =       1     8   ⁢           ⁢   f       ⁢       (       1         V   r       I   pp       -     r   s         -     g   d       )     .               (   11   )               
A plot of the effective capacitance as a function of the LED current for different frequencies is shown in plot  500  of  FIG. 5 . It is evident from the plot that the effective capacitance at 200 kHz is high, since the impedance of the capacitance is much smaller than that of the dynamic resistance.
 
     The capacitor-less LED drive circuit  100  shown in  FIG. 1  was connected in the laboratory using off-the-shelf components to test the proposed design experimentally. The LED used is the sum of three series packages of 11 parallel LEDs per package, giving a total of 33 LEDs. The output voltage is measured across the LED packages. The components used are as follows: L is an inductor of 470 μH, Q 1  is an N-MOS power transistor BUZ71, Vpulse is the switching control pulse with an amplitude of 10V, and D 1  is a silicon fast-switching diode 1N914. The inductor&#39;s series resistance is measured, and its value is around 4Ω. We assume the ac source is rectified and provides a DC output called Vin with nominal voltage of 35V. The LED&#39;s I-V characteristics are shown in plot  400 , which has been used to extract the value of R LED  for different DC current values. 
     The behavior of the circuit was studied by varying the duty cycle of Vpulse from 18% to 44% at three different frequencies (100 KHz, 150 KHz and 200 KHz). The maximum duty cycle was set to 44% because this duty cycle will produce the maximum current that can be handled by the LEDs. The output voltage was probed across the LEDs for the DC output and ripple voltage, and results were plotted as shown in plot  600  of  FIG. 6 . It is clear from plot  600  that as the duty cycle increases, the DC output voltage increases. The ripple voltage is decreasing with the increase of frequency. 
     The deviation between theoretical and experimental results is shown in plot  700  of  FIG. 7 . It is evident from plot  700  that a designer should select the switching pulse duty cycle to be greater than 30% to minimize the error and use higher frequencies to minimize the ripple voltage. 
     From plot  600 , the DC voltage is linearly changing with the duty cycle for D&gt;30%. Also, the error curve in plot  700  shows that for duty cycle greater than 30%, the error is less than 3%. However, the error is much greater with less than 30% duty cycle, and this is due to the long OFF period of the buck switch, resulting in non-linear behavior of the LED voltage. If the voltage across the LED is below a certain value, there will be no diffusion capacitor and the LED&#39;s voltage will drop logarithmically, causing the large error shown. This value can be estimated from the knees of each curve in plot  700 , and it depends on the forward current as well, since it depends on how deep the LED is in the conduction region. 
     Plots  800  and  900  of  FIGS. 8 and 9 , respectively, show the ripple voltage at 100 kHz, with a duty cycle of 18% and 40%, respectively. The non-linearity is clearly shown in plot  800 , where the off period was long enough to drive the LED to the weak conduction region, while the ripple of plot  900  is almost linear. It is clear that the ripple is linear for higher duty cycle. 
     To see the changes on the DC output voltages and ripple, the frequency was swept at a fixed duty cycle of 40% from 50 kHz to 300 kHz, and the output was probed. The result is shown in plot  1000  of  FIG. 10 . It is clear that the ripple voltage is decreasing as the frequency is increasing, and the DC voltage is almost constant. The minimum ratio of ripple voltage to DC voltage is around 1.4%, and it can be decreased further by increasing the frequency. 
     Efficiency is an important factor in an LED drive. The efficiency was found by measuring the DC output voltage, the output current, the DC input voltage and the input current for each duty cycle for different frequencies. Experimental results are displayed in plot  1100  of  FIG. 11 , and show that the average efficiency is 85%. The efficiency can be improved further using an inductor with smaller internal resistance and transistor with smaller ON resistance. 
     Because of the slight changes in the DC output voltage, the efficiency is barely changing with the change of the frequency, as shown in plot  1200  of  FIG. 12 . The average of the efficiency over the frequency range was about 88%. Increasing the frequency further will lead to smaller ripple voltage and smaller components for better integration. However, increasing the switching frequency will reduce the efficiency of the drive because of the switching power loss for light loads. As for LED lighting applications, the LED load needs to draw high current specially when using a capacitor-less drive. This is because it is better to use many parallel LEDs for higher summation of LED capacitance, which gives this method one more advantage. 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.

Technology Classification (CPC): 8