Abstract:
A current regulator for non-isolated, line voltage LED Driver circuits solves the problem of LED flicker caused by line noise in the visible frequency range. In one aspect of the invention, LED flicker frequency may be increased beyond the line frequency. In another aspect, line voltages in excess of LED voltage ratings may be tolerated. The driver may be implemented using discrete circuit elements or in software and utilizes separate current regulation for different segments of an AC line voltage cycle.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to the field of light-emitting diode (L.E.D. or LED) driver circuits. In particular, the present invention is directed to a half- or quarter-cycle current regulator for non-isolated, line voltage L.E.D. driver (“ballast”) circuits. 
     BACKGROUND 
     LEDs are solid-state devices that produce light when electrical current flows therethrough. They are typically polarized, low-voltage devices, although they may be combined into arrays requiring higher voltages. Despite having minimum and operating voltage specifications, LEDs are typically specified and regulated with respect to the amount of current required for operation. Unlike conventional loads that require a fixed voltage, LEDs typically require a near-constant current for ideal operation. 
     An LED light source that runs directly from a commercial alternating current (AC) 120 volt (V) power supply (“line voltage”) usually requires a current-regulated ballast circuit for ideal operation. This circuit is responsible for both current and temperature regulation, and should be immune to voltage spikes and other noise on the AC line within a predetermined design range. 
     A non-isolated linear LED driver can provide excellent affordability and reliability and can be made to have a very small profile. Without the additional bulk and cost of a transformer, inductors, and, particularly, electrolytic capacitors, the driver can be made very compact and the detrimental effects of capacitor aging can be minimized. 
     However, without electrolytic capacitors to filter (or “buffer”) the line voltage, the circuit is subject to any noise that might be present on the incoming power lines (or “mains”). Particularly, noise in the visible frequency range, which extends up to about 100 Hz, may be detrimental to the performance of LEDs. 
     There are several ways that ideal line conditions may be disturbed such that alternating cycles and rising or falling edges may become asymmetrical from their counterparts. These conditions may conspire to create an undesirable visible flicker in the LED light output. 
     For example, if an LED load draws current from an AC power source that is also connected to a circuit having a poor power factor, the rising and falling edges of the line voltage may become mismatched or asymmetrical. Other sources of line distortion, such as fluorescent lighting ballasts, can easily cause mismatches between edges within the same cycle. 
     Wall dimmers often use inexpensive DIAC-TRIAC circuits. This type of wall dimmer implements phase-cut style dimming where the rising edge is delayed for some time according to the setting of the adjustment potentiometer. These devices often exhibit asymmetrical behavior in the different operating quadrants of the TRIAC which may result in each alternative half-cycle on the line having slightly differing durations. When a wall dimmer using phase-cut style dimming is connected to the same AC power source as an LED driver, this difference in half-cycles may manifest itself in the LED light output as a disturbing flicker. 
     One solution for dealing with line voltage variations is to integrate the voltage across a sense resistor provided in series with the LED load and apply the output of the integration (in the form of a voltage) to the gate of a metal-oxide-semiconductor field-effect transistor (“MOSFET”) provided in series with the LED load. This works to an extent, but the time constant necessary for good regulation is longer than several cycles of the line voltage; therefore, this type of implementation cannot compensate for short-term fluctuations such as half-cycle or quarter-cycle asymmetry. 
     In order to compensate for such short-term fluctuations, a massive electrolytic capacitor with a rating about equal to the voltage drop across the LED load may be connected in parallel with the LED array. However, due to their large size and cost, utilizing an electrolytic capacitor in such a way can be detrimental to the compactness and/or price, and thus marketability, of a LED driver. 
     SUMMARY OF THE DISCLOSURE 
     Embodiments of the present invention variously address problems of LED flicker caused by line noise in the visible frequency range. In one exemplary embodiment, flicker frequency may be increased beyond the line frequency (at the expense of line distortion). In another exemplary embodiment, line voltages in excess of LED voltage ratings may be used. 
     In one implementation, the present disclosure is directed to a circuit for use with an alternating current voltage source and one or more light-emitting diodes. The circuit includes a rectifier, a sensor, an electrically variable resistor, and a regulator capable of applying a voltage, which includes at least two integrators, a timer, and at least one switch, wherein the rectifier rectifies the alternating current voltage source to produce a rectified voltage source and provides it to the one or more light-emitting diodes; the sensor detects a circuit condition and provides a sensor output related to the alternating current voltage source; at least one of the integrators provides an integration output based at least on the sensor output; the at least one switch selects the integration output based on a timing signal from the timer; the regulator applies a voltage corresponding to the selected integration output to the electrically variable resistor; and the electrically variable resistor reacts to the applied voltage in order to limit the influence of undesirable variations in the rectified voltage source on the one or more light-emitting diodes. 
     In another implementation, the present disclosure is directed to a method. The method includes synchronizing a timer with an alternating current voltage source waveform, the timer having at least two increments corresponding to separate portions of a repeating wave cycle in the alternating current voltage source waveform; monitoring a circuit condition; and for each timer increment: generating values in accordance with the monitoring; integrating at least one of the values to produce an integration output; and applying a voltage corresponding to the output to an electrically variable resistor; wherein: the integration output is selected from each of the integration outputs based upon the timer increment; and the electrically variable resistor reacts to the applied voltage in order to mitigate undesirable variations in the voltage source in order to protect a load. 
     In still another implementation, the present disclosure is directed to a circuit. The circuit includes a rectifier, a load, an electrically variable resistor comprising at least one pin, a sensor, at least two integrators, and at least one switch, wherein the rectifier, the load, the electrically variable resistor, and the sensor are arranged in series; the at least one switch selects at least one of the at least two integrators, electrically connecting a selected integrator with the at least one pin of the electrically variable resistor; and the electrically variable resistor moderates a circuit condition of the load. 
     In yet another implementation, the present disclosure is directed to a method. The method includes detecting a repeating wave cycle corresponding to an alternating current voltage source waveform, determining a plurality of segments of the repeating wave cycle, and moderating a circuit condition of a load independently for each the segment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a circuit diagram for a LED driver system utilizing two integrators according to an exemplary embodiment of the invention; 
         FIG. 2  is a circuit diagram for an exemplary integrator according to an embodiment of the present invention; 
         FIG. 3  is a circuit diagram for an exemplary high-speed cut-out according to an embodiment of the present invention; 
         FIG. 4  is a circuit diagram for a LED driver system utilizing four integrators according to an alternative embodiment of the present invention; and 
         FIG. 5  is a circuit diagram for a LED driver system utilizing a microprocessor according to a further alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention overcome disadvantages of using a large electrolytic capacitor as discussed above by providing a current regulator configured to adapt to line voltage variations by switching between separate integrators corresponding to the different parts of the incoming line voltage wave. In various embodiments, the integrators may be switched on and off in sequence using edge and/or zero-crossing detection methods, analog switches, and a one- or two-bit counter (or timer), or by any other suitable methods known in the art. In other embodiments, the integrators may be implemented with multi-layer ceramic capacitors (MLCC) or any other capacitors known in the art. However, it should be noted that no electrolytic capacitors are required and that the teachings of the present invention may be implemented in most instances with capacitors having a voltage rating not greater than about 10 V, and in other embodiments employing capacitors with a maximum voltage rating not exceeding about 25 V. 
     As shown in  FIG. 1 , an AC voltage source  100  may be connected to a bridge rectifier  104 , a fuse  150 , a LED load  108 , a MOSFET  112 , and a resistor  116 . Integrators  120 ,  124  may be alternatively switched in and out of the circuit by a detector  128  and switches  130 ,  131 . As the integrators are switched in and out, they produce outputs based on a current reference  138  and the voltage across resistor  116 . An optional high-speed cut-out  132 , shown in detail in  FIG. 3 , and switches  134 ,  135  may be used to protect the LED load  108  from undesirable conditions (discussed further herein below). The integrator output selected by switches  130 ,  131  is provided to the gate of the MOSFET  112  as long as the high-speed cut-out  132  is not activated ( FIG. 1  depicts the case where the cut-out is inactive). In this arrangement, MOSFET  112  acts as a variable resistor in order to moderate the amount of current passing through the LED load  108 . 
     In one exemplary embodiment, two distinct integrators  120 ,  124  may be provided—one for each half-cycle of the line voltage wave. This approach helps compensate for general line and/or component asymmetry. For this implementation, a circuit as shown in  FIG. 1  may be used, and the integrators  120 ,  124  may be enabled in alternate intervals corresponding to detected half-cycles of the line voltage wave. While an integrator is inactive, it is disconnected from the resistor  116  and it substantially retains the last output it generated while it was active. 
     In an alternative embodiment, two distinct integrators  120 ,  124  may be again provided—but in this embodiment, one for the first and third quarters of the line voltage wave and one for the second and fourth quarters of the line voltage wave. This approach helps compensate for asymmetry between rising and falling halves of the rectified circuit voltage. For this implementation, a circuit as shown in  FIG. 1  also may be used, and the integrators  120 ,  124  may be enabled in alternative intervals corresponding to detected half-cycles of the rectified circuit voltage waveform. While an integrator is inactive, it is disconnected from the resistor  116  and it substantially retains the last output it generated while it was active. 
       FIG. 2  shows an exemplary analog integrator  220  including an operational amplifier (or “op-amp”)  204 , a capacitor  208 , and a resistor  216 , as is known in the art, that may be used to implement the integrators  120 ,  124  in the circuit of  FIG. 1 . A current reference  138  and, in alternative time periods (as determined by detector  128 ), the voltage across the resistor  116 , may be provided as inputs to the integrator  220  on the “left” pin  201  and the “lower-right” pin  203  respectively; the output is selectively provided to the gate of the MOSFET  112  via the “upper-right” pin  202 , as described above. The current reference  138  is determined at design time (i.e., predetermined by the time of circuit fabrication) based on ideal/desired conditions of the LED load. 
       FIG. 3  shows an exemplary analog high-speed cut-out  332  including a comparator  356  and a voltage divider  318 , connected to “upper” pin  301 , that may be used to implement the high-speed cut-out  132  shown in the circuit of  FIG. 1 . An over-volt reference  338  and the output of the voltage divider  318  may be provided as inputs to the comparator  356 . The output of the comparator  356  and the output of an optional temperature sensor  330  may be input to an “OR” logic gate  354 . The OR gate  354  outputs its result on pin  302 . The “upper” pin  301  may be connected to the circuit of  FIG. 1  between the comparator  104  and the LED load  108 , while the “lower” pin  302  may be operatively connected to the switches  134 ,  135  in  FIG. 1 . This arrangement results in the high-speed cut-out  332  selectively toggling switches  134 ,  135  in order to protect the LED load  108  from undesirable conditions (discussed further herein below). The over-volt reference  338  may be determined at design time (i.e., predetermined by the time of circuit fabrication) based on ideal/desired conditions of the LED load. 
     In another alternative embodiment, as shown in  FIG. 4 , four distinct integrators  420 ,  422 ,  424 ,  426  may be provided—one for each of the four quarters of the line voltage wave cycle. This approach helps compensate for asymmetry between any of the four quarters of the line voltage wave. For this implementation, a circuit as shown in  FIG. 4  may be used, and the integrators  420 ,  422 ,  424 ,  426  may be sequentially enabled in alternating intervals corresponding to quarter-cycles of the line voltage wave. 
     As shown in  FIG. 4 , an AC voltage source  400  may be connected to a bridge rectifier  404 , a fuse  450 , a LED load  408 , a MOSFET  412 , and a resistor  416 . Integrators  420 ,  422 ,  424 ,  426  may be sequentially, alternately switched in and out of the circuit by a counter or timer  428  and switches  430 .  FIG. 2  shows an analog integrator  220 , described above, which may be used to implement the integrators in the circuit of  FIG. 4 . As the integrators are switched in and out, they produce outputs based on a current reference  438  and the voltage across resistor  416 . While an integrator is inactive, it is disconnected from the resistor  416  and it substantially retains the last output it generated while it was active. An optional high-speed cut-out  432 , shown in detail in  FIG. 3 , and switch  434  may be used to protect the LED load  408  from undesirable conditions (discussed further herein below). The integrator output selected by switches  430  is provided to the gate of the MOSFET  412  as long as the high-speed cut-out  432  is not activated ( FIG. 4  depicts the case where the cut-out is inactive). In this arrangement, MOSFET  412  acts as a variable resistor in order to moderate the amount of current passing through the LED load  408 . 
     According to one aspect of the invention, a microprocessor arrangement  520  as shown in  FIG. 5  may be used to implement select parts of the circuit including at least the integrators  120 ,  124 ,  420 ,  422 ,  424 ,  426 , the switches  130 ,  131 ,  134 ,  135 ,  430 ,  434 , the detector  128 , the counter  428 , the comparator  304 , the references  138 ,  338 ,  438 , and the OR gate  354 . A suitable microprocessor arrangement  520  may comprise an “upper” pin  501 , a “right” pin  502 , a “lower-right” pin  503 , a “lower-left” pin  505 , an about 1.8 V to 5 V voltage regulator  544 , a microprocessor  540 , and an optional over-voltage detector  518 . The microprocessor  540  may be an analog-enabled digital processor or any other suitable processor known in the art. This approach eliminates the need for select discrete analog circuit elements and, as a result, may allow for more cost savings and result in a more compact implementation. 
     In an exemplary embodiment comprising a microprocessor, a circuit like that shown in  FIG. 4  may be used (with the microprocessor-implemented parts being replaced by the microprocessor arrangement  520 ). The voltage regulator  544  of an exemplary microprocessor arrangement  520  may be connected between the “upper” pin  501  of the microprocessor arrangement  520  and the microprocessor  540  and may provide a regulated voltage of about 1.8 V to 5 V to the microprocessor  540 , as is known in the art. The “upper pin”  501  may be connected to the circuit of  FIG. 4  between the rectifier  404  and the LED load  408 , while the “lower-left” pin  505  may be connected to the circuit of  FIG. 4  between the resistor  416  and the rectifier  404  in order to provide a common ground for the microprocessor arrangement  520 . The “lower-right” pin  503  of the microprocessor arrangement  520  may be connected to the circuit of  FIG. 4  between the resistor  416  and the MOSFET  412  in order to provide the microprocessor  540  with the voltage across the resistor  416 . Outputs of the microprocessor  540  may be generated via software in accordance with the functionality described above and selectively provided to the gate of the MOSFET  412  via the “right” pin  502  of the microprocessor arrangement  520  in accordance with the desired regulator functionality (half cycle, quarter cycle, etc.), as described above. The software may be stored in random-access memory (RAM), read-only memory (ROM), or any other suitable machine readable hardware storage known in the art and accessible to the microprocessor  540 . 
     An optional over-voltage detector  518  may be connected between “upper” pin  501  and microprocessor  540  of microprocessor arrangement  520 . In the event that the over-voltage detector provides an indication of undesirable circuit conditions, such as a voltage spike, in its output to microprocessor  540 , microprocessor  540  may apply a gate voltage to MOSFET  412  having a substantially equivalent voltage to the voltage across resistor  416 , effectively resulting in electrical isolation of LED load  408 . For the over-voltage detector  518 , a circuit as shown in  FIG. 3  may be used; alternatively, the over-voltage detector may be implemented in software. 
     According to an aspect of the invention, briefly referenced above, a high-speed cutout may be used to account for abnormal line voltage, current, or temperature. This can help to protect the LED load from damage. The high-speed cutout may be implemented with analog switches that disconnect the integrators from the gate of the MOSFET and substantially simultaneously short (or connect) the MOSFET&#39;s gate to its source (see  FIG. 3  and  FIG. 4  for exemplary embodiments). The high-speed cutout may also be implemented with an over-voltage detector and software (as described above in reference to  FIG. 5 ) or by using any other methods known in the art. 
     By enabling separate current regulation for distinct segments of the AC line voltage cycle, embodiments of the present invention diminish or eliminate flickering resulting from undesirable line disturbances. In order to account for the use of a Triode for Alternating Current (“TRIAC”) dimmer, a circuit like the one shown in  FIG. 1  may be used to attempt to ensure a match between each half of the line voltage cycle. In the case of extreme asymmetry in the line voltage, a circuit like the one shown in  FIG. 4  may be used in an attempt to ensure that each quarter cycle of the line voltage provides nearly identical current to the LEDs. 
     In addition to precluding the visible effects of line voltage noise, embodiments of the present invention provide other advantageous arrangements. Using a circuit like the one shown in  FIG. 4 , it is possible, by setting a suitable overvolt reference  338 , to use the high-speed cutout  432  to generate a LED drive frequency that is about double the line voltage frequency by engaging the high-speed cutout between the first and second cycles and third and fourth cycle of the line voltage. Of course, this is done at the expense of power factor and line distortion. However, this corresponds to the time of greatest heat dissipation and power loss in the MOSFET. Accordingly, this technique may be used to improve efficiency. Any timing errors or asymmetry in the line voltage would ordinarily negatively impact the effectiveness of this method, but these effects may be compensated for by using separate current regulation for each quarter cycle of the AC line voltage cycle, as described above. 
     Another potential advantage enabled by embodiments of the present invention is related to the fact that if there is a significant mismatch between the LED load voltage and the line voltage, a large power dissipation would usually occur in the MOSFET in the form of heat. By utilizing a high-speed cut-out with a suitable over-voltage reference, the LED load may be electrically isolated during these periods of high voltage and dissipation. This allows for efficient drive of a low voltage, low power LED array directly from the line. Using separate current regulation for each quarter cycle of the AC line voltage cycle again compensates for any low frequency line anomalies that could otherwise potentially induce flickering. 
     This arrangement may be further extended to create a multi-voltage device that could operate, for example, using either AC 120 V or AC 240 V nominal line voltages. The LED array may be driven at line voltage frequency for AC 120 V operation and switched to being driven at double the line frequency when connected to AC 240 V by utilizing a high-speed cutout with a suitable over-voltage reference. The high-speed cutout may be employed between the first and second quarters and between the third and fourth quarters of the AC 240 V repeating wave cycle, where the elevated power dissipation (due to increased voltage) would be unwanted or wasteful, and the quarter-cycle regulation, as described above, reduces or eliminates any visible artifacts that may have been caused by frequency conversion and/or line noise. 
     Though exemplary embodiments have been described with reference to MOSFETs, any device with suitable electrically variable resistance characteristics may be used including, but not limited to, bipolar junction transistors (BJT), vacuum tubes, a plurality of transistors, any suitable combination thereof, or any other electrically variable resistor. Likewise, though exemplary embodiments have been described with reference to a voltage divider, any suitable means of providing suitable voltages may be used. Further, though exemplary embodiments have been described with reference to integrators, any suitable discrete circuit elements or mathematical operations (in the case of using a microprocessor) may be used. 
     Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.