Abstract:
An analog electronic circuit for driving a string of LEDs including input terminals for accepting connection to AC voltage, a current regulation circuit operatively coupled to receive an AC voltage from the input terminals and to provide an output for connection to drive the string of LEDs. Included is a current regulation circuit configured to limit the current flow through the string of LEDs on a half-cycle basis to a predetermined value. Also disclosed are an overvoltage circuit configured to switch off electrical connection between the AC voltage and the string of LEDs upon the AC reaching a predetermined high voltage value on a half-cycle basis in order to limit power. Overtemperature and power factor correction are also addressed. Also improving efficiency by shorting part of the LED string during the lower voltage phase of the input AC voltage.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of, and claims a priority benefit, under 35 U.S.C section 120, from nonprovisional U.S. application Ser. No. 14/611,053 filed on Jan. 30, 2015; that in turn claims a priority benefit, under 35 U.S.C section 120, from U.S. Ser. No. 14/227,996 filed on Mar. 27, 2014; that in turn claims a priority benefit, under 35 U.S.C section 120, of U.S. Ser. No. 13/068,844, filed on Mar. 3, 2011, now U.S. Pat. No. 8,704,446; that in turn claims a priority benefit, under 35 U.S.C section 119(e), from U.S. Provisional Application No. 61/310,218, filed on Mar. 3, 2010. This application also claims a priority benefit, under 35 U.S.C section 119(e), from U.S. Provisional Application No. 61/986,664, filed on Apr. 30, 2014. All contents of these applications are hereby, herein incorporated by reference in their entireties. 
    
    
     FIELD 
     This disclosure concerns analog circuitry for reliably powering LEDs from AC mains. 
     BACKGROUND 
     It has been predicted that solid-state lighting using light emitting diodes will eventually take over most of the applications now occupied by conventional lighting technology. A major attraction of LED lighting is reduced energy costs due to having inherently greater efficiency than incandescent, fluorescent and high-energy discharge lighting. Other attractions are that LEDs potentially have a much greater life span than the alternatives and do not contain hazardous chemicals such as the mercury used in fluorescent bulbs. 
     Two current disadvantages of LED lighting are the high cost of the LEDs themselves and the fact that many implementations do not live up to the often-claimed 50K+ hour lifetimes. To address this second issue the driving circuitry sophistication needs to be improved while keeping the cost low and, for practical reasons, the space taken by the controller small. Reliability issues with LED driving circuitry include failures in components such as large electrolytic capacitors used to produce DC voltages for LEDs. Their limited life becomes even shorter as ripple current increases, calling for even larger capacitors. Other contributors to a shorter lifetime are LEDs being stressed by overheating, overvoltage, or current spikes in excess of their maximum ratings. As the price of LEDs comes down the cost of the driving circuitry becomes relatively more important to the total consumer price, but the sophistication of the drive circuit needs to be higher than many circuits currently in use to ensure a long lifetime. 
     LED current is often regulated with a high frequency switching regulator that uses an inductor and capacitor as storage elements and a flyback diode to recirculate current between switching cycles. Switching regulator circuits are often chosen due to having higher efficiency than most non-switching designs. However, switching regulators have a number of disadvantages that can require additional circuit costs. Switchers create high frequency electromagnetic interference (EMI) that needs to be filtered in order to meet FCC regulations, for example. Also, the switching power supplies can create harmonic distortion in the current drawn from the power line. This is primarily seen as peak currents much greater than the root-mean-square (RMS) current and is drawn primarily at the peak of the AC voltage sine wave due to the capacitive current inrush on each AC cycle. This phenomenon undesirably lowers the Power Factor. 
     Power Factor is the ratio of real power in watts to apparent power in voltamps (VA). If the effective load of an LED lamp is inductive or capacitive then the Power Factor will be less than the ideal 1.0. Additional circuitry may be needed to correct the Power Factor (PF) of the lamp to meet utility company regulations. 
     In a lighting system that uses either a switcher or a conventional power supply to produce a DC rail, the PF is typically much less than optimum due to the power supply&#39;s input and output filter capacitors. As mentioned, these capacitors draw large peak current near the peaks of the input line voltage and much less between peaks. These distortions show up in the voltage and current frequency spectrums of the system as increased odd harmonics. In the usual lighting installation the power supplied is singlephase 120 VAC or 220 VAC connected phase to neutral. In this case the harmonic distortions will be additive on the neutral and can cause the neutral current to be up to 1.73 times greater than the phase current. This can cause the neutral to overheat even when the load is within the rating of the service. There is a need for circuits for driving LEDs that control the current and do not have inherent EMI and PF problems. 
     SUMMARY 
     This disclosure includes several versions of a simple but sophisticated, low cost light emitting diode (LED) driver circuit designed to interface directly with the AC mains voltages. An analog electronic circuit can take unfiltered mains voltage and apply it to a string of LEDs through a current regulator that can keep the LED current constant once it reaches a desired level. This happens on a half cycle-by-half cycle basis. The current regulator can have a high impedance, low voltage control point configured to be driven by one or more open collector control signals. If there is more than one control signal they can be wire-ORed through respective isolating diodes. In these circuits any of the wire-ORed signals can be used to independently reduce or shut down the current. 
     In some versions the circuitry can have a power limit protection via voltage sensing, an overtemperature circuit, a power factor correction circuit, and/or a dimming circuit. These features can be present in any combination. Other ancillary circuits disclosed include providing higher efficiency and implementing a 3-way bulb replacement. All of these circuits have embodiments where circuitry can be free of any requirement for a steady DC voltage to power either the LEDs or the various control circuits. 
     A low voltage control point is a circuit node not requiring a high-voltage circuit to drive it. In this context, low voltage is in contrast to the high voltage of the AC mains used to power the circuits of the embodiments. In many circuits a low-voltage control point may nominally be about 5 volts. A high impedance control point is a circuit node that can be taken to ground without excessive current flow. As an example, the transistor 2N3900 has a specified maximum collector current of 100 mA and a maximum emitter to collector voltage of 18V. This would be more than sufficient to drive a low-voltage, high-impedance node to ground without a heat sink or other special considerations. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a block diagram of a circuit using unfiltered AC that controls the current through LEDs and protects against overvoltage; 
         FIG. 2  shows voltage and current waveforms during the operation of the circuit of  FIG. 1 ; 
         FIG. 3  is a detailed schematic of a circuit corresponding to the block diagram of  FIG. 1 ; 
         FIG. 4  shows the voltage and current waveforms of  FIGS. 1 and 3  in the case of an overvoltage condition; 
         FIG. 5  is a schematic of a first alternate circuit embodiment of the block diagram of  FIG. 1  using a comparator; 
         FIG. 6  is a schematic of a second alternate circuit embodiment of the block diagram of  FIG. 1  using two comparators, with one half of a dual comparator IC used in the current regulation circuit and the other half in used in voltage detection; 
         FIG. 7  is a block diagram based on the block diagram of  FIG. 1  with additional strings of LEDs added, each string with its own current regulation and voltage detection; 
         FIG. 8  is a schematic of an LED driver with a single overvoltage detection circuit controlling two separate LED strings; 
         FIG. 9  is a block diagram of a circuit for controlling the current through LEDs and providing over power protection in a non-filtered, non-rectified, symmetric, two-phase scheme; 
         FIG. 10  is a detailed schematic of a circuit corresponding to the block diagram of  FIG. 9 ; 
         FIG. 11  shows a block diagram based on the bock diagram of  FIG. 1  with an additional, optional control block; 
         FIG. 12  is a schematic of an LED driver with current regulation, voltage protection and overtemperature detection circuit; 
         FIG. 13  is a schematic of an LED driver with the addition of PWM intensity modulation; 
         FIG. 14  is a schematic of an LED driver with the addition of a power factor correction circuit; 
         FIG. 15  shows voltage and current waveforms illustrating the operation of the power correction circuitry of  FIG. 14 ; 
         FIG. 16  is a schematic of an LED driver circuit with selective shorting of one LED for improved over-all efficiency; 
         FIG. 17  is a schematic of an LED driver circuit with current regulation, voltage detection and two LED strings to implement a 3-way Edison bulb replacement. 
     
    
    
     DETAILED DESCRIPTION 
     Introduction 
     The circuitry described can provide low cost methods of connecting Light Emitting Diodes to standard mains level AC service while providing current regulation and optionally, overvoltage protection. They have the advantage of simplicity and potentially much lower cost than other regulated methods. These circuits have a relatively high Power Factor due to requiring no large reactive components. In some versions additional circuitry is included to further improve the power factor. The circuits shown and described include those with inherently lower harmonics than switching regulators, consequently having low EMI. 
     Many alternate designs are presented. These designs do not attempt to provide a steady, level, DC supply to strictly regulate the current and voltage applied to the lighting elements. Instead, embodiments of the circuitry are exposed to, and operate over, the complete 360-degree sine wave of the power source. In this document “directly from AC mains” means a circuit capable of operating at 110 VAC to 250 VAC without requiring the AC to be converted to DC before the circuit can use the voltage, and also without needing the AC voltage to be stepped down to a lower voltage. Rectification, either half-wave or full wave, may be present and while no large filter capacitors are required, small noise reducing and stabilizing capacitors may be present. 
     The following will be better understood by consulting  FIG. 1  and  FIG. 2 . The block diagram drawing of  FIG. 1  shows four major sections, (1) a full wave bridge rectifier ( 600 ) getting input directly from the mains voltage, (2) a string of LEDs ( 601 ), (3) a current regulator ( 102 ), and (4) an overvoltage detector ( 103 ). If the AC voltage were filtered to a steady DC level this circuitry might seem conventional, but these teachings involve circuits not requiring a DC rail either for the LED current or to power control circuitry. In fact, the control circuits in the presented embodiments are designed to be de-powered and re-powered 120 times a second. The powering down occurs during the time the sine wave voltage is about + or −3 volts of its zero crossing. 
     Current Control 
     Consulting  FIG. 2  the AC input sine wave V AC  starts a new cycle at time T 0 . When V AC  reaches a high enough level (˜3V), the circuitry in blocks  102  and  103  become powered-on and monitors the current and voltage. Since the circuit is a straightforward analog circuit without memory there is no turn-on discontinuity or problem. 
     Inherent in the nature of diodes, no current flows through the LED series string ( 601 ) until the input voltage is greater than the sum of the minimum forward bias voltages of the string of LEDs. This level is marked as V fwr bias  in  FIG. 2 . The input sine wave V AC  reaches this at time T 2  as seen in the I LED . This is the first time current that flows through the LEDs. As the voltage increases along a sine wave ramp the current correspondingly ramps up in a sine wave ramp. The current will be below the current regulation point over the range where the applied voltage is too small to achieve the desired current regulation point. 
     The current regulator  102  has a predetermined setting to a desired regulated value of current through the LED string. This level is shown as I REG  in  FIG. 2 . When the current reaches the set point of regulation at time T 3 , the current is held to that value by the current regulator as seen by the flat top of the  FIG. 2  I LED  waveform. While the AC voltage exceeds the voltage required to produce the set point current, power is dissipated in the current regulator. At time T 4 , V AC  falls below the quantity required to produce the set point amount of current and the sequence of actions reverses. 
     A decreasing amount of current flows through the LEDs until the applied voltage is less than the sum of the forward bias voltages V fwr bias  at time T 5 . At about three volts the control circuitry stops functioning. Again, this causes no discontinuity. The current I LED  through the LEDs stays at zero thorough the end of the half-cycle at time T 7 . These steps reoccur for each half cycle. The current I LED  is shown flowing in  FIG. 2  during both phases of the AC input due to the full wave rectifier between the AC input and the rest of the circuit. 
     The voltage detector  103  circuit is discussed below in the context of a fleshed out schematic. 
     Specific Circuitry 
       FIG. 3  shows a detailed schematic of a circuit corresponding to the block diagram of  FIG. 1 . The current regulator section  102  is formed around a precision adjustable shunt voltage regulator U 1 . The shunt regulator is a three terminal Texas Instruments TL431. It varies its conduction of current between its cathode and anode to keep its control reference input equal to a fixed internal reference voltage. In this circuit it is configured with high-voltage NPN transistor Q 1  and resistors R 10  and R 11  as a constant current sink from the cathode of Q 1  back to the voltage source return. 
     The voltage, V SENSE  across the sense resister R 10  is compared within the shunt regulator with an internal voltage reference (typically 2.50V or 1.49V) and when the sensed voltage begins to exceed this voltage the shunt regulator begins to reduce the base current available to the NPN transistor Q 1  and this folds back the current flow of the LED string using this negative feedback effect. The current regulation point is set by sense resistor R 10  and by the formula: I_setpoint=Vref/Rsense. This circuit can variously be called a current regulator a constant current sink or a current limiter. In most applications of a circuit like this the goal is constant current. In this application it is a constant value or less. 
     Q 1  should have a collector emitter breakdown voltage rating higher than the highest expected peak spike or surge it will be exposed to from the mains. In the  FIG. 3  circuit, that quantity is limited by MOV 1 . In a nominally 117 V environment, the MOV&#39;s clamping voltage can be 230 volts. In that case a FZT458 with a breakdown voltage of 400V would be suitable as Q 1 . 
     Voltage detection and Power Protection 
     One element in  FIG. 1  that has not been discussed is the overvoltage detector (OVD)  103 . It is connected the voltage supplying the LEDs and measures the voltage to detect it exceeding a predetermined limit. When it does, the voltage detector shuts off the current regulator completely via an open collector control point  500 . 
     The purpose of the overvoltage detection circuit is not to protect any component directly from too high a voltage. Reducing the current to zero does not change the voltage across Q 1 . As mentioned, the MOV and Q 1  breakdown voltage are chosen to accomplish that protection. A large current will pass through the MOV until the voltage spike has passed, on a cycle-by-cycle basis and if the total duration on is long enough to overheat the fuse, the fuse will open. The fuse also protects against over current conditions due to a failure in the circuit by opening the path to the mains protecting the circuit. This fuse use can be a onetime acting component or a resettable fuse that will automatically close once the over current condition has passed. 
     The voltage detection circuit is to protect the power transistor from being required to dissipate power beyond its specifications when the AC mains voltage surges or spikes. The overall function of this aspect of the circuitry is better understood while consulting  FIG. 4 . These waveforms are similar to the previous waveform figures in  FIG. 2  but with the addition of portraying a voltage spike on the AC voltage. 
     In the first two half-cycles the voltage and resulting current are as in  FIG. 2 . However in the third half-cycle at time T spike-on  V AC  input spikes significantly above its nominal value. This is detected by circuit  103  that completely shuts off the current regulator until the voltage falls back below the OVD&#39;s cut-off point at T spike-off . The current shut off prevents Q 1  from being required to dissipate more power than it is specified to handle. A spike is shown for ease of explanation, but exceeding a power dissipation specification for a few milliseconds is normally not a big problem. The OVD is more important in a surge, in a flood of spikes, or a longer length overvoltage condition. 
     Details of OVD Circuit 
     The OVD shown in  FIG. 3  is connected to the bridge rectifier through bias resistor R 12 . The Zener diodes Z 1 , Z 2  and Z 3  are stacked together to set the voltage detection point. The stack of three Zeners is used in this example since they can have a lower total cost than one large voltage Zener due to the way the semiconductors are manufactured. For a nominal 117 VAC application, the set point voltage should be 165V. To avoid the OVD circuit turning on with normal voltage variations, but to ensure that it turns on before Q 1 &#39;s maximum power dissipation is exceeded, the set point voltage can be set about 10% higher than this at 182V. The bias resistor R 12  sets the nominal Zener current and absorbs the excess voltage during a voltage surge. Zener diode Z 4  limits the peak voltage at the gate of an N-channel MOSFET U 2  below its maximum rating and gate resistor R 13  going from the MOSFET&#39;s gate to the voltage return pulls the gate voltage back down to zero when the overvoltage condition passes. MOSFET part number ZXMN2A02N8 would be a suitable component. 
       FIG. 5  shows circuit very similar to  FIG. 3  but with the current regulator  602  created from a comparator and an NPN transistor. This current regulator replaces the adjustable shunt voltage regulator used in the current regulator circuitry shown in  FIG. 3 . The function of this circuit is described next. As the LED current increases due to the increasing sine wave of the AC input voltage, the voltage drop V COMPARE  across sense resistor R 10  increases. The voltage across R 10  is applied to the inverting input of comparator U 4 . The noninverting input of U 4  is connected to a voltage reference Zener Z 5  to set the maximum voltage across V REF  (typically about 2V). Resistor R 11  supplies bias current for the current regulator circuit  602 . The voltage from R 11  also powers the comparator and raises V REF  via its biasing resistor R 22 . The output of the comparator will initially be high impedance since no or low current flowing in R 10 , its negative input voltage V COMPARE  is lower than V REF . This high impedance output allows the NPN transistor Q 1  (a FZT458 or equivalent) to be turned on by current flowing into the base through R 11  and R 22 . This pulls its collector down close to its emitter potential. LED current will then flow once the sine wave voltage from the bridge output is high enough to supply the minimum required voltage across the LEDs  601  for them to begin conducting. When the LED current passing through sense resistor R 10  causes V COMPARE  to exceed the reference voltage V REF , the output of comparator U 4  will go low and begin to reduce the base current available to the NPN transistor Q 1 . This negative feedback effect folds back the current flow to the LEDs and limits it to a maximum current. The maximum current ILED peak  is set by the value of the sense resistor R 10  and the voltage V REF  by the formula:
 
 I LED peak   =V   ref   /R   sense .
 
     When the AC mains voltage sine wave drops far enough back towards zero, the LED current reduces due to Q 1  increasing resistance caused by U 4  starting to turn it off, and Vcompare will begin to reduce below the reference voltage. Then the comparator U 4  output will again go high allowing increase base current to Q 1  and begin reducing the voltage drop collector to emitter of Q 1  to control the current flow. Capacitor C 10  supplies filtering across the comparator&#39;s power connection&#39;s to prevent oscillations. It is not intended to keep a steady DC supply for the comparator during the AC cycle. As mentioned elsewhere, in many embodiments there is no requirement to keep a steady DC supply on any components. The purpose of the overvoltage detection circuit  103  as in  FIG. 5  is to protect the LEDs and power transistors from the effects of voltage surges originating from the AC line. It is the same circuit as shown in  FIG. 3  as explained above. 
       FIG. 6  shows an alternate circuit with similar operation as the circuit of  FIG. 5 . An open collector (drain) comparator IC with two comparators is used in both the LED current regulator  614  and voltage limiter  615  circuits. One comparator  306  is set up as before as the core of the current limiter as in  FIG. 5  and the second comparator  500  replaces the N-Channel MOSFET from  FIG. 5  to perform the voltage limiting function. The voltage reference V COMPARE  used by the current regulator also supplies the reference level at the non-inverting pin of the comparator  500 . The overvoltage signal is produced by the same method with stacked Zener diodes Z 1  Z 2  Z 3  through defining the overvoltage level and Z 4  providing voltage limiting to the inverting pin of  500 . Resistor R 12  connects the Zener string to the sensed voltage at the output of the bridge rectifier  600  and also limits the Zener current. Bleed resistor R 13  pulls the inverting input back down towards ground after each half sine wave phase to reset the overvoltage circuit  615 . 
     Initially with low to normal voltages, the voltage at the inverting input of  500  will be less than the reference voltage at the noninverting input and this will result in a high impedance output. The output of the comparator is tied to the base of NPN transistor Q 1  and, when high, it does not affect the operation of the current regulator  614 . When the inverting input to  500  exceeds the reference voltage V REF , then the output of  500  comparator will go low and pull the base of Q 1  low that turns off Q 1  and therefore the LED&#39;s  100  current flow. The LED current flow will remain off, protecting Q 1  from excessive power dissipation, until the overvoltage condition clears and the output of  500  goes back to a high impedance state. The output pins of comparators  306  and  500  are tied together at the base of the NPN transistor Q 1  and either one pulling low will turn off the LED current. Thus the LEDs and Q 1  are protected from excessive current and/or voltage and the maximum power that any circuit component dissipates is limited. 
       FIG. 7  is an expansion of the circuit of  FIG. 3 , into multiple strings of LEDs. In this case the same fuse F 1  and bridge rectifier  600  are used to drive all of the LED strings with associated circuitry  900   1  to  900   n  in parallel as shown. An example where this can be useful is in the replacement of linear fluorescent bulbs with LED equivalents. For instance if the LED luminance requires 40 LEDs per foot for an equivalent output then two strings could be used for a 24″ replacement bulb and four strings for a 48″ replacement bulb. Separate Voltage Detectors could be an advantage if the strings are widely separated and the driving voltage is lower due to IR voltage drop on the connecting cable between them. Also, if the strings were in separate enclosures daisy chained together by a cable one less cable wire would be needed. 
       FIG. 8  shows a schematic of an alternate circuit for driving multiple LED strings.  FIG. 8  shows that a single overvoltage detection circuit  598  can be used to control multiple LED current regulator circuits that are each controlling individual LED strings via respective control points. In this circuit there are two distinct LED strings  603  and  604 , each current-controlled by distinct instance of the circuit  102 . In contrast to the block diagram of  FIG. 7 , however, they share a common overvoltage circuit  598 . This OVD circuit differs from the OVD circuit in  FIG. 3  and other, previous figures. A voltage divider of R 45  and the sum of R 47  and R 48  is used to bring the sensed voltage into a lower range and allow the use of a single low voltage Zener D 40 . Alternatively, a high voltage Zener or several Zeners in series could be used. Another refinement seen in  FIG. 8  is the use of two resistors in series in several places including R 40  and R 41 . This avoids a single point of failure of a shorted resistor putting an excessive voltage into the circuit. 
     Non-Rectified Embodiment 
     There are ways to take advantage of these teachings using circuits without any rectification at all.  FIG. 9  shows another way to apply the same core circuitry. In this case, rather than having a full wave bridge rectifier, there are dual current regulators and dual OVD circuits, one per phase. 
     The choice to use the non-rectified embodiment really depends on the type of lighting that is being manufactured using this method. When using a string of LEDs  100 , the number of LEDs used will depend on the forward voltage at the desired LED current. The total voltage drop across the string  100  needs to be less than the peak voltage of the AC source at its lowest nominal level. For an 117 VAC source, this might be taken as 10% below or 117V*1.414/1.10=150V. Lower than this will decrease the amount of dimming during a brownout (voltage droop) condition but will also reduce the efficiency during normal voltage conditions. Lower LED voltage drop also relates to fewer LEDs used in series, which will reduce the lumens output during normal voltage conditions. This is one of the tradeoff decisions to be made when creating a light source using these teachings. 
     A dual phase current regulator with overvoltage detection used with a string of AC LEDs is shown in block diagram form in  FIG. 9 . An AC LED is a type of LED that illuminates when current flows in either direction. A standard LED only operates in one direction. Alternatives to AC LEDs are back-to-back LEDs or back-to-back LED strings could be used in this circuit. There is no rectification or step down of the raw AC mains. Here a dual phase control circuit is shown as a phase A section and a phase B section. Each section has a respective current regulator  102 A  102 B and overvoltage detection circuit  103 A  103 B. These can be identical circuits to the current regulator  102  of  FIG. 3  previously discussed. 
     The AC LED string is represented by a string of pairs of LEDs in parallel in opposite directions. 
     During Phase A the current flows as shown by arrows  2000 . During that phase the current regulator  102 A and voltage detector  103 A are active and control the current seen by the AC LEDS. The voltage detector  103 B and the current regulator  102 B of the Phase B side are not functioning during Phase A since they are biased opposite to that required to operate. Diode D 2 , shown dashed, allows the current path  2000  to get current “backwards” through the phase B side during Phase A. It is shown dashed because some implementations of the current regulator  102 B may have an inherent diode path in this direction and a discrete D 2  would not be required. As clearly seen in  FIG. 9 , the mains waveform, the LEDs, and the phase A/phase B circuits are completely symmetric. Therefore the operation during Phase B is a mirror image of the operation in Phase A 
     Detailed Two-Phase Circuit 
       FIG. 10  shows a circuit representing the scheme of  FIG. 9  at a deeper lever of detail. As mentioned above, when  102 A is actively regulating current, the voltage is sourced via  102 B with the current path  599  shown in  FIG. 10 . The source current flows through the U 1 B anode to cathode diode and then through the Q 1 B base/emitter (P/N) junction to the LED string. Resistor R 25  supplies bias current to power U 1 A that sources from U 1 B&#39;s cathode during this phase. When the phase switches, current flows in the other direction through R 25  to power U 1 B coming from U 1 A&#39;s cathode, and importantly, the current  599  flows in the opposite way through the LEDs. The parts list for the Dual Phase AC LED Interface shown in  FIG. 10  is seen in table 1. 
     Other LED Circuits Using a Control Point 
     In many of the figures described above the LED current can be shut off by an overvoltage circuit pulling down the circuit point formed by the base of NPN transistor Q 1  and the cathode of the shunt regulator U 1 , as seen  FIG. 3 , for example. This point is the wire-ORed control point, as mentioned above. Its characteristics are a high impedance, low voltage point, that when taken to ground shuts off the current regulator.  FIG. 11  is a block diagram level drawing illustrating a generic use of a low voltage control point for shutting down the regulator u[on an overvoltage condition, or controlling the regulator via another arbitrary control circuit using diode isolated wired-OR logic. 
     Overtemperature 
     As an example of the use of another control block, an overtemperature circuit is seen in  FIG. 12  that is formed similarly to the overvoltage detection circuit, but with an NTC thermistor R 32  in series with the sensing resistor voltage divider R 30  R 31  as seen in this schematic. The top of the voltage divider R 30  is connected to the control point  599 A where there is a fairly constant 3V during the time when the current regulator is turned on. 
     The LED current is reduced or cut off for the whole portion of the phase that the bridge voltage is high enough to turn on the regulator. The circuit gradually transitions the current lower as the thermistor resistance drops low enough to start turning on transistor Q 5 . The result is a reduction in power drawn by the LED string and dissipated by the current regulator output transistor. With the component values shown, the light will still illuminate but at a reduced lumen output during this state until the thermistor temperature reaches 100 C at which point the current regulator and light output will be completely shut down. As long as both the Overvoltage Detection circuit and the Overtemperature Limit circuit are open collector type outputs either or both circuits conducting and pulling the control point low will shut down the LED current. 
     Dimming Control 
     Also, a PWM signal could drive the same control point at a repetition rate greater than the input line voltage frequency to control the percentage of time that an LED string is on. A schematic of an example embodiment of a PWM control is seen in  FIG. 13 . This can be used to enable functions such as dimming the light or controlling the color of the light if different color light strings are individually controlled. The PWM signal  709  can be created by a linear circuit  701  that converts a 0-10V input to a proportionally (as seen in  FIG. 13 ) or logarithmically related pulse width modulated signal. In  FIG. 13  a PWM control is shown in conjunction with an overvoltage circuit. Alternatively, a microcontroller could perform the translation and produce the PWM signal (not shown). Another method would be to use a wireless module such as Bluetooth or Zigbee to bring the desired dimming level into an enclosed fixture or lamp and drive a PWM signal to the current regulator control points. 
     Power Factor Control 
     The control point technique can also be used to improve the power factor of a design; this is shown in the schematic of  FIG. 14  and the waveforms of  FIG. 15 . A power factor enhancement correction circuit  802  is shown working in conjunction with an overvoltage circuit. The power factor enhancement circuit controls a small number of LEDs D 60 , D 61  that are electrically separate from the primary string of LEDs  601 . The theory of operation of the power factor circuit is to draw some current and produce some light at parts of the half cycle where the V AC  is below the V fwd bias  of the primary string of LEDs. 
     Near the beginning of each half cycle voltage phase at time T 1 , as seen in  FIG. 15 , a current I PF  starts to flow through the short string. This is due to the much lower forward bias voltage required by the short string of LEDs. As seen in  FIG. 15 , when the V AC  reaches V PF , which is the sum of the forward bias voltages of the short string current I PF  starts flowing. The circuit that controls I PF  includes an N channel MOSFET U 60 . A particular example MOSFET is ZXMN2A02N8. MOSFET U 60  is turned on by the voltage across current sense resistor R 10 , pulling the MOSFET&#39;s drain low and bringing the base to emitter voltage of U 60  near zero. This turns off the power factor enhancement circuit. The NPN transistor Q 2  is turned on by the input voltage, supplying base current via base resistors R 9  and R 62 . This could be one resistor, but two are shown in  FIG. 14  to handle single fault failure modes. When Q 2  turns on, it draws current from the input source via R 63 , which dissipates the excess power. 
       FIG. 15  shows current and voltage waveforms related to the power factor correction circuit. This V AC  waveform is similar to the V AC  waveform of  FIG. 2  but shown on an enlarged timescale. Below the V AC  waveform is the I LED  current waveform, again, the same as the waveform shown in  FIG. 2 , but on an enlarged timescale. With a power enhancement circuit, this represents the current through the main LED string. Below that current waveform is I PF , this represents the current through the smaller string. As seen in  FIG. 14  that is diodes D 60  and D 61 . The total current drawn from the AC source is shown below that waveform as I LED W/PF , which signifies the sum of current through the two LED strings. Because the total current drawn with power factor circuit is somewhat closer to a sine wave than the original I LED  the power factor is increased. This also provides an increase in efficiency. 
     Modularity Using the Common Control Point 
     Since the circuits described above all take advantage of a single open collector driven control point that can be diode-ORed together, there is an inherent support for modularity. A system might be composed of separately packaged modules that snap together mechanically and pass the control point between them. A user or configurer could add or subtract distinct strings of LEDs, overvoltage, overtemperature, and PWM modules to produce a desired instance of a system. 
     Improved Efficiency Circuit 
     An efficiency improvement circuit is shown in  FIG. 16  that shorts one LED in an LED string at the leading lower voltage part of the bridge AC voltage phase. This allows the balance of the LED string to turn on earlier in the phase. The bridge voltage is sensed by the same type of circuit used for overvoltage detection but it&#39;s output is used to turn off the transistor switch Q 7  that is shorting across the extra LED  710  in the string. This increases the lumen output of the string during the higher voltage period of the bridge AC voltage. The net result is a longer ‘on’ time for a slightly reduced version of the LED string and additional output during the peak periods. The current limiting circuit&#39;s sink transistor has less voltage across it during the peak periods as well so the total ‘lost’ power is reduced. Efficiency=Lumens/Watts is improved. Although  FIG. 16  shows a single LED, it can be multiple LEDs. In an alternate embodiment, more than one voltage point could be detected for a ladder of separately short-able LED segments. The core concept of these improved efficiency circuits could be applied to any of the preceding embodiments. 
     In Addition—3 Way Edison Bulb 
     A 3-way Edison bulb can be produced with two LED strings that are individually powered by each contact on the bottom of the base as shown in  FIG. 17 . Alternatively, another single string of LEDs could be used with the input driven by either/both contacts, but a sensing circuit detects which combination of contacts are powered and controls a PWM signal into its current regulator&#39;s control point to create the three different amounts of illumination. That alternate embodiment achieves a similar result. 
     Reference Number Table 
     Table 1 shows part numbers, reference number, and corresponding figure numbers. 
     
       
         
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Reference # 
                 Description 
                 Part # 
                 Used in FIGS. 
               
               
                   
               
             
             
               
                 C1 
                 Capacitor, High Frequency Filter 
                 1 nF 
                 13, 16 
               
               
                 C10 
                 Capacitor, High Frequency Filter 
                 1 nF 
                 6, 8, 14, 17 
               
               
                 C10′ 
                 Capacitor, High Frequency Filter 
                 1 nF 
                   8, 
               
               
                 C11 
                 Capacitor, High Frequency Filter 
                 1 nF 
                 12, 17 
               
               
                 C20 
                 Capacitor, High Frequency Filter 
                 1 nF 
                 16 
               
               
                 C3 
                 Capacitor, High Frequency Filter 
                 1 nF 
                 13 
               
               
                 D10 
                 isolation diode 
                 low current Schottky 
                 11, 13 
               
               
                   
                   
                 diode - MBR0520 
                   
               
               
                 D10′ 
                 isolation diode 
                 low current Schottky 
                 13 
               
               
                   
                   
                 diode - MBR0520 
                   
               
               
                 D15 
                 isolation diode 
                 low current Schottky 
                 11, 13 
               
               
                   
                   
                 diode - MBR0520 
                   
               
               
                 D15′ 
                 isolation diode 
                 low current Schottky 
                 13 
               
               
                   
                   
                 diode - MBR0520 
                   
               
               
                 D40 
                 Zener Diode Reference 
                 A 6.2 V Zener diode 
                 8, 11, 13, 14 
               
               
                   
                   
                 such as the BZX84C6V2 
                   
               
               
                 D50 
                 Zener Diode Reference 
                 A 10 V Zener diode 
                 13 
               
               
                   
                   
                 such as BZX84C10 
                   
               
               
                 D60 D61 
                 LEDs 
                 Can be same LEDs used 
                 14 
               
               
                   
                   
                 in LED string such 
                   
               
               
                   
                   
                 as 24 V XLAMP type 
                   
               
               
                 D70 
                 Zener Diode 
                 A 6.2 V Zener diode 
                 17 
               
               
                   
                   
                 such as the BZX84C6V2. 
                   
               
               
                 710 
                 LED 
                 LED diode such as 
                 17 
               
               
                   
                   
                 24 VXLAMP. 
                   
               
               
                 D71 
                 Zener Diode 
                 6.2 V Zener diode such 
                 16 
               
               
                   
                   
                 as the BZX84C6V2. 
                   
               
               
                 D73 
                 Zener Diode 
                 6.2 V Zener diode 
                 17 
               
               
                   
                   
                 BZX84C6V2. 
                   
               
               
                 D8 
                 Diode 
                   
                 12 
               
               
                 D9 
                 Diode 
                   
                 12 
               
               
                 F1 
                 Fuse 
                   
                 1, 3, 5, 6, 7, 13, 
               
               
                   
                   
                   
                 16, 17 
               
               
                 MOV1 
                 Metal Oxide Varistor 
                 FZT458 
                 1, 3, 5, 6, 7, 13, 
               
               
                   
                   
                   
                 16, 17 
               
               
                 MOV2 
                 Metal Oxide Varistor 
                 FZT458 
                 17 
               
               
                 Q1 
                 High Voltage NPN Transistor 
                 Q2N2222 
                 3, 5, 6, 8, 12, 
               
               
                   
                   
                   
                 13, 16, 17 
               
               
                 Q1′ 
                 High Voltage NPN Transistor 
                 Q2N2222 
                  13, 
               
               
                 Q1A 
                 High Voltage NPN Transistor 
                 Q2N2222 
                 10 
               
               
                 Q1B 
                 High Voltage NPN Transistor 
                 Q2N2222 
                 10 
               
               
                 Q2 
                 High Voltage NPN Transistor 
                 Q2N2222 
                 14, 17 
               
               
                 Q3 
                 High Voltage NPN Transistor 
                 Q2N2222 
                   8, 14, 
               
               
                 Q4 
                 High Voltage NPN Transistor 
                 Q2N2222 
                 8, 12, 13, 14, 16 
               
               
                 Q5 
                 High Voltage NPN Transistor 
                 Q2N2222 
                 12 
               
               
                 Q6 
                 High Voltage NPN Transistor 
                 Q2N2222 
                 16 
               
               
                 Q7 
                 P MOSFET 
                 A P-channel MOSFET 
                 16 
               
               
                   
                   
                 such as RFD15P05 
                   
               
               
                 Q8 
                 trans. In 3-way circuit 
                 Low voltage PNP 
                 17 
               
               
                   
                   
                 transistor - BC848C 
                   
               
               
                 Q9 
                 trans. In 3-way circuit 
                 Low voltage PNP 
                 17 
               
               
                   
                   
                 transistor - BC848C 
                   
               
               
                 R1 
                 resistor 
                 56 
                 17 
               
               
                 R3 
                 resistor 
                 47 K 
                 17 
               
               
                 R5 
                 resistor 
                 18 
                 8, 14, 16 
               
               
                 R9 
                 resistor 
                 22 K 
                 14 
               
               
                 R10 
                 Sense Resistor 
                 47 
                 3, 5, 6, 8, 12, 
               
               
                   
                   
                   
                 14, 16 
               
               
                 R10′ 
                 Sense Resistor 
                 47 
                  8 
               
               
                 R10A 
                 Sense Resistor 
                 47 
                 10 
               
               
                 R10B 
                 Sense Resistor 
                 47 
                 10 
               
               
                 R12 
                 OVD Bias Resistor 
                 Around 56 K 
                 3, 5, 6 
               
               
                 R12A, R12B 
                 Resistor 
                 Around 56 K 
                 10 
               
               
                 R13 
                 Gate Bleed Resistor 
                 100 K  
                 3, 5, 6 
               
               
                 R13A 
                 Gate Bleed Resistor 
                 100 K  
                 10 
               
               
                 R13B 
                 Gate Bleed Resistor 
                 100 K  
                 10 
               
               
                 R18 
                 Resistor 
                 47 K 
                 13, 17 
               
               
                 R18′ 
                 Resistor 
                 47 K 
                  13, 
               
               
                 R19 
                 Resistor 
                 30 
                 17 
               
               
                 R21 
                 Resistor, NPN Base 
                  1 K 
                 5, 6 
               
               
                 R22 
                 Resistor, V---Reference Bias 
                  1 K 
                 5, 6 
               
               
                 R25 
                 Resistor 
                 68 K 
                 10 
               
               
                 R31 
                 Resistor 
                 3.9 K  
                 12 
               
               
                 R32 
                 Thermistor 
                 220 K@25 C 
                 12 
               
               
                 R33 
                 Resistor, Bleed 
                 47 K 
                 12 
               
               
                 R39 
                 Resistor 
                 39 
                 13 
               
               
                 R39′ 
                 Resistor 
                 39 
                 13 
               
               
                 R40 
                 Resistor 
                 43 K 
                  8 
               
               
                 R41 
                 Resistor 
                 43 K 
                  8 
               
               
                 R43 
                 Resistor 
                 43 K 
                 8, 14, 16 
               
               
                 R44 
                 Resistor 
                 43 K 
                 8, 14, 16 
               
               
                 R45 
                 Resistor 
                 22 K 
                 8, 12, 13, 14, 16 
               
               
                 R46 
                 Resistor 
                 47 K 
                 8, 12, 13, 14, 16 
               
               
                 R47 
                 Resistor 
                 220 K  
                 8, 12, 13, 14, 16 
               
               
                 R48 
                 Resistor 
                 221 K  
                 8, 12, 13, 14, 16 
               
               
                 R11 
                 Bias Resistor for Current 
                 5.6 K  
                 3, 5, 6 
               
               
                   
                 Regulators 
                   
                   
               
               
                 R110 
                 Resistor 
                 18 
                  8, 12 
               
               
                 R49 
                 Resistor 
                 43 K 
                 12 
               
               
                 R50 
                 Resistor 
                 47 K 
                 13 
               
               
                 R51 
                 Resistor 
                 2.2 K  
                 13 
               
               
                 R52 
                 Resistor 
                 10 K 
                 13 
               
               
                 R53 
                 Resistor 
                 1 M 
                 13 
               
               
                 R54 
                 Resistor 
                 18 K 
                 13 
               
               
                 R55 
                 Resistor 
                 1 M 
                 13 
               
               
                 R56 
                 Resistor 
                 2.7 K  
                 13 
               
               
                 R57 
                 Resistor 
                 10 K 
                 13 
               
               
                 R62 
                 Resistor 
                 22 K 
                 14 
               
               
                 R63 
                 Resistor 
                 2.2 K  
                 14 
               
               
                 R64 
                 Resistor 
                 4.7 K  
                 14 
               
               
                 R66 
                 Resistor 
                 4.7 K  
                 14 
               
               
                 R70 
                 Resistor 
                 10 K 
                 17 
               
               
                 R71 
                 Resistor 
                 24 K 
                 17 
               
               
                 R72 
                 Resistor 
                 36 K 
                 16 
               
               
                 R73 
                 Resistor 
                 6.8 K  
                 16 
               
               
                 R74 
                 Resistor 
                 10 K 
                 17 
               
               
                 R75 
                 Resistor 
                 24 K 
                 17 
               
               
                 R80 
                 Resistor 
                 450 K  
                 17 
               
               
                 R81 
                 Resistor 
                 33 K 
                 16 
               
               
                 R82 
                 Resistor 
                 33 K 
                 16 
               
               
                 R84 
                 Resistor 
                 450 K  
                 17 
               
               
                 U1 
                 Shunt Voltage Regulator 
                 TL431 
                 3, 8, 12, 13, 16, 17 
               
               
                 U1′ 
                 Shunt Voltage Regulator 
                 TL431 
                 13 
               
               
                 U1A, U1B 
                 Shunt Voltage Regulator 
                 TL431 
                 10 
               
               
                 U2 
                 MOSFET, N---Channel 
                 ZXMN2A02N8 
                 3, 5 
               
               
                 U2A 
                 MOSFET, N---Channel 
                 ZXMN2A02N8 
                 10 
               
               
                 U2B 
                 MOSFET, N---Channel 
                 ZXMN2A02N8 
                 10 
               
               
                 U3 
                 Shunt Voltage Regulator 
                 TL431 
                 8, 14, 17 
               
               
                 U4 
                 Comparator (single) 
                 LM393A 
                  5 
               
               
                 U60 
                 MOSFET, N---Channel 
                 ZXMN2A02N8 
                 14 
               
             
          
           
               
                 100 
                 String of Light Emitting Diodes (LED) or AC LEDs 
                  9, 10 
               
             
          
           
               
                 306 
                 Dual Open Collector Voltage 
                 LM393A 
                  6 
               
               
                   
                 Comparator (A side) 
                   
                   
               
               
                 500 
                 Dual Open Collector Voltage 
                 LM393A 
                  6 
               
               
                   
                 Comparator (B side) 
                   
                   
               
               
                 600 
                 Bridge Rectifier 
                   
                 1, 3, 5, 6, 7, 8, 
               
               
                   
                   
                   
                 11, 12, 13, 14, 
               
               
                   
                   
                   
                 16, 17 
               
             
          
           
               
                 601 
                 String of Light Emitting Diodes (LED) 
                 1, 3, 5, 6, 7, 11, 
               
               
                   
                   
                 12, 14, 16 
               
               
                 603 
                 String of Light Emitting Diodes (LED) 
                  8, 13 
               
               
                 603′ 
                 String of Light Emitting Diodes (LED) 
                 13 
               
               
                 604 
                 String of Light Emitting Diodes (LED) 
                  8 
               
               
                   
               
             
          
         
       
     
     Rectification is turning an AC source into a voltage or current that only flows in one direction. This may be by a half-wave rectifier or a full-wave rectifier. Constant sink current regulators, as shown in these figures, can be implemented with a shunt voltage regulator or a comparator circuit. It can also be embodied in a single integrated circuit or entirely built from transistors. Protecting from excessive power dissipation can be done by many means. Circuits in these figures demonstrate power limitation via constant current and bounded voltage. Alternatives include constant voltage and bounded current and by directly sensing temperature of the component being protected. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, operation, or other characteristic described in connection with the embodiment may be included in at least one implementation of the invention. However, the appearance of the phrase “in one embodiment” or “in an embodiment” in various places in the specification does not necessarily refer to the same embodiment. 
     As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms “includes,” “comprises,” “including” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.