WO2014088947A1

WO2014088947A1 - Dimmable led having constant voltage and linear current control using headroom control

Info

Publication number: WO2014088947A1
Application number: PCT/US2013/072616
Authority: WO
Inventors: Junhua Huang; Julian ZHU
Original assignee: General Electric Company
Priority date: 2012-12-04
Filing date: 2013-12-02
Publication date: 2014-06-12
Also published as: TWI607667B; TW201431435A; CN103857127A; CN103857127B

Abstract

A driver circuit (500) for an LED lamp includes a first stage (504, 510) that is configured to receive an AC input voltage (526) and produce a regulated DC voltage (514). A linear current regulator (508) is coupled to the first stage (504) and configured to regulate a lamp current supplied to the LED lamp (506). A feedback circuit (510, 512)) is coupled to the first stage and is configured to monitor the regulated DC voltage (514) and operate the first stage to maintain the regulated DC voltage at a generally constant voltage. The driver circuit (500) also includes a voltage drop monitoring circuit (528) for monitoring the voltage drop (headroom) of the linear current regulator (508) and is coupled to the feedback circuit (510, 512). The voltage drop monitoring circuit (528) is configured to adjust the generally constant voltage produced by the first stage (504, 510). The voltage drop monitoring circuit (528) monitors the voltage drop of the linear current regulator (508) and reduces the generally constant voltage as the voltage drop increases.

Description

DIMMABLE LED HAVING CONSTANT VOLTAGE AND LINEAR CURRENT CONTROL USING HEADROOM

CONTROL

BACKGROUND TO THE INVENTION

[0001] The aspects of the present disclosure relate generally to LED light sources, and in particular to drive circuits for LED lamps.

DESCRIPTION OF RELATED ART

[0002] A light emitting diode (LED) is a light source constructed from semiconductor materials, typically gallium arsenide. Like other diodes a LED is created by doping the semiconductor with various impurities to create a p-n junction. When a current is applied, charge carriers flow into the junction where positively charged holes combine with negatively charged electrons causing the electrons to fall to a lower energy level thereby releasing energy as visible or infrared light. As in all diodes, current flows easily from the positively doped p side to the negatively doped n side of the device but not in the opposite direction. Therefore LEDs are typically driven from a direct current (DC) power source.

[0003] It is common in lighting applications to construct LED lamp assemblies, or LED lamps, using multiple individual LEDs, or LED elements. Incorporating multiple LED elements into a single LED lamp assembly provides many advantages, such as increased light output, and graceful failure modes. The graceful failure is achieved because individual elements typically fail at different timse so light output is gradually degraded over time as individual elements fail. By connecting the individual LED elements in series or strings the LED lamp assembly can be driven with higher voltage and lower current levels providing flexibility in the driver design and allowing greater efficiency in the driver circuitry. By having multiple strings of LEDs, where the strings are connected in parallel, sometimes referred to as a series-parallel block arrangement, the LED lamp assembly can be made more fault tolerant while maintaining other benefits of series connected assemblies.

[0004] In many LED applications, such as traffic lighting or domestic lighting applications, the drive circuitry is powered from the locally available AC power grid. These applications often provide dimming functionality through phase angle dimming methods where the AC input power is modified by triggering current conduction at different angles in the sinusoidal waveform thereby reducing the amount of power contained in the input power. In these applications the AC grid power signal, which may be a phase angle dimmed power signal, is converted to DC power and regulated by the LED driver circuitry such that a steady brightness is produced by the LED fixture. However, the power regulation included in typical LED drivers effectively removes the phase control dimming and operates the LED lamp at a constant brightness in spite of any phase control dimming in the input power. Therefore, to maintain dimming performance, special dimming detection and power control circuitry must be included in LED drivers resulting in higher driver cost and larger driver size.

[0005] Typical driving circuits used for driving LED lamps fall into two categories: single stage, and multiple stage, of which the double, or two, stage drivers are most common. Each type has its particular set of advantages and drawbacks. Single stage driving circuits are lower cost, smaller in size, and can provide high driver efficiency. However the single stage drivers suffer from a tradeoff between power factor and LED ripple current. Single stage drivers also have worse dimming performance than multi-stage drivers. Double stage and other multistage drivers have excellent dimming performance and can achieve high power factor while still maintaining a low ripple current. However multi-stage and double stage drivers are more costly to manufacture, are larger in size, and have lower overall driver efficiencies than single stage drivers.

[0006] Accordingly, it would be desirable to provide LED driver circuitry that addresses at least some of the problems identified above.

SUMMARY OF THE INVENTION

[0007] As described herein, the exemplary embodiments overcome one or more of the above or other disadvantages known in the art.

[0008] One aspect of the exemplary embodiments relates to a driver circuit for a LED lamp. In one embodiment, the driver circuit includes a first stage that is configured to receive an AC input power and produce a regulated DC voltage. A linear current regulator is coupled to the first stage and configured to regulate a lamp current supplied to the LED lamp. A feedback circuit is coupled to the first stage and is configured to monitor the regulated DC voltage and operate the first stage such to maintain the regulated DC voltage at a generally constant voltage. The driver circuit also includes voltage drop monitoring and includes a voltage drop monitoring circuit coupled to the feedback circuit. The voltage drop monitoring circuit is configured to adjust the generally constant voltage produced by the first stage. The voltage drop monitoring circuit monitors the voltage drop of the linear current regulator and reduces the generally constant voltage as the voltage drop increases.

[0009] Another aspect of the exemplary embodiments relates to an electric lighting device. In one embodiment, the electric lighting circuit includes a driver circuit configured to receive an AC input power and produce a lamp current, and a LED lamp coupled to the lamp current. The driver circuit includes a first stage that is configured to receive an AC input power and produce a regulated DC voltage. A linear current regulator is coupled to the first stage and configured to regulate a lamp current supplied to the LED lamp. A feedback circuit is coupled to the first stage and is configured to monitor the regulated DC voltage and operate the first stage to maintain the regulated DC voltage at a generally constant voltage. The driver circuit also includes voltage drop monitoring and includes a voltage drop monitoring circuit coupled to the feedback circuit. The voltage drop monitoring circuit is configured to adjust the generally constant voltage produced by the first stage. The voltage drop monitoring circuit monitors the voltage drop of the linear current regulator and reduces the generally constant voltage as the voltage drop increases.

[0010] Another aspect of the exemplary embodiments relates to a method for operating an LED driver circuit where a first power stage is used to convert an AC input power to a DC output voltage. In one embodiment, a feedback signal is created from the DC output voltage and is used to regulate the DC output voltage at a generally constant voltage. The DC output voltage is provided to an LED lamp and a linear current source is used to regulate the lamp current. The voltage drop of the linear current source is monitored and the feedback signal is adjusted to reduce the DC output voltage when the voltage drop exceeds a threshold voltage.

[0011] These and other aspects and advantages of the exemplary embodiments will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Moreover, the aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings illustrate presently preferred embodiments of the present disclosure, and together with the general description given above and the detailed description given below, serve to explain the principles of the present disclosure. As shown throughout the drawings, like reference numerals designate like or corresponding parts. [0013] Figure 1 illustrates a functional block diagram of an exemplary LED driver circuit according to one embodiment of the present technology.

[0014] Figure 2 illustrates a schematic diagram for an exemplary power conversion stage incorporating aspects of the present disclosure.

[0015] Figure 3 illustrates a schematic diagram for exemplary dimming detection circuit incorporating aspects of the present disclosure.

[0016] Figure 4 illustrates a schematic diagram for an exemplary linear constant current regulator with dimming functionality incorporating aspects of the present disclosure.

[0017] Figure 5 illustrates a functional block diagram of an exemplary LED driver circuit according to one embodiment of the present technology.

[0018] Figure 6 illustrates a schematic diagram of an exemplary feedback circuit incorporating aspects of the present disclosure.

[0019] Figure 7 illustrates a flow chart for an exemplary method of driving an LED lamp with voltage drop monitoring according to one embodiment of the present technology.

[0020] Figure 8 illustrates a flow chart for a method of dimming an LED lamp incorporating aspects of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Referring now to Figure 1 there can be seen a functional block diagram of a one and a half stage LED driver system 100 according to one embodiment of the present disclosure. The LED driver system 100, which in one embodiment comprises a one and a half stage driver, is applied to provide power to an LED lamp or lamp assembly 106 which includes one or more light emitting diodes elements. Input power for the LED driver system 100 is supplied by an input power source or signal 102 such as the local power grid or other suitable AC power source. Typical grid power appropriate for input power 102 may be about 1 10 volts root mean square (Vrms) to about 250 Vrms at about 50 to 60 Hertz. The input power signal 102 may also be supplied by a dimming controller, in which case the input power signal 102 may comprise a phase control dimming signal such as forward phase control diming signal, reverse phase control dimming, or other suitable AC dimming signal that reduces input power when dimming is desired. A first stage 104 is coupled to the input power signal 102 and configured to output a voltage regulated power 114 which may be supplied to the LED lamp assembly 106 via output terminals 131 and 132. The first stage 104 is an active power conversion circuit that includes an AC to DC conversion circuit such as a bridge diode to convert the generally sinusoidal input power signal 102 to a full or half wave rectified DC signal. In certain embodiments the first stage 104 also includes a switching converter configured to provide power factor correction and to regulate the voltage regulated power 1 14, in the form of a DC output voltage of the first stage. The LED lamp 106 receives an electric current from the voltage regulated power 114 which is returned through a linear current regulator 108.

[0022] In one embodiment, the exemplary LED driver system 100 also includes a dimming detection circuit 110 that is configured to monitor the input voltage 122 and generate a dimming signal 120 proportional to the dimming level indicated by a phase angle dimming level of the input power signal 102. The dimming signal 120 is received by the linear current regulator or control circuit 108 which adjusts the lamp current 116 accordingly to achieve a desired dimmed light output from the LED lamp 106. To maintain a constant light output the lamp current 116 is maintained at a constant value, however due to environmental and other factors the voltage drop across the LED lamp 106 can vary. When the voltage across the LED lamp 106 drops, the voltage drop across the linear current regulator 108 increases proportionally. In the case of a linear regulator, as compared to a switching regulator, the power dissipated by a linear regulator is proportional to the voltage drop across the regulator and the current passing through the regulator. Thus, as the voltage drop across the linear current regulator 108 increases the power dissipated increases proportionally. Similarly, when the required dimming level indicated by the input power signal 102 is low, the voltage drop across the linear current regulator 108 will increase, further increasing the power dissipated by the linear current regulator 108. Any power dissipated in the linear current regulator 108 cannot be used to generate light. Thus, it is desirable to reduce the power dissipated by the linear current regulator as much as possible.

[0023] In one embodiment, to help reduce wasted power, the linear current regulator 108 generates a voltage drop signal 118 when the voltage drop across the linear current regulator 108 exceeds a threshold amount. The voltage drop signal 118 is provided to the first stage 104 where it is used to lower the voltage regulated power 114, or DC output voltage, in order to reduce power dissipated by the linear current regulator 108.

[0024] Referring now to Figure 2 there can be seen an illustration of a schematic diagram for an exemplary power conversion stage 200 that provides a means of converting an AC input power such as the AC input power signal 102 described above with respect to Figure 1, to a regulated DC voltage. The power conversion stage 200 receives input power at input terminals 1 and 2, generally referred to as supply terminal pin 1 and return terminal pin 2, of the input connector CO 1 through a fuse F201 that is use to protect the power conversion stage 200 from surges or other abnormalities that may cause excessive current to flow into the power conversion stage 200. An input filter 220 is included to condition the input power and to prevent harmonics and switching noise created within the power conversion stage 200 from being transmitted back to the input power through the input connector CON 1. The input filter 220 includes a capacitor C210 connected in parallel with the input terminals 1 & 2 of connector CO 1 and is followed by a pair of inductors L201 and L202, each connected in series with the supply terminal pin 1 of the input connector CO 1 and the return terminal pin 2 of the input connector CO 1 respectively. Each inductor L201, L202 is connected in parallel with a resistor R201 and R202 respectively.

[0025] A bridge diode circuit BD1 is used to convert the AC power to a full wave rectified DC power and a capacitor C201 connected in parallel across the DC power provides some initial filtering to help smooth the full wave rectified signal produced by the bridge diode circuit BD1. A flyback converter, generally indicated by reference numeral 222, converts and regulates the rectified DC power produced by the bridge diode BD1 into a generally constant voltage V201 at output terminals V+ and V-. The flyback converter 222 transfers power from the primary side to the secondary or output side through a transformer T201 with primary winding T201-1 and secondary winding T201-2. The diode D201 rectifies the output power and a filter capacitor C208 connected in parallel with the output voltage V201 provides filtering and smoothing of the output voltage V201. A series connected resistor R216 and capacitor C207 are connected in parallel with the diode D201 to provide additional conditioning on the output side T201-2 of the transformer T201. Power in the primary side T201-1 is controlled by a switching device Q201 which is duty cycle modulated by an integrated control circuit U201 through a driving resistor R213. In the exemplary embodiment shown in Figure 2, the switching device Q201 is illustrated as an n-channel metal oxide semiconductor field effect transistor (MOSFET). Alternatively other types of switching devices may be advantageously employed. The drain of switching device Q201 is connected back to the positive output of the bridge diode BD1 through a diode D202 and resistor R212 to protect the switching device Q201 from excessive voltages and to provide a current circulation path when the switching device Q201 is not conducting. A capacitor C206 is also coupled in parallel with R212.

[0026] In the exemplary flyback regulator 222 the output voltage V201 is regulated by the integrated circuit U201, which in this embodiment comprises one of the many standard power factor correction (PFC) controller integrated circuits. The integrated circuit U201, or PFC controller, regulates the duty cycle of the switching device Q201 such that the power factor of the power conversion stage 200 is near unity and to maintain the DC voltage V201 at a generally constant value. The PFC controller U201 receives operating power on pin 8 through resistors R203 and R204 and filtering capacitors C202 and C205. Operating power is also provided to the PFC controller U201 from the tertiary winding T201-3 of the transformer T201 through the diode D203 and resistor R211. The tertiary winding T201-3 of the transformer T201, which in one embodiment is a flyback transformer, is used to provide a zero crossing detection signal (ZCD) to pin 5 of the PFC controller U201 through a current limiting resistor R215 and a parallel connected filtering capacitor C209. A current sensing resistor R208 is coupled in series between the switching device Q201 and circuit ground 224 and provides a current sensing signal at pin 4 of the PFC controller U201. A high power factor is achieved by keeping the current drawn from the input power in phase with the voltage of the input power. To achieve this, PFC controller U201 senses the voltage of the input power through a multiplier input at pin4 of the PFC controller U201. In the exemplary flyback converter 222, the input voltage is sensed at the full wave rectified DC voltage output from the bridge diode BD1 through resistors R205 and R206, and is filtered with a parallel connected resistor R207 and capacitor C204 to create a multiplier input signal at pin 4 of the PFC controller U201.

[0027] Regulation of the voltage V201 is achieved by using a resistor divider network

R208, R209 to feed the output voltage V201 back to the summing node of an operation amplifier (op-amp) type compensation circuit exposed at pin 1 of the PFC controller U201. This creates a feedback control loop that acts to regulate the output voltage V201 at a generally constant level. Stability of the control loop is achieved by integral control created by a capacitor C203 connected between the summing node 226 and the compensation input pin 2 of the PFC controller U201. This feedback control loop allows the PFC controller U201 to modulate the duty cycle of switching device Q201 such that the output voltage V201 is maintained at a generally constant level according to the feedback signal at node 226. Thus the voltage at which the output voltage V201 is regulated by adjusting the feedback signal applied to node 226. As will be discussed in more detail below, this functionality can be used to adjust the output voltage V201 as desired to reduce power dissipated in other portions of an LED driver system such as the LED driver system 100 described above.

[0028] Referring now to Figure 3 there can be seen an exemplary dimming detection circuit 300 that incorporates a comparator circuit to generate a DC dimming signal V303 that corresponds to a dimming level indicated by the input voltage signal V302. The diming signal detection circuit 300 illustrates and example of a means for generating a DC dimming signal V303 that is indicative of a dimming level of a phase angle dimming signal. In the exemplary dimming detection circuit 300 the input voltage signal V302 is a rectified input power signal such as the signal produced by the bridge rectifier circuit BD1 described above with reference to Figure 2. In many applications, such as where LED lighting is being retrofitted into earlier incandescent lighting fixtures, the input power, such as input power signal 102 of Figure 1, may comprise a phase angle dimming signal as is known in the art. The phase angle dimming signal may be of any appropriate type such as forward phase angle control or reverse phase angle control dimming. When the input power is a phase angle dimming signal the dimming signal V303 will be a full wave rectified phase angle dimming signal. A comparator A301 is used to provide a charging signal to charge capacitor C301 through resistor R305 while the resistor R306 provides a discharge path. An operating voltage source V301 provides operating power to the comparator A301 and is also used to generate a reference voltage for the non-inverting input of the comparator A301 via a resistor divider network formed by resistors R304 and R303. The input voltage signal V302 is applied to the inverting input of the comparator A301 via a second resistor divider network formed by resistors R301 and R302. Thus the DC dimming signal V303 varies inversely with the dimming level indicated by the input voltage signal V302, i.e. when a bright light output is required the DC dimming signal V303 is a low voltage and vice versa.

[0029] Referring now to Figure 4 there can be seen an exemplary embodiment of a linear current regulation circuit 400 that may be used to regulate the current flowing through an LED lamp assembly LA401 at a generally constant amount as controlled by a dimming signal V303. The linear current regulation circuit 400 illustrates one example of regulating the current through the LED lamp assembly LA401 at a generally constant amount in accordance with the DC dimming signal V303. Lamp current is supplied by a voltage source V201, such as for example the power conversion circuit 200 described above. Alternatively other voltage regulation circuits can be used to provide the voltage source V201, also referred to as a regulated voltage.

[0030] In one embodiment, the current regulation circuit 400, also referred to as a controller, includes a pair of transistors Q401 and Q402, where the collector of transistor Q401 receives current from the LED lamp assembly LA401 such that the transistor Q402 controls the current flowing through the LED lamp assembly. An operating voltage V401 supplies current to the base of the transistor Q402 through a resistor R403. A resistor R405 is coupled between the emitter of the control transistor Q402 and a circuit ground 402 such that an increase in current flowing through the control transistor Q402 raises the voltage at the emitter 404. A second transistor Q401 is coupled between the base 406 of the control transistor Q402 and circuit ground 402 with the control terminal, or base, of transistor Q401 coupled to the emitter of the control transistor Q402 through a resistor R406. A DC dimming signal V303, such as the DC dimming signal V303 generated by the exemplary dimming detection circuit 300 described above, is applied to theh base of transistor Q401 through a resistor R402 and a pair of series connected diodes D401 and D402.

[0031] Operation of the linear current regulation circuit 400 can be understood by first assuming the resistor R406 is shorted and the DC dimming signal is removed by removing the diode D401. This reduces the DC dimming circuit 400 to a linear circuit where the lamp current ILED can be calculated as ILED=0.7 volts / R405 Ω where 0.7 volts is the base to emitter voltage drop of the control transistor Q402 and the symbol Ω indicates the resistance value of each resistor in ohms. Now, looking at the full linear current regulation circuit 400, including the diode D401 and resistor R406, the lamp current can be calculated as ILED = (0.7v - R406) R405n, where VR₄O6 is the voltage across the resistor R406. Thus, the lamp current, I_LED goes down as the DC dimming signal V303 goes high. In certain embodiments, the resistor R405 has a low resistance such as several ohms, and resistor R406 and resistor R402 are relatively large such as several kilo ohms. The voltage across the resistor R406 may be approximated as: V_R406 = V303 * R406 Ω / (R402 Ω + R406 Ω) and the lamp current can be approximated as: II.ED 0.7- V303*R406 Ω /(R406 Ω -H1402 Ω))/ 405 Ω yielding a lamp current that is a linear function of the DC dimming signal V303. In the exemplary circuit 400 two diodes D401 and D402 are coupled in series with the DC dimming signal V303 to improve the dimming curve. Each diode, D401, D402 has a voltage drop of about 0.7 volts so the DC dimming signal V303 does not affect the linear current regulation circuit 400 until the DC dimming signal V303 exceeds about 1.4 volts. For example, with the exemplary circuits illustrated above in Figures 3, and 4, the lamp current is reduced linearly when the AC dimming signal V302 has a remaining angle of less than about 144 degrees. The remaining angle refers the portion of the sinusoidal cycle that a phase angle dimming signal is transmitting power to a load.

[0032] Figure 5 illustrates a functional block diagram of a one and a half stage LED driver circuit 500 using voltage drop monitoring to lower power dissipation while maintaining steady LED light output and dimming functionality. The exemplary LED driver circuit 500 includes a first stage voltage regulator 504 having an input 526 and an output 514. The first stage 504 is configured to convert an AC input power 502 on input 526 into a regulated DC voltage on output 514. In certain embodiments the first stage 504 includes a PFC controller configured to maintain a current drawn from the input power 502 in phase with the voltage of the input power 502, such that the power factor of the driver circuit 500 is maintained at or close to unity. A LED lamp or lamp assembly 506 receives the regulated DC voltage on output 514 from the first stage 504. A linear current controller 508 is coupled in series between the LED lamp assembly 506 and a return 520 of the first stage 504 to maintain the lamp current 516, which is the current flowing through the LED lamp, at a generally constant amount. The DC voltage on output 514 is regulated by a regulator 510, also referred to as a feedback control circuit, that receives a feedback signal 522 and operates the first stage 504 via a control signal 524 such that the DC voltage on output 514 is maintained at about a constant voltage. The regulator 510 receives the feedback signal from a summer 512 that combines the DC voltage on output 514 with a voltage drop signal 534 created from the voltage drop of the linear current regulator 518 by a voltage drop sensing circuit 528. As described above, power dissipated by the linear current regulator 508 is proportional to the voltage drop 518 across the regulator 508. The voltage monitoring circuit 528 produces a voltage drop signal 534 when the voltage drop 518 across the current regulator 508 exceeds a threshold amount. When the voltage drop signal 534 is added to the DC voltage on output 514 in the summer 512 it modifies the feedback signal such that the DC voltage on output 514 is regulated at a lower value. In this fashion the voltage drop monitoring or sensing circuit 528 acts along with the summer 512 to reduce power dissipation in the linear current regulator 508. To accommodate applications requiring dimming functionality, the one and a half stage LED driver circuit 500 includes a dimming detection circuit 536 which receives the input power signal 532 and produces a dimming signal 530 proportional to the dimming level indicated by the input power signal 532. The dimming signal 530 is received by the linear current regulator 508 where it is used to adjust the lamp current 516 accordingly.

[0033] It is desirable to drive LED lamps with a constant current to maintain a steady light output. However, the forward voltage drop across LEDs varies due to environmental and manufacturing factors. For example the forward voltage drop of a LED is reduced as temperature increases. The linear current regulator 508 described operates by varying its voltage drop inversely with the voltage drop of the LED. This means when the temperature rises, the LED voltage drop will go down and the linear current regulator 508 will increase its voltage drop in order to maintain a constant current. Using voltage drop monitoring, a voltage monitoring circuit 528 is used to feed back a voltage drop signal 534 to the voltage regulator 510, which in turn reduces the amount of power dissipated in the current regulator 508. The increased voltage drop on the linear current regulator 508 results in increased power dissipated. Table 1 below provides some representative values to illustrate the power dissipated in a drive circuit that does not employ voltage drop monitoring to reduce power dissipation in the current regulator as is described herein. Referring to Table 1, in this example, the DC voltage on output 514 is approximately 14 volts and the desired LED current is approximately 0.5 amps. As seen in Table 1 when the LED voltage drop is 1 1 volts the linear current regulator 508 dissipates 1.5 watts. When the LED voltage goes up the power dissipated by the linear current regulator 508 goes down. Thus, when the LED voltage is approximately 13 volts the power dissipated in the linear current regulator 508 is 0.5 watts.

[0034]

[0035] Table 2 provides representative values for a LED driver circuit 500 that includes the novel voltage drop monitoring method described above. As seen in Table 2 when the LED voltage is at or above approximately 12 volts the voltage drop of the current regulator 508 is below the threshold voltage and the voltage monitoring circuit 528 does not generate any voltage drop signal. When the LED voltage falls below approximately 12 volts, such as to 1 1 volts, the voltage drop of the current regulator 508 exceeds the threshold voltage and the voltage monitoring circuit 528 begins to generate a voltage drop signal that causes the first stage voltage on output 514 to be regulated at a lower value, such as about 13 volts. Reducing the first stage voltage on output 514 results in a proportional drop in the power dissipated in the linear current regulator 508, which in this example is about 1 watt.

[0036]

(watts)

13 1 1 0.5 1

14 12 0.5 1

14 13 0.5 0.5

[0037] Referring now to Figure 6 there can be seen an exemplary feedback control circuit 510 and summer 512 used to produce a control signal 524 as may be used to operate a first stage voltage regulator such as first stage 504 described above. The LED lamp D600 receives DC voltage 612 which produces a current through the lamp that is regulated by the current regulating transistor Q402, such as the control transistor Q402 described above with reference to Figure 4. A resistor divider network, including resistors R603 and R604 connected in series between the DC power 612 and circuit ground 614, creates a voltage feedback 616 that is coupled to the feedback control circuit 510 through resistor R602. Zener diode Z601 is coupled between the current regulator transistor Q402 and the summer 512 so that when the voltage drop across the current regulator transistor Q402 exceeds the breakdown voltage of the Zener diode D601 a voltage drop signal is applied to circuit node 616 where it is added to the voltage feedback through the resistor R602. An operational amplified A601 receives a reference voltage V601 and produces the regulation or control signal 524. Stabilization and optimization is provided in exemplary the feedback control circuit 510 by capacitor C601 coupled in parallel with a series connected resistor R601 and capacitor C602.

[0038] Figure 7 illustrates a flow chart of an exemplary method for driving a LED lamp incorporating aspects of the voltage drop monitoring technology described herein. In one embodiment, the method uses a one and a half stage driver circuit that includes a first stage to convert 702 an AC input power to a DC output power. A feedback signal is created 704 from the DC output voltage, and this feedback signal is used to regulate 706 the voltage of the DC output voltage at a generally constant voltage corresponding to the feedback signal. In certain embodiments the first stage includes power factor correction to maintain the current drawn by the first stage in phase with the voltage of the input power such that the power factor of the LED driver is at or close to unity. The DC output power is provided 708 to an LED lamp, such as for example a LED lamp. In one embodiment, the LED lamp can comprise multiple LED elements arranged in a serial-parallel block arrangement or other suitable LED lamp. The LED lamp current, i.e. the current flowing through the LED lamp, is maintained 710 at a generally constant amount by a linear current source, also referred to herein as a linear current regulator. Power dissipated in the linear current source is proportional to the lamp current and the voltage drop across the linear current source. Since this power is not used to produce light, it is essentially wasted energy. It is therefore desirable to reduce the power dissipated by the linear current source as much as possible. A reduction in the linear current source power can be achieved by monitoring 712 the voltage drop across the linear current source. When the voltage drop exceeds 714 a threshold voltage, the voltage drop signal is used to adjust 716 the feedback signal created in 704 above. As described above, increasing the feedback signal will lower the voltage at which the DC output voltage is regulated. The adjusted feedback signal is then used to regulate 706 the DC output voltage at a generally lower voltage.

[0039] Figure 8 illustrates a flow chart showing a method 800 for including dimming functionality in the exemplary LED driving method 700 illustrated in Figure 7. In certain embodiments the AC input power comprises a dimming signal such as for example a forward or reverse phase control diming signal. To achieve dimming functionality the AC input power is monitored 802 for a dimming signal. The dimming signal is converted to create 806 a dimming voltage, generally a DC dimming voltage, where a voltage of the dimming signal corresponds to the input dimming signal. The DC dimming voltage is used to adjust the amount of lamp current provided by the linear current source.

[0040] Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Moreover, it is expressly intended that all combinations of those elements, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

CLAIMS What is claimed is:

1. A driver circuit for a LED lamp, the driver circuit comprising:

a first stage configured to receive an AC input power and produce a regulated DC voltage; a linear current regulator coupled to the first stage and configured to regulate a lamp current; a feedback circuit coupled to the first stage, wherein the feedback circuit is configured to monitor the regulated DC voltage and operate the first stage to maintain the regulated DC voltage at a generally constant voltage; and

a voltage drop monitoring circuit coupled to the feedback circuit and configured to adjust the generally constant voltage, wherein the voltage drop monitoring circuit monitors a voltage drop of the linear current regulator and reduces the generally constant voltage as the voltage drop increases.

2. The driver circuit of claim 1, wherein the voltage monitoring circuit is configured to reduce the generally constant voltage when the voltage drop exceeds a threshold voltage, and not reduce the generally constant voltage when the voltage drop is below the threshold voltage.

3. The driver circuit of claim 2, wherein the voltage drop monitoring circuit comprises a zener diode.

4. The driver circuit of claim 1, wherein the first stage comprises a bridge diode circuit.

5. The driver circuit of claim 1, wherein the first stage is configured to maintain a power factor of the driver circuit at or close to unity.

6. The driver circuit of claim 1, wherein the linear current regulator comprises a first and second transistor, and wherein the first transistor controls the lamp current, the second transistor controls a base current of the first transistor, and the lamp current is coupled to the base of the second transistor via a resistor.

7. The driver circuit of claim 1, wherein the AC input power comprises a dimming signal, and the driver circuit comprises a dimming controller, wherein the dimming controller is coupled to the AC input voltage and adapted to operate the linear current regulator such that the lamp current corresponds to the dimming signal.

8. The driver circuit of claim 7, wherein a control output of the dimming controller is coupled to the linear current regulator and,

wherein the dimming controller monitors the dimming signal and the control output comprises a DC voltage that is inversely proportional to a dimming level indicated by the dimming signal.

9. The driver circuit of claim 8 wherein the dimming controller is coupled to the linear current regulator through one of more series connected diodes.

10. An electric lighting device, comprising:

a driver circuit configured to receive an AC input power and produce a lamp current; and an LED lamp coupled to the lamp current, wherein the driver circuit comprises:

a first stage coupled to the AC input power and configured to produce a regulated DC voltage;

a linear current regulator coupled between the first stage and the LED lamp, the linear current regulator is configured to regulate the lamp current at a generally constant amount; a feedback circuit coupled to the first stage, wherein the feedback circuit is configured to monitor the regulated DC voltage and operate the first stage such that the regulated DC voltage is maintained at a generally constant voltage; and a voltage drop monitoring circuit coupled to the feedback circuit and configured to adjust the generally constant voltage,

wherein the voltage drop monitoring circuit monitors a voltage drop of the linear current regulator and reduces the generally constant voltage when the voltage drop increases.

11. The electric lighting device of claim 10, wherein the LED lamp comprises one or more LED diodes coupled in a series-parallel block arrangement.

12. The electric lighting device of claim 10, wherein the voltage drop monitoring circuit is adapted to reduce the generally constant voltage when the voltage drop exceeds a threshold voltage.

13. The electric lighting device of claim 12, wherein the voltage drop monitoring circuit comprises a zener diode.

14. The electric lighting device of claim 10, wherein the first stage is configured to maintain a power factor of the driver circuit at or close to unity.

15. The electric lighting device of claim 10, wherein the first stage comprises a flyback type power converter.

16. The electric lighting device of claim 10, wherein the AC input power comprises a dimming signal, the driver circuit comprises a dimming controller, and

wherein the dimming controller is coupled to the AC input voltage and adapted to operate the linear current regulator to adjust the lamp current relative to the dimming signal.

17. A method for operating an LED driver circuit, the method comprising:

using a first power stage to convert an AC input power to a DC output voltage;

creating a feedback signal from the DC output voltage; using the feedback signal to regulate the DC output voltage;

providing the DC output voltage to an LED lamp;

using a linear current source to regulate a lamp current;

monitoring a voltage drop of the linear current source; and

adjusting the feedback signal to reduce the DC output voltage when the voltage drop exceeds a threshold voltage.

18. The method of claim 17, comprising:

monitoring the AC input power for a dimming signal; and

adjusting the lamp current based at least in part on the dimming signal.

19. The method of claim 18, further comprising:

creating a DC dimming voltage corresponding to a dimming level indicated by the dimming signal; and

adjusting the lamp current relative to the DC dimming voltage.

20. The method of claim 19, wherein a magnitude of the DC dimming voltage is inversely proportional to the lamp current.