Title of Invention: AC DRIVEN SOLID STATE LIGHTING APPARATUS WITH LED STRING INCLUDING SWITCHED SEGMENTS Abstract: A diode selection circuit for a light emitting apparatus according to some embodiments includes a plurality of light emitting devices coupled in series. The diode selection circuit includes a comparator configured to receive a rectified AC input signal and a reference voltage and to generate a control signal in response to comparison of the rectified AC input signal with the reference voltage, a voltage controlled current source configured to supply a current to the plurality of light emitting diodes that is proportional to the rectified AC input signal, and a switch configured to receive the control signal and to shunt current away from at least one of the plurality of light emitting devices in response to the control signal. Detailed Description: RELATED APPLICATIONS This application is a continuation of U.S. patent application Ser. No. 12/775,842 filed May 7, 2010, the disclosure of which is incorporated herein by reference in its entirety. FIELD OF THE INVENTION The present invention relates to solid state lighting, and more particularly to lighting fixtures including solid state lighting components: BACKGROUND Solid state lighting arrays are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region. Solid state lighting panels are commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices. In addition, there has been increased interest in the use of solid state lighting panels as backlights for larger displays, such as LCD television displays. For smaller LCD screens, backlight assemblies typically employ white LED lighting devices that include a blue-emitting LED coated with a wavelength conversion phosphor that converts some of the blue light emitted by the LED into yellow light. The resulting light, which is a combination of blue light and yellow light, may appear white to an observer. However, while light generated by such an arrangement may appear white, objects illuminated by such light may not appear to have a natural coloring, because of the limited spectrum of the light. For example, because the light may have little energy in the red portion of the visible spectrum, red colors in an object may not be illuminated well by such light. As a result, the object may appear to have an unnatural coloring when viewed under such a light source. The color rendering index (CRI) of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. The color rendering index ranges from essentially zero for monochromatic sources to nearly 100 for incandescent sources. Light generated from a phosphor-based solid state light source may have a relatively low color rendering index. For large-scale backlight and illumination applications, it is often desirable to provide a lighting source that generates a white light having a high color rendering index, so that objects and/or display screens illuminated by the lighting panel may appear more natural. Accordingly, to improve CRI, red light may be added to the white light, for example, by adding red emitting phosphor and/or red emitting devices to the apparatus. Other lighting sources may include red, green and blue light emitting devices. When red, green and blue light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the red, green and blue sources. Although solid state light sources having high CRI and/or high efficiency have been demonstrated, one problem with the large-scale adoption of such light sources in architectural applications is that solid state lighting devices are typically designed to be driven using direct current (DC) power, while electric power is distributed using alternating current (AC), which is more efficient for long distance power distribution. Typically, a solid state lighting source is provided or coupled with a power converter that converts AC power into DC power, and the DC power is used to energize the light source. However, the use of such power converters increases the cost of the lighting source and/or the overall installation, and introduces additional efficiency losses. Some attempts at providing AC-driven solid state lighting sources have involved driving an LED or string or group of LEDs using a rectified AC waveform. However, because the LEDs require a minimum forward voltage to turn on, the LEDs may turn on for only a part of the rectified AC waveform, which may result in visible flickering, may undesirably lower the power factor of the system, and/or may increase resistive loss in the system. Other attempts at providing AC-driven solid state lighting sources have involved placing LEDs in an anti-parallel configuration, so that half of the LEDs are driven on each half-cycle of an AC waveform. However, this approach requires twice as many LEDs to produce the same luminous flux as using a rectified AC signal. SUMMARY A circuit for a light emitting apparatus according to some embodiments includes a plurality of light emitting devices coupled in series. The circuit includes a comparator configured to receive a rectified AC input signal and a reference voltage and to generate a control signal in response to comparison of the rectified AC input signal with the reference voltage, a voltage controlled current source configured to supply a current to the plurality of light emitting diodes that is proportional to the rectified AC input signal, and a switch configured to receive the control signal and to shunt current away from at least one of the plurality of light emitting devices in response to the control signal. The voltage controlled current source may include a first transistor, an emitter resistor coupled to an emitter of the first transistor, a diode coupled to a base of the first transistor, a first resistor coupled to a first terminal of the diode and a second resistor coupled to a second terminal of the diode, and a terminal configured to receive the rectified AC input signal coupled to the emitter resistor and to the first resistor. The diode may include a Zener diode. A collector of the first transistor may be coupled to the series of light emitting devices. In some embodiments, the circuit may further include a second transistor including a base coupled to a collector of the first transistor and a collector coupled to the emitter of the first transistor. The circuit may further include an adjustable current sink coupled to the base of the first transistor. The adjustable current sink may include a third transistor having a collector coupled to the base of the first transistor and an emitter coupled to ground, and a diode coupled to a base of the third transistor. The base of the third transistor is configured to receive a pulse width modulation (PWM) control signal configured to control a conductivity of the third transistor. The circuit may be configured to shunt current away from the at least one light emitting device in response to a level of the rectified AC input signal being below a threshold level. The switch may include a field effect transistor, and the control signal may be applied to a gate of the field effect transistor. The switch may further include a second transistor coupled in a cascode configuration with the field effect transistor so that conductivity of the second transistor may be controlled by the field effect transistor. The second transistor may include a bipolar transistor including a base, a collector and an emitter, and a drain of the field effect transistor may be coupled to the emitter of the bipolar transistor, and the collector of the bipolar transistor may be coupled to an anode of the at least one light emitting device. The circuit may further include a resistor coupled between the drain of the field effect transistor and the emitter of the bipolar transistor. The diode selection circuit may further include a voltage divider ladder configured to generate a plurality of reference voltages, a plurality of comparators configured to receive the rectified AC input signal and a respective one of the plurality of reference voltages and to generate respective control signals in response to comparison of the rectified AC input signal with the respective reference voltages, and a plurality of switches configured to receive respective ones of the control signals and to shunt current away from respective ones of the plurality of light emitting devices in response to the control signals. The switch may include a field effect transistor including a gate terminal and source/drain terminals, one of the source/drain terminals may be coupled to an anode of the at least one light emitting device and another of the source/drain terminals may be coupled to an cathode of the at least one light emitting device. The circuit may further include a slew rate control capacitor coupled between the gate terminal of the field effect transistor and the anode of the at least one light emitting device. The circuit may further include a voltage divider ladder configured to generate a plurality of reference voltages, a plurality of comparators configured to receive the rectified AC input signal and a respective one of the plurality of reference voltages and to generate respective control signals in response to comparison of the rectified AC input signal with the respective reference voltages, and a plurality of switches configured to receive respective ones of the control signals and to shunt current away from respective ones of the plurality of light emitting devices in response to the control signals. The switch may include a bipolar transistor including a base coupled to an output of the comparator and having an emitter coupled to ground and a collector coupled to an anode of the at least one light emitting device. A lighting apparatus according to some embodiments includes a terminal configured to receive an AC power signal, a full wave rectifier configured to generate a rectified AC input signal in response to the AC power signal, a reference voltage generator configured to generate a DC reference voltage in response to the rectified AC input signal, a plurality of light emitting devices coupled in series, a voltage controlled current source configured to supply a current to the plurality of light emitting diodes that is proportional to the rectified AC input signal, and a diode selection circuit including a comparator configured to receive the rectified AC input signal and the reference voltage and to generate a control signal in response to comparison of the rectified AC input signal with the reference voltage. The diode selection circuit includes a switch configured to receive the control signal and to shunt current away from at least one of the plurality of light emitting devices in response to the control signal. The voltage controlled current source may include a first transistor, an emitter resistor coupled to an emitter of the first transistor, a diode coupled to a base of the first transistor, a first resistor coupled to a first terminal of the diode and a second resistor coupled to a second terminal of the diode, and a terminal configured to receive the rectified AC input signal coupled to the emitter resistor and to the first resistor. The diode may include a Zener diode. A collector of the first transistor may be coupled to the series of light emitting devices. In some embodiments, the circuit may further include a second transistor including a base coupled to a collector of the first transistor and a collector coupled to the emitter of the first transistor. The circuit may further include an adjustable current sink coupled to the base of the first transistor. The adjustable current sink may include a third transistor having a collector coupled to the base of the first transistor and an emitter coupled to ground, and a diode coupled to a base of the third transistor. The base of the third transistor is configured to receive a pulse width modulation (PWM) control signal configured to control a conductivity of the third transistor. The diode selection circuit may be configured to shunt current away from the at least one light emitting device in response to a level of the rectified AC input signal being below a threshold level. The switch may include a field effect transistor, and the control signal may be applied to a gate of the field effect transistor. The switch may further include a second transistor coupled in a cascode configuration with the field effect transistor so that conductivity of the second transistor may be controlled by the field effect transistor. The second transistor may include a bipolar transistor including a base, a collector and an emitter, and a drain of the field effect transistor may be coupled to the emitter of the bipolar transistor, and the collector of the bipolar transistor may be coupled to an anode of the at least one light emitting device. The switch may include a field effect transistor including a gate terminal and source/drain terminals, one of the source/drain terminals may be coupled to an anode of the at least one light emitting device and another of the source/drain terminals may be coupled to an cathode of the at least one light emitting device. The lighting apparatus may further include a current supply circuit coupled to an anode of a first one of the plurality of light emitting devices, the current supply circuit including a bipolar transistor including a collector coupled to the anode of the first light emitting device, an emitter resistor coupled to an emitter of the bipolar transistor, and a bias circuit coupled to a base of the bipolar transistor. Other apparatus and/or methods according to embodiments of the invention will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional apparatus and/or methods be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate certain embodiment(s) of the invention. In the drawings: FIGS. 1A and 1B illustrate a solid state lighting apparatus in accordance with some embodiments of the invention. FIG. 1C is a schematic circuit diagram illustrating series interconnection of light emitting devices (LEDs) in a solid state lighting apparatus. FIG. 2 is a schematic circuit diagram illustrating a drive circuit for a solid state lighting apparatus in accordance with various embodiments of the invention. FIGS. 3A, 3B and 3C are schematic circuit diagrams illustrating voltage controlled current sources for a solid state lighting apparatus in accordance with various embodiments of the invention. FIG. 3D is a graph of simulated output current of voltage controlled current sources for a solid state lighting apparatus in accordance with various embodiments of the invention. FIG. 4A is a graph illustrating output current, and FIG. 4B is a graph illustrating input and output voltage waveforms, for solid state lighting systems in accordance with various embodiments of the invention. FIG. 5 is a schematic circuit diagram illustrating a diode selection circuit according to some embodiments of the invention. FIG. 6 is a graph illustrating a rectified voltage signal according to some embodiments. FIGS. 7, 8 and 9 are schematic circuit diagrams illustrating diode selection circuits according to further embodiments of the invention. FIG. 10 is a schematic circuit diagram illustrating a voltage controlled current sources with an adjustable current sink for a solid state lighting apparatus in accordance with some embodiments of the invention. DETAILED DESCRIPTION OF EMBODIMENTS Embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” “comprising,” “includes” and/or “including” when used herein, 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. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Referring to FIGS. 1A and 1B, a lighting apparatus 10 according to some embodiments is illustrated. The lighting apparatus 10 shown in FIGS. 1A and 1B is a “can” lighting fixture that may be suitable for use in general illumination applications as a down light or spot light. However, it will be appreciated that a lighting apparatus according to some embodiments may have a different form factor. For example, a lighting apparatus according to some embodiments can have the shape of a conventional light bulb, a pan or tray light, an automotive headlamp, or any other suitable form. The lighting apparatus 10 generally includes a can shaped outer housing 12 in which a lighting panel 20 is arranged. In the embodiments illustrated in FIGS. 1A and 1B, the lighting panel 20 has a generally circular shape so as to fit within an interior of the cylindrical housing 12. Light is generated by solid state lighting devices (LEDs) 22, 24, which are mounted on the lighting panel 20, and which are arranged to emit light 15 towards a diffusing lens 14 mounted at the end of the housing 12. Diffused light 17 is emitted through the lens 14. In some embodiments, the lens 14 may not diffuse the emitted light 15, but may redirect and/or focus the emitted light 15 in a desired near-field or far-field pattern. Still referring to FIGS. 1A and 1B, the solid-state lighting apparatus 10 may include a plurality of first LEDs 22 and a plurality of second LEDs 24. In some embodiments, the plurality of first LEDs 22 may include white emitting, or near white emitting, light emitting devices. The plurality of second LEDs 24 may include light emitting devices that emit light having a different dominant wavelength from the first LEDs 22, so that combined light emitted by the first LEDs 22 and the second LEDs 24 may have a desired color and/or spectral content. For example, the combined light emitted by the plurality of first LEDs 22 and the plurality of second LEDs 24 may be warm white light that has a high color rendering Index. The chromaticity of a particular light source may be referred to as the “color point” of the source. For a white light source, the chromaticity may be referred to as the “white point” of the source. The white point of a white light source may fall along a locus of chromaticity points corresponding to the color of light emitted by a black-body radiator heated to a given temperature. Accordingly, a white point may be identified by a correlated color temperature (CCT) of the light source, which is the temperature at which the heated black-body radiator matches the hue of the light source. White light typically has a CCT of between about 2500K and 8000K. White light with a CCT of 2500K has a reddish color, white light with a CCT of 4000K has a yellowish color, and while light with a CCT of 8000K is bluish in color. “Warm white” generally refers to white light that has a CCT between about 3000K and 3500K. In particular, warm white light may have wavelength components in the red region of the spectrum, and may appear yellowish to an observer. Warm white light typically provides a relatively high CRI, and accordingly can cause illuminated objects to have a more natural color. For illumination applications, it is therefore desirable to provide a warm white light. In order to achieve warm white emission, conventional packaged LEDs include either a single component orange phosphor in combination with a blue LED or a mixture of yellow/green and orange/red phosphors in combination with a blue LED. However, using a single component orange phosphor can result in a low CRI as a result of the absence of greenish and reddish hues. On the other hand, red phosphors are typically much less efficient than yellow phosphors. Therefore, the addition of red phosphor in yellow phosphor can reduce the efficiency of the package, which can result in poor luminous efficacy. Luminous efficacy is a measure of the proportion of the energy supplied to a lamp that is converted into light energy. It is calculated by dividing the lamp's luminous flux, measured in lumens, by the power consumption, measured in watts. Warm white light can also be generated by combining non-white light with red light as described in U.S. Pat. No. 7,213,940, entitled “LIGHTING DEVICE AND LIGHTING METHOD,” which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference. As described therein, a lighting device may include first and second groups of solid state light emitters, which emit light having dominant wavelength in ranges of from 430 nm to 480 nm and from 600 nm to 630 nm, respectively, and a first group of phosphors which emit light having dominant wavelength in the range of from 555 nm to 585 nm. A combination of light exiting the lighting device which was emitted by the first group of emitters, and light exiting the lighting device which was emitted by the first group of phosphors produces a sub-mixture of light having x, y color coordinates within a defined area on a 1931 CIE Chromaticity Diagram that is referred to herein as “blue-shifted yellow” or “BSY.” Such non-white light may, when combined with light having a dominant wavelength from 600 nm to 630 nm, produce warm white light. Blue and/or green LEDs used in a lighting apparatus according to some embodiments may be InGaN-based blue and/or green LED chips available from Cree, Inc., the assignee of the present invention. Red LEDs used in the lighting apparatus may be, for example, AlInGaP LED chips available from Epistar, Osram and others. In some embodiments, the LEDs 22, 24 may have a square or rectangular periphery with an edge length of about 900 μm or greater (i.e. so-called “power chips.” However, in other embodiments, the LED chips 22, 24 may have an edge length of 500 μm or less (i.e. so-called “small chips”). In particular, small LED chips may operate with better electrical conversion efficiency than power chips. For example, green LED chips with a maximum edge dimension less than 500 microns and as small as 260 microns, commonly have a higher electrical conversion efficiency than 900 micron chips, and are known to typically produce 55 lumens of luminous flux per Watt of dissipated electrical power and as much as 90 lumens of luminous flux per Watt of dissipated electrical power. The LEDs 22 in the lighting apparatus 10 may include white/BSY emitting LEDs, while the LEDs 24 in the lighting apparatus may emit red light. The LEDs 22, 24 in the lighting apparatus 10 may be electrically interconnected in respective strings, as illustrated in the schematic circuit diagram in FIG. 1G. As shown therein, the LEDs 22, 24 may be interconnected such that the white/BSY LEDs 22 are connected in series to form a first string 34A. Likewise, the red LEDs 24 may be arranged in series to form a second string 34B. Each string 32, 34 may be connected to a respective anode terminal 23A, 25A and a cathode terminal 23B, 25B. Although two strings 34A, 34B are illustrated in FIG. 1C, it will be appreciated that the lighting apparatus 10 may include more or fewer strings. Furthermore, there may be multiple strings of white/BSY LEDs 22, and multiple strings of red or other colored LEDs 24. Referring now to FIG. 2, a drive circuit 100 for driving a plurality of solid state light emitting devices in a solid state lighting apparatus is illustrated. In particular, the drive circuit 100 is configured to drive a plurality of LEDs, or sets of LEDs, that are connected in series. As illustrated in FIG. 2, the sets of LEDs may include and sets S1, S2, . . . SN. Each set of LEDs S1 to SN may include one or more LEDs connected in series and/or parallel. For example, as illustrated in set S1, the LEDs in a set may be connected in respective series strings that are themselves connected in parallel. The total number of LEDs in series may be selected so that the circuit has a suitably high efficiency when the input voltage to the string is at the maximum line voltage. Efficiency may also be increased by keeping the voltage across the selected LEDs close to an applied AC voltage, as discussed below. An alternating current signal VAC is applied to a full wave rectifier bridge B1. The output of the full wave rectifier bridge B1 is a rectified AC signal Vrect. The rectified AC signal Vrect may be applied to a current source, such as a series resistor or a voltage controlled current source, which supplies current to the first set S1 of LEDs. Subsequent sets of solid state lighting devices S2 to SN are connected in series with the first set S1. Control lines L2, L3, . . . LN are coupled to anodes of respective sets of LEDs S2 to SN. The drive circuit 100 includes a diode selection circuit 10, a power supply 12, and a voltage divider 14. The rectified signal Vrect is divided by the voltage divider 14 and supplied as a control signal to the diode selection circuit 10. The rectified signal Vrect is also supplied to a power supply 12, which responsively generates a substantially steady reference voltage Vref, which is also supplied to the diode selection circuit 10. In some embodiments, the power supply 12 may generate more than one reference voltage, e.g., a family of reference voltages of various levels. The diode selection circuit 10 is configured to dynamically adjust the number of LEDs that are being driven by the rectified AC voltage Vrect in response to a level of the rectified AC signal Vrect, so that a voltage drop of diodes that are being driven at any given point in time is less than the level of the rectified AC signal Vrect. In this manner, the AC waveform may be more fully utilized, potentially increasing light output from the circuit, improving the power factor of the circuit, and or reducing visible flicker of light generated by the circuit. In general, the diode selection circuit is designed to keep the instantaneous LED string voltage close to but always slightly less than Vrect, which may reduce or minimize the voltage dropped across the current source 20, thereby increasing or maximizing efficiency, and more fully utilizing the AC waveform to generate more light. In some embodiments, compared to apparatus including non-switched LED strings, the THD may improve from 85% to 25%. In operation, when the level of the rectified AC voltage Vrect is low, only the first group LEDs S1 may be driven by the rectified AC signal Vrect, while the other sets of LEDs S2 to SN may be switched out of the circuit and/or bypassed, so that they are not being driven by the rectified signal Vrect. As the rectified AC signal Vrect increases, successive sets of LEDs S2 to SN may be switched into the circuit so that they are being driven by the rectified AC signal Vrect until all of the sets of LEDs S1 to SN have been switched into the circuit and are being driven by the rectified AC signal Vrect. In a switching arrangement that utilizes a resistor (Rseries) as the current source, the voltage across Rseries starts off small, and increases until the next switching threshold is reached, at which point it returns to zero and the process begins again. This is during the first half-cycle of Vrect(t), where Vrect(t) is rising. During the second half-cycle of Vrect(t), Vrect(t) is falling and the LED current is the mirror image of the first half-cycle. It is therefore sufficient to consider only the first half-cycle. Assuming a Binary Weighted Switching approach in which LEDs are controllably switched in groups of 1, 2, 4, 8, etc., . . . , the LED current is given by: Vrect(t)=Vpeak*|sin(2*□*Fac*t)|−2*Vf Vr_series=Vrect(t)−floor([Vrect(t)−Vdrop]/Vled)*Vled ILED(t)=[Vrect(t)−floor([Vrect(t)−Vdrop]/Vled)*Vled]/Rseries Rather than using a series resistor, some embodiments drive the bank of dynamically switched LEDs with a Voltage Controlled Current Source (or sink)—a VCCS, as illustrated in FIG. 3A. According to some embodiments, current to the LED string is provided by a voltage controlled current source (VCCS) 20A. The current source 20A includes a PNP transistor Q1 having an emitter resistor RE coupled to the emitter thereof and a bias circuit including resistors RH and RL and a zener diode VZ coupled to the base thereof. Output current IOUT is sourced by the current source 20 according to the following equations: I SOURCE = V Z R E + Vrect  ( R H R H + R L )  ( 1 R E ) I SOURCE = I 0 + kVrect Accordingly, the output current is proportional to the rectified input voltage Vrect. This circuit may therefore further regulate the output current to help reduce spikes as sets of LEDs are switched into/out of the circuit. It will be appreciated that the zener diode could be replaced with another type of diode that has a voltage drop that is substantially matched to the base-emitter junction of the transistor Q1. A more general illustration of a VCCS 20B is shown in FIG. 3B, and will be discussed below. The VCCS 20B illustrated in FIG. 3B includes resistors R1, R2 and R3, diode D1 and PNP transistor Q1. Assuming D1 and Q1 are matched and thermally coupled, Vf(D1)=Vbe(Q1). Assuming transistor Q1 has sufficient gain, then the voltage across resistor R3 is the same as the voltage across resistor R1. Referring to FIG. 3C, a Sziklai pair 122 may be used in a VCCS current source 120 according to some embodiments. The Sziklai pair 122 includes a PNP transistor Q1 and an NPN transistor Q2. The collector of the PNP transistor Q1 is coupled to the base of the NPN transistor Q2, and the emitter of the PNP transistor Q1 is coupled to the collector of the NPN transistor Q2, as shown in FIG. 3C. The emitter of the NPN transistor Q2 is coupled to the switched LED string 124, which is controlled by the diode selection circuit 10. A Sziklai pair may have a low saturation voltage and temperature coefficient, but very high gain (and the high power transistor Q2 is NPN). Darlington transistors may be used, but the saturation voltage is larger and there are two base-emitter junctions requiring thermal compensation. MOSFET current sources may also be used in some embodiments. If we then assume that Vrect(t)>>Vf(D1), the voltage across R1 is given by: Vr1=Vrect(t)−Vrect(t)*R2/(R1+R2) which re-arranges to give: Vr1=Vrect(t)*R1/(R1+R2) Furthermore: Vr3=Isource*R3=Vr1 Thus: Isource*R3=Vrect(t)*R1/(R1+R2) Isource=[Vrect(t)/R3]*[R1/(R1+R2)] Isource=[Vrect(t)/R3]*[1/(R1+R2)/R1] Isource=[Vrect(t)/R3]*[1/(1+R2/R1)] Isource=Vrect(t)/[R3*(1+R2/R1)] which can be written as: Isource(t)=Vrect(t)/Req Req=R3*(1+R2/R1) Although it is called a constant current source, it is clear that the current Isource is directly proportional to Vrect(t). Hence the term Voltage Controlled Current Source (VCCS) is more properly used to describe the current source 20B. Typical circuit designs attempt to remove the dependence on Vrect(t). However, in some embodiments, it may be more appropriate not to have a constant current. If D1 is used to compensate for the static and dynamic base-emitter voltage Vbe of Q1 (−2.2 mV/K), a Voltage Controlled Current Source (VCCS) is provided. The VCCS has a minimum dropout voltage of: Vdropout=Vcesat(Q1)+Isource*R3 Where Vcesat(Q1) increases with increasing Isource. It is sufficient to use Isource_max when calculating Vcesat(Q1) and the voltage drop across R3. If a string of dynamically switched LEDs is driven with a VCCS, and the circuit is designed so that Vdrop>Vdropout (in other words the dynamic switching controller ensures the VCCS always has enough headroom to regulate Isource), then the LED current is always proportional to Vrect(t). As noted above: Vrect(t)=Vpeak*|sin(2*□*Fac*t)|=2*Vf Assuming Vpeak*|sin(2*□*Fac*t)|>>2*Vf, then: Vrect(t)=Vpeak*|sin(2*□*Fac*t)| and Isource(t)=Vrect(t)/Req Thus: Isource(t)=Vpeak*|sin(2*□*Fac*t)|/Req Isource(t)=[Vpeak/Req]*|sin(2*□*Fac*t)| Isource(t)=Ipeak*|sin(2*□*Fac*t)| where Ipeak=Vpeak/Req and Req=R3*(1+R2/R1) The input current is therefore substantially sinusoidal, although the foregoing analysis has ignored the bridge rectifier, D1 and Vdrop voltages, which cause a small amount of dead time and prevent Isource(t) from being perfectly sinusoidal. A simulation comparing a VCCS with a resistor current source is shown in FIG. 3D. Curve 301 is a graph of Isource(t) for a dynamically switched LED driver circuit including a VCCS current source with an equivalent resistance Req=2.22 k□, while curve 302 is a graph of Isource(t) for a dynamically switched LED driver circuit including a series resistor current source having a resistance R series=270□□. Both circuits use the same Dynamically Switched LED string which is non-linear (but not binary), and designed for minimum loss with 7 switches. Two LEDs are always in series, to provide for a free low voltage supply (and to allow for flicker-free dimming). For a resistor current source using Rseries=270□, the total harmonic distortion is 77.4%. For the VCCS with Req=2.22 k□□, the □input current is very close to a pure sinusoid, and has 2.4% THD. The power factor is greater than 0.99. In embodiments in which the LED current is set using a Voltage Controlled Current Source, such as the VCCS circuits 20A, 20B illustrated in FIG. 3A and/or FIG. 3B, to set the LED current proportional to Vrect, the output current waveform may exhibit some spikes at the switching instant, as illustrated in FIG. 4A, which is a graph 403 of simulated output current for a circuit according to some embodiments. Current spikes are visible in the output current signal 403 at the switching instants. When a set of k LEDs is switched in (or out) of the string, the LED string voltage rises (or falls) by k*Vled. This occurs fairly quickly—shorting a set is as fast as the shorting switch itself. The time taken removing the short circuit from a set is governed by the current source charging the stray capacitance of the set, which is fairly small. Once a shorting switch is turned OFF (and assuming there are not minority carrier storage-time issues), the current source sets the slew rate as it charges up the stray capacitance. However, the current source is haversinusoidal, so over one quarter AC line period the current ranges from near zero to 1.4*Irms. The uppermost LED anode connects to the collector of the PNP current source transistor Q4, so changes in the LED string voltage change the PNP collector voltage an equal amount. This feeds current I=Cm*dV/dt from the collector to the base through the BJT Miller capacitance Cm, and causes the current source to misbehave, which is what creates the spikes shown in FIG. 4B. It may be possible to reduce or avoid these spikes by designing the VCCS so they are reduced, or by controlling the slew rate of the switches. Simulated example voltage and current waveforms for a drive circuit 100 according to some embodiments are illustrated in FIG. 4B. Referring to FIG. 4B, curve 401 is a rectified AC sine wave Vrect, while curve 402 is a graph of the voltage across the LEDs that are switched into the circuit at any given instant, that is, the sum of the voltage drops of all of the LEDs that are energized at a particular instant. As illustrated in FIG. 4A, as the rectified AC signal Vrect increases, successive groups of LEDs are switched into the circuit, and the output voltage increases in steps as successive sets of LEDs are energized. According to some embodiments, the numbers of LEDs that are switched on at any given step and the threshold levels of Vrect that result in such sets being switched in our chosen so that the level of Vrect remains greater than the sum of the forward voltages of the energized LEDs at each successive step. Accordingly, as the level of Vrect rises, all LEDs that are switched into the circuit may remain energized when a successive set of LEDs is switched into the circuit. As the level of Vrect falls, successive sets of LEDs may be switched out of the circuit to ensure that the level of the voltage across the energized diodes remains below Vrect at substantially each time instant. Referring again to FIG. 2, once Vrect exceeds the voltage required to turn on the LEDs in set S1, the LEDs are forward biased and current begins to flow through the LEDs. The voltage dropped across these resistors is equal to Vrect-Vied (where Vled is the voltage across the LEDs in S1), so the current ramps up with the same slope as Vrect. When Vrect is sufficiently above Vled (as a function of losses in the current source), the diode selection circuit 100 switches the next set S2 of LEDs in series, so the total LED string voltage increases by Vnew_led_string, which may be chosen such that: Vrect(t=Tswitch)>Vled+Vnew_led_string. This ensures that when the new LED string is switched in series, the current through the current source is still slightly positive. We can define dV=Vrect(Tswitch)−(Vled+Vnew_led_string). If dV>0, current through the LEDs drops down to a positive, non-zero value when the new set is switched into the series. If dV=0, the current drops down to zero when the new set is switched into the series. If dV<0, the current tries to drop below zero, but the LEDs are unipolar so current cannot flow, and current continues to not flow until Vrect rises enough to make Vled>0. Thus, when dV<=0, current in the LEDs is equal to zero, so dV should be made greater than zero keep current flowing through the diodes, regardless of current control methodology. If not, the current drops to zero at the switching edge, and stays there until Vrect is greater than the voltage drop of all LEDs switched into the series. Furthermore, as dV becomes more negative the zero current period gets longer. So, the switching edge may give rise to notches (down/up to 0 A) in the input current waveform, if dV is not chosen well. With a resistor as the input current source, all that is needed is dV>=0. With a current source, however, there is a finite dropout voltage Vcsdo, that may, for example, be about 1V, below which the current source is OFF. The constraint on dV when using a Voltage Controlled Current Source (VCCS) is therefore dV>=Vcsdo. A drive circuit 100A including a diode selection circuit 10A is illustrated in more detail in FIG. 5. As illustrated therein, the diode selection circuit 10A includes a voltage divider ladder 18 including resistors R51 to R54 that generate a plurality of reference voltages Vref-1, Vref-2 and Vref-3 from the reference voltage Vref. The diode selection circuit 10A further includes a plurality of comparators C51 to C53 and switching transistors Q51 to Q53 having gates coupled to respective outputs of the comparators C51 to C53. In the embodiments illustrated in FIG. 5, sources of the switching transistors Q51 to Q53 are coupled to ground, and drains of the switching transistors Q51 to Q53 are coupled to emitters of respective shunting bipolar transistors Q54 to Q56 through respective resistors R56, R57 and R58. Collectors of the bipolar shunting transistors Q54 to Q56 are coupled to respective control lines L1, L2, and L3, which are in turn coupled to anodes of respective ones of the sets S2, S3 and S4 of LEDs. Reference voltages Vrect-1 to Vrect-3 generated by the voltage divider ladder 18 are applied to the non-inverting inputs of the comparators C51 to C53. In particular, the lowest reference voltage Vref-1 is applied to the non-inverting input of the comparator C51, the reference voltage Vref-2 is applied to non-inverting input of the comparator C52, and the highest reference voltage Vref-3 is applied to the non-inverting input of the comparator C53. An optional voltage divider 16 supplies a bias voltage to the bases of the respective bipolar shunting transistors Q54 to Q56. One or more of the sets S2 to S4 of series connected LEDs may be switched out of the drive circuit based on the level of the rectified AC input signal Vrect. In particular, based on a comparison of the level of Vrect′ with one of the reference voltages Vref-2 to Vref-3, the drive current of the LEDs may be shunted through one of the bipolar shunting transistors Q54 to Q56 through a respective emitter resistor R56 to R58 to ground, bypassing one or more sets of LEDs S2 to S4. Current through the remaining LEDs may be controlled by the bias level of the bipolar shunting transistor Q54 to Q56 through which the LED drive current is shunted. Operation of the drive circuit 100A and the diode selection circuit 10A illustrated in FIG. 5 will now be described further reference to FIG. 6, which is a graph of one cycle of the scaled AC rectified signal Vrect′. Referring to FIGS. 5 and 6, when the value of Vrect′ is lower than the lowest reference voltage Vref-1, the non-inverting inputs to the comparators C51 to C53 are all less than the corresponding reference voltages Vref-2 to Vref-3 that are input into the non-inverting inputs to the comparators C51 to C53. Accordingly, each of the comparators C51 to C53 generates a high output voltage, which switches each of the respective switching transistors Q51 to Q53 to an ON state. In particular, the transistor Q51 is switched on, which also is emitter switches the bipolar shunting transistor Q54 to a conductive state. When the bipolar shunting transistor Q54 is conductive, the anode of the second set S2 of LEDs is coupled to ground though the switching transistor Q51, thereby bypassing the subsequent sets S2 to S4 of LEDs. When the level of Vrect exceeds the forward voltage drop of the LEDs in the first set S1, the LEDs in the first set S1 will turn on and begin to generate light. As the level of Vrect′ continues to increase, it reaches the lowest reference voltage Vrect-1. At that point, the output of the comparator C51 switches to low, thereby turning off the switching transistor Q51 and the bipolar shunting transistor Q54. However, because the voltage Vrect′ is still less than the second reference voltage Vref-2, comparators C52 and C53 continue to output a high level, so that the third and fourth sets S3 and S4 of LEDs continue to be switched out of the circuit and not energized. Thus, when the level of Vrect′ is between Vref-1 and Vref-2, only the first and second sets S1 and S2 of LEDs may be energized. As Vrect′ continues to increase, it reaches the value of Vref-2, at which point the second comparator C52 also switches to a low output, turning off switching transistor Q52 and bipolar shunting transistor Q55. This switches the third set S3 of LEDs into the drive circuit, while continuing to bypass the fourth set S4 of LEDs. Finally, as Vrect′ reaches the value of the third reference signal Vref-3, the third comparator C53 also switches to low output, turning off transistors Q53 and Q56 and switching the fourth set S4 of LEDs into the drive circuit. The bipolar transistors Q54 to Q56 provide a means both for controlling current flow through the LEDs that are currently switched into the drive circuit, as well as providing an inexpensive and reliable transistor for sustaining the voltage of LEDs that are in the ON state. For example, if the transistor Q54 to Q56 were not included and the MOS transistors Q51 to Q53 were directly coupled to the output lines L1 to L3, when the Vrect′ greater than Vrect-3 and all three of the switching transistors Q51 to Q53 are switched off, the transistor Q51 would have to be capable of sustaining entire voltage drop over sets S2, S3 and S4 of LEDs. This may require a larger and more expensive MOS transistor. Furthermore, it might be difficult to regulate current at the output of the energized LEDs. The bipolar shunting transistors Q54 to Q56 may be biased in a linear mode and emitter-switched by the switching transistors Q51 to Q53. A circuit according to some embodiments can drive LEDs with a relatively high power factor. In general, “power factor” refers to how closely aligned the output current and voltage waveforms are over each cycle. Instead of switching current on and off only near peaks of the rectified waveform when an entire string is energized, current is drawn in steps as the rectified waveform changes. Thus, the output current may follow the input voltage waveform more closely in circuits according to some embodiments. A drive circuit 100B including a diode selection circuit 10B according to further embodiments is illustrated in FIG. 7. The drive circuit 100B includes similar elements as the drive circuit 100A illustrated in FIG. 5, including a bridge rectifier B1, a power supply 12, and a voltage divider 14. Operation of these elements is similar to the operation of like elements in drive circuit 100A, and need not be described again in detail. The diode selection circuit 10B includes a voltage divider ladder 18 that is configured to generate a range of reference voltages Vref-1 to Vref-3, which are supplied to the non-inverting inputs of respective comparators C71 to C73, as illustrated in FIG. 7. A scaled version Vrect′ of the rectified input voltage Vrect is supplied to the inverting inputs of the comparators C71 to C73. The outputs of the comparators C71 to C73 are coupled to respective gates of shunting MOS transistors Q71 to Q73 through bias resistors R75, R76 and R77. Drain and source terminals of the shunting transistors Q71 to Q73 are coupled to anode and cathode contacts, respectively, of respecting
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