This invention pertains to the field of driver circuits for driving LEDs (Light Emitting Diodes), which may or may not couple with dimmers.
The use of high-brightness LEDs in lighting applications is growing rapidly as a result of inherent benefits to LED technology, such as long lifetimes, good efficiency, and non-toxic material usage. LED lighting solutions, however, still need to offer better performance at better value. Because LEDs prefer to be driven in a different fashion as compared to traditional incandescent bulbs, performance depends heavily on the LED driver circuit.
Traditional LED driver ICs (integrated circuits) suffer in performance and features in several ways. First, the driver efficiency generally falls short of the target. Similarly, the power factor for existing solutions can be quite poor, especially in a dimming configuration. When using the TRIAC-based wall dimmers that are typical in existing installations, conventional solutions may cause annoying flicker while dimming.
Further difficulties arise when retrofitting existing applications with LED fixtures. Retrofitting requires compatibility with the large installed base of dimmers, particularly leading-edge TRIAC-based dimmers. Because these dimmers were commonly designed for current levels much higher than those consumed by LED applications, many problems occur with existing LED driver solutions.
TRIAC-based dimmers function by allowing current to pass during a fraction of the half-cycle of the input AC mains voltage. One of the most common types of TRIAC dimmers is the leading-edge type, which initially turns on at some point past the zero-crossing of the AC waveform (in both the upward direction and the downward direction), and then turns off at the next zero-crossing.
Most leading-edge TRIAC-based dimmers were designed for use with incandescent light bulbs. In order to turn on and power the bulb, the TRIAC requires a latching current to flow through the load. Subsequently, to maintain the TRIAC's On state until the next AC zero-crossing, a holding current must be present. This TRIAC behavior matches well with the strongly positive temperature coefficient of incandescent bulbs. When cold and unpowered, an incandescent bulb presents a filament resistance which is a fraction of its value when powered. As current and power dissipation increase, temperature and hence resistance increase greatly. By its nature, the incandescent bulb provides a large latching current at the time of turn on, and maintains a lesser holding current while lit. Since one of the advantages of LED-based incandescent bulb replacements is power efficiency, it naturally draws less current than the hot incandescent bulb, and much less than the cold incandescent bulb.
TRIAC-compatible LED drivers may dissipate more power. Even with the degraded efficiency due to a bleed of either constant current or constant resistance, many driver solutions fail in terms of gross functionality with digitally-controlled TRIAC-based dimmers. The digitally-controlled TRIAC-based dimmers require low load impedance even in the standby state. Therefore, when the dimmer is not explicitly powering the driver yet, the dimmer needs to keep standby circuits inside the dimmer alive. In U.S. Pat. No. 8,581,498, a dimmer compatibility device has been disclosed to address these dimmer compatibility problems.
Typical LED driver solutions that use standard closed loop control on the LED current suffer dimmer compatibility problems. The combination of dimmer and current control loop dynamics results in various problems such as flicker, flashing, always-On, cycling, never-On, and other obnoxious behaviors in commercially available LED bulbs.
Unfortunately, a reduction of complexity in order to avoid dimmer and current control dynamics can suffer from output current inaccuracy. In U.S. Pat. No. 8,525,438, a simple high power factor LED driver over entire dimming range has been disclosed. However, the nature of its simplicity leads to output current variation with changes in component values and AC line voltage.
The LED's forward voltage Vf can vary at the same current. As a consequence, standard current drive LED circuits must be overdesigned. The power electronics and heat sinking must be able to handle any possible output power, as present at the maximum Vf for a given current. When the electronics and heat sinking are designed to handle this maximum power, then the electronics and heat sinking become overdesigned for the nominal and minimum Vf case. Accordingly, it is desirable to design a driver for LEDs where the driver is able to optimize the design of the power electronics and heat sink.
A method and apparatus are disclosed for controlling gain factor in a driver device for a lighting device according to the input voltage signal to reduce output current or output power dependency on the amplitude of the input voltage signal. One method incorporating an embodiment of the present invention couples an input voltage signal to the input port of the driver device (also called a power converter system in this disclosure) and an output branch to the output port of the driver device. The input voltage signal corresponds to a rectified voltage signal. The gain factor is adjusted according to the input voltage signal to reduce output current or output power dependency on input voltage amplitude of the input voltage signal, wherein the gain factor corresponds to a ratio of the output current to the input voltage of the driver device. In another embodiment, the gain factor is configured according to a reference voltage and the output voltage across the output port to deliver a substantially constant output power.
Various exemplary system configurations are disclosed. For example, the gain factor can be inversely proportional to the input voltage amplitude; this will cause substantially constant output current for a driver device incorporating non-isolated buck power conversion or cause substantially constant output power for a driver device incorporating isolated flyback or non-isolated buck-boost power conversion. The input voltage amplitude can be determined based on the input voltage signal using a peak detector.
When the reference voltage (Vref) and the output voltage (Vout) are used, the gain factor can be configured to include a term proportional to (1+(Vref−Vout)/Vref).
The gain factor adjustment can be implemented using a variable resistance. The variable resistance can be used with a resistor to form a voltage divider and to form a control signal across the variable resistance for controlling power conversion, where the resistance of the resistor is substantially larger than the variable resistance. The variable resistance can be implemented by imposing a force current and a force voltage on a circuit. For example, the circuit may comprise one or more MOSFETs (metal-oxide-semiconductor field-effect-transistor).
A driver device according to the present invention may also be used with a dimmer compatibility device to sink required current for proper dimmer operation by connecting the dimmer compatibility device and the driver device to the dimmer.
It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the systems and methods of the present invention, as represented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely a representative of selected embodiments of the invention. References throughout this specification to “one embodiment,” “an embodiment,” or similar language mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.
Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures, or operations are not shown or described in detail to avoid obscuring aspects of the invention. The illustrated embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of apparatus and methods that are consistent with the invention as claimed herein.
Output Current(t)=G0*Vin(t)=G0*Vpk*y(t) (1)
With the power converter system of
The output current 12 flows to the load that incorporates capacitor 13 and LED string 14. The capacitor 13 sinks most of the AC attribute of the output current in output branch, leaving the mostly DC current to flow through LED string 14.
Examination of the relationship between output current and input voltage reveals a dependence on the amplitude Vpk. This output dependence on input amplitude is undesirable. The name for this dependence is “line sensitivity.” How well a power converter system, such as system 11 in
In order to avoid said poor line regulation, the gain factor is modified, as shown in
Output Current(t)=G0*f1(Vpk)*Vpk*y(t) (2)
Power converter system 31 of
which is independent of Vpk. While peak detection 22 is shown external to power converter system 31, the peak detection circuit may also be incorporated into the power converter system as part of the power converter system.
Noteworthy is that the peak voltage Vpk directly affects the gain factor. Such a change in the gain factor based on a parameter is called “feedforward.” So long as the peak remains consistent, there is substantially no dynamic introduced into the current control. Line regulation of the output current in this case does not utilize additional feedback. If feedback were to be used, then control signals in a closed loop change the power conversion to counteract input voltage amplitude differences. This kind of closed loop feedback makes sense when the input voltage is always a full sine wave; it becomes problematic with dimming systems.
The off-line LED power converters under discussion typically control their inductor current. When an inductor directly connects the output of such a converter, the converter effectively controls the output current. Such is the case in a so-called “buck” converter, whose inductor is at the output. In buck converter applications that demand constant output current, there is no need for output voltage to be used in the gain factor as discussed above.
Yet there are other scenarios wherein the output voltage matters. For example,
Output Current(t)=G0*f2(Vout, Vref)*Vpk*y(t) (4)
In eqn. (4), f2(.) is a function with Vref and Vout as variables.
G0*(1+((Vref−Vout))/Vref). (5)
With this gain factor, the output current (12) in the output branch becomes:
Just as in the power converter system (31) of
The power converter system in
Gain=G0*f1(Vpk)*f2(Vref ,Vout). (7)
In other words, the gain factor according to the system of
Output Current(t)=G0*Vpk*f1(Vpk)*f2(Vref,Vout)*y(t) (8)
All of the power converter systems as shown in
An adjustable resistance device can be implemented by imposing a force current (81) and a force voltage on a device 83 as shown in
In the above example, the control voltage(t) becomes:
In
A few key assumptions must hold true for device 104 to work as a synthetic resistor.
Assuming all of the above conditions hold, then the resistor divider consisting of Rext and the synthetic resistor (i.e., MOSFET 105) form the desired scaling factor shown in
As discussed previously, control of the current in a magnetic storage device within a flyback or buck-boost converter amounts to output power control rather than output current control. These power circuits are so-called “indirect” converters. The principle of delivering power for these circuits is by first loading energy into a magnetic storage device that is connected to an input source, and then releasing the energy to the output with the input disconnected. This process repeats at a given frequency.
Power converter system 111 incorporates an exemplary gain factor G1/Vpk. To represent power control, the gain equation includes term G1 of units Watts per Volts. By using an embodiment disclosed above, gain factor can incorporate the term, 1/Vpk so that converter output power becomes independent of the amplitude of input voltage. In this case, the output current is not under control. Instead, output power is controlled and the magnitude of output current depends on the output voltage as shown in eqn. (12) and eqn. (13).
Consideration of the foregoing disclosure reveals an embodiment of a variable multiplier. In
For power converters such as a buck whose output current is under control, the determining elements of adjustable resistor 83 combine to effect power control. Yet there is no direct sense of power. This method is different from the more standard, expensive, analog to digital conversion method with subsequent digital multiplication. More typically for true power control, sensed power is used in a discrete time control feedback loop, with analog to digital and digital to power conversions required. Instead, the present invention uses feedforward to change the control signal, and as a result can achieve low cost power regulation without introducing system dynamics that interfere with dimmer compatibility.
As shown in
This present invention manages the broad area of problems in conventional power converter systems for LED lighting control. It combines dimmer compatibility with high power factor drive and high output current accuracy. The resulting solution is a clean architecture that adds accuracy not provided by U.S. Pat. No. 8,525,438, while maintaining the ability to use methods of U.S. Pat. No. 8,581,498 to ensure dimmer compatibility.
The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described examples are to be considered in all respects only as illustrative and not restrictive. Therefore, the scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
The present invention is related to U.S. Pat. No. 8,581,498, issued on Nov. 12, 2013, entitled “Control of bleed current in drivers for dimmable lighting devices” and U.S. Pat. No. 8,525,438, issued on Sep. 3, 2013, entitled “Load driver with integrated power factor correction”. The U.S. Patents are hereby incorporated by reference in their entireties.