RESPONSE FOR IMPROVED LIGHT

QUALITY

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates in general to the field of electronics, and more specifically to a lighting and power control system with increased dynamic response for improved light quality.

DESCRIPTION OF THE RELATED ART

Lighting and power control systems convert power received from a power source, such as a voltage supply, into power suitable for a load. Lighting and power control systems often provide power factor corrected and regulated output voltages to many devices that utilize a regulated output voltage and/or current. The goal of power factor correction technology is to make the power control and conversion system appear resistive to the power source.

Figure 1 depicts a lighting and power control system 100 is a two stage system that converts a supply voltage VSUPPLY into a regulated bus voltage VBUS and converts the bus voltage VBUS into a regulated current ILED for an LED load 102. The LED load 102 includes one or more light emitting diodes (not shown). The supply voltage VSUPPLY is, for example, a rectified public utility mains voltage, which is a 50 Hz/220 V line voltage in Europe and a 60 Hz/110 V line voltage in the United States of America (USA). Thus, in at least one embodiment, the supply voltage VSUPPLY is nominally a 100 Hz rectified sine wave in Europe and a 120 Hz rectified sine wave in the USA. The rectification of the supply voltage VSUPPLY creates what is commonly referred to as "double-line frequency noise" at the frequency of the supply voltage VSUPPLY. The double-line frequency noise presents as a ripple in the supply voltage VSUPPLY.

Input power converter stage 104 is the first stage of the two stage lighting and power control system 100. The input power converter stage 104 includes an input stage controller 106 to generate a control signal CSo that controls conversion of the time- varying supply voltage VSUPPLY into a regulated direct current (DC) voltage VBUS by input power stage 108. Input power stage 108 is a boost-type switching power converter. The input stage controller 106 includes a proportional- integral (PI) controller 110 that generates the control signal CSo. The PI controller 1 10 is well-known type of controller that compares the bus voltage VBUS with a reference voltage VREF and generates the control signal CSo so that the bus voltage VBUS tracks the reference voltage VREF. The reference voltage VREF is set to a desired value of the bus voltage VBUS. The PI controller 110 attempts to control the input power stage 108 so that the input power stage 108 meets power factor correction

requirements while regulating the bus voltage VBUS. The bandwidth of the PI controller 1 10 is generally about 10-20 Hz to avoid following the 100-120 Hz double line frequency ripple and thereby meet power factor correction requirements. However, because of the relatively low bandwidth, dynamic response is slow, so voltage disturbances, such as the double line frequency ripple are transferred to the boost capacitor 1 11, which can lead to undesired flicker and stroboscopic light effects of the LED load 102.

Output power converter stage 112 is cascaded with the input power converter stage 104 to form the second stage of the two stage lighting and power control system 100. The output power converter stage 112 includes an output stage controller 1 14 to generate a control signal CSi that controls conversion of the bus voltage VBUS into a regulated constant current ILED by output power stage 116. Output power stage 116 is a buck-type switching power converter. The output stage controller 114 includes a PI controller 118 that generates the control signal CSi. The PI controller 118 compares the current ILED with a reference current IREF and generates the control signal CSi so that the current ILED tracks the reference current IREF. The reference current IREF is set to a desired value of the current ILED. The bandwidth of the PI controller 1 18 is typically around 500 Hz. The bandwidth of the PI controller 118 is adequate to attenuate the double line frequency ripple in LED current to levels meeting requirements for visible flicker and stroboscopic light artefacts. However, double line frequency components are still present in LED current.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a lighting and power control system includes one or more inputs to receive a rectified time-varying supply voltage and includes a load. The lighting and power control system also include a first power converter stage providing a regulated voltage. The lighting and power control system also includes a second power converter stage, coupled to the first power converter stage and the load, to convert the regulated voltage generated by the first power converter stage into a regulated current for the load. The second power converter stage includes an inverted notch filter to convert the voltage generated by the first power converter stage to current for the load with a reduced ripple component at the notch frequency, the notch frequency being approximately equal to a frequency of the time-varying supply voltage.

Preferentially, the first power converter stage may comprise a notch filter to (i) attenuate a control loop error signal, derived from a reference signal and a sampled output signal of the first power converter stage at the notch frequency and (ii) dynamically respond to the attenuated output signal with a first control feedback system to generate a control signal to control generation of power by an input power stage.

In another embodiment of the present invention, a method includes receiving a rectified time-varying supply voltage. The method further includes converting the rectified time varying supply voltage into a regulated voltage using a first power converter stage. The method includes converting the regulated voltage into a regulated current for a load using a second power converter stage coupled to the first power converter stage and the load. The method comprises generating an error signal representing a difference between a second reference signal and a second power converter stage feedback signal representing the power for the load. The method comprises filtering the error signal with an inverted notch filter, wherein the inverted notch filter has a notch frequency approximately equal to a frequency of the time-varying supply voltage. The method comprises processing an output signal of the inverted notch filter with a second control feedback system to generate a second power supply stage control signal, wherein the second control feedback system has a bandwidth greater than the rectified time-varying supply voltage. Additionally, the method comprises controlling an output power stage with the second power supply stage control signal to convert the power generated by the input power stage to the power for the load

Preferentially, the method may further include attenuating a control loop error signal, derived from a reference signal and a sampled output signal of a first power converter stage with a notch filter having at the notch frequency. The method may also include dynamically responding to the attenuated output signal with a first control feedback system by generating a control signal to control generation of power by an input power stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element.

Figure 1 (labeled prior art) depicts two stage lighting and power control system.

Figure 2 depicts a lighting and power control system configured with notch and/or inverted notch filters in respective control loops.

Figure 3 depicts a control loop with a notch filter in an input power converter stage.

Figure 4 depicts a composite magnitude-frequency response Bode plot for input stage PI controllers.

Figure 5 depicts a control loop with an inverted notch filter in an output power converter stage.

Figure 6 depicts a composite magnitude-frequency response Bode plot for output stage PI controllers.

DETAILED DESCRIPTION

In at least one embodiment, a lighting and power control system includes a two stage power converter system providing on its input power factor correction that meets regulatory requirements on harmonic currents and suppresses on its output double line frequency noise to provide a stable output current. In at least one embodiment, a stable output current improves output light quality by, for example, reducing visible flicker and stroboscopic effects. In at least one embodiment, to simultaneously achieve both regulatory acceptable power factor correction and output current stability, an input power converter stage of the lighting and power control system includes a notch filter, a control feedback system, and an input power stage configured in a control loop. In at least one embodiment, by applying a notch filter at the input power converter stage with a notch frequency at the double line frequency, the double-line-frequency component on capacitor 207 can be suppressed for input stage controller 208 but, in at least one embodiment, not removed by the notch filter from capacitor 207 at voltage node Vbus. Additionally, by applying the notch filter in input stage controller 208 of the input power converter stage 202, the bandwidth of the input power converter stage 202 can be designed at higher levels. Increasing the bandwidth increases the dynamic frequency response of the control feedback system, without sacrificing power factor correction, i.e. harmonic currents and total harmonic distortion. In at least one embodiment, the notch filter has a notch frequency at a frequency of a supply voltage, i.e. at a double line frequency of a mains supply voltage. The control feedback system generates a control signal to control power conversion to an input power stage, which completes the control loop. Additionally, not only is power factor correction maintained, because of, for example, the higher dynamic response of the control feedback system relative to conventional systems, the input power converter stage generates a more stable regulated bus voltage as an input to an output power stage relative to conventional power converter systems.

In at least one embodiment, the control feedback system is a type of proportional-integral controller or a compensator, such as a lead-lag compensator. In at least one embodiment, the dynamic response of the control feedback system is, for example, at or above 60 Hz including, for example, 75-85 Hz. In at least one embodiment, the control loop is a negative feedback system, and the notch filter filters an error signal. The control feedback system responds to a notch filtered output signal at the higher dynamic response frequency and generates an output signal that meets or exceeds regulatory harmonic currents requirements, such as EN 61000-3-2 requirements in Europe, and exceeds a power factor PF of 0.9 at least at 100% of rated output power. Additionally, total harmonic distortion (THD) is below 20%.

In at least one embodiment, the lighting and power control system also includes an output power converter stage that represents the second stage in the two stage lighting and power control system. In at least one embodiment, the output power converter stage also includes components in a negative feedback control loop similar to the control loop of the input power converter stage, except the control loop of the output power converter stage replaces the notch filter with an inverted notch filter (also sometimes referred to as an "inverse notch filter"). In at least one embodiment, the inverted notch filter also has a notch frequency at the double line frequency. Utilizing the inverted notch filter increases the open loop gain of the output power converter stage at the double line frequency, which allows the output power converter stage to suppress the double line frequency ripple noise. In at least one embodiment, in a lighting and power control system, reducing the noise improves the quality of light output by, for example, reducing light flicker and stroboscopic effects in the visible light output. In at least one embodiment, the light is generated by a load that includes one or more light emitting diodes (LED's).

In at least one embodiment, the lighting and power control system includes the input and output power converter stages except that only one of the power converter stages includes the control loop with the notch filter (input power converter stage) or inverted notch filter (output power converter stage).

Figure 2 depicts a lighting and power control system 200 that includes power converter stages 202 and 204, which has a sufficiently high bandwidth to provide power factor correction that meets regulatory requirements and suppresses double line frequency noise to provide a stable output current. The lighting and lighting and power control system 200 is a two stage system that converts a supply voltage VSUPPLY into a regulated bus voltage VBUS and converts the bus voltage VBUS into a regulated current ILED for a load 206. In at least one embodiment, the load 206 includes one or more LED's (not shown). The supply voltage VSUPPLY is, for example, a rectified public utility mains voltage, which is a 50 Hz/220 V line voltage in Europe and a 60 Hz/1 10 V line voltage in the United States of America (USA). Thus, in at least embodiment, the supply voltage VSUPPLY is nominally a 100 Hz rectified sine wave in Europe and a 120 Hz rectified sine wave in the USA. As previously described, the rectification of the supply voltage VSUPPLY creates what is commonly referred to as "double-line frequency noise" at the frequency of the supply voltage VSUPPLY. The double-line frequency noise manifests as a ripple in the supply voltage VSUPPLY. In at least one embodiment, the regulatory requirements for power factor are at least 0.9 at 100% rated output power and in addition harmonic currents requirements are found in jurisdictional specific requirements documents such as EN 61000-3-2 for Europe.

Input power converter stage 202 is the first stage of the two stage lighting and power control system 200. The input power converter stage 202 includes an input stage controller 208 to generate a control signal CSoo that controls conversion by input power stage 202 of the time-varying supply voltage VSUPPLY into a regulated DC voltage VBUS. The type of input power stage 202 is a matter of design choice and is, for example, a boost-type switching power converter. The link capacitor 207 holds the bus voltage VBUS during operation of the power converter system 200.

In at least one embodiment, to increase dynamic response, provide sufficient power factor correction to meet regulatory requirements, and suppress the double line frequency noise, the input power converter stage 202 of the lighting and power control system 200 includes an error generator 209, a notch filter 210, a control feedback system 212, and an input power stage 214 configured in a control loop 216. Operation of an exemplary control loop 216 is described in more detail with reference to Figure 3. The error generator 209 generates an error signal ERRi representing a difference between a reference voltage VREF and the output voltage VBUS. The reference voltage VREF is a desired value of the output voltage bus and can be fixed or programmable. The notch filter 210 receives and filters the error signal ERRi at a notch frequency equal to the frequency of the rectified supply voltage VSUPPLY, i.e. at a double line frequency of a mains power supply (not shown).

Cascading the notch filter 210 with the control feedback system 212 with a notch frequency equal to the double line frequency, in at least one embodiment, attenuates the open loop gain at least -20dB relative to conventional technology, which results in an open loop gain of, for example, -40dB. Because the open loop gain of the input stage controller 208 changes about -20dB per decade of frequency change, OdB will be at about 80Hz when the open loop gain is-20dB at 100Hz while maintaining a phase margin larger than 30degrees. Because of increased bandwidth, the dynamic response of the input stage controller 208 is significantly improved (faster) as compared to conventional technology. As subsequently described in more detail, Bode plots 402 and 404 of Figure 4 depict the open loop gains of the respective input power converter stages 104 and 202.

The control feedback system 212 processes the filtered error signal ERR_Fi, and generates the control signal CSoo so that the input power stage 214 generates the bus voltage VBUS to track the reference voltage VREF and the supply voltage VSUPPLY. Increasing the dynamic response time of the input stage controller 208 allows the control feedback system to follow the supply voltage VSUPPLY more closely. Following the supply voltage VSUPPLY more closely allows the control feedback system 212 to generate the control signal CSoo so that the input power stage 214 operates with sufficient power factor correction to meet regulatory requirements. In at least one embodiment, the dynamic response frequency of control feedback system is at or above 60 Hz including, for example, 75-85 Hz.

In at least one embodiment, the output power converter stage 204 represents the second stage in the lighting and power control system 200. In at least one embodiment, the output power converter stage 204 also includes components in a negative feedback control loop 220 similar to the control loop 216 of the input power converter stage 202, except the control loop 220 of the output power converter stage 204 replaces the notch filter 210 with an inverted notch filter 222 (also sometimes referred to as an "inverse notch filter"). Operation of an exemplary control loop 220 is subsequently described in more detail with reference to Figure 5. In general, the output power converter stage 204 includes an error generator 224 that generates an error signal ERRo representing a difference between a current reference signal IREF and the output current ILED. In at least one embodiment, the inverted notch filter 222 also has a notch frequency at the double line frequency. In at least one embodiment, the inverse notch filter 222 is replaced with a comb filter starting at an attenuation frequency of a mains power supply frequency, e.g. 50 Hz instead of 100 Hz for a 50 Hz mains power supply frequency and at 60 Hz instead of 120 Hz for a 60 Hz mains power supply frequency.

The control feedback system 226 processes the filtered error signal ERR_Fo and generates the output power stage control signal CSoi to control conversion by the output power stage 2 18 of the bus voltage VBUS into a constant current output ILED. Utilizing the inverted notch filter 222 increases the open loop gain of the output power converter stage 204 at the double line frequency, which allows the output power converter stage 204 to suppress the double line frequency ripple noise more effectively than conventional technology. In at least one embodiment, the load 206 includes one or more LED's, and reducing the double line frequency noise improves the quality of light output by, for example, reducing visible light flicker and visible stroboscopic effects in the light output of load 206. In general and in at least one embodiment, the control feedback system 212 is a type of proportional- integral (PI) controller or a compensator. Exemplary controllers are a proportional-integral (PI) controller, a proportional- integral-derivative (PID) controller, a proportional-integral-integral (PII) controller, and a proportional-integral-integral-derivative (PUD) controller. An exemplary compensator is a lead-lag compensator. In at least one embodiment, the bandwidth of the control feedback system 212 is well above 100 Hz and is, for example, 500 Hz.

The implementation of the components of the control loops 2 16 and 220 are a matter of design choice. In at least one embodiment the components of control loop 216 and 220 are implemented as software stored in a memory (not shown) and executed by a microcontroller (not shown). In at least one embodiment, the components are implemented in hardware or hardware and software executing on the microcontroller. In at least one embodiment, the input power converter stage 202 and the output power converter stage 204 are implemented with digital signal processing, analog signal processing, or a combination of analog and digital signal processing.

In some embodiments of the lighting and power control system 200, only the input power converter stage 202 or the output power converter stage 204 include the respective error generator 209 and notch filter 210 or the error generator 224 and the inverted notch filter 222. These two optional configurations are indicated by the dotted lines outlining the respective error generator 209, notch filter 210, error generator 224, and the inverted notch filter 222. Figure 3 depicts a control loop 300 , which represents an exemplary control loop 216 configuration with corresponding exemplary components. Figure 4 depicts a composite magnitude- frequency response Bode plot 400 of the PI controller 1 10 (Figure 1) and the PI controller 306 with a notch filter 304 for a 100 Hz supply voltage VSUPPLY. PI controller 306 represents one embodiment of the control feedback system 212. Referring to Figures 3 and 4, the error compensator 302 subtracts the feedback signal VBUS from the reference voltage VREF to generate the error signal ERRi. In at least one embodiment, the control loop 300 is implemented as a digital signal processor with the functional units shown in Figure 3. The feedback signal VBUS is a sample of the bus voltage VBUS and may also be a scaled version of the bus voltage VBUS. The notch filter 304 filters the error signal ERRi with a notch frequency of 100 Hz equal to the double line frequency noise for an exemplary 100 Hz supply voltage VSUPPLY. The particular design of the notch filter 304 is a matter of design choice and, for example, is designed using well-known design techniques.

The Bode plots 402 and 404 illustrate the comparative effect of the notch filter 304 on open loop gain and, thus, the bandwidth of PI controller 306 and the bandwidth of PI controller 1 10. Bode plot 402 indicates a stable open loop gain and, thus, stable bandwidth for PI controller 1 10 of approximately 10 Hz, and Bode plot 404 indicates a stable open loop gain and, thus, stable bandwidth for PI controller 306 of approximately 80 Hz. The particular component values of the input power converter stage 202 and output power converter stage 204 system is a matter of design choice. For the systems to be sufficiently stable under typical operational conditions, conventional design techniques can be used to generate and analyze phase- frequency Bode plots. It is a matter of design choice for a particular desired bandwidth to select a particular phase margin to provide sufficient stability of the system. Furthermore, gain margin can also be evaluated and set to increase confidence in the stability of the system.

As previously described, the increased bandwidth allows the input power converter stage 202 to provide sufficient power factor correction and not affect the double line frequency noise at the notch filter frequency of 100 Hz. The difference in gain between control system 106 with gain plot 402 and control system 300 with plot 404 amounts to more than 20dB, as Figure 4 shows, in favor of the system 300 with inverted notch. The PI controller 306 processes the filtered error signal ERR_Fi, and generates a power converter control signal CSoo to control the input power stage 308, which represents one embodiment of input power stage 202. The input power stage 308 then converts the supply voltage VSUPPLY into the bus voltage VBUS using, for example, well-known boost-type switching power converter technology. The z ^"1 delay 3 10 depicts an inherent delay for a digital version of the control loop 300.

Figure 5 depicts a control loop 500 , which represents an exemplary control loop 220 configuration with corresponding exemplary components. Figure 6 represents a composite magnitude-frequency response Bode plot 600 of the PI controller 1 18 (Figure 1) and the PI controller 506 with an inverted notch. PI controller 506 represents one embodiment of the control feedback system 220. Referring to Figures 5 and 6, the error compensator 502 subtracts the feedback signal ILED from the reference current IREF to generate the error signal ERRo. In at least one embodiment, the control loop 500 is implemented as a programmed digital signal processor with the functional units as shown in Figure 5. The feedback signal ILED is a sample of the output current ILED and may also be a scaled version of the output current ILED. The inverted notch filter 504 filters the error signal ERRi with a notch frequency of 100 Hz equal to the double line frequency noise for an exemplary 100 Hz supply voltage VSUPPLY. The particular design of the inverted notch filter 504 is a matter of design choice and, for example, is designed using well-known design techniques.

Bode plot 602 represents the magnitude versus frequency response of output stage controller 1 14, and Bode plot 604 represents the magnitude versus frequency response of output stage controller 500, i.e. PI controller 506 plus the inverted filter 504 (Figure 5). The Bode plots 602 and 604 illustrate the comparative effect of the inverted notch filter 504 on the double line frequency noise attenuation combined with the PI controller 506 and the much lower attenuation by PI controller 1 1 8 only without an inverted notch filter. Bode plot 602 indicates no particular open loop gain increase or noise suppression at the double line frequency for a 100 Hz supply voltage VSUPPLY. Bode plot 604 indicates significant open loop gain increase at the double line frequency gain and, thus, suppression of the double line frequency noise of approximately 1 00 Hz. As previously described, the increased open loop gain allows the output power converter stage to significantly suppress the double line frequency noise. The PI controller 506 processes the filtered error signal ERR_Fo, and generates a power converter control signal CSoi to control the output power stage 508, which represents one embodiment of output power stage 308. The output power stage 508 then converts the bus voltage VBUS into the constant current ILED using, for example, well-known switching power converter technology. The z ^"1 delay 5 10 depicts an inherent delay for a digital version of the control loop 500. Thus, in at least one embodiment, a lighting and power control system includes a two stage power converter system that has a sufficiently high dynamic response frequency to provide power factor correction that meets regulatory requirements and suppresses double line frequency noise to provide a stable output current.

Although embodiments have been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.