1. Field of the Invention
The present invention relates to electronic ballasts for controlling a gas discharge lamp, such as a fluorescent lamp, and more specifically, to a method of controlling the gas discharge lamp to avoid flickering and flashing of the lamp during low temperature conditions.
2. Description of the Related Art
In order to reduce energy consumption of artificial illumination sources, the use of high-efficiency light sources is increasing, while the use of low-efficiency light sources (i.e., incandescent lamps, halogen lamps, and other low-efficacy light sources) is decreasing. High-efficiency light sources may comprise, for example, gas discharge lamps (such as compact fluorescent lamps), phosphor-based lamps, high-intensity discharge (HID) lamps, light-emitting diode (LED) light sources, and other types of high-efficacy light sources. Lighting control devices, such as dimmer switches, allow for the control of the amount of power delivered from a power source to a lighting load, such that the intensity of the lighting load may be dimmed from a high-end (i.e., maximum intensity) to a low-end (i.e., minimum) intensity. Both high-efficiency and low-efficiency light sources can be dimmed, but the dimming characteristics of these two types of light sources typically differ.
Because of the increase in use of high-efficiency light sources, fluorescent lamps are often being installed in outdoor installations where the lamp may be subject to low operating temperatures. However, typical fluorescent lamps may not operate correctly and may flicker if the fluorescent lamps are dimmed in cold ambient temperatures. As the fluorescent lamp is dimmed towards the low-end intensity, the magnitude of a lamp voltage required to drive the fluorescent lamp increases. In addition, as the temperature of the lamp decreases, the magnitude of the lamp voltage required to drive the fluorescent lamp increases even further. These increases in the lamp voltage required to drive the fluorescent lamp can cause instability in the intensity of the fluorescent lamp, particularly near the low-end intensity of the lamp, which may thus produce visible flickering or flashing of the fluorescent lamp. Thus, there is a need for a load control device for high-efficiency light sources that is able to stably dim the light sources to low intensities without flicker in low temperature conditions.
According to an embodiment of the present invention, an electronic ballast circuit for driving a gas discharge lamp is operable to control the lamp to avoid flicking and flashing of the intensity of the lamp during low temperature conditions. The ballast circuit comprises an inverter circuit for receiving a DC bus voltage and for generating a high-frequency inverter output voltage, a resonant tank circuit for receiving the inverter output voltage and generating a sinusoidal voltage for driving said lamp, and a control circuit operatively coupled to the inverter circuit for adjusting an intensity of the lamp between a minimum intensity and a maximum intensity. The control circuit receives a control signal representative of a lamp temperature of the lamp, and increases the minimum intensity of the lamp if the lamp temperature of the lamp drops below a cold temperature threshold. In addition, the ballast circuit may further comprise a temperature sensing circuit operable to generate the control signal representative of the lamp temperature of the lamp. The temperature sensing circuit may be operatively coupled to the control circuit, such that the control circuit is operable to increase the minimum intensity of the lamp if the lamp temperature of the lamp drops below the cold temperature threshold.
In addition, a method of driving a gas discharge lamp to avoid flicking and flashing of the lamp during low temperature conditions is also described herein. The method comprises the steps of: (1) generating a high-frequency output voltage having an operating frequency; (2) adjusting the operating frequency so as to control an intensity of the lamp between a minimum intensity and a maximum intensity; (3) generating a temperature control signal representative of a lamp temperature of the lamp; (4) determining if the lamp temperature of the lamp is below a cold temperature threshold; and (5) increasing the minimum intensity of the lamp if the lamp temperature of the lamp is below the cold temperature threshold.
Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings.
The invention will now be described in greater detail in the following detailed description with reference to the drawings in which:
The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.
The dimmer switch 104 typically includes a bidirectional semiconductor switch 105B, such as, for example, a thyristor (such as a triac) or two field-effect transistors (FETs) coupled in anti-series connection, for providing a phase-controlled voltage VPC (i.e., a dimmed-hot voltage) to the hybrid light source 100. Using a standard forward phase-control dimming technique, a control circuit 105C renders the bidirectional semiconductor switch 105B conductive at a specific time each half-cycle of the AC power source, such that the bidirectional semiconductor switch remains conductive for a conduction period TCON during each half-cycle. The dimmer switch 104 controls the amount of power delivered to the hybrid light source 100 by controlling the length of the conduction period TCON. The dimmer switch 104 also often comprises a power supply 105D coupled across the bidirectional semiconductor switch 105B for powering the control circuit 105C. The power supply 105D generates a DC supply voltage VPS by drawing a charging current ICHRG from the AC power source 102 through the hybrid light source 100 when the bidirectional semiconductor switch 105B is non-conductive each half-cycle. An example of a dimmer switch having a power supply 105D is described in greater detail in U.S. Pat. No. 5,248,919, issued Sep. 29, 1993, entitled LIGHTING CONTROL DEVICE, the entire disclosure of which is hereby incorporated by reference.
Alternatively, the dimmer switch 104 could comprise a two-wire analog dimmer switch having a timing circuit (not shown) and a trigger circuit (not shown). The timing circuit conducts a timing current from the AC power source through the hybrid light source 100 when the bidirectional semiconductor switch 105B is non-conductive each half-cycle. The timing current is used to control when the bidirectional semiconductor switch 105B is rendered conductive each half-cycle.
The hybrid light source 100 comprises, for example, a screw-in Edison base 110 for connection to a standard Edison socket, such that the hybrid light source may be coupled to the AC power source 102. The screw-in base 110 has two input terminals 110A, 110B (
The fluorescent lamp 106 and halogen lamp 108 may be surrounded by a housing comprising a light diffuser 114 (e.g., a glass light diffuser) and a fluorescent lamp reflector 115. The fluorescent lamp reflector 115 directs the light emitted by the fluorescent lamp 106 away from the hybrid light source 100. The halogen lamp 108 is mounted to a post 116, such that the halogen lamp is situated beyond the terminal end of the fluorescent lamp 106. The post 116 allows the halogen lamp to be electrically connected to the hybrid light source electrical circuit 120. A halogen lamp reflector 118 surrounds the halogen lamp 108 and directs the light emitted by the halogen lamp in the same direction as the fluorescent lamp reflector 115 directs the light emitted by the fluorescent lamp 106.
The hybrid light source 100 provides an improved color rendering index and correlated color temperature across the dimming range of the hybrid light source (particularly, near a low-end lighting intensity LLE) as compared to a stand-alone compact fluorescent lamp.
The hybrid light source 100 is further operable to control the fluorescent lamp 106 and the halogen lamp 108 to provide high-efficiency operation near the high-end intensity LHE.
Because the fluorescent lamp 106 cannot be dimmed to very low intensities without the use of more expensive and complex circuits, the fluorescent lamp 106 is controlled to be off at a transition intensity LTRAN, e.g., approximately 8% (as shown in
The structure and operation of the hybrid light source 100 is described in greater detail in commonly-assigned, co-pending U.S. patent application Ser. No. 12/205,571, filed Sep. 8, 2008; U.S. patent application Ser. No. 12/553,612, filed Sep. 3, 2009; and U.S. patent application Ser. No. 12/704,781, filed Feb. 12, 2010; each entitled HYBRID LIGHT SOURCE. The entire disclosures of all three applications are hereby incorporated by reference.
Since the fluorescent lamp 106 is turned on at the transition intensity LTRAN in the middle of the dimming range of the hybrid light source 100 as shown in
The control circuit 160 is operable to determine the desired total lighting intensity LDESIRED of the hybrid light source 100 in response to a zero-crossing detect circuit 164 (i.e., as determined by the user actuating the intensity adjustment actuator of the dimmer switch 104). The zero-crossing detect circuit 164 provides a zero-crossing control signal VZC, representative of the zero-crossings of the phase-controlled voltage VPC, to the control circuit 160. A zero-crossing is defined as the time at which the phase-controlled voltage VPC changes from having a magnitude of substantially zero volts to having a magnitude greater than a predetermined zero-crossing threshold VTH-ZC (and vice versa) each half-cycle. Specifically, the zero-crossing detect circuit 164 compares the magnitude of the rectified voltage to the predetermined zero-crossing threshold VTH-ZC (e.g., approximately 20 V), and drives the zero-crossing control signal VZC high (i.e., to a logic high level, such as, approximately the DC supply voltage VCC1) when the magnitude of the rectified voltage VRECT is greater than the predetermined zero-crossing threshold VTH-ZC. Further, the zero-crossing detect circuit 164 drives the zero-crossing control signal VZC low (i.e., to a logic low level, such as, approximately circuit common) when the magnitude of the rectified voltage VRECT is less than the predetermined zero-crossing threshold VTH-ZC. The control circuit 160 determines the length of the conduction period TCON of the phase-controlled voltage VPC in response to the zero-crossing control signal VZC, and then determines the target lighting intensities for both the fluorescent lamp 106 and the halogen lamp 108 to produce the target total lighting intensity LTOTAL of the hybrid light source 100 in response to the conduction period TCON of the phase-controlled voltage VPC. Alternatively, the zero-crossing detect circuit 164 may provide some hysteresis in the level of the zero-crossing threshold VTH-ZC.
The low-efficiency light source circuit 150 comprises a full-wave rectifier 152 for generating a rectified voltage VRECT (from the phase-controlled voltage VPC) and a halogen lamp drive circuit 154, which receives the rectified voltage VRECT and controls the amount of power delivered to the halogen lamp 108. The low-efficiency light source circuit 150 is coupled between the rectified voltage VRECT and the rectifier common connection (i.e., across the output of the front end circuit 130). The control circuit 160 is operable to control the magnitude of the halogen voltage VHAL to thus control the intensity of the halogen lamp 108 to the target halogen lighting intensity corresponding to the present value of the desired total lighting intensity LDESIRED of the hybrid light source 100, e.g., to the target halogen lighting intensity as shown in
The high-efficiency light source circuit 140 comprises a fluorescent drive circuit (e.g., a dimmable electronic ballast circuit 142) for receiving the phase-controlled voltage VPC (via the RFI filter 130) and for driving the fluorescent lamp 106. Specifically, the phase-controlled voltage VPC is coupled to a voltage doubler circuit 144, which generates a bus voltage VBUS across two series connected bus capacitors CB1, CB2. The first bus capacitor CB1 is operable to charge through a diode D1 during the positive half-cycles, while the second bus capacitor CB2 is operable to charge through a diode D2 during the negative half-cycles. The ballast circuit 142 includes an inverter circuit 146 for converting the DC bus voltage VBUS to a high-frequency inverter output voltage VINV (e.g., a square-wave voltage). The inverter output voltage VINV is characterized by an operating frequency fOP (and an operating period TOP=1/fOP). The ballast circuit 142 further comprises an output circuit, e.g., a resonant tank circuit 148, for filtering the inverter output voltage VINV to produce a substantially sinusoidal high-frequency AC voltage VSIN, which is coupled to the electrodes of the fluorescent lamp 106. The high-efficiency lamp source circuit 140 further comprises a lamp current measurement circuit 170 (which provides a lamp current feedback signal VFB
The control circuit 160 is operable to control the inverter circuit 146 of the ballast circuit 140 to control the intensity of the fluorescent lamp 106 to the target fluorescent lighting intensity LFL corresponding to the present value of the desired total lighting intensity LDESIRED of the hybrid light source 100, e.g., to the target fluorescent lighting intensity LFL as shown in
The hybrid light source electrical circuit 120 further comprises a temperature sensing circuit 180 that is coupled to the control circuit 160. The temperature sensing circuit 180 generates a measured temperature control signal VTEMP that is representative of a measured temperature TM measured by the temperature sensing circuit. Since the hybrid light source electrical circuit 120 is housed in the enclosure 112 in close vicinity to the fluorescent lamp 106, the measured temperature TM measured by the temperature sensing circuit 180 is representative of the lamp temperature TL of the fluorescent lamp 106. For example, the temperature sensing circuit 180 may be located close to the connection points between the dimmable electronic ballast circuit 142 and the fluorescent lamp 106. The temperature sensing circuit 180 may comprise for example a negative-temperature-coefficient (NTC) thermistor (not shown) coupled in series with a resistor (not shown), where the supply voltage VCC is coupled across the series combination of the NTC thermistor and the resistor. The impedance of the NTC thermistor changes as a function of the measured temperature TM, such that the measured temperature control signal VTEMP may be generated at the junction of the NTC thermistor and the resistor. Alternatively, the temperature sensing circuit 180 could comprises a temperature sensor integrated circuit (not shown).
The control circuit 160 is operable to adjust the minimum fluorescent intensity LFL-MIN of the fluorescent lamp 106 in response to the measured temperature control signal VTEMP (i.e., the measured temperature TM measured by the temperature sensing circuit 180).
If there is presently a change in the desired total lighting intensity LDESIRED of the hybrid light source 100 at step 220 (i.e., as determined from the zero-crossing control signal VZC of the zero-crossing detect circuit 164), the control circuit 160 determines if the new desired total lighting intensity LDESIRED is less than the transition intensity LTRAN at step 222. If so, the control circuit 160 sets the target fluorescent lighting intensity LFL equal to 0% at step 224 (i.e., the fluorescent lamp 106 is off), and the fluorescent lamp control procedure 200 exits. If the desired total lighting intensity LDESIRED is greater than or equal to the transition intensity LTRAN at step 222, the control circuit 160 determines the target fluorescent lighting intensity LFL as a function of the desired total lighting intensity LDESIRED (e.g., according to the graph shown in
The ballast 300 further comprises a control circuit 360 for controlling the intensity of the lamp 306 to a target intensity LTARGET between a low-end (i.e., minimum) intensity LLE (e.g., 1%) and a high-end (i.e., maximum) intensity LHE (e.g., 100%). The control circuit 360 may comprise, a microprocessor, a microcontroller, a programmable logic device (PLD), an application specific integrated circuit (ASIC), or any suitable type of controller or control circuit. The control circuit 360 is coupled to the inverter circuit 346 and provides a drive control signal VDRIVE to the inverter circuit for controlling the magnitude of a lamp voltage VL generated across the lamp 306 and a lamp current IL conducted through the lamp. Accordingly, the control circuit 360 is operable to turn the lamp 306 on and off and adjust (i.e., dim) the intensity of the lamp. The control circuit 360 receives a lamp current feedback signal VFB-IL, which is generated by a lamp current measurement circuit 370 and is representative of the magnitude of the lamp current IL. The control circuit 360 also receives a lamp voltage feedback signal VFB-VL, which is generated by a lamp voltage measurement circuit 372 and is representative of the magnitude of the lamp voltage VL. The ballast 300 also comprises a power supply 362, which receives the bus voltage VBUS and generates a DC supply voltage VCC (e.g., approximately five volts) for powering the control circuit 360 and other low-voltage circuitry of the ballast.
The ballast 300 may comprise a phase-control circuit 390 for receiving a phase-control voltage VPC (e.g., a forward or reverse phase-control signal) from a standard phase-control dimmer (not shown). The control circuit 360 is coupled to the phase-control circuit 390, such that the microprocessor is operable to determine the target intensity LTARGET for the lamp 306 from the phase-control voltage VPC. The ballast 300 may also comprise a communication circuit 392, which is coupled to the control circuit 360 and allows the ballast to communicate (i.e., transmit and receive digital messages) with the other control devices on a communication link (not shown), e.g., a wired communication link or a wireless communication link, such as a radio-frequency (RF) or an infrared (IR) communication link. Examples of ballasts having communication circuits are described in greater detail in commonly-assigned U.S. Pat. No. 7,489,090, issued Feb. 10, 2009, entitled ELECTRONIC BALLAST HAVING ADAPTIVE FREQUENCY SHIFTING; U.S. Pat. No. 7,528,554, issued May 5, 2009, entitled ELECTRONIC BALLAST HAVING A BOOST CONVERTER WITH AN IMPROVED RANGE OF OUTPUT POWER; and U.S. Pat. No. 7,764,479, issued Jul. 27, 2010, entitled COMMUNICATION CIRCUIT FOR A DIGITAL ELECTRONIC DIMMING BALLAST, the entire disclosures of which are hereby incorporated by reference.
According to the second embodiment of the present invention, the control circuit 360 infers the lamp temperature TL of the fluorescent lamp 306 from the magnitude of the lamp voltage VL. Since the lamp voltage VL is dependent upon the lamp temperature TL of the fluorescent lamp 306, the lamp voltage feedback signal VFB-VL generated by the lamp voltage measurement circuit 372 is representative of the lamp temperature TL of the fluorescent lamp 306. Accordingly, the control circuit 360 is operable to increase the low-end intensity LLE if the magnitude of the lamp voltage VL exceeds a maximum lamp voltage limit VL-LIMIT (e.g., approximately 270 VRMS). For example, the control circuit 360 may increase the low-end intensity LLE so as to limit the magnitude of the lamp voltage VL to the maximum lamp voltage limit VL-LIMIT.
If the lamp voltage feedback signal VFB-VL is less than the maximum lamp voltage limit VL-LIMIT at step 412, and the low-end intensity LLE is not equal to a normal low-end intensity LLE-N (e.g., approximately 1%) at step 416, the control circuit 360 decreases the low-end intensity LLE by the predetermined value ΔLLE at step 418, and the lamp voltage monitor procedure 400 exits. The control circuit 360 will continue to decrease the low-end intensity LLE by the predetermined value ΔLLE at step 418 each time that the lamp voltage monitor procedure 400 is executed. When the low-end intensity LLE is equal to the normal low-end intensity LLE-N at step 416, the lamp voltage monitor procedure 400 simply exits.
The method of the present invention for controlling a fluorescent lamp during low temperature conditions could be used in any dimmable electrical ballast to minimize flickering and flashing of the lamp during low temperature conditions. Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.
This application is a non-provisional application of commonly-assigned U.S. Provisional Application No. 61/321,316, filed Apr. 6, 2010, and U.S. Provisional Application No. 61/374,884, filed Aug. 18, 2010, both entitled METHOD OF CONTROLLING AN ELECTRICAL DIMMING BALLAST DURING LOW TEMPERTATURE CONDITIONS, the entire disclosures of which are hereby incorporated by reference.
Number | Date | Country | |
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61321316 | Apr 2010 | US | |
61374884 | Aug 2010 | US |