Patent Application
20100109534
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
This application claims benefit of the following patent application which is hereby incorporated by reference: JP2008-277425, filed Oct. 28, 2008.
Not Applicable
Not Applicable
The present invention relates generally to an electronic ballast for powering a discharge lamp. More particularly, the present invention relates to an electronic ballast with controlled filament preheating using half-wave lamp current detection.
An electronic ballast as known in the art for lighting a hot-cathode type discharge lamp, such as a high-pressure discharge lamp, also referred to as an HID (High-intensity discharge lamp), typically includes a power converting circuit for receiving a DC power input and outputting AC power, and a control circuit for controlling the power converting circuit.
In an example of an electronic ballast of this type as previously known in the art, the control circuit performs a filament heating operation by increasing the output frequency of the power converting circuit to heat each filament of the discharge lamp upon startup of the discharge lamp. This heating operation takes place after a starting operation wherein the output voltage of the power converting circuit is relatively higher, and before the start of steady-state lamp.
The electronic ballast of this example can stabilize lamp discharge after a shift to the steady-state operation and reduce lamp fade-out as compared to the case where filament heating not performed.
Referring to
However, where multiple discharge lamps may be connected to the ballast, a particular duration of the filament heating operation that accommodates all discharge lamps is likely to be excessive for some lamps. Because the filament heating operation allows the power converting circuit to output larger power than during steady-state operation, to inhibit a negative effect on the life of the discharge lamp, the duration of the filament heating operation needs to be reduced as much as possible while yet properly heating the filaments.
The present invention was made in consideration of the above problems, with an object to provide an electronic ballast and a lighting fixture in which the output current to the discharge lamp in shifting to steady-state operation can be provided in a positive-negative symmetrical state, while performing a filament heating operation of a suitable but not excessive duration.
According to an aspect of the present invention, an electronic ballast includes a power converting circuit for receiving DC power and outputting AC power, a starting circuit connected between output ends of the power converting circuit together with a discharge lamp so as to generate a high voltage for starting the discharge lamp, and a control circuit for controlling the power converting circuit.
The control circuit at ballast startup first performs a starting operation to allow the discharge lamp to start with a high voltage generated by the starting circuit. The control circuit then shifts to a filament heating operation to make an output frequency of the power converting circuit higher than that in the steady state to heat each filament of the discharge lamp. The control circuit shifts to a steady-state operation to allow the power converting circuit to output AC power for maintaining the lighting of the discharge lamp to the discharge lamp.
The ballast further includes a half-wave discharge detecting circuit for detecting half-wave discharge (rectification) in the discharge lamp. When the half-wave discharge detecting circuit detects half-wave discharge while in the filament heating operation, the control circuit performs a half-wave discharge improving process. The process involves determining peak values for the positive and negative polarities of an output current of the power converting circuit with respect to ground, and then increasing a lower peak value of the two peak values by a predetermined amount to reduce the disparity and thereby resolve the half-wave discharge.
The output current to the discharge lamp in shifting to steady-state operation can be provided in a positive-negative symmetrical state, while performing a filament heating operation of a suitable but not excessive duration for the discharge lamp, in comparison with the case without performing the half-wave discharge improving process.
According to another aspect of the present invention, the power converting circuit includes a step-down chopper circuit for stepping down the received DC power and a full bridge circuit for converting the DC power output from the step-down chopper circuit. The control circuit controls output power of the power converting circuit by a duty ratio associated with turning on/off of at least one switching element in the full bridge circuit.
Alternatively, the power converting circuit may include a half bridge circuit, wherein the control circuit controls output power of the power converting circuit by a duty ratio associated with the turning on/off of a switching element in the half bridge circuit.
According to another aspect of the present invention, the half-wave discharge improving process is realized by superimposing a DC component on the output current of the power converting circuit. Alternatively, the half-wave discharge improving process may be realized by increasing the amplitude of the output current from the power converting circuit.
According to another aspect of the present invention, the control circuit through the filament heating operation maintains a constant adjustment amount of a small peak value obtained by the half-wave discharge improving process.
According to yet another aspect of the present invention, the control circuit sets the adjustment amount of the small peak value obtained by the half-wave discharge improving process to be a half of a difference of peak values between polarities of an output current of the power converting circuit. The adjustment amount is so set by the control circuit when half-wave discharge is detected by the half-wave discharge detecting circuit for the first time after starting the filament heating operation.
According to yet another aspect of the present invention, the control circuit sets the adjustment amount of the small peak value obtained by the half-wave discharge improving process in accordance with the duration of the filament heating operation from detection of half-wave discharge by the half-wave discharge detecting circuit for the first time after starting the filament heating operation. For example, the control circuit may make the adjustment amount larger with an increase in the duration of the filament heating operation from detection of the half-wave discharge by the half-wave discharge detecting circuit for the first time after starting the filament heating operation.
According to another aspect of the present invention, the control circuit changes the adjustment amount of the small peak value obtained by the half-wave discharge improving process as needed in accordance with a difference of peak values between polarities of the output current of the power converting circuit. The control circuit may make the adjustment amount of the small peak value obtained by the half-wave discharge improving process larger with an increase in the difference of peak values between the polarities of the output current of the power converting circuit. The control circuit may further be prevented from increasing the adjustment amount of a small peak value obtained by the half-wave discharge improving process more than a predetermined upper limit value.
According to another aspect of the present invention, the control circuit causes the power converting circuit to stop outputting AC power to the discharge lamp if the half-wave discharge detecting circuit detects half-wave discharge in terminating the filament heating operation. Accordingly it is made possible to prevent an excessive electrical stress from being applied to the discharge lamp resulting from continuously supplying power in a state of having half-wave discharge in the discharge lamp.
According to another aspect of the present invention, the control circuit allows the process to return to the starting operation if the half-wave discharge detecting circuit detects half-wave discharge in terminating the filament heating operation.
According to another aspect of the present invention, if the half-wave discharge detecting circuit detects half-wave discharge in terminating the filament heating operation, the control circuit allows the process to return to the starting operation after causing the power converting circuit to stop outputting AC power to the discharge lamp over a predetermined period of time. Accordingly, half-wave discharge in the discharge lamp is less likely in a subsequent filament heating operation, in comparison with processes where the power converting circuit is not caused to stop outputting AC power to the discharge lamp prior to returning to the starting operation.
According to another aspect of the present invention, the control circuit counts the number of times of returning from the filament heating operation to the starting operation, and causes the power converting circuit to stop outputting AC power when a predetermined upper limit number of times are reached. It is thus made possible to prevent an electrical stress from being wastefully applied to the discharge lamp and circuit components resulting from repeating the starting operation and the filament heating operation without limitation.
According to yet another aspect of the present invention, a fixture main body may be incorporated with various embodiments of the ballast as herein described.
a and 24b are explanatory diagrams of examples of a waveform in a lamp current, showing a case of having an insufficient duration of the filament heating operation in
Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. The term “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, charge, temperature, data or other signal. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa. With regards to the discharge lamp of this application, “igniting,” “starting,” and “discharging” of the lamp may be considered synonymous unless otherwise stated.
As shown in
One of the output ends of the above full bridge circuit, that is a connection point between the switching elements Q1 and Q2, is connected to one end (i.e. one of the filaments) of the discharge lamp La via a first inductor L1. The other output end of the above full bridge circuit, that is a connection point between the switching elements Q3 and Q4, is connected to the other end (i.e. the other filament) of the discharge lamp La via a second inductor L2. The first inductor L1 serves as a so-called autotransformer having a tap which is connected to ground via a series circuit including a first capacitor C1 and a resistor R1. Further connected in parallel with a series circuit formed of the first inductor L1 and the discharge lamp La is a second capacitor C2. That is, each of the inductors L1 and L2 and each of the capacitors C1 and C2 constitute a resonant circuit (referred to as a “load circuit” hereinafter) together with the discharge lamp La.
The present embodiment further includes a half-wave discharge detecting circuit 2 for detecting an power converting circuit output current Ila provided to the discharge lamp La (referred to as a “lamp current” hereinafter) and detecting half-wave discharge (rectification) in the discharge lamp La on the basis of the detected lamp current Ila, and a control circuit 3 for driving on and off each of the switching elements Q1-Q4.
The half-wave discharge detecting circuit 2 detects a peak value (or absolute value) in each polarity of the lamp current Ila so as to calculate a difference ΔI of peak values between the polarities (referred to as an “asymmetrical current value” hereinafter) and compares an absolute value thereof to a predetermined current threshold Ir (refer to
The control circuit 3 drives on and off the switching elements Q1-Q4 so that the switching elements Q1-Q4 diagonally positioned from each other are turned on simultaneously and the switching elements Q1-Q4 serially connected from each other are alternately turned on and off. DC power received from the DC power source E is therefore converted into AC power, and frequency of this AC power corresponds to a frequency in polarity inversion by the above on/off driving (referred to as an “operating frequency”).
Operation of the control circuit 3 of an embodiment of the present invention will be explained below in detail with reference to
When power is supplied (S1), the control circuit 3 begins a starting operation in order to initiate discharge in the discharge lamp La (S2). During a starting control period P1 to perform the starting operation, the control circuit 3 changes an operating frequency periodically in a range from several tens of kHz to several hundreds of kHz. During this starting control period P1, the operating frequency is used at a resonant frequency (or very near to the resonant frequency) for the resonant circuit, the resonant circuit including a primary winding portion of the first inductor L1 serving as an autotransformer, that is, the portion between a connection point of the switching elements Q1 and Q2 and the tap, and the first capacitor C1. A resonant voltage occurring at this time is boosted by the first inductor L1 further serving as the autotransformer, whereby a voltage Vla output to the discharge lamp La (referred to as a “lamp voltage” hereinafter) reaches a voltage required for starting (e.g. 3 to 4 kV), so that the discharge lamp La is started. The first inductor L1 and the first capacitor C1 in this example constitute a starting circuit.
Referring to
After continuing the above starting operation for a predetermined period of time, the control circuit 3 terminates the starting operation. The process then continues to an filament heating control period P2 to perform an filament heating operation by reducing the operating frequency to a lower frequency (e.g. to about several tens of kHz) than that of the starting operation. The operating frequency during the filament heating operation is still a relatively high frequency which is close to a resonant frequency of the load circuit connected between the output ends of the full bridge circuit, in comparison with an operating frequency during a steady state operation to be described later. In this period P2 each filament of the discharge lamp La is heated.
When the filament heating period P2 has begun, the control circuit 3 starts counting a predetermined filament heating time during which the filament heating operation should be maintained (S3), followed by referring to an output from the half-wave discharge detecting circuit 2 (S4). If no half-wave discharge is detected, the filament heating operation is performed over a predetermined period of time (S5), followed by determining whether or not counting the filament heating time is completed (S6), and returning to step S4 if the count indicates that the filament heating time is incomplete. Reference to an output from the half-wave discharge detecting circuit 2 is periodically made at every predetermined time mentioned above until a half-wave discharge is detected or the filament heating time is complete.
In contrast, if the half-wave discharge is detected in step S4, a filament heating operation including a half-wave discharge improving process for resolving half-wave discharge in an early stage is performed (S7), followed by determining whether or not counting the filament heating time is completed (S8) and returning to step S7 when the filament heating time has not been completed.
When the filament heating time is counted as being completed in either of step S6 or step S8, the process proceeds to steady state operation (S9).
During a steady state control period P3 to perform the steady state operation, the control circuit 3 supplies to the discharge lamp La rectangular wave AC power for maintaining lighting of the discharge lamp La by reducing the operating frequency to be much lower (e.g. several hundreds Hz) than that during the filament heating operation. During the steady state operation, the control circuit 3 also performs PWM control of the power supplied to the discharge lamp La by turning on and off each of the switching elements Q3 and Q4 in one of the series circuit with a predetermined duty ratio without constantly turning them on even in a period during which the diagonally positioned switching elements Q1 and Q2 are turned on.
An embodiment of the half-wave discharge improving process as shown in step S7 may now be explained in further detail. In a period during which the half-wave discharge detecting circuit 2 detects half-wave discharge, the control circuit 3 receives from the half-wave discharge detecting circuit 2 information regarding a peak value of the lamp current Ila in each of the positive and negative polarities with respect to ground (i.e. asymmetrical current value ΔI). For a pair of the switching elements Q1-Q4 diagonally positioned from each other and having a polarity with a lower peak of the lamp current Ila with respect to the opposite pair of diagonally positioned switching elements, an on-time is extended by the control circuit 3 by a predetermined adjustment amount. The control circuit 3 further shortens the on-time by the same adjustment amount in the other pair.
Also, in a period during which the half-wave discharge detecting circuit 2 does not detect half-wave discharge, the above adjustment amounts are set to 0, which means an on-time duty ratio of 0.5 is set for each of the switching elements Q1-Q4. That is, in the above half-wave discharge improving process, the operating frequency as a whole is unchanged regardless of the presence or absence of half-wave discharge detected by the half-wave discharge detecting circuit 2. If a value other than 0 is set for the adjustment amount in the above half-wave discharge improving process, a DC current with a magnitude corresponding to the adjustment amount and in a direction corresponding to the switching elements Q1-Q4 with extended on-time is superimposed on the lamp current Ila, and the magnitude of this DC component is made larger with an increase in absolute value of the adjustment amount. For example, if a direction extending from left to right as shown in
On the other hand, a negative DC component with a magnitude corresponding to an adjustment amount is observed in each of the lamp voltage Vla and the lamp current Ila when longer on-time (with on-time duty ratio of 0.6 for example) is set for the switching elements Q2 and Q3 each of which corresponds to a negative polarity as shown in
Detection of half-wave discharge similar to step S4 and the above changes in the adjustment amount are carried out as needed until the filament heating operation is finished by completion of counting the filament heating time in step S8. That is, in the case where half-wave discharge is not later detected after an initial detection of the half-wave discharge and applied adjustment, the adjustment amount returns to 0. In the case where the half-wave discharge is detected again thereafter, the adjustment amount is set to any values corresponding to the asymmetrical current value ΔI, other than 0.
In the following explanation, and with reference generally to
Note that to avoid the adjustment amount being excessively increased, the above absolute value of the adjustment amount may also be set so that the magnitude of the DC component generated in the lamp current Ila becomes slightly smaller than a half of the absolute value of the asymmetrical current value ΔI obtained when the half-wave discharge is detected for the first time.
According to the above configuration, the half-wave discharge improving process makes it easier to heat one of the filaments with a lower temperature corresponding to a polarity with a smaller amount of the lamp current Ila in the discharge lamp La. Lamp flickering and fade out after proceeding to steady state operation may thereby be suppressed by providing an output current to the discharge lamp in a positive-negative symmetrical state in proceeding to the steady state operation while desirably keeping the duration of the filament heating operation relatively short.
With reference now to
Alternatively, a step-down chopper circuit 4 as shown in
As shown in
A pulse generating circuit (not shown) may also be arranged as a starting circuit to generate a high voltage pulse for starting the discharge lamp La during the starting operation. The aforementioned pulse generating circuit can be realized by well known techniques, so a detailed drawing and explanation thereof may be omitted.
In place of setting a duration for the filament heating operation to be constant, the filament heating operation may also be continued until at least no half-wave discharge is detected by the half-wave discharge detecting circuit 2. That is, a step is arranged to refer to an output of the half-wave discharge detecting circuit 2 prior to step S8 for determining completion of counting the filament heating operation, and the process proceeds to step S8 when no half-wave discharge is detected in this step, whereas the process continues the filament heating operation without proceeding to step S8 when the half-wave discharge is detected.
In addition, and as shown in
Furthermore, as shown in
In addition, as shown in
Furthermore, the method to detect the half-wave discharge by the half-wave discharge detecting circuit 2 is not limited to one based on the difference of peak values between polarities as stated above, and may also be realized by, for example, comparing a smaller peak value out of peak values of both polarities of the lamp current Ila (referred to as a “small peak value” hereinafter) to a predetermined current threshold so as to detect the half-wave discharge in a period during which the small peak value is maintained to be less than the current threshold for the predetermined determination time or longer, and prevent detection of the half-wave discharge in a period other than the above period. The current threshold used in this case is assumed to correspond to a minimum value required for the lamp current Ila to sufficiently increase the temperature of filaments of the discharge lamp La in the filament heating time under an assumed environment (referred to as a “minimum current value” hereinafter).
The half-wave discharge improving process may also be realized by making the amplitude of the lamp current Ila larger in place of generating a DC component in the lamp current Ila as stated above. For example, the amplitude of the lamp current Ila is increased only in a period during which the half-wave discharge detecting circuit 2 detects the half-wave discharge. An amount increased in the amplitude of the lamp current Ila in the half-wave discharge improving process (simply referred to as “increased amplitude” hereinafter) is set to, for example, a half of the absolute value of the asymmetrical current value AI obtained when the half-wave discharge is detected for the first time. Also considered as a method to make the amplitude of the lamp current Ila larger is, in addition to changing the operating frequency, changing an output voltage Vout_sd of the step-down chopper circuit 4 in the example of
Furthermore, the half-wave discharge improving process may also be realized by setting a larger value for the absolute value of the adjustment amount Iadj and the increased amplitude Iinc with an increase in the absolute value of the asymmetrical current value ΔI as shown in
The absolute value of the adjustment amount Iadj and the increased amplitude Iinc may also be determined by a feedback control in which a small peak value is used as a lower limit current value. Furthermore, in the case where the adjustment amount per unit time and an adjustment period of the increased amplitude are constant, the process may also be realized without proceeding to step S8 until a difference between the small peak value and a lower limit current value becomes a predetermined threshold or less, that is, preventing the filament heating operation from terminating even if counting the filament heating time is completed.
The absolute value of the adjustment amount and the increased amplitude Iinc in the half-wave discharge improving process may also be gradually increased in accordance with the duration of the filament heating operation from detection of the half-wave discharge by the half-wave discharge detecting circuit 2 for the first time after starting the filament heating operation. This increase may be realized in a stepwise manner with respect to the above duration as shown in
It is also desirable for the control circuit 3 to increase the absolute value of the adjustment amount Iadj and the increased amplitude Iinc in a range less than a predetermined upper limit value in the case where the absolute value of the adjustment amount Iadj and the increased amplitude Iinc are changed as needed as stated above. The above upper limit value may be appropriately determined in accordance with a rated current value of the circuit components and the discharge lamp La.
The aforementioned various embodiments of the ballast of the present invention can be used for lighting, for example, each of the lighting fixtures 5 shown in
Thus, although there have been described particular embodiments of the present invention of a new and useful Electronic Ballast with Controlled Filament Preheating Using Half-Wave Lamp Current Detection, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2008-277425 | Oct 2008 | JP | national |
