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Pre-heating lamp filaments helps extend the life of gas-discharge lamps. Furthermore, the pre-heating of lamp filaments may be necessary to ignite certain types of high impedance gas-discharge lamps, such as the T5 model gas-discharge lamp. During a pre-heat period, ballast circuits may be coupled to pre-heat components to transmit pre-heat voltages to the lamp filaments of gas-discharge lamps 16.
Referring now to
To pre-heat the lamp filaments 4A of gas-discharge lamps 4, the inductive component L_resonant of the resonant circuit 5 is magnetically coupled to secondary windings 7. During the pre-heat period, inverter 3 is operated at a high switching frequency, well above the resonant frequency of the resonant circuit 5. Consequently, a relatively low AC voltage is generated by the inverter 3 and an associated pre-heat voltage is coupled from the inductive component 6 to each of the secondary windings 7.
Unfortunately, while ballast circuit 1 is very useful to operate high impedance gas-discharge lamps 4, the pre-heat voltage continues to be coupled to the lamp filaments 4A after the lamp filaments 4A have been ignited. This reduces the efficiency of ballast circuit 1 during steady state operation and may cause an over current problem. This is particularly troublesome for high impedance lamps which are particularly sensitive to over-current problems.
The prior art ballast circuit 1 utilizes capacitors C1, C2, and C3 to reduce the magnitude of the pre-heat voltage during steady state operation. However even then, the pre-heat voltage causes unacceptable inefficiencies and lamp pin current problems when operating high impedance gas-discharge lamps 4. Furthermore, the selection of capacitance values for capacitors C1, C2, and C3 requires a delicate balance between providing a pre-heat voltage at a high level during the pre-heat period to pre-heat the filaments 4A and maintaining the pre-heat voltage at a low level during steady-state operation to not cause over-current problems. This makes the sizing of capacitors C1, C2, and C3 particularly difficult and sometimes impractical.
The prior art solves this problem by coupling the prior art ballast circuit 1 to expensive filament shutdown circuits which significantly increase the cost of the ballast circuit 1.
What is needed, then, is a filament cutback circuit that will reduce the pre-heat voltage to the lamp filament after lamp ignition without significantly increasing the cost of the ballast circuit.
This invention is directed to a ballast circuit that reduces a pre-heat voltage to lamp filaments of gas discharge lamps after the lamp filaments have been pre-heated. The ballast circuit does not require expensive electronic components and is capable of reducing the pre-heat voltage to acceptable levels during steady state operation.
The ballast circuit may include an inverter with switch devices that convert a DC voltage into a pulsed voltage. A resonant circuit filters the pulsed voltage into the required AC voltage for powering the gas-discharge lamps. A pre-heat circuit is coupled to transmit a pre-heat voltage to the lamp filaments. The pre-heat circuit may include a primary pre-heat winding coupled to a resonant inductive component to receive the pre-heat voltage and may be magnetically coupled to secondary pre-heat windings which couple the pre-heat voltage to the lamp filaments of the gas-discharge lamps.
After the lamp filaments have been pre-heated, the ballast circuit attempts to ignite the lamps. A filament cutback circuit senses the ignition of the lamps and suppresses the pre-heat voltage during steady-state operation. To accomplish this, the filament cutback circuit may include a cutback circuit inductive component magnetically coupled to the resonant inductive component and receive a filament cutback control voltage associated with the AC voltage. This filament cutback control voltage charges a chargeable component to a switch threshold level during lamp ignition. Once the chargeable component reaches the switch threshold level, a switch device may be placed into a conducting switch state which causes the filament cutback circuit to reduce the pre-heat voltage. The chargeable component may be maintained above the switch threshold level during steady state operation. In this manner, the filament cutback circuit suppresses the pre-heat voltage after pre-heating the lamp filaments to eliminate the inefficiencies caused by the over current pin problem during steady state operation.
Referring now to
Inverter 20 receives a DC voltage 24 at V_rail and converts the DC voltage 24 into an AC voltage 22 that powers the lamps 18. Inverter 20 utilizes an inverter drive circuit (not shown), inverter switch devices, Q1, Q2, and a resonant circuit 26 that includes a resonant inductive component, L_resonant, and a capacitive resonant component C_resonant. Resonant circuit 26 may be tuned to the appropriate frequency for powering the gas discharge lamps 18. In this particular embodiment, the resonant circuit 26 is coupled between the inverter switches Q1, Q2 at terminal 25. As is known in the art, inverter switches Q1, Q2 are switched at a switching frequency to generate a pulsed voltage 25A.
DC blocking capacitor, C_dc_block, blocks the DC components of the pulsed voltage 25A. Resonant circuit 26 then filters the pulsed voltage 25A to provide an AC voltage 22 at the appropriate frequency for powering the gas discharge lamps 18.
To pre-heat the lamps 18, a pre-heat voltage 12 is received by the lamp filaments 16. In this embodiment, the lamp filaments 16 are connected in series. As shown in
During the pre-heat period, the pre-heat circuit 11 receives the pre-heat voltage 12 from the resonant inductive component 28 in the inverter 20. Consequently, the pre-heat voltage 12 is associated with a voltage across the resonant inductive component L_resonant, and is related to AC voltage 22. Upon receiving the pre-heat voltage 12, the pre-heat circuit 11 couples the pre-heat voltage 12 to the lamp filaments 16, utilizing a pre-heat component which in this embodiment is primary pre-heat winding, T_preheat. Secondary pre-heat windings T_preheat_A, T_preheat_B, and T_preheat_C, are magnetically coupled to primary pre-heat winding T_preheat to receive pre-heat voltage 12. Secondary pre-heat windings T_preheat_A, T_preheat_B, and T_preheat_C are coupled to the lamp filaments 16 which are pre-heated by the pre-heat voltage 12.
A control circuit (not shown) may be utilized to control the switching frequency of switches, Q1, Q2 in the inverter 20. When the inverter 20 is pre-heating the lamp filaments 16, the switching frequency is relatively high and away from the resonant frequency of resonant circuit 26. Accordingly, the AC voltage 22 is small and the pre-heat voltage 12 is coupled via primary pre-heat winding T_preheat to secondary pre-heat windings T_preheat_A, T_preheat_B, and T_preheat_C. A pre-heat capacitor, C_preheat, is provided between the resonant inductive component L_resonant and the primary pre-heat winding, T_preheat, so that the pre-heat voltage 12 is substantially constant during the pre-heat period. Pre-heat capacitor C_preheat is also provided to keep the pre-heat voltage 12 to the primary pre-heat winding, T_preheat, relatively low. This allows a reduction in the size of the ballast circuit 10 which helps reduce cost.
A filament cutback circuit 40 may be coupled across the primary pre-heat winding, T_preheat. Filament cutback circuit 40 includes a switch control circuit 45 that receives a filament cutback control signal 44 utilizing a cutback circuit inductive component, L_resonant_1, magnetically coupled to the resonant inductive component, L_resonant. Filament cutback control signal 44 charges a chargeable component, which in this case, is capacitor C4. Resistors R1, R2 behave as a voltage divider so that the voltage level of filament cutback control signal 44 is maintained at a desired level across the capacitor C4.
Filament cutback control signal 44 is associated with the AC voltage 22, and may even be the AC voltage 22 itself. In addition, filament cutback control signal 44 may lead or lag the AC voltage 22 and may have different voltage levels than the AC voltage 22. However, the voltage level of the filament cutback control signal 44 should follow the same pattern as the level of the AC voltage 22 to the filament cutback circuit 40 and may be the AC voltage 22 itself. In this embodiment, the filament cutback control signal 44 is related to a voltage 30 across the resonant inductive component, L_resonant. The voltage level of the filament cutback circuit 40 can also be controlled via the turns ratio from the resonant inductive component, L_resonant, and cutback circuit inductive component, L_resonant_1. This helps assure that low voltage components can be utilized by the ballast circuit 10 and reduces the cost of the filament cutback circuit 40.
Upon receiving the filament cutback control signal 44 from the resonant inductive component, L_resonant, a rectifying diode D5 transmits only a single half-cycle of the filament cutback control signal 44 which charges the capacitor C4. In the illustrated embodiment, diode D5 is a fast switching diode. During the non-charging half cycle of the filament cutback control signal 44, the capacitor C4 may at least partially discharge through resistor R2.
Capacitor C4 is coupled to the switch device, Q_cutback, at switch state input terminal 52. A charging characteristic of the chargeable component is selected so that the chargeable component does not reach a switch threshold level during the pre-heat period. In this case, the charging characteristic is the capacitance value of capacitor C4.
During the pre-heat period, the switch device 40 is off and therefore does not inhibit the coupling of the pre-heat voltage 12 to the secondary pre-heat windings T_preheat_A, T_preheat_B, and T_preheat_C. After the pre-heat period, ballast circuit 10 ignites the gas discharge lamps 18. To accomplish this, the switching frequency of switches Q1, Q2 is lowered so that the AC voltage 22 operates at or above a lamp ignition power level. This causes filament cutback control signal 44 to be received at a high voltage level which charges capacitor C4 at or above the switch threshold level. Upon reaching the switch threshold level, the switch state input terminal 52 places the switch device, Q_cutback, into a conductive switch state and the primary pre-heat winding, T_preheat is shorted, thereby suppressing the pre-heat voltage 12 to the secondary pre-heat windings T_preheat_A, T_preheat_B, and T_preheat_C.
The capacitance of the capacitor C4 may be selected so that the switch device Q_cutback is maintained at or above the switch threshold level after lamp ignition and during steady-state operation. In this manner, the pre-heat voltage 12 to the lamp filaments 16 is reduced after lamp filaments 16 have been pre-heated. This operates a high impedance lamp 18, such as a T5 model gas-discharge lamp, with improved efficiency and reduces the pin current unbalance problem caused by the pre-heat voltage 12 during steady-state operation.
It should be understood however that other embodiments of the filament cutback circuit 40 may suppress the pre-heat voltage 12 when the switch device 40 is in a non-conducting switch state. For example, the switch device, Q_cutback, may be connected in series with the primary pre-heat winding, T_preheat, and suppress the pre-heat voltage 12 when turned off.
In addition, filament cutback circuit 40 may reduce the pre-heat voltage 12 when coupled to other pre-heat components utilized to couple the pre-heat voltage 12 to the lamp filaments 16. For example, if the filament cutback circuit 40 is coupled across secondary pre-heat winding, T_preheat_A, filament cutback circuit 40 suppresses the pre-heat voltage 12 to this individual lamp filament 16A.
In the embodiment of
However, the use of a primary pre-heat winding, T_preheat, and secondary pre-heat windings T_preheat_A, T_preheat_B, and T_preheat_C, are not required for the operation the filament cutback circuit 40. For example, if the ballast circuit 10 is coupled to a single lamp filament 16A in a solitary lamp, then the secondary pre-heat windings, T_preheat_A, T_preheat_B, and T_preheat_C, of the solitary lamp 18 may be magnetically coupled to resonant inductive component L_resonant instead of the primary pre-heat winding, T_preheat. Filament cutback circuit 40 could then be coupled across the secondary pre-heat winding at the solitary lamp filament 16A and would control the pre-heat voltage 12 across the single lamp filament 16A. In fact, filament cutback circuit 40 may be coupled to any pre-heat component in the pre-heat circuit 11 so long as filament cutback circuit 40 is arranged in a manner that reduces the pre-heat voltage 12 to a desired lamp filament(s) when the switch device, Q_cutback, is activated.
Referring again to
To assure that low voltage components may be utilized with the filament cutback circuit 40, terminal 40A of the pre-heat circuit 11 may be coupled to a tap 72 on the resonant inductive component, L_resonant. This tap 72 is positioned on the resonant inductive component, L_resonant, such that the diodes, D1, D2, D3, D4 and switch device, Q_cutback may be low voltage components. The use of low voltage components helps reduce the cost of the filament cutback circuit.
The table below shows parameter values for one embodiment of the ballast circuit 10 in
By using the design parameters shown above, the pre-heat voltage 12 is approximately 8 volts during the pre-heat period and approximately 0.3 volts during steady-state operation. This 0.3 volt pre-heat voltage 12 is not coupled from the primary pre-heat winding, T_preheat, but instead is naturally caused by the lamp current during steady-state operation. The equivalent impedance of the filament cutback circuit 40 is approximately 10.6 k′Ω during the pre-heat period. This is relatively low when compared to previous filament shutdown circuit designs. The equivalent impedance is even smaller during steady state operation. Because of this relatively low impedance, the magnetic core coupling the primary pre-heat winding T_preheat and secondary pre-heat windings T_preheat_A, T_preheat_B, and T_preheat_C can be relatively small. This further reduces the cost of the ballast circuit 10.
Referring to
Filament pre-heat voltage 12 is thus reduced whenever the switch device Q_cutback is placed into the conducting state. Filament cutback circuit capacitor C5 may have a capacitance value that is at least 50 times greater than the capacitance value of the pre-heat capacitor C_preheat. In this manner, rectifier 70 and diodes D1, D2, D3, and D4 are not required which further reduces the cost of the filament cutback circuit 40.
Thus, although there have been described particular embodiments of the present invention of a new and useful BALLAST CIRCUIT FOR A GAS DISCHARGE LAMP THAT REDUCES A PRE-HEAT VOLTAGE TO THE LAMP FILAMENTS DURING LAMP IGNITION 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.
This application claims benefit of the following patent application(s) which is hereby incorporated by reference: U.S. Provisional Application No. 61/168,876, filed Apr. 13, 2009, entitled “Ballast Circuit for a Gas Discharge Lamp that Reduces a Pre-Heat Voltage to the Lamp Filaments During Lamp Ignition.”
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