Ballast with relamping circuitry

An apparatus for detecting the reconnection of a lamp filament to an electronic ballast driving a fluorescent lamp. A control circuit controls an inverter circuit to providing power to a filament control circuit. The filament control circuit preheats and powers the lamp filament of the one or more lamps. A pulse generator generates an input signal as a function of the number of lamp filament connected to the filament control circuit. A current sensor generates a first voltage indicative of whether a lamp filament has been reconnected to the circuit. A peak detector generates a peak voltage signal when the first voltage indicates a reconnection of a lamp filament has occurred. A sensing circuit generates a command signal to provide to the control circuit to supply power to the filament control circuit to preheat and power the lamp when the peak detector generates the peak voltage signal.

TECHNICAL FIELD

The present invention relates to ballasts for powering gas discharge lamps. In particular, the invention relates to an electronic ballast for powering multiple series-connected fluorescent lamps having filaments connected in parallel. The ballast includes relamping circuitry for detecting the reconnection of a lamp filament in order to energize the reconnected lamp.

BACKGROUND OF THE INVENTION

Electronic ballasts for gas discharge lamps are often classified into two groups according to how the lamps are ignited: (1) a preheat type ballast; and (2) an instant start type ballast. In preheat ballasts, the lamp filaments are preheated at a relatively high level (e.g., 7 volts peak) for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts peak) is applied across the lamp in order to ignite the lamp. In instant start ballasts, the lamp filaments are not preheated, so a higher starting voltage (e.g., 1000 volts peak) is required in order to ignite the lamp. It is generally acknowledged that instant start operation offers certain advantages, such as the ability to ignite the lamp at a lower ambient temperatures and greater energy efficiency (i.e., light output per watt) due to no expenditure of power on filament heating during normal operation of the lamp. On the other hand, instant start operation usually results in considerably lower lamp life than preheat operation.

Because a significant amount of power can be unnecessarily expended heating the lamp filaments after the lamp is ignited, it is desirable to have preheat type ballasts in which filament power is minimized or eliminated once the lamp has ignited. One approach for preheating ballasts employs switching circuitry such as a filament control circuit that disconnects the source of filament power from each of the filaments after the lamp ignites. However, when such switching circuitry is used with ballasts driving multiple fluorescent lamps, there have been problems preheating and igniting lamps which have been disconnected from the ballast and then reconnected back to the ballast. One solution to ignite such reconnected lamps has been to cycle the power supplied to the ballast (i.e., turn the power off, and then back on).

In ballast circuits driving three (3) or more lamps, the outermost lamps are usually connected directly to the ballast circuit. Thus, disconnecting the outer lamps may cause an open circuit which can be detected. When an outer lamp is reconnected, it closes the circuit so that preheating and/or ignition can be initiated. However, the inner lamps, such as the middle lamp in a three lamp circuit, are connected with one or more of the outer lamps but are not directly connected to the ballast circuit. Hence, removing and reconnecting an inner lamp may not close an open circuit so that its reconnection is difficult to detect. Accordingly, re-igniting a disconnected and reconnected inner lamp has typically required cycling of the power. To avoid the need for cycling the ballast power when an inner lamp of a plurality of lamps connected to the ballast circuit is taken out and then reconnected to the circuit, there is a need for a ballast circuit that detects the reconnection of an inner lamp to preheat and/or ignite the reconnected lamp without requiring cycling of the power.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a ballast circuit is provided for detecting the reconnection of a lamp filament to a power bus in an electronic ballast driving a fluorescent lamp. The ballast circuit includes an inverter control circuit that controls an inverter circuit to provide power to the power bus. The ballast circuit also includes a filament control circuit that interconnects the power bus and the lamp filament to preheat and power the lamp filament and to inhibit the inverter circuit when sensing that the lamp filament has been disconnected from the filament control circuit. The ballast circuit also includes a pulse generating circuit coupled to the lamp filament that generates an input signal indicative of a reconnection of the lamp filament to the filament control circuit. The ballast circuit further includes a detection circuit coupled to the pulse generating circuit that detects the reconnection of the lamp filament and is operative to produce a command signal that is provided to the inverter control circuit to cause the inverter circuit to supply power to the filament control circuit to preheat the lamp filament and supply power to the lamp.

In accordance with another aspect of the invention, a detection circuit is provided for detecting the reconnection of a lamp filament in an electronic ballast that includes a filament control circuit for preheating and powering lamp filaments of a plurality of fluorescent lamps. The ballast includes an inverter control circuit that controls an inverter circuit to provide an AC voltage signal to power the filament control circuit to preheat and power each lamp filament of the plurality of lamps. The ballast also includes a pulse generating circuit coupled to the plurality of lamps to generate an input signal indicative of a reconnection of one of the lamp filaments to the filament control circuit. The ballast also includes a current sensor that is connected to the pulse generating circuit and responsive to the input signal for generating an input voltage signal that has a first magnitude when the filament is disconnected from the filament control circuit and has a second magnitude when the filament is reconnected to the filament control circuit. The ballast also includes a peak detection circuit connected to the current sensor that senses a magnitude of the input voltage signal, and generates a detected voltage signal as a function of the sensed magnitude of the input voltage signal. The detected voltage signal has a peak magnitude when the input voltage signal has the second magnitude. The ballast further includes a sensing circuit connected to the peak detection circuit that senses a magnitude of the detected voltage signal, and generates a command signal that is provided to the inverter control circuit to supplying power to the filament control circuit to preheat and power the lamp filament when the detected voltage signal has the peak magnitude.

In accordance with yet another aspect of the invention, a method is provided for detecting the reconnection of a lamp filament to a power bus in a ballast circuit driving a fluorescent lamp. The method includes supplying an alternating current (AC) signal to the lamp via an inverter circuit. The method also includes preheating and powering the lamp filament when the lamp filament is connected to the power bus. The method also includes generating an input signal that has a first magnitude when the lamp filament is disconnected from the power bus and has a second magnitude when the lamp filament is reconnected to the power bus. The method also includes generating a detection signal as a function of the magnitude of the generated input signal. The generated detection signal has a peak magnitude when the generated input signal has the second magnitude. The method further includes supplying the AC signal to preheat and power the lamp filament when the detection signal has the peak magnitude.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1illustrates a ballast circuit100for powering a plurality of gas discharge lamps102,104,106. The ballast circuit100includes a control circuit107connected to and controlling an inverter108to supply power to output terminals110,112,114,1.15,116,117via an isolation transformer122, and via a filament control circuit124.

The inverter108receives a substantially direct current (DC) input voltage, VDC, from a DC bus125via input terminals126,128, and is responsive to a control signal129from the control circuit107to provide an alternating current (AC) output voltage at output bus terminal130for powering the lamps102,104,106. The DC input voltage can be provided from a DC source (not shown) such as a rectified input AC source, a battery, or any other source of DC power. As known to those skilled in the art, the AC output voltage at inverter output bus terminal130has a high frequency (e.g., 20,000 hertz or greater) at or near to the natural resonant frequency of an inductor119and a capacitor120of a resonant tank circuit121connected to the inverter108, the isolation transformer122, and circuit ground133.

The isolation transformer122provides the increased voltage necessary for igniting the lamps102,104,106and minimizes power dissipation. The isolation transformer122includes a primary winding132connected between the inductor119and capacitor120and connected to circuit ground133via a DC blocking capacitor123such that the primary winding132and the DC blocking capacitor123form a series combination that is connected in parallel with the capacitor120of the resonant tank circuit121. A secondary winding134of the isolation transformer122outputs the increased voltage via terminals110,112. More specifically, the isolation transformer122is responsive to the AC output voltage at bus terminal130, and the resulting voltage across the capacitor120, to provide an increased AC output voltage at the secondary winding134for preheating and/or igniting the lamps102,104,106.

The filament control circuit124coupled to bus terminal130supplies a preheat voltage to lamps102,104,106via output terminals114,115,116,117to preheat the filaments135–140of lamps102,104,106. As explained above, in order minimize the amount of power expended on heating lamp filaments in a preheat ballast, it is desirable to preheat lamp filaments prior to ignition by supplying a preheat voltage during a preheat mode in which the voltage applied across each of the lamps is substantially less than an ignition voltage required to ignite the lamp. Output terminals110,112,114,115,116,117and input terminal126are adapted for connection to the filaments135–140of the lamps102,104,106. More specifically, input terminal126is connected to a connection141of a first filament135of lamp102via a current limiting resistor142, and output terminal110is connected to a connection143of the first filament135of lamp102. Output terminals116,117are connected to a second filament138of lamp102via connections144,146, respectively, and to a second filament139of lamp104via connections148,150. Output terminals114,115are connected to a first filament136of lamp104via connections152,154, and to a first filament137of lamp106via connections156,158. A second filament140of lamp106is connected to output terminal112via connection162, and to circuit ground133via a current limiting resistor159and connection160. Thus, as can be seen, the first filament135of lamp102is connected in series with output terminal110and input terminal126via resistor142. The first filament136of lamp104is connected in parallel with the first filament137of lamp106via output terminals114,115. The second filament138of lamp102is connected in parallel with the second filament139of lamp104via output terminals116,117. The second filament140of lamp106is connected in series with the secondary winding134of the isolation transformer122.

In this particular circuit100, the filament control circuit124is configured to provide the preheat voltage to filaments136–139. For example, the preheat voltage produced across output terminals114,115preheats filaments136and137of the second and third lamps104,106, and the preheat voltage across output terminals116,117preheats filaments138and139of the first and second lamps102,104. After filament preheat is complete, the filament control circuit124shuts down, and only re-activates when power to ballast circuit100is cycled.

In operation, when either of the outer lamps102,106are removed an open circuit occurs between terminals126and141or between terminal160of the filament140of the outer lamp106and ground133. This causes the voltage across the resistor159to fall to zero. When the outer lamps102,106are reconnected, the circuit is closed. As such a voltage appears across the resistor159which can be used to re-trigger the control circuit107to start the ballast again. Notably, resistor159should be of sufficiently high value so that the isolation between the input and the output because of the presence of isolation transformer122remains substantially unaffected. However, as noted above, the filaments136,139of middle lamp104are connected in parallel with the filaments137,138of lamps106,102, respectively. Because of this parallel connection, the control circuit107cannot detect an open circuit when lamp104is removed or a closed circuit when lamp104is reconnected. As explained in more detail below in reference toFIG. 2, the filament control circuit124includes a shut down circuit164responsive to the removal of the any of the three lamps to generate a fault signal166. The control circuit107connected to the shut down circuit164is responsive to the fault signal166to shut down the inverter108. As a result, minimal, if any, voltage is present across the primary and secondary windings132,134of the isolation transformer122, and, thus, the filament control circuit124is de-energized and shuts down. The middle lamp104becomes a floating system. That is, even when the middle lamp104is reconnected, the filament control circuit124remains deactivated until the ballast power is cycled.

Referring now toFIG. 2, a combination block and schematic diagram illustrates components of a ballast circuit200according to one embodiment of the invention. As described above, the inverter circuit108is responsive to an input DC voltage signal received via input terminal126,128to generate an output AC voltage signal, as indicated by reference character204, for powering the lamp filaments135–140via a filament control circuit206(e.g., filament control circuit124inFIG. 1). In this embodiment, the inverter circuit108includes switching transistors such as MOSFETs208,210, connected between DC input terminals126,128. MOSFETs208,210are driven by first and second control signal212,214, respectively, supplied from a control circuit216(e.g., control circuit107inFIG. 1) to generate the output AC voltage signal204. The control circuit216can be a L6569 Half Bridge Driver manufactured by STMicroelectronics of Plan les Ouates, Geneva, Switzerland.

A drain218of the MOSFET208is coupled to input terminal126. A gate220of the MOSFET208connected to the control circuit216is responsive to the first control signal212generated by the control circuit216to turn the MOSFET208on and off. For example, when the magnitude of the first control signal212is equal to or greater than a threshold voltage (i.e., first control signal has at least a minimum magnitude), the MOSFET208turns on and positive current flows through the ballast circuit200via a power bus222. A drain218of the MOSFET210is coupled to a source224of MOSFET208. A gate220of the MOSFET210connected to the control circuit216is responsive to the second control signal214generated by the control circuit216to turn the MOSFET210on and off. For example, when the magnitude of the second control signal214is equal to or greater than a threshold voltage (i.e., second control signal has at least a minimum magnitude), the MOSFET210turns on and negative current flows through the circuit via power bus222. By selectively activating MOSFETs208,210in an alternating fashion, the control circuit216causes the inverter circuit108to generate the output AC signal to preheat, ignite and operate lamps102,104,106.

As described above, the filament control circuit206provides a preheat voltage to the filaments136–139to preheat the lamps102,104,106prior to ignition. In this embodiment, the filament control circuit206includes a second transformer225, a capacitor226, a switching device228(e.g., a MOSFET), and a diode230. The second transformer225has a primary winding232, a first auxiliary winding234, and a second auxiliary winding236. The primary winding232is connected to the inverter circuit108and circuit ground133, via capacitor226and the switching device228, and is responsive to the output AC voltage signal204from inverter108to generate the preheat voltage across each of the first and second auxiliary windings234and236. The MOSFET228is connected between the capacitor226and circuit ground133. More specifically, a drain238of the MOSFET228is connected to capacitor204and a source240of the MOSFET228is connected to circuit ground133. A pulse generator241supplies a pulse signal242to a gate244of the MOSFET228to turn the MOSFET228on and off. For example, the pulse generator241is configured to generate the pulse signal242when the DC input voltage between input terminals126,128reaches a threshold value. When the pulse signal242is supplied to the gate244of the MOSFET228, the MOSFET228turns on and current flows thru the primary winding232of the second transformer224. As a result, current flows through each of the first and second auxiliary windings234,236producing the preheat voltage across each of the first and second auxiliary windings234,236.

The filaments138and139of the first and second lamps102,104, respectively, are connected in parallel with each other, via connections144,146and connections148,150, respectively, and with the first auxiliary winding234. The filaments136and137of the second and third lamps104,106, respectively, are connected in parallel with each other, via connections152,154and connections156,158, respectively, and with the second auxiliary winding236. When the pulse signal242being applied to the gate224of MOSFET228is removed, the MOSFET228turns off and current stops flowing to the primary winding232of the second transformer225, and, thus, no voltage is generated across the first and secondary auxiliary windings234,236. Notably, as illustrated in phantom lines, the filament control circuit206may also include third and fourth auxiliary windings245,246for preheating the remaining filaments135and140of outer lamps102,106, respectively. However, for purposes of illustration the filament control circuit108is described herein as supplying a preheat voltage to filament138of outer lamp102, to filament137of outer lamp106, and to filaments136,139of middle lamp104.

The shut down circuit164includes a current sensing resistor247, and generates a fault signal248representative of the voltage drop across the resistor247. The control circuit216connected to the shutdown circuit164is responsive to the fault signal248(e.g., fault signal166inFIG. 1) having a magnitude greater than a specified value (e.g., 1V) to shut down the ballast200. For example, as known to those skilled in the art, when any one of the lamps102,104,106is removed from the circuit200, the MOSFETS208,210go into hard switching. As a result, the current through the inverter108increases resulting in current spikes within the ballast circuit200. These current spikes cause the voltage drop across resistor247to increase beyond the specified value. The control circuit216is responsive to the increased voltage to inhibit operation of the inverter circuit108by preventing control signals212and214(i.e., gate-drive signals for MOSFETs208,210) from being supplied to the inverter circuit108. This terminates AC power from being supplied to the lamps102,104,106.

According to the present invention, a detection circuit252connected to the filament control circuit206and the control circuit216is responsive to an input signal indicative of the reconnection of one or more lamps102,104,106to generate a command signal254provided to the control circuit216to override the fault signal248to operate the inverter108without cycling of the power to the ballast.

Referring now toFIG. 3, a schematic diagram illustrates components of a detection circuit252of the ballast circuit200for detecting the disconnection and reconnection of any of lamps102,104,106according to one embodiment of the invention. In this particular embodiment, the detection circuit252senses a magnitude of an input voltage signal generated within the ballast circuit200and generates the command signal254provided to the control circuit216as a function of the magnitude of the sensed voltage.

A pulse generating circuit300connected to the filament control circuit206and the lamps102,104,106generates an input signal, as indicated by reference character301, indicative of a disconnection or reconnection of a lamp filament from the filament control circuit206. The pulse generating circuit300includes a pulse transformer302, having a primary winding304and first and second auxiliary windings306,308. The primary winding304is connected to a second pulse generator310supplying a pulse signal312of high frequency. The pulse transformer302is responsive to the pulse signal312supplied to the primary winding304to generate an output voltage across each of the first and second auxiliary windings306,308. The pulse generator310is, for example, an astable multivibrator 555 timer capable of providing a high frequency voltage signal. The first and second auxiliary windings306,308of the pulse transformer302are connected in series with the first and second auxiliary windings234,236, respectively, of the filament control circuit206(seeFIG. 2). As a result of the output voltage signal generated across the first and second auxiliary windings306,308, a current is continuously supplied to filaments138and139of the first and second lamps102,104and to filaments136and137of the second and third lamps104,106. As known to those skilled in the art, when a circuit includes resistive elements (e.g., filaments) connected in parallel, and one of the resistive elements is removed, the effective resistance of the circuit increases. FromFIG. 3it can be seen that the filament138of lamp102is connected in parallel with the filament139of lamp104. Accordingly, if filament139of lamp104is disconnected, the current through first auxiliary winding306of pulse transformer302is reduced because the corresponding effective resistance on the secondary side increases. When the second filament139of lamp104is reconnected, the corresponding effective resistance on the secondary side decreases, and, thus, current through the first auxiliary winding306increases.

As a result of the current supplied to the first and/or second auxiliary windings306,308, there is a reflected current (e.g., input signal301) in the primary winding304. The primary winding304of the pulse transformer302is connected to output terminal316of the pulse generator310and circuit ground133via a filtering capacitor318and a current sensing resistor320(e.g., current sensing resistor247ofFIG. 2). Thus, the magnitude of the current flowing through the resistor320corresponds to the number of filaments connected to the filament control circuit206. For example, if the filament139of lamp104is disconnected, the current through resistor320is reduced, and, thus, the voltage drop across resistor320decreases. When the filament139of lamp104is reconnected, the reflected current onto the primary winding304increases resulting in an increased voltage drop across the current sensing resistor320.

A peak detector circuit322connected to the current sensing resistor320detects when the voltage drop across the current sensing resistor320increases. In this embodiment, the peak detector322includes a first operational amplifier (opamp)324having a first input terminal (non-inverting terminal)326, a second input terminal328(inverting terminal), and an output terminal330. The non-inverting terminal326is connected to the filtering capacitor318and the current sensing resistor320via an input resistor332. The inverting terminal328is tied to the output terminal330so that the first opamp324acts as a voltage follower. Thus, the first opamp324receives an input voltage at the non-inverting terminal326determined as a function of the magnitude of the voltage drop across the current sensing resistor320, and is responsive to the input voltage at the non-inverting input terminal326to generate an output voltage signal Vout, as indicated by reference character334. In other words, the output voltage signal334follows the voltage across the current sensing resistor320. A diode336connected to the output terminal330is forward biased by the output voltage signal334and charges a capacitor338. The capacitor338continues to charge until the inverting and non-inverting terminals are at same voltage. In other words, when the voltage at the non-inverting input terminal326exceeds the voltage at the inverting input terminal328, the capacitor338continues to charge until the voltage across the capacitor338is equal to the input voltage at the non-inverting input terminal326. Because the output voltage signal334follows the voltage across the current sensing resistor320, the voltage across capacitor338decreases when a filament is removed and increases (i.e., peaks) when a filament is connected.

A sensing circuit340connected to the peak detection circuit322is responsive to the output voltage signal334to generate a command signal341(e.g., command signal254ofFIG. 2) provided to the control circuit216to control operation of the inverter circuit108. The sensing circuit340includes a second operational amplifier (opamp)344having a first input terminal (non-inverting terminal)346, a second input terminal348(inverting terminal), and an output terminal350. In this particular embodiment, the first and second opamps324,344include positive voltage input terminals351,352, respectively, that are tied together and connected to a DC voltage source349(e.g., 15 volt DC source), and negative voltage input terminals353,354that are both connected to ground133. The non-inverting terminal346is connected to the peak detector322via a resistor network355. The resistor network355comprises resistors356,357connected in series with each other and connected in parallel with resistors358,360. The values of the resistors356,357,358,360in the resistor network348determine the input voltages supplied to the non-inverting terminal346and the inverting terminal348. The inverting terminal344is connected to the peak detector322via the resistor network355, and a delay capacitor362connected in parallel with resistor360. The non-inverting terminal346and the inverting terminal348are connected to the resistor network355such that the effective resistance ultimately causes input voltage at the inverting terminal348to be greater than the input voltage at the non-inverting terminal346. However, the inverting terminal348is also connected to the capacitor362, which operates to delay this condition. That is, the delay capacitor362slowly charges so that the input voltage at the non-inverting terminal346is initially greater than the input voltage at the inverting terminal348, and the opamp316is responsive to the voltage difference to generate an output voltage signal, as indicated by341, having a peak magnitude (e.g., 5 volts), which is indicative of the reconnection of a filament. After a delay, the capacitor362charges so that the input voltage at the inverting terminal348becomes greater than the input voltage at the non-inverting terminal346, at which time the output voltage signal341(i.e., command signal) goes low (e.g., 0 volts). Thus, in operation the command signal341generated by the sensing circuit340can have two different states. For example, when the detection signal has a peak magnitude (i.e., filament connected), the command signal341generated by the sensing circuit340has a first state (e.g., peak magnitude). In contrast, when the detection signal has a minimum magnitude (i.e., filament disconnected), the command signal341generated by the sensing circuit340has a second state (e.g., low magnitude). The control circuit112is responsive to the command signal341having a peak magnitude to activate the MOSFETs208,210(SeeFIG. 2) to supply power to the lamps102,104,106. Notably, it can be seen that after implementation of this circuit200, resistors142and159(seeFIG. 1), which can be used to detect the removal of the outer lamps102,106can be eliminated from the circuit. That is, because the outer lamp filaments137,138are connected in parallel with the middle lamp filaments136,139, respectively, the re-lamping of the outer lamps102,106will also get detected in the same way as re-lamping of the middle lamp.