Abstract:
A fault protection circuit for fluorescent lamp ballasts cuts off ballast power to the lamp in response to fast rise-time changes but not slower changes in either current output or voltage output of the ballast. The fault protection circuit is compatible with different ballast circuit topologies and features a floating reference level which renders the circuit insensitive to the lamp load wattage, making the circuit particularly useful in so-called universal ballasts.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to the field of ballast circuits for powering arc discharge lamps, and is more particularly directed to a fault protection circuit for turning off ballast output power to the lamp load in the event of lamp malfunction. 
     2. State of the Prior Art 
     Arc discharge lamps such as fluorescent lamps require relatively high operating voltages, particularly when first initiating the arc discharge in the lamp. Once started, the lamp current must be externally limited to a normal level because arc lamp impedance characteristically drops after start-up. Many so called ballast circuits have been devised for supplying the necessary voltages and currents for powering such lamps. 
     Arc discharge lamps, for example fluorescent lamp tubes, are subject to certain types of malfunction characteristic of this type of lamp. If the lamp tube is loosened from its electrical socket the lamp connectors or pins may become sufficiently exposed to sustain an arc discharge between the exposed metal parts. Such an arc discharge is undesirable not only because it can severely stress the electrical components of the ballast circuit, but may also pose a fire and electrical shock hazard. Under such circumstances, it is, of course, desirable to shut down electrical power to the lamp immediately. Another type of lamp malfunction occurs with aging of the discharge lamp, manifested by partial rectification of the A.C. current delivered by the ballast to the lamp load. In this case too it is desirable to interrupt electrical power to the lamp load in order to call the attention of maintenance personnel to the need for replacing the lamp. Ballast output protection is particularly needed in arc discharge lamp fixtures installed in aircraft cabin lighting. Aircraft vibration may shake individual lamps loose from their sockets exposing the metal contacts and resulting in arcing. Electrical arcs are clearly undesirable in an environment exposed to combustible fumes from jet fuel. 
     Various schemes have been devised for sensing abnormal operating conditions of the lamp load in arc discharge fixtures and shutting down the output of the ballast in such event. For example, it is known that certain failure modes, particularly arcing, result in abnormally high transient voltages and currents through the lamp load. This knowledge has been exploited in the past to design protection circuits responsive to such higher than normal voltages or currents. In one known type of protection circuit a voltage comparator has one input connected for sensing the lamp voltage and the other input referenced to a fixed, preset voltage threshold. The output of the comparator is connected for switching off the ballast output in the event that the lamp voltage exceeds the treshhold voltage. This type of output protection circuit works well in ballasts intended for use with a known lamp load, i.e. a given lamp wattage. If the lamp wattage is known in advance, then the normal operating lamp voltage can be anticipated and a reliable reference voltage can be preset for the comparator circuit. 
     The conventional idea of sampling the output voltage or current of the lamp load and comparing that information against a fixed reference for sensing an over voltage or over current condition is sound. However, the use of a fixed reference will not work when the output lamp load changes, or the ballast is designed for bright and dim lamp operation. This shortcoming is particularly evident with so called universal ballasts designed to supply a range of lamp loads, allowing an end user to install lamps of different wattage as needed. Universal ballasts simplify inventories of electrical supply vendors and facilitate ballast replacement in installed light fixtures, and have come into widespread usage. 
     What is needed is an output protection circuit which can sense abnormal or unsafe operating conditions at the output of an arc discharge lamp ballast intended for operation with variable lamp loads. Such a protection circuit should be reliable, relatively simple and economical. 
     SUMMARY OF THE INVENTION 
     The aforementioned need is addressed by this invention which provides a floating reference fault detection circuit for arc discharge lamp ballasts useful with ballasts intended for supplying variable lamp loads. The improved fault detection circuit has a comparator with first and second inputs of opposite polarity and a comparator output, and a sampling circuit connected for deriving a sample signal related to the lamp voltage or lamp current delivered by the ballast output The sample signal is provided to both of the comparator inputs, and a biasing circuit is connected for holding one of the inputs at a bias above the sample signal relative to the other of the comparator inputs, thereby to hold the comparator output in a normal state. A discriminator circuit is connected for passing relatively fast rise-time waveforms only and not slower change sin the waveforms to the other of the comparator inputs, such that a relatively fast rise-time component waveform of sufficient amplitude in the sample signal overcomes the bias and changes the comparator output to a shut down state. The bias voltage is summed to the sample signal voltage at the comparator input and consequently provides a floating reference voltage independent of the absolute value of the voltage or current at the ballast output. As a result, abnormal fast rise-time components of sufficient amplitude of the sample signal can be reliably detected relative to the bias level irrespective of slower fluctuations and changes in lamp voltage or current, and irrespective of changes in lamp load wattage. 
     The sample signal may be a rectified A.C. signal derived from the voltage or current supplied at the ballast output to a lamp load. The sampling circuit may be a voltage divider, such as a resistive voltage divider, connected for sampling the alternating voltage at the ballast output and a full wave rectifier for rectifying the sampled voltage to derive the sample signal. Alternately, the sampling circuit may include a current transformer connected for sampling an alternating lamp current and a circuit for converting the sampled current to a sample voltage, which is then rectified by a full wave rectifier to derive the sample signal. The discriminator circuit may be a low pass filter, such as a low pass capacitor connected across; the comparator input to remove higher frequency components in the sample signal. The biasing circuit may be a resistive voltage divider connected for holding one of the comparator inputs at a bias voltage relative to the other comparator input. A latching diode may be connected between the comparator output and a comparator input for latching the output in the shut down state. 
     The bias between the comparator inputs may be fixed, but since it is added to the sample signal at the comparator inputs the bias provides a floating reference level substantially independent of the voltage or current through the lamp load. 
     It is contemplated that the fault protection circuit is to be part of a ballast unit, and therefore the invention contemplates the combination of the floating reference fault protection circuit with a ballast circuit intended for powering an arc discharge lamp load, to provide in combination an output protected ballast. 
     In a broader sense, the invention is a fault detection circuit for detecting relatively fast rise-time changes in either current or voltage output of the ballast, the fault detection circuit having an output connected for turning off electrical power from the ballast to the lamp load in response to the fast rise-time changes. The fault detection circuit includes a filter, such as a low pass capacitor, selected to pass waveforms representative of undesirable lamp conditions to an input, such as a comparator input of the fault detection circuit. One comparator input is held at a bias relative to the other comparator input, and the filter is connected such that the bias is overcome by fast rise-time changes of sufficient amplitude to derive a fault output by a change of state of the comparator output. 
     The invention also includes a method for protecting the output of a ballast circuit for powering a fluorescent lamp load. The novel method comprises the steps of sampling detecting relatively fast rise-time changes in current or voltage output of the ballast in excess of a predetermined amplitude and interrupting electrical power from the ballast circuit to the lamp load in response to relatively fast rise-time changes of sufficient amplitude substantially independently of the absolute value of the lamp current or lamp voltage. The sufficiency of the amplitude is predetermined by a bias level against which the fast rise-time changes are compared but not in response to slower changes or fluctuations in the current or voltage. 
     For greatest sensitivity to different modes of lamp failure, fast rise-time components in both lamp current and lamp voltage may be sampled against a floating bias or reference voltage, and a shut down output may be derived upon sensing either a fast over-voltage or over-current event in the sample signal against the floating reference. For example, two separate floating reference comparator circuits may be used, one comparator receiving a sampled current input and the other a sampled voltage input, and the outputs of the two comparators connected in a logic OR circuit to derive a single fault or ballast shut-down output. 
     The method for protecting the output of a ballast circuit for powering a fluorescent lamp load according to this invention may also be said to have the steps of deriving a sample signal related to the voltage or current across the lamp load, applying the sample signal as a first comparator input signal, deriving a second comparator input signal by filtering fast rise time waveforms from the sample signal and adding a bias level, comparing the first and second comparator input signals; and shutting off power to the lamp load if the first comparator input signal sufficiently exceeds the second comparator input signal. 
     These and other improvements, features and advantages will be better understood from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block circuit diagram illustrating a typical electronic ballast equipped with the fault protection circuit according to this invention; 
     FIG. 2 is a more detailed circuit diagram of the ballast output fault protection circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, FIG. 1 is a block diagram of a typical electronic ballast  10  supplied by a power source  12  and powering an arc discharge lamp load  14  connected to the ballast output  16 . The internal operation of the electronic ballast  10  need not be described in detail inasmuch as such units are known and understood. In general, the ballast circuit receives low voltage low frequency AC electrical power from power source  12  through an electromagnetic interference filter  18 . The AC power is converted to DC power by rectifier  20 , regulated by a high frequency power factor correction circuit  22  which cooperates with power factor controller  24 . High frequency AC power is delivered to the lamp load by a half-bridge output drive  26  through an output inductor/capacitor  28 . In effect, the ballast circuit delivers high frequency high voltage power to the arc discharge lamp load, makes a power factor correction to minimize adverse impact on the quality of the AC power delivered to other systems also supplied by power source  12 , and by means of EMI filter  18  keeps spurious signals generated by the ballast circuit from passing back to the power source and possibly interfering with operation of other devices. The power delivered to the lamp load at the ballast output  16  is typically high frequency A.C. such as 25 kHz. 
     The present invention improves upon the ballast circuit  10  by providing a floating reference fault detection circuit generally designated by numeral  30  in FIG.  1  and shown in greater detail in FIG.  2 . Turning now to FIG. 2, the fault detection circuit  30  includes a voltage comparator  32  which has a positive input  34 , a negative input  36 , and a comparator output  38 . The comparator is connected to a suitable voltage supply V cc  and a circuit ground Gnd. A resistive voltage divider comprised of resistors R 1 , R 2  is connected across the output  16  of the ballast  10 , i.e. across the lamp load  14 . The relative values of resistors R 1 , R 2  are such as to derive a lower voltage A.C. sample signal usable as an input to the fault protection circuit The R 1 , R 2  voltage divider may be connected across the ballast output, i.e. across the lamp load  14 . This sample signal input is rectified by full wave rectifier  40  to produce a high frequency single polarity rippling D.C. sample signal. The D.C. ripple output of the rectifier is connected to both the positive and negative inputs of the comparator. The positive comparator input receives the unfiltered output of rectifier  40 . The negative comparator input, however, receives the rectified sample signal filtered by low pass capacitor  42 . As a result, higher frequency components of the rectified sample signal are filtered out and removed before reaching the negative input of the comparator. These high frequency components do however, reach the positive input of the comparator. 
     A biasing circuit includes a resistive divider R 3 , R 4  connected to keep the negative input of comparator  32  at a bias voltage above the positive input. The bias voltage applied to the negative input is added to and floats on the filtered DC sample signal delivered by rectifier  40 . As a result, in a normal condition of the circuit  30 , i.e. in the absence of high frequency components of the sample signal, the negative input is at all times biased above the positive input and holds the comparator output  38  in a normal state. Diode  45  isolates the positive and negative comparator inputs from each other to maintain the bias level. 
     Upon occurrence of a lamp malfunction of the type mentioned earlier, resulting in higher voltage fast rise-time transients, these fast rise components are also rectified with the sample signal. The positive input receives the sample signal including the unfiltered and unmodified high voltage transients. The fast rise-time transients, however, are filtered out by the low pass capacitor  42  before reaching the negative input of the comparator, so that the negative input  34  receives the sample signal minus its higher frequency components. This condition upsets the voltage relationship between the two comparator inputs because the positive input voltage is raised to the extent of the transients filtered from the negative input As a result, a fast rise transient of sufficient amplitude, i.e. of amplitude exceeding the bias level of the negative input  34 , raises the voltage at the positive input  36  sufficiently to overcome the bias and causes the comparator output to change to a fault output. A latching diode  44  is connected between output  38  and the positive input  36  so as to latch and hold the comparator  32  in a fault state. As a result, the two possible states of the voltage comparator provide an indication of a normal operating condition of the lamp load or an abnormal, fault condition of the lamp load. These two states of the comparator output  38  are then used so as to turn off the output of the ballast  10  in the event of a fault condition. Ballast output may be turned off by any convenient means depending on the topology of the particular ballast circuit. In the case, of an electronic ballast with a power factor controller, the controller integrated circuit typically used for this purpose includes a control input which can be brought high or low to turn the ballast output on or off. If such a control input is unavailable, ballast output may be turned off by, for example, driving low the base of a suitably chosen output drive transistor. The precise manner in which the ballast output is controlled by the comparator output may be left to the designer of a given ballast system. It will be appreciated that the fault detection circuit  30  can be adapted for use with a wide range of arc discharge lamp ballasts of different topologies. 
     In an alternate form of the invention, the fault detection circuit  30  derives the sample input from ballast output current i.e. from the lamp current flowing through the lamp load  14 , rather than from the lamp voltage supplied by the ballast output  16 . In that case, a current transformer indicated as T 1  in FIG. 2 is used to sample the lamp current, and current sample is then applied to a suitable resistive divider, such as R 1 , R 2 , to convert the current sample to a voltage sample which is then processed as described earlier in connection with FIG.  2 . For current sampling the primary winding of T 1  may be connected to the ballast output  16  in series with the lamp load. 
     It has been found that different modes of lamp failure affect lamp voltage and lamp current differently. Some types of failure, for example, an intermittent connection between the lamp and its socket in the fixture may be detected by sensing the presence of voltage transients. On the other hand, immersion of the lamp in water seems better sensed by sampling transients in lamp current 
     For this reason, and for optimum sensitivity to different forms of lamp failure, two fault detection circuits  30 , one configured for sampling lamp current, the other configured for sampling lamp voltage are used and the comparator outputs  38  of the two circuits are connected in a logical OR configuration so as to derive a fault output in the eve nt of a change in either comparator output  38 , i.e. resulting from a sufficient transient in either voltage or current. In this OR configuration, an isolation diode  46  is connected in series with the output of each comparator in order to isolate the two fault detection circuits  30  from each other. 
     From the foregoing, it will be appreciated that the improved fault detection circuit relies upon the relatively fast rise-time of certain voltage o r current components sensed at the ballast output as a result of a faulty operating condition of the lamp load. The fast rise-time characteristic of these components is exploited in order to maintain a floating reference or bias at the comparator inputs  34 ,  36  which is largely independent from the absolute values of the normal operating lamp voltage or lamp current. During normal lamp operation, the lamp voltage and lamp current may fluctuate for various reasons, but such normal fluctuations are relatively slow. The value of low pass capacitor  42  is chosen so as to discriminate between the faster rise-time of the fault indicative transients and the slower normal fluctuations in the sample signal delivered to the comparator inputs. 
     Triggering of the comparator to a fault or shut-off output in circuit  30  therefore requires two conditions: a transient of sufficiently fast rise-time to be filtered by low pass capacitor  42 , and of sufficient amplitude to overcome the relative bias between the two comparator inputs. To some extent, these two requirements are interdependent since the extent of filtering by low pass capacitor  42  is a function of the rise-time of the transient, so that slower rise-time transients will be less diminished in amplitude by the capacitor  42 , and thus, the circuit will be somewhat less sensitive to slower transients because a smaller voltage difference will be induced between the two inputs by such transients. Very fast transients, on the other hand, may be entirely filtered out by capacitor  42 , inducing a greater voltage difference between the inputs  34 ,  36 . Consequently, faster transients will trigger tie fault output of the comparator at a smaller transient voltage than a slower transient. 
     While a particular embodiment of the invention has been described and illustrated for purposes of clarity and example, it must be understood that many changes, substitutions and modifications to the described embodiments will be apparent to those having no more than ordinary skill in the art without thereby departing from the invention as defined in the following claims.