Abstract:
A ballast for a fluorescent lamp incorporates in an integrated circuit (IC) complex circuit functions, such as driving a switching arrangement that provides AC power to the lamp. Beneficially, such IC&#39;s may be widely available and inexpensive. Additional circuitry complements such IC by protecting against one or more of the following three conditions: (1) the lamp starting to significantly rectify current in either direction, (2) the lamp voltage exceeding a predetermined level for a prolonged duration, and (3) the power mains supply voltage falling below a predetermined level.

Description:
FIELD OF THE INVENTION 
     This invention relates to fluorescent lamp ballasts incorporating an integrated circuit. More particularly, the invention relates to such ballasts including additional circuitry for protecting the ballast when one or more of the following three conditions occur: (1) the lamp starting to significantly rectify current in either direction, (2) the lamp voltage exceeding a predetermined level for a significant time such as half a minute, and (3) the power mains supply voltage falling below a predetermined level. 
     BACKGROUND OF THE INVENTION 
     Ballasts, or power-supply, circuits for fluorescent lamps can benefit from incorporating complex circuit functions in integrated circuit (IC) form. Widely available, low cost IC&#39;s can include various functions, such as driving a switching arrangement that provides AC power for the lamp. It would be desirable to inexpensively complement the widely available IC&#39;s, with protection from one or more of the following three conditions: (1) the lamp starting to significantly rectify current in either direction, (2) the lamp voltage exceeding a predetermined level for a prolonged duration, and (3) the power mains supply voltage falling below a predetermined level. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment of the invention protects from all of the foregoing three conditions relating to (1) lamp rectification, (2) excessive lamp voltage, and (3) low line voltage comprises the following circuitry. That embodiment comprises a gas discharge lamp ballast in which a resonant load circuit for at least one lamp includes a DC blocking capacitor connected between a reference node and the at least one lamp. A switching arrangement includes first and second switches serially connected between a rail node at a DC potential and the reference node, for supplying AC current to the load via a midpoint node between the first and second switches. This embodiment protects against all three conditions in a ballast having an integrated circuit including (1) a driver for the switching arrangement including control means to create a frequency sweep from a pre-heat frequency, through a substantially lower, resonant frequency, to a still lower operating frequency, (2) a pre-heat pin for triggering the control means to re-start a frequency sweep in response to a re-start signal that exceeds a threshold level, (3) a shut-down pin associated with an internal shut-down latch for shutting down the driver in response to a shut-down signal that exceeds a threshold level, and (4) a pin at a preset voltage during normal operation and whose impedance to the reference node determines frequency of operation of the switching arrangement. 
     The ballast also includes first through fourth protection circuits: 
     (1) The first protection circuit compares a first voltage representing an average voltage on the midpoint node with a second voltage representing the voltage of the DC blocking capacitor, and for sending a shut-down signal to the shut-down pin when one of the first and second voltages exceeds the other by respective predetermined amounts. 
     (2) A second protection circuit has an output coupled to the pre-heat pin for detecting a brief period of substantially excessive lamp voltage when a lamp has not yet started in response to current spikes through a switch of the switching arrangement and, in turn, for supplying the pre-heat pin with a re-start signal. 
     (3) A third protection circuit detects a longer period of less excessive lamp voltage; the third protection circuit including a DC amplifier with a response time substantially longer than the brief period for amplifying a signal representing the output of the second protection circuit and providing the resulting signal to the shut-down pin. The first and third protection circuits share an auxiliary circuit that prevents each of them from continually sending a shut-down signal to the shut-down pin. 
     (4) A fourth protection circuit lowers the mentioned impedance when a voltage representing the magnitude of an AC input voltage falls below the mentioned preset voltage by a predetermined amount. 
     A ballast may incorporate any one or any combination of the foregoing protection circuits. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In the drawings, in which like reference numerals refer to like parts: 
     FIG. 1 is a simplified schematic diagram, partly in block, of a ballast for a fluorescent lamp including an integrated circuit for driving a half bridge switching arrangement; 
     FIG. 2 is a schematic diagram of portions of the illustrated parts of the ballast of FIG. 1 together with additional circuitry for implementing protection against lamp rectification; 
     FIG. 3 is a schematic diagram of portions of the illustrated parts of ballast of FIG. 1 together with additional circuitry for implementing protection against excessive lamp voltage; and 
     FIG. 4 is a schematic diagram of portions of the illustrated parts of the ballast of FIG. 1 together with additional circuitry for implementing protection against low line voltage. 
     FIGS. 5A-5C shows waveforms of envelopes of ballast current versus rectified line input voltage over time. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an integrated circuit (IC)  10  in a ballast circuit  12  for a fluorescent lamp  14 . IC  10  may comprise chip no. L6574 manufactured by ST Micro electronics of Italy, for instance. Circuit  12  includes a switching arrangement comprising a pair of switches  16  and  18 , such as MOLESTS. Ballast  12  may include a resonant inductor  20  and a resonant capacitor  22 . An AC to DC converter  24  receives AC power from mains  26 , and produces a DC voltage between a positive rail node  28  and a reference node  30 . 
     FIG. 1 is simplified in various respects, so as, for instance, to represent lamp  14  as a single lamp, and to omit circuitry whose implementation will be routine to those of ordinary skill in the art. It will be routine to those of ordinary skill in the art to include various modifications, such as including plural lamps. 
     Protection Against Lamp Rectification 
     Circuitry for protecting the ballast against the lamp voltage rectifying utilizes a shut-down pin  8  of IC  10  (FIG.  1 ). When the voltage of pin  8  reaches a predetermined level e.g., 0.6 volts), a latch (not shown) in IC  10  shuts down the IC, and hence stops ballast operation. A capacitor  31  protects pin  8  from glitches. 
     FIG. 2 shows a node  48 , which may be connected to pin  8  of the IC via a AN diode  49  (FIG.  1 ), and which receives the output of a circuit  51 . Circuit  51  compares the voltage at point A with the voltage at point B. The voltage on point A represents the average of the voltage on a midpoint node  52  between switches  16  and  18 . The voltage on node A may be provided by a resistive voltage divider including resistances  54  and  56 . A capacitor  58  smoothest the AC voltage on midpoint  52  to provide an averaged voltage. The voltage on node B may be provided by a resistive voltage divider including resistances  60  and  62 . A capacitor  64  smoothest the voltage signal obtained from DC blocking capacitor  66 . Capacitors  58  and  64  also desensitize circuit  51  from glitches. 
     Preferably, the voltage on node A is substantially proportional to half the voltage on the positive rail node  28 , while the voltage on node B is substantially proportional to half the voltage on the DC blocking capacitor. If the duty cycle of the two switches  18  and  16  is other than 50 percent, this variation affects the voltages at A and B equally, so any effect of this variation is cancelled out. 
     Circuit  51  may include a pair of PAP transistors  51   a  and  51   b  having their bases connected to nodes B and A, respectively, and their other nodes connected as shown. When either the voltage at point A exceeds the voltage at point B, or vice versa, by respective predetermined amounts (typically the same), one of transistors  51   a  and  51   b  turns on. This causes the voltage on pin  8  to rise at a rate determined by a resistor  66  and a capacitor  68 . Once the voltage on pin  8  exceeds a predetermined amount (e.g., 0.6 volts), the IC ( 10 , FIG. 1) shuts down. Transistors  51   a  and  51   b  perform the well-known function of a comparator. This function can be provided in many other ways that will be routine to those of ordinary skill in the art. 
     When the IC shuts down, a PAP transistor  70  discharges capacitor  68 . It does so by having its base connected to pin  2  of the IC (FIG.  1 ), which is normally at a preset level (e.g., 2.5 volts) during normal ballast operation. When the ballast shuts down, the voltage on pin  2  drops, turning on transistor  70  and discharging capacitor  68 . Thus, whenever power to the ballast is toggled or the lamp is replaced, the voltage on capacitor  68  rapidly decays so that an elevated voltage on capacitor  68  does not prevent the lamp from restarting. 
     Alternatives to the pair of PAP transistors  51   a  and  51   b  can be found in circuitry textbooks under the classification of comparators. A resistor  72  (FIG. 1) is connected from pin  8  to reference node  30  to make sure the voltage on such pin is reset to zero when driving current is removed. If a lamp is replaced after the ballast has been shut down from a voltage on pin  8 , resistor  72  assures that the IC automatically restarts when the IC is powered up. Resistor  72  also removes leakage current from transistors  51   a  and  51   b,  which might otherwise accidentally trigger a shut down at high temperatures. 
     Protection Against Excessive Lamp Voltage 
     In addition to pin  8  of the IC (FIG.  1 ), pin  9  of the IC (FIG. 1) is also used to protect against excessive lamp voltage. The occurrence of the voltage on pin  9  exceeding a threshold level triggers a control circuit in the IC (not shown) to operate switches  16  and  18  at a frequency sweep that starts at a pre-heat frequency, drops through a substantially lower resonant frequency, and reaches a still lower operating frequency. 
     FIG. 3 shows circuitry cooperating with pins  8  and  9  to protect against excessive lamp voltage. Circuitry  74  very rapidly detects highly excessive lamp voltage (e.g., 400 percent overload) that manifests as spikes of current through a resistor  76 . The voltage on resistor  76  is conveyed to pin  9  via a resistor  78  and a capacitor  80 , and consists essentially of a series of positive spikes at the operating frequency of the switches. If the lamp voltage is highly excessive for a brief period of time, then even a single spike will trigger pin  9  to restart the frequency sweep just described, with the result of rapidly shutting down power to the lamp. 
     Circuitry  82  detects a more modest excess of lamp voltage (e.g., 15 percent overload) over a longer period of time, which may be on the order of half a minute. Circuitry  84  uses an operational amplifier (op amp)  84  internal to the IC (FIG.  1 ), and whose pins are numbered  5 ,  6  and  7  (FIGS.  1  and  3 ). Pin  5  is the output, while pin  7  is an inverting input and pin  6  is a non-inverting input. Connecting a capacitor  86  across pins  5  and  6  makes the op amp an integrator. Adding resistors  88  and  90  turns the integrator into a DC amplifier with a very slow response time, on the order of half a minute. 
     The thus-described DC amplifier receives the output of pin  9  on its inverting input  7 , and provides an output to pin  8  via AN diode  50  (FIG. 1) to shut down the ballast only if a modest overvoltage persists for a predetermined, sustained period of time (e.g., half a minute). If such modest overvoltage is briefer, the ballast does not shut down. AN diode  50  is associated with AN diode  49  so as to make pin  8  receive whichever voltage is highest on the anodes of the diodes. 
     If circuitry  82  turns off the ballast, transistor  70  turns on and prevents capacitor  68  (FIG. 2) from getting charged during the shutdown, which it otherwise would. This enables automatic-restart of the ballast by merely replacing a lamp. 
     The foregoing operation of transistor  70  was previously described in connection with protection against rectification of lamp voltage (FIG.  2 ). The FIG.  2  and FIG. 3 circuits cooperate by both using transistor  70  for discharging capacitor 68. 
     Protection Against Low Line Voltage 
     Pin  4  of the IC (FIG. 1) cooperates with the circuitry of FIG. 4 to protect against low line voltage. Pin  4  is normally at a preset reference potential, such as 2.5 volts. The frequency of operation of switches  16  and  18  (FIG. 1) is determined by the impedance between pin  4  and reference node  30 . In the absence of low line voltage, resistor  91  (FIG. 1) determines that impedance. FIG. 4 shows a circuit that increases the switching frequency to momentarily reduce the output current of the ballast to essentially zero when the line voltage becomes undesirably low. This prevents burn out of the switches. 
     In FIG. 4, AN diodes  92 - 95  implement AC to DC converter  24  (FIG.  1 ), in the form of a full-wave rectifier. A capacitor  97  smoothest out the rectified voltage on positive rail node  28 . Because resistors  98  and  99  form a voltage divider, the voltage at their common node  160  represents the magnitude of the AC input voltage. Such voltage is smoothed by a capacitor  102 . A AN diode  104  “sees” the voltage at node  160  on its anode, and sees the pin  4  voltage on its cathode. When the node  160  voltage falls below the preset voltage on pin  4  by about 0.7 volts, the diode becomes forward biased and starts conducting. This, in turn, places resistor  99  between pin  4  and reference node  30 , thereby increasing the conductance from pin  4  to node  30 . The result is a decrease of impedance between pin  4  and node  30 , so that the switching frequency is abruptly increased. Alternatives to using a AN diode  104  include the combination of a diode and a resistor in series, or the use of a transistor to incorporate gain to accentuate the frequency shifting effect in a manner that will be apparent to those of ordinary skill in the art. 
     FIGS. 5A-5C shows that increasing the frequency of switching reduces ballast current. These figures compare envelopes  106  and  108  of ballast current with rectified line voltage waveforms  110  and  112 . Only envelopes of ballast current are shown, since the actual current waveforms oscillate much more rapidly than the line voltage. For line voltage  110 , corresponding envelope  106  falls to essentially zero due to higher switching frequency when diode  104  (FIG. 4) becomes forward biased, which may occur when voltage  110  falls below 1.8 volts, for instance. This occurs over intervals  106   a,  centered near the zero crossings of voltage  110 . When the line voltage falls further, to that of waveform  112 , corresponding envelope  108  contains larger periods  108   a  of essentially zero magnitude. In this way, as the line voltage falls further and further, the ballast current remains essentially zero for larger and larger portions of the line voltage cycles. Owing to this, the DC potential on the positive rail node  28  does not fall so sharply with declining line voltage, which beneficially prevents burn out of the switches. 
     Other Advantages 
     When using the circuitry described herein, the ballast will be protected against degassing of a lamp. Such degassing causes high lamp voltage, which repeatedly causes a trip on pin  9  from circuitry  74  (FIG.  3 ). This causes the IC to preheat and go through the mentioned frequency sweep again. All this time, a sufficient imbalance in voltage exists between nodes A and B (FIG.  2 ), so that capacitor  68  (FIG. 2) charges up, eventually causing a trip on pin  8  that stops the ballast. This sequence of events also happens if a lamp is removed while running or degausses while running. 
     Using the specific IC mentioned above, exemplary component values for a fluorescent lamp  14  rated at 26-watts, with a DC potential on rail  28  of 470 volts, and with pre-heat, resonant and operating frequencies of 87 kHz, 57 kHz, and 45 kHz, respectively, are as follows: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Resistances 
                 Ohms 
               
               
                 54 
                 474K 
               
               
                 56 
                 10k 
               
               
                 60 
                 475K 
               
               
                 62 
                 10k 
               
               
                 66 
                 200k 
               
               
                 72 
                 100k 
               
               
                 76 
                 .56 
               
               
                 78 
                 1k 
               
               
                 88 
                 110k 
               
               
                 90 
                 7.5k 
               
               
                 91 
                 68.1k 
               
               
                 98 
                 1.2 M 
               
               
                 99 
                 33.2K 
               
               
                 Capacitances 
                 Microfarads 
               
               
                 22 
                 3.3nf 
               
               
                 31 
                 470 pf 
               
               
                 58 
                 .1 uF 
               
               
                 64 
                 .1 uF 
               
               
                 66 
                 .1 uF 
               
               
                 68 
                 100 uF 
               
               
                 80 
                 470 pF 
               
               
                 86 
                 22 uF 
               
               
                 102  
                 .1 uF 
               
               
                 Transistors 
                 Description 
               
               
                 16 
                 A 3NB50, n-channel, enhancement mode 
               
               
                   
                 MOSFET, sold by ST Microelectronics, an 
               
               
                   
                 international company 
               
               
                 18 
                 Same as 16 
               
               
                  51a 
                 A PNP model 3906 transistor sold by Rohm 
               
               
                   
                 of Kyoto Japan. 
               
               
                  51b 
                 Same as 51a 
               
               
                 70 
                 Same as 51a 
               
               
                   
               
             
          
         
       
     
     While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. For instance, with a different IC than the specific one described, the pin numbering may differ. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention.