Patent Document

RELATED APPLICATION 
   This application is based on and claims benefit of U.S. Provisional Application No. 60/507,637, filed Sep. 30, 2003, entitled Simplified Topology for HID Lamps, to which a claim of priority is hereby made. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to ballasts for HID lamps, and related more particularly to a simplified topology for a ballast to drive an HID lamp. 
   2. Description of Related Art 
   High intensity discharge (HID) lamps are highly desirable for commercial markets due to their superior light efficiency in terms of lumens produced per watt and the variety of power ranges in which they are available. As HID lamp technology as developed, manufacturers and ballast designers have come to focus on reliability and efficiency in providing a robust and simple HID lighting system. For example, lamp manufacturers typically suggest that an HID lamp be driven in an AC mode, at a low frequency and with constant power to maximize reliability. 
   An HID lamp having a power range of 35–150 W may use less than 100 volts for ignition, which is followed by an operating region in which voltage may drop to a few tens of volts, and current may drop to a few amperes. In this operating region, it is desirable to maintain a constant power output. 
   When the HID lamp is ignited, a high voltage (HV) generator is placed in series with the main power supply to obtain a few kilovolts for a cold strike ignition or a few tens of kilovolts for a hot strike ignition, primarily near the point at which ignition occurs. The HV generator is fairly well known, with a number of variations presently available in conventional designs. 
   Lamp manufacturers suggest that lamps be driven with an AC signal before and after ignition due to the different consumption or lifetime of lamp cathode and anode terminals if a DC signal is used. Low frequency AC signals of from about 50 to about 500 Hz is suggested due to mechanical instabilities of the lamp arc upon ignition, to avoid degradation to the lamp components or eventual complete failure of the lamp. 
   In addition to the practical limitations illustrated by the suggestions of the lamp manufacturers, international regulations call for a limit on the harmonic current drawn by a converter connected to an HID lamp. Accordingly, to satisfy the practical considerations and the international regulations regarding operation of an HID lamp, low frequency AC signals should be used with a limitation on the harmonic current drawn by the lamp ballast or converter. 
   Another factor that is important for lamp operation and power converter regulation is the power factor obtained by the ballast or converter that is connected to the public power system. It is desirable that any ballast or converter for a lamp exhibit a high power factor, such as one as close as possible to unity, so that the load appears as close as possible to purely resistive as seen by the main power lines. Power factor correction circuitry draws a sinusoidal current from the main power lines that is in phase with the sinusoidal voltage supply to obtain a high power factor. A simple and inexpensive technique for power factor correction would be highly desirable. 
   Another important factor that contributes to longevity of an HID lamp is to regulate the power supply to the lamp during normal running conditions. Accordingly, it is desirable to provide constant power to the lamp during normal operation to maintain an even light output and extend the longevity of the lamp. 
   The above constraints are desirable in addition to obtaining a low frequency AC modulation for the lamp for the practical reasons discussed above. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided a ballast for driving an HID lamp that provides low frequency modulation, constant power to an inverter section and draws sinusoidal current from the input main power supply. The power converter detects conditions in a transformer used to convert input power to supply the inverter. The power converter control also senses an output voltage of the inverter to ensure constant power is delivered to the load. Sensing the inverter voltage output also permits the control to react to the lamp ignition. The voltage sense of the inverter output provides all the feedback needed to maintain constant power at the lamp. 
   The various rules used to drive the simplified HID ballast are established based on a sense of the voltage output of the inverter. One of the rules includes the on time for the switch coupled to the transformer to switch power delivered to the transformer to obtain a constant power output. Switching the switch is also conducted in accordance with obtaining a high power factor by drawing a sinusoidal input current from the input power supply. Another rule is the provision of constant lamp power while output voltage varies over a range. Each of these rules in conjunction provides a robust HID ballast control that is simple to implement, such as through a micro-controller, while providing a flexible design that can be used with a variety of HID lamps. 
   In accordance with one embodiment of the present invention, a ballast power converter is driven in critical mode with a variable switching frequency. The variable switching frequency depends upon the current in the transformer when transferring input power to the inverter. The switch coupled to the transformer for controlling the power transfer is operated based on the magnetizing current of the transformer going to zero. 
   The HID lamp is driven in a half bridge inverter configuration by a high voltage driver coupled to an oscillator or timer IC. The use of two capacitors split across the lamp load permits the use of the half bridge topology to avoid a full bridge switching configuration to reduce the cost of the overall solution. Ideally, the capacitors balance the voltage on either side of the lamp connections, but in practice are prone to some variation. By introducing a feedback signal from the power converter and one of the capacitors, the high voltage driver IC can be operated to drive the half bridge to balance the voltage between the two capacitors. 
   Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an abstract circuit diagram of an HID ballast with a simplified topology according to the present invention. 
       FIG. 2  is a graph showing switch on time versus output voltage for a switch transformer according to the present invention. 
       FIG. 3  is a graph illustrating a sinusoidal input current drawn to improve the power factor of the circuit according to the present invention. 
       FIG. 4  is a graph illustrating constant output power versus output voltage in accordance with the present invention. 
       FIG. 5  is a circuit block diagram of an electronic ballast control in accordance with the present invention. 
       FIG. 6  is a circuit block diagram of an electronic ballast control according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 1 , an abstract circuit diagram of an electronic ballast control for an HID is illustrated generally as circuit  10 . Circuit  10  includes a transformer TX 1  that receives a full wave rectified signal from the diode bridge consisting of diodes D 2 –D 5 . In accordance with a known result, if a flyback converter is operated in critical conduction mode, the output of such converter provides constant power if the main switch is driven with a constant on interval and a variable off interval, while receiving a constant input voltage. With these conditions, the output voltage and output current of the flyback converter follow a constant power law. 
   However, when the voltage received by the flyback converter is not constant, but rather a fullwave rectification signal like that delivered by the diode bridge consisting of diodes D 2 –D 5  in circuit  10 , a different control approach is taken. A modified control approach is especially indicated if constant power is to be delivered by the power converter while obtaining a close to unity power factor with a low total harmonic distortion (THD). A high power factor, e.g., close to unity, is achieved when the input current is fully sinusoidal and in phase with the input voltage. Under these conditions, the circuit load appears fully resistive to the input power supply or input power mains. 
   In accordance with the present invention, an energy transfer device such as a transformer TX 1  in circuit  10  is switched in critical conduction mode. Switch M 1  is illustrated as a MOSFET, but it should be apparent that any type of appropriate switch may be used, including IGBTs and other bipolar switches. A controller  12  is connected to an indicator such as an auxiliary winding L 1  of transformer TX 1  to determine the occurrence of the demagnetization of transformer TX 1 , or a zero crossing of current in the transformer. Controller  12  is also connected to the output voltage of the power converter, Vo to contribute to controlling the power converter for constant power. 
   A voltage V 1  supplied to the bridge rectifier is a sinusoidal input main voltage, typically 110 or 230 volts, so that transformer TX 1  receives a fullwave rectified signal. Capacitors C 1  and C 2  provide a filtering function in conjunction with diode D 1 , so that voltage Vo is a DC voltage supplied to the inverter composed of switches M 2  and M 3 . The HID lamp is shown schematically as a resistor R 2  and an igniter  14  in series with the lamp. 
   Controller  12  detects the demagnetization of transformer TX 1  through auxiliary winding L 1 . The signal provided by auxiliary winding L 1  also serves to indicate to controller  12  the point at which the off period for switch M 1  occurs. Voltage Vo supplies information about the power provided by the power converter to determine the point at which point switch M 1  is turned on. 
   Referring to  FIG. 2 , the on time of switch M 1  varies as the output voltage changes. Accordingly, unlike prior flyback power converter designs, the switch on time for switch M 1  changes as the output voltage changes to achieve constant power. The relationship shown in the graph in  FIG. 2  between the on time of switch M 1  and the output voltage Vo obtains a suitable compensation for receiving a fullwave rectified voltage input on transformer TX 1 , rather than a fixed DC input. Controller  12  can be implemented to operate switch M 1  in accordance with the graph in  FIG. 2  in the form of a rule for operating the power converter with constant power output. 
   Referring to  FIG. 3 , input current drawn by the power converter from the main power supply lines is sinusoidal on average and in phase with the input voltage to achieve a high power factor. Although the sinusoidal shape of the input current may not comply perfectly with the sinusoidal input voltage, the result is still a very good power factor with low THD for the simplified power converter design. 
   In  FIGS. 2 ,  3 , an exemplary embodiment is indicated where the lamp power is set to approximately 70 W and the input voltage is approximately 230 VRMS. However, it should be apparent that this exemplary embodiment represents only one of many general cases that may be easily realized by the power converter according to the present invention. 
   Referring now to  FIG. 4 , a graph illustrating the power output to the lamp as a function of output voltage is illustrated. As can be seen from the graph in  FIG. 4 , power is constant over output voltage Vo, and maintained at 70 W for the purposes of the exemplary circuit operation. 
   Referring for a moment to  FIG. 1 , the inverter that includes switches M 2  and M 3  provides low frequency modulation to the lamp represented by resistor R 2 . Switches M 2  and M 3  are arranged in a half bridge switching arrangement, where the maximum peak voltage across the lamp is limited to Vo/2. 
   The voltage used to ignite the lamp represented by resistor R 2  may be customized by changing the turn ratio of transformer TX 1  to achieve the desired voltage prior to ignition. Alternately, the time interval Ton, which represents the on time for switch M 1 , can be modified to generate the appropriate voltage on the output of the converter for lamp ignition, typically around 100 volts. After lamp ignition, controller  12  senses output voltage Vo and modifies switch M 1  on time Ton as appropriate to conform with the rule described in the graph in  FIG. 2 . This simple feedback arrangement obtains all the functionality needed to completely operate the HID lamp in this simplified topology. For example, as the ignited lamp begins to absorb additional current, a decrease in output voltage Vo is observed when switch M 1  on time Ton is constant. Controller  12  senses the decrease in output voltage Vo and increases switch M 1  on time Ton accordingly to maintain constant power to the lamp. 
   The rules associated with the control illustrated in  FIGS. 2–4  can be easily stored in a micro-controller memory or other programmable device to provide a simple and low cost solution in conjunction with the HID ballast topology for realizing an inexpensive but robust lighting control system. Another exemplary embodiment of controller  12  is a ROM or one time programmable memory used to drive a micro-controller or an analog PWM circuit used to operate switch M 1 . 
   Referring now to  FIG. 5 , a more complete circuit block diagram of a lighting ballast circuit  50  is illustrated. Circuit  50  includes a full bridge rectifier composed of diodes D 1 –D 4  that provides a fullwave rectified input voltage pulse train from the line input voltage supply. The output of the full bridge rectifier is typically on the order of 85 to 265 VRMS, dependent upon the voltage supplied on the input main lines. The power converter includes switch M 1 , transformer TX 1  and diode D 5 , which is driven in critical conduction mode through variable switching frequency. A power output of transformer TX 1  through diode D 5  is the output power supplied to the inverter composed of switches M 2  and M 3  for supplying power to lamp and igniter  54 . Transformer TX 1  has an auxiliary winding L 1  that can be connected to controller  52  to sense the magnetizing current in transformer TX 1 . When the magnetizing current in transformer TX 1  goes to zero, it signals the beginning of a switching interval for switch M 1 . The switching interval starts with a switch on period Ton that is determined by controller  52 . Controller  52  provides switch on time modulation for switch M 1  according to the relationship illustrated in the graph in  FIG. 2 , for example. The modulation of switch M 1  for switch on time Ton is derived with the power converter output voltage feedback Vo. The feedback signal Vo can be conditioned in any particular way so that it is sufficient for reception by controller  52 . For example, in circuit  50 , a resistor divider R 5 , R 6  provides an appropriate voltage to controller  52  based on output voltage Vo. A capacitor C 3  is used to filter the signal provided by output voltage Vo. 
   As output voltage Vo changes over time, the on time Ton for switch M 1  also changes. For example, if output voltage Vo begins to decrease, switch M 1  on time Ton is increased to deliver additional energy to transformer TX 1  to increase output voltage Vo. If output voltage Vo increases over a desired amount, switch M 1  on time Ton is decreased to correspondingly decrease the amount of energy transferred in transformer TX 1 . 
   The power converter illustrated in circuit  50  includes capacitors C 1  and C 2 , which represent a divider arrangement to provide a balanced voltage value on either side of terminal  56  of lamp  54 . By arranging capacitor C 1  and C 2  in this form, lamp  54  can be driven with the half bridge composed of switches M 2  and M 3  to permit a reduction in the number of switches used to provide the ballast circuit solution. Switches M 2  and M 3  operate in a half bridge configuration to drive lamp  54  at a low frequency to improve the longevity of lamp  54 , while contributing to regulating light output. The switching half bridge composed of switches M 2  and M 3  usually operates at a frequency in the range of from about 200 to 500 Hz, as suggested by lamp manufacturers in typical arrangements. Due to the low frequency used to drive lamp  54 , a resonant filter, typically composed of an LC circuit, is not needed to operate lamp  54 . 
   A timer IC  58  provides the switching signals for the low frequency modulation of switches M 2  and M 3 . The switching signals are delivered to the high voltage IC driver  57 , which operates high and low side switches M 2 , M 3 , respectively. Driver  57  typically includes a level shift circuit for shifting a drive voltage supplied to switch M 2  to an appropriate voltage reference, while also providing switching signals to switch M 3 . Driver  57  can include a number of features, such as automatic deadtime insertion, fault detection and overtemperature detection, for example. Since driver  57  includes a level shift circuit, additional isolation transformers for driving switch M 2  are unnecessary. 
   Referring now to  FIG. 6 , another embodiment of the ballast lighting system of the present invention is illustrated as a circuit  60 . In circuit  60 , capacitor C 1  and C 2  represent practical capacitors with voltage mismatch variations that typically occur in real world applications. A balanced voltage on capacitor C 1  and C 2  can be obtained by drawing on the functionality of timer IC  68 . Output voltage Vo is provided to timer IC  68  to measure the total voltage observed on the combination of capacitor C 1  and C 2 . A feedback from capacitor C 2  is also provided to timer IC  68 . Timer IC  68  operates to provide switching signals to driver  57  so that switches M 2  and M 3  are operated in low frequency modulation drive lamp  64 . By modifying the low frequency on time of switches M 2  and M 3  in the half bridge arrangement, output voltage Vo can be modified so that the voltage at capacitor C 2  is one half of voltage Vo. This same voltage is present on capacitor C 1 . The balance of the voltages on capacitors C 1  and C 2  improves the ballast performance and provides a better overall system efficiency. 
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Technology Category: 5