Abstract:
An electronic ballast includes a microprocessor which is programmed to read a voltage value corresponding to an output of the electronic ballast, output a signal which controls an amount of power outputted by the electronic ballast in accordance with the voltage value, read an external voltage value, and select one of a trimming mode and a normal mode as an operating mode based on the external voltage value. The microprocessor is also programmed, when operating in the trimming mode, to set an internal reference value, compare the voltage value corresponding to the output of the electronic ballast with the internal reference value, trim an amount of power outputted by the electronic ballast to a resistor corresponding to an impedance of a High Intensity Discharge lamp by adjusting the signal based upon the comparison, and store a result of the signal adjustment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of High Intensity Discharge (HID) lamps, and more particularly, to electronic ballast of such lamps. 
     2. Description of the Related Art 
       FIG. 1A  is a schematic diagram of a conventional electronic ballast of an HID lamp. In this ballast, a microprocessor (MPU) reads a scaled down voltage of the HID lamp, and outputs a pulse width modulation (PWM) signal to a power converter switch driver, which drives a power converter switch. The MPU varies the duty cycle of the PWM signal in accordance with the scaled down voltage of the HID lamp, and may set the duty cycle based on values in a lookup table of the MPU, for example. The power converter switch provides power to the HID lamp in accordance with the PWM signal. 
     The power outputted from the ballast to the HID lamp is a function of the duty cycle of the PWM signal. However, due to component tolerances of the ballast, such as a voltage divider resistor tolerance, a tolerance of an analog to digital converter, a power inductor tolerance, and circuit delay, the output power can widely vary from one ballast to another. For example, the output power of a 70 W ballast can vary between 60 W and 80 W. Thus, the output of the ballast is not only a function of the duty cycle of the PWM signal, but also is a function of the component tolerances. 
     To minimize the output power variations among ballasts, components with tight tolerances can be used. However, a disadvantage of such a design is the associated increase in cost. 
       FIG. 1B  is a schematic diagram of a second type of conventional electronic ballast of an HID lamp. In this ballast, a MPU does not output a PWM signal directly to a power converter switch driver, as in the ballast shown in  FIG. 1A . Instead, the MPU outputs a PWM signal to an input of an operational amplifier. The duty cycle of the PWM varies in accordance with the scaled voltage of the HID lamp, and may be set, for example, based on values in a lookup table of the MPU. The second type of the conventional electronic ballast has the same component tolerance issue as the first type of conventional electronic ballast. However, to minimize the output power variation, a potentiometer is connected to a second input of the operational amplifier, and is used to trim the output of the power converter switch. 
       FIG. 1C  is a schematic diagram of the second type of conventional electronic ballast when it is in a trimming mode. To trim the output of the second type of ballast, the ballast output is connected to a fixed resistor, rather than an HID lamp. Typically, the resistance of the resistor corresponds to an HID lamp impedance at a nominal wattage. An operator measures the output power of the ballast, and turns the potentiometer to trim the output power until he or she determines that it has reached an acceptable value. 
     A disadvantage of this ballast is that the potentiometer can be adjusted to compensate for error at only one set point, typically the impedance at nominal lamp wattage. However, the lamp impedance is not a constant value during the entire time the lamp is in operation. Thus, the MPU cannot provide an accurate ballast output throughout the entire time the lamp is in operation. 
     SUMMARY OF THE INVENTION 
     A feature of the present invention is that it allows an electronic ballast output to be automatically and effectively trimmed, without the above-noted drawbacks of the related art. 
     This may be implemented with an electronic ballast which includes a microprocessor which is programmed to read a voltage value corresponding to an output of the electronic ballast, output a signal which controls an amount of power outputted by the electronic ballast in accordance with the voltage value, read an external voltage value, and select one of a trimming mode and a normal mode as an operating mode based on the external voltage value. The microprocessor is also programmed, when operating in the trimming mode, to set an internal reference value, compare the voltage value corresponding to the output of the electronic ballast with the internal reference value, trim an amount of power outputted by the electronic ballast to a resistor corresponding to an impedance of an HID lamp by adjusting the signal based upon the comparison, and store a result of the signal adjustment. 
     The microprocessor may be programmed to set the internal reference proportional to the external voltage value. The signal may be a PWM signal. When operating in the trimming mode, the microprocessor may adjust a duty cycle of the PWM signal based upon the comparison. When in the trimming mode, the microprocessor may adjust the duty cycle of the PWM signal if a difference between the voltage value corresponding to the output of the electronic ballast and the internal reference value is outside a preset range. When in the trimming mode, the microprocessor may adjust the duty cycle of the PWM signal by applying a duty cycle offset to a prior duty cycle of the PWM signal. 
     When in the trimming mode, the microprocessor may store an offset value in a memory. The offset value may be one of a final adjusted duty cycle and a final duty cycle offset. The external voltage source may be provided in the electronic ballast. 
     When operating in the normal mode, the microprocessor may re-scale the result of the signal adjustment performed in the trimming mode based upon a difference between the impedance of the resistor and an impedance of an HID lamp connected to the electronic ballast in the normal mode. When operating in the normal mode, the microprocessor may output a signal which controls an amount of power to an HID lamp in accordance with a voltage corresponding to the electronic ballast output and the result of the signal adjustment performed in the trimming mode. 
     The foregoing and other objects, features, aspects and advantage of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram of a first type of conventional electronic ballast of an HID lamp; 
         FIG. 1B  is a schematic diagram of a second type of conventional electronic ballast of an HID lamp; 
         FIG. 1C  is a schematic diagram of a second type of conventional electronic ballast when it is in the trimming mode; 
         FIG. 2A  is a schematic diagram of an embodiment of an electronic ballast of the present invention when it is in the trimming mode; 
         FIG. 2B  is a schematic diagram of an embodiment of an electronic ballast of the present invention when it is in the normal mode; 
         FIGS. 3A and 3B  are schematic diagrams of embodiments of an electronic ballast of the present invention; and 
         FIG. 4  is a flow chart depicting an embodiment of an algorithm which is performed by the MPU. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2A  is a schematic diagram of an embodiment of an electronic ballast of the present invention when it is in the trimming mode. As shown in  FIG. 2A , a voltage divider network made up of resistors R 1 -R 4  is connected to output terminals LAMP 1  and LAMP 2  of the ballast, and a fixed resistor RL is connected across the terminals LAMP 1  and LAMP 2 . The resistance of the resistor RL corresponds approximately to an impedance of an HID lamp at a nominal wattage. A MPU of the ballast includes a sensing pin which receives an external voltage. The external voltage may be provided, for example, by a voltage source onboard the ballast, or from a production fixture. A specific voltage of the external voltage, which is read via an internal analog to digital converter, tells the MPU that it is in the trimming mode. The MPU then generates an initial duty cycle of the PWM signal to output an amount of power to the resistor RL of the electronic ballast terminals. The voltage divider network provides a scaled down voltage value of the ballast output, corresponding to a voltage across the resistor RL, to the MPU. The MPU reads the scaled down voltage value via an internal analog to digital converter, and generates an second duty cycle of the PWM signal, which may be set, for example, by picking up the value composed of the on-width of the duty cycle of the PWM signal from a lookup table of the MPU in accordance with the scaled down voltage value. 
     The external voltage is also utilized for a target voltage value. The MPU reads the external voltage, and sets an internal reference value proportional to the external voltage value. The internal reference value is compared with the scaled down voltage value. If a difference between the internal reference value and the scaled down voltage value is outside a preset range in the MPU, the MPU adjusts the duty cycle of the PWM signal based upon the difference. This can be performed, for example, by applying a duty cycle offset to the second duty cycle of the PWM signal. 
     When the duty cycle of the PWM signal is adjusted, this causes the scaled down voltage value to move closer to the internal reference value. The MPU measures the new scaled down voltage value and compares it to the internal reference value. If the difference between the internal reference value and the new scaled down voltage value is still out of the preset range, the MPU again adjusts the duty cycle of the PWM signal based upon the difference, for example, by increasing or decreasing the duty offset. This process repeats until the difference between the internal reference value and the scaled down voltage is within the preset range. 
     When the difference between the internal reference value and the scaled down voltage is within the preset range, the duty cycle of the PWM signal is a final value. The MPU stores the final duty cycle offset in a memory, such as an EEPROM. 
     However, if the MPU is unable to adjust so that the difference between the internal reference value and the scaled down voltage is within the preset range, the MPU may shut off the ballast, and signal an operator that a target scaled down voltage has not been achieved. 
     As an alternative to comparing the scaled down voltage value directly with the internal reference value, the MPU may compare ratios. For example, the internal reference value may be 1.9V, and a desired scaled down lamp voltage may be 0.95V. Thus, in this case, a desired ratio of the internal reference value and the desired scaled down lamp voltage would be 1.9/0.95=2. If the initial scaled down lamp voltage is 1.0V, the initial ratio would be 1.9/1.0=1.9. If the preset range is set 0.00 to 0.05, in this case, the difference between the desired ratio of 2 and the measured ratio of 1.9 would be 2.0−1.9=0.1, which is out of the preset range. Thus, the MPU would apply an offset to the duty cycle of the PWM signal to lower the lamp voltage. 
     In an alternative embodiment, the MPU can have an internal reference value set in its code instead of being set by reading the external voltage value. But in this case, the tolerance of an analog to digital converter, among other factors, has to be very small. On the other hand, if the internal reference value is generated by reading a external voltage, the tolerance of an analog to digital converter, among other factors, can be relatively compensated for. 
       FIG. 2B  is a schematic diagram of an embodiment of an electronic ballast of the present invention when it is in the normal mode. An HID lamp is connected across the terminals LAMP 1  and LAMP 2 . A specific voltage of the external voltage tells the MPU that it is in the normal mode. The voltage divider network provides a scaled down voltage value of the ballast output, corresponding to a voltage across the HID lamp, to the MPU. The MPU then generates an adjusted duty cycle of the PWM signal by picking up a value from a lookup table of the MPU composed of an on-width of the duty cycle in accordance with the scaled down voltage value and then combines the value from the lookup table with the stored final duty cycle offset value obtained in the trimming mode. The lookup table may contain a number of values related to voltage values corresponding to an electronic ballast output through an entire operating range. For example, throughout the operating range of the electronic ballast, its output may vary from A volts to Z volts. A corresponding value in the lookup table for generating a on-width of the PWM signal may vary accordingly from a to z. 
     The final duty cycle offset obtained in the trimming mode is based on the resistance of the fixed resistor RL, which corresponds approximately to an impedance of an HID lamp at nominal wattage. However, the impedance of the HID lamp may vary during the time the HID lamp is in operation. Therefore, if the MPU directly applies the stored final duty cycle offset in the memory to the values from the lookup table corresponding to the scaled down voltage, the ratio between the stored final duty cycle offset and the value at the nominal impedance is different from the one between the stored final duty cycle and values at any non-nominal impedance. To have more accurate operation, the MPU may be programmed to compensate the final duty offset in accordance with the scaled down voltage to have the same ratio between a compensated final duty cycle offset and a value from lookup table in the entire operating range (that is, from impedance R A  to impedance R Z , as the lamp voltage varies from A volts to Z volts). 
     If the PWM signal has a fixed frequency throughout an entire operating range, the MPU may be able to directly apply the stored final duty cycle offset to the cycle of the PWM signal instead of applying the compensated final duty cycle to the values from lookup table. 
       FIGS. 3A and 3B  are schematic diagrams of embodiments of a ballast of the present invention. In the ballast shown in  FIG. 3A , the MPU outputs the PWM signal directly to a power converter switch driver of the ballast. The power converter switch driver drives a power converter switch to output power via the output terminals LAMP 1  and LAMP 2 . However, in the ballast shown in  FIG. 3B , the MPU outputs the PWM signal to analog circuitry, the PWM signal is smoothed by a CR circuit, then output to an operational amplifier, where it is compared to another input signal. The output of the operational amplifier then controls the power converter switch driver. Both of the ballasts have lookup tables which have a number of values composed of the on-width of the PWM signal. 
       FIG. 4  is a flow chart depicting an embodiment of an algorithm which is performed by the MPU during a trimming mode, based upon a program it executes. The algorithm begins with the MPU reading an external voltage to judge if it is in the trimming mode or in the normal mode. After the MPU judges that it is in the trimming mode, the MPU sets an initial duty cycle of the PWM signal and the ballast outputs the power to the resistor RL. Then the MPU reads a scaled down voltage, corresponding to a voltage across the resistor RL. The MPU then sets a second duty cycle of the PWM signal based upon the scaled down voltage. The initial duty cycle of the PWM may be set to be the same as the second duty cycle of the PWM signal. 
     The MPU then reads the external voltage again, sets the internal reference value, and compares it to the scaled down voltage value. If a difference between the internal reference value and the scaled down voltage value is out of the preset range, the MPU adjusts the duty cycle of the PWM signal based upon the difference. This can be performed, for example, by applying a duty cycle offset to the second duty cycle. 
     The MPU continues to read the scaled down voltage value and compare it to the internal reference value until the difference between the internal reference value and the scaled down voltage value is in the preset range. When the difference between the internal reference value and the scaled down voltage value is within the preset range, the duty cycle of the PWM signal is at a final value, and the MPU stores the value of the final duty cycle or the final duty cycle offset in a memory. 
     Thus, using the above-described apparatus and method, the ballast output can be automatically and effectively trimmed to an acceptable level. 
     The foregoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. 
     The illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The illustrations are not intended to serve as a complete description of all of the elements and features of apparatus and systems that utilize the structures or methods described herein. Many other embodiments may be apparent to those of skill in the art upon reviewing the disclosure. Other embodiments may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive. 
     One or more embodiments of the disclosure may be referred to herein, individually and/or collectively, by the term ‘invention’ merely for convenience and without intending to voluntarily limit the scope of this application to any particular invention or inventive concept. Moreover, although specific embodiments have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all subsequent adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the description.