Patent Publication Number: US-6337544-B1

Title: Digital lamp signal processor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to fluorescent lamp ballasts and more specifically to a digital control circuit that can achieve a real lamp power calculation in real time. 
     2. Description of the Background of the Invention 
     Analog ballast may achieve low cost and low power consumption. However, the performance of an analog ballast is limited due to effects of parasitic components and noise sensitivity on accuracy. Furthermore, the functionality, flexibility, and programmability of the analog ballast are also limited, since analog ballasts use multiple resistors and capacitors, which are difficult to implement using standard integrated circuit (IC) processing technology. Additionally, analog ballasts are complex and bulky. 
     Most dimmable high frequency (HF) electronic ballasts use analog ICs to control various operations of fluorescent lamps. These control operations may include preheat, ignition, burn standby, power regulation, and dimming. Some electronic ballasts may use standard CPUs or micro-controllers to control the operation of fluorescent lamps. For those ballasts the functionality, flexibility, and programmability is much improved. 
     However, due to the speed limitation, a standard CPU cannot process alternating current (AC) lamp signals in real time in order to obtain the required information, such as the phase of a current or voltage, peak current or voltage, real power, etc. This information is very important for a dimmable ballast control. Therefore, these kinds of digital ballasts have to sample more signals and require complicated signal condition circuits that are very difficult to integrate. 
     Furthermore, standard off-the-shelf micro-controllers with on-chip Analog-to-Digital (A/D) converters use slow on-chip A/D converters and are too slow to process the output of a high-speed A/D converter. Having slow on-chip A/D converters, the analog input signals are filtered externally, requiring additional external components. Additionally, filtering removes useful information from the input analog signal thereby limiting what can be regulated, e.g., real lamp power may not be regulated. 
     What is needed is a lamp signal processor able to use fast A/D converters, which may achieve the real lamp power calculation in real-time. 
     SUMMARY OF THE INVENTION 
     To achieve the real-time lamp signal processing, the present invention introduces a specific digital ballast control IC, designed and used in conjunction with analog digital ballasts. 
     The inventive digital lamp signal processor senses lamp current and lamp voltage in real time. These two signals are sufficient to obtain information, such as real lamp power calculation, necessary to control ballast operation and fault detection. The invention measures the phase of lamp current and voltage, the peak current and voltage, and calculates the average lamp current and voltage. 
     The inventive digital lamp signal processor eliminates the effect of parasitic capacitance of power wiring and a signal condition circuit. It may detect and control hard switching, and apply the over current and voltage protection. The invention directly processes AC signals allowing for simple and easily integrated single chip design. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects and advantages of the present invention may be more readily understood by one skilled in the art with reference to the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings wherein like elements are designated by identical reference numerals throughout the several views, and in which: 
     FIG. 1 is a system architecture diagram of the electronic ballast using an inventive digital lamp signal processor. 
     FIG. 2 is an architectural diagram of the inventive digital lamp signal processor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventive digital ballast circuit  1  uses the digital lamp signal processing (DLSP) technology to process an AC lamp current  2  and lamp voltage  3  in real time. Although currents other than lamp current, e.g., inductor current, or a combination of currents simultaneously, e.g., lamp and inductor currents, may be processed by the DLSP, for simplicity the term lamp current will be used to describe all situations. The ballast circuit  1  needs only to sense lamp current i l    2  and lamp voltage v l    3  signals to obtain the information required for the ballast operation control and fault detection. 
     The inventive DLSP when used in conjunction with the sampling of i l    2  and v l    3 , may achieve the control of the peak lamp current and voltage, the real lamp power and of the rectified average lamp current and voltage. The DLSP may also detect the ignition fault, the capacitor mode and lamp presence, as well as the proportion of negative and positive lamp current for end of lamp life. 
     Furthermore, since the ballast may process AC signals directly, the signal condition circuits are very simple and easy to integrate into a single chip. Therefore, the cost, size, and component count of the inventive ballast are reduced significantly. 
     FIG. 1 shows the inventive low-voltage digital lamp signal processor circuit  20  utilized in a ballast circuit  1  comprising a fast A/D converter  23  to over-sample, e.g., 32× over-sampling. Analog input signals, e.g., lamp voltage  3 , lamp current  2 , half-bridge power switch current  4 , may be received from a power stage  5 . Digital output signals of the A/D converter  23  are sent to the DLSP circuit  20 , which on a per-cycle basis calculates the power by multiplying and averaging the digital signals corresponding to the two input analog signals  2 ,  3 ; and an average value of each input signal. The DLSP circuit  20  further rectifies the input signals received from the A/D converter  23  followed by calculating average values of the rectified input signals and their peak values, and by detecting phases of the two input signals. 
     The Pulse Width Modulation (PWM) circuit  31  generates output signals  6 . The frequency and duty-cycle of the PWM signals depend on the outcome of operations performed in the DLSP circuit  20 . By varying the frequency/duty-cycle of the PWM signals, the lamp power or lamp current may be regulated at a selected level. 
     The PWM circuit  31  generated signals G 1 , G 2 , G E1 , G E2    6  are created by a low-voltage integrated circuit with a voltage of 3.3V or less and are referenced to a ground level. The level shifters  8  may be used to perform the function of level-shifting of signals  6  before they may be applied to the gates of the power switches T 1 , T 2 , T E1 , T E2    7  in order to control the ON/OFF state of these switches. 
     The regulator  9  generates the supply voltages for the low voltage, i.e., 3.3V or less, integrated circuit  10  and the supply voltage of the high voltage, i.e., 12V or higher, integrated circuit  11  from the pre-conditioner power factor correction (PFC) circuit  12  output voltage. The power-on reset (POR) circuit  13  generates a reset pulse, which may be applied to a reset pin of the low voltage integrated circuit  10  when both integrated circuits  10 ,  11  are powered on. 
     The micro-controller unit (MCU)  14  may be used to set the following functions and parameters: 
     1. the sequence of the ballast operation, e.g., electrode heating, ignition, lamp output regulation; 
     2. the mode of operation, e.g., symmetric PWM or asymmetric PWM, frequency shift or PWM control; 
     3. electrode pre-heat period; 
     4. slow signal processing, e.g., filtering for compensation of feedback loop; and 
     5. slow protection, e.g., detection of end-of-life for lamp. 
     FIG. 2 shows the inventive DLSP circuit  20  comprising a digital subtract circuit  21  used to receive data sampling current  2  and voltage  3  signals from a high-speed A/D converter (ADC) circuit  23  and to remove the offset created by the analog data sampling. Changing of the offset value allows the DLSP circuit  20  to process signed and unsigned data. The digital subtract  21  may also be used to extract a peak value of lamp current  2  and voltage  3  which are used for over-voltage protection, large and small current and voltage operation model switching control, etc. 
     First In First Out (FIFO)  32  by 8 bit buffers  22  may be used to store sampled current and voltage data. The use of FIFO buffers  22  eliminates the need for more than one ADC circuit  23 , which is the most: expensive and complicated part of the circuit to be implemented in the inventive ballast circuit. The FIFO buffers  22  may be implemented using DRAM, SRAM, or flip-flop transistors. A digital multiplier circuit  24  may be connected to the FIFO buffers  22  and is used to multiply the lamp current and voltage values stored there, in order to obtain the dynamic lamp power used for ballast control. 
     A digital average circuit  25  is provided for calculating the average lamp power and for sending the results to the power registers  26 . The DLSP may also be controlled to calculate the average current and voltage information that is very important for digital ballast operation. A control logic circuit  27  is used to generate the control signals, such as large/small signal switching (LS_S)  28 , ADC clock (ADCLK)  29 , and current/voltage switching (IV_S)  30 . 
     While the invention has been particularly shown and described with respect to illustrative and preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention, which is limited only by the scope of the appended claims.