Abstract:
A full wave sense amplifier for sensing a periodic current flowing through a discharge lamp is disclosed. The full wave sense amplifier comprises a first circuit for sensing the positive going portion of the periodic current. The amplifier also includes a second circuit for sensing the negative going portion of the periodic current. Finally, an output circuit for combining the negative going portion and the positive going portion into a current flow signal is provided.

Description:
TECHNICAL FIELD 
     The present invention relates to voltage or current sense amplifiers, and more particularly, to a sense amplifier that can sense the full wave of the alternating current (AC) in a discharge lamp. 
     BACKGROUND 
     A discharge lamp, such as a cold cathode fluorescent lamp (CCFL), has terminal voltage characteristics that vary depending upon the immediate history and the frequency of a stimulus (AC signal) applied to the lamp. Until the CCFL is “struck” or ignited, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage. Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to a run voltage that is approximately ⅓ of the strike voltage over a relatively wide range of input currents. When the CCFL is driven by an AC signal at a relatively high frequency, the CCFL (once struck) will not extinguish on each cycle and will exhibit a positive resistance terminal characteristic. 
     Driving a CCFL with a relatively high frequency square-shaped AC signal will produce the maximum useful lifetime for the lamp. However, since the square shape of an AC signal may cause significant interference with other circuits in the vicinity of the circuitry driving the CCFL, the lamp is typically driven with an AC signal that has a less than optimal shape such as a sine-shaped AC signal. 
     Typically, the lamp is driven by an inverter, which converts a DC signal to an AC signal, filters the AC signal, and transforms the voltage to the higher voltages required by a CCFL. Examples of such inverters are shown in U.S. Pat. No. 6,114,814 to Shannon et al., assigned to the assignee of the present invention and herein incorporated by reference in its entirety. Also, the MPN1011, MP1015, and MP1018 products from Monolithic Power Systems, Inc. are exemplary of the type of inverter-used to drive a CCFL. 
     In order to most efficiently deliver power to the lamp, it is necessary to monitor the current delivered to the lamp. Therefore, a sense amplifier is used to monitor the lamp current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an exemplary schematic of a current controlled integrated circuit coupled to another tank circuit on a primary side of the step-up transformer for driving the discharge lamp; 
     FIGS. 2A and 2B are schematic diagrams of prior art half-wave sense amplifiers; and 
     FIG. 3 is a schematic diagram of a full-wave sense amplifier formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     As noted above, inverters for driving a CCFL typically comprise a DC to AC converter, a filter circuit, and a transformer. Examples of such circuits are shown in U.S. Pat. No. 6,114,814 to Shannon et al., assigned to the assignee of the present invention and herein incorporated by reference in its entirety. In addition, other prior art inverter circuits, such as a current-fed push-pull (Royer) oscillator, a constant frequency half-bridge (CFHB) circuit, or an inductive-mode half-bridge (IMIB) circuit, may be used to drive a CCFL. The present invention may be used in conjunction with any of these inverter circuits, as well as other inverter circuits. 
     The disclosure herein teaches a method and apparatus for monitoring the current drawn by (or delivered to) a lamp. In accordance with the invention, a sense amplifier is used to monitor the full wave of the current, not just the negative going or positive going portions. Prior to proceeding with the description of the sense amplifier, a brief description of the operation of an inverter and lamp combination is provided. It can be appreciated that this is but one embodiment of an inverter, and therefore, the sense amplifier of the present invention may be used with nearly any inverter design using current monitoring. 
     In one embodiment, the present invention is an integrated circuit (IC) that includes four power MOSFETs arranged in an H-bridge circuit. The IC in combination with a separate output network inverts a direct current (DC) signal into an alternating current (AC). The IC operates near the resonant frequency of the output filter network comprising inductive and capacitive elements. 
     Losses in the filter network can be minimized by designing for a low loaded Q (to minimize current circulating through the tank components and the switches) and a high unloaded Q (which means the inductors and capacitors have low loss). Nevertheless, the harmonic content of the output waveform should be maintained at a low level to ensure that the inverter does not interfere with the operation of nearby circuits. 
     In one typical circuit, the H-bridge circuit generates an AC signal by periodically inverting a DC signal. The control circuitry regulates the amount of electrical power delivered to the load by modulating the pulse width (PWM) of each half cycle of the AC signal. Since the PWM provides for a symmetrical AC signal during normal operation, even harmonic frequencies in the AC signal are canceled out. By eliminating the even harmonics and generally operating at the resonant frequency of the filter (load), the designed loaded Q value of the filter may be fairly low and losses in the filter may be minimized. Also, since the CCFL is connected directly across the secondary winding of the step-up transformer, except for the fraction of a second required to strike an arc inside the lamp, the step-up transformers secondary winding generally operates at the run voltage of the CCFL. Further, it will be seen further below that the control circuitry will selectively increase the width of the pulses provided to the load during striking of the load, relative to normal operation. 
     Turning now to FIG. 1, an exemplary schematic  100 ′ displays the power control embodiment of an integrated circuit  104  (IC) coupled to a load that includes a tank circuit  108  and a lamp  106  such as a CCFL. A DC power supply  102 , i.e., a battery, is connected to IC  104 . A boost capacitor  120   a  is connected between a BSTR terminal and an output terminal  110   a , which is connected to another terminal labeled as OUTR. Similarly, another boost capacitor  120   b  is connected between a BSTL terminal and an output terminal  110   b  that is connected to another terminal identified as OUTL. The boost capacitors  120   a  and  120   b  are energy reservoirs that provide a source of power to operate circuitry inside the IC  104  that can float above the operating voltage of the rest of the circuitry. 
     An end of inductor  116  is connected to the output terminal  110   a  and an opposite end of the inductor is coupled to an end of a capacitor  118  and an end of a primary winding of a step-up transformer  114 . An opposite end of the capacitor  118  is coupled to another end of the primary winding of the step-up transformer  114  and the output terminal  110   b . An end of a secondary winding for the step-up transformer  114  is connected to a lamp terminal  112   a  and another end of the secondary winding is connected to a lamp terminal  112   b.    
     A reactive output network or the “tank” circuit  108  is formed by the components connected between the output terminals  110   a  and  110   b  and the primary winding of the step-up transformer  114 . The tank circuit is a second-order resonant filter that stores electrical energy at a particular frequency and discharges this energy as necessary to smooth the sinusoidal shape of the AC signal delivered to the lamp  106 . The tank circuit is also referred to as a self-oscillating circuit. 
     Further, a circuit for current sensing is included. Note that the second terminal of the secondary winding is directly connected to ground. The other lamp terminal  112   b  is coupled to an anode of a diode  107  and a cathode of a diode  105 . The cathode of the diode  107  is coupled to an end of a sense resistor  109  and a V sense  terminal at the IC  104 . The anode of the diode  105  is coupled to the other end the sense resistor  109  and ground. In this case, the IC  104  monitors the voltage across the sense resistor  109  so that the amount of current flowing into the lamp  106  may be approximated and used to control the amount of electrical power used to drive the lamp. 
     The signal carried on the V sense  terminal is provided to a half-wave sense amplifier  201 . In one prior art embodiment, the sense amplifier is shown in FIG.  2 A. The half-wave sense amplifier  201  comprises an operational amplifier  203 , an output transistor  205 , a current source  207 , and a resistor  209 . Because of the arrangement of the diodes  105  and  107 , the current I L , and thus the voltage V across the sense resistor  109 , only captures the positive going half of the current through the lamp. It should be noted that sensing of the current and sensing of the voltage is synonymous. In other words, it can be appreciated that sensing the voltage across the sense resistor  109  is the same as sensing the current drawn by the lamp. 
     In operation, current flowing out of the diode  107  will travel through the sense resistor  109 , causing a voltage to be placed on the non-inverting input of the operational amplifier  203 . The output of the operational amplifier is provided to the gate of the output transistor  205 . The drain of the output transistor  205  is connected to the inverting input of the operational amplifier  203  and one terminal of the resistor  209 . The other terminal of the resistor  209  is connected to ground. 
     The source of the output transistor is connected to current source  207 . The amount of current drawn from the current source  207  is thus indicative of the current drawn by the lamp. Note that the current drawn from the current source  207  is the inverse of the current drawn by the lamp because of the inverting action of the operational amplifier  203 . 
     FIG. 2B shows another embodiment of a prior art sense amplifier  201 . In this arrangement, the summing node is at the inverting input to the operational amplifier  203 . A feedback capacitor C f  and feedback resistor R f  are placed between the output of the operational amplifier  203  and the inverting input. Again, this arrangement still only looks at half of the current being provided to the lamp. 
     In accordance with the present invention, turning to FIG. 3, a full wave sense amplifier  301  is shown. Note initially, that the full wave sense amplifier  301  is intended for use without the diodes  105  and  107  of FIGS. 1B and 2. Thus, these elements are removed and the signal V sense  is taken directly from node  112   b  of FIG.  1 . 
     The full wave sense amplifier  301  includes the operational amplifier  203 , the output transistor  205 , resistor  209 , and current source  207  of the prior art half wave amplifier  201 . Additionally, the full wave sense amplifier  301  also includes a second operational amplifier  307 , a second output transistor  305 , and an input resistor  303 . 
     The signal V sense  is provided through input resistor  303  to the inverting input of the second operational amplifier. The non-inverting input of the second operational amplifier  307  is grounded. The output of the second operational amplifier is connected to the gate of the second output transistor  305 . The drain of the second output transistor  305  is connected to the inverting input of the second operational amplifier  307 . The source of the second output transistor  305  is connected to current source  207 . 
     As noted above, the diodes  105  and  107  are eliminated. This results in the signal V sense  following the current drawn by the lamp. It can be appreciated that the current drawn by the lamp across the resistor  109  is the voltage signal V sense . The positive going half of the signal V sense  is captured by the operational amplifier  203  and first output resistor  205 . The negative going half of the signal V sense  is captured by the second operational amplifier  307  and second output transistor  305 . The result of this arrangement is the full wave of the signal V sense  is captured by the current drawn from current source  207 , as seen in FIG.  3 . 
     There are some advantages to sensing the full sinusoidal wave of the lamp current. Sensing both half-cycles effectively doubles the sampling rate of the loop and allows for a faster loop time constant and therefore tighter control of the loop. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.