Patent Publication Number: US-2003227783-A1

Title: Circuit and method for controlling display power to a plasma display

Description:
[0001] This application claims benefit under 35 U.S.C. 119 (a) of co-pending European Application Serial No. 02425113.4 filed Mar. 1, 2002, in the European Patent Office, entitled “Power Circuit for a Plasma TV Display, Plasma Television Set Containing Said Circuit and Respective Powering Method” which is hereby incorporated by reference.  
       [0002] Be it known that we, Antonio Canova, Lorenzo Cincinelli and Mauro Piazzesi, Italian citizens residing in Arezzo, Italy, have invented a new and useful “Circuit and Method for Controlling Display Power to a Plasma Display.” 
       BACKGROUND OF THE INVENTION  
       [0003] This invention relates to a power circuit for a plasma television display. The invention also relates to a plasma television containing said circuit and a method for controlling display power via the power circuit.  
       [0004] Plasma televisions have been known in the art for many years and are being increasingly more popular on the market thanks to recent technological perfecting. Patents related to plasma display technology include the following U.S. Pat. Nos. 4,130,777; 4,233,623; 4,329,626; 5,808,420; 6,211,867.  
       [0005] Plasma displays present considerable electrical power problems due to their high capacitance load. Currently, these types of displays are powered by means of PWM converters, generally of the flyback type for generating high voltage and of the forward type for generating low voltage. The use of these types of converters for powering plasma displays presents considerable problems and shortcomings. Particularly, high dV/dt and dl/dt switching (hard switching) with generation of high frequency harmonics cause problems of electromagnetic interference and the performance is not always high.  
       SUMMARY OF THE INVENTION  
       [0006] One object of the present invention is to provide a power circuit for plasma displays which overcomes the shortcomings of traditional power systems. Another object of the invention is the realization of a display for televisions and the like, with a new power circuit.  
       [0007] Essentially, according to the invention, a plasma display is powered by a power circuit comprising a resonant converter and, preferably, a resonant converter in a series-parallel configuration. The invention is therefore based on a new use of the resonant converter, characterized in that said converter is used in a plasma display power circuit.  
       [0008] A number of advantages are obtained by using a resonant converter to power a plasma display, some of which are listed below:  
       [0009] the converter MOSFETs switch at zero voltage and zero current (Zero Voltage Switching, Zero Current Switching) which entails highly efficient switching;  
       [0010] the current waveform is nearly sinusoidal and this entails a considerable reduction of conducted and irradiated noise; minimal electromagnetic shielding is required;  
       [0011] the circuit is cost-effective and employs a low number of components, because it exploits the parasitic capacitance of semiconductors (MOSFETs) of the half bridge of the converter and the leakage inductance of the transformer;  
       [0012] a single winding is used, which additionally simplifies assembly and reduces device weight.  
       [0013] According to a particularly advantageous embodiment of the invention, the power circuit comprises a current control loop and a voltage control loop between the load applied to the converter and the converter itself. The loops control the working frequency of the converter. By means of this arrangement it is possible to limit the current output from the power circuit to the display at turn-on, in spite of the high capacitance of the load applied to the converter. Power is voltage controlled when the capacitance of the display is charged at the required voltage. The current control ring will start working again in the event of over-current.  
       [0014] In practice, the current control loop and the voltage control loop are connected to the converter according to an alternative configuration, by which the current control ring is active when a voltage lower than a predetermined value is present between the output terminals of the converter, while the voltage control loop is active when a voltage exceeding said predetermined value is present between the output terminals of the converter. For this purpose, corresponding one-way components, essentially consisting of respective diodes, are advantageously arranged on the output of respective operational amplifiers inserted in said two control loops.  
       [0015] Advantageously, the voltage and current control loops are galvanically isolated from the switching control circuit of the converter, e.g. by interposing an optical coupler, i.e. a photocoupler.  
       [0016] The circuit according to the invention can be used to implement a method for powering a plasma display, e.g. a television display, characterized in that: said display is powered by a resonant converter; in that the output current and voltage of said converter are controlled by a current control loop and a voltage control loop, the power current being limited by said current control ring until the output voltage of said converter has reached a predetermined value. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1 is a simplified circuit diagram of the power circuit according to the invention applied to a plasma display.  
     [0018]FIG. 2 shows the current and voltage waveform patterns at the output of the power circuit according to the invention when the display is turned on;  
     [0019]FIG. 3 is a more detailed schematic of one embodiment of the circuit according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0020]FIG. 1 schematically shows a power circuit of a plasma display according to the invention. The power circuit is generally indicated by reference numeral  1  and the plasma display is generally indicated by reference numeral  3  and schematically illustrated in the form of a capacitive system consisting of the parallel arrangement of a plurality of capacitors  5 A,  5 B, . . .  5 N and a generic load  7 .  
     [0021] The power circuit  1  comprises a resonant converter in series-parallel configuration, indicated by reference numeral  9 . This type of converter is generally known and operation of the device will not be described in detail herein. For a detailed theoretical description of the operation of this type of converter, reference can be made to M. K. Kazimierczuk, “ Class D Voltage Switching Power Inverter ”, IEE Proc. Volume 138, November 1991; M. K. Kazimierczuk and W. Szaraniec, “ Class D Voltage Switching Inverter with Only One Shunt Capacitor ”, IEE Proc. Volume 139, September 1992; Marian K. Kazimierczuk, Dariusz Czarkowski, “ Resonant Power Converters ”, Editor John Wiley &amp; Sons Inc., 1995.  
     [0022] In the diagram of FIG. 1, references  11 A and  11 B indicate two MOSFETs in half-bridge configuration, whose switching is controlled by a converter control circuit  13 . This may be, for example, an integrated circuit of the IR21 571 family made by International Rectifier (USA), or an L6598 integrated circuit made by STM (Italy), or other equivalent circuit. The half-bridge arrangement of the MOSFET&#39;S is connected to a converter output circuit wherein reference numeral  15  indicates the output transformer and reference numerals  17 ,  19  indicate the two rectifier diodes on the secondary winding output of the transformer  15 . Reference numeral  21  indicates the resonant capacitor in series with the primary winding of the transformer  15 . The resonant capacitance in parallel of the primary winding of the transformer  15  is represented by the internal parasitic capacitance of the two MOSFETs  11 A,  11 B.  
     [0023] A current sensor is arranged on the negative output terminal of the converter  9 , specifically a reading resistor  23 , which reads the current output by the converter  9  to the display  3 . The voltage drop at the terminals of the reading resistor  23  is proportional to the current I output by the converter  9  and is used in a current control loop, generally indicated by reference numeral  25 , to limit the maximum value of output current.  
     [0024] The current control loop  25  comprises a first operational amplifier  27  to whose inverting terminal a voltage, which is proportional to the voltage upstream to the reading resistor  23 , is applied. A reference voltage, determined by the voltage Vref, is applied to the non-inverting terminal of the operational amplifier  27 . Reference numerals  29  and  31  indicate a resistor and a capacitor of the reaction branch of the operational amplifier  27 , between the inverting terminal and the output. The output of the operational amplifier  27  is connected, via a diode  33 , to a photocoupler  35 . The output signal of the operational amplifier  27  is thus transmitted to the control circuit  13 , for the purposes described below, following galvanic uncoupling.  
     [0025] In addition to the current control loop described above, the power circuit  1  comprises a voltage control loop, generally indicated by reference numeral  37 . The loop  37  comprises a second operational amplifier  39 , on whose inverting input a voltage, which is proportional to the voltage applied by the converter  9  to the load, i.e. the plasma display  3 , is applied. A reference voltage is applied to the non-inverting input of the operational amplifier  39 . A reaction branch, comprising a resistor  41  and a capacitor  43 , is arranged between the inverting input and output of the operational amplifier  39 . Similarly to the output of the first operational amplifier  27 , the second operational amplifier output  39  is connected to the photocoupler  35  via a diode  45 .  
     [0026] The operation of the circuit  1  described so far is as follows. The capacitors  5 A . . .  5 N are discharged when the display  3  is turned on. Due to the very high capacitance of the capacitors (in the order of several thousands of microfarads), the switching of the power circuit  1  would short-circuit the converter  9 , with consequent irreversible damage to internal components. The purpose of the current control loop  25  is to avoid this event, by imposing a maximum current value. When the display  3  is turned on, the output of the first operational amplifier  27  is kept at a low level by the high voltage drop on the reading resistor  23 , because the voltage applied to the inverting terminal (voltage upstream to the reading resistor  23 ) prevails on the reference voltage applied to the non-inverting terminal. Consequently, the diode  33  is switched to conducting and a signal is sent to the control circuit  13  by the photocoupler  35 . The signal tends to reduce the switching frequency of the MOSFETs  11 A,  11 B, distancing it from the working frequency, which in turn is higher than the resonance frequency.  
     [0027] Following a current peak output by the power circuit  1  caused by the inevitable delay in the operation of the current control loop  25 , the signal output by the first operational amplifier  27  keeps the value of the current I at a constant and controlled level with a gradual increase of output voltage of the first operational amplifier  27  and a consequent reduction of the emission of the photodiode of the photocoupler  35 , due to the gradual accumulation of charge in the capacitors  5 A . . .  5 N of the display  3 . The switching frequency of the converter  9  is consequently reduced.  
     [0028] During this initial phase, the output of the second operational amplifier  39  is high and the diode  45  is blocked. Consequently, the voltage control loop  37  is not active.  
     [0029] The capacitors  5 A . . .  5 N of the display  3  are gradually charged as the current controlled by the current control loop  25  flows through the display  3 . Consequently, the voltage V between the output terminals of power circuit  1  increases gradually until the voltage applied to the inverting terminal of the second operational amplified  39  prevails on the reference voltage applied to the corresponding non-inverting terminal. Consequently, the output voltage of the second operational amplifier  39  is lowered and the diode  45  switches to conduction. T he corresponding increase of the output voltage of the first operational amplifier  27  blocks the diode  33 . In this way, the voltage control loop  37  comes into operation while the current control loop  25  is deactivated and the switching frequency of the converter  9  reaches running value.  
     [0030] During normal operation of the device, the switching frequency control and, consequently, the power conditions, is governed by the voltage control loop  37 , unless an over-current occurs, in which case the current control loop  25  limits the current I to a maximum value.  
     [0031]FIG. 2 shows the waveforms of the current I and the voltage V when the display  3  is switched on. The converter is switched on in instant t1. A current peak due to the delay in current control loop operation occurs between instant t1 and t2. Between instant t2 and instant t3, the current I is kept at a level I1 which is essentially constant and is returned to a minimum value I2 in instant t3. At the same time, the voltage V increases from the value V1 to the value V2, which is reached in instant t3 in an essentially linear fashion. Instant t3 is when the running voltage across the capacitors  5 A . . .  5 N of the display  3  is reached.  
     [0032]FIG. 3 shows a more detailed circuit diagram of a form of embodiment of the power circuit  1  shown in FIG. 1. Equal reference numerals indicate parts which are either equal or corresponding to those in FIG. 1.  
     [0033] The center-tapped transformer and the arrangement of the two diodes  17 ,  19  can clearly be replaced with equivalent arrangements, e.g. by a non-center-tapped transformer and a diode bridge rectifier.  
     [0034] It should be understood that the drawing shows only a possible embodiment of the invention which may vary in its forms and arrangements without departing from the scope of the concept underlying the invention. The possible presence of reference numbers in the enclosed claims has only the purpose of facilitating the reading of the claims in view of the above description and of the enclosed drawings and does not limit their scope of protection.