Abstract:
For achieving a desired course in the demagnetizing circuit (I 1 ) and power dissipation that is as low as possible during continuous operation of a color television set, a demagnetizing circuit for controlling the demagnetizing current (I 1 ) includes two transistors (T 1 , T 2 ) that are controlled via a common or via two separate capacitive voltage dividers (C 1 -C 4 ). A rectified alternating voltage is applied to the capacitive voltage dividers (C 1 -C 4 ). The demagnetizing current (I 1 ) controlled by the transistors (T 1 , T 2 ) is supplied to a demagnetizing coil (R 4 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the field of demagnetizing circuits, and in particular to a demagnetizing circuit for demagnetizing color picture tubes.  
           [0002]    Color picture tubes must be demagnetized in order to obtain sufficient color purity. For this reason, a demagnetizing coil is used through which a fading high-amplitude alternating current is sent when the equipment is turned on. However, the leakage current flowing through the demagnetizing coil during continuous operation should be as low as possible in order to reduce power dissipation.  
           [0003]    In conventional demagnetizing circuits, a positive temperature coefficient (PTC) thermistor in series with the demagnetizing coil is employed for obtaining the decreasing amplitude in the alternating current. The PTC thermistor is a resistor with a resistance that is a function of temperature, wherein the resistance increases as the temperature increases. The resistance of the PTC thermistor is thus very low when the equipment is turned on, that is, when it is cold, but it is substantially higher when it is warmed up in the operating mode.  
           [0004]    A problem with a PTC thermistor is that it suffers from the disadvantage that during continuous operation of the equipment, leakage current flowing through the demagnetizing coil and the PTC thermistor causes continuous power dissipation of approximately 2 W. This is particularly troubling in standby mode operation because the power consumption should be as low as possible in that mode of operation. Therefore, in expensive television sets the demagnetizing current, (i.e., the current flowing through the demagnetizing coil and the PTC thermistor) is turned off during continuous operation using an additional circuit (e.g., that includes a triac or optical coupling device).  
           [0005]    Therefore, there is a need for a demagnetizing circuit in which the desired current flow can be achieved with a reduced complexity control circuit and without substantial power dissipation occurring during continuous operation.  
         SUMMARY OF THE INVENTION  
         [0006]    Briefly, according to an aspect of the present invention, a demagnetizing circuit for controlling a demagnetizing current applied to a demagnetizing coil includes at least two transistors that are controlled by at least one capacitive voltage divider. A rectified alternating voltage is applied to the capacitive voltage divider, which applies control signals to the transistors to control the demagnetizing current supplied to the demagnetizing coil.  
           [0007]    The demagnetizing circuit uses MOS or bipolar transistors rather than a PTC thermistor. Thus, with modest complexity in terms of the control, not only can a demagnetizing current with fading amplitude be produced, but the demagnetizing current returns to zero. As a result, after the demagnetizing no power dissipation occurs, which is particularly advantageous when the equipment is in standby mode.  
           [0008]    The transistors are controlled via a capacitive circuit that may include a single capacitive voltage divider, or at least two separate capacitive voltage dividers. Ideally, the inverse diode generally present in MOS transistors is also used. When using bipolar transistors that are not equipped with such inverse diodes, discrete diodes must be provided.  
           [0009]    In accordance with one exemplary embodiment of the present invention, the complexity of the control can be further reduced when the source and gate terminals for the two MOS transistors are connected to one another so that the demagnetizing circuit can be operated with just one capacitive voltage divider.  
           [0010]    In another aspect of the present invention, a demagnetizing circuit may retroactively actuate or activate the demagnetizing even after the equipment has been turned on, so that demagnetizing can also be performed during continuous operation of the equipment. This is particularly desirable when the equipment remains powered up for an extended period and is merely switched to standby outside of operating times. In this embodiment, an additional transistor is used (e.g., a small-signal transistor) and a corresponding voltage must be applied to this additional transistor to switch this transistor to the conducting state to initiate the demagnetizing. For instance, this can occur with a voltage that is low in the equipment standby mode and is high in the operating mode.  
           [0011]    The invention is particularly suitable for demagnetizing color picture tubes in television equipment. However, the invention is not restricted to this field of application; rather, it can be used in general whenever demagnetizing is to be performed using a demagnetizing coil.  
           [0012]    These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    [0013]FIG. 1 is a schematic illustration of a demagnetizing circuit in accordance with a first exemplary embodiment of the present invention;  
         [0014]    [0014]FIG. 2 illustrates a plot of various voltage potentials and the demagnetizing current, both as a function of time, for the circuit illustrated in FIG. 1;  
         [0015]    [0015]FIG. 3 is a schematic illustration of a demagnetizing circuit in accordance with a second exemplary embodiment of the present invention;  
         [0016]    [0016]FIG. 4 is a schematic illustration of a demagnetizing circuit in accordance with a third exemplary embodiment of the present invention; and  
         [0017]    [0017]FIG. 5 is a schematic illustration of a demagnetizing circuit in accordance with a fourth exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]FIG. 1 is a schematic illustration of a demagnetizing circuit that includes two MOS transistors for controlling demagnetizing current. The demagnetizing circuit includes two terminals AC 1  and AC 2  for applying an alternating voltage. Interposed between the two terminals AC 1  and AC 2  is a series connection comprising a first MOS transistor T 1 , two resistors R 3  and R 4 , and a second MOS transistor T 2 . The resistor R 3  limits the current, while the resistor R 4  corresponds to the ohmic equivalent resistance of a demagnetizing coil provided for demagnetizing. The drain terminals of the two transistors T 1  and T 2  are labeled D in FIG. 1, the source terminals are labeled S, and the gate terminals are labeled G.  
         [0019]    The two transistors T 1  and T 2  are each controlled via a capacitive voltage divider C 1 , C 2 , and C 3 , C 4 , respectively. The capacitors C 2  and C 4  are each connected to a control terminal ST, while the capacitors C 1  and C 3  connect the gate terminals G and the source terminals S of the transistors T 1  and T 2 . Clipper diodes D 1  and D 2  and discharge resistors R 1  and R 2  are connected parallel to capacitors C 1  and C 3 , respectively.  
         [0020]    Connected to the control terminal ST is the output of a bridge rectifier BR, the inputs of which are connected to the two terminals AC 1  and AC 2 . The bridge rectifier BR ensures that only positive half-waves are applied to the control terminal ST. Also connected to the control terminal ST is an electrolyte capacitor C 5 , the other end of which is connected to a ground. This electrolyte capacitor C 5  smoothes the voltage rectified by the bridge rectifier BR. Both the bridge rectifier BR and the electrolyte capacitor C 5  are components of a power supply unit that is to be connected to the demagnetizing circuit, as illustrated in FIG. 1.  
         [0021]    The function of the demagnetizing circuit illustrated in FIG. 1 is explained in the following referring to FIG. 2 for the terminal AC 1  and transistor T 1 . FIG. 2 illustrates a plot of the demagnetizing current I 1  flowing through the demagnetizing coil R 4  and the voltage potentials V 1  and V 3  with respect to the half-waves applied to terminal AC 1  shown in FIG. 1.  
         [0022]    When the power supply is on, the electrolyte capacitor C 5  is charged to the peak value of the supply voltage. During the next zero crossing of the supply voltage, the transistor T 1  is first conductive, since the gate terminal G of the transistor T 1  is positively biased relative to the source terminal S by the capacitive voltage dividers C 1  and C 2  located between the source terminal S of the transistor T 1  and the positive pole of the electrolytic capacitor C 5 . The gate/source voltage Vgs of the transistor T 1  is Vgs=V 3 *C 2 /(C 1 +C 2 ) (if the effect of the clipper diode D 1  is ignored).  
         [0023]    The clipper diode D 1  protects against exceeding the permissible gate/source voltage and ensures that the transient characteristics remain constant regardless of the height of the current supply voltage, whereby the amplitude of the demagnetizing current is reduced from one half-wave to the other.  
         [0024]    The values of the capacitors C 1  and C 2  should be such that the transistor T 1  can be fully turned on even at the smallest supply voltage at which the equipment can run.  
         [0025]    If the voltage applied to terminal AC 1  rises to its peak value during a half wave, the gate/source voltage of the transistor T 1  decreases since the voltage drops via the capacitive voltage dividers C 1 , C 2 . During the next maximum power, the voltage on the terminal AC 1  reaches the value of the voltage potential V 3  on the electrolyte capacitor C 5 , since V 3  equals the peak value of the voltage applied to terminal AC 1 . In contrast, the voltage potential V 1  does not quite reach the peak value, since the transistor T 1  begins to block shortly before the peak value is achieved. The voltage V 1  stabilizes at a value at which the transistor T 1  just remains conductive. The demagnetizing current I 1  flows in the forward direction through the current-conducting path of the transistor T 1  and over the resistors R 3  and R 4 , while the demagnetizing current flows in the reverse direction through the transistor T 2  (through the integrated reverse-conducting inverse diode).  
         [0026]    Once the peak value of the supply voltage is exceeded, the voltage applied to the terminal AC decreases again. Initially the voltage V 1  largely retains its value, and not until the voltage applied to the terminal AC 1  is less than the voltage V 1  does the transistor T 1  become completely conductive again and the voltage V 1  drops with the voltage applied to the terminal AC 1 .  
         [0027]    The process repeats itself with the next half wave, whereby in this case the process plays out with respect to the voltage applied to the terminal AC 2  in the lower area of the circuit (i.e., in the components T 2 , C 3  and C 4 , D 2 , and R 2 ). The voltage applied to the terminal AC 1  remains at zero, while the voltage applied to the terminal AC 2  changes in accordance with a sinusoidal half wave.  
         [0028]    These processes repeat themselves during the subsequent half waves, whereby however the capacitors C 1  and C 3  are gradually discharged through the resistors R 1  and R 2 . The voltages V 1  and V 2  therefore increase less and less, so the demagnetizing current I 1  flowing through the resistors R 3  and R 4  gradually decreases. In particular the voltages V 1  and V 2  and the demagnetizing current I 1  decrease exponentially, as shown in FIG. 2 (only V 1  is illustrated in FIG. 2), whereby the period T of the demagnetizing current I 1  is 20 ms at 50 Hz supply voltage. In this manner, the desired course for the demagnetizing current I 1  as described at the beginning is obtained using the demagnetizing circuit illustrated in FIG. 1.  
         [0029]    [0029]FIG. 3 illustrates a simplified exemplary embodiment of the present invention. The resistors in the electric circuit (i.e., the current-limiting resistor R 3  and the ohmic resistance of the demagnetizing coil) are divided equally on the upper and lower parts of the circuit (R 3 =R 4 ). The source and gate terminals of the two transistors T 1  and T 2  are connected to one another. A common capacitive voltage divider C 1 , C 2  is provided for the two transistors T 1  and T 2  (with clipper diode D 1  connected parallel to the capacitor C 1  and with a parallel-connected discharge resistor R 1 ), so that the complexity of the control circuit is half that of the exemplary embodiment illustrated in FIG. 1.  
         [0030]    [0030]FIG. 4 illustrates an exemplary embodiment equivalent to the exemplary embodiment illustrated in FIG. 1, wherein bipolar transistors are used rather than MOS transistors. If the bipolar transistors do not already contain reverse-conducting inverse diodes like the MOS transistors, these must be additionally provided. Therefore, the embodiment illustrated in FIG. 4 includes additional diodes D 3  and D 4  associated with the bipolar transistors T 1  and T 2 , respectively. Since bipolar transistors by nature have a limit in the base voltage, in contrast to FIG. 1 and FIG. 2 the limiting diodes D 1  and D 2  can be omitted, at least in a narrow region of the supply voltage. The additional resistors R 5  and R 6  included in the embodiment illustrated in FIG. 4 act as voltage dividers for the base voltage of the transistors T 1  and T 2 .  
         [0031]    [0031]FIG. 5 illustrates a demagnetizing circuit that makes it possible to retroactively demagnetize, even after the supply voltage has been turned on. Thus, with this circuit it is possible to demagnetize even when the equipment is operating.  
         [0032]    In the exemplary embodiment illustrated in FIG. 5, current-limiting resistors R 31  and R 32  are divided equally on the upper and lower part of the circuit. The gate terminals for the two transistors T 1  and T 2  are connected to one another as in FIG. 3. The electric circuit for the demagnetizing current I 1  runs in a corresponding half wave from the first supply voltage terminal AC 1  via the first current-limiting resistor R 31 , the first transistor T 1 , the demagnetizing coil and its ohmic resistor R 4 , the second transistor T 2 , and the second current-limiting resistor R 32  to the second supply voltage terminal AC 2 . The current I 1  flows in the reverse direction in the subsequent half wave.  
         [0033]    Referring still to FIG. 5, the control member that ensures the exponential damping of the demagnetizing current I 1  includes capacitor C 2 , discharge resistor R 1 , and clipper diode D 1 . In this embodiment, the capacitor C 1  is not required since the gate/source capacitors of the two transistors T 1  and T 2  are formed by the parasitic input capacitors of these transistors.  
         [0034]    For reasons of clarity, neither the bridge rectifier BR nor the electrolyte capacitor C 5  are illustrated in FIG. 5. In this exemplary embodiment, terminal K 1  acts as control terminal ST; the power supply is to be connected thereto with the connection point between the bridge rectifier BR and the electrolyte capacitor C 5 . In contrast to the preceding exemplary embodiments, the capacitor C 2  is not connected directly to the electrolyte capacitor C 5 , but rather via an additional resistor R 7 .  
         [0035]    The connection point for the resistor R 7  to the capacitor C 2  is connected to a ground via a series connection out of another capacitor C 6  and the collector-emitter segment of another transistor T 3 . The transistor T 3  is a small-signal transistor that must, however, be voltage-stable up to approximately 300 V. Connected parallel to the capacitor C 6  is another discharge resistor R 9 , and resistor R 8  is interposed between the connection point of the resistors R 7 , R 9  and the ground.  
         [0036]    The demagnetizing circuit illustrated in FIG. 5 functions as follows. In steady-state after the first demagnetizing (i.e., after the supply voltage has been turned on) the gate terminals G that are connected to one another and that are from the two MOS field effect transistors T 1  and T 2  discharge to the source potential so that the transistors T 1  and T 2  block and no more demagnetizing current I 1  flows. The collector of the transistor T 3  applies a voltage that is somewhat lower than the high voltage on the control terminal (i.e., on the terminal K 1 ). The voltage is lowered by the voltage dividers formed from the resistors R 7  and R 8  and this ensures that the permissible collector voltage of the transistor T 3  is not exceeded.  
         [0037]    If at some later time additional demagnetizing must be performed, the transistor T 3  is switched to the conducting state by applying a suitable voltage to the base terminal K 2 . Thus, in the embodiment of the transistor T 3  illustrated in FIG. 5 wherein T 3  is configured as an npn-transistor, a positive voltage must be applied to the terminal. This can occur, for example, by a voltage that is too low in the standby mode and is too high in the operating mode. A switched mode power supply control component such as TDA 16847 is particularly suitable for producing this voltage, since it has an output for power measurement at which a power-dependent voltage can be produced by simple wiring, but not a frequency or supply voltage-dependent voltage.  
         [0038]    By turning on the transistor T 3 , its collector is pulled to the ground, whereby the resulting negative voltage jump is transmitted via the capacitor C 6  so the voltage potential on the connection point of the capacitors C 2  and C 6  also drops almost to the ground potential (since the capacitors C 2  and C 6  are selected with C 2 &lt;&lt;C 6 , the voltage is only slightly capacitively divided). However, the voltage jump is also transmitted via the capacitor C 2  to the gate terminals G of the two transistors T 1  and T 2 . The gate terminals are held at ground potential by the diode D 1 . The capacitor C 6  is charged relatively rapidly via the resistor R 7  so that the voltage on the connection point of the capacitors C 2  and C 6  increases. This increase in voltage is transmitted by the capacitive voltage divider formed by the capacitor C 2  and the parasitic gate capacitors of the transistors T 1  and T 2 , to the gate terminals G of the two transistors. The limiting Zener diode D 1  prevents the permissible gate voltage from being exceeded. The transistors T 1  and T 2  are now conductive and a demagnetizing process is initiated as described above.  
         [0039]    The control circuit is re-set in preparation for another demagnetizing by turning the transistor T 3  off again. The charged capacitor C 6  is then gradually discharged through the resistance R 9 . Once the capacitor C 6  is discharged, the circuit is again ready for a new demagnetizing process.  
         [0040]    Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.