Abstract:
A computer monitor power supply system includes a comparator, a relay, and an inductor connected in series between the AC input and a flyback transformer circuit. The inductor includes two windings which are automatically switched between a parallel connection and a series connection upon switching between a 110 V and 220 V power input so as to reduce the amount of low frequency harmonics.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a power supply system for a computer monitor, and more particularly to a power supply system for both 110 V and 220 V AC sources which eliminates harmonics efficiently. 
     The ideal AC source voltage waveform is a pure 60 Hz sine wave. If a sine wave has a distortion (as shown in FIG. 1), it indicates the presence of a harmonic. The harmonic can be 120 Hz, 180 Hz, 240 Hz, . . . , that is, beyond the 60 Hz fundamental frequency. 
     When the 60 Hz square wave shown in FIG. 2a is analyzed by Fourier Series, the waveform shown in FIG. 2b results. FIGS. 4a to 4d show the input characteristics of a computer monitor power supply circuit in which the electric current waveform (see FIG. 4d) greatly differs from the sine wave, indicating the presence of significant harmonic current. Except for resistance type electrical heating apparatus, the input current of most electronic apparatus, and particularly ones using a switching power supply or phase control, carries surges. If the use of these electrical apparatus reaches a certain amount, as shown in FIG. 3, the harmonic current malfunctions shown in the following Table I will occur in the whole power supply system when the total load current of the end users contains a certain amount of harmonics. 
     
                                           TABLE I__________________________________________________________________________Interfered Apparatus              Malfunction Phenomenon__________________________________________________________________________Power System      Series reactance              burnout overheat, vibration, noise due to              overcurrent      Fuse    broken              melted due to overcurrentMotor circuit breaker              false actionElectric home appliance      Audio equipment              noise      Television set              picture jumpingOthers     Motor   noise, vibration      Elevator              vibration, shut down      Control Circuit              false action      Harmonic filter              trip off due to overcurrent__________________________________________________________________________ 
    
     Therefore, the IEC issued standard number IEC-1000-3-2 in 1993 to define harmonic current levels in public low voltage power supply systems. Monitors are classified as D type electric appliances, and the harmonic current of monitors is defined as shown in the following Table II.. 
     
                       TABLE II______________________________________          Maximum allowable                       Maximum allowable          harmonic current                       harmonicOder of harmonic          per Watt     current(n)            (mA/W)       (A)______________________________________3              3.4          2.305              1.9          1.147              1.0          0.779              0.5          0.4011              0.35        0.3313 ≦ n ≦ 39          3.85/n       0.15 × 15/n(odd oder harmonic effective)______________________________________ 
    
     In the conventional monitor power supply circuit shown in FIG. 4a, the load of the monitor must be capacitive because the rectified output is directly connected to the filter capacitor, and therefore an inductor is needed for neutralization to make the load close to pure resistance, so as to reduce harmonic current. The methods shown in the following Table III are commonly adopted. 
     
                                           TABLE III__________________________________________________________________________MethodItem      1. Series Inductor             2. Converter                        3. Active Filter__________________________________________________________________________harmonic improvement     barely acceptable             good       excellentCost      low     2.5 times above method                        3.5 times above method             1          1Efficiency     90 ˜ 95%             80 ˜ 85%                        80 ˜ 85%Power Factor     0.65 ˜ 0.75             0.8 ˜ 0.9                        0.9 ˜ 0.99Interference prevention     not needed             difficult to overcome                        difficult to overcome88 V ˜ 264 V input     not allowed             barely allowed                        allowedControl circuit     FIG. 5a FIG. 5b    FIG. 5c__________________________________________________________________________ 
    
     The aforesaid second method and third method have the drawbacks of high cost and serious electromagnetic interference, and therefore they are not popularly accepted. The aforesaid first method costs less, but is not suitable for a power range of from 110 V grade (110 V±20%) to 220 V grade (220 V±20%), and therefore the inductor used in a monitor to eliminate harmonics must be relatively changed subject to local power voltage. This limitation causes a great trouble to manufacturers and distributors. In order to facilitate the manufacturing process and sales management and to prevent a wrong use, a harmonic eliminating device should be applicable to both 110 V grade and 220 V grade power supplies, i.e., applicable to the currents ranging from range within AC 80 V˜AC 264 V. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished with the above-described circumstances in view. It is the main object of the present invention to provide a power supply circuit for a computer monitor which is applicable to both 110 V and 220 V currents, and which effectively eliminates harmonics. 
     According to the present invention, the inductor unit is made having two independent windings of same design, and the iron core is shaped like &#34;Λ&#34;. 
     When the power voltage input is 110 V grade, the two windings are connected in parallel as shown in FIG. 6a. When power voltage input is 220 V grade, the two windings are connected in series as shown in FIG. 6b. Either in parallel connection or series connection (the current output of the former is 2i, and the latter is i), the distribution and direction of current i of the windings are the same, i.e., the distribution and direction of the magnetic flux Φ within the iron cores are the same, and therefore the present invention is practical. In other words, when the power supply voltage of the monitor is at 110 V grade, the inductors are connected in parallel to reduce harmonic current from the monitor. When the power supply voltage of the monitor is at 220 V grade, the inductors are connected in series, and harmonic current from the monitor can also be reduced. Because the power supply circuit of a monitor uses the switching power supply circuit, doubling the source voltage cannot reduce the load current to one half, or reduce harmonic current proportionally. For example, FIG. 7a shows the harmonic current value measured from the monitor at AC 110 V, and FIG. 7b shows the harmonic current value measured from the monitor at AC 220 V. The former is 1.225 A, and the latter is 0.7174 A (not one half of the former). Further, different order harmonic currents do not change in the same ratio. Assume the monitor uses a power supply circuit constructed according to the present invention, the inductance for parallel connection is Lp=12 mH, and the inductance for series connection is Ls=48 mH, so that the value measured at AC 110 V is as shown in FIG. 8a, and the value measured at AC 220 V is as shown in FIG. 8b. It is apparent that FIG. 8b passes the limited values shown in Table II, but FIG. 8a does not pass. Therefore, the inductance of the inductors must be increased to such a level that the value measured at AC 110 V passes the limited values. At this stage the value of Ls surpasses 48 mH, so of course the value measured at AC 220 V also undoubtedly meets the requirement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a distorted waveform of a city power voltage. 
     FIG. 2a illustrates the waveform. of a 60 Hz square wave. 
     FIG. 2b is a spectral analysis waveform chart obtained from the 60 Hz square wave. 
     FIG. 3 illustrates the distribution of a power supply to subscribers. 
     FIGS. 4a to 4d illustrate the input characteristics of a conventional monitor power supply circuit. 
     FIG 5a is a circuit diagram of a harmonic current eliminating circuit constructed according to the first prior art method (series inductor type). 
     FIG. 5b is a circuit diagram of a harmonic current eliminating circuit constructed according to the second prior art method (converter type). 
     FIG. 5c is a circuit diagram of a harmonic current eliminating circuit constructed according to the third prior art method (active filter type). 
     FIG. 6a is a current, magnetic flux distribution chart showing the inductors connected in parallel for a 110 V AC source. 
     FIG. 6b is a current, magnetic flux distribution chart showing the inductors connected in series for a 220 V AC source. 
     FIG. 7a is a harmonic value chart measured from a monitor at AC 110 V FIG. 7b is a harmonic value chart measured from a monitor at AC 220 V. 
     FIG. 8a is a harmonic value chart measured from a computer monitor with the present invention at AC 110 V. 
     FIG. 8b is a harmonic value chart measured from a computer monitor with the present invention at AC 220 V. 
     FIG. 9 is a circuit block diagram of a computer monitor. 
     FIG. 10 is a circuit diagram of an integrated circuit UC3842A used in a computer monitor power supply circuit. 
     FIG. 11a illustrates the structure and application example of the integrated circuit UC3842A. 
     FIG. 11b is a supply current versus supply voltage operating curve obtained from the integrated circuit UC3842A. 
     FIG. 12a is a circuit diagram of the first embodiment according to the present invention (with a voltage-doubler rectified circuit at 110 V AC input). 
     FIG. 12b is a circuit diagram of the second embodiment of the present invention (with a voltage-doubler rectified circuit at 110 V AC input). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 9, a computer monitor generally comprises a power supply circuit 10, a synchronizing signal processor 20, a microprocessor 25, a deflection 30, a deflection driver 35, a high voltage circuit 40, a video pre-amplifier 50, and a CRT driver circuit 55. The main point of the present invention is the controlling operation of the power supply circuit 10. In this example, the power supply circuit 10 is a flyback type of power supply circuit. The synchronizing signal processor 20, the microprocessor 25, the deflection 30, the deflection driver 35, the high voltage circuit 40, the video pre-amplifier 50 and the CRT driver circuit 55 are known parts and not within the scope of the invention, and therefore they are not described in detail. 
     The operation principle of the power supply circuit 10 is outlined hereinafter. 
     (1) As illustrated in FIG. 10, the IC UC3842A I 802 , used in the power supply circuit 10, is a high performance current mode controller. The internal structure of the IC UC3842A I 802  is shown in FIG. 11a. 
     (2) Rectifier circuit: 
     Referring also to FIG. 12b, AC current is sent through resistor R 803  to diode D 801  for rectification, and then filtered through capacitor C 805 , and then sent to a starting circuit and a driving circuit. 
     (3) Starting Circuit: 
     Electric current is sent through resistor R 811 , resistor R 812  and diode D 805  to charge capacitor C 815  to 16 V. Because C 815  is connected to the 7 th  pin of IC UC3842A I 802 , IC UC3842A I 8021  is started when capacitor C 815  is charged to 16 V, and a PWM (pulse width modulation) signal is sent out through the 6 th  pin. The start current and working current of IC UC3842A I 802  are smaller than 1 mA and 17 mA respectively. Referring to FIG. 7b, after IC UC3842A I 802  has been started the working current is greater than the charging current through R 811  →R 812 , and IC UC3842A I 802  will shut down if the end voltage of capacitor C 815  drops below 10 V and current compensation is not obtained from diode D 810 . 
     (4) Driving circuit: 
     When the 6 th  pin of IC UC3842A I 802  outputs a pulse through transistor R 815  to transistor Q 805 , transistor Q 805  is turned on if the pulse is at high potential, or turned off if at low potential. When transistor Q 805  is on, current i goes through C 809  →T 801  →Q 805  →R 831  →C 809 . At this moment, 1/2 Li 2  energy will be stored in the ferrite core of transformer T 801  via the primary inductance L. After that, the transistor is off, and the magnetic energy 1/2 Li 2  will be transferred to electrical energy and released from the secondary windings of T 801  to generate several DC voltage outputs. 
     (5) Voltage feedback: 
     Voltage generated from the feedback winding at the secondary end of transformer T 801  is rectified by diode D 812 , then filtered by capacitor C 816 , then sent through diode D 810  to charge capacitor C 815  (please refer to the explanation on the aforesaid &#34;starting circuit&#34;), causing capacitor C 815  to be maintained within 10 V˜35 V (for example, 12 V) for the working of IC UC3842A I 802 . Voltage from capacitor C 815  is connected to resistor R 825  and a voltage divider circuit formed of resistor R 827  and resistor R 826 , and then connected to the second pin of IC UC3842A I 802 . When the voltage obtained from the voltage divider surpasses 2.5 V, the 6 th  pin of IC UC3842A I 802  stops outputting, so that output voltage is maintained at a constant value. 
     (6) Current-limiting protection: 
     In order to prevent an overcurrent during working of transistor Q 805 , resistor R 831  is connected in series to the source pin of transistor Q 805  to limit the voltage drop of Is×R 831  to below 1 V (wherein current Is is the source current of transistor Q 805 ). If the voltage drop surpasses 1 V, the 3rd pin of IC UC3842A I 802  detects an overload, causing IC UC3842A I 802  to shut down. 
     (7) Frequency control: 
     When IC UC3842A I 802  is working, its 8 th  pin provides a 5 V reference voltage to make the oscillator circuit of resistor R 817  and capacitor C 820  work. The oscillation frequency is determined by resistor R 817  and capacitor C 820 . When resistor R 817  =4.3 K and capacitor C 820  =0.022 μF, the frequency is about 20 KHz (see specification manual of IC UC3842A I 802 ). 
     (8) Error compensation: 
     IC UC3842A I 802  comprises an internal amplifier. The gain of the internal amplifier of IC UC3842A I 802  is controlled by resistor R 823 , and its response speed is controlled by capacitor C 818 . The voltage feedback value at the second pin of IC UC3842A I 802  is 2.5 V±Δv, where Δv=error value. The value of Δv is directly proportional to the output voltage error at each output end of the transformer T 801 , i.e., the greater the value of Δv (error value), the higher the output voltage error will be. In contrast, when the error value is reduced, the working frequency of IC UC3842A I 802  is increased, causing transistor Q 805  to increase its loss and temperature. The output voltage error is proportional to resistor 1/R 823  and capacitor C 818 . 
     (9) Induction voltage elimination: 
     Because transformer T 801  is an inductive load, when transistor Q 805  is changed from the &#34;On&#34; state to an &#34;Off&#34; state, a reverse electromotive force is produced at the primary end of transformer T 801 . The electromotive force is provided by the induced voltage of L×(di/dt), in which L is inductance of the primary wiring of transformer T 801  ; I=conduction current at transistor Q 805  ; and t=fall time of transistor Q 805 . The induced voltage is as high as several thousand volts, which surpasses the breakdown voltage of transistor Q 805 . In order to protect transistor Q 805 , snubbers are provided to absorb the energy of L×(di/dt), enabling it to be reduced to within the rating voltage of transistor Q 805 . Capacitor C 814 , resistor R 853  and diode D 807  form one circuit to absorb the energy, capacitor C 813 , resistor R 813  and diode D 806  form the other. 
     The present invention uses a comparator to detect the voltage value of the power supply, and to output the detected result to drive relay SR 801 , causing it to change the windings of inductor T 802  to a series connection or parallel connection. When power consumption is over 100 W, a voltage doubler rectifier circuit is used for AC 110 V power input, and a bridge rectifier circuit is used for AC 220 V power input (see FIG. 12a). For cost&#39;s sake, when power consumption is below 100 W, a bridge rectifier circuit without a voltage doubler is used (see FIG. 12b). The bridge rectifier circuit with voltage doubler function and the bridge rectifier circuit without voltage doubler function work in the same manner, and the only difference between these two types of bridge rectifier circuits is the &#34;voltage detection&#34; that must be used. The operation of the &#34;voltage detection&#34; is described hereinafter by way of example with reference to FIG. 12a. 
     According to the present example, IC I 801 , uses LM339 which has four comparators A;B;C;D, and whose output port is an open collector type. Comparators B;C and comparator D are connected in parallel to increase the sink current at the output port, so as to facilitate the driving of relay SR 803 . Therefore, comparators B;C;D are regarded as a single comparator BCD. Further, V 1  is assigned to the voltage at the first pin of IC I 801 , V 2  is assigned to the voltage at the second pin of IC I 801 , and so on. 
     (a) Initial state of power voltage input: 
     When power is input through P805A, electric current passes through bridge rectifier D 801 , resistor R 811 , resistor R 812  and diode D 805 , if power switch S 801  is closed, so as to charge capacitor C 815 . When the end voltage of capacitor C 815  reaches 16 V, IC I 802  is started, and feedback voltage from the secondary end of transformer T 801  is sent through diode D 812  to charge capacitor C 815  to, for example, 12.5 V. When capacitor C 815  is initially charged, the 6 th  pin of IC I 801  has a voltage, but the 3 rd  pin (Vcc) has no voltage. In order to prevent a damage due to a higher voltage at the input port than at the power supply, diode D 804  is connected to a Vcc clamp for protection. When comparator A is in the initial state, V 1  =0 V because V 6  &gt;V 7  ; when capacitor C 816  is fully charged to 12.5 V, V 5  =V 7  and V 7  &gt;0 V because V 4  =V 1  =0 V, and the output of comparator BCD is 12.5 V. Therefore, relay SR 803  does no work, and transistor Q 801  is turned on to energize the coil of relay SR 801 . Because the operation time from the state where the coil of relay SR 801  is initially energized to the state where the contacts are closed is normally over 10 ms and the operation time of IC I 801  is several μs, the power supply voltage input becomes stable before the contacts of relay SR 801  are closed, as described in (b). 
     (b) Power supply voltage input is stable: 
     Resistor R 808  is the positive feedback resistor of comparator A that achieves a hysteresis comparison effect. When at the initial stage, V1=0 V, one end of resistor R 808  is grounded, causing resistor R 808  and resistor R 838  to be connected in parallel (indicated by R 808  //R 838 ), i.e., ##EQU1## as shown in FIG. 12a, because R 808  =R 838  =R 839  =100 K, and thus, V 7  =4.17 V. ##EQU2## as shown in FIG. 12a, because R 810  =100 K, R 812  =2.7 M, and R 813  =2 M, if 
     (I) V AC  =110 V, resulting in V 6  =3.24±0.6.5 V=3.89 V˜2.49 V; in which case V 6  &lt;V 7 , causing V 1  =12.5 V. At this time, resistor R 808  and resistor R 807  are connected in series to Vcc, and resistor R 839  is in parallel connection with R 808  +R 807  to increase V 7  to 8.34 V, so as to achieve hysteresis (refer to explanation (c)). At the same time, V 4  of comparator BCD=V 1  =12.5 V, V 5  =V 7  =8.34 V, so that V 4  &gt;V 5 , causing V 2  =0 V, and therefore relay SR 803  is electrically connected. Because resistor R 809  is short-circuited when V 2  =0 V before the contacts of relay SR 803  are closed, resistors R 809  and R 810  are connected in parallel, causing V 6  to drop its potential to a half level. In order to prevent the potential of V 6  from being doubled to cause relay SR 803  to be alternatively closed and opened due to voltage multiplication rectification after relay SR 803  is closed, the aforesaid design is provided to keep the potential of V 6  unchanged after relay SR 803  has been electrically connected, ie., ##EQU3## Because V AC  =110 V, resistor R 809  =100 K, V 6  =3.89 V˜2.49 V, V 6  &lt;V 7 , and SR 803  is remained unchanged. At this time, transistor Q 801  is off, relay SR 801  does no work, and inductor T 802  is connected in parallel. 
     (II) V AC  =220 V, resulting in V 6  =6.48±1.30 V=7.78 V˜5.18 V; in which case V 6  &gt;V 7 , V 1  remains unchanged, i.e., V 1  =0 V. Because V 4  =V 1  =6 V, and V 6  =V 7  =4.17 V, so that V 4  &lt;V 5 , the output of comparator BCD is 12.5 V, and relay SR 803  does no work. At this time, transistor Q 801  is turned on, relay SR 801  works, and inductor T 802  is changed to series connection. 
     (c) Power supply voltage input is unstable: 
     The power supply voltages are ideally 110 V or 220 V. However, the power supply voltage maybe within 132 V˜176 V when the power supply system falls, in which case: 
     (I) when the power supply voltage rises from 88 V to over 132 V (but below 176 V), 2-times double voltage rectification control is adopted, and the inductor is connected in parallel. 
     (II) when the power supply voltage drops from 264 V to 176 V (above 132 V), 1-time double voltage rectification control is adopted, and the inductor is connected in series. 
     It can been seen that the overlapped part above 132 V and below 176 V must thus be controlled by means of the comparator&#39;s hysteresis characteristic, i. e., the voltage range of the overlapped part becomes greater when the value of resistor R 808  is reduced, and vice versa. Further, because resistor R 809  is affected by diode D 803 , the hysteresis characteristic is provided only at one side, and it is effective only when the voltage is increased from 132 V. According to the measuring result from the circuit shown in FIG. 12a, a 2-times double voltage rectification control and inductor connected in parallel result when voltage is increased from 88 V to 194 V, or a 1-time double voltage rectification control and inductor connected in series result when voltage drops from 264 V to 148 V. 
     It is to be understood that the drawings are designed for purposes of illustration only, and are not intended as a definition of the limits and scope of the invention disclosed.