JPH10200835A - Electromagnetic focus circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an overcurrent from flowing to a focus coil by detecting the current flowing to the focus coil and turning OFF the switch of a current supply path when the value exceeds a set reference value. SOLUTION: A switch control part 7 detects the current flowing to the focus coil 3 of this electromagnetic focus circuit for ON/OFF controlling the switches 2, 4 and 5 at a prescribed timing with horizontal synchronizing signals S1 as reference by a current detection resistor 10 serially provided between a variable voltage power supply 1 and the switch 4 corresponding to the timing set by a data setting part 8. The detected result and the reference voltage of a reference power supply 13 are compared in a comparator 12, and when the output of the current detection resistor 10 becomes larger than the reference voltage, a normally closed switch 11 for forcibly turning OFF the switch provided in a circuit for supplying the current to the focus coil 3 is controlled to be opened, the switch 2 is turned OFF and the overcurrent is prevented from flowing to the focus coil.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic focusing circuit of a display device using a cathode ray tube.

[0002]

2. Description of the Related Art Heretofore, there has been known an electromagnetic focus circuit of a cathode ray tube which drives a focus coil by a switching circuit with less power. FIG. 10 shows a configuration diagram of a conventional electromagnetic focus circuit. In the electromagnetic focus circuit shown in FIG. 1, a switch 2, a focus coil 3, and a switch 4 are connected in series between both ends of a variable voltage power supply 1. An intermediate point between the variable voltage power supply 1 and the switch 2 and an intermediate point between the focus coil 3 and the switch 4 are connected via a switch 5, and an intermediate point between the switch 2 and the focus coil 3 is connected to the variable voltage power supply. A diode 6 is connected in the opposite direction between the cathode of the first switch and the midpoint of the switch 4. The switches 2, 4, and 5 are ON / OFF controlled by the switch control unit 7, and ON / OFF timing is set for the switch control unit 7 by the data setting unit 8.

FIG. 11 is a timing chart showing operation waveforms of the electromagnetic focus circuit. The switch control unit 7 controls ON / OFF of the switches 2, 4, and 5 at the timing shown in FIG. 11 in accordance with the timing set by the data setting unit 8 based on the horizontal synchronization signal. At the timing when the switches 2 and 4 are simultaneously turned on, a power supply voltage is applied from the variable voltage power supply 1 to the focus coil 3, and the current flowing through the focus coil 3 increases linearly in proportion to time. Next, switch 2 is turned off
At the moment, the diode 6 is turned on by the back electromotive force of the focus coil 3, the focus coil 3 is short-circuited by the diode 6 and the switch 4, and a constant current continues to flow. Next, when the switch 4 is turned off and the switch 5 is turned on, the focus coil 3 is connected to the variable voltage power supply 1 by the diode 6 and the switch 5 in a direction opposite to the above-mentioned direction, and the current value is attenuated in inverse proportion to time. I do. The moment the current stops flowing through the focus coil 3, the back electromotive force of the focus coil 3 also becomes zero and the diode 6 is turned off.

As described above, the switches 2, 4, and 5 are turned on.
Focus coil 3 by repeating / OFF control
A current as shown in FIG. 11 can be generated.
By controlling the current flowing through the focus coil 3 as shown in FIG. 11, the power supplied from the variable voltage power supply 1 to the focus coil 3 is regenerated by the diode 6 and the switch 5 to the variable voltage power supply 1. 1
The average value of the power supplied from is very small.

[0005]

However, in the above-mentioned conventional electromagnetic focus circuit, the voltage of the variable voltage power supply 1 is too high with respect to the cycle of the horizontal synchronizing signal S1 because the circuit does not have a function of limiting the current. Thus, there is a problem that a large current flows through the focus coil 3. Further, if the switch 4 and the switch 5 are simultaneously turned on, there is a problem that the variable voltage power supply 1 is short-circuited. Also, due to the loss of the switch, the variable voltage power supply 1
However, there is a problem that the current during the period of regenerating the current attenuates quickly.

The present invention has been made in view of the above circumstances, and an overcurrent flows in a focus coil, a power supply of a focus coil is short-circuited, or a current waveform of a focus coil deviates from an ideal waveform. It is an object of the present invention to provide a highly reliable electromagnetic focus circuit that can reliably prevent the occurrence of the electromagnetic focus circuit.

[0007]

In order to solve the above-mentioned problems, an electromagnetic focus circuit according to the present invention detects a current flowing through a focus coil and supplies a current to the focus coil when the value exceeds a set value. By turning off a switch provided on the line, an excessive current is prevented from flowing.

The cycle of the horizontal synchronizing signal is measured, the magnitude of the current flowing through the focus coil is determined using a reference value adapted to the measured cycle value, and the switch is turned off when the detected current value exceeds the reference value. This prevents an excessive current from flowing. Since a reference value adapted to the cycle measurement value is used, it is possible to stably prevent an overcurrent from flowing through the focus coil regardless of the cycle of the horizontal synchronization signal.

When one of the switches is ON, the other switch is ON during a signal for controlling a switch that may cause a short circuit of the power supply of the focus coil when the switches are simultaneously ON.
By preventing the power supply from becoming short, the power supply is prevented from being short-circuited.

[0010] By measuring the cycle of the horizontal synchronizing signal and correcting the switching timing according to the measured value of the cycle, it has the effect of realizing high-precision focusing performance at various horizontal synchronizing frequencies. In addition, an error in the focus current due to the current being attenuated quickly during the period of regeneration from the focus coil to the power supply is reduced.

An input means capable of adjusting the timing from the outside is attached to the arithmetic means for setting the timing of turning on / off the switch, and the switching timing is adjusted by the timing adjustment data provided from the input means, so that the focus coil can be controlled. An error in the focus current due to the current attenuating quickly during the period of regeneration to the power supply is reduced.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 shows the configuration of an electromagnetic focus circuit according to an embodiment of the present invention. Parts having the same functions as the respective parts of the electromagnetic focus circuit in FIG. 10 described above are denoted by the same reference numerals.

A current detecting resistor 10 is inserted in series between the cathode of the variable voltage power supply 1 and the switch 4, and a switch control unit 7
A normally-closed switch 11 for forcibly turning off the switch 2 is provided on a line for controlling the switch 2. The switch 2 is provided on a line for supplying power from the variable voltage power supply 1 to the focus coil 3, and is set so as to be forcibly turned off in synchronization with the opening control of the normally closed switch 11. The normally closed switch 11 is ON / OFF controlled by the output of the comparator 12. One of the comparison input terminals of the comparator 12 is connected to the anode of a reference power supply 13 for generating a reference voltage for detecting the occurrence of overcurrent, and the other comparison input terminal is connected to the switch 4 of the current detection resistor 10. The connected terminal is connected. The comparator 12 compares the voltages of the comparison input terminals, and controls the opening of the normally closed switch 11 when the voltage of the terminal to which the current detection resistor 10 is connected exceeds the reference voltage of the reference power supply 13.

The operation of the electromagnetic focus circuit configured as described above will be described with reference to FIG. The switch control unit 7 controls ON / OFF of the switches 2, 4, and 5 at the timing shown in FIG. 2 based on the horizontal synchronization signal in accordance with the timing set by the data setting unit 8. That is, at the timing when the switches 2 and 4 are simultaneously turned on, the variable voltage power supply 1 and the focus coil 3 are connected, and the current increases linearly in proportion to time. Next, at the moment when the switch 2 is turned off, the diode 6 is turned on by the back electromotive force of the focus coil 3, the focus coil 3 is short-circuited by the diode 6 and the switch 4, and a constant current continues to flow. Next, switch 4 is turned off.
F. When the switch 5 is turned on, the focus coil 3 is connected to the variable voltage power supply 1 by the diode 6 and the switch 5,
The current value attenuates in inverse proportion to time. At the moment when the current stops flowing through the focus coil 3, the back electromotive force of the focus coil 3 becomes zero and the diode 6 is turned off.

The current flowing through the focus coil 3 is detected by a current detection resistor 10. This detection result is compared with the reference voltage of the reference power supply 13 by the comparator 12, and when the output of the current detection resistor 10 becomes larger than the reference voltage, the normally closed switch 11 is opened to turn off the switch 2 and the overcurrent is reduced. The flow to the focus coil 3 is prevented. In this manner, a current as shown in FIG. 2 can be generated while preventing an overcurrent from flowing through the focus coil 3.

(Embodiment 2) In this embodiment, a reference voltage applied to a comparator 12 having a function of judging whether or not a switch 2 is forcibly turned off is appropriately set in accordance with the cycle of a horizontal synchronization signal. Set a proper value. FIG. 3 shows the configuration of the electromagnetic focus circuit according to the present embodiment. Parts having the same functions as the respective parts of the electromagnetic focus circuit shown in FIGS. 1 and 10 are denoted by the same reference numerals.

The comparator 12 has one comparison input terminal receiving the current detection value of the current detection resistor 10 and the other comparison input terminal receiving the reference voltage value output from the D / A converter 14. The input (reference voltage command value) of the D / A converter 14 that determines the reference voltage value is given from the microcomputer 15. The microcomputer 15 includes the switch control unit 7
Are input in parallel, the cycle of the horizontal synchronization signal is measured, and a reference voltage value adapted to the measurement cycle is instructed to the D / A converter 14.

The operation of the electromagnetic focus circuit configured as described above will be described with reference to FIG. The switch control unit 7 controls ON / OFF of the switches 2, 4, and 5 according to the timing set by the data setting unit 8 with reference to the horizontal synchronization signal, as shown in FIG. A switch 11 provided between the switch 2 and the switch control unit 7 turns on / off a signal for controlling the switch 2 based on an output of the comparator 12.

At the timing when the switches 2 and 5 on the line for supplying current to the focus coil 3 are simultaneously turned on, the variable voltage power supply 1 and the focus coil 3
Are connected, and the current increases linearly in proportion to the time.
Next, at the moment when the switch 2 is turned off, the diode 6 is turned on by the back electromotive force of the focus coil 3, the focus coil 3 is short-circuited by the diode 6 and the switch 4, and a constant current continues to flow. Next, when the switch 4 is turned off and the switch 5 is turned on, the focus coil 3 is connected to the variable voltage power supply 1 by the diode 6 and the switch 5, and its current value attenuates in inverse proportion to time. Focus coil 3
When the current stops flowing, the back electromotive force of the focus coil 3 becomes zero, and the diode 6 is turned off.

The current flowing through the focus coil 3 is detected by a current detection resistor 10, and the detection result is compared with a reference voltage of a D / A converter 14 by a comparator 12. The output of the current detection resistor 10 is larger than the reference voltage. When this happens, the switch 2 is turned off by opening control of the switch 11 to prevent an overcurrent from flowing.

The microcomputer 15 measures the cycle of the horizontal synchronization signal, determines a reference voltage value adapted to the frequency, and dynamically sets the reference voltage value using the D / A converter 14. Overcurrent protection can be performed stably regardless of the cycle.

(Embodiment 3) In this embodiment, when one of the switches 4 is ON, the other switches between signals for controlling the switches 4 and 5, which may cause a short circuit of the variable voltage power supply 1 when they are simultaneously turned on. Is controlled so as not to be turned ON.

FIG. 5 shows the configuration of the electromagnetic focus circuit according to the present embodiment. Parts having the same functions as the respective parts of the electromagnetic focus circuit in FIG. 10 described above are denoted by the same reference numerals.

The switch control section 7 supplies an ON / OFF control signal to the switches 4 and 5 via the lines L1 and L2, respectively. Line L for giving a signal to switch 5
2 is provided with a normally closed switch 16. Normally closed switch 1
6 is an ON / OFF signal having a logic opposite to the signal state of the other line L1.
OFF control is performed. Therefore, O
When the N control signal is output, the normally closed switch 1
Switch 5 is forcibly turned off by opening 6
When the signal for controlling the switch 4 to be turned off is output, the switch 5 is turned on / off by the signal output from the switch control unit 7 by closing the normally closed switch 16.
Perform F control.

The operation of the electromagnetic focus circuit configured as described above will be described with reference to FIG. The switch control unit 7 controls ON / OFF of the switches 2, 4 and 5 as shown in FIG. 6 in accordance with the timing set by the data setting unit 8 based on the horizontal synchronization signal. A signal for controlling ON / OFF of the switch 5 is supplied through a normally closed switch 16 provided on the line L2.
The switch 5 can be turned off by the control of.
At the timing when the switches 2 and 4 are simultaneously turned on, the variable voltage power supply 1 and the focus coil 3 are connected and the current increases linearly in proportion to the time. Next, at the moment when the switch 2 is turned off, the diode 6 is turned on by the back electromotive force of the focus coil 3, the focus coil 3 is short-circuited by the diode 6 and the switch 4, and a constant current continues to flow. Next, when the switch 4 is turned off and the switch 5 is turned on, the focus coil 3 is connected to the variable voltage power supply 1 by the diode 6 and the switch 5, and its current value attenuates in inverse proportion to time. At the moment when the current stops flowing through the focus coil 3, the back electromotive force of the focus coil 3 becomes zero and the diode 6 is turned off.

Here, in the circuit shown in FIG. 5, when the switches 4 and 5 are simultaneously turned on, the variable voltage power supply 1 is short-circuited. To prevent the switches 4 and 5 from being turned on at the same time, the switch 5 is forcibly turned on.
The normally closed switch 16 for FF is controlled to be ON / OFF by the control signal of the other switch 4. Specifically, as shown in FIG. 6, the normally closed switch 16 is ON / OFF controlled by a logic opposite to that of the switch 4.

Therefore, when a control signal for turning on the switches 5 and 4 at the same time is generated as shown by a broken line in FIG.
Is forcibly turned off irrespective of the control signal output from the switch control unit 8, it is possible to prevent the variable voltage power supply 1 from being short-circuited.

(Embodiment 4) In this embodiment, the ON / OFF of each switch is adapted to the cycle of the horizontal synchronizing signal.
By shifting the timing, the current waveform of the focus coil can be partially corrected.

FIG. 7 shows the configuration of the electromagnetic focus circuit according to the present embodiment. Parts having the same functions as the respective parts of the electromagnetic focus circuit in FIG. 10 described above are denoted by the same reference numerals.

The switch control unit 7 for controlling the ON / OFF timing of each of the switches 2, 4, and 5 constituting the switching circuit receives data of the ON / OFF timing of each of the switches 2, 4, and 5 from the microcomputer 17. The microcomputer 17 measures the cycle of the horizontal synchronizing signal, and adjusts the switching timing during the period in which the current is regenerated from the focus coil 3 to the variable voltage power supply 1 in accordance with the cycle.

Hereinafter, the contents of correction of the switch switching timing by the microcomputer 17 will be described with reference to FIG. The switch control unit 7 controls ON / OFF of the switches 2, 4, and 5 in accordance with the timing set by the microcomputer 17 with reference to the horizontal synchronization signal as shown in FIG. Switch 2
The variable voltage power supply 1 and the focus coil 3 are connected at the timing when the switch 4 and the switch 4 are simultaneously turned on, and the current increases linearly in proportion to the time. Next, switch 2
At the moment of the FF, the diode 6 is turned on by the back electromotive force of the focus coil 3, the focus coil 3 is short-circuited by the diode 6 and the switch 4, and a constant current continues to flow. Next, when the switch 4 is turned off and the switch 5 is turned on, the focus coil 3 is connected to the variable voltage power supply 1 by the diode 6 and the switch 5, and its current value attenuates in inverse proportion to time. At the moment when the current stops flowing through the focus coil 3, the back electromotive force of the focus coil 3 becomes zero and the diode 6 is turned off.

The current waveform shown by the solid line as the current of the focus coil 3 in FIG. 8 is an ideal waveform having no loss in the switch. In the actual circuit, due to the loss of the switch and the like, the current attenuation during the period in which the focus coil 3 regenerates to the variable voltage power supply 1 becomes larger than the ideal waveform, as shown by the uncorrected waveform indicated by the one-dot chain line in FIG.

The microcomputer 17 measures the period of the horizontal synchronizing signal S1 and corrects the switching timing of the switches 2, 4, and 5 to the position shown by the dotted line in FIG. 8 during the period when the current waveform deviates from the ideal waveform. ing. As a result, the current waveform of the focus coil 3 becomes like the corrected waveform shown by a dotted line in the current waveform of the focus coil 3 in FIG. The post-correction waveform is closer to the ideal waveform indicated by the solid line compared to the pre-correction waveform, so that the error of the focus current can be reduced. Further, since the horizontal synchronization signal is input to the microcomputer 17 and the ON / OFF cycle of the switch change is adjusted so as to match the cycle, high-accuracy focusing accuracy can be realized at various horizontal synchronization frequencies.

(Embodiment 5) FIG. 9 shows a configuration of an electromagnetic focus circuit according to an embodiment. Microcomputer 18 for setting timing in switch control unit 7
And the phase adjustment value input means 19 is connected to
A phase adjustment value for adjusting the N / OFF timing is input. The microcomputer 18 is ON / O of each switch.
The timing of shifting the timing of the FF from the position shown by the solid line in FIG. 8 to the position shown by the dotted line in FIG. 8 is set. How much the ON / OFF timing of each switch is shifted from the reference pattern is input from the phase adjustment value input means 19 in the form of a phase adjustment value.

FIG. 8 is a timing chart of the electromagnetic focus circuit according to the present embodiment. The phase adjustment value is input to the microcomputer 18 from the phase adjustment value input means 19, and each switch is turned on at the timing shown by the dotted line in FIG.
The timing of turning on / off is set in the switch control unit 7. Therefore, the ON / OFF timing of each switch is corrected as shown by the dotted line in FIG. 8, and the focus current is corrected as shown by the corrected waveform shown by the broken line in the current waveform of the focus coil in FIG. Error from the ideal waveform is reduced.

The present invention is not limited to the above-described embodiment. An electromagnetic focus circuit may be formed by appropriately combining the first, second, third, fourth and fifth embodiments.
The circuit element of the illustrated electromagnetic focus circuit may be replaced with another element having the same function.

[0037]

As described above, according to the present invention, when the value of the current flowing through the circuit exceeds a predetermined value, the switch on the line supplying the current to the focus coil can be turned off. An overcurrent can be reliably prevented from flowing. Further, according to the present invention, since the ON / OFF operation of both switches is exclusively controlled for a pair of switches in which the power supply of the focus coil is short-circuited when turned on at the same time, it is ensured that the power supply of the focus coil is short-circuited. Can be prevented. Further, according to the present invention, since the timing for controlling the plurality of switches is adaptively switched according to the cycle of the horizontal synchronization signal, it is possible to realize high-precision focus performance at various horizontal synchronization frequencies.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an electromagnetic focus circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram showing waveforms at various parts of the electromagnetic focus circuit according to the first embodiment.

FIG. 3 is a configuration diagram of an electromagnetic focus circuit according to a second embodiment of the present invention.

FIG. 4 is a diagram showing waveforms at various parts of an electromagnetic focus circuit according to a second embodiment.

FIG. 5 is a configuration diagram of an electromagnetic focus circuit according to a third embodiment of the present invention.

FIG. 6 is a diagram showing waveforms at various parts of the electromagnetic focus circuit according to the third embodiment.

FIG. 7 is a configuration diagram of an electromagnetic focus circuit according to a fourth embodiment of the present invention.

FIG. 8 is a diagram showing waveforms at various parts of the electromagnetic focus circuit according to the fourth and fifth embodiments.

FIG. 9 is a configuration diagram of an electromagnetic focus circuit according to a fifth embodiment of the present invention.

FIG. 10 is a configuration diagram of a conventional electromagnetic focus circuit.

FIG. 11 is a diagram showing waveforms at various parts of a conventional electromagnetic focus circuit.

[Explanation of symbols]

 Reference Signs List 1 variable voltage power supply 2, 4, 5 switch 3 focus coil 6 diode 7 switch control unit 8 data setting unit 10 current detection resistor 11, 16 normally closed switch 12 comparator 13 reference power supply 14 D / A converter 15, 17, 18 micro Computer 19 Phase adjustment value input means

Claims (11)

[Claims] 1. A focus coil and a power supply thereof are connected via a switching circuit including a plurality of switches, and the switches are turned on in synchronization with a horizontal synchronization signal.
In an electromagnetic focus circuit for supplying a current to the focus coil by controlling the current / off, a current detection means for detecting a current flowing in the focus coil, and a current detected by comparing a current detection value of the current detection means with a reference value. An electromagnetic focus circuit, comprising: current cutoff means for turning off a switch for supplying a current to the focus coil when a detected value exceeds a reference value. 2. The electromagnetic focus circuit according to claim 1, wherein a period of the horizontal synchronizing signal is measured to generate a reference value adapted to the measured period value, and a reference adapted to the current interrupting means is adapted to the period of the horizontal synchronizing signal. An electromagnetic focus circuit comprising a reference value optimizing means for setting a value. 3. A focus coil and a power supply thereof are connected via a switching circuit including a plurality of switches, and the switches are turned on in synchronization with a horizontal synchronization signal.
In the electromagnetic focus circuit that supplies current to the focus coil by performing the ON / OFF control,
Then, a means is provided for forcibly turning off one of the pair or two switches when an ON command is issued simultaneously to the pair or two switches where the power supply is short-circuited. Electromagnetic focus circuit. 4. A focus coil and a power source thereof are connected via a switching circuit including a plurality of switches, and the switches are turned on in synchronization with a horizontal synchronization signal.
An electromagnetic focus circuit that supplies a current to the focus coil by performing the on / off control, measures a cycle of the horizontal synchronizing signal, and switches a timing of turning on / off the switch according to the cycle measurement value; An electromagnetic focus circuit comprising: switch control means for controlling ON / OFF of a switch based on timing set by the means. 5. The electromagnetic focus circuit according to claim 4, wherein the calculating means corrects the ON / OFF control timing of the switch so that the current waveform of the focus coil approaches an ideal waveform having no switch loss. An electromagnetic focus circuit characterized by the following. 6. A focus coil and a power supply thereof are connected via a switching circuit including a plurality of switches, and the switches are turned on in synchronization with a horizontal synchronization signal.
In an electromagnetic focus circuit that supplies current to the focus coil by performing / OFF control, input means for inputting timing adjustment data for adjusting switch switching timing, and the timing adjustment data input from the input means are taken into account. Turn the switch ON / OF
Calculating means for setting the timing of the F control, and turning on / off the switch based on the timing set by the calculating means.
An electromagnetic focus circuit, comprising: a switch control means for performing OFF control. 7. A focus coil, a power supply for supplying power to the focus coil, and a plurality of switches, wherein power is supplied from the power supply to the focus coil by a combination of ON / OFF of the switches, and A switching circuit for regenerating current from the coil to the power supply; data setting means for setting a timing for controlling a switch of the switching circuit; and a switch synchronized with a horizontal synchronization signal based on the timing set by the data setting means. Switch control means for switching the current, a current detection resistor inserted in series between the focus coil and the power supply to detect a current flowing through the focus coil, a detection value and a reference value detected by the current detection resistor And if the detected value exceeds the reference value, the focus coil A current cut-off means for forcibly turning off a switch on a line for supplying a current to the electromagnetic focus circuit. 8. The D / A according to claim 7, wherein a DC voltage corresponding to the input data is output and the DC voltage is set as a reference voltage in the current cutoff means.
An electromagnetic focus circuit, comprising: a converter; and a calculation unit that measures a cycle of a horizontal synchronization signal and outputs reference voltage instruction data corresponding to the cycle measurement value to the D / A converter. 9. A combination of a focus coil, a power supply for supplying power to the focus coil, and a plurality of switches, wherein power is supplied from the power supply to the focus coil by a combination of ON / OFF of the switches, A switching circuit for regenerating current from the coil to the power supply; data setting means for setting a timing for controlling a switch of the switching circuit; and a switch synchronized with a horizontal synchronization signal based on the timing set by the data setting means. And a switch control means for switching the power supply on and off, wherein the power supply is short-circuited when turned on at the same time. ON / OFF control of the other of the switches And a short-circuit prevention switch whose ON / OFF is controlled by a reverse logic. 10. A focus coil, a power supply for supplying power to the focus coil, and a plurality of switches, wherein the power is supplied from the power supply to the focus coil by a combination of ON / OFF of the switch. A switching circuit for regenerating a current from the coil to the power supply; and a switch ON / O during a period in which the period of the horizontal synchronizing signal is measured and the current waveform of the focus coil is affected by switch loss in the measurement period.
A calculating means for setting a timing at which the FF timing is corrected so that the current waveform of the focus coil approaches an ideal waveform having no switch loss; and a switch is turned on based on the timing set by the calculating means by inputting a horizontal synchronizing signal. And a switch control means for performing / OFF control. 11. A focus coil, a power supply for supplying power to the focus coil, and a plurality of switches, wherein power is supplied from the power supply to the focus coil by a combination of ON / OFF of a switch, and the focus coil is provided. A switching circuit for regenerating current from the coil to the power supply, input means for a phase adjustment value for adjusting ON / OFF control timing of each switch included in the switching circuit, and timing for controlling each switch of the switching circuit And a switch control means for switching a switch based on the timing set by the calculation means.