Patent Number: 
Section: description

An electron beam irradiation apparatus having a power supply according to an embodiment of the present invention will be described below with reference to FIGS. 1 and 2. In FIGS. 1 and 2, like components in the present invention are designated by the same reference numerals as those shown in FIGS. 3 and 4. The electron beam irradiation apparatus has the same structure as that shown in FIG. 3 except for a power supply. A power supply shown in FIGS. 1 and 2 comprises a step-down transformer 36 for lowering an AC voltage of a commercial power supply from 6600 V to 3300 V, an inverter device 37 for transforming the lowered voltage of the commercial AC power supply into a desired AC voltage having a desired frequency, a LC filter 38 connected to the downstream side of the inverter device 37, a transformer 27 for stepping up the AC voltage outputted from the inverter device 37, and rectifying devices 28 for rectifying the stepped up AC voltage. The high DC voltage Vo is produced by a plurality of rectifying devices 28 connected in series. The rectifying devices 28 produces a high voltage of about 800 kV which is the sum of 20 kV produced by each rectifying device. The high DC output voltage Vo is applied via voltage dividing resistances 30 to each of accelerating electrodes in an accelerating tube 13. The inverter device 37 adopts a so-called pulse-width modulation control, which controls the turn-on pulse width and the turn-off pulse width of the carrier frequency signal to form the desired waveform of the AC output voltage. In this control, a cycle of the carrier frequency signal can be a unit of the control. Thus, the AC output voltage can be switched on and off within each cycle of the carrier frequency signal as a unit. For example, if the carrier frequency is 2 kHz, then the cycle time of the carrier frequency signal is 0.5 millisecond, and hence the output voltage can be adjusted within the cycle of 0.5 millisecond as a unit. Specifically, when the inverter device 37 is commanded to stop off the output in a certain cycle, the inverter device 37 can output zero voltage at the next cycle. The LC filter 38 is connected to the downstream side of the inverter device 37 and employed for preventing the carrier frequency signal having a high frequency from being transmitted into the step-up transformer 27. The DC output voltage Vo is detected by a voltage detector 29 and the signal from the voltage detector 29 is transmitted into a controller in the inverter device 37. In the controller of the inverter device 37, the DC output voltage Vo is adjusted so as to be kept at a certain set value, e.g. 800 kV. Specifically, the output voltage is compared with the set value in each cycle of the carrier frequency signal in the inverter device 37 and controlled to make the deference between the output voltage and the set value zero by the feedback control. Accordingly, even if the input voltage of the commercial AC power supply varies, the output voltage can be controlled to follow the variation of the input voltage within a cycle of the carrier frequency signal. Current detectors 26, 26a are disposed in the downstream side of the inverter device 37. The detected results by the current detectors are fed back to the controller in the inverter device 37. Accordingly, if an electric discharge in the parts to which a high voltage is applied in the power supply, an electric discharge at the accelerating electrodes in the accelerating tube 13, or a short-circuit occurs, then the current detectors 26, 26a can detect the abnormal current produced by the electric discharge or short-circuit. Then, the signal from the current detectors 26, 26a is transmitted into the controller in the inverter device 37. As described above, since the output voltage can be controlled at each cycle of the carrier frequency signal in the inverter device 37, as soon as the abnormality is detected by the current detector 26, 26a, the inverter device 37 can stop off the output. Specifically, the output can be stopped off within one or two cycles from the time when the abnormal current is detected. For example, if the carrier frequency is 2 kHz, the output can be stopped off within 1 millisecond. Thus, the power supply according to the present invention can prevent the rectifying devices such as diodes from being broken. The primary of the step-down transformer 36 is connected to the commercial power supply of 6600 V AC. The secondary of the step-down transformer 36 lowers the voltage in electric power to 3300 V AC and then supplies the power to the inverter device. The transformer 36 has a delta connection at the primary, and a star connection and a delta connection in parallel at the secondary. As shown in FIG. 2, two rectifying circuits each comprising a converter are disposed in the inverter device and connected in series to DC output device forming a DC power supply. The DC supply supplies a direct-current power to a single inverter. Since a large number of semiconductor elements are used as the rectifying elements and the switching elements in the inverter device 37, it is preferable to use the voltage of about 3300 V because of a relatively low allowable voltage range. Inasmuch as the step-down transformer 36 has the delta-star connection and the delta-delta connection in parallel, the formation of the harmonics at the commercial AC power supply side can be completely suppressed. Hence, the power supply according to the present invention can dispense with the harmonic suppression filter 22 shown in FIG. 4 used in the conventional system. As described above, the conventional harmonic suppression filter 22 is employed for suppressing the 23rd or lower-order harmonics, for example, and requires a large space and cost. By removing such a filter, the cost reduction and the compactness of an electron beam irradiation apparatus can be achieved. As described above, according to the present invention, a power supply in an electron beam irradiation apparatus comprises an inverter device. Therefore, the output can be controlled within each cycle of the carrier frequency signal in the pulse-width modulation control, and hence the power supply can ensure high stability and safe operation in which the output can be stopped off instantaneously. Thus, the electron beam irradiation apparatus can be operated stably and safely. Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.