KR20190122554A - Laser apparatus and power supply apparatus thereof - Google Patents

Abstract

Provided is a laser apparatus with improved reliability. A high frequency power supply (400) applies a high frequency voltage (V_RF) to a resonant circuit (210) including capacities of one pair of discharge electrodes (202, 204). An overvoltage suppression circuit (500) suppresses an overvoltage between both ends of the resonant circuit (210). A protection circuit (550) stops application of the high frequency voltage (V_RF) from the high frequency power supply (400) when the abnormality is detected.

Description

LASER APPARATUS AND POWER SUPPLY APPARATUS THEREOF

This application claims the priority based on Japanese Patent Application No. 2018-081678 for which it applied on April 20, 2018. The entire contents of that application are incorporated herein by reference.

The present invention relates to a laser device.

As industrial processing tools, laser processing apparatuses are widely used. As a laser processing apparatus, a high output gas laser such as a CO ₂ laser is used. 1 is a block diagram of a laser device 100R. The laser device 100R includes a laser resonator 200 and a power supply device 250R. The laser resonator 200 includes a pair of discharge electrodes 202 and 204, a total reflection mirror 206, and a partial reflection mirror 208.

The pair of discharge electrodes 202 and 204 are provided in a gas chamber filled with a laser medium gas such as CO ₂ . A capacitance C exists between the pair of discharge electrodes 202 and 204. The capacitance C and the inductor L (inductor element or parasitic inductor) form a resonant circuit 210 having a resonant frequency f _RES .

The power supply device 250R applies a high frequency voltage V _RF to the resonance circuit 210. The frequency f _RF (hereinafter referred to as a synchronous frequency) of the high frequency voltage V _RF is set near the frequency f _RES of the resonant circuit. By applying a high frequency voltage V _RF , a discharge current flows between the pair of discharge electrodes 202 and 204. The laser medium gas is excited by the discharge current, and an inversion distribution is formed. The guided emission light is amplified by reciprocating in the optical resonator formed by the total reflection mirror 206 and the partial reflection mirror 208 and passing through the laser medium gas. A portion of the amplified light is taken out as output from the partial reflector 208.

The power supply device 250R includes a DC power supply 300 for generating a stabilized DC voltage V _DC , and a high frequency power supply 400 for converting the DC voltage V _DC into a high frequency voltage V _RF .

Patent Document 1: Japanese Patent Application Laid-Open No. 9-129953 Patent Document 2: Japanese Unexamined Patent Publication No. 2015-32746 Patent Document 3: Japanese Unexamined Patent Publication No. 2017-69561

MEANS TO SOLVE THE PROBLEM As a result of examining the laser apparatus 100R of FIG. 1, the present inventors came to recognize the following subjects.

If contact failure or the like occurs in the discharge electrodes 202 or 204, the operation is performed in the open state. In the open state, since the capacitance C becomes very small, the resonance frequency of the resonance circuit becomes a very high value f _RES '. In this state, if the high frequency voltage V _RF of the synchronizing frequency f _O (f _O <f _RES ') is continuously applied, a very high high voltage exceeding the amplitude of the high frequency voltage V _RF occurs at the resonant frequency f _RES '. do. When this high voltage is applied to the semiconductor element (that is, the power transistor) inside the high frequency power source 400, the reliability is lowered.

The present invention has been made in such a situation, and one of the exemplary objects of the predetermined aspect is to provide a laser device having improved reliability.

Certain aspects of the invention relate to a laser device or a power supply thereof. The laser device includes a laser resonator including a pair of discharge electrodes, and a power supply device for driving the pair of discharge electrodes. The power supply apparatus includes a high frequency power supply for applying a high frequency voltage to a resonant circuit including a capacitance of a pair of discharge electrodes, an overvoltage suppression circuit for suppressing overvoltage between both ends of the resonant circuit, and when an abnormality is detected, A protection circuit for stopping the application is provided. The time required for the protection circuit to detect an abnormality is shorter than the time that the overvoltage suppression circuit can withstand the overvoltage.

According to this aspect, by providing the overvoltage suppression circuit, when the resonance frequency of the resonant circuit deviates greatly from the set value, the overvoltage can be suppressed and the semiconductor element included in the high frequency power source or the like can be protected. While the overvoltage suppression circuit suppresses the high voltage, the protection circuit determines whether there is an abnormality, and when an abnormality occurs, the semiconductor device of the high frequency power supply can be protected by stopping the device. Deterioration of reliability can be prevented.

The capacity of the overvoltage suppression circuit may be smaller than 1/5 of the capacity of the pair of discharge electrodes. As a result, the influence of the overvoltage suppression circuit on the resonance frequency of the resonance circuit can be reduced.

The overvoltage suppression circuit may include at least one of a voltage suppressor, a slow local arc device, and a gas arrester (surge arrester).

The overvoltage suppression circuit may include a plurality of elements connected in series. When the capacitance of each element is large, by connecting them in series, the capacitance of the overvoltage suppression circuit can be reduced.

The overvoltage suppression circuit may include a capacitor whose capacitance is 1/10 or less of the capacitance of the pair of discharge electrodes. In this case, since the capacitor becomes a load, excessive resonant frequency can be prevented, and overvoltage can be suppressed.

The overvoltage suppression circuit may include an LCR load. In this case, even when an abnormality occurs in the discharge electrode and becomes an open state, the resonance frequency can be prevented from being excessively increased due to the LCR load, and the overvoltage can be suppressed.

The protection circuit may determine the abnormality based on the presence or absence of the output light of the laser device. In the case where the laser device is non-light emitting, it may be determined that it is in a no load state. In the case of a CO ₂ laser, an infrared detection element may be used.

The protection circuit may determine the abnormality based on the current component of the resonance frequency. The current flowing through the load (resonance circuit) or the output of the high frequency power source is monitored, the component of the resonant frequency is extracted from the detected value, and when the current of the resonant frequency is small, it may be determined as a no-load state.

The protection circuit may determine the abnormality based on a current component other than the resonance frequency. The current flowing through the load (resonance circuit) or the output of the high frequency power source is monitored, and components other than the resonance frequency are extracted from the detected value, and when the current other than the resonance frequency is large, it may be determined as a no-load state.

The protection circuit may determine the abnormality based on the drop width of the input voltage of the high frequency power supply after short circuit. When the laser emits light normally, the electric charge accumulated in the output capacitor (bank capacitor) of the DC power supply is discharged and the DC voltage is lowered. Therefore, the voltage of the bank capacitor is monitored, and when the voltage drop is small, it can be determined that it is in the no-load state.

The protection circuit may determine the abnormality based on the current flowing through the overvoltage suppression circuit. When a current flows through the surge protection device constituting the overvoltage suppression circuit, it can be determined as a no-load state.

The protection circuit may determine an abnormality based on noise at a frequency higher than the resonance frequency. When the current becomes high frequency, the radiated noise or conducted noise of the high frequency increases. This noise is detected by the antenna, and when the noise increases, it can be determined that it is a no-load state.

The protection circuit may determine the abnormality based on the voltage between the pair of discharge electrodes. If a high frequency voltage is applied but no sufficient voltage is detected between both ends of the resonant circuit, it can be determined as a no-load state.

However, any combination of the above components, or components or representations of the present invention, which are mutually substituted between a method, an apparatus, a system, and the like, are also effective as aspects of the present invention.

According to the predetermined aspect of the present invention, the reliability of the laser device can be improved.

1 is a block diagram of a laser device.
2 is a block diagram of a laser apparatus according to the embodiment.
3A to 3D are circuit diagrams showing an example of the configuration of the overvoltage suppression circuit.
4 is a circuit diagram showing a specific configuration example of a power supply device.
5 is a diagram illustrating a configuration example of a high frequency power supply including a protection circuit.
6 is a diagram showing a factory value of a laser having a laser device.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated referring drawings based on suitable embodiment. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and overlapping descriptions are appropriately omitted. In addition, embodiment is not limited to an invention, but is an illustration, and all the features and its combination which are described in embodiment are not necessarily essential to the invention.

2 is a block diagram of the laser device 100 according to the embodiment. The laser device 100 includes a laser resonator 200 and a power supply device 250. The functions of the laser resonator 200 and the power supply device 250 are the same as in FIG. 1.

The laser resonator 200 includes a pair of discharge electrodes 202 and 204, and the capacitance C therebetween forms the resonant circuit 210 together with the inductor L. The resonant frequency of this resonant circuit 210 is f _RES .

In addition to the power supply device 250 of FIG. 1, the power supply device 250 includes an overvoltage suppression circuit 500 and a protection circuit 550.

The DC power supply 300 generates the DC voltage V _DC stabilized at a predetermined voltage level and supplies it to the high frequency power supply 400. The high frequency power supply 400 generates a high frequency voltage V _RF having the same frequency (synchronization frequency) f _RF as the resonance frequency f _RES and supplies it to the laser resonator 200. The configuration of the high frequency power supply 400 is not limited, but may include an inverter for converting a DC voltage V _DC to an AC voltage V _AC , and a transformer for boosting the output voltage V _AC of the inverter.

The overvoltage suppression circuit 500 is configured to suppress overvoltage between both ends of the resonance circuit 210.

The protection circuit 550 monitors the operation of the laser device 100, and stops the application of the high frequency voltage V _RF by the high frequency power supply 400 when an abnormality is detected. The abnormality to be detected by the protection circuit 550 is an abnormality in which an overvoltage is generated between both ends of the resonance circuit 210, in other words, an abnormality in which the suppression operation by the overvoltage suppression circuit 500 actually occurs. As such an abnormality, an abnormality in which the resonant frequency of the resonant circuit 210 becomes higher than its set value (i.e., synchronous frequency) f _RES is exemplified. For example, the contact failure of the discharge electrodes 202 and 204, the inductor L This is caused by misalignment of the wires, misalignment of the wirings connecting them, and the like. However, the spectrum of the voltage ΔV between the both ends of the resonant circuit 210 includes other frequency components in addition to the synchronous frequency f _RF , and the output terminal of the high frequency power supply 400 and the input terminal of the laser resonator 200. Note that there is a parasitic impedance (not shown) between the waveforms, so that the waveforms of the high frequency voltage V _RF and the voltage ΔV between both ends do not necessarily coincide.

The time required for the protection circuit 550 to detect an open abnormality is designed to be shorter than the time for the overvoltage suppression circuit 500 to withstand the overvoltage.

That is the configuration of the laser device 100. According to this laser apparatus 100, the following effects can be acquired.

According to this embodiment, by providing the overvoltage suppression circuit 500, when the resonant frequency of the resonant circuit 210 greatly deviates from the set value f _RES , the overvoltage generated between the both ends can be suppressed, and the high frequency power supply ( 400 can be protected.

Here, while the overvoltage state of the voltage DELTA V between both ends is suppressed by the overvoltage suppression circuit 500, current continues to flow in the overvoltage suppression circuit 500. If this state continues for a long time, there is a possibility that the heat generation of the overvoltage suppression circuit 500 becomes large and the overvoltage suppression function is reduced or the overvoltage suppression function is completely lost. As a result, the overvoltage is applied to the high frequency power supply 400 again, which may lower the reliability of the semiconductor device. Therefore, when the protection circuit 550 is provided in addition to the overvoltage suppression circuit 500, and the overvoltage suppression circuit 500 suppresses the high voltage, the protection circuit 550 determines whether there is an abnormality, and an abnormality occurs. By stopping the high frequency power supply 400 or the DC power supply 300, the cause of the overvoltage can be eliminated to protect the semiconductor device of the high frequency power supply 400, and the reliability of the overvoltage suppression circuit 500 is lowered. Can be prevented.

The present invention is directed to various apparatuses and methods that are understood as block diagrams and circuit diagrams of FIG. 2 or derived from the above description, and are not limited to specific configurations. Hereinafter, not to narrow the scope of the present invention but to help understand the nature and operation of the present invention and to clarify them, more specific structural examples and embodiments will be described.

3A to 3D are circuit diagrams showing an example of the configuration of the overvoltage suppression circuit 500. The overvoltage suppression circuit 500 of FIG. 3A includes a gas arrester 502. If the voltage between the terminals of the gas arrestor 502 exceeds the operation start voltage, the gas arrester 502 is short-circuited, and the voltage? V between both ends of the overvoltage suppression circuit 500 is suppressed.

Here, the capacitance between both ends of the overvoltage suppression circuit 500 is preferably smaller than 1/5 of the capacitance of the pair of discharge electrodes. This is because when the capacitance of the overvoltage suppression circuit 500 is excessively large, the resonance frequency f _RES of the resonance circuit 210 is shifted, which affects the circuit operation. In this respect, when the overvoltage suppression circuit 500 is constituted by the gas arrestor 502 alone as shown in Fig. 3A, the capacitance may be excessively large.

In such a case, as shown in Fig. 3B, a plurality of overvoltage suppression elements (surge protection elements) may be connected in series. As a result, the capacitance between both ends of the overvoltage suppression circuit 500 becomes the combined capacitance of each of the plurality of overvoltage suppression elements, so that the capacitance of the individual overvoltage suppression elements can be made smaller.

More specifically, the overvoltage suppression circuit 500 of FIG. 3B includes a gas arrester 502 and a varistor 504 connected in series. In this configuration, when a high voltage ΔV is applied between both ends of the overvoltage suppression circuit 500, the voltage between the terminals of the gas arrestor 502 exceeds the operation start voltage and becomes a short circuit state, and the high voltage ΔV becomes the varistor ( 504). As a result, a current flows according to the I-V characteristic of the varistor 504, so that the high voltage ΔV can be suppressed. Instead of the varistor 504, a general overvoltage suppression element can be used. For example, an SPD (zinc oxide type arrester) or a transistor may be used.

Although the overvoltage suppression circuit 500 of FIGS. 3A and 3B operates in response to an overvoltage, the present invention is not limited thereto, and the overvoltage suppression circuit 500 is in an abnormal state of opening of the laser resonator 200. A circuit may be used to prevent the occurrence of overvoltage in the circuit. Specifically, the over-voltage prevention circuit 500 than is, in synchronization with the frequency f _RF, and a sufficiently high impedance compared to the resonance circuit 210, in synchronization with a frequency higher than the frequency f _RF, may have a low impedance. The overvoltage suppression circuit 500 of FIG. 3C includes a capacitor 506. The capacitance of the capacitor 506 is 1/5 or less, preferably 1/10 or less of the capacitance of the pair of discharge electrodes 202 and 204. Even if an opening abnormality occurs, the capacitor 506 remains as a load, so that the resonance frequency can be prevented from becoming excessively high, and the overvoltage can be suppressed.

The overvoltage suppression circuit 500 of FIG. 3D includes an LCR load circuit. Even in the open state, excessive resonant frequency can be prevented by the LCR load, and overvoltage can be suppressed.

However, the overvoltage suppression circuit 500 may have a structure in which some of the circuits illustrated in FIGS. 3A to 3D are connected in parallel.

4 is a circuit diagram illustrating a specific configuration example of the power supply device 250. The laser device 100 receives a light emission period (excitation period) and a control signal (excitation signal) S1 indicating the stop period, and intermittently operates on the basis of the excitation signal S1. For example, the excitation signal S1 is a repetitive frequency of about several kHz and a pulse signal of about 5% duty ratio.

The high frequency power supply 400 includes an H bridge circuit (full bridge circuit) 402 and a boost transformer 404. The high frequency power supply 400 includes two sets of the H bridge circuit 402 and the step-up transformer 404, and they are connected in parallel. Of course, this set 401 may be configured in only one system. The H bridge circuit 402 switches and applies an alternating voltage V _AC to the primary winding of the boost transformer 404 while the excitation signal S1 indicates the excitation section (for example, high). . The switching frequency of the H bridge circuit 402 is a synchronous frequency f _RF and is set at, for example, about 2 MHz. As a result, the high frequency voltage V _RF which boosted the _AC voltage V _AC generate | occur | produces in the secondary winding of the voltage rising transformer 404.

The DC power supply 300 includes a bank capacitor 302 and a charging circuit 304. The bank capacitor 302 is provided between the DC links 306. The charging circuit 304 charges the bank capacitor 302 and keeps the voltage V _DC of the bank capacitor 302 constant.

During the excitation section, the H-bridge circuit 402 switches so that the energy (charge) accumulated in the bank capacitor 302 is released, and the voltage level of the DC voltage V _DC is lowered. The charging circuit 304 supplies a charging current to the bank capacitor 302 so as to compensate for the drop in the voltage level of the DC voltage V _DC . That is, the DC power supply 300 also intermittently operates in synchronization with the excitation signal S1.

However, the DC power supply 300 may be configured as a DC / DC converter that operates normally even during the excitation period.

In FIG. 4, the protection circuit 550 is configured as part of the high frequency power supply 400. At the output of the high frequency power supply 400, a current sensor CT is provided, and a current flowing through the laser resonator 200 is monitored. Specifically, current sensors CT1 and CT2 are provided at the outputs of each of the two boost voltage transformers 404, and the protection circuit 550 detects an abnormality based on the outputs of the current sensors CT1 and CT2. In the abnormal state, the H bridge circuit 402 is stopped.

5 is a diagram illustrating a configuration example of the high frequency power supply 400 including the protection circuit 550. The protection circuit 550 includes a high frequency current detection circuit (high frequency current detection substrate) 560, a preamplifier circuit (preamp substrate) 570, and a drive signal generation circuit (drive signal generator substrate) 580A, 580B. do.

The high frequency current detection circuit 560 receives the outputs of the two current sensors CT1 and CT2 and removes the frequency component of the synchronous frequency f _RF (2 MHz). The high frequency current detection circuit 560 includes, for example, band removing filters 562 and 564.

The preamp circuit 570 processes the electric potential value detected by the current sensors CT1 and CT2. The level determiner 572 (574) compares the output of the band elimination filter 562 (564) with a threshold value. The level determiner 576 (578) compares the output of the current sensor CT1 (CT2) with a predetermined threshold value. The current difference determiner 579 detects the difference between the outputs of the two current sensors CT1 and CT2 and compares it with the threshold value.

The drive signal generation circuit 580A is a block that generates a drive signal for controlling the H bridge circuit 402. The drive signal generation circuit 580A determines that the opening is abnormal (circuit open) when the output side of the band rejection filter 562 (564) is large, that is, when a frequency component higher than the synchronous frequency f _RF is included. To generate an interlock.

When the output side of the current sensor CT1 (CT2) is larger, the drive signal generation circuit 580A determines that it is an overcurrent state and generates an interlock.

The drive signal generation circuit 580A determines that the current is unbalanced when the difference is greater than the threshold, and generates an interlock.

When an interlock occurs due to any one of the factors, the high frequency power supply 400 (switching operation of the H bridge circuit 402) is stopped. In addition, the stop instruction is supplied to a programmable logic controller (PLC) 590. A PLC is a controller which has a function as a sequencer or a state machine, and controls the whole power supply device 250 collectively. Upon receiving the stop instruction, the PLC 590 instructs the drive signal generation circuit 580B to stop. The drive signal generation circuit 580B is a block for controlling the DC power supply 300, and stops the generation operation of the DC voltage V _DC in response to the stop instruction.

The above is an example of the structure of the protection circuit 550. Subsequently, a modification of abnormality detection by the protection circuit will be described.

(Modification 1)

The protection circuit 550 determines the abnormality based on the presence or absence of the output (laser light) of the laser device 100. That is, even when the high frequency power supply 400 is in the operating state, if no laser light is detected, it can be determined that the opening is abnormal. In this modification, the protection circuit 550 can be comprised with an optical sensor.

(Modification 2)

When the DC power supply 300 includes the bank capacitor 302 and the charging circuit 304 as shown in FIG. 4, when the laser resonator 200 normally emits light, the voltage of the bank capacitor 302 (V _DC ) Decreases the predetermined voltage width, but when the laser resonator 200 does not emit light normally, the reduction width of the voltage V _DC of the bank capacitor 302 becomes small. Therefore, the protection circuit 550 may determine an abnormality based on the fall width of the input voltage (output voltage V _DC of the DC power supply 300) of the high frequency power supply 400 before and behind a short.

(Modification 3)

The protection circuit 550 may determine abnormality when the current flowing through the overvoltage suppression circuit 500 exceeds a threshold value.

(Modification 4)

When the current flowing through the resonant circuit becomes a high frequency, the radiated noise or conducted noise of the high frequency increases. The protection circuit 550 detects this high frequency noise with an antenna, and may determine that it is no-load state (open abnormality) when the high frequency noise increases.

(Modification 5)

The protection circuit 550 may monitor the potential difference ΔV between the pair of discharge electrodes 202 and 204 and determine an abnormality based on the potential difference ΔV.

However, the protection circuit 550 may use some of the abnormality detection methods described herein.

Although the case where the overvoltage suppression circuit 500 was provided in the power supply device 250 was demonstrated in embodiment, it is not limited to this, The overvoltage suppression circuit 500 may be provided in the laser resonator 200 side.

(Usage)

Subsequently, the use of the laser device 100 will be described. 6 is a diagram illustrating a laser processing apparatus 900 including the laser apparatus 100. The laser processing apparatus 900 irradiates the laser pulse 904 to the object 902 and processes the object 902. The type of the object 902 is not particularly limited, and the type of processing is also exemplified, but is not limited thereto.

The laser processing apparatus 900 includes a laser apparatus 100, an optical system 910, a control apparatus 920, and a stage 930. The object 902 is placed on the stage 930 and fixed as necessary. The stage 930 positions the object 902 in accordance with the position control signal S2 from the control device 920, and relatively scans the irradiation position of the object 902 and the laser pulse 904. The stage 930 may be one axis, two axes XY, or three axes XYZ.

The laser device 100 oscillates in accordance with the trigger signal (excitation signal) S1 from the control device 920 to generate the laser pulse 906. The optical system 910 irradiates the laser pulse 906 to the object 902. The configuration of the optical system 910 is not particularly limited, and may include a mirror group for guiding the beam to the object 902, a lens or aperture for beam shaping, and the like.

The controller 920 collectively controls the laser processing apparatus 900. Specifically, the control device 920 outputs the excitation signal S1 intermittently to the laser device 100. The control device 920 also generates a position control signal S2 for controlling the stage 930 in accordance with data (recipe) describing the processing.

Although this invention was demonstrated using the specific phrase based on embodiment, embodiment is only showing the one aspect of the principle and application of this invention, and embodiment is an idea of this invention prescribed by the Claim. In the range which does not deviate, various modifications and a change of arrangement are recognized.

100 laser unit
200 laser resonators
202, 204 discharge electrodes
206 Total Reflector
208 Partial Reflector
210 resonant circuit
250 power supply
300 DC power
302 Bank Capacitors
304 charging circuit
400 high frequency power supply
402H bridge circuit
404 boost transformer
500 overvoltage suppression circuit
502 gas arrestor
504 varistors
550 protection circuit
560 high frequency current detection circuit
570 preamp circuit
580 driving signal generating circuit
590 PLC

Claims (6)

A power supply device for driving a laser resonator including a pair of discharge electrodes,
A high frequency power supply for applying a high frequency voltage to the resonant circuit including the capacitance of the pair of discharge electrodes;
An overvoltage suppression circuit for suppressing overvoltage between both ends of the resonance circuit;
A protection circuit for stopping the application of the high frequency voltage when an abnormality is detected,
And the time required for the protection circuit to detect the abnormality is shorter than the time that the overvoltage suppression circuit can withstand the overvoltage. The method of claim 1,
And the capacitance of the overvoltage suppression circuit is less than one fifth of the capacitance of the pair of discharge electrodes. The method according to claim 1 or 2,
The overvoltage suppression circuit includes at least one of a voltage suppressor, a sustained-circuit device, a gas arrester, a capacitor whose capacitance is 1/10 or less of the capacitance of the pair of discharge electrodes, and an LCR load. The method according to claim 1 or 2,
The overvoltage suppression circuit includes a plurality of elements connected in series. The method according to claim 1 or 2,
The protection circuit,
(i) presence or absence of output light of the laser device,
(ii) a component of the resonant frequency of the resonant circuit among the currents flowing in the high frequency power source,
(iii) components other than the resonant frequency of the resonant circuit among the currents flowing in the high frequency power source,
(iv) a drop width of the input voltage of the high frequency power supply after a short circuit;
(v) current flowing through the overvoltage suppression circuit,
(vi) noise of a frequency higher than the resonance frequency of the resonance circuit,
(vii) voltage between the pair of discharge electrodes
And determining an abnormality based on at least one of the following. A pair of discharge electrodes,
A high frequency power supply for applying a high frequency voltage to the resonant circuit including the capacitance of the pair of discharge electrodes;
An overvoltage suppression circuit for suppressing overvoltage between both ends of the resonance circuit;
A protection circuit for stopping the application of the high frequency voltage when an abnormality is detected,
And the time required for the protection circuit to detect the abnormality is shorter than the time that the overvoltage suppression circuit can withstand the overvoltage.

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