Patent ID: 12233490

EMBODIMENTS

Preferred embodiments of the present disclosure will be described below with reference to the accompanying drawings.

FIGS.1-4are views for explaining a welding power supply according to a first embodiment.FIG.1is a block diagram of a welding power supply A1and shows the overall configuration of the welding system.FIG.2Ais a circuit diagram showing an example of a charging circuit63of the welding power supply A1.FIG.2Bis a circuit diagram showing an example of a discharging circuit64of the welding power supply A1.FIG.3is a functional block diagram showing an example of the internal configuration of a polarity switching controller83.FIG.4is a time chart illustrating variation with time of signals and currents for explaining the output polarity switching process. Note that the vertical axis and the horizontal axis in the time chart may be appropriately magnified or reduced for easier understanding. Also, the waveforms shown in the time chart may be simplified or exaggerated for easier understanding.

As shown inFIG.1, the welding system includes a welding power supply A1and a welding torch B. The welding system is an AC TIG (tungsten inert gas) welding system in which the welding torch B utilizes a non-consumable electrode. The welding power supply A1converts the AC power inputted from a commercial power supply D and outputs the converted power via the output terminals a1and b1. The output terminal a1is connected to a workpiece W by a cable. The output terminal b1is connected to the electrode of the welding torch B by a cable. The welding power supply A1supplies power to allow arc generation between the tip of the electrode of the welding torch B and the workpiece W. The heat from the arc enables welding.

The welding power supply A1includes a rectifying/smoothing circuit1, an inverter circuit2, a transformer3, a rectifying/smoothing circuit5, a restriking circuit6, an inverter circuit7, a controlling circuit8, and a current sensor91.

The rectifying/smoothing circuit1converts the AC power inputted from the commercial power supply D into DC power and outputs the DC power. The rectifying/smoothing circuit1includes a rectifying circuit that rectifies an AC current and a smoothing capacitor for smoothing the rectified current. The configuration of the rectifying/smoothing circuit1may be varied.

For example, the inverter circuit2is a single-phase full-bridge type PWM control inverter and has four switching elements. The inverter circuit2converts the DC power inputted from the rectifying/smoothing circuit1into high-frequency power by switching the switching elements based on output control driving signals inputted from the controlling circuit8, and outputs the high-frequency power. The inverter circuit2may be a half-bridge inverter or another type of inverter circuit as long as it can convert DC power to high-frequency power.

The transformer3transforms the high-frequency voltage outputted from the inverter circuit2and outputs it to the rectifying/smoothing circuit5. The transformer3includes a primary winding3a, a secondary winding3band an auxiliary winding3c. The input terminals of the primary winding3aare connected to respective output terminals of the inverter circuit2. The output terminals of the secondary winding3bare connected to respective input terminals of the rectifying/smoothing circuit5. The secondary winding3bis provided with a center tap separately from the two output terminals. The center tap of the secondary winding3bis connected to an output terminal b1via a connection line4. The output voltage from the inverter circuit2is transformed in accordance with the winding turns ratio of the primary winding3aand the secondary winding3band inputted into the rectifying/smoothing circuit5. The output terminals of the auxiliary winding3care connected to respective input terminals of the charging circuit63. The output voltage from the inverter circuit2is transformed in accordance with the winding turns ratio of the primary winding3aand the auxiliary winding3cand inputted into the charging circuit63. Since the secondary winding3band the auxiliary winding3care insulated from the primary winding3a, the current inputted from the commercial power supply D is prevented from flowing to the circuits on the secondary side or the charging circuit63.

The rectifying/smoothing circuit5converts the high-frequency power inputted from the transformer3into DC power and outputs the DC power. The rectifying/smoothing circuit5includes a full-wave rectifying circuit that rectifies high-frequency current, and DC reactors for smoothing the rectified current. The configuration of the rectifying/smoothing circuit5may be varied.

The inverter circuit7may be a single-phase full bridge inverter of PWM control and has two switching elements Q1and Q2. In the present embodiment, the switching elements Q1and Q2are IGBTs (Insulated Gate Bipolar Transistors). Each of the switching elements Q1and Q2may be a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a bipolar transistor, or the like. The switching element Q1and the switching element Q2are connected in series to each other, with the emitter terminal of the switching element Q1and the collector terminal of the switching element Q2connected to each other. The collector terminal of the switching element Q1is connected to the input terminal on the positive electrode side of the inverter circuit7. The emitter terminal of the switching element Q2is connected to the input terminal on the negative electrode side of the inverter circuit7. The connection point of the switching element Q1and the switching element Q2is connected to the output terminal of the inverter circuit7. A diode is connected in reverse parallel to each of the switching elements Q1and Q2. Switching driving signals outputted from the controlling circuit8are inputted into the gate terminals of the switching element Q1and switching element Q2. The output terminal of the inverter circuit7is connected to the output terminal a1. In the inverter circuit7, the switching elements Q1and Q2are switched based on switching driving signals inputted from the controlling circuit8so as to alternately change the potential of the output terminal of the inverter circuit7(the potential of the output terminal a1) between the potential of the output terminal on the positive electrode side and the potential of the output terminal on the negative electrode side of the rectifying/smoothing circuit5. By this operation, the inverter circuit7performs alternate switching between the forward polarity (where the potential of the output terminal a1connected to the workpiece W is higher than the potential of the output terminal b1connected to the electrode of the welding torch B) and the reversed polarity (where the potential of the output terminal a1is lower than that of the output terminal b1). That is, the inverter circuit7converts the DC power inputted from the rectifying/smoothing circuit5into AC power and outputs the AC power. The inverter circuit7may have a configuration different from that described above as long as it converts DC power to AC power.

The restriking circuit6is arranged between the rectifying/smoothing circuit5and the inverter circuit7. The restriking circuit6applies a high voltage across the output terminals a1and b1of the welding power supply A1at the time of switching the output polarity of the welding power supply A1. Such a high voltage is applied to achieve reliable restriking at the time of polarity switching and hereinafter referred to as “restriking voltage”. Arc extinction is likely to occur when the output polarity switches from the forward polarity to the reversed polarity. In the present embodiment, therefore, the restriking circuit6applies the restriking voltage only when the output polarity switches from the forward polarity to the reversed polarity and does not apply the restriking voltage when the output polarity switches from the reversed polarity to the forward polarity. The restriking circuit6includes a diode61, a restriking capacitor62, a charging circuit63and a discharging circuit64.

The diode61and the restriking capacitor62are connected in series to each other and in parallel to the input side of the inverter circuit7. The diode61has an anode terminal connected to the input terminal on the positive electrode side of the inverter circuit7and a cathode terminal connected to one of the terminals of the restriking capacitor62. One of the terminals of the restriking capacitor62is connected to the cathode terminal of the diode61, and the other terminal of the restriking capacitor62is connected to the input terminal on the negative electrode side of the inverter circuit7. The restriking capacitor62has a predetermined capacitance and is charged with a restriking voltage that will be added to the output from the welding power supply A1. The restriking capacitor62is charged by the charging circuit63and discharged by the discharging circuit64. Cooperating with the diode61, the restriking capacitor62absorbs the surge voltage at the time of switching the inverter circuit7. That is, the restriking capacitor62also functions as a snubber circuit for absorbing surge voltage.

The charging circuit63is a circuit for charging the restriking capacitor62for generating the restriking voltage and connected in parallel to the restriking capacitor62.FIG.2Ashows an example of the charging circuit63. As shown inFIG.2A, the charging circuit63according to the present embodiment includes a rectifying/smoothing circuit63cand a boost chopper63d. The rectifying/smoothing circuit63cincludes a rectifying circuit that performs full-wave rectification of AC voltage and a smoothing capacitor for smoothing the rectified voltage. The rectifying/smoothing circuit63cconverts the high-frequency voltage inputted from the auxiliary winding3cof the transformer3into DC voltage. The circuit configuration of the rectifying/smoothing circuit63cmay be varied.

The boost chopper63draises the DC voltage inputted from the rectifying/smoothing circuit63cand outputs it to the restriking capacitor62. The boost chopper63dincludes a coil and a diode that are connected in series between the input terminal and the output terminal. (One of the terminals of the coil is connected to the anode terminal of the diode: the coil arranged on the input terminal side whereas the diode is arranged on the output terminal side.) A switching element63bis connected in parallel to the connection point of the coil and the diode. A capacitor is connected in parallel to the cathode terminal of the diode. The circuit configuration of the boost chopper63dmay be varied. Although the switching element63bis a MOSFET in the present embodiment, the switching element63bmay be an IGBT, a bipolar transistor or the like.

The boost chopper63dis provided with a drive circuit63afor driving the switching element63b. The drive circuit63aoutputs a pulse signal for driving the switching element63bbased on a charging circuit driving signal inputted from a charge controller86. While the charging circuit driving signal is OFF (e.g. low-level signal), the drive circuit63adoes not output a pulse signal. During this period, the switching element63bis maintained in the off state. Thus, the DC voltage inputted from the rectifying/smoothing circuit63cis applied as it is to the restriking capacitor62to charge the restriking capacitor62. On the other hand, while the charging circuit driving signal is ON (e.g. high-level signal), the drive circuit63aoutputs a predetermined pulse signal to the switching element63b. This actuates the boost chopper63dso that the DC voltage inputted from the rectifying/smoothing circuit63cis raised and applied to the restriking capacitor62to charge the restriking capacitor62. That is, based on the charging circuit driving signal, the charging circuit63performs switching between the state for applying the DC voltage from the rectifying/smoothing circuit63cas it is to the restriking capacitor62and the state for applying the DC voltage after the DC voltage is raised. Note that the drive circuit63amay be dispensed with, and the charge controller86may directly input a pulse signal as the charging circuit driving signal into the switching element63b. The configuration of the charging circuit63may be varied. For example, the charging circuit63may be provided with an isolated forward converter instead of the boost chopper63d.

The discharging circuit64discharges the restriking voltage charged in the restriking capacitor62. The discharging circuit64is connected between the connection point of the diode61and the restriking capacitor62and the connection line4that connects the center tap of the secondary winding3band the output terminal b1.FIG.2Bshows an example of the discharging circuit64. As shown in the figure, the discharging circuit64includes a switching element64aand a current limiting resistor64b. In the present embodiment, the switching element64ais an IGBT. The switching element may be a bipolar transistor, a MOSFET or the like. The switching element64aand the current limiting resistor64bare connected in series to each other and connected in series to the restriking capacitor62. The collector terminal of the switching element64ais connected to one of the terminals of the current limiting resistor64b, and the emitter terminal of the switching element64ais connected to the connection line4via the connection line64c. Note that the current limiting resistor64bmay be connected to the emitter side of the switching element64a. The discharge controller85, which will be described later, inputs a discharging circuit driving signal to the gate terminal of the switching element64a. While the discharging circuit driving signal is ON (e.g. high-level signal) , the switching element64ais in the on state. In this state, the restriking voltage charged in the restriking capacitor62is discharged via the current limiting resistor64b. While the discharging circuit driving signal is OFF (e.g. low-level signal) , the switching element64ais in the off state. In this state, discharge of the restriking voltage is interrupted. In this way, based on the discharging circuit driving signal, the discharging circuit64is switched between the state for discharging the restriking capacitor62and the state for not discharging the restriking capacitor62. The configuration of the discharging circuit64may be varied.

The current sensor91detects the output current lout from the welding power supply A1. In the present embodiment, the current sensor91is arranged on the connection line71that connects the output terminal of the inverter circuit7and the output terminal a1. In the present embodiment, current may flow from the inverter circuit7toward the output terminal a1(which is referred to as “positive” state) , or may flow from the output terminal a1toward the inverter circuit7(which is referred to as “negative” state). The current sensor91detects the instantaneous value of the output current and inputs it to the controlling circuit8. The current sensor91may have any configuration as long as it detects the output current lout from the connection line71. Further, the position of the current sensor91is not limited to the illustrated one. For example, the current sensor91may be placed on the connection line4.

The controlling circuit8controls the welding power supply A1and its function may be implemented by a microcomputer, for example. To the controlling circuit8, the instantaneous value of the output current is inputted from the current sensor91. The controlling circuit8outputs a driving signal to each of the inverter circuit2, the inverter circuit7, the charging circuit63and the discharging circuit64. The controlling circuit8includes a current controller81, a target current setter82, a polarity switching controller83, a discharge controller85and a charge controller86.

The current controller81controls the inverter circuit2for achieving feedback control with respect to the output current lout from the welding power supply A1. The current controller81converts the instantaneous value signal of the output current (inputted from the current sensor91) into an absolute value signal by using an absolute value circuit. Based on the deviation between the absolute value signal and the output current set value inputted from the target current setter82, the current controller81generates an output control driving signal for controlling the switching elements of the inverter circuit2by PWM control, and outputs the output control driving signal to the inverter circuit2. As will be described later, while a stop signal inputted from the polarity switching controller83is ON (e.g. high-level signal), the current controller81stops outputting the output control driving signal. Accordingly, the inverter circuit2stops switching the switching elements to stop high-frequency power output.

The polarity switching controller83controls the inverter circuit7in switching the output polarity of the welding power supply A1. The polarity switching controller83generates switching driving signals that are pulse signals for controlling the switching elements Q1and Q2such that the output polarity is switched, and outputs the switching driving signals to the inverter circuit7. Specifically, the polarity switching controller83generates a switching driving signal S1that is inputted to the switching element Q1to control the switching of the switching element Q1, and a switching driving signal S2that is inputted to the switching element Q2to control the switching of the switching element Q2. When the switching driving signal S1is ON (high level signal), the switching element Q1is ON, with the emitter terminal and the collector terminal electrically connected to each other. When the switching driving signal S1is OFF (low level signal), the switching element Q1is OFF, with the emitter terminal and the collector terminal disconnected. When the switching driving signal S2is ON (high level signal) , the switching element Q2is ON, with the emitter terminal and the collector terminal electrically connected to each other. When the switching driving signal S2is OFF (low level signal), the switching element Q2is OFF, with the emitter terminal and the collector terminal disconnected. Thus, when the switching driving signal S1is ON and the switching driving signal S2is OFF, the potential of the output terminal a1(workpiece W) is higher than the potential of the output terminal b1(welding torch B) (i.e., forward polarity). When the switching driving signal S1is OFF and the switching driving signal S2is ON, the potential of the output terminal a1(workpiece W) is lower than the potential of the output terminal b1(welding torch B) (i.e., reversed polarity). In the present embodiment, in switching the output polarity, the polarity switching controller83sets both of the switching elements Q1and Q2to ON for a certain period of time by setting both switching driving signals S1and S2to ON such that a short circuit is caused (a short-circuit period). The switching driving signals S1and S2are outputted to the discharge controller85and the charge controller86as well.

The polarity switching controller83also generates a stop signal for making the current controller81stop outputting the output control driving signal, and outputs the stop signal to the current controller81. In the present embodiment, in switching the output polarity, the polarity switching controller83sets the stop signal to ON before the short-circuit period to make the current controller81stop outputting the output control driving signal.

Generation of the switching driving signals S1, S2and the stop signal by the polarity switching controller83is described below in more detail.

In switching the output polarity, the polarity switching controller83first stops the output of the inverter circuit2. Specifically, the polarity switching controller83sets the stop signal to ON to make the current controller81stop outputting the output control driving signal, to thereby stop the output of the inverter circuit2. Since the output of the inverter circuit2is stopped, the absolute value of an instantaneous value of the output current gradually decreases. When the absolute value of the instantaneous value of the output current decreases to or below a “short-circuit switching current value”, the polarity switching controller83sets both switching elements Q1and Q2to ON to cause the short-circuit state. When the absolute value of the instantaneous value of the output current further decreases to or below a “polarity switching current value”, the switching element that was ON before the short-circuit state is switched to OFF. The “polarity switching current value” is a threshold for the output current determined in advance such that when the output current is not higher than this threshold, switching the switching element Q1or Q2to OFF will not generate a surge voltage exceeding an acceptable range. The “short-circuit switching current value” is a threshold for the output current that is determined to decrease the output current lout to a certain degree in advance so that an unacceptably large surge voltage can be avoided even when the output current set value is considerably large.

Specifically, the short-circuit switching current value is determined such that when the output current is not higher than this value, the surge voltage lies within the acceptable range. It is now assumed that the polarity switching current value is 200 A. In this case, when the output current set value is 300 A, the surge voltage in switching off a switching element lies within the acceptable range even if stopping the output of the inverter circuit2and switching to the short-circuit state are performed simultaneously. However, when the output current set value is 500 A, the surge voltage in switching off a switching element exceeds the acceptable range if stopping the output of the inverter circuit2and switching to the short-circuit state are performed simultaneously. The threshold of the output current below which the surge voltage will lie within the acceptable range even if stopping the output of the inverter circuit2and switching to the short-circuit state are performed simultaneously, which may be 350 A in this example, is set as the short-circuit switching current value. The short-circuit switching current value may be determined appropriately based on experiments or simulations.

FIG.3is a functional block diagram showing an example of the internal configuration of the polarity switching controller83. The polarity switching controller83includes comparing circuits831and832, a signal generating circuit833, a timer circuit834and a stopping circuit835.

The comparing circuit831compares the instantaneous value of the output current (hereinafter referred to as “output current instantaneous value”) with the polarity switching current value I1. Since the output current lout is an alternating current that periodically reverses the direction, the output current instantaneous value, which is detected by the current sensor91, can be a negative value. To perform switching from the short-circuit state to the reversed polarity, the comparing circuit831compares the output current instantaneous value with the polarity switching current value I1. To perform switching from the short-circuit state to the forward polarity, the comparing circuit831compares the output current instantaneous value with ‘−I1’ (i.e., negative of the polarity switching current value I1). The comparison results are outputted to the signal generating circuit833, the timer circuit834and the stopping circuit835.

The comparing circuit832compares the output current instantaneous value with the short-circuit switching current value I2. To perform switching from the forward polarity to the short-circuit state, the comparing circuit832compares the output current instantaneous value with the short-circuit switching current value I2. To perform switching from the reversed polarity to the short-circuit state, the comparing circuit832compares the output current instantaneous value with ‘−I2’ (i.e., negative of the short-circuit switching current value I2) . The comparison results are outputted to the signal generating circuit833.

The signal generating circuit833generates switching driving signals S1and S2based on the comparison results inputted from the comparing circuits831and832. Specifically, the signal generating circuit833generates a pulse signal that becomes ON when the output current instantaneous value increases to or above −I2and becomes OFF when the output current instantaneous value decreases to or below the polarity switching current value I1, and outputs this pulse signal as the switching driving signal S1. Also, the signal generating circuit833generates a pulse signal that becomes ON when the output current instantaneous value decreases to or below the short-circuit switching current value I2and becomes OFF when the output current instantaneous value increases to or above −I1, and outputs this pulse signal as the switching driving signal S2.

The timer circuit834counts a predetermined time period T. The timer circuit834starts counting the time when the output current instantaneous value decreases to or below the polarity switching current value I1and outputs a timing signal to the stopping circuit835when the predetermined time T has elapsed. The timer circuit834also starts counting the time when the output current instantaneous value increases to or above −I1and outputs a timing signal to the stopping circuit835when the predetermined time T has elapsed.

The stopping circuit835generates a stop signal based on the comparison results inputted from the comparing circuit831and the timing signal inputted from the timer circuit834. Specifically, the stopping circuit835generates a pulse signal that becomes ON when a timing signal is inputted from the timer circuit834and that becomes OFF when the output current instantaneous value decreases to or below the polarity switching current value I1or increases to or above −I1.

The internal configuration of the polarity switching controller83is not limited to that shown inFIG.3.

FIG.4is a time chart illustrating variation with time of signals and currents for explaining the polarity switching process in the welding power supply A1. In the figure, (a) shows the stop signal outputted from the stopping circuit835, (b) shows the switching driving signal S1generated by the signal generating circuit833, (c) shows the switching driving signal S2generated by the signal generating circuit833, (d) shows the output current instantaneous value (the instantaneous value of the output current lout) detected by the current sensor91, and (e) shows the current Ien and the current Iep flowing through the switching element Q1and the switching element Q2, respectively. Note that when the current Iep (the current Ien) has a negative value, the current Iep (the current Ien) does not actually flow through the switching element Q2(Q1) but flows through a diode connected in reverse parallel to the switching element Q2(Q1). (The current flows in the opposite direction of the arrow shown inFIG.1).

Until the time t1, the output polarity is the forward polarity, and the output current lout is controlled to the set value (see (d) inFIG.4) At time t1, the stop signal is switched from OFF to ON (see (a) inFIG.4). In response to this, the inverter circuit2stops the high-frequency power output. As a result, the output current instantaneous value decreases to (or below) the short-circuit switching current value I2at time t2(see (d) inFIG.4). In response to this, the switching driving signal S2becomes ON (see (c) inFIG.4). Note that the driving signal S1is ON before time t1(see (b) inFIG.4). That is, at time t2, both switching driving signals S1and S2are ON so that the short-circuit state is achieved. Note that, during the period from time t1to time t2, the short-circuit state is not achieved although the output of the inverter circuit2is stopped. (The switching element Q1is ON, whereas the switching element Q2is OFF.) During this period, the current Iep through the switching element Q2is zero because the switching element Q2is OFF, and the current Ien flowing through the switching element Q1corresponds to the output current lout (see (e) inFIG.4).

Because of the short-circuit state achieved at t2, the output current instantaneous value further decreases to (or below) the polarity switching current value I1at time t3(see (d) inFIG.4) . In response to this, the switching driving signal S1becomes OFF (see (b) inFIG.4) , the stop signal becomes OFF (see (c) inFIG.4) , and the timer circuit834starts counting the time. The time period from time t2to time t3is the short-circuit period. During the short-circuit period, the current Iep flowing through the switching element Q2gradually increases from zero. The current Ien flowing through the switching element Q1, which is equivalent to the sum of the output current Iout and the current Iep, decreases gradually (see (e) inFIG.4) . Since the current Iep does not increase during the period from time t1to time t2and increases only during the period from time t2to time t3(short-circuit period) , the current Iep at time t3is low as compared with the case where the current Iep would increase throughout the period from time t1to time t3. In this manner, the output current instantaneous value decreases to the short-circuit switching current value I2during the period from time t1to time t2, in which the switching element Q2is OFF and the current Iep does not increase.

At time t3, a surge voltage is applied to the switching element Q1because of the switching driving signal S1becoming OFF as described above. However, since the current Ien flowing through the switching element Q1does not deviate largely from the polarity switching current value I1, the surge voltage advantageously lies within the acceptable range.

At time t3, the output polarity is switched to the reversed polarity because of the switching driving signal S1becoming OFF. Thereafter, the output current decreases rapidly to zero and then changes its flow direction and increases to the negative value whose absolute value is equal to that of the output current set value (see (d) inFIG.4) . When the output current instantaneous value becomes zero in this process, a restriking voltage is applied by the restriking circuit6, which achieves reliable restriking and prevents arc extinction.

During the period from time t3to time t4, the output polarity is the reversed polarity, and the output current lout is controlled to the opposite (negative value) of the output current set value (see (d) inFIG.4). At time t4, the stop signal is switched from OFF to ON (see (a) inFIG.4). In response to this, the inverter circuit2stops the high-frequency power output. As a result, the output current instantaneous value increases to or above −I2at time t5(see (d) inFIG.4). In response to this, the switching driving signal S1becomes ON (see (b) inFIG.4). Note that switching driving signal S2is ON before time t5(see (c) inFIG.4). That is, at time t5, both switching driving signals S1and S2are ON so that the short-circuit state is achieved. Note that, during the period from time t4to time t5, the short-circuit state is not achieved although the output of the inverter circuit2is stopped. (The switching element Q2is ON, whereas the switching element Q1is OFF.) During this period, the current Ien through the switching element Q1is zero because the switching element Q1is OFF, and the current Iep flowing through the switching element Q2equals to |Iout| (see (e) inFIG.4), where |*| denoted the absolute value.

Because of the short-circuit state achieved at t5, the output current instantaneous value further increases to or above −I1at time t6(see (d) inFIG.4). In response to this, the switching driving signal S2becomes OFF (see (c) inFIG.4), the stop signal becomes OFF (see (a) inFIG.4), and the timer circuit834starts counting the time. The time period from time t5to time t6is the short-circuit period. During the short-circuit period, the current Ien flowing through the switching element Q1gradually increases from zero. The current Iep flowing through the switching element Q2is equivalent to the current Ien minus the output current lout (i.e., the sum of the opposite of the output current lout and the current Ien) and decreases gradually (see (e) inFIG.4). Since the current Ien does not increase during the period from time t4to time t5and increases only during the period from time t5to time t6(short-circuit period), the current Ien at time t6is low as compared with the case where the current Ien increases throughout the period from time t4to time t6. In this manner, the output current instantaneous value increases to −I2during the period from time t4to time t5, in which the switching element Q1is OFF and the current Ien does not increase.

At time t6, a surge voltage is applied to the switching element Q2because of the switching driving signal S2becoming OFF as described above. However, since the current Iep flowing through the switching element Q2does not deviate largely from the polarity switching current value I1, the surge voltage advantageously lies within the acceptable range.

At time t6, the output polarity is switched to the forward polarity because of the switching driving signal S2becoming OFF. Thus, the output current increases rapidly and changes its direction to reach the output current set value (see (d) inFIG.4).

Referring again toFIG.1, the discharge controller85controls the discharging circuit64. Based on the switching driving signal inputted from the polarity switching controller83, the discharge controller85generates a discharging circuit driving signal for controlling the discharging circuit64and outputs it to the discharging circuit64. While the discharging circuit driving signal is ON, the discharging circuit64discharges the restriking voltage charged in the restriking capacitor62. The discharge controller85generates the discharging circuit driving signal in such a manner that the discharging circuit driving signal is ON when the output current lout from the welding power supply A1changes from positive to negative. Specifically, the discharge controller85generates a pulse signal that switches to ON when the switching driving signal S1is switched from ON to OFF and that switches to OFF after the lapse of a predetermined time period since the pulse signal was switched to ON. The discharge controller85outputs this pulse signal as the discharging circuit driving signal. The time period is set to cover the timing at which the output current lout changes from positive to negative. The manner in which the discharge controller85generates the discharging circuit driving signal is not limited to the above. It is only required that the restriking voltage is applied when the output current lout changes from positive to negative, so that the discharging circuit driving signal is only required to become ON before the output current lout changes from positive to negative and become OFF after the output current lout is changed from positive to negative.

The charge controller86controls the charging circuit63. The charge controller86generates a charging circuit driving signal for controlling the charging circuit63based on the switching driving signal inputted from the polarity switching controller83and the instantaneous value of the voltage between the terminals of the restriking capacitor62inputted from a voltage sensor, and outputs the charging circuit driving signal to the charging circuit63. While the charging circuit driving signal is ON, the charging circuit63charges the restriking capacitor62. The charging circuit63needs to charge the restriking capacitor62with the restriking voltage after the completion of discharge by the discharging circuit64and before the timing of next discharge. When the restriking capacitor62is charged to the target voltage, further charging is not necessary. Thus, the charge controller86generates the charging circuit driving signal so that the charging circuit driving signal is ON from when the discharge by the discharging circuit64is completed till when the restriking capacitor62is charged to the target voltage. Specifically, the charge controller86generates a pulse signal that switches to ON after the lapse of a predetermined time period since the switching driving signal S1was switched from ON to OFF and that switches to OFF when the voltage between the terminals of the restriking capacitor62reaches the target voltage. The charge controller86outputs this pulse signal as the charging circuit driving signal. The manner in which the charge controller86generates the charging circuit driving signal is not limited to the above. The charging circuit driving signal may be switched to ON at any time after the completion of discharge by the discharging circuit64and before the timing of next discharge as long as it allows the restriking capacitor62to be properly charged to the target voltage.

The functions of the controlling circuit8may be implemented by a microcomputer (which may operate on a program made up of modularized sections) or a digital or analog circuit including logic circuits.

The operation and advantages of the welding power supply according to the present embodiment are described below.

According to the present embodiment, in switching the output polarity, the polarity switching controller83first stops the output of the inverter circuit2. Since the welding power supply is not in the short-circuit state at this time, stopping the output of the inverter circuit2causes the output current to decrease. In this state, no current is flowing through the switching element that is in the off state. When the absolute value of the output current instantaneous value decreases to or below the short-circuit switching current value, the polarity switching controller83sets both switching elements Q1and Q2to the on state to cause a short circuit. During the short-circuit state, current flows also through the switching element that was OFF before the polarity switching. The amount of current increase due to such current flow is small, however, as compared with the case where the output of the inverter circuit2is stopped at the same time as causing a short circuit, whereby the amount of current flowing through the other switching element that was ON before the polarity switching is also small. Thus, even when the output current set value is considerably large, subsequently switching one of the switching elements to OFF causes only a small amount of current to flow through that switching element. Accordingly, the surge voltage is prevented from exceeding the acceptable range.

Also, in switching the output polarity, the polarity switching controller83stops the output of the inverter circuit2by setting the stop signal to ON to cause the current controller81to stop outputting the output control driving signal. Thus, the output current Iout decreases quickly, so that the time taken for the output polarity switching process is reduced. Also, the polarity switching controller83sets the short-circuit period to switch the output polarity. This causes the output current Iout to decrease quickly, so that the time taken for the output polarity switching process is further reduced.

Also, in switching the output polarity, the polarity switching controller83first stops the output of the inverter circuit2and then performs switching to the short-circuit state when the absolute value of the output current instantaneous value decreases to or below the short-circuit switching current value. As described before, the short-circuit switching current value is a threshold below which the surge voltage generated in switching a switching element to OFF will lie within the acceptable range even if stopping the output of the inverter circuit2and switching to the short-circuit state are performed simultaneously. Thus, by the polarity switching controller83not switching to the short-circuit state until the absolute value of the output current instantaneous value decreases to or below the short-circuit switching current value, the surge voltage reliably lies within the acceptable range. Also, by the polarity switching controller83switching to the short-circuit state when the absolute value of the output current instantaneous value decreases to or below the short-circuit switching current value, the time taken for the output polarity switching process is shortened.

In the present embodiment, in switching the output polarity, the output of the inverter circuit2is stopped by making the current controller81stop outputting the output control driving signal. However, the present disclosure is not limited to this, and the output of the inverter circuit2may not be stopped but reduced. Specifically, the polarity switching controller83may make the target current setter82change the output current set value to zero, and the current controller81may control the output current lout to zero, so that the output from the inverter circuit2is reduced. The output current set value may not be set to zero but set to the polarity switching current value I1or the short-circuit switching current value I2. To reduce the output current lout quickly, it is desirable to stop the output of the inverter circuit2.

In the present embodiment, to switch from the short-circuit state to the forward polarity, the polarity switching controller83compares the output current instantaneous value with −I1(I1is the polarity switching current value). However, the present disclosure is not limited to this, and values other than −I1may be used as the polarity switching current value. That is, the polarity switching current value for switching from the short-circuit state to the forward polarity and the polarity switching current value for switching from the short-circuit state to the reversed polarity may have different absolute values. For example, the absolute value of the polarity switching current value for switching to the forward polarity may be set larger than that for switching to the reversed polarity. In this case, a larger surge voltage is generated in switching to the forward polarity, which may promote the charging of the restriking capacitor62. Similarly, the short-circuit switching current value for switching from the reversed polarity to the short-circuit state and that for switching from the forward polarity to the short-circuit state may have different absolute values.

Although the present embodiment utilizes a DC power supply circuit including the rectifying/smoothing circuit1, the inverter circuit2, the transformer3and the rectifying/smoothing circuit5, the present disclosure is not limited to this. The configuration of the DC power supply circuit for supplying DC power to the inverter circuit7may be varied. For example, the AC voltage from the commercial power supply D may be raised by a transformer, or a DC voltage may be raised by a DC/DC converter or a boost chopper.

FIGS.5-7show other embodiments of the present disclosure. In these figures, the elements that are identical or similar to those of the foregoing embodiment are denoted by the same reference signs as those used for the foregoing embodiment.

FIGS.5and6are views for explaining a welding power supply A2according to a second embodiment of the present disclosure.FIG.5is a block diagram of a welding power supply A2and shows the overall configuration of the welding system.FIG.6is a functional block diagram showing an example of the internal configuration of the polarity switching controller83. The welding power supply A2differs from the welding power supply A1(seeFIGS.1and3) of the first embodiment in that the polarity switching current value I1and the short-circuit switching current value I2are changed in accordance with the output voltage Vout.

As shown inFIG.5, the welding power supply A2includes a voltage sensor92. The voltage sensor92is provided for detecting the output voltage Vout of the welding power supply A2and detects the voltage across the output terminal a1and the output terminal b1. In the present embodiment, the condition where the potential of the output terminal a1is higher than that of the output terminal b1is defined as positive, whereas the condition where the potential of the output terminal a1is lower than that of the output terminal b1is defined as negative. The voltage sensor92detects the instantaneous value of the output voltage Vout and inputs it into the controlling circuit8. The voltage sensor92may have any configuration as long as it detects the output voltage Vout. Though description is omitted, the welding power supply A1according to the first embodiment also includes the voltage sensor92.

As shown inFIG.6, the polarity switching controller83further includes threshold setting circuits836and837. The threshold setting circuit836sets the polarity switching current value I1in accordance with the output voltage Vout. Specifically, the instantaneous values of the output voltage Vout (hereinafter referred to as “output voltage instantaneous values”) detected by the voltage sensor92are inputted to the threshold setting circuit836, which calculates the average of the output voltage Vout (hereinafter referred to as the “output voltage average value”). The output voltage average value is obtained by calculating the integral of the absolute value of the output voltage instantaneous values over a predetermined period of time and dividing the result of the integration by the period of time. The predetermined period of time may be the time period corresponding to one cycle of the output voltage Vout but is not limited to this. The threshold setting circuit836may calculate an effective value instead of the above-noted average value. The threshold setting circuit836compares the output voltage average value obtained in this way with a threshold V0. When the output voltage average value is equal to or lower than the threshold V0, the threshold setting circuit836sets I1Has the polarity switching current value I1and outputs it to the comparing circuit831. When the output voltage average value is larger than the threshold V0, the threshold setting circuit836sets I1Lsmaller than I1Has the polarity switching current value I1and outputs it to the comparing circuit831.

When the switching element that was ON before the short-circuit state is switched to OFF to change the output polarity, a current flows through the switching element, and this current becomes larger as the impedance of the external load of the welding power supply A2is larger. This is because, a larger impedance of the external load causes a larger current to be regenerated during the short-circuit state (the current flowing through the switching element that was OFF before the short-circuit state). Accordingly, when the impedance of the external load is large, a large current maybe flowing through the switching element even when the absolute value of the output current instantaneous value decreases to the polarity switching current value I1. In such a case, a surge voltage exceeding the acceptable range may be generated when the switching element is switched to OFF. According to the present embodiment, such a situation is avoided by changing the polarity switching current value I1to a smaller value when the impedance of the external load is large. Since a large impedance of the external load leads to a large output voltage Vout, the threshold setting circuit836according to the present embodiment compares the output voltage average value with the threshold V0. In accordance with the comparison results, the threshold setting circuit836switches the polarity switching current value I1between I1Hand I1Lfor output to the comparing circuit831.

The threshold setting circuit837operates similarly to the threshold setting circuit836and sets the short-circuit switching current value I2in accordance with the output voltage average value. Specifically, the threshold setting circuit837compares the output voltage average value with a threshold V0. When the output voltage average value is equal to or lower than the threshold V0, the threshold setting circuit837sets I2Has the short-circuit switching current value I2and outputs it to the comparing circuit832. When the output voltage average value is larger than the threshold V0, the threshold setting circuit837sets I2Lsmaller than I2Has the short-circuit switching current value I2and outputs it to the comparing circuit832.

In the present embodiment again, the effects similar to those of the first embodiment are achieved. Further, according to the present embodiment, the threshold setting circuit836sets I1Has the polarity switching current value I1when the output voltage average value is equal to or lower than the threshold V0and sets I1Lsmaller than I1Has the polarity switching current value I1when the output voltage average value is larger than the threshold V0. That is, the polarity switching controller83changes the polarity switching current value I1to a smaller value (I1L) when the impedance of the external load is large. Thus, even when the impedance of the external load is large and a large current is regenerated by the short-circuit state, only a small amount of current flows through a switching element when the switching element is switched to OFF, because the polarity switching controller83switches the switching element to OFF after the absolute value of the output current instantaneous value decreases to or below I1L. Thus, the surge voltage is prevented from exceeding the acceptable range. Also, similarly to the threshold setting circuit836, the threshold setting circuit837sets the short-circuit switching current value I2in accordance with the output voltage average value. Thus, the polarity switching controller83can set the short-circuit switching current value I2in relation to the polarity switching current value I1. Since the short-circuit switching current value I2is changed in relation to the polarity switching current value I1, the surge voltage reliably lies within the acceptable range, while the time taken for switching the output polarity can be made as short as possible.

Although the threshold setting circuit836switches the polarity switching current value I1between two values in the present embodiment, the present disclosure is not limited to this. That is, the threshold setting circuit836may switch the polarity switching current value I1among three or more values or may change the polarity switching current value I1linearly in accordance with the output voltage average value. Similarly, although the threshold setting circuit837switches the short-circuit switching current value I2between two values in the present embodiment, the present disclosure is not limited to this. That is, the threshold setting circuit837may switch the short-circuit switching current value I2among three or more values or may change the short-circuit switching current value I2linearly in accordance with the output voltage average value. Note that threshold setting circuit836and the threshold setting circuit837may employ different switching methods. That is, the polarity switching current value I1and the short-circuit switching current value I2may not be changed in relation to each other. Further, the short-circuit switching current value I2may be fixed while the polarity switching current value I1maybe made changeable and vice versa.

FIG.7is a block diagram of a welding power supply A3according to a third embodiment of the present disclosure and shows the overall configuration of the welding system. Note that illustration of the commercial power supply D is omitted inFIG.7. The welding power supply A3differs from the welding power supply A1according to the first embodiment (seeFIG.1) in that the inverter circuit7is a full-bridge inverter.

The inverter circuit7according to the present embodiment is a single-phase full-bridge type PWM control inverter and has four switching elements Q1-Q4. The switching element Q1and the switching element Q2are connected in series to each other, with the emitter terminal of the switching element Q1and the collector terminal of the switching element Q2connected to each other. The collector terminal of the switching element Q1is connected to the input terminal on the positive electrode side of the inverter circuit7. The emitter terminal of the switching element Q2is connected to the input terminal on the negative electrode side of the inverter circuit7. Similarly, the switching element Q3and the switching element Q4are connected in series to each other, with the emitter terminal of the switching element Q3and the collector terminal of the switching element Q4connected to each other. The collector terminal of the switching element Q3is connected to the input terminal on the positive electrode side of the inverter circuit7. The emitter terminal of the switching element Q4is connected to the input terminal on the negative electrode side of the inverter circuit7. The connection point of the switching element Q1and the switching element Q2is connected to the output terminal a1via the connection line71. The connection point of the switching element Q3and the switching element Q4is connected to the output terminal b1via a connection line. A diode is connected in reverse parallel to each of the switching elements Q1-Q4.

A switching driving signal S1outputted from the controlling circuit8is inputted into the gate terminals of the switching element Q1and switching element Q4. A switching driving signal S2outputted from the controlling circuit8is inputted into the gate terminals of the switching element Q2and switching element Q3. When the switching driving signal S1is ON and the switching driving signal S2is OFF, the switching elements Q1and Q4are in the on state whereas the switching elements Q2and Q3are in the off state, so that the inverter circuit7provides the forward polarity (where the potential of the output terminal a1connected to the workpiece W is higher than the potential of the output terminal b1connected to the electrode of the welding torch B). When the switching driving signal S1is OFF and the switching driving signal S2is ON, the switching elements Q1and Q4are in the off state whereas the switching elements Q2and Q3are in the on state, so that the inverter circuit7provides the reversed polarity (where the potential of the output terminal a1connected to the workpiece W is lower than the potential of the output terminal b1connected to the electrode of the welding torch B). When both switching driving signals S1and S2are ON, all of the switching elements Q1-Q4are in the on state, so that the short-circuit state is established.

In the restriking circuit6of the present embodiment, the connection line64cis connected to the input terminal on the positive electrode side of the inverter circuit7. In the transformer3of the present embodiment, the secondary winding3bis not provided with a center tap, and the connection point of the switching element Q3and the switching element Q4is connected to the output terminal b1via a connection line.

In the third embodiment again, the effects similar to those of the first embodiment are achieved. According to the present embodiment, since the connection line64cis connected to the input terminal on the positive electrode side of the inverter circuit7, the restriking circuit6can apply the restriking voltage not only when the output polarity switches from the forward polarity to the reversed polarity but also when the output polarity switches from the reversed polarity to the forward polarity. Further, arc extinction at the time of switching to the reversed polarity is prevented by increasing the charging amount of the restriking capacitor62. This may be achieved by setting the polarity switching current value for switching to the reversed polarity larger than the absolute value of the polarity switching current value for switching to the forward polarity and thereby increasing the surge voltage applied at the time of switching to the reversed polarity.

Although the above description of the first through the third embodiments relate to the welding power supplies A1-A3used for a TIG welding system, the present disclosure is not limited to this. The welding power supply according to the present disclosure maybe used for other semiautomatic welding systems. Also, the welding power supply according to the present disclosure may be used for a fully automatic welding system using a robot or a shielded metal arc welding system.

The welding power supply according to the present disclosure is not limited to the foregoing embodiments. The specific configuration of each part of the welding power supply of the present disclosure may be varied in many ways.