JP2688152B2 - Welding transformer control method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for controlling a welding transformer in an inverter type DC resistance welding machine, and more specifically, to a method for controlling a welding transformer during a target welding current increase / decrease period.
The present invention relates to a welding transformer control method and device for controlling a pulse width for controlling a secondary current.

[0002]

2. Description of the Related Art Conventionally, in an inverter type DC resistance welding machine, increasing / decreasing control of a target welding current by increasing / decreasing an ON period of a switching element in an inverter, such as slow-up control and secondary current waveform control. It is known to control welding current. When performing such welding current control, the pulse widths of the switching element control pulses for setting the positive period (denoted as A phase) and the negative period (denoted as B phase) of the primary current of the welding transformer are as shown in FIG. As shown in FIGS. 8A and 8B, the same pulse width is controlled within the same cycle.

However, as shown in FIGS. 8 (a) and 8 (b), the switching element control pulse a and the switching element control pulse a which form one cycle having the same pulse width are followed by the switching element control pulse a having the same pulse width. When the fluctuation of the pulse width in each cycle is large, the magnetic flux of the iron core of the welding transformer is biased as shown in FIG. 8 (e), as controlled by the switching element control pulses c and d forming the next one cycle. Magnetization is likely to occur, and when bias magnetization occurs, the primary current shown in FIG.
The primary current increases in parts a and b. 8 (c) shows the primary voltage of the welding transformer as shown in FIG.
(F) shows the current flowing through the work, that is, the welding current.

[0004]

Due to the above-mentioned magnetic bias, the welding current cannot be suddenly changed,
As a result, there is a problem in that ideal welding current control accompanied by a sudden change in welding current, that is, ideal welding control cannot be performed.

An object of the present invention is to provide a welding transformer control method and apparatus capable of suppressing the occurrence of magnetic bias and rapidly changing the welding current.

[0006]

A welding transformer control method according to the present invention is a method for controlling ON / OFF of a switching element of an inverter by a switching element control pulse to energize a primary current of a welding transformer in a positive or negative direction. A method for controlling a welding transformer in an inverter type DC resistance welding machine for controlling a welding current value for energizing a work by controlling each period, which is a primary current of the welding transformer in a process of increasing or decreasing a target welding current value. Set the pulse width of the first switching element control pulse that determines the forward direction energization period to the target welding current value.
Second switch that determines the negative direction energization period based on
The pulse width of the switching element control pulse is set to the first pulse immediately before.
The pulse width and 1 cycle of the control pulse for the switching element
The pulse of the second switching element control pulse before
The feature is that it is decided based on the width .

In the method for controlling a welding transformer according to the present invention, in the method for controlling a welding transformer according to claim 1,
When the pulse width of the first switching element control pulse immediately before is A _n and the pulse width of the second switching element control pulse one cycle before is B _n-1 , the pulse width of the second switching element control pulse is and obtaining the B _n by _{B n = 2A n -B n-} 1.

The controller for the welding transformer of the present invention controls on / off of the switching element of the inverter by the switching element control pulse to control the positive / negative direction energization period of the primary current of the welding transformer. In a controller for a welding transformer in an inverter type DC resistance welding machine for controlling a welding current value to be applied to a work, a forward direction of a primary current of the welding transformer in a target welding current value increasing / decreasing process is determined. Control means for controlling the pulse width of the switching element control pulse on the basis of the deviation between the target welding current value and the detected welding current value, and the negative direction of the primary current of the welding transformer in the process of increasing or decreasing the target welding current value. The pulse width of the second switching element control pulse that determines the energization period is set to the pulse width of the first switching element control pulse immediately before. Characterized in that a second control means for controlling based on the pulse width of the second switching element control pulses of the previous cycle.

[0009]

According to the method for controlling a welding transformer of the present invention, the pulse width of the first switching element control pulse that determines the positive direction energization period of the primary current of the welding transformer during the process of increasing or decreasing the target welding current value is Determined based on the target welding current value
The second switching element control that determines the negative direction energization period
The pulse width of the control pulse is the first switching just before
Pulse width of element control pulse and one cycle before
Based on the pulse width of the second switching element control pulse
Is determined according to the above, the bias will not occur before it occurs.
The first and second switching element control pulse
The width of the rule will be determined, and the first switching element
The magnetic flux based on the pulse width of the slave control pulse and the first switch.
One cycle is constructed in cooperation with the switching element control pulse.
Based on the pulse width of the second switching element control pulse
The magnetic flux becomes 0 on average, and the magnetic bias is suppressed.

When the pulse width of the first switching element control pulse immediately before is A _n and the pulse width of the second switching element control pulse one cycle before is B _n-1 , the second switching element control pulse Since the pulse width B _n of the first switching element control pulse is B _n = 2A _n −B _n-1 , the magnetic flux based on the pulse width of the first switching element control pulse and the first switching element control pulse cooperate with each other for one cycle. And the magnetic flux based on the pulse width of the second switching element control pulse, which becomes 0, averages 0, and the magnetic bias is suppressed.

According to the controller for a welding transformer of the present invention, the pulse width of the first switching element control pulse is controlled by the first control means based on the deviation between the target welding current value and the detected welding current value. The pulse width of the second switching element control pulse is controlled by the second control means based on the pulse width of the first switching element control pulse immediately before and the pulse width of the second switching element control pulse one cycle before. Therefore, the first switching element
The magnetic flux based on the pulse width of the slave control pulse and the first switch.
One cycle is constructed in cooperation with the switching element control pulse.
Based on the pulse width of the second switching element control pulse
The magnetic flux becomes 0 on average, and the magnetic bias is suppressed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method and apparatus of the present invention will be described below with reference to embodiments.

FIG. 1 is a block diagram showing the configuration of an inverter type DC resistance welding machine to which an embodiment of the present invention is applied.

The inverter type DC resistance welding machine to which the present embodiment is applied converts AC power supplied from an AC power supply 11 to DC by a rectifier circuit 12, and outputs output power of the rectifier circuit 12 to switching elements S1 to S4 such as transistors. The power is converted into AC power by the inverter 13 having S4, and the primary current is supplied to the welding transformer 14. Transformer for welding 1
The secondary current of No. 4 is full-wave rectified by the full-wave rectifier 15, and a full-wave rectified output, that is, a DC welding current is supplied to the welding gun 16 to resistance-weld the workpiece 17.

Further, in the inverter type DC resistance welding machine to which this embodiment is applied, the welding current is supplied to the current detection coil 18
Detected by the smoothing circuit 19, the detected welding current is smoothed by the smoothing circuit 19, and the smoothed output is converted into digital data by the A / D converter 20.

Furthermore, the inverter type DC resistance welding machine to which the present embodiment is applied receives the output from the A / D converter 20 and controls the switching elements S1 to S4 to be turned on / off, thereby basing the welding condition on the welding condition. Welding current control means that performs slow-up control to slow up the welding current, welding current waveform control to control the welding current waveform during energization, and pulse target welding current value increase / decrease by controlling the switching element control data. In the process, the pulse width of the switching element control pulse a (see FIGS. 7A and 7B) that determines the positive direction energization period of the primary current of the welding transformer 14 is set between the target welding current value and the detected welding current value. By controlling the switching element control data and the first control means that substantially controls the deviation based on the deviation, the process of increasing or decreasing the target welding current value can be performed. Based on the pulse width of the switching element control pulse a immediately before and the pulse width of the switching element control pulse b one cycle before, the pulse width of the switching element control pulse b that determines the negative direction energization period of the primary current of the welding transformer is determined. A control device 21 having a second control means for substantially controlling the function is provided.

The control unit 21 is a central processing unit (CPU) 2
11, switching elements S1 to S for the target welding current
ROM 212 storing a program for performing pulse width calculation for substantially calculating the switching element control pulse width by calculating switching element control data corresponding to the ON period of No. 4 and the welding current control, a work area, etc. A RAM 213 having a data storage area in
A timer / counter 214 for counting time and an input / output port 215 are provided, and the welding current is controlled based on the combination information with the welding gun and the object to be welded inputted from the input device 30.

Further, the inverter type DC resistance welding machine to which the present embodiment is applied is a switching element control pulse for controlling the switching elements S1 and S4 in response to the switching element control data output from the control device 21 and the frequency setting timing pulse. a and switching element S2
And a PWM modulator 31 for generating a switching element control pulse b for controlling S3 and the switching element control pulses a and b respectively for the drivers 27 and 28.
After amplification, the switching elements S1 to S4 are turned on / off.

Here, the pulse width of the switching element control pulse a sets the period of the A phase of the primary current, and the pulse width of the switching element control pulse b sets the period of the B phase of the primary current.

The PWM modulator 31 converts the switching element control data output from the controller 21 into the D / A converter 2.
4 converts the signal into an analog signal, receives a frequency setting timing pulse output from the control device 21, generates a timing pulse by the one-shot multivibrator 22, and converts the timing pulse into a timing pulse by the sawtooth wave generation circuit 23 that receives the timing pulse. The synchronized sawtooth wave output is generated, and the level of the analog signal converted by the D / A converter 24 and the level of the sawtooth wave generated by the sawtooth wave generation circuit 23 are compared and switched by the comparison circuit 25. The switching element control pulse a is generated by the multiplexer 26 which generates the element control pulses a and b and is controlled based on the signal from the control device 21.
And b are supplied to the drivers 27 and 28 separately.

The operation of the present embodiment configured as described above is
The slow-up control of the patterns shown in FIGS. 3 and 6 will be described as an example with reference to the flowcharts of FIGS. 4 and 5.
First, before describing the slow-up control, the pulse width control of the switching element control pulse will be described with reference to FIG.

The object to be welded and the type of welding current control are instructed from the input device 30. Based on an instruction from the input device 30, the target welding current value data to be full-wave rectified and output from the welding transformer 14 for the object to be welded is stored in the ROM 2
12, the timing pulse of the cycle T and the switching element control data based on the target welding current value data are output from the control device 21 in synchronization with the timing pulse. The one-shot multivibrator 22 that has received the timing pulse outputs a pulse having a period T shown in FIG. 2A in synchronization with the timing pulse. From the sawtooth wave generation circuit 23 at the timing based on the output pulse from the one-shot multivibrator 22, FIG.
The sawtooth wave shown in (b) is output.

On the other hand, the D / A converter 24 receiving the switching element control data outputs an analog signal shown in FIG. 2C in synchronization with the timing pulse. The level of the sawtooth wave output from the sawtooth wave generation circuit 23 shown in FIG. 2B and the output level of the D / A converter 24 shown in FIG.
The level of the conversion output from the D / A converter 24 is compared with the level of the conversion output from the D / A converter 24 while the level of the sawtooth wave is below the level of the conversion output from the D / A converter 24. The duty ratio based on the PWM modulation output shown in FIG. 2D, that is, the switching element control pulses a and b are output from the comparison circuit 25.

The switching element control pulses a and b output from the comparison circuit 25 are separately supplied to the drivers 27 and 28 by the multiplexer 26, and the switching element control pulse a is amplified by the driver 27 and the switching elements S1 and S4. The switching elements S1 and S4 are controlled to be in the ON state during the output period of the switching element control pulse a. The switching element control pulse b is amplified by the driver 28 and applied to the switching elements S2 and S3 to control the switching elements S2 and S3 to be in the ON state during the output period of the switching element control pulse b.

Therefore, the pulse widths of the switching element control pulses a and b are changed based on the switching element control data. By controlling the switching elements S1 to S4 by the switching element control pulses a and b, a primary current having an AC waveform flows through the welding transformer 14. The secondary current of the transformer for welding 14 based on the primary current is full-wave rectified and flows as a direct-current welding current to the object to be welded 17 through the welding gun 16, and the object to be welded 17 is resistance-welded. In this way, the pulse widths of the switching element control pulses a and b are controlled based on the switching element control data so that the welding current becomes the target welding current.

Next, returning to the slow-up control, the type of the slow-up control and the combination of the workpiece 17 and the welding gun are instructed from the input device 30. The slow-up control based on this instruction is assumed to be the slow-up control of the pattern shown in FIG. As is clear from FIG. 3, period 0
~t until _a the slow-up period, a constant welding current period from the period t _a to t _b, from the time t _b until t _c is a pattern of slow down period. T _1, t ₂ at predetermined equal intervals 2T during the slow-up period, ..., the target welding current value i _1, i ₂ at t _a, ..., i _s, the target welding current value i _s in a constant welding current period, A target welding current value i _s (not shown) at predetermined uniform intervals 2T during the slowdown period,
The ROM 212 stores i _s-1 , i _s-2 , ..., 0.

When the slow-up control is instructed,
The first target welding current value i ₁ is read, the switching element control data corresponding to the pulse width of the first switching element control pulses a and b for the target welding current value i ₁ is initialized, and the B-phase flag is reset. Is initialized (step S1). Then, it is determined whether or not it is in the welding current increase / decrease section (step S2).

When it is determined in step S2 that the welding current is in the increase / decrease section, it is determined whether or not A-phase energization is performed (step S3). The determination of the A-phase energization is made by checking the B-phase flag. When the B-phase flag is reset, the A-phase energization is determined. Since it is just after the start, the B-phase flag is reset by the initial setting, it is determined that the A-phase is energized, the switching element control data for the initially set A-phase energization is output, and the pulse is output. width switching elements S1 and S4 by the switching element control pulses a of 0 to a ₁ is controlled, the time counting started at the same time (step S4).

Following the execution of step S4, the B-phase flag is set (step S5), and the time period from step S1 is counted until the period T (= a _{0 to} a ₄ ) elapses (step S6), and then step S3. Is executed.

In this case, since the B flag is set, execution of step S3 determines that the phase B is energized. Here, since it is the first B-phase energization after the start, the switching element control data (corresponding to B _n-1 in FIG. 6B) for initializing the B-phase energization is output, and the pulse width _{a 4 ~a 6 (= 2a 1} ) ( which is described later (1) obtained as B _n-1 = 0 from the equation) switching elements S2 and S3 are controlled by the switching element control pulses b of, is started counting at the same time (Step S7).

Following step S7, the B-phase flag is reset (step S8), and then the detected welding current value is read (step S9). Then, the next target welding current value is read (step S10). Step S10
Is executed for the first time, the target secondary current value read is i ₂ . Following step S10, it is determined whether the welding time has ended (step S11). If it is finished, the welding is finished.

When it is determined in step S11 that the welding time has not ended, the next A based on the difference between the detected welding current value read in step S9 and the target welding current value read in step S10. Switching element control data for phase energization is calculated and stored (step S12) (corresponding to A _n in FIG. 6B). Following step S12, switching element control data for the next B-phase energization is calculated and stored (step S13).

In step S13, the pulse width of the switching element control data b for the next B-phase energization is calculated by setting the pulse width of the switching element control pulse a in step S4 to A _n, and the switching element control for the immediately preceding B-phase energization is performed. Set the pulse width of pulse b to B _n-1
Then, the pulse width B _n of the switching element control pulse b at the time of the next B-phase energization is

[0034]

(Equation 1)

Is calculated by When this is done, magnetic bias can be suppressed as described below. Although the pulse width of the switching element control pulse has been described for the sake of explanation, the switching element control data is calculated so as to have a pulse width of the switching element control pulse. Next, the period T (= a
Waiting for _{4 to} a ₈ ) (step S14).

After step S14, the process is repeated from step S2. By this execution, the pulse width of the switching element control pulse a in step S4 is the pulse width (corresponding to A _n in FIG. 6B) stored in the execution of step S12 immediately before that, and as a result, the secondary current value is controlled to a target welding current value read in step S 10. Further, the pulse width of the switching element control pulse b in step S7 is the pulse width calculated by the above equation (1) and stored in the execution of step S13 immediately before that (B _{n in} FIG. 6B).
The pulse width is different from the pulse width of the immediately preceding switching element control pulse a.

In this way, the detected welding current value is controlled to the target welding current values i ₁ , i ₂ , i ₃ , ... Every time step S4 is executed until the target welding current value reaches the target welding current value i _s. It is repeatedly executed from step S3 following step S2. Therefore, the detection welding current sequential target welding current value i _1, i _2, ... is controlled in i _s, so that the slow-up control is performed.

In the above control, the pulse width of the switching element control pulse a is A _n-1 (= a _{0 to} a ₁ ), and the pulse width of the switching element control pulse b is sequentially as shown in FIG. 6B. Is B _n-1 (= a _{4 to} a ₆ ), and the pulse width of the switching element control pulse a is A _n (= a ₈ to)
a ₁₁ ), the pulse width of the switching element control pulse b is B
_n (= a _{12 to} a ₁₆ ), and so on, the pulse width of the switching element control pulse a and the pulse width of the switching element control pulse b immediately after that which constitute one cycle are as described above (1). Since it is controlled by the relation of expressions,
As shown in FIG. 6 (a), the magnetic flux is at the 0 point on average, and the demagnetization is suppressed.

That is, t = a₀At pulse width A
_n-1When the switching element control pulse a of is output
Is t = a₁The magnetic flux is Φ_AReach t = a_FourIn the
Ruth width B_n-1The switching element control pulse b of
T = a₆Magnetic flux is (−Φ_AReach
You. That is, when one cycle is output, the magnetic flux is Φ_AOr
(-Φ_A) And its center becomes 0. Also,
t = a₈At pulse width A_nSwitching element control
When pulse a is output, t = a₉Magnetic flux Φ =
0, t = a₁₁The magnetic flux is Φ_BReach t
= A₁₂With pulse width B _nSwitching element control pulse b
Is output, t = a₁₄Through Φ = 0,
t = a₁₆Magnetic flux is (−Φ_B) Is reached.

[0040] In step S2, when it is judged entered the constant welding current countdown is started, counting period whether reaches a predetermined time period (t _b -t _a) is checked (step S16). When counting the predetermined time period (t _b -t _a) is judged not been performed in step S16, similar to steps S to step S3~ step S6
17 to step S20 are executed.

When it is determined in step S17 that the phase B is energized, steps S21 to S10 are the same as steps S7 to S10 and steps S12 and S12.
Step S26 is executed. The pulse width of the switching element control pulse b in step S21 is controlled to be the same as the pulse width of the switching element control pulse a. Therefore, in each cycle, the pulse widths of the switching element control pulses a and b are the same and do not change,
Demagnetization does not occur. The reason why steps S23 to S25 are executed here is that the actual welding current value may not match the target welding current value due to a change on the secondary side.

When it is determined that the predetermined period has elapsed by executing step S26, step S16 is executed,
Counting until reaching a predetermined time period (t _b -t _a), it is performed repeatedly step S17~ step S26, during which detects the welding current value is controlled to a target welding current value i _s. When timing is judged to have reached the predetermined time period (t _b -t _a) In step S16, step S3~ step S
15 is executed.

In this case, the target welding current value read in step S9 is gradually decreased, and the target welding current value is gradually decreased to be controlled in the slow down state. In this case, when the welding time ends, the slowdown control ends (step S11). Also calculated et al is in the above-described (1) relationship pulse width of the switching element control pulses a constituting one cycle and the pulse width of the immediately following switching element control pulse b in slowdown state, biased magnetization is generated There is no such thing.

When the pulse widths of the switching element control pulses a and b are controlled as in this embodiment, the switching element control pulse a, the switching element control pulse b, the primary current of the welding transformer 14 and the welding current The magnetic flux and welding current of the iron core of the transformer 14 are shown in FIG.
7 (a) to 7 (f), there is no demagnetization, there is no increase in the primary current of the welding transformer 14 that has occurred conventionally, and there is a sudden change in the welding current. Can be given.

Further, although the case of the slow-up control is exemplified in the above-mentioned embodiment, the present method can be applied to the case of the slow-up control by another pattern, or can be applied to the case of the welding current waveform control. it can.

[0046]

As described above, according to the welding transformer control method of the present invention, the first direction of the forward direction of the primary current of the welding transformer in the process of increasing or decreasing the target welding current value is determined.
Welding target pulse width of switching element control pulse of
The second based on the current value and the negative direction energization period.
The pulse width of the switching element control pulse immediately before
The pulse width of the first switching element control pulse and 1
Second switching element control pulse before cycle
Since it is determined on the basis of the pulse width, the bias magnetism can be suppressed, the welding current can be changed rapidly, and good welding quality can be obtained.

Further, the first pulse width of the switching element control pulses just before the A _n, the second pulse width of the switching element control pulses in one cycle before B
_{When n−1} , the pulse width B _n of the second switching element control pulse is set to B _n = 2A _n −B _n−1. Therefore , the magnetic flux based on the pulse width of the first switching element control pulse and the first The magnetic flux based on the pulse width of the second switching element control pulse, which forms one cycle in cooperation with the first switching element control pulse, becomes 0 on average and bias magnetism is suppressed, and the welding current changes rapidly. It is also possible to obtain good welding quality.

According to the controller for a welding transformer of the present invention, the pulse width of the first switching element control pulse is controlled by the first control means based on the deviation between the target welding current value and the detected welding current value. , The pulse width of the second switching element control pulse is controlled by the second control means based on the pulse width of the first switching element control pulse immediately before and the pulse width of the second switching element control pulse one cycle before. , The first switching element system
Magnetic flux based on the pulse width of the control pulse and the first switch
Forming one cycle in cooperation with the switching element control pulse
2 based on the pulse width of the switching element control pulse
The bundle has an average value of 0, demagnetization is suppressed, the welding current can be rapidly changed, and good welding quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an inverter type DC resistance welding machine to which an embodiment of the present invention is applied.

FIG. 2 is a timing diagram for explaining the operation of the PWM modulator according to the embodiment of the present invention.

FIG. 3 is a schematic diagram showing an example of a slow-up control pattern according to an embodiment of the present invention.

FIG. 4 is a flowchart for explaining the operation of the embodiment of the present invention.

FIG. 5 is a flowchart for explaining the operation of the embodiment of the present invention.

FIG. 6 is a schematic diagram showing a change in magnetic flux of the iron core of the welding transformer and a switching element control pulse, which is provided for explaining the operation in one embodiment of the present invention.

FIG. 7 is a switching element control pulse for explaining the operation in one embodiment of the present invention, primary and secondary of a welding transformer.
It is a schematic diagram which shows the secondary current waveform and the magnetic flux change of the iron core of the transformer for welding.

FIG. 8 is a switching element control pulse in a conventional example,
It is a schematic diagram which shows the primary and secondary current waveform of a transformer for welding, and the magnetic flux change of the iron core of a transformer for welding.

[Explanation of symbols]

 12 ... Rectifier circuit 13 ... Inverter 14 ... Welding transformer 16 ... Welding gun 17 ... Welding object 18 ... Current detection coil 19 ... Smoothing circuit 21 ... Control device 22 ... One shot multivibrator 23 ... Sawtooth wave generation circuit 25 ... Comparator circuit 26 ... Multiplexer 27, 28 ... Driver

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenji Miyanaga 1-10-1 Shin-Sayama, Sayama-shi, Saitama Honda Engineering Co., Ltd. (56) References JP-A 1-222477 (JP, A) JP-A 1 -27786 (JP, A)

Claims (3)

(57) [Claims] 1. A welding current value for energizing a work is controlled by controlling ON / OFF of a switching element of an inverter by a switching element control pulse to control a positive / negative direction energization period of a primary current of a welding transformer. A method for controlling a welding transformer in an inverter type DC resistance welding machine, which is a pulse width of a first switching element control pulse that determines a positive direction energization period of a primary current of the welding transformer in a process of increasing or decreasing a target welding current value. Is determined based on the target welding current value, and the pulse width of the second switching element control pulse that determines the negative-direction energization period is the pulse width of the first switching element control pulse immediately before and the second switching element control one cycle before. A method for controlling a welding transformer, characterized in that it is determined based on the pulse width of a pulse. 2. The method for controlling a welding transformer according to claim 1, wherein the pulse width of the first switching element control pulse immediately before is A _n, and the pulse width of the second switching element control pulse one cycle before. Where B _n-1 is the pulse width B of the second switching element control pulse
The method of welding transformer and obtains the _n by _{B n = 2A n -B n-} 1. 3. A welding current value for energizing a work is controlled by controlling ON / OFF of a switching element of an inverter by a switching element control pulse to control a positive / negative direction energization period of a primary current of a welding transformer. In a controller for a welding transformer in an inverter type DC resistance welding machine, a pulse width of a first switching element control pulse that determines a positive direction energization period of a primary current of the welding transformer in a process of increasing or decreasing a target welding current value is targeted. First control means for controlling on the basis of the deviation between the welding current value and the detected welding current value, and second switching element control for determining the negative direction energization period of the primary current of the welding transformer in the process of increasing or decreasing the target welding current value. The pulse width of the pulse is the pulse width of the first switching element control pulse immediately before and the second pulse one cycle before.
And a second control means for controlling the switching element control pulse based on the pulse width of the switching element control pulse.