JPH0556234B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a welding power supply device that uses an inverter circuit to convert the output of a DC power supply into high-frequency AC and then converts it back into DC.

<Conventional technology> A DC power supply circuit that rectifies a commercial AC power source to obtain a DC output, an inverter circuit that converts the output of this DC power supply circuit to high-frequency AC, and a transformer that converts the output of this inverter circuit to a voltage suitable for welding. The inverter-type welding power supply device is equipped with a rectifier circuit that re-rectifies the output of this transformer to obtain a DC output. The circuit can be made smaller, and the entire welding machine can be made smaller and lighter. However, since the inverter used in this type of welding power source is structured to take out the output using a transformer, if an imbalance occurs between the positive and negative output waveforms of the inverter circuit, the difference between the positive and negative outputs will correspond to the difference between the positive and negative outputs. The DC component of the transformer saturates the transformer core, causing a large excitation current to flow. Particularly in a welding power source, when an inverter system using a semiconductor switching element, which has a large output current and is susceptible to overcurrent, is adopted, it is important to prevent magnetic saturation.

Conventionally, as a means to prevent this magnetic saturation,
The output circuit of the inverter uses a capacitor-coupled system in which a DC blocking capacitor is connected in series with the primary winding of the output transformer.

<Problems to be Solved by the Invention> However, in the case of this capacitor-coupled type, when the inverter is once in operation, then stopped, and then restarted, the residual charge that had been charged in the capacitor during the previous operation is removed. Due to the charge, the voltage applied to the transformer becomes an unbalanced alternating voltage between positive and negative, shifted by the terminal voltage of the capacitor. As a result,
On the other hand, the installation of a DC blocking capacitor sometimes induced saturation of the transformer core upon restart.

For this reason, it is necessary for the output transformer to have an iron core with a sufficiently large cross-sectional area to prevent this transient saturation, and it is necessary to use an inverter system to increase the frequency handled and make it smaller and lighter. However, the disadvantage was that the effects offset each other, making it of little practical value.

<Means for Solving the Problems> The present invention provides a welding power source having a capacitor-coupled transformer output type inverter circuit as described above, in which the residual charge of the DC blocking capacitor is discharged immediately after the inverter is stopped, Another object of the present invention is to provide a welding power supply device that prohibits restarting the inverter while the capacitor is discharging.

<Example> FIG. 1 shows a connection diagram of a welding power source of the present invention.
In the figure, reference numeral 1 denotes an AC power source, and usually a single-phase or three-phase commercial AC power source is used. 2 is a rectifier circuit for converting power from the AC power supply 1 into DC; 3 is an inverter circuit for converting the DC output of the rectifier circuit 2 into high-frequency AC; switching elements 31 and 34; switching elements 32 and 33; By alternately conducting and cutting off each pair as a set, an AC output is obtained between each series connection point of switching elements 31 and 34 and switching elements 32 and 33. 35 is a DC blocking capacitor, 36 is a transformer for taking out the output, and the capacitor 35 and transformer 36 are connected in series.
This constitutes a capacitor-coupled transformer output type output circuit.

4 is the output of the inverter circuit 3, that is, the transformer 36
5 is a rectifier circuit that rectifies the output of the rectifier again and converts it into direct current, 5 is a DC reactor provided to obtain transient characteristics of the output current suitable for welding, and 6 is an output terminal.

Reference numeral 7 denotes an inverter start/stop circuit for starting and stopping the inverter circuit 3, and for example, a high level signal (H
signal), and when stopped, a low level signal (L signal) is output.

8 is the output signal of the inverter start/stop circuit 7.
This is an inverter control circuit that controls conduction and interruption of the switching elements 31 to 34 of the inverter 3 in accordance with the frequency and conduction ratio that are separately determined by receiving the signals from the terminals. The frequency and conduction ratio of the output signal of this inverter control circuit are determined based on predetermined reference values or values determined by feedback signals from welding load conditions such as welding voltage, current, and speed. .

Reference numeral 9 denotes a time limit circuit that supplies a start prohibition command to the E terminal of the inverter control circuit 8 for a certain period of time from when the H signal of the inverter start/stop circuit 7 falls, that is, from when the inverter start signal disappears when the inverter is stopped. A mono-multivibrator can be used that generates an H signal pulse for a certain period of time after the input signal changes from an H signal to an L signal.

10 is a NOT circuit that inverts the output signal of the inverter start/stop circuit 7, and 11 is the output of the timer circuit 9.
An OR circuit whose input is the output of the NOT circuit 10, 1
2 is a discharge circuit connected in parallel to the capacitor 35 operated by the H signal output of the OR circuit 11, and the discharge current limiting resistor R and the OR circuit 11
and a switch means S that is closed by the H signal.

In the embodiment shown in the figure, when the inverter start/stop circuit 7 generates an H signal using a push button switch or the like when welding is started, the inverter control circuit 8 sends a switching command signal of a frequency and conduction time ratio determined by an output setting means (not shown) to the inverter circuit. AC voltage is supplied to each of the switching elements 31 to 34 of 3, and an alternating voltage is generated in the transformer 36 in accordance with the conduction order of the switching elements. This AC voltage is converted into DC by a rectifier circuit 4, and is supplied to the welding load from an output terminal 6 via a DC reactor 5. At this time, since the input signal to the time limit circuit 9 is the H signal, the time limit is not started and its output remains the L signal, and no operation prohibition command is issued to the inverter control circuit 8. Further, since the output signal of the inverter start/stop circuit 7 is an H signal, the output of the NOT circuit 10 is an L signal, and as a result, both input signals of the OR circuit 11 become L signals, and the switch means S of the discharge circuit 12 is turned on. remains open. Therefore, the capacitor 35 is connected in series with the transformer 36 to the output circuit of the inverter circuit 3, and even if an unbalance occurs in the conduction period of the switching elements 31 to 34, the DC voltage due to the unbalance is transferred to the capacitor. 35
As a result, an alternating current voltage with positive and negative balance is applied to the transformer, and the iron core of the transformer does not become saturated.

During such an operation, when the output of the inverter start/stop circuit 7 changes from an H signal to an L signal in order to finish welding, the inverter control circuit 8 immediately stops its operation because the input signal of the A terminal becomes an L signal. As a result, the inverter circuit 3 is stopped and the switching elements 31 to 34 are all cut off.

On the other hand, since the input signal of the NOT circuit 10 becomes an L signal, its output signal becomes an H signal, and the output of the OR circuit 11 also changes from an L signal to an H signal. As a result, the switch means S of the discharge circuit 12 is closed and the capacitor 35 starts discharging through the resistor R.
Further, when the output signal of the inverter start/stop circuit 7 falls from the H signal to the L signal, the time limit circuit 9 is activated and outputs the H signal for a certain period of time. This H signal output is supplied to the E terminal of the inverter control circuit 8 and acts to inhibit the restart of the inverter control circuit with priority over the A terminal input, and is also supplied to the OR circuit 11 to control the discharge circuit 12. Keep the switch means closed.

As a result, by pressing the welding start switch again, the output signal of the inverter start/stop circuit 7 becomes H again.
As a signal, the inverter control circuit 8 is not restarted for a certain period of time determined by the time limit circuit 9, and the discharge of the capacitor 35 is not interrupted. Therefore, if the time limit circuit 9 is set to be longer than the time required for discharging the capacitor 35, the terminal voltage of the capacitor 35 will be at a sufficiently low value when the inverter circuit 3 can be restarted. Therefore, the iron core of the transformer will not be transiently saturated due to the residual voltage of the capacitor 35 at the time of restart.

In FIG. 1, the NOT circuit 10 is provided to keep the capacitor 35 in a discharged state at all times when the inverter is stopped. This is because, in the case of a power outage accident, the timer circuit 9 does not operate, and the capacitor 35 is not sufficiently discharged. When the power is turned on again in such a state, the capacitor can be discharged before the inverter is started for the first time.

Note that when the switch means S is a normally closed contact of a relay, which naturally closes when the power is cut off, neither the NOT circuit nor the OR circuit is necessary, and the H output signal of the timer circuit 9 is Therefore, it is only necessary to directly close the switch means S.

In Figure 1, a time limit circuit is used that outputs a prohibition signal for a certain period of time in response to an inverter stop command, but the circuit that prohibits restarting is not limited to this, and checks the discharge state of the capacitor to ensure that it is sufficiently discharged. The prohibition may be canceled upon detection. The discharge state of a capacitor can be easily determined by checking its discharge current or terminal voltage.

FIG. 2 is a connection diagram showing the main parts of an example of a method for detecting the terminal voltage of a capacitor, and the parts not directly related to the restart inhibition circuit are the same as those in FIG. 1 and are therefore omitted.

In the figure, a voltage detector 13 is connected to both ends of the resistor R of the discharge circuit, and detects the terminal voltage of the capacitor 35 only while the switch means S is closed and the capacitor 35 is discharging. Reference numeral 14 denotes a reference voltage source whose output value e _r is set to a low voltage that does not saturate the transformer core when restarting the inverter. 15 is a comparator which compares the output e _r of the reference voltage source 14 and the output e _c of the voltage detector 13, and outputs an H signal when e _r < e _c , and outputs an H signal when e _r > e _c . Outputs L signal. 16 is NOT circuit, 17
is an AND circuit.

In the example shown in the figure, when the inverter start/stop circuit 7 outputs a start signal (H signal) with the capacitor 35 discharged, the _{comparator 15} at this time
OR circuit 11 because it outputs an L signal for
The output of is an L signal and does not close the switch means S. Further, since the output of the NOT circuit 16 becomes an H signal, the AND circuit 17 becomes an H signal, and the inverter control circuit 8 is activated and the inverter circuit outputs high frequency alternating current.

When the output of the inverter start/stop circuit 7 changes from an H signal to an L signal when welding is stopped, the AND circuit 17 immediately closes and cuts off the start signal to the inverter control circuit 8.

On the other hand, since the output of the NOT circuit 10 becomes an H signal, the output signal of the OR circuit 11 becomes an H signal, closing the switch means S and completing the discharge circuit of the capacitor 35. The terminal voltage e _c of the capacitor 35 decreases according to a time constant determined by its capacitance and the resistance value of the discharge circuit. Since the comparator 15 outputs an H signal while the terminal voltage detected by the voltage detector 13 is higher than the output e _r of the reference voltage source 14, NOT
The L signal converted by circuit 16 is input to AND circuit 17 .

Therefore, even if the output of the inverter start/stop circuit 7 becomes an H signal again during the period e _c > e _r during discharge of the capacitor, the AND circuit 17 will not close and will prohibit restarting the inverter control circuit 8. become.

Also, while e _c > e _r, the H signal from the comparator 15 continues to be supplied to the OR circuit 11, so the switch means S
will continue to be closed regardless of the output state of the inverter start/stop circuit 7.

As the discharge of the capacitor 35 progresses, its terminal voltage e _c
When the output level of the comparator 15 becomes lower than the output e _r of the reference voltage source 14, the output level of the comparator 15 is inverted from the H signal to the L signal. As a result, the output of the NOT circuit 16 becomes an H signal, and the AND circuit 17 receives an H signal from one input signal.
Since it becomes a signal, it is opened and closed only by the output of the inverter start/stop circuit 7, and the inverter start inhibited state is released. At this time, OR
The circuit 11 also opens and closes only in response to the output of the inverter start/stop circuit 7, and operates to open the switch means S and cut off the capacitor discharge circuit when the inverter is started. It goes without saying that the same operation can be obtained even if the voltage detection circuit 13 in FIG. 2 is replaced with a current detection circuit that detects the current flowing through the discharge resistor R.

By the way, when the inverter is constructed of semiconductor switching elements, a DC voltage is applied to the capacitor due to the difference in leakage current of the semiconductor switching elements. That is, due to the nature of the semiconductor switching element, its effective resistance cannot become infinite even when the circuit is cut off, and a leakage current inevitably occurs.
Furthermore, this leakage current varies considerably from element to element, and also varies greatly depending on the operating temperature.
When all the semiconductor switching elements are in the cut-off state, the inverter circuit forms a bridge circuit made up of resistors whose value corresponds to the effective resistance of the semiconductor switching elements at the time of cut-off, and its output A DC voltage corresponding to the difference in leakage current is applied to the capacitor 35 and the transformer 36 corresponding to the section. This voltage is different from the DC voltage that is generated due to the difference in conduction time width when the inverter is in operation, and is also generated when the inverter is stopped. Therefore, even if the capacitor is always connected to the discharge circuit only when the inverter is stopped, as in the examples shown in Figures 1 and 2, the voltage across the resistor of the discharge circuit, that is, the terminal voltage of the capacitor This occurs and cannot be eliminated depending on the discharge circuit.

To solve this problem, it is possible to connect a resistor in parallel with one of the switching elements to compensate for the difference in leakage current, but the effective resistance of the semiconductor switching element during cut-off is quite high. In order to compensate for these differences, the resistance value of the resistor that must be connected in parallel needs to be very large, and in addition, the large resistance value is required in order to cope with the leakage current that changes greatly depending on the operating temperature. Since it is necessary to keep the value variable, it is not practical.

Therefore, in the present invention, by connecting a resistor with a value that allows a current sufficiently larger than the leakage current of each semiconductor switching element to flow in parallel to each semiconductor switching element, the influence of the leakage current of each semiconductor switching element is reduced. The problem was solved by providing a means to suppress the problem to an almost negligible extent.

FIG. 3 shows an embodiment thereof, in which only the inverter circuit 3 is shown and the others are omitted. In the figure, R31 to R34 are switching elements 3
1 to 34 are resistors connected in parallel,
The value is selected to be a value sufficiently lower than the minimum value of the effective resistance value when the switching elements 31 to 34 are cut off, for example, a value of about 1/10 to 1/100. By doing so, the influence of the difference in leakage current is extremely reduced to the extent that it hardly has any influence.

In each of the above embodiments, the switching means S provided in the discharge circuit of the capacitor is illustrated as a mechanical switch, but the switching means S may be of any type having a bidirectional switching function, such as a relay contact, a bidirectional thyristor, etc. , unidirectional semiconductor switching elements such as unidirectional thyristors and transistors connected in antiparallel, or a rectifier that double-wave rectifies the terminal voltage of a capacitor, and one unidirectional device connected so as to short-circuit its DC output via a resistor. Any method such as combination with a semiconductor switching element may be used.

Further, even if the state of the output level of each circuit is different from that of the above embodiments, it is clear that by making slight changes, it is possible to easily obtain functions equivalent to those of each embodiment, so detailed illustrations will be omitted.

<Effects of the Invention> As described above, in the present invention, the unbalanced operation of the inverter is prevented by the series capacitor, and the remaining charge in the capacitor is forcibly discharged when the inverter is stopped, so that the steady output is reduced. The core of the output transformer does not saturate even during transient times immediately after startup, and the rated capacity of the semiconductor switch used in the inverter can be used effectively, and the cross-sectional area of the transformer core has almost no margin. Since it is no longer necessary to take a load, the advantages of the inverter system, which is compact and lightweight, can be fully utilized.

[Brief explanation of the drawing]

Figure 1 is a connection diagram showing an embodiment of the present invention, Figure 2 is a connection diagram showing an embodiment of the present invention.
The figure is a connection diagram showing only the essential parts of the restart prohibition circuit in another embodiment of the invention, and FIG. 3 is a connection diagram showing only the inverter circuit in another embodiment of the invention. 1... AC power supply, 2, 4... Rectifier circuit, 3...
Inverter circuit, 7... Inverter start/stop circuit, 8... Inverter control circuit, 9... Time limit circuit, 12... Discharge circuit, 13... Voltage detection circuit,
31 to 34... switching element, 35... capacitor, 36... transformer.

Claims (1)

[Scope of Claims] 1. A rectifier circuit that rectifies a commercial AC power source to obtain DC, an inverter circuit using a semiconductor switching element that converts the output of the rectifier circuit into high-frequency AC, and a rectifier circuit provided at the output section of the inverter circuit. a series circuit consisting of a capacitor and a primary winding of an output transformer; a rectifier circuit connected to a secondary winding of the output transformer; A welding power supply device equipped with a discharge circuit for discharging electric charge. 2. A rectifier circuit that rectifies a commercial AC power source to obtain DC, an inverter circuit using semiconductor switching elements that converts the output of the rectifier circuit into high-frequency AC, and a capacitor and output transformer provided at the output section of the inverter circuit. a series circuit consisting of a primary winding of the inverter, a rectifier circuit connected to the secondary winding of the output transformer, and a rectifier circuit connected in parallel to the capacitor for discharging residual charge in the capacitor when the inverter is stopped. A welding power supply device comprising a discharge circuit and a prohibition circuit that prohibits restarting of the inverter circuit while the capacitor is discharging. 3. The welding power supply device according to claim 2, wherein the prohibition circuit is activated by an inverter stop signal and comprises a time limit circuit that prohibits restarting of the inverter circuit for a certain period of time. 4. The prohibition circuit includes a detection circuit that detects the terminal voltage or discharge current of the capacitor, and prohibits the restart of the inverter circuit until the output of the detection circuit drops below a certain value after the inverter is stopped. Claim 2, which is a circuit that
Welding power supply device as described in section. 5. Claims 2 to 5, wherein the discharge circuit is a circuit that continuously maintains the capacitor in a discharged state while either the inverter stop signal or the inhibition circuit restart prohibition signal is output. The welding power supply device according to any one of Item 4. 6. The welding power supply device according to any one of claims 2 to 5, wherein each of the switching elements is connected in parallel with a resistor whose resistance value is smaller than the resistance value when each switching element is cut off.