JPS6247106B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention connects a switching circuit to the primary side of an output transformer, and a rectifier output circuit and a high frequency circuit to the secondary side, and controls the switching frequency to control the magnitude of the rectifier output. The present invention relates to a welding power supply device in which the voltage is changed, and particularly to an improvement in a control circuit that prevents damage to semiconductor elements in a switching circuit due to biased magnetization of an output transformer.

For example, in a DC TIG welding power supply device in which a switching circuit is connected to the primary side of an output transformer and a rectifier output circuit is connected to the secondary side, the detected amount of the rectifier output on the secondary side is fed back to the primary side to control the frequency of the switching circuit. There are things that can be controlled. A DC TIG welding power supply equipped with this switching circuit and feedback control system has the advantage of not requiring a large-capacity reactor compared to some other types of power supplies, and dramatically improving response and accuracy. ing. However, while having these advantages,
Because excessive current may flow through the switching element due to the so-called biased magnetism of the output transformer, some kind of switching element protection method is required to maintain high operational stability and equipment reliability. come.

In particular, in devices equipped with a high-frequency circuit for arc starting, noise that tends to cause biased magnetization enters the switching control circuit from the high-frequency circuit at the time of start-up, so it is extremely important to take measures to protect the switching element at the time of start-up. becomes important.

Therefore, in order to meet this demand, the present applicant previously proposed in Japanese Patent Application No. 56-56016 a welding power supply device that can prevent destruction of switching elements while maintaining high-speed and high-precision control characteristics. proposed.

To summarize the proposed welding power supply device, it includes a control circuit that feeds back the detected amount of rectified output to control the frequency of the switching circuit, and a control circuit that operates with priority over this control circuit to control the current flowing through the semiconductor element that performs switching to a predetermined level. Another control circuit is provided to turn off the semiconductor element when the value is reached.

First, in order to facilitate understanding of this invention, the first
An example of the power supply device, which is the premise of the present invention, will be explained with reference to the drawings and FIG.

In FIG. 1, reference numeral 1 denotes a DC power source obtained by rectifying and smoothing a commercial power source, which is supplied to a switching circuit 2. The switching circuit 2 includes two switching transistors 2 that are push-pull connected to the primary winding of the output transformer 3.
The switching current is supplied to the primary side of the output transformer 3. Further, the rectifier output circuit 4 connected to the secondary winding of the output transformer 3 includes a rectifier diode bridge 4.
0 and a reactor 41, the output end of which is connected to an electrode torch 5 and a base material 6. and,
A high frequency circuit 7 is provided between the electrode torch 5 and the base material 6 to apply a high frequency to them at the start of welding to properly start the arc. This high frequency circuit 7 is composed of a high frequency coupling coil 70, a high frequency generation circuit 71, and a bypass capacitor 72 that prevents high frequency waves from flowing to the output transformer side.

A first circuit that controls the frequency of the switching circuit 2.
The control circuit 8 includes a first current detector 80 that detects a rectified output current, and an error amplification control circuit 8 that compares the output of this current detector 80 with a reference setting voltage Ed.
1. A signal transmission circuit 82 using a photo coupler that transmits the output of this error amplification control circuit 81 to a switching control circuit 83 with the primary and secondary sides of the output transformer insulated, and a switching circuit that controls the switching circuit 2. It is constituted by a control circuit 83. The switching control circuit 83 controls the switching transistors 20 and 2 at a frequency corresponding to the output level of the signal transmission circuit 82.
A two-phase clock generation circuit (not shown) that alternately turns on and off 1, and an off-time generation circuit (not shown) that interposes a rest period between two-phase clocks, which was previously proposed by the applicant. Contains.

It operates with priority over the first control circuit 8, and when the current flowing through the switching transistors 20, 21 reaches a predetermined value, the switching transistors 20, 21
The second control circuit 9 that turns off the switching current includes a second current detector 90 that detects the switching current, that is, the current flowing through the transistors 20 and 21, and compares the output of this current detector 90 with a constant reference voltage Ek. Comparing amplifier 91. Note that the reference voltage Ek is a voltage that sets the limit of the allowable current flowing through the transistors 20 and 21, and is determined in consideration of the characteristics of the current detector 90, the maximum characteristics of the transistors 20 and 21, and the like.

Next, the operation of this power supply device will be explained. First, when starting an arc, a high frequency is generated from the high frequency generation circuit 71, and the high frequency is applied between the electrode torch 5 and the base material 6 via the coupling coil 70. generate an arc. When an arc occurs, the operation of the high frequency generation circuit 71 is stopped. When an arc is formed in this way, the rectified output current is detected by the first current detector 80, and the output of this current detector 80 and the first reference setting voltage Ed are error amplified by the error amplification control circuit 81. Furthermore, the output of the error amplification control circuit 81 is sent to the switching control circuit 8 via the signal transmission circuit 82.
3, and the switching circuit 2 is controlled by this switching control circuit 83. In other words, the first control circuit 8 has a rectified output that is equal to the reference setting voltage.
Constant feedback control will be performed to ensure that the size corresponds to Ed. On the other hand, the current flowing through the switching circuit 2 is detected by a second current detector 90, and the output of this current detector 90 is compared with a constant reference voltage Ek by a comparison amplifier 91. When this second control circuit 9 operates,
In other words, when an abnormality in the circuit such as a magnetic bias phenomenon occurs and an abnormal current exceeding the allowable maximum current corresponding to the reference voltage Ek flows through any transistor, the output of the comparator amplifier 91 becomes high level and the switching control circuit Transistors 20,2 through 83
1 is turned off, and therefore the operation of the first control circuit 8 is also stopped. This state continues while the output of comparison amplifier 91 is at a high level. As long as the above abnormal current is temporary, the transistors 20, 2
When the current flowing through the first control circuit 1 returns to a state below the allowable maximum current, the off state of the transistors 20 and 21 is released, and the first control circuit 8 becomes functional again, returning to the original steady state. . In this way, when an abnormal current begins to flow through a transistor, the second control circuit 9 operates in priority to the operation of the first control circuit 8, and forcibly turns off the transistor to prevent destruction of the transistor. I'm doing it like that.
Even while performing such an operation, power is supplied to the load, making it possible to continue the welding operation.

Note that when an abnormal current flows, it is only necessary to protect the transistors 20 and 21, so instead of clamping the current so that it is completely off as described above, a constant safe current is applied so that it remains slightly active. It may also be clamped.

As described above, by providing the second control circuit 9 which operates preferentially to the first control circuit 8, destruction of the transistor due to biased magnetism can be prevented. However, in general, in order to completely guarantee this prevention of destruction, the control speed of the control circuit 9 or the responsiveness of the transistor must be such that the rising slope of the switching current, that is,
Must be sufficiently faster than di/dt. FIG. 2 shows the transistor 2 of the switching circuit 2.
0 and 21, the output level EM of the current detector 90 corresponding to the reference voltage Ek (corresponding to the maximum allowable current) and the maximum rated current of the transistor is as shown in the figure. If set, the currents A and B will not destroy the transistor, but the current C generated due to biased magnetism etc. may destroy the transistor. That is, the current A does not operate the control circuit 9, so there is no destruction of the transistor, and the current B also does not operate the control circuit 9.
operates, but its control speed is sufficiently high relative to di/dt, so that it does not destroy the transistor, whereas the control speed of current C is high enough relative to di/dt to It is not possible to follow the change, that is, the current value exceeds the maximum rated current value within the period of the control delay time T, and there is a risk that the transistor will eventually be destroyed within this period T. This breakdown is particularly likely to occur during startup in a power supply device equipped with the above-mentioned high-frequency circuit for arc starting. For this reason, high-speed performance of the control circuit 9 or the transistor is required, but in reality there are no elements that fully meet these requirements, and as long as low-cost, general-purpose elements are used, the above-mentioned It cannot be said that there is no risk of destruction.

Therefore, the main purpose of the present invention is to further ensure the prevention of destruction of switching elements such as transistors in a power supply device equipped with a high-frequency circuit for arc starting, while using common low-cost elements. be.

To summarize the present invention, in a welding power supply apparatus having a high-frequency circuit for arc starting and a feedback system for performing switching control by feedback of a detected amount of rectified output, the control circuit for the feedback system (a first control circuit), a control circuit for turning off the semiconductor element (second control circuit) that operates with priority over this control circuit, and a reference value for turning off the semiconductor element in the second control circuit, at the rated current when starting the arc. A third control circuit is provided which sets the current to a sufficiently smaller value, and after starting the arc, sets the current to a value that is higher than the rated current and lower than the maximum allowable current. Note that when starting the arc, the reference value is set to a value ten parts smaller than the rated current, but this sufficiently small value is defined as the maximum current that flows between when the off signal is output and when the semiconductor element actually turns off. This is the value to keep the current below the allowable current.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram of a welding power supply device according to an embodiment of the present invention. Note that the same parts as those of the power supply device shown in FIG. 1 are given the same reference numerals, and the description of those parts will be omitted below.

The difference in the configuration from FIG. 1 is that the second control circuit 9 is provided with two reference setting voltages Ek ₁ and Ek ₂ that are selected by a changeover switch 92.
The third control circuit 10 is provided, which switches to the reference set voltage Ek ₁ at the time of starting the arc, and switches to the reference set voltage Ek ₂ after starting the arc.
Of the two reference setting voltages, the voltage Ek ₁ is set to a value sufficiently smaller than the output of the current detector 90 corresponding to the rated current of the transistors 20 and 21, and the voltage
Ek ₂ is the current detector 90 corresponding to the maximum allowable current
is set to a value smaller than the output of the current detector 90 and larger than the output of the current detector 90 corresponding to the rated current.
That is, in the second control circuit 9, when the voltage Ek ₁ is selected by the changeover switch 92, the transistor 2
A current sufficiently smaller than the rated current of transistors 20 and 21 is the reference value for turning off the transistor, and when voltage Ek ₂ is selected, a current smaller than the maximum allowable current of transistors 20 and 21 and larger than the rated current is the reference value for turning off the transistor. becomes.

The third control circuit 10 receives the output of the current detector 80 and controls the changeover switch 92, while
In this embodiment, the high frequency generation circuit 71 is also controlled at the same time. A circuit diagram of this control circuit 10 is shown in FIG. The output from the current detector 80 is the reference voltage
It is led to a comparator 12 which performs a comparison with Es.
This reference voltage Es is set to a voltage value corresponding to the welding current after starting the arc, and the comparator 12 does not output when the current is small before starting the arc, but when the current reaches a certain value after starting the arc. Comparator 12 outputs. This comparator 12 constitutes the arc starting state detection means of the present invention. The output of the comparator 12 turns on the transistor 13 that controls the relay 14 and also stops driving the high frequency generation circuit 71. The relay 14 sets the changeover switch 92 to voltage Ek ₁ when the transistor 13 is off, but
When the transistor 13 is turned on, the changeover switch 92
Switch from voltage Ek ₁ to voltage Ek ₂ . This relay 14 constitutes the switch operating means of the present invention.

From the above configuration, when the high frequency generation circuit 71 is driving, that is, when starting the arc, the second
The reference setting voltage of the control circuit 9 becomes Ek ₁ , and when the high frequency generation circuit 71 is not driven, that is, after arc starting, the reference setting voltage becomes Ek ₂ .

Next, the operation of this power supply device will be explained.

First, when the high frequency generation circuit 71 is driven, the high frequency is transmitted to the electrode torch 5 via the coupling coil 70.
is applied between the base material 6 and an arc is generated. As described above, the generation of this high frequency continues until the output of the current detector 80 reaches the reference voltage Es, and during that time the welding current increases. If biased magnetization of the output transformer 3 does not occur at the time of starting the arc, the reference voltage will be switched from Ek ₁ to Ek ₂ without the second control circuit 9 operating, and the operation of the high frequency generation circuit 71 will be stopped. and enters a steady state.
In this steady state, the reference voltage in the first control circuit 9 is equal to or higher than the rated current of the transistors 20 and 21, so each transistor can be operated near its maximum rating.

On the other hand, when the arc is started, high frequency waves flow into the first control circuit 8 and disturb the switching timing.
When this causes biased magnetization in the output transformer 3, the current flowing through the transistors 20 and 21 increases rapidly from a low current state. However, since the reference voltage of the second control circuit 9 at the time of starting the arc is the voltage Ek ₂ corresponding to a current sufficiently smaller than the rated current, the sudden increase in current due to the above-mentioned biased magnetization is detected at an early timing. Then, the transistors 20 and 21 are turned off. In other words, even if di/dt is steep, the reference voltage Ek ₂ is so low that an abnormal state is detected near the beginning of its rise.
Therefore, the transistors 20 and 21 are turned off without a current exceeding the maximum allowable current flowing within the transistor control delay time T.

In this way, at the time of starting the arc, which tends to be biased due to the high frequency from the high frequency generation circuit 71,
By setting the reference voltage of the second control circuit 9 low, destruction of the transistor can be completely prevented. Note that if biased magnetization occurs after arc starting, di/dt will not become so steep, so even if the reference voltage of the second control circuit 9 is Ek ₂ near the voltage value corresponding to the rated current. , the transistor can be turned off so that the maximum allowable current is not exceeded within the transistor control delay time T.

Although the above embodiment is a power supply device applied to a DC TIG welding machine, the present invention can also be applied to other welding machines such as a plasma welding machine.

As detailed above, according to the present invention, even if biased magnetization occurs during arc starting due to the high frequency for arc starting, and furthermore, even if an abnormality occurs in the switching current, the semiconductor element is always activated at a stage where di/dt is small. can be turned off. Therefore, the device can be sufficiently protected without increasing the response speed of the control circuit and semiconductor elements, and the elements used can be general-purpose and low-priced ones.The control circuit can also have a simple configuration, so it is not expensive. A reliable and low-cost power supply device can be obtained. Furthermore, since the semiconductor element can be operated near its maximum rating, there is also the advantage of improving efficiency.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a DC TIG welding power supply device which is the premise of this invention, and FIG. 2 shows the switching current waveform of the same power supply device. Further, FIG. 3 is a block diagram of a DC TIG welding power supply device according to an embodiment of the present invention, and FIG. 4 is a circuit diagram of a third control circuit of the same power supply device. 2... Switching circuit, 20, 21... Switching transistor (semiconductor element), 3... Output transformer, 4... Rectifier output circuit, 7... High frequency circuit for arc starting, 8... First control circuit, 9...
...Second control circuit, 10...Third control circuit.

Claims (1)

[Claims] 1. A switching circuit using at least two semiconductor elements, an output transformer in which the switching circuit is connected to the primary side, and a rectifier output circuit and a high-frequency circuit for arc starting are connected to the secondary side. , a first control circuit that controls the frequency of the switching circuit by feeding back the detected amount of the rectified output of the output transformer; and a first control circuit that operates with priority over the first control circuit and detects the current flowing through the semiconductor element. a current detector and a comparator that compares the current value detected by the current detector with a predetermined reference value, and when the current detected by the current detector reaches the predetermined reference value, the semiconductor element A welding power supply device comprising: a second control circuit that outputs an off signal to the first control circuit to turn off the flowing current, wherein the second control circuit adjusts the predetermined reference value to
a first value that makes the current flowing through the semiconductor element less than or equal to its maximum allowable current from when the off signal is output until the semiconductor element actually turns off; and a first value that is less than or equal to the maximum allowable current of the semiconductor element and greater than or equal to the rated current. further includes an arc starting state detecting means for detecting when the arc starts and after the arc starts; and when the arc starting state detecting means detects the arc starting time, 1, and a switch actuating means for operating the switch means to select the second value when detecting after arc starting. A power supply device for welding.