JP7220013B2 - welding power supply - Google Patents

Description

The present invention relates to a welding power supply.

In an inverter type welding power supply, many inventions have already been made to prevent damage due to excessive current flowing through a switching element due to biased magnetism of a transformer. Patent Literature 1 proposes a method of stopping for a certain period of time when the current flowing through the switching element becomes abnormal, and then restarting from the opposite polarity of the detected abnormality. Further, in Patent Document 2, the excitation current of the transformer is calculated according to the turns ratio of the transformer from the primary current flowing through the transformer and the output current of the welding power source. A method of stopping the driving of the switching element during the period has also been proposed.

JP-A-62-107867 Japanese Patent No. 2973564

In an inverter type welding power supply, if the transformer continues to be magnetized and an excessive current repeatedly flows through the switching element in a short period of time, there is a problem that the switching element is damaged due to secondary breakdown.

Patent Literature 1 proposes a method for restarting as soon as possible when overcurrent protection due to biased magnetization of a transformer is activated and temporarily stopped. Further, in Patent Document 2, when bias magnetism occurs in the transformer, only the driving of the switching element is stopped for a period of half a cycle. Therefore, in Patent Literatures 1 and 2, when the biased magnetization state of the transformer continues, it is not possible to prevent overcurrent from repeatedly flowing through the switching element in a short period of time, and failure due to secondary breakdown of the switching element cannot be prevented.

SUMMARY OF THE INVENTION It is an object of the present invention to prevent failures due to secondary breakdown of switching elements even if the state of biased magnetization of a transformer continues.

In order to solve the above-mentioned problems, the invention of claim 1 is
an inverter unit that converts DC power into high-frequency power;
a transformer for converting the high-frequency power generated by the inverter unit into a predetermined voltage;
a current detector that detects the current flowing through the primary winding of the transformer;
an overcurrent detector that generates an overcurrent signal when the current value detected by the current detector exceeds a reference value;
a welding output unit that rectifies the high-frequency power converted to the predetermined voltage by the secondary winding of the transformer into DC power;
and
When the overcurrent detector generates the overcurrent signal, the inverter section temporarily stops operation and then restarts, and when the overcurrent signal is generated a predetermined number of times within a predetermined time, the inverter section stops operating. ,
This welding power supply is characterized by:

The invention of claim 2 is
When restarting after temporarily stopping the operation of the inverter unit, the ON time of the switching element of the inverter unit is gradually extended.
The welding power source device according to claim 1, characterized by:

According to the present invention, it is possible to prevent failure due to secondary breakdown of the switching element even if the transformer remains in a state of magnetism bias.

1 is a connection diagram and a block diagram of each function of a welding power source apparatus according to an embodiment of the present invention; FIG. FIG. 5 is a timing chart when restarting after the inverter section is temporarily stopped by an overcurrent signal according to the embodiment of the present invention; FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment]
FIG. 1 is a connection diagram and a block diagram of each function of a welding power supply according to an embodiment of the present invention. Each block will be described below with reference to FIG. The DC power supply 1 outputs a DC voltage and a DC current to the inverter unit 2, and includes, for example, a rectifying circuit for rectifying AC power of commercial frequency and a smoothing capacitor for smoothing.

Inverter unit 2 converts the DC voltage and DC current input from DC power supply 1 into high-frequency DC voltage and DC current, and outputs the DC voltage and DC current to transformer 3 . The inverter unit 2 is a full-bridge inverter and has four switching elements 21 to 24 . In this embodiment, IGBTs (insulated gate bipolar transistors) are used as the switching elements 21 to 24 . The switching elements 21 to 24 are not limited to IGBTs, and may be MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or the like.

The transformer 3 applies the high-frequency voltage input from the inverter unit 2 to the primary winding, transforms the voltage into a voltage corresponding to the turns ratio between the primary winding and the secondary winding, and outputs the voltage to the welding output unit 4. .

The welding output unit 4 includes diodes 51 and 52 and a DC reactor 6, converts the high-frequency voltage input from the secondary winding of the transformer 3 to DC, smoothes the current, and outputs the arc load as the welding current Iw. output to 7.

The current detector CT detects the primary winding current Ip of the transformer 3 and outputs it to the overcurrent detector 8 as the primary current value Id.

The overcurrent detection unit 8 generates an overcurrent signal OCP that switches from Low to High when the absolute value of the primary current value Id exceeds the overcurrent detection reference value Vs, and outputs the overcurrent signal OCP to the control unit 25 of the inverter unit 2 . As shown in FIG. 2, the waveform of the primary current value Id has both positive and negative polarities. The detection unit 8 switches the overcurrent signal OCP from Low to High, and when the primary current Id returns from the lower limit value −Vs to the upper limit value Vs, the overcurrent detection unit 8 switches the overcurrent signal OCP from High to Low. switch.

The inverter section 2 is composed of blocks of switching elements 21 to 24 , a control section 25 and a driving section 26 .

The control unit 25 of the inverter unit 2 outputs a signal PWM for driving the switching elements 21 to 24 to the drive unit 26, and performs the following processing based on the overcurrent signal OCP of the overcurrent detection unit 8 to generate a stop signal. St is output to the drive unit 26 .
1) At the moment when the overcurrent signal OCP of the overcurrent detection unit 8 switches from Low to High, the control unit 25 outputs a stop signal St (High) to the drive unit 26, and the drive unit 26 stops the inverter unit 2 according to the stop signal St. , the driving of the switching elements 21 to 24 is temporarily stopped, and the inverter section 2 is temporarily stopped.
2) After a predetermined period of time has passed since the inverter unit 2 temporarily stopped operating, the control unit 25 switches the stop signal St output to the driving unit 26 from High to Low. When the stop signal St becomes Low, the driving unit 26 restarts the driving of the switching elements 21 to 24 of the inverter unit 2. At the time of restart, the time width (maximum duty ratio) in which the switching elements 21 to 24 can be turned on is set. The control unit 25 performs control to gradually widen the alternating-current conversion frequency (inverter frequency) every half cycle, and performs so-called soft-start restart.
3) When the overcurrent signal OCP, which is the output of the overcurrent detection unit 8, switches from Low to High more than a predetermined number of times within a predetermined time, the control unit 25 switches the stop signal St from Low to High, and the driving unit 26 switches from Low to High. The driving of the switching elements 21 to 24 of the inverter section 2 is stopped, and the operation of the inverter section 2 is stopped. The high state of the stop signal St in this case is maintained until the operator turns off the power switch (not shown) of the welding power supply, so that the inverter section 2 keeps its operation stopped.

The drive section 26 of the inverter section 2 drives the switching elements 21 to 24 based on the drive signal PWM from the control section 25 . However, the switching elements 21 to 24 of the inverter section 2 are driven only when the stop signal St from the control section 25 is Low, and the driving of the switching elements 21 to 24 is stopped when the stop signal St is High.

As described above, the inverter unit 2 turns on/off the switching elements 21 to 24 to convert the DC voltage and DC current input from the DC power supply 1 into high-frequency DC voltage and DC current, and outputs the DC voltage and DC current to the transformer 3. . In addition, when the current flowing through the switching elements exceeds the reference value due to the overcurrent signal OCP of the overcurrent detector 8, an overcurrent protection operation is performed to temporarily stop the driving of the switching elements 21 to 24. If the overcurrent protection operation is performed more than the number of times, the inverter unit 2 stops operating.

FIG. 2 is a timing chart for explaining the operation when the transformer 3 according to the embodiment of the present invention is magnetized and the overcurrent detector 8 generates the overcurrent signal OCP. Hereinafter, the operation when the operation of the inverter unit 2 is restarted after being temporarily stopped by the overcurrent signal OCP will be described with reference to FIG.

4A shows the primary winding current Ip flowing from the inverter section 2 to the transformer 3. FIG. The AC conversion frequency (inverter frequency) of the inverter unit 2 in the embodiment of the present invention is 50 kHz, and a high-frequency AC current of 50 kHz flows through the primary winding of the transformer 3 as shown in FIG. . FIG. 4(b) shows the primary current value Id, which is the output of the current detector CT, and has a waveform corresponding to the primary winding current Ip of the transformer 3. FIG. Before the time t0, the transformer 3 is not biased, and the primary winding current Ip of the transformer 3 has a well-balanced positive and negative waveform, as shown in FIG.

Time t0: At time t0, due to factors such as variations in the switching speeds of the switching elements 21 to 24, the transformer 3 gradually begins to be magnetized. As a result, the magnetizing inductance of the transformer 3 becomes smaller, and as shown in FIG. 3(a), the positive and negative primary winding current Ip of the current transformer 3 is out of balance, and either the positive or negative current becomes large. Although FIG. 4(a) shows a case where the current on the positive side increases, the current on the negative side may increase.

Time t1: At time t1, the magnetic bias of the transformer 3 increases, and as shown in FIG. As shown in FIG. 4(c), the overcurrent detector 8 switches the overcurrent signal OCP from Low to High. Then, as shown in (d) of the figure, the control section 25 of the inverter section 2 immediately switches the stop signal St from Low to High. Since the stop signal St has switched to High, the driving section 26 temporarily stops driving the switching elements 21 to 24 of the inverter section 2 until time t2. Since the inverter unit 2 is temporarily stopped, the primary winding current Ip and the primary current value Id of the transformer 3 become zero as shown in (a) and (b) of FIG. The overcurrent signal OCP switches from High to Low.

Time t1 to t2: During the period from time t1 when the inverter unit 2 is temporarily stopped by the overcurrent signal OCP to time t2 when it is restarted, the control unit 25 keeps the stop signal St high as shown in FIG. , the drive unit 26 stops driving the switching elements 21 to 24 of the inverter unit 2, so that the inverter unit 2 is temporarily stopped. The pause period from time t1 to t2 is set to a short period of 100 μS in the embodiment of the present invention and is intended to demagnetize the current transformer used in the current detector CT. If the current detector CT is a current shunt or the like that does not have biased magnetism, it can be set to a period shorter than 100 μs.

Time t2: At time t2, the stop signal Ts switches from High to Low as shown in FIG. Since the stop signal St has switched to Low, the drive unit 26 resumes driving the switching elements 21 to 24 of the inverter unit 2, and the inverter unit 2 restarts. However, the control unit 25 of the inverter unit 2 does not restart the time width in which the switching elements 21 to 24 can be turned on at the maximum duty, but from 10% to 100% of the maximum duty as described below in increments of 10%, It is gradually increased every half cycle of the AC conversion frequency (inverter frequency).

Time t2 to t3: The time width in which the switching elements 21 to 24 can be turned on at time 2 when the inverter unit 2 restarts is 10% of the maximum duty, and 10 times every half cycle of the AC conversion frequency (inverter frequency). increase by percentage. In the embodiment of the present invention, the maximum duty is set to 8 μS, increases from 0.8 μS of 10% at time t2 in increments of 10 μS in increments of 0.8 μS, and at time t3 after 100 μS, 100% of 8 μS. Therefore, as shown in FIG. 1(a), the primary winding current Ip of the transformer 3 has a current waveform whose width and amplitude gradually increase from time t2 to t3.

As described above, when the overcurrent detection unit 8 generates the overcurrent signal OCP (switching from Low to High), the control unit 25 of the inverter unit 2 switches the stop signal St from Low to High and waits for 100 μs. After the temporary stop, the stop signal St is switched from High to Low, restarted by soft start in a period of 100 μs, and overcurrent protection operation is performed. However, during a short period of about 100 μs, the transformer 3 is not demagnetized, and the biased magnetization state of the transformer 3 continues. Therefore, if this overcurrent flow is repeated too frequently, a high temperature portion called a heat spot is generated due to the overcurrent repeatedly flowing through the semiconductor chips of the switching elements 21 to 24, resulting in secondary breakdown. will occur, and the switching elements 21 to 24 will be damaged.

As a countermeasure, in the embodiment of the present invention, the control unit 25 of the inverter unit 2 switches the stop signal St to High when the overcurrent protection operation is repeated eight times in 3 ms, and changes the High state to Hold. Therefore, the driving section 26 stops the operation of the switching elements 21 to 24 of the inverter section 2 because the stop signal St is kept at High. The reason why it is set to 8 times within 3 mS is that the period during which the maximum collector current of the IGBTs used in the switching elements 21 to 24 can flow continuously is limited to several mS. Therefore, the maximum ON time Ton of the switching elements 21 to 24 is 8 μS, the temporary stop time T during overcurrent protection is 100 μS, the repetition detection time Ts of the overcurrent protection operation is 3 mS, and the number of repetition detections Ns of the overcurrent protection operation is 8. , the time during which a current close to the maximum collector current flows through the switching elements 21 to 24 can be obtained by the following equation.
(Ton/T)×Ts×Ns=(8/100)×3mS×8=1.92mS
The secondary breakdown of the IGBT is prevented by suppressing the time during which the current close to the maximum collector current is flowing within several milliseconds. The control unit 25 of the inverter unit 2 keeps the stop signal St at High until the operator turns off the power switch of the welding power source. Part 2 remains stationary. By this measure, the operation of the inverter section 2 can be stopped before the secondary breakdown of the switching elements 21 to 24 occurs, and damage due to the secondary breakdown of the switching elements 21 to 24 can be prevented.

As described above, the transformer 3 after the temporary stop of the overcurrent protection has not yet been sufficiently demagnetized and is in a state of biased magnetization. , the overcurrent signal OCP immediately returns to the time t1 in FIG. Therefore, by gradually increasing the duty to the maximum duty in the period from time t2 to t3, this is to prevent frequent stoppages due to overcurrent due to biased magnetization of the transformer 3 again. , it is possible to reduce the frequency of occurrence of temporary stop due to overcurrent protection, prevent the overcurrent detection unit 8 from generating the overcurrent signal OCP more than eight times in 3 ms, and the secondary breakdown of the switching elements 21 to 24. In order to prevent this, there is an effect that the frequency of the inverter unit 2 falling into a stopped state can be reduced.

1 DC power supply 2 Inverter unit 3 Transformer 4 Welding output unit 6 DC reactor 7 Arc load 8 Overcurrent detectors 21 to 24 Switching element 25 Control unit 26 Drive unit 51, 52 Diode CT Current detector Ip Primary winding current Id Primary Current value Iw Welding current St Stop signal Vs Overcurrent detection reference value PWM Switching element drive signal

Claims (2)

an inverter unit that converts DC power into high-frequency power;
a transformer for converting the high-frequency power generated by the inverter unit into a predetermined voltage;
a current detector that detects the current flowing through the primary winding of the transformer;
an overcurrent detector that generates an overcurrent signal when the current value detected by the current detector exceeds a reference value;
a welding output unit that rectifies the high-frequency power converted to the predetermined voltage by the secondary winding of the transformer into DC power;
and
When the overcurrent detector generates the overcurrent signal, the inverter section temporarily stops operation and then restarts, and when the overcurrent signal is generated a predetermined number of times within a predetermined time, the inverter section stops operating. ,
A welding power supply device characterized by: When restarting after temporarily stopping the operation of the inverter unit, the ON time of the switching element of the inverter unit is gradually extended.
The welding power source device according to claim 1, characterized in that:

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