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Inverter source of welding current. Encyclopedia of radio electronics and electrical engineering

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Welding, casting, galvanizing and other works require a powerful specialized voltage or current source (sometimes of a special shape). When analyzing the structure of such sources, it was noticed that their functional schemes practically coincide. This article provides an example of the implementation of a bridge-type voltage converter based on a universal microcontroller control module. This converter is suitable not only for use in inverter welding current sources, but also in induction heating systems, uninterruptible power supplies for electronic equipment, power sources for electroplating, frequency converters, power supplies for ultrasonic generators.

The proposed inverter source of welding current is powered by an AC voltage of 170...240 V and is designed for a load current of up to 150 A for 70% of the working time. The main difference between this source and the classical ones is the absence of a separate storage capacitor charging unit, as well as the ability to adapt to solving various problems without changing the control unit circuit, but only by replacing the microcontroller program.

Functionally, the welding power source consists of a power source of its components, a trinistor controlled rectifier, a high-frequency IGBT bridge inverter with optoisolated control units, and an external welding unit. The schematic diagram of the listed blocks is shown in fig. 1. The rectifier and inverter are controlled and controlled by a microcontroller control and management unit, the circuit of which is shown in fig. 2. The numbering of elements on these diagrams is continuous.

Rice. 1. Schematic diagram of the external welding unit (click to enlarge)

Rice. 2. Scheme of the microcontroller control and management unit

When the contacts of the circuit breaker SA1 are closed, the AC mains voltage is supplied to the diode bridge, consisting of diodes VD1, VD5 and diodes of the main power rectifier VD11, VD12. The rectified current charges the capacitor C4 to the amplitude of the mains voltage. Thermistor RK1 JNR10S470L limits the charging current.

Through resistors R1, R2, R5 and R6, the voltage from capacitor C4 enters the power supply circuit of the voltage converter controller DA1 TOR233R.

From the moment of start until the voltage on the capacitor C10 rises to 5 V, the DA1 microcircuit operates in a self-oscillating mode. When this voltage is reached, the output circuit of the parallel integrated regulator DA2 TL431ALP opens, which causes current to flow through the resistor R9 and the emitting diode of the optocoupler U1. The opened phototransistor of this optocoupler closes the control circuit of the DA1 microcircuit, ensuring that its output key is closed and the accumulation of energy in the magnetic circuit of the pulse transformer T1 is stopped. As long as this key is closed, the accumulated energy through the secondary windings of the transformer enters their loads. All secondary windings are galvanically separated from each other and from the mains voltage. For more information on the operation of the TOPSwitch-FX Family Design Flexible, EcoSmart®, Integrated Off-line Switcher document, pdf.datasheetbank.com/pdf/ Power-Integrations/233 232. pdf.

The control and monitoring unit is made on the DD1 ATmega48-20AU microcontroller. The C34R59 circuit delays the startup of the microcontroller until a stable supply voltage level is established. At the end of the pulse generated by this circuit, the internal clock RC generator of the microcontroller starts working at a frequency of 8 MHz. This frequency is set when programming the microcontroller configuration.

The mains sinusoidal voltage through the resistors R34 and R35 is supplied to the VD24 diode bridge. The rectified pulsating current flows through the emitting diode of the optocoupler U7, shunted by the resistor R38. Near the transition of the instantaneous value of the mains voltage through zero, the current through the emitting diode stops for a while, and the output transistor of the optocoupler U7 closes, which leads to the supply of a high logic level clock signal to the input PD2 of the microcontroller DD1.

Processing this event, the microcontroller sets a low level signal at its PB3 output with a specified delay. This causes current to flow through the circuit consisting of the emitting diode of the optocoupler U2 and the resistor R14. The phototransistor of the optocoupler U2 opens, and the signal from the resistor R15 opens the p-channel field-effect transistor VT1. Through the opened transistor and resistors R16 and R17, the + 12 V voltage from the rectifier on the VD6 diode enters the circuits of the control electrodes of the trinistors Vs 1 and VS2. Trinistors open.

AC mains voltage is also supplied to the power bridge rectifier formed by diodes VD11 and VD12 and trinistors VS1 and VS2. From the moment they open and until the anode-cathode voltage polarity changes, causing the SCRs to close, the storage capacitor C17 is charged. With each transition of the supply voltage through zero, the microcontroller reduces the opening delay, so charging occurs smoothly. Its duration (in the variant under consideration is about 5 s) is programmed.

In the event of an emergency, the microcontroller does not generate a signal at the PB3 output that allows the opening of the trinistors, as a result of which they remain closed. Circuits R18C15 and R20C16 exclude false opening of trinistors under the influence of interference.

Having completed the smooth charging of the storage capacitor C17, the program begins to generate pulses for controlling the bridge inverter keys at the outputs PB1 and PB2 of the microcontroller, following at a frequency of 20 kHz (it is set by software). The duty cycle of the pulses is regulated by a variable resistor R33 in the range of 0,1 ... 0,9.

From these outputs, control signals mutually delayed by half a frequency period of 20 kHz enter the IGBT VT3-VT6 control units made on optocouplers U2-U5. Since these nodes are identical, in the diagram of Fig. 1 shows only one of them in detail, built on the U3 optocoupler.

It is powered from winding IV of transformer T1 by a rectified diode VD9 with a voltage of 25 V. Timing diagrams explaining its operation are shown in fig. 3. The emitter of the IGBT VT5 controlled by this node is connected to the output of the integrated negative voltage regulator DA3. Due to this, the gate-emitter voltage of the IGBT, depending on the state of the optocoupler, changes from +18 V, at which the IGBT is fully open, to -7 V (the IGBT is securely closed).

Rice. 3. Timing charts

The pulses from the PB2 output of the microcontroller through the resistor R60 are fed to the series-connected emitting diodes of the optocouplers U3 and U4, which control the IGBTs VT5 and VT2, respectively. Therefore, these IGBTs open at the same time. IGBT VT3 and VT4 remain closed at this time, since there is no pulse at the PB1 output. The current flows through the circuit positive plate of capacitor C17, open IGBT VT2, current transformer T4, winding I of transformer T5 (in the direction from end to beginning), open IGBT VT5, current transformer T3, negative plate of capacitor C17. This induces voltages on the secondary windings of the transformer T5, applied positively to the anode of the VD21 diode and minus to the anode of the VD22 diode. Welding current flows through winding II of transformer T5, open diode VD21, inductor L2 and through the welding circuit.

In the next half-cycle of the inverter, the program generates a pulse at the output of PB1 of the microcontroller, which opens IGBT VT3 and VT4. There is no pulse at the PB2 output, so IGBTs VT2 and VT5 are closed. The current flows through the circuit positive capacitor C17, open IGBT VT4, winding I of transformer T5 (from beginning to end), current transformer T4, open IGBT VT3, current transformer T2, negative capacitor C17. This induces voltages on the secondary windings of the transformer T5, applied positively to the anode of the VD22 diode and minus to the anode of the VD21 diode. Welding current flows through winding III of transformer T5, open diode VD22, inductor L2 and welding circuit.

Regulate the welding current with a variable resistor R33 mounted on the front panel of the inverter. A voltage is supplied to the ADC2 input of the microcontroller through the integrating circuit R46C30, depending on the position of the slider of this variable resistor. Resistors R41, R42, R45, R47 serve to eliminate the possibility of damage to the ADC2 input of the microcontroller in the event of an open circuit in the variable resistor R33. The ADC of the microcontroller converts the voltage applied to the ADC2 input into a code, and the program processes it and, depending on the result, changes the duty cycle of the pulses at the outputs PB1 and PB2.

Current transformers T2 and T3 serve as IGBT load-fault and through-current protection sensors. In the event of an emergency, the voltage on the secondary windings of these transformers increases. After rectification by VD25 or VD26 diode assemblies, it is fed through a resistive divider R48R49 (capacitor C29 suppresses interference) to the non-inverting input of the DA7.1 comparator. The exemplary voltage at its inverting input forms a resistive divider R54R55 with an interference suppression capacitor C32 (it is also applied to the non-inverting input of the DA7.2 comparator). When the signal received at input 5 exceeds the exemplary voltage (this occurs when more than 2 A flows through the primary windings of transformers T3 or T30), a high-level pulse is formed at the output of comparator DA7.1. Through the R58C35 integrating circuit, which avoids false positives, it enters the inverting input of the DA7.2 comparator. If the duration of the emergency pulse exceeds 5 ms, then a signal will be sent to the input PD3 of the microcontroller from the output of the comparator DA7.2, which will prohibit the program from generating control pulses at the outputs PB1 and PB2.

The current transformer T4 serves as a sensor for the operating current in winding I of the transformer T5. The voltage of the secondary winding of the transformer T23, rectified by the bridge of the diodes of the VD27 and VD4 assemblies, through the integrating circuit R52C31, will go to the ADC1 input of the microcontroller. It will be measured and processed by software. When the measured current exceeds 25 A, the program corrects the duty cycle of the IGBT control pulses.

Overheating protection is made on the thermistor RK2 KTY81/210. Its resistance and the signal level at the ADC0 input of the microcontroller depend on the temperature. If the permissible temperature is exceeded, the program reduces the duty cycle of the pulses at the outputs PB1 and PB2 or stops their formation altogether until the thermistor cools down.

After power is supplied to the microcontroller and its internal clock generator is started, the program waits for the signal to arrive at the PD2 input of the transition of the instantaneous value of the mains voltage through the zero level. Upon receiving such a signal, it starts two internal timers. The contents of the counting register of one of them is used to control the charging rate of the capacitor C17.

The second timer serves the protection of the inverter. It restarts the microcontroller in the absence of a zero-voltage signal for 10 ms, as a result of which the program starts again. After 9,95 ms from the moment the zero-crossing signal is received, the program sends a signal to open the trinistors, setting a high level at the PB3 output of the microcontroller. Upon receipt of the next such signal, the level at the output of PB3 goes low. The next signal to open the SCRs will be given in 9,9 ms, so they will remain open 0,5 ms longer. Due to the gradual increase in the duration of the open state of the trinistors, the capacitor C17 is smoothly charged. After about 5 s, the microcontroller will give a signal to open the trinistors continuously. It will be removed only in the event of a power failure in the supply network or in an "Accident" situation.

Until the capacitor C17 is fully charged, the program does not generate IGBT control signals. Upon completion of its charging, sequences of pulses appear at the outputs PB1 and PB2 of the microcontroller, following with a period of 50 μs, mutually shifted by half a period (25 μs). The duration of the pulses depends on the voltage supplied to the ADC2 input of the microcontroller. Its minimum value is 2,5 µs, the maximum is 22,5 µs (the remaining 2,5 µs of the half-cycle is the minimum pause required to ensure that previously opened IGBTs are closed).

The action of emergency protection is based on the termination of the formation of IgBt control signals in the situations "Accident", "Accident 2" and "Overheat 2". The "Emergency" situation occurs when the voltage at the ADC1 input of the microcontroller rises. This voltage is converted into a binary code. Depending on its value, the duration of the IGBT control signals first gradually decreases, and if this does not work, the formation of pulses completely stops.

When a high logic level signal arrives at the PD3 input, the "Alarm 2" situation occurs without delay. The condition for the occurrence of the "Overheat 2" situation is an increased voltage at the ADC0 input of the microcontroller. It is also converted into a binary code, the result of the analysis of which is a decrease in the duration of the control pulses or their complete shutdown. After elimination of the causes of emergencies, the operation of the inverter source is automatically resumed.

The download file of the weld.hex microcontroller program is attached to the article. The microcontroller configuration must be set as follows: extended byte - 0xFF, high byte - 0xDD, low byte - 0xE2. The programmer is connected to the XP9 connector.

Structurally, the main part of the parts of the welding source is placed on a printed circuit board with dimensions of 140x92,5 mm, the drawing of the printed conductors of which is shown in fig. 4.

Rice. 4. PCB

On the bottom side of the printed circuit board (Fig. 5) there are elements for surface mounting, as well as diodes VD11 and VD12, trinistors VS1 and VS2, IGBT VT2-VT5. On the upper side (Fig. 6) - the rest of the elements. Power circuits are made with hanging wires with a cross section of at least 2,5 mm². The magnetic cores of current transformers T2, T3, T4 of size K20x12x6 made of 2000NM1 ferrite with secondary windings containing 200 turns of PEV-2 wire with a diameter of 0,25 mm are put on these wires.

Rice. 5. Elements on the printed circuit board

Rice. 6. Elements on the printed circuit board

Transformer T1 is mounted on the top side of the PCB. Its magnetic circuit is a ring of size K24x13x7,5 made of permalloy MP140, insulated with a layer of varnished cloth. Winding data are given in table. 1, and the order in which the windings are wound corresponds to their numbers in the diagram. Winding turns I, VI and VII are evenly distributed around the entire perimeter of the magnetic circuit. Each of the other windings is wound on its own segment of the magnetic circuit and does not overlap. All windings are insulated with varnished cloth.

Table 1

Choke L1 - EC24.

Capacitor C17 is fixed above the top surface of the board on stands 20 mm high. They press mounting petals to its terminals with wires soldered to them, connected to the terminals of the capacitor. To connect power wires with IGBT VT2-VT5 terminals, VS1 and VS2 trinistors, VD11 and VD12 diodes, contact pads with holes are provided on the printed circuit board. These elements are pressed against the heat sink block through insulating gaskets, as shown in fig. 7.

Rice. 7. Heat sink unit

Output transformer T5, inductor L2, rectifier diodes VD21, VD22 are located on a separate heat sink unit. The winding data of the transformer T5 are given in Table. 2. Its magnetic core is Gammamet GM414 class. 2 standard sizes OL64x40x30. The primary winding is isolated from the magnetic circuit and secondary windings by double layers of varnished fabric.

Table 2

The L2 inductor winding is wound on a ShLM20x32 magnetic circuit made of electrical steel 0,08 mm thick with a package of five soft copper tapes 0,1 mm thick and a width slightly less than the height of the magnetic circuit window. The package, insulated with varnished cloth, made seven turns. The magnetic circuit is assembled with a non-magnetic gap 1,8 mm long.

Between the heat sinks there are two 80x80 mm fans from the computer power supply connected to the XP1 and XP2 connectors. One fan blows around transformer T5, inductor L2 and capacitor C17. Its air flow is directed towards the transformer T5. The second fan is located between the heat sinks. Its air flow is directed towards the diodes VD21 and VD22.

Network cable PVA 2x2,5 mm² connected to terminals 1 and 3 (upper) of the circuit breaker SA1. To terminals 2 and 4 (bottom) of this switch, two wires with a cross section of 1,5 mm are connected². One of the wires from terminal 2 is connected to the anode of the VS2 trinistor, and the other to the cathode of the VD12 diode (there is no connection between them through the printed conductors). One of the wires from terminal 4 goes to the anode of the VS1 trinistor, and the second one goes to the cathode of the VD11 diode. There is no connection between them through printed conductors either. A variable current regulation resistor R33 is installed on the front panel of the case and connected to the XP8 connector with a three-wire harness. Thermistor RK2 is fixed on the heat sink with a clamping bracket.

The microcontroller program can be downloaded from ftp://ftp.radio.ru/pub/2017/03/weld.zip.

Authors: A. Zharkov

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