# Overview of the standard scheme of inverter-type welding

The steady downward trend in prices for inverter-type welding machines has led to a significant increase in the popularity of this equipment both among professionals and among those who use welding only for their own needs. It is quite understandable that many users who have such a device are interested in its structure and principle of operation, because information of this kind will help repair equipment in the event of a malfunction or even improve an inexpensive model with “truncated” functionality. As we will see later, it is not at all difficult to deal with these issues, it is enough to have basic knowledge of electrical engineering.

Invertor welding machine.

## General information

The electrical circuit of various models of welding inverters may differ in some details, but in general terms, all these devices work according to the same principle. The main task of each of them is to convert the electrical energy coming from the network so as to obtain a large current at the output. The conversion process is divided into several stages:

Throttle circuit welding inverter.

• rectification of alternating current coming from the mains;
• DC conversion back to AC, but with a much higher oscillation frequency;
• amplification of alternating high-frequency current by lowering its voltage;
• straightening amplified high frequency alternating current.

Anyone who is at least a little versed in computer “hardware” probably knows that the switching power supply unit of a personal computer works in the same way. The central point of this circuit is an increase in the frequency of the alternating current, and this is exactly the task that the inverter performs. What is it for? The fact is that the dimensions and weight of a transformer depend not only on its power, but also on the frequency of the current for which it is designed to be converted. The lower the frequency, the more massive and larger the transformer is. This dependence is very significant. So, for example, with a fourfold increase in the frequency of an alternating current, the dimensions of a transformer are halved. Inverter circuit raises the frequency of electric current from 50 Hz to 60-80 kHz, so that the gain in weight and size is quite tangible. As a result, we get a light and compact welding machine, for the production of which much less materials are needed, including expensive copper.

Next, we consider in detail the main blocks of the inverter apparatus and their interrelations.

## Power unit: mains rectifier

The scheme of the inverter welding machine.

The peculiarity of the inverter circuit is that its work requires a constant current. Therefore, the alternating current of the ordinary power supply, supplied with a voltage of 220 V and a frequency of 50 Hz, is primarily subjected to rectification. The rectifier circuitry includes a diode bridge and two capacitors, whose task is to smooth the pulsations. Due to the high power of the current, the diode bridge heats up sufficiently during operation, so it is equipped with a radiator with a thermal fuse. The latter performs the opening of the circuit when heated to a temperature of 90 degrees.

At the output of the diode bridge, a pulsating direct current with a voltage of 220 V is obtained, but on capacitors it increases by 1.41 times and is already 310 V. Taking into account the possibility of an initial voltage jump in the direction of an increase, the capacitors that withstand the voltage up to 400 In (corresponds to the initial voltage of 280 V).

The mains rectifier is connected to the power source through an electromagnetic compatibility filter, which prevents high-frequency interference from the operation of the inverter into the power grid.

Inverter welding machine power supply circuit.

Immediately after turning on the welding machine, the charging current supplied to the capacitors can reach a value that is sufficient for disabling the diode bridge. To prevent this from happening, all types of welding inverters are equipped with a soft start circuit. It is realized by means of a relay and a resistor, the power of which is about 8 W, and the resistance is about 50 Ohms (in different models of welding inverters, the characteristics of the resistor may differ from those indicated). The resistor is connected to the rectifier circuit, and at the time of switching on the welding machine, it weakens the starting current. After the equipment enters the operating mode, a relay is triggered, which closes the resistor terminals so that the current flows already “past” it.

## Inverter: working principle

The electrical circuit of the inverter, which is equipped with welding machines of this type, includes two key transistors, which are connected according to the principle of "slanting bridge". Their peculiarity is that they can switch with a very high frequency, from 60 to 80 kHz. In this case, the direct current flowing into the inverter is transformed into an alternating current having the same frequency. From the usual current in the electrical network, it also differs by its characteristic: it is not a sinusoidal, but rectangular.

Key transistors are installed on the radiator, which allows to avoid overheating. Protection against excessive voltages is provided by an RC snubber circuit.

## High Frequency (Pulse) Transformer

The principle of operation of the inverter.

The main part of any welding machine is a step-down transformer. Its design in inverter devices is almost the same as usual, but at the same time it is more compact. Another important difference is the presence of an additional secondary winding, which is used to power the control circuit.

The primary winding of a high-frequency transformer is supplied by an alternating current produced by the inverter with a voltage of 310 V and a frequency of several tens of kilohertz. At the output of the secondary winding, which has a smaller number of turns, the voltage decreases to 60-70 V, and the current increases to 110-130 A. It remains for him to pass one more, last stage.

## Output rectifier

The current coming from the high-frequency transformer must be turned into a constant current — just such a current is needed for welding. To this end, an inverter welding machine is equipped with an output rectifier, the electrical circuit of which consists of dual diodes with a common cathode. They differ from ordinary diodes in high speed. The open-close cycle of these elements is only 50 nanoseconds (this characteristic is called the recovery time). This quality is necessary for working with ultra-high frequency currents.

The diodes of the output rectifier are also installed on the radiator, and for their protection this unit is equipped with an RC circuit.

## Starting circuit apparatus

Ways to connect a welding inverter.

At the moment of switching on the device from the mains rectifier, power is supplied to the control circuit through a 15-volt stabilizer.

After the control circuit launches the inverter key transistors, a voltage appears on the additional secondary winding of the high-frequency transformer. It is rectified by diodes and through all the same stabilizer begins to power the control circuit, while it is disconnected from the network rectifier.

## Control scheme

Coordination of the inverter-type current converter works by the control circuit. Its main element is a PWM controller chip. The task of this chip is the switching of the key transistors of the inverter. Their operation is controlled by the PWM controller not directly, but by means of two successive elements: a field-effect transistor and an isolation transformer.

Current conversion in welding inverter.

From the field-effect transistor, a high-frequency (about 65 kHz) current with a rectangular characteristic enters the primary winding of the isolation transformer. The transformer converts the voltage of this current to the value that is needed to control the key transistors of the inverter. The signals on them come from two secondary windings of an isolation transformer, with each of the windings connected to one transistor.

In addition to these elements, the electrical circuit of the control and monitoring board contains auxiliary transistors, which help the key transistors of the inverter circuit to close, and the zener diodes that protect them from voltage surges. There is also an analyzer-current limiter. The main element of the analyzer is a transformer, which is included in the primary circuit of the high-frequency transformer installed in the power unit. The analyzer-limiter controls the current in the converter of the welding machine and uses the signals from the primary winding of the power transformer to adjust the welding current and the formation of pulses transmitted to the PWM controller chip.

To regulate the welding current, a variable resistor is turned on in the electrical circuit of the control unit, the resistance of which is set by turning the knob on the control panel of the inverter's welding machine.

## Control of output and mains voltage

The functionality of the welding inverter.

In addition to all the above, the task of the control circuit of the welding machine is to monitor the voltage in the network and at the output rectifier. To do this, its electrical circuit is completed with an operational amplifier. Some of its elements are connected to a network rectifier in order to detect voltage surges in the power grid. In the event of violations, these elements reproduce the current and voltage protection signals that go to the summing module and then to the PWM controller pulse generator. The operation of the generator, therefore, of the whole circuit is blocked.

Similarly, the operating voltage at the output of the converter is monitored. Its value may deviate from the norm in the event of a malfunction in the operation of the diode bridge of the network rectifier or other elements. In this case, the control circuit is also disabled.

The blocking of the circuit is accompanied by the supply of voltage to the signal diode, which notifies the user of the welding machine about the malfunctions.

## Instructions for repairing welding machine-inverter

Like any equipment, inverter welding machines can fail. The following symptom is often observed: the device seems to be completely intact (the “normal” display is on, the fan can be heard in the case), but a spark does not appear when the electrode contacts the metal. Sometimes you can hear an unusual hum. In some cases, the repair of the device can be carried out on its own, without involving specialists from the service company.

The scheme of welding thin metal using inverter welding.

According to the instructions, first of all, it should be checked with a multimeter the state of the thermal fuses installed on the radiators of various elements in the power unit. The temperature at which their contacts open is typically 90 degrees. Separate types of such fuses are disposable, after triggering they have to be changed. Others open the circuit when overheating, but when the radiator cools down, they restore the connection again. Such elements can be installed on the primary windings of power transformers. Their triggering often leads astray electrical amateurs who think that a break has occurred in the winding. If you find a faulty thermal fuse, you can try to short its contacts. This option is suitable as a temporary "treatment", it will allow you to finish the work, if it is urgent.

Since overheating protection is now partially absent, the welding machine should be operated very carefully, completely. And upon completion of work, you should immediately move to the radio parts store to purchase a spare part.

Another "sensitive" place of welding inverters is an output rectifier, more precisely, the diodes included in its composition. The currents with which they have to work reach 130 A and sometimes cause a breakdown in these diodes.

It is easy to verify the inoperability of the output rectifier with a multimeter, but without the “continuity” of each diode separately, it is impossible to determine which of them is broken. Diodes (three dual diodes are used here) will have to be soldered and removed from the heatsink to which they are screwed. The radiator will also have to be removed.

Control of the welding inverter.

Soldering diodes and other elements can be difficult. In modern welding inverters, soldering is done very qualitatively, with a large amount of solder, especially in those places where there are high-current currents. In addition, lead-free solder is used, the melting point of which is higher than that of ordinary lead-tin. Therefore, for soldering diodes and other elements it is better to use a powerful 50 W soldering iron, a 40-watt may not be enough. The task is complicated by the fact that you need to unsolder three outputs simultaneously, so you cannot do without a good warm-up. To remove the solder, you can use desolder or copper braid.

After the punched diode is detected (both parts can be punched in dual diodes), you should buy a new one, the same or similar. The user should pay attention to an important fact: the output rectifier diodes are fast, their recovery time is only 50 ns. Only such elements can operate with an alternating current frequency of 60-80 kHz. Conventional diodes can not be installed here. In foreign specifications, high-speed diodes can be referred to as Hyper-Fast, Ultra-Fast, Stealth Diode, Super-Fast, High Frequency Secondary Rectifier, etc.

Before mounting diodes or key transistors, a fresh layer of heat conductive paste (KPT-8 or similar) should be applied to the radiator. Paste must be applied in sufficient quantities, but not too copiously. It provides heat removal from the element in the direction of a copper or aluminum radiator.

Soldering diodes should be done very carefully. Due to the large current strength in poor-quality connections, strong heating and significant power losses will be observed.

It happens that due to negligence during the dismantling of the radiator, the copper tracks and the “patches” of the board were damaged, they are enlarged with copper tinned wire and are properly soldered.