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Posted on 30 August 2019

High Current Rectifier Diodes for Welding Applications

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The trend is to increase frequency from 1 kHz today to 10 kHz or even 20 kHz

Resistance welding, although not a high profile power electronics application, has specific requirements which have led to the development of a family of application specific devices. The ever increasing demands are pushing the limits for new developments, both in terms of electrical parameters as well as reliability.

By Björn Backlund, ABB Switzerland Ltd, Semiconductors and Ladislav Radvan, ABB s.r.o., Semiconductors

 

In this article, we’ll take a look at the application itself, the requirements the power semiconductors must meet and the components which have been developed to meet these demands. Finally, we’ll take a look at the trends in new product releases in the near future.

Resistance welding

Resistance welding is a technique used mainly for joining sheets of metal. It involves the generation of heat by passing a current through the resistance caused by the contact between the metal surfaces. Small pools of the molten metal are formed at the weld area when the high current is passed through and after the area cools, a solid joint is established. In comparison with other welding methods, resistance welding is very efficient, as it causes little pollution and limited work piece deformation. It has high production rates, can easily be automated and requires no filler materials. It is used extensively in the automotive industry, since most cars have several thousand spot welds made by industrial robots.

Resistance welding of a car body

Welding circuit principle

The electrical circuit of the resistance welding system consists of 4 parts:
• A frequency converter generates a single phase 560V square wave current with the desired frequency from the standard 400V 3 phase AC input.
• A transformer with a secondary voltage in the range of 6.3 – 20 V, with 10 V being the most common. The secondary current is often in the range of 10 – 15 kA and even higher for Aluminium welding.
• A diode rectifier to convert the quasi-square wave current to DCcurrent. This is done since the welding quality is better when using DC instead of AC. The connection of choice is type M2 to reduce the number of diodes required for the rectification. When higher currents are needed, the capability is increased by having two or more diodes connected in parallel.
• A welding gun with water-cooled electrodes between which the metal sheets to join are pressed.

The welding cycle and diode load

In the automotive industry, a typical welding cycle consists of 5 phases with a total cycle time usually in the range 4 – 8 s. The 5 phases are:
• Welding time: when the welding current flows through the system causing the metal at the joint to partially melt.
• Holding time: when the metal, still under pressure from the welding gun, cools down to a temperature where the joint is established.
• Gun opening time: when the pressure is released and the gun prepares to move to the next welding spot.
• Gun moving time: when the gun moves to the next welding spot
• Gun closing time: when the parts to be joined are pressed together.

A typical welding circuit

The duty cycle, often referred to as ED from the German word “Einschaltdauer”, is defined as the ratio between welding time and total cycle time, consisting of the sum of the welding time and the time between the welding intervals. For the welding sequence described earlier, the definition of duty cycle is as per Figure 3.

Typical welding cycle

The current through the diodes during the welding time is shown in Figure 4. The output current IOUT is the sum of currents ID1 and ID2 and, in the example, is 20 kA. The current waveforms of both diodes, ID1 and ID2, are rectangular pulses with the magnitude 20 kA, 10 kA and 0 with an average current of 10 kA per diode. T is the operation period.

The current flowing through the diodes during welding

Required performance of diodes for welding applications

For each car model, a customised welding system is designed with the requirement that it should run without failure as long as the given car model is produced. Each welding cycle represents a load cycle for the diodes and the expected lifetime is generally more then 10 million cycles. This means that the load cycling capability of the diode is crucial for the choice of component and this capability is determined by the temperature swing the diode undergoes during the cycle. To keep the temperature swing as low as possible during the cycle, the diodes must be designed for lowest possible losses and thermal impedance.

In the automotive industry, the transformer, the rectifier and the welding gun are often placed on a robot arm. Thus, size and weight is of great importance. Since increased frequency reduces the size of the transformer at equal power, the trend is to increase the frequency from the 1 kHz used today to about 10 kHz or possibly even 20 kHz.

ABB welding diodes

ABB’s Welding Diodes portfolio

ABB has developed a comprehensive welding diode range that is shown in Table 1. Due to the low voltage, it is possible to use thin silicon to reduce conduction losses. Since the requirements for air strike and creepage distance are low, thin housings with low thermal resistance are used, see Figure 5. An added advantage is the small size and low weight of the diodes, a welcome feature for equipment mounted on a robot arm.

Welding diodes from ABB

The quest for lower thermal impedance lead to the introduction of the housing-less welding diode (HLWD) some years ago. Although more susceptible to environmental conditions, the HLWD has found its place in the welding industry due to improved thermal performance.

To meet the demands of higher frequencies, a new group of high frequency welding diodes with high current capability combined with excellent reverse recovery characteristics is currently in development. These new features will enable operation with high efficiency at frequencies around 10 kHz. ABB’s new high frequency welding diodes will be available in sealed and HLWD versions.

 

 

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