Technical information

Notes on Rating
3-2
1070 080 059-101 (2001.04) GB
3.1 Load diagrams
Various types of semiconductors are used as power breakers in the welding
systems. Generally, semiconductors have an almost unlimited life if used in
accordance with the specifications.
Unfortunately, however, the relatively large semiconductor chips have to
sustain mechanical stress due to thermal loads. This mechanical stress may
reduce semiconductor life.
Solder fatigue and defective wire bonds have been reported in literature as
factors affecting the life of IGBTs (used as power breakers in the PSI 6000
MF inverters).
As regards the weld diodes of the MF transformers (PSG 3000), the loads
acting on these large silicon wafers may have the effect that crystal impuri-
ties as they are found in every silicon crystal may lead to diode failure.
Semiconductor manufacturers state life characteristics for calculation pur-
poses. There, varying with the junction temperature gradient, the number of
temperature cycles available until the calculated end of the semiconductor
service life is stated. Every weld spot represents one temperature cycle.
We have taken these conditions into account in the rating of our systems.
Our PSI 6000 load diagrams also include peak current curves indicating the
calculated service life in each case.
The temperature gradient, which - among other contributing factors - de-
pends on the welding current, secondary resistance, weld time and ambient
temperature is taken into account in the service life calculation. Because our
curves shown are based on 45°C ambient temperature, while 25°C would be
a realistic assumption, you may expect a service life three times as long as
the one stated.
On principle, service life is extended with the lower temperatures achieved
with water cooling as compared to air cooling. However, depending on the
application, the temperature may drop more steeply during the cooling time
after a weld spot due to the superior cooling capacity of water, which in turn
shortens the life time due to the higher temperature gradient. However, it is
possible to have the cooling water running only during the weld time, which
also allows you to save water.
If you need further information in respect of any specific application, please
contact us.
The load diagrams show the inverter output currents or, respectively, the pri-
mary currents of the welding transformers.
To determine secondary currents, multiply the primary currents by the trans-
formation ratio.