History

Problem

After completing my studies at TU Wien (Vienna University of Technology), I was a researcher, developer and inventor for a large Austrian company whose competition had succeeded in reducing the size of their direct voltage supplies, so-called converters, which made the device significantly smaller.

However, if an inverter connects direct voltage in alternating positive and negative directions to a transformer, there is a danger of sudden saturation of the transformer core, at which point the transformer causes a short circuit. The solutions dating from 1970 to 1990 are currently being redeveloped digitally. Inverter converters require a transformer with an increased air gap so the core slowly saturates, over-dimensioning and additional components, and must be regulated on a quasi-stationary basis relative to the switching frequency, whereas the larger forward converters do not have this limitation. The saturation is prevented in that at least 50% of the cycle time does not transmit power but rather the transformer core is demagnetised. Due to the high filtering requirement to bridge the long pulse pauses, the forward converter is inefficient for larger capacities. The duration of the voltage pulses can, however, be set as desired from one voltage pulse to the next and, in particular, every voltage pulse can be switched off at any time without any problem if the load current exceeds a maximum value and load short circuit is easy to control.

In the case of dc voltage conversion with transformers connected to voltage-fed inverters (INVT), on the other hand, any big change in the pulse duration can lead to saturation of the transformer and an operational interruption. My client also developed an industrially significant application process that required a dynamic converter.

For higher capacities, the safety problem of DC-DC conversion is solved using an intermediate circuit that is material-intensive and complicated in regard to control technology, which doubles the total cost and reduces the level of efficiency. However, the single-stage INVT converter of the competitor was not easier or faster to control. So-called additional losses increase with increasing performance. In order to control complicated consumer processes (solar converters, electro-welding, plasma curing...) or to supply these with sufficient constant direct current (performance processors, robots, placement machines...), a large temporary energy storehouse and a further control stage are also required here.

Solution

The great success of my first solution concept that worked for the FET and IGBT transistors of the time (phase-shift full-bridge inverter, transformer with air gap, PWM according to DE19634713A1 (1995)) proved that the wandering of the magnetising current could be prevented and the limitation to quasi-stationary setting of the duration of the primary voltage pulses could be overcome without requiring a measurement to do so. Free of pressure to perform and practical constraints, I developed the approach further. My patent AT511298B1 from 2011 describes the pulse-to pulse current limitation.

However, the transistors have to switch exactly and the application of this solution concept is limited to FET and IGBT inverters. If the magnetising current wanders, large changes to the pulse width or duty cycle are dangerous. The tendency of the magnetising current to wander increases with the switching frequency. The advantage of WBG transistors with a switching frequency many times higher than FET and IGBT cannot be exploited in this way. Even though WGB transistor devices are smaller due to the higher switching frequency, the regulation/limitation of the output variables is difficult again.

The need for more powerful DC voltage sources is increasing, e.B. due to the high luminous efficacy of light-emitting diodes and wear-free inverter drives. Furthermore, the direct current consumers present ever increasing requirements regarding the reliability and quality of the direct current supplying them. The saturation problem and the associated additional losses stand in the way of this development, precisely because the circuit breakers have become so fast.

With my Austrian application A 60209/2021 of 18/08/2021, this deficiency is remedied and a generally applicable, highly efficient solution concept is described that no longer requires any air gap in the transformer core or any further over-dimensioning and has the actuating speed and load short circuit strength of a forward converter.

A 60209/2021 represents a new solution concept that is able to use the advantages of the WBG circuit breaker, offers the highest level of efficiency possible and remedies the power limitation of direct DC voltage conversion. Finally, the conversion of the current three-phase network to a more efficient and space-saving DC grid is conceivable.