GALVANICALLY ISOLATED BIDIRECTIONAL DC-DC CONVERTER MODULE

Partner Call open until: May 25, 2021

Start of the project: Q3 2021

Objectives

This project aims to address circuit design, modeling with simulation as well as the creation of the concept for the control of a galvanically isolated, bidirectional and highly dynamic 45kW DC-DC converter. 

The aim of the cooperative research project is the circuit design, the modeling with simulation and the creation of a concept for the control of a galvanically isolated DC-DC converter. The DC-DC converter shall interconnect the two voltage levels 760V and 80V over galvanic isolation. The required power range is 45kW. The device must be able to be operated completely bidirectionally and should have a very high dynamic range. The converter shall also have a very high power-density, which is supported by the possibility of water cooling. The high power-density will inevitably be accompanied by a high efficiency. In this respect, a value in the range of 97% should be targeted. 

To achieve these goals, it can be assumed that the solution will be based exclusively on wide-bandgap transistors.  

In terms of dynamics, it should be emphasized that a bandwidth of about 500kHz is to be aimed for. Up to this frequency the converter shall behave as an ohmic (very low ohmic resistance). In addition, care must be taken to keep the voltage ripple on the secondary side (80V level) extremely low (Vripple,pp < 0.1%).

Due to these demanding goals, the following challenges arise in particular: 

Comparative analysis of possible circuit topologies:
Two topologies are basically available for the solution. One is the so-called dual-active bridge and the other is the series resonant converter. The dual-active bridge has the advantage of better controllability. The series resonance converter in turn has the advantage of almost sinusoidal currents and the fact that the transistor switch-off transition only takes place at low currents corresponding to practically lossless zero-voltage switching. Dual-active bridges have basically also soft-switching turn-off transients. Due to the higher currents to be switched, there are, however, arising switching losses in the transistors.

Here, the most suitable topology shall be selected in the course of detailed simulations.           

Development of the high-frequency transformer:
The specified dynamics of the converter requires operation at a high switching frequency (>500kHz). Thus, the design of the transformer has to meet particular requirements due to skin and proximity effects. A good solution will probably only be possible as part of an iterative process in combination with 3D field simulations. The cooling concept will have to include both the ferrite core and the windings. For the solution path, it is proposed to compare the approach of a multilayer planar transformer with a transformer wound with fine litz wires.  

Precision of control / regulation:
The high switching frequency requires the very precise setting of the switching times with a resolution in the range of 10ns. Especially with the series resonant converter topology, a solution must be found to determine the ideal switching time so that zero-voltage switching at low turn-off current values can be ensured over a wide load range.  For this purpose, corresponding broadband current and voltage measurement methods must also be developed. In principle, a way must be found to determine which current voltage values are to be measured in order to be able to set the control correspondingly precisely over the entire load range.

Broadband precision current measurement:
Within the scope of the project, it is to be clarified whether it will be possible to measure the current via the high-frequency transformer (~ 500kHz) precisely enough. in this respect, corresponding analyses are to be carried out. 

Expected results

  • Simulation models and simulation results to support the topology selection and the associated measurement technology
  • Decision regarding the topology suitable for the specification
  • Selection of the entire measurement data acquisition
  • Detailed control concepts including verification in the context of simulations