Design and Control of High-Density High-Voltage Smart Converter for Food Ohmic Heating

The design of an innovative converter for ohmic heating of fluid foods is presented. Application-specific requirements are addressed with new solutions and verified on a commercial-size 60-kW prototype. A multioutput high-voltage (0.8 to 4 kV) transformer with automatic tap change enriches a basic H-bridge topology, enabling processing of a wide set of foods and allowing grounding one of the outputs. High power density is achieved by power switches Pareto optimization in the reliability-efficiency objectives and by pushing the switching frequency up to 30 kHz, solving the electrode corrosion problem, too. Reproducible and reliable operation is assured by three different control strategies: load variability is dealt with high-dynamics power control, repeatable output is guaranteed by input voltage compensation, and optimal working point is ensured by control of the transformer primary current dc component. This last control also allows removing the output capacitors, improving density metrics with respect to the state-of-the-art. Analytical models for all control strategies are given to support dynamic performance tuning while guaranteeing stability.