99% Efficient 10 kV SiC-Based 7 kV/400 V DC Transformer for Future Data Centers

Abstract

The power supply chain of data centers from the medium voltage (MV) utility grid down to the chip level voltage consists of many series connected power conversion stages and accordingly shows a relatively low efficiency. Solid-State Transformers (SSTs) could improve the efficiency by substantially reducing the number of power conversion stages and/or directly interfacing the MV AC grid to a 400 V DC bus, from where server racks with a power consumption of several tens of kilowatts could be supplied by individual SSTs. The recent development of SiC MOSFETs with a blocking voltage of 10 kV enables the realization of a simple and hence highly reliable two-stage SST topology, consisting of an AC/DC PFC rectifier and a subsequent isolated DC/DC converter. In this context, an isolated 25 kW, 48 kHz, 7 kV to 400 V series resonant DC/DC converter based on 10 kV SiC MOSFETs is realized and tested in this paper. To achieve zero voltage switching (ZVS) of all MOSFETs, a special modulation scheme to actively control the amount of the switched magnetizing current on the MV and LV side is implemented. Furthermore, the design of all main components and especially the electrical insulation of the employed medium frequency (MF) transformer is discussed in detail. Calorimetric efficiency measurements show that a fullload efficiency of 99.0 % is achieved, while the power density reaches 3.8 kW/L (63 W/in 3 ).

Dielectric Losses in Dry-Type Insulation of Medium-Voltage Power Electronic Converters

“…Zero Voltage Switching (ZVS) is achieved for all semiconductors and, therefore, the SST can be operated at 48 kHz. More details about this SST can be found in [10], [51]. The MV/MF transformer, which is operated inside the DC-DC converter, is subject to all identified critical aspects with respect to dielectric losses: high-frequency, high switching speed, large common-mode (CM) voltages, and large differential-mode (DM) voltages.…”

“…The stray inductance of the transformer, together with the series capacitor located on the LV side, is used as a resonant tank [5], [23]. The DC-DC converter is operated at the resonance frequency and ZVS is achieved with the magnetizing current of the transformer [10]. This implies that the switched current is nearly load-independent and that the switching transitions feature a limited speed, which is beneficial for the dielectric losses [9], [23].…”

“…The transformer core is grounded with respect to the LV side such that no MV insulation is required for the LV winding. More details about the transformer design and construction can be found in [10], [23]. Fig.…”

“…Medium-Frequency (MF) energy conversion [2], [5], [9]. The design of such MV/MF converters is typically examined at system level [3], [4], [6], [7], [10] and component level [9], [11]- [13] while the electrical insulation is often neglected [14]- [17].…”

“…• The online or offline monitoring of the insulation losses allows the detection of insulation degradation and represents a valuable diagnostic tool (e.g., quality control and preventive maintenance) [19], [34], [36], [37]. • Even if the dielectric losses are usually small compared to the total losses of a converter, this may not be the case for converters simultaneously operated at high voltages and high-frequencies [5], [10], [15], [35]. • The insulation temperature increase, due to the converter losses, can drive the dielectric itself into a highloss regime, with a subsequent thermal breakdown of the insulation [35], [38].…”