PCIM EUROPE 2021 - BEST PAPER AWARD WINNER A resonant DC/DC converter with high efficiency and power
In the solid state transformers with medium voltage SiC-Devices, it is possible to achieve very high efficiency, although designing a sufficiently high switching frequency in order reduce filter elements, volume and mass of the transformer.
The solid state transformer (SST) is a converter that is able to transfer energy between different electrical networks, by promoting a galvanic isolation and adding functions, such as: active energy control, harmonic current distortion reduction and voltage regulation. The patent named “Power converter circuits having a high frequency link” was granted to McMurray at 1968, it is considered the first document in the area of SST systems. The same author has claimed that its initial use may be found in special applications where cost and efficiency are secondary to size and weight. Fifty years later, Mcmurray would be very impressed that, with the dawn of better semiconductors devices such as SiC-Devices, nowadays, his invention is very attractive for cost, efficiency, size and weight all together.
As a focus of the paper “Characterization of a 300 kW isolated DC/DC converter using 3.3 kV SiC-MOSFETs”, which was awarded with the Best Paper Award at PCIM 2021, the solution of solid state transformers is restated to be very attractive, in particular, as their efficiency can be further improved depending on the characteristics of the devices.
Converter topology and soft switching mode
A very interesting DC/DC converter concept called as R-SAB topology (Figure 1) has been used due to its intrinsic capability to reach quasi-ZCS at the turn-off and ZVS at the turn-on of the MOSFETs, depending on the transformer design, operational conditions and devices output capacitances. It includes two full H-Bridges (one implemented with MOSFETs and another with diodes) connected through a series resonant circuit formed by the leakage inductance (Ls) of a medium frequency transformer (MFT) and on the resonant capacitor (Cr).
Usually, the majority of high voltage converters are hard switched, which means that power modules will present high commutation losses, as these devices turn-on and turn-off at the nominal current values. Indeed, at this point, the concept of soft switching becomes indispensable for reasonably meet switching losses constraints. Under soft switching mode (Figure 2), the turn-off behavior of the transistor is controlled by the resonant external circuit, as it forces the device’s current to decrease, achieving the event equivalent to zero current switching (quasi-ZCS), at the end of the half-cycle. Meanwhile, the turn-on can be performed when the device’s voltage is crossing zero, event called zero voltage switching (ZVS). In discontinuous conduction mode (DCM), this behavior relies upon the discharging of the device’s output capacitance by means of the magnetizing current circulation.
Device’s output capacitance influence
Under DCM mode, the current and voltage behavior during the switching transients will depend on the magnetizing current (imag), dead time (tdead) and the equivalent capacitance (Ceq) considering all semiconductor devices and transformer. As per Figure 3, the amount of equivalent capacitance influences the device’s charging and discharging effects during the dead time, period when only the magnetizing current is circulating. At the end of the dead time, certain amount of energy is left in the device’s output capacitances, leading to a surge current event and causing switching losses. Therefore, larger output capacitances lead to larger voltage switching levels, causing higher current peaks and, consequently, higher switching losses. These elements play an important role for achieving perfect ZVS operation, but also as origin of high frequency oscillations in the circuit which may disturb the converter operation and limit the output power, other than, indeed, improve de converter efficiency.
R-SAB efficiency characterization
In order to characterize the 300 kW R-SAB prototype, an opposition method has been used where the converter losses are measured both electrically (measurement of the total input power) and thermally (measurement of the coolant), as shown in Fig. 1. The voltage source (VDC) imposes the voltage (Vin) on the input DC-Bus while the current-source regulates the output current (Iout) flowing in the converter.
Thus, the two power supplies provide only the losses of the converter. Three water-cooling circuits are used in parallel (at the rectifier, transformer and inverter) to estimate the losses by calorimetry. The experimental results presented in the full paper consider a fixed switching frequency of 15 kHz and 50 % inverter duty cycle.
The full paper presents an experimental comparison, regarding three different 3.3 kV SiC-Devices modules where the influence of the output parasitic capacitances on the converter efficiency is deeply analyzed. In that matter, Figure 4 shows the switching waveforms of SiC-MOSFETs for the three different device scenarios experimented: 750A SiC-MOSFETs, 375A SiC-MOSFETs and 750 A Standalone SiC-Diodes. As expected, these electrical experimental results confirm the improvement of switching behavior. Hence, the voltage switching steps have decreased from 1100 V to 800 V and, finally, to 500 V, approximately.
At last but not least, as it is highlighted in Figure 5, the maximum efficiency has been improved up to 99.33 % and the nominal efficiency up to 99.02 % at the best scenario using the full SiC-Diodes at the secondary. As per results shown in the full paper, a better overall behavior has been achieved as the inverter losses have decreased due to the lower equivalent capacitance of the full SiC-Diodes module, resulting in lower switching losses. In other hand, the rectifier losses have decreased by means of the lower on-resistance of the SiC-Diodes, resulting in lower conduction losses.
This work is supported by the Shift2Rail Joint Undertaking (JU) under grant agreement No 881772. The JU receives support from the European Union’s Horizon 2020 research and innovation program.