The Swiss Rectifier

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The Swiss Rectifier

In this newsletter, we present a new rectifier converter topology, the Swiss Rectifier, which was just recently proposed in [1-3]. What was the motivation for the new topology? The easiest approach to three-phase mains rectification is a simple diode bridge. However, a diode bridge has a low power factor and introduces unwanted low frequency current harmonics into the grid. Additionally, many applications require a controlled output voltage. Therefore, Power Factor Correction (PFC) rectifiers are typically used, e.g. a 3-phase six-switch buck-type PFC rectifier in case an output control down to low voltages and/or a current limitation in case of a load short circuit is required.

 Compliance with guidelines concerning the mains’ behavior of 3-phase systems (for example EN 61000-3-2 and 61000-3-4) and buck-type DC output voltage regulation capability can also be achieved with the converter structure shown in Fig. 1, which is referred to as the Swiss Rectifier [1-3]. The Swiss Rectifier employs active third harmonic current injection and allows to achieve efficiencies superior to that of conventional buck-type systems.

 

Fig. 1: Circuit topology of the three-phase Swiss Rectifier with LC input filter.
SwissFig1 

By proper modulation of the power transistors T+ and T- the DC output current IDC can be distributed to the three phases such that after the low-pass filtering sinusoidal mains phase currents will result. We have prepared a simulation model of the Swiss Rectifier as a GeckoCIRCUITS applet which you can open by clicking on the yellow Java applet box. Please feel free to “play” with the GeckoCIRCUITS simulation model. Once you’ve examined it, you should carry on reading below about the implementation of the converter’s controller.

 

 

The Swiss Rectifier allows the currents in the positive and negative active switches, iT+ and iT-, to be formed proportionally to the two phase voltages involved in the formation of the diode bridge output voltage. If the difference of iT+ and iT- is now fed back into the remaining third mains phase via a current injection network (formed with three four-quadrant switches gated at twice the mains frequency), a sinusoidal input current shape can be assured for three mains phases while the simultaneously occurring voltage conversion guarantees a constant system output voltage.

For a given DC current IDC the mains current amplitude can be selected through proper modulation of T+ and T-. This also changes the voltage occurring across the two freewheeling diodes DF+ and DF- which finally is low-pass filtered by the inductors L and the output capacitor C. The output voltage can be adjusted starting from zero up to

SwissEqn1.

A circuit implementation with a single output inductor is feasible. However, in the circuit of Fig. 1, the total DC inductance is split evenly between the positive and negative output bus in order to provide attenuation of conducted common mode noise emissions.

 

Fig. 2: Control structure for the Swiss Rectifier.
SwissFig2 

A possible implementation of a control scheme for the Swiss Rectifier is shown in Fig. 2. The control comprises a superimposed output voltage controller R(s) and a subordinate output current controller G(s). Finally, a feed-forward loop adds normalized modulation functions defined by the voltages u+ and u- of the positive and negative diode bridge output bus and the system output voltage reference value u*pn to the DC current controller output signal. This results in a formation of iT+ and iT- directly proportional to u+ and u-. The switchover of the current injection circuit is with low frequency, following the rectifier input voltages uCF,a, uCF,b, uCF,c in such a way that the injection always occurs into the mains phase with lowest absolute voltage value (cf. Tab. I and/or Fig. 3).

 

TABLE I: Modulation of the current injection circuit (cf. Fig. 3).
SwissTable1

In case the PWM modulator for T+ and T- is operated with in-phase triangular carrier signals, the ripple Δiy of the injected current iy is minimized. If it is operated by using carriers with a phase difference of 180°, the system is permitted to work with minimized DC current ripple ΔiDC. The GeckoCIRCUITS simulation results are shown in Fig. 4. The converter specifications, as given in Tab. II, are considered in the simulation; operation with in-phase or interleaved PWM carriers and a load step (from 3.75 kW to 7.5 kW) are presented. The results demonstrate that the line currents ia, ib , ic follow the sinusoidal input phase voltages ua, ub , uc even in case of load steps, which underlines the high performance of the proposed rectifier topology.

TABLE II: Swiss Rectifier prototype specifications.
SwissTable2

 

Fig. 3: Mains sectors 1 to 12 defined by the different relations of the instantaneous values of the mains phase voltages ua, ub, uc.
SwissFig3

Fig. 4: Simulation results of the Swiss Rectifier operating with (I) in-phase or (II) interleaved PWM carriers; (III) load step from 50% to 100% of the rated output power (3.75 kW to 7.5 kW).
SwissFig4

References
[1] J. W. Kolar and T. Friedli, “The Essense of Three-Phase PFC Rectifier Systems”, in Proc. 33rd IEEE Int. Telecom. Energ. Conf (INTELEC 2011) , Oct. 9-13, pp. 1-27, 2011.
[2] J. W. Kolar, M. Hartmann and T. Friedli, “Three-Phase Unity Power Factor Mains Interfaces of High Power EV Battery Charging Systems”, in Power Electronics for Charging Electric Vehicles ECPE Workshop, Mar. 2011.
[3] T. Soeiro, T. Friedli and J.W. Kolar, “SWISS Rectifier – A Novel Three-Phase Buck-Type PFC Rectifier Topology for Electric Vehicle Battery Charging”, Proceedings of the 27th Applied Power Electronics Conference and Exposition (APEC 2012), Orlando Florida, USA, February 5-9, 2012.

 

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