Patent Description:
In general, the power supply industry continuously demands converters with high efficiency, high power density, and low cost in order to achieve less energy consumption, smaller installation space, and cost effectiveness. In addition, higher power processing is required in many newly developed applications, such as, electrical vehicles (EV) and data centers. By using higher power rated converters, charging time of EVs and the size of power racks in data centers can be significantly reduced.

Multi-phase converter topologies are commonly employed to increase processing power. In multi-phase converters, each phase delivers a portion of the total power. Since the current stress of each phase is only a fraction of the total current, the conduction loss as well as the temperature of components can be easily limited. In addition, the operation of each phase of the multi-phase converter can be interleaved, so that switching ripples of the phases can cancel each other and the filter size can be reduced significantly. Furthermore, multi-phase converters with multiphase transformers can inherently achieve current sharing without any additional control. For these reasons, multi-phase DC/DC converters are attractive candidates in high power applications. See, for example, <NPL>; and <NPL>.

<NPL>, relates in general to bi-directional operation of on-board charger applications, and is directed to a two stage structure utilizing SiC and GaN devices. As described, bi-directional power flow as well as zero voltage switching is achieved with the proposed structures. In addition, a variable DC-link voltage operation is adopted to deal with the battery voltage range so that a CLLC resonant converter can always work at its optimized point.

<CIT> relates to the problem of reducing a loss by performing soft switching operations of a primary-side switch and a secondary-side switch even in the case where a voltage difference between a primary-side DC voltage and a secondary-side DC voltage is great or even under a light load, in a DC/DC converter. Proposed is a control device for the DC/DC converter, a phase difference between drive signals of primary-side switches outputted from a primary-side pulse modulator and drive signals of secondary-side switches outputted from a secondary-side pulse modulator is controlled by a phase control section. Further, between a duty of the primary-side switches and a duty of the secondary-side switches, the duty is adjusted by a duty control section in accordance with a voltage difference between a primary-side DC voltage and a secondary-side DC voltage, and when turning on or turning off the primary-side switches and the secondary-side switches, the soft switching operations are performed.

Although isolated multi-phase DC/DC converters show advantages in high power applications, it is still important to further improve efficiency, power density, and cost effectiveness.

The present invention relates to an electrical circuit for converting electrical power according to independent claim <NUM>.

The present disclosure provides an isolated multi-phase DC/DC (direct current/direct current) converter with a reduced number of blocking capacitors that prevent saturation of the transformer. By using the fact that the current in a phase is equal to the sum of the currents in the other phases of the transformer, the blocking capacitor of the phase can be eliminated, because the DC components of the other phases can be eliminated by their own blocking capacitors. The DC/DC converter of the present disclosure can achieve both cost effectiveness and higher power density, because one blocking capacitor bank at the primary side and one blocking capacitor bank at the secondary side can be eliminated in a multiphase transformer.

The present disclosure will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this disclosure are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. <FIG> and their description serve as comparative examples which are useful to understand the present invention.

<FIG> respectively illustrate Y-connection windings <NUM> and Δ-connection windings <NUM>. As shown in <FIG>, in the case of Y-connection, the right end of all windings 110_1 through 110_N is connected together at a same node <NUM>, namely, a neutral point, while the left end of windings 110_1 through 110_N is a free end for current input/out. As shown in <FIG>, in the case of Δ-connection, one end of a winding is connected to the other end of a neighboring winding in sequence at nodes 121_1 through 121_N. For example, the right end of a first winding 120_1 is connected to the left end of a second winding 120_2 at node 121_2, while the left end of first winding 120_1 at node 121_1 is connected to the right end of a last winding 120_N. In this manner, all windings 120_1 through 120_N are electrically coupled in series to form a closed loop with nodes 121_1 through 121_N serving as points for current input/output. Almost all the multi-phase transformers employ at least one of these two winding configurations to their primary and secondary sides.

<FIG> respectively illustrate a multi-core three-phase transformer <NUM> and a single-core three-phase transformer <NUM> that can be employed in a three-phase converter. It should be noted that both the primary (left) and secondary (right) sides of transformers <NUM> and <NUM> use windings with the Y-connection configuration as shown in <FIG>.

As shown in <FIG>, windings <NUM>, <NUM>, and <NUM> are connected in Y-connection configuration with terminals P1, P2, and P3, thereby forming the primary side of transformer <NUM>, while windings <NUM>, <NUM>, and <NUM> are connected in Y-connection configuration with terminals S1, S2, and S3, thereby forming the secondary side of transformer <NUM>. As depicted by dashed lines in <FIG>, three pairs of windings (i.e., the winding pair of windings <NUM> and <NUM>, the winding pair of windings <NUM> and <NUM>, and the winding pair of windings <NUM> and <NUM>) are magnetically coupled to serve as a three-phase transformer that can be employed to an isolated three-phase converter. As shown in <FIG>, three-phase transformer <NUM> can be implemented with three independent magnetic cores <NUM>, <NUM>, and <NUM>. As shown in <FIG>, three-phase transformer <NUM> is substantially the same as three-phase transformer <NUM> shown in <FIG>, except that three-phase transformer <NUM> is implemented with a single magnetic core <NUM> that integrates the magnetic couplings of the winding pair of windings <NUM> and <NUM>, the winding pair of windings <NUM> and <NUM>, and the winding pair of windings <NUM> and <NUM>. It is appreciated that single magnetic core <NUM> and independent magnetic cores <NUM>, <NUM>, and <NUM> are made of any suitable magnetic material (e.g., iron, cobalt, and nickel) with a high magnetic permeability so as to confine and guide magnetic fields.

An isolated DC/DC converter may utilize a transformer to provide galvanic isolation. To guarantee reliable operation of the DC/DC converter, it is extremely important to avoid saturation of the magnetic cores of the transformer by excessive flux density. To minimize the maximum flux density in the magnetic core, the DC component of the magnetizing current of the transformer should be zero. Note that magnetizing current is the current drawn by the primary side of a transformer that is being magnetized or energized at a specific voltage, but the secondary side is not loaded.

One method to eliminate the DC component of the magnetizing current is connecting a blocking capacitor in series with the transformer. In a steady state condition when charge balance of the capacitor is met, i.e., the charging current of the capacitor is equal to the discharging current during a switching cycle, the capacitor blocks the DC component of the magnetizing current. As a result, the transformer operates reliably without excessive magnetic flux.

<FIG> illustrates a single-phase transformer <NUM> with blocking capacitors CP and CS. Blocking capacitors CP and CS are respectively connected in series with a primary winding <NUM> and a secondary winding <NUM>. In one embodiment, blocking capacitors CP and CS can be a capacitor bank <NUM> including a plurality of capacitors connected in parallel and in series. For example, as shown in <FIG>, capacitor bank <NUM> includes a total of six capacitors with three of the parallel-connected capacitor pairs being connected in series. It should be noted that transformer <NUM> as shown in <FIG> includes an ideal transformer with turns ratio NP:NS = n:<NUM>, and a magnetizing inductor LM connected in parallel with primary winding <NUM>. The following equation represents the steady state condition when charge balance of the blocking capacitor is met, and hence, the DC component of magnetizing current iLM is zero. <MAT>
where TS, iP, iS, and <iLM>TS represent the switching period of the converter, the current flowing through capacitor CP, the current flowing through capacitor Cs, and the DC component of magnetizing current iLM which is equal to the average magnetizing current over switching period TS. In one embodiment, switching period TS may be <NUM> to <NUM> times to the inverse of a resonant frequency of the serially connected inductor LM and capacitor CP (where <MAT>). For example, switching period TS may be about <NUM> over several hundreds of kHz.

This method can be used to prevent saturation of transformer <NUM>, because it does not require any additional control. However, in high power as well as high voltage applications, a plurality of capacitors are connected in series and in parallel to meet required voltage and current stresses, because commercially available capacitors have limited voltage and current ratings.

<FIG> illustrates a three-phase transformer <NUM> that employs windings with the Y-connection configuration for both the primary and secondary sides as shown in <FIG>. Each winding is coupled with a capacitor or a capacitor bank in series to eliminate the DC component of the magnetizing current in the three-phase transformer <NUM>. In this example, six capacitor banks <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are required to prevent saturation of transformer <NUM>, which results in extra cost and lower power density of the DC/DC converter.

<FIG> illustrates a bi-directional isolated three-phase DC/DC converter <NUM> with a three-phase transformer <NUM> and blocking capacitors <NUM>. Converter <NUM> delivers energy back and forth between two voltage sources V<NUM> and V<NUM>. All three phases of the primary side are coupled to primary voltage source V<NUM> and all three phases of the secondary side are coupled to secondary voltage source V<NUM>.

Each phase of converter <NUM> includes two switches coupled to a primary side of transformer <NUM> and two switches coupled to a corresponding secondary side of transformer <NUM>. Specifically, as shown in <FIG>, switches Q<NUM> and Q<NUM> are connected to phase A of the primary side of transformer <NUM> through an external inductor LPA; switches Q<NUM> and Q<NUM> are connected to phase B of the primary side of transformer <NUM> through an external inductor LPB; and switches Q<NUM> and Q<NUM> are connected to phase C of the primary side of transformer <NUM> through external inductor LPC. Likewise, as shown in <FIG>, switches QA and QB are connected to phase A of the secondary side of transformer <NUM>; switches QC and QD are connected to phase B of the secondary side of transformer <NUM>; and switches QE and QF are connected to phase C of the secondary side of transformer <NUM>.

External inductors LPA, LPB, and LPC are connected in series with a respective winding of three-phase transformer <NUM> via a respective one of capacitors <NUM> to control the slope of the current through transformer <NUM>. If the leakage inductance of each winding of the transformer <NUM> is sufficiently large to control the slope of the current, external inductors LPA, LPB, and LPC become optional and can be removed.

Converter <NUM> in <FIG> is called an isolated three-phase dual-active-bridge converter, if its operating switching frequency is selected to be much higher (e.g., at least one order of magnitude higher) than a resonant frequency of the serially connected inductor and capacitor. On the other hand, converter <NUM> is also called an isolated three-phase resonant converter, if its operating switching frequency is designed to be close to the resonant frequency. If the number of phases of an isolated DC/DC converter is greater than two, the converter is generally called an isolated multi-phase DC/DC converter. Also, it should be noted that blocking capacitors <NUM> should be implemented to prevent the DC component of the magnetizing current of transformer <NUM>.

<FIG> illustrates a schematic diagram of a multi-phase transformer <NUM> without blocking capacitors. Primary side windings <NUM> and secondary side windings <NUM> of multi-phase transformer <NUM> can be a Y-connection configuration or Δ-connection configuration. A magnetic core <NUM> of transformer <NUM> can include a single core or multiple cores. The primary side has N phases, and each end of primary side windings <NUM> are referred to as P<NUM>, P<NUM>,. and PN, where <NUM> ≤ K < N-<NUM>. The secondary side has M phases, and each end of secondary side windings <NUM> are referred to as S<NUM>, S<NUM>,. and SM, where <NUM> ≤ I < M-<NUM>. <FIG> illustrates a schematic diagram of multi-phase transformer <NUM> with blocking capacitors <NUM> and <NUM>. In order to avoid the DC component of the magnetizing current of the transformer <NUM>, each end of primary side windings <NUM> is connected in series with a blocking capacitor <NUM>, and each end of secondary side windings <NUM> is connected in series with a blocking capacitor <NUM>.

<FIG> illustrates a schematic diagram of a multi-phase transformer <NUM> with a reduced quantity of blocking capacitors <NUM> and <NUM>, in accordance with an embodiment of the claimed invention. Multiphase transformer <NUM> as shown in <FIG> is substantially the same as multi-phase transformer <NUM> in <FIG>, except that two blocking capacitors originally coupled to terminals PN-K and SM-I of multi-phase transformer <NUM> in <FIG> are removed to obtain multi-phase transformer <NUM> in <FIG>. Primary side windings <NUM> and secondary side windings <NUM> of multi-phase transformer <NUM> can be a Y-connection configuration or Δ-connection configuration. It is appreciated that primary side windings <NUM> and secondary side windings <NUM> do not have to have the same connection configurations. For example, primary side windings <NUM> may have a Y-connection configuration, while secondary side windings <NUM> may have a Δ-connection configuration, and vice versa. One of primary side windings <NUM> and one of secondary side windings <NUM> may form a winding pair through a magnetic core <NUM>. Accordingly, primary and secondary side windings <NUM> and <NUM> (with or without magnetic core <NUM>) may form one of Y-Y winding pairs, Y-A winding pairs, Δ-Y winding pairs, and Δ-Δ winding pairs.

As shown in <FIG>, although two capacitors are removed, it is still possible to block any DC component of the magnetizing current of transformer <NUM>, because the average current through the winding for terminal PN-K is the sum of the average currents through all other windings at the primary side of transformer <NUM> and the average current through the winding of terminal SM-I is the sum of the average currents through all other windings at the secondary side of transformer <NUM>. It should be noted that all windings except windings for terminals PN-K and SM-I are serially coupled to blocking capacitors <NUM> and <NUM> that eliminate DC components of the current. As a result, fewer electronic components are required in a DC/DC converter, which can be manufactured at a lower total cost and a smaller physical size. Although only one blocking capacitor is removed from a phase of either the primary side or the secondary side, it is appreciated that, under certain conditions, more than one blocking capacitors may be removed from each of the primary and secondary sides.

<FIG> illustrates a schematic diagram of an isolated multiphase DC/DC converter <NUM> including a multi-phase transformer <NUM>, in accordance with an embodiment of the present disclosure. Converter <NUM> can employ one or more of inverter or rectifier circuits <NUM> and <NUM> at its primary and secondary sides to operate as an isolated multi-phase DC/DC converter. It is appreciated that the concept of reduced quantity of blocking capacitors can be applied to any suitable multi-phase, multi-port transformer for any suitable multi-phase, multi-output converter. For example, <FIG> illustrates a schematic diagram of an N-phase, three-port transformer with one of the blocking capacitors being removed at each of the primary, secondary, and tertiary sides.

<FIG> illustrates the primary side of a Y-Y connected three-phase transformer <NUM> in an arrangement which is not covered by the scope of claim <NUM>, with each of three terminals PA, PB, and PC at the primary side being connected to a respective one of blocking capacitors <NUM>, <NUM>, and <NUM>, similar to that as shown in <FIG>. It is noted that the DC component of the current in each phase winding is zero due to charge balance of blocking capacitors <NUM>, <NUM>, and <NUM>. In contrast, <FIG> illustrates the primary side of a Y-Y connected three-phase transformer <NUM>, with only two terminals PA and PB at the primary side being respectively connected to blocking capacitors <NUM> and <NUM>, in accordance with an embodiment of the present invention. Note that blocking capacitor <NUM> originally present in transformer <NUM> of <FIG> is absent from transformer <NUM> of <FIG>.

By applying Kirchhoff's current law to a neutral point <NUM> of the Y-connected windings as shown in <FIG>, the following equation is obtained:
<MAT>
where iPA, iPB, and iPC respectively represent the currents in terminals PA, PB, and PC. One can take an average of currents iPA, iPB, and iPC in Equation (<NUM>) over a switching period TS, which gives:
<MAT>
<MAT>
where <iPA>, <iPB>, and <iPC> respectively represent the average values of currents iPA, iPB, and iPC over switching period Ts. Due to charge balances of capacitors CPA and CPB, the average values of currents iPA and iPB are both zero, i.e.:
<MAT>
<MAT>.

By substituting Equations (<NUM>) and (<NUM>) into Equation (<NUM>), the average value of current iPC becomes:
<MAT>.

Therefore, the presence or absence of a blocking capacitor at terminal PC does not make a difference for transformer <NUM>, especially when charge balance of the capacitors is met. Although the present disclosure describes the removal of capacitor CPC in transformer <NUM> having a Y-connected configuration, it is appreciated that any arbitrary one of capacitors CPA, CPB, and CPC can be removed from transformer <NUM> to achieve the same results. It is also appreciated that the removal of one of capacitors CPA, CPB, and CPC works in the same manner for transformers having a Δ-connected configuration.

<FIG> respectively illustrate a multi-phase transformer <NUM> with a reduced quantity of blocking capacitors, in accordance with an embodiment of the present disclosure. As shown in <FIG>, each of terminals PA and PB at the primary side and terminals SA and SB at the secondary side includes a blocking capacitor connected therewith in series, while terminal PC at the primary side and terminal SC at the secondary side (both of phase C) do not have a blocking capacitor connected therewith. This is the case for a dual-active bridge (DAB) DC/DC converter, where the removed blocking capacitors at the primary and secondary sides must be for the same phase, because the resonant frequency for a DAB converter is much smaller than the switching frequency. It is appreciated that, in alternative embodiments, the blocking capacitors for either phase A or phase B at both primary and secondary sides can be removed.

For blocking capacitors having a capacitance that is synchronous to the switching frequency (namely, the resonant frequency is commensurate with the switching frequency), the removed blocking capacitors at the primary and secondary sides must be of different phases. For example, as shown in <FIG>, the blocking capacitor of terminal PC (phase C) at the primary side is removed, while the blocking capacitor of terminal SB (phase B, different from phase C) at the secondary side is removed. It is appreciated that, in alternative embodiments, if the blocking capacitor for phase A at the primary side is removed, the blocking capacitor for either phase B or phase C at the secondary side can be removed.

For three-phase transformer <NUM> as shown in <FIG>, this results in the saving of one-third (<NUM>/<NUM>) or <NUM>% on the total material cost for blocking capacitors. For a five-phase transformer, for example, this results in the saving of one-fifth (<NUM>/<NUM>) or <NUM>% on the total material cost for blocking capacitors.

Claim 1:
An electrical circuit (<NUM>) for converting electrical power, the electrical circuit (<NUM>) comprising:
a primary circuit comprising a first quantity of windings (<NUM>); and
a secondary circuit comprising a second quantity of windings (<NUM>) and magnetically coupled to the primary circuit,
wherein the primary circuit includes a first quantity of terminals (P1, P2, PN-K, PN), each (P1, P2, PN) being electrically coupled to a corresponding one of the first quantity of windings (<NUM>) of the primary circuit via a primary blocking capacitor (<NUM>), except that one of the terminals (PN-K) of the primary circuit is electrically coupled directly to the corresponding one of the first quantity of the windings (<NUM>) of the primary circuit without a primary blocking capacitor (<NUM>),
wherein the first quantity of windings (<NUM>) of the primary circuit are connected with each other in a Y-connection configuration or a Δ-connection configuration,
wherein the secondary circuit includes a second quantity of terminals (S1, S2, SM-I, SM), each (S1, S2, SM) being electrically coupled to a corresponding one of the second quantity of windings (<NUM>) of the secondary circuit via a secondary blocking capacitor (<NUM>), except that one of the terminals (SM-I) of the secondary circuit is electrically coupled directly to the corresponding one of the second quantity of windings (<NUM>) of the secondary circuit without a secondary blocking capacitor (<NUM>),
wherein the second quantity of windings (<NUM>) of the secondary circuit are connected with each other in a Y-connection configuration or a Δ-connection configuration,
wherein the first quantity is at least three, and the second quantity is at least three.