FIELD OF THE INVENTION

The present invention relates to a DC to DC converter. It will be especially useful with regard to high voltage applications. The present invention also relates to a method of converting a high voltage DC current to a low voltage DC current.

BACKGROUND TO THE INVENTION

In today's energy-saving conscious world, the efficiency of processing electric power is of vital importance. Electric power is typically transmitted to load centers on ac transmission lines. However, in certain systems, such as railway systems, it is desirable to transmit power over high-voltage dc transmission lines. The electrical energy being supplied to low- voltage equipment in those systems usually goes through multiple power conversion stages for converting the high dc voltage into a low dc voltage. However, the overall efficiency is low with such a configuration.

Although much progress has been made in low- voltage dc-dc power conversion, the evolution of high-voltage power conversion technology is less impressive. Because high- voltage switching devices have high on-resistance, the available dc-dc converters operating at high input voltage generally dissipate a substantial amount of energy in their switching devices.

The energy supplied to the low- voltage equipment in systems powered by a high dc voltage, such as the railway system, typically goes through multiple power conversion stages. As illustrated in Fig. 1, the electric power in the railway system is transmitted to the trains through high dc voltage (e.g. 1500V) overhead lines and is inverted into a 3-phase ac voltage (e.g. 440V, 60Hz). The ac voltage is further transformed and rectified into a dc voltage (e.g. 11 OV) for charging the backup batteries and powering various control units on the train. It is energy-inefficient to use a low-frequency ac voltage as the means to perform the dc-dc conversion from the high voltage to low voltage (i.e. from 1500V to 110V). A more efficient approach is to perform the power conversion process through a high-frequency ac voltage. As high-voltage switching devices have high on-resistance, available dc-dc converters operating at high input voltage generally dissipate a substantial amount of energy in the switching devices. The on-resistance ( r _on ) of high-voltage devices increases with their voltage-rating (BV) according to a nonlinear relationship: r _on ∞ BV ^k , where k e [1.6, 2.6].

This leads power electronics designers to explore new converter circuits that could reduce the device voltage requirement, so that switches of low on-resistance could be used.

A typical dc-dc converter for high-voltage applications is shown in Fig. 2. It consists of two power conversion stages. In the first stage, the dc input voltage is transformed into a high- frequency (HF) ac voltage which is applied across the primary of a HF transformer. The transformer is used for electrical isolation and providing an HF ac voltage across its secondary as determined by the turns ratio m. In the second stage, consisting of a rectifier, output filter and snubber, the output (load) voltage is obtained as a regulated dc voltage whose value is determined by the values of m and of the duty cycle.

In a conventional full-bridge (FB) converter, the four primary-side switches sustain the input voltage when they are off. In high-voltage applications, each switch can be realized by connecting two switches in series. Thus, each switch sustains only one-half of the input voltage in voltage-balanced networks. However, the equipment cost has to include that of the eight switches. This approach does not work well as far as dynamic balancing is concerned, since no switches are identical.

In order to improve the efficiency in systems with a high input voltage, a three-level topology was proposed. A typical structure is shown in Fig. 3. Based on the concept of the neutral-point-clamped inverters, a three-level (TL) converter has been introduced. By using two dc-link capacitors to split the input voltage, operating the outer two and inner two switches in anti-phase, and using an additional "flying " capacitor and two extra diodes to clamp the voltage on the transistors in the off-state, the voltage stress on each switch results in only one-half of the input voltage. SUMMARY OF THE INVENTION

It would be desirable to realize dc-dc power conversion in a more energy efficient manner. For example, one efficient approach would be to perform the ultimate dc-dc power conversion in one step.

Furthermore, it would be desirable for high voltage dc to dc converters to make use of low voltage switches which have lower on-resistance than high voltage switches, (and therefore lower conduction losses).

A first aspect of the present invention provides A DC to DC converter comprising :-

a pair of input terminals,

a plurality of capacitors connected in series between said input terminals,

a plurality of switch pairs, each respective switch pair being connected in parallel across a respective one of said capacitors, each switch pair comprising two switches and a mid-point between said two switches,

one or more transformers comprising a plurality of primary windings and a plurality of secondary windings, the primary windings being coupled to the midpoints of the switch pairs,

and an output section coupled to the one or more transformers, said output section comprising an AC to DC converter.

The capacitors, which are in parallel between the input terminals and the switch pairs, enable the input voltage to be split between the switch pairs. For example, if there are n capacitors, each capacitor in parallel with a respective switch pair, and the input voltage at the terminals is V, then the voltage across each switch pair will be V/n (if the capacitors are of equal value - which is the preferred arrangement). This enables switching devices of low voltage rating and low on-resistance to be used, resulting in a significant improvement in the power conversion efficiency. The invention enables high voltage DC-DC conversion using low voltage switches. The terms 'high voltage' and 'low voltage' are relative and will depend upon the application. In general the idea is that the series capacitors split the input voltage and so each switch needs to withstand only a fraction of the input voltage. The greater the number of switch pairs (and associated capacitors), the lower the switch voltage can be. In one embodiment, the input voltage is 2.2kV, there are three switch pairs and each switch is a 800V switch; however this is an example only and the invention is not limited to this.

Preferably the AC to DC converter comprises a current multiplier coupled to said secondary winding s .

The multi-phase arrangement of the current multiplier on the secondary side of the transformer makes it possible for the circuit to have a high current output. It is very expandable and by adding further legs to the multiplier it would be possible to further increase the current output.

Preferably said current multiplier comprises a plurality of legs, each leg having diode and an inductor in series, each leg being coupled to the said secondary windings and connected in parallel to a pair of output terminals. Preferably the inductor is not in parallel with a switch; in one preferred embodiment the inductor and diode are the only elements on the leg.

The invention will have a significant impact on saving energy. It may be used for high voltage DC-DC conversion, but features low voltage stress on the switches and high output current capacity.

Preferably for each switch pair, the two switches are arranged to operate in anti-phase. This is a convenient way of producing the AC current and ensures that stress is applied to only one of the switches at any one time.

Preferably the switches are configured for zero volt switching or alternatively zero current switching. For example, each switch of the switch pair may have a diode and capacitor in parallel with it; this facilitates zero volt switching. Zero voltage switching (ZVS) is a switching technique in which the switch is activated or deactivated when the voltage across it is zero. Zero current switching (ZCS) is a switching technique in which the switch is activated or deactivated when the current through it is zero.

Preferably the total number of primary windings of the one or more transformers is equal to the number of capacitors connected between the input terminals of the converter.

Preferably the total number of primary windings of the one or more transformers is equal to the number of legs of said current multiplier.

Preferably the number of primary transformer windings is equal to the number of secondary transformer windings.

In a preferred embodiment there are n capacitors in series, n switch pairs, n primary windings, n secondary windings and n legs of the current multiplier; where n is a predetermined integer number equal to 2 or greater. That is there is the same number of capacitors in series, switch pairs, primary windings, secondary windings and current multiplier legs. The integer n may be 2, 3, 4 or greater. In general the larger n, the lower the voltage rating of the switches may be compared to the input voltage.

Preferably the converter has one or more blocking capacitors on the primary side of the one or more transformers. More specifically, the blocking capacitors are preferably coupled to the primary transformer windings, preferably between the switch pairs and the primary transformer windings. The blocking capacitors may help to create the conditions for zero voltage switching.

The present invention has a respective capacitor in parallel with the two switches of each respective switch pair. In contrast, the three-level converter of Fig.3 has a dc-link capacitor in parallel with only one switch of switch pair.

Moreover, the converter of Fig. 3 has clamping diodes in parallel with the switch pairs and connected to the midpoint of the switch pairs. The present invention preferably does not have such clamping diodes. The three level converter of Fig. 3 has a free-wheeling capacitor in parallel with the switches of more than one switch pair and connected between said plurality of capacitors and said switch pairs. The converter of the present invention preferably does not have capacitors in parallel with the switches of more than one switch pair and connected between said plurality of capacitors and said switch pairs.

A second aspect of the present invention provides an apparatus comprising a DC-DC converter according to the first aspect of the present invention.

A third aspect of the present invention provides a train or tram comprising a DC-DC converter according to the first aspect of the present invention.

A fourth aspect of the present invention provides a method of converting a high voltage DC current to a low voltage DC current using the converter according to the first aspect of the present invention, comprising the steps of inputting the high voltage DC current to the input terminals, switching the switches of the switch pairs to convert the high voltage DC current to a high voltage AC current, transforming the high voltage AC current to a low voltage AC current with the transformer and converting the low voltage AC current to a low voltage DC current with the AC to DC converter.

Switching the switch pairs preferably takes place over a series of time intervals. 'Switching a switch pair', or 'switching the switches of a switch pair', means that the switches are switched. E.g. if the switch pair comprises a first switch and a second switch, then switching of the switch pair means switching from a state in which the first switch is on and the second switch is off to a state in which the first switch is off and the second switch is on, or vice versa. Preferably one switch pair is switched in each time interval. Preferably only one switch pair is switched in any one time interval, i.e. there is preferably no simultaneous switching of different switch pairs. This has the advantage of balancing the excitation of the various transformer windings. A switching cycle is a cycle in which all of the switch pairs are switched. Preferably the switching cycle comprises n time intervals, where n is the number of switch pairs.

A fifth aspect of the present invention provides a DC to DC converter comprising:- an input section comprising an AC to DC converter,

one or more transformers comprising a plurality of primary windings and a plurality of secondary windings,

the primary windings being coupled to the AC to DC converter and the secondary windings being coupled to a current multiplier, wherein said current multiplier comprises a plurality of legs, each leg having diode and an inductor in series, each leg being coupled to said secondary windings and connected in parallel to a pair of output terminals and wherein on each of said legs the diode is not in parallel with a switch.

The current multiplier has an advantageous design and increases the amount of current available at the output from the DC-DC converter. The diodes on the legs of the current multiplier may, for example, be fast recovery diodes (FRD) or Schottky diodes (SD). The voltage rating of the FRD diodes is generally higher than that of SD. However, the voltage drops of the SD are lower than that of FRD.

A sixth aspect of the present invention provides a DC to DC converter comprising:-

an input section comprising an AC to DC converter,

one or more transformers comprising a a plurality of primary windings and a plurality of secondary windings,

the primary windings being coupled to the AC to DC converter and the secondary windings being coupled to a current multiplier, wherein said current multiplier comprises a plurality of legs, each leg having diode and an inductor in series, each leg being coupled to said secondary windings and connected in parallel to a pair of output terminals and wherein on each of said legs the diode is a fast recovery diode or a Schottky diode.

A seventh aspect of the present invention provides a method of converting a high voltage DC current to a low voltage DC current using the conductor according to the fifth or sixth aspects of the present invention, comprising the steps of inputting the high voltage DC current to the input section and converting the high voltage DC current to a high voltage AC current, transforming the high voltage AC current to a low voltage AC current with the one or more transformers and converting the low voltage AC current to a low voltage DC current with the current multiplier.

An eighth aspect of the present invention provides a DC to DC converter comprising:-

a pair of input terminals,

a pair of output terminals,

a plurality of capacitors connected in series between said input terminals,

a plurality of switch pairs, each respective switch pair being connected in parallel across a respective one of said capacitors, each switch pair comprising two switches and a mid-point between said two switches,

a transformer having a plurality of primary and secondary windings, the primary windings being coupled to the midpoints of the switch pairs,

the secondary windings being coupled to a current multiplier;

said current multiplier comprising a plurality of legs, each leg having diode and an inductor in series, each leg being coupled to said secondary windings and connected in parallel to said output terminals.

The diode may be any suitable diode. Possible examples include a fast recovery diode or a Schottky diode. A MOSFET could also be used as a diode, although that would require more complicated auxilliary circuitary in order to act as a diode. Other possible diodes will be apparent to a person skilled in the art.

The invention is especially applicable to a DC-DC converter structure for high voltage applications. According to a preferred embodiment, the structure preferably exhibits three distinct features. First, the voltage stress on the primary switches is only a fraction of the input voltage, so that switches of low voltage rating and thus of low on-resistance can be used. This leads to reduced conduction loss. Second, all the switches are soft- switched, so that the switching loss can be reduced. Third, the rectifier is a current multiplier, so that the output current capacity, and thus the power handling capacity of the converter are increased.

In order to achieve high output current capability with the need of using the converter for a dc-dc conversion of a very high input voltage, a preferred embodiment of the present invention presents the following structure. The power primary-side switches are arranged in multiple switch pairs. The mid-points of the switch pairs are connected to the primary side of a multiphase HF transformer. The preferred embodiment preferably achieves the following goals:

1) The voltage stress on each primary-side switch is only a fraction of the input voltage.

2) All the switches are turned on and off with ZVS.

3) The output rectifier is a current multiplier that has a high output current capability.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which ; -

Fig. 1 shows a block diagram of the electrical network on the train;

Fig. 2 shows a typical structure of a prior art high- voltage dc-dc converter;

Fig. 3 shows the typical structure of a prior art three-level converter;

Fig. 4 shows the circuit structure of a high-voltage dc-dc converter according to a first embodiment of the present invention;

Fig. 5 shows the circuit structure of the kth switch pair of the converter of Fig. 4;

Fig. 6 shows an illustration of the voltage stress on the switching devices;

(a) shows the case when switch Sw is on and switch SkD is off,

(b) shows the case when switch Sw is off and switch S _kD is on; Fig. 7 shows the circuit structure of a multiphase transformer with dc blocking capacitors; Fig. 8 shows an example of a dc-dc converter according to an embodiment of the present invention in which n, the number of switch pairs, is equal to 3; Fig. 9 shows a timing diagram of one-third of the switching cycle of a dc-dc converter according to an embodiment of the present invention; Fig. 10 shows the modes of operation of the converter of Fig. 9;

(a) shows Mode 0 [before to],

(b) shows Mode 1 [to, ti],

(c) shows Mode 2 [ti, t ₂ ], (d) shows Mode 3 [t ₂ , t ₃ ],

(e) shows Mode 4 [t ₃ , t ₄ ],

(f) shows Mode 5 [t ₄ , ts], and

Fig. 11 shows the timing diagram of the whole switching cycle of the converter of Fig. 9.

DETAILED DESCRIPTION

A. Circuit Structure

A high voltage DC to DC converter according to an embodiment of the present invention is shown in Fig. 4. There is a pair of input terminals which receives an input voltage V; There are n input dc-link capacitors C ₁ , C ₂ , ..., C _n connected in series across the input voltage V ₁ . They preferably have the same capacitances, such that, due to the symmetry of the circuit, they split equally the input voltage. The voltage on each capacitor is V ₁ I n.

There are n switch pairs, SPl, SP2, ..., SPw , each switch pair connected in parallel across a respective capacitor C ₁ , C ₂ , ..., C ₃ . The structure of the Mi switch pair is shown in Fig. 5.

Each switch pair is formed by two switching devices, namely Sw and SkD, which are operated in anti-phase.

Fig. 6 illustrates the voltage stress on the switching devices. As shown in Fig. 6(a), when Sw in SPfc is on, the voltage stress on SkD is v, I n. As shown in Fig. 6(b), when SkD is on, the voltage stress on Sw is V ₁ 1 n. The operation is similar for the other switch pairs. Thus, the voltage stress on all switches in off-state is only one-wth of the input voltage. No clamping diodes, as in TL converters, are needed for this purpose. The switching patterns applied to the switch pairs have a phase difference of 360° / n. Fig. 7 illustrates the circuit structure of the multiphase transformer with dc blocking capacitors. As shown in Fig. 7, the mid-points of the switch pairs in Fig. 4 are connected to respective primary transformer windings TrI, Tr2, ..., Tm through dc blocking capacitors C _M , C^ ₂ , ..., Cd _cn - The dc blocking capacitors preferably have relatively large capacitances which are equal to each other. The steady-state dc voltage of capacitors Cdci, Cda, ■ ■ ■, is V ₁ I n and Cdcn 's voltage is (n - IJv, / n. The voltages produced at the nodes Ml, M2, ..., Mn have phase differences of 360°/ n . The turns ratio of the transformer is m : 1.If a Delta transformer is used as in Fig. 8, then the mid-points of the switch pairs may be coupled to the angles of the Delta transformer.

As shown in Fig. 5, each switch of each switch pair has a built-in diode-capacitor pair; Dku -C sku and DkD - CskD for the &th switch pair. The diode-capacitor pairs are used to provide ZVS for all primary-side switches, in order to reduce the switching losses. A dead time is added between the gate signals applied to the switches in each switch pair.

The secondary- side circuit has a plurality of secondary transformer windings coupled to respective legs of a multiphase rectifier, which is also known as a "current multiplier".

The output current is shared by a plurality of identical parallel diode-inductor branches (three in the illustrated embodiment): D _\ - L ₁ , D ₂ - L ₂ , ..., D _n - L _n (Li = L ₂ = ... = L _n ). This structure has several advantages over mainstream rectifiers which have one output inductor and two or four diodes. Firstly, the output circuit of the present embodiment consists of n identical parallel-connected inductors, which share the load current. Thus, each output inductor carries only one-wth of the load current. Secondly, the current stress in each diode is reduced by one-nth. Theoretically, compared to a conventional rectifier, it will have lower average current through each device, and thus secondary conduction loss.

As the current multiplier outputs to a pair of output terminals. The output voltage is determined by the voltages applied to the transformer primary windings, and therefore can be adjusted by controlling the duty cycle of the switches in the switch pairs. The circuit operation is explained by way of example only, for a converter with n = 3, where n = the number of switch pairs. It will be appreciated that the present invention could be implemented with a greater or lesser number of switch pairs (although at least three is preferred). Furthermore, as will be appreciated by a person skilled in the art the devices of the switching cycle could be modified from the description below while still obtaining the advantages of the invention.

B. Illustration of the circuit operation with n = 3

According to a preferred embodiment the converter is arranged to go cyclically through 15 modes in one switching cycle. An example of the circuit is shown in Fig. 8. Due to the switching operation symmetry, it is sufficient to analyze the first five modes which cover a period of T _s I 3. At any time, three out of the six primary switches (i.e., S _\ ~ Se) are switched on. The timing diagram of one-third of the switching cycle is given in Fig. 9. Fig. 10 shows the modes of operation. Fig. 11 shows the timing diagram of the whole switching cycle. The voltage v _ab in the figure shows the voltage between nodes a and b. As the durations of [to, t ₄ ] and [ts, ^] are equal, the average value of v _ab is V ₁ /3. As the steady-state average voltage across each transformer winding is zero, the average voltage of C _xγ is V ₁ /3.

Similarly, by considering the nodes b, c, and a, c, the average voltage of C _γz is V ₁ /3 and C _7x is 2 V ₁ /3.

During the operation in the first T _s I 3, S ₄ and Se are turned on, and S ₃ and S ₅ are turned off.

Mode 0 [before to][Fig. 10(a)]: S ₂ , S ₄ , and Se are turned on. The converter is in the freewheeling stage. The voltages across the transformer's windings are zero. The currents in the primary and secondary windings keep constant. The three output inductors share i _o . D _x and D _y are on, in order to provide the constant current i _o I 3 in iu and i _Ly, , respectively.

Mode 1 [to, ti][Fig. 10(b)]. At to, S ₂ turns off with ZVS because of the presence of Csi- The current i _x = — i _o I 3 begins to charge Cs ₂ and discharge Cs ₁ . As typically for a transition from a freewheeling stage towards an energy transfer stage (passive topology to active topology transition), only the energy in the transformer leakage inductances is available for achieving such a function. As v _γz = v _C3 - v _cz = 0 , the winding YZ can be considered as a short-circuit, and thus its leakage inductance does not participate in providing energy for the above operation, V _XY increases from zero to a positive value and vzx goes from zero to a negative value. As D _x , D _y , and D _z are turned on, the voltages on the secondary windings are all zero. Mode 1 ends when Cs ₁ is completely discharged and Cs ₂ is charged up to v,- / 3.

Mode 2 [t _\ , t ₂ ][Fig. 10(c)]: At t _l5 as Vs ₁ = 0, D _\ starts conducting and Si is turned on with ZVS (as the two stages, the first one with D _\ conducting, and the second one with Si conducting, are the same from the point of view of the equations governing their operation, they are considered here as the same topology). D _x , D _y , and D _z are still turned on because the increasing primary currents are still not sufficient to provide the load current. The voltages across the secondary windings are zero. As typically found in FB and TL converters with ZVS, this phenomenon causes a loss of the secondary (effective) duty-cycle. Even if the primary circuit is in the "on" topology, the secondary one is still in the freewheeling state.

Mode 2 ends when the primary current ix reaches the reflected secondary current i _u I m = i _o I 3m , and thus i _x = I _LX and io _x = 0. A new energy transfer stage begins.

Mode 3 [t ₂ , ? ₃ ][Fig. 10(d)]: This is the first energy transfer stage in the cycle. The currents in the primary and secondary windings are almost constant.

Mode 4 [t ₃ , t ₄ ][Fig. 10(e)]: According to the PWM action, the "on" stage is ended at t ₃ = to + D T _s I 3. At t ₃ , S _\ is turned off with ZVS due to the presence of Cs ₁ . ix divides into two currents for charging Cs ₁ from zero and discharging Csi from v,7 3.

In this mode, the energy of the output inductor, which is reflected to the primary side, is sufficient for assuring ZVS of the switches, even at light load. This is typical for all FB and TL converters at the transition from a transfer energy ("active") stage to a freewheeling one ("passive" stage). The currents remain almost unchanged.

Mode 4 ends when Cs; is charged up to V ₁ I 3 [i.e., v _csl (t ₄ ) = V ₁ I 3 ] and Cs2 is completely discharged [i.e., v _CS2 (t ₄ ) = O ]. The converter enters into the freewheeling stage.

The currents in the transformers windings are kept constant during the transition due to the leakage inductances of the windings.

Mode 5 [t ₄ , ts][Fig. 10(f)]. As at t ₄ , vcsiiU) = 0, Z) ₂ starts conducting and S ₂ turns on with ZVS. As these two topologies are similar from the point of view of the equations governing their operation, they will be considered here as the same mode. That is, the converter operates in the first freewheeling stage of a switching cycle. The currents in the primary windings remain constant. D _y and D _z are conducting in order to provide the constant current i _o l 3 in i _Ly and i _Lz , respectively.

For the next two T _s I 3 intervals, the converter will operate in a similar way, the transition taking place in the switch pair SP2, and then in the switch pair SP3, arriving in the last Mode at the diagram described in Mode 0. In summary, a high-voltage dc/dc converter according to the present invention has low- voltage stress on the primary switches and an output current multiplier. By use of a particular structure or the primary side, the voltage stress on the power switches is reduced to only 1/n of the input voltage, allowing for the use of transistors of low-voltage rating and low on-state resistance. The conduction losses may be considerable reduced. This novel primary structure preferably matches the output current multiplier, which allows for an increase in the load and power handling capacity. The output voltage may be by adjusting the duty cycle of the switches in each switch pair. All the switches are preferably turned-on/off with soft- switching, allowing for minimal switching losses.

While certain embodiments have been described above, they should not be taken to limit the scope of the invention. Various modifications and alternative designs may be apparent to a person skilled in the art, but still fall within the spirit and scope of the invention as defined by the appended claims.