Patent Description:
The invention deals with voltage balancing in split DC-link, as used (for example) in conjunction with Three-Level (<NUM>) Neutral Point Clamped (NPC) topologies.

As of today, voltage balancing is accomplished by means of a dedicated balancing circuit, so-called 4th leg (buck-boost topology), driven in Discontinuous Conduction Mode (DCM). Other architectures may feature a neutral leg. Further, UPS are known for exploiting the balancing capability of the battery-line DC/DC converter implemented as a dual-boost converter and operated as a <NUM> converter.

UPS or photovoltaic inverters may face DC-link unbalance issues linked to the limited power capability of a 4th leg converter. The root cause may either be DC offset in the load current (e.g. half-wave rectifiers) but more often the presence of even harmonics on the load current (as motor soft-starters or non-linear load such as medical equipment). In double-conversion operation, the UPS may ensure balanced DC-link voltage with the aid of the rectifier, for example introducing DC offset and/or even harmonics in the input current. However, during stored-energy operation, the UPS is limited by the power capability of the 4th leg converter.

<NPL>, describes a three-level bi-directional buck-boost converter, analyzes the operating principle of this topology and a method to balance the DC-linked capacitor voltages based on adjusting the width of phase-shifted PWM.

<NPL>" describes a framework for a power conversion architecture of <NUM>-NPC and bidirectional dc-dc converters designed essentially for voltage and power balancing functionality. The paper focusses on bidirectional three-level buck-boost with high conversion ratio, along with functionality for dc split-bus neutral-point and power balancing in <NUM>-NPC converters.

<NPL> describes a topology of single-phase transformerless PV inverter capable of operating from two PV sources.

There may be a desire to provide an improved double boost converter.

The problem is solved by the subject-matter of the independent claims. Embodiments are provided by the dependent claims, the following description and the accompanying figures.

Embodiments and examples not covered by the claims are presented to illustrate, and facilitate the understanding of, the claimed invention.

The described embodiments similarly pertain to the voltage balancing circuit for a double-boost DC/DC converter, the double boost converter, the method for controlling the voltage balancing circuit for the double-boost DC/DC converter, the controller for the voltage balancing circuit, the uninterruptable power supply (UPS), the usage of a voltage balancing circuit and the program element. Synergetic effects may arise from different combinations of the embodiments although they might not be described in detail.

Technical terms are used by their common sense. If a specific meaning is conveyed to certain terms, definitions of terms will be given in the following in the context of which the terms are used.

According to a first aspect, a double-boost DC/DC converter is provided in accordance with claim <NUM>.

The proposed double-boost DC/DC converter allows for a converter operation and a DC source boost operation without generating undesired voltages, in particular undesired common mode voltages, at low effort in terms of hardware and controlling, and thus at low costs.

According to an embodiment, the switches comprise a transistor and an anti-parallel diode, and the directional devices are diodes. The diodes are directed such that they work as freewheeling diodes. For operating the circuit with the above-mentioned effects both in converter mode and in DC source mode, the switches have to be controllable and to provide freewheeling capability. Therefore, in this disclosure, the term "switch" is used for a controllable element such as a transistor and an associated freewheeling diode.

According to an embodiment, the directional devices further comprise transistors and the diodes are anti-parallel diodes with respect to these transistors. In this case, the directional devices have the same transistor-diode configuration as the inner switches, and are therefore called "outer switches" in this disclosure. The anti-parallel diodes are also called freewheeling diodes herein.

According to the invention, the inner switches comprise a first inner switch and a second inner switch, wherein the switches are configured to switch repeatedly according to the following scheme:.

The sequence of switching of the first and the second inner switch may be reversed such that S3 is performed prior to S2. That is, one of the inner switches is turned off later with a pre-defined delay with respect to the other inner switch. This measure allows voltage balancing without impressing an undesired common mode (CM) voltage on the DC source terminals. During step S2 current flowing into the inductance between the midpoints is stored and in S3, the current is dispensed again, before in step S4 the first and the second inner switch are switched on again such that no CM voltage is impressed on the DC source terminals.

According to an embodiment, the length of the third time interval is equal or greater than the second time interval. By configuring that twice the time extension fully fits within the freewheeling phase, it is ensured that the current will be fully dispensed, i.e. until <NUM> A is reached, before the first and the second inner switch are switched on again and thus it ensures that no CM voltage is impressed on the DC source terminals.

According to an embodiment, the DC source is a battery or a solar panel. The types of possible DC sources is, however, not limited thereto.

The double boost converter provides energy from a DC source, e.g. as part of a UPS or a solar inverter.

According to an embodiment, the double-boost converter is operated as a buck-boost converter for DC-link voltage balancing when no current is exchanged with the DC source.

That is the DC-link voltage is also balanced when the double-boost converter for providing DC source current is operated as a buck-boost converter. This is achieved by a configuring the controlling of the switches in a usual way.

A method for controlling a double-boost DC/DC converter is provided in claim <NUM>, comprising the steps:.

As mentioned in the explanations above with respect to the double-boost DC/DC converter, steps S2 and S3 may be inverted. For further explanations, it is referred to the description of the voltage balancing circuit and the figures.

The controller of the double-boost DC/DC converter is configured to perform the steps of the first operation mode specified in claim <NUM>:
S1: the first and the second inner switch are switched on, S2: after a first time interval, the second inner switch is switched off, S3: after a further second time interval, the first inner switch is switched off, S4: after a further third time interval, jump to S1, which are the steps of the described method. The controller may comprise circuits without programmable logics or may be or comprise a micro controller, a field programmable gate array (FPGA), an ASIC, a Complex Programmable Logic Devices (CPLD), or any other programmable logic devices known to person skilled in the art.

According to the invention, the controller is configured to drive the double-boost DC/DC converter as a buck-boost converter for DC-link voltage balancing when no current is exchanged with the DC source. That is, the controller is configured such that it provides two operation modes. The first one is an operation as a double-boost converter, where the controller is configured to perform steps S1 to S4. The second one is an operation as buck-boost converter without involving the DC source. In this operation mode, the switches are controlled in another way. In particular, the upper outer directional device and inner switch are turned on contemporarily while the lower directional device and inner switch are turned off, and vice versa. "Upper" means the part between the midpoint of the inner switches and the positive DC bus providing the connection of the positive DC capacitor and the upper directional device. The "lower" switches are accordingly the switches between the midpoint of the inner switches and the negative DC bus at the negative DC capacitor.

According to a fifth aspect, an uninterruptable power supply (UPS) comprising a voltage balancing circuit and / or a controller as described herein is provided.

According to a sixth aspect, a usage of a voltage balancing circuit according to the first aspect or an embodiment of the first aspect in an UPS, a photovoltaic / solar inverter, a Battery Energy Storage System (BESS), a converter interfacing a DC microgrid to an AC grid, or any suitable alternative energy storage system is provided.

According to a seventh aspect, a program element is provided that, when executed on a controller according to the fourth aspect or an embodiment of the fourth aspect, instructs the controller to perform the steps of the method according to the third aspect. The computer program element may be part of a computer program, but it can also be an entire program by itself. For example, the computer program element may be used to update an already existing computer program to get to the present invention.

A computer readable medium may be provided on which the program element is stored. For a UPS, the DC source may be a battery, whereas in other applications such as or a photovoltaic / solar inverter the DC source may, for example be a solar panel, a boost DC/DC converter, and an inverter. If the latter is a <NUM> NPC (<NUM> level neutral point clamped) / TNPC (transistor neutral point clamped) inverter, a double-boost DC/DC converter may be used which, with the benefit of the invention, may offer balancing capability.

These and other features, aspects and advantages of the present invention will become better understood with reference to the accompanying figure and the following description.

The figures are merely schematic and not to scale. In principle, identical or similar parts are given the same reference signs.

With the disclosed invention, the balancing capability may be enhanced with the contribution of the boost converter. As a matter of fact, the disclosed invention removes the need for a dedicated 4th leg converter, hence potentially reducing cost/footprint. At the same time, one peculiar aspect of the disclosed invention is the absence of Common Mode (CM) voltage at the DC source terminals. While some batteries are floating w. ground, and pretty tolerant of CM voltage, alternative energy storage technologies (especially those requiring an interfacing converter) may exhibit a CM voltage limit. Finally, these peculiarities also facilitate common DC source operation of multiple UPS systems. Therefore, cost and footprint are reduced, and the balancing capability of the UPS system is improved. The invention further provides an improved application compatibility, as it mitigates the limits of the current solution and facilitates compatibility with alternative energy storage system as well as paralleling of UPS systems at their DC port.

<FIG> show a schematic of the topology of the voltage balancing circuit (<NUM>). The circuit is based on a converter topology that comprises a DC link consisting of a positive DC capacitor <NUM> and a negative capacitor <NUM>. In between these capacitors is the midpoint <NUM>. Parallel to the DC link <NUM>, <NUM>, a series of switches <NUM>, <NUM>, <NUM>, and <NUM> are connected. The switches <NUM> and <NUM> that connect the DC link to the series of switches are referred to as "outer directional devices" or "outer switches". Correspondingly, the switches <NUM> and <NUM> between the outer switches are referred to as "inner switches". A switch consists of a transistor and an antiparallel freewheeling diode. The transistors are designated T1, T2, T3 and T4 corresponding to the switches <NUM>, <NUM>, <NUM>, and <NUM>. The inner switches <NUM>, <NUM> are connected to each other at midpoint <NUM>. The DC source <NUM> may be, here as an example, a battery. Battery <NUM> is connected over inductances <NUM> and <NUM>, respectively, to the switches. In more detail, inductance <NUM> is connected to the connection point of the outer switch <NUM> and the inner switches <NUM>, and inductance <NUM> is connected to the connection point of the inner switch <NUM> and the outer switch <NUM>. The particularity in the depicted circuit <NUM> is the inductance L4 <NUM> that is arranged between the midpoint <NUM> of the DC link <NUM>, <NUM> and the midpoint <NUM> of the inner switches <NUM>, <NUM>. The inductance <NUM> enables the voltage balancing functionality as explained in the following. Inductors <NUM> and <NUM> as shown are coupled, that is, they are arranged as a single differential choke assembly with separate windings. However, in some embodiments, the windings may be separate.

<FIG> show the flow of the current in voltage balancing circuit <NUM> and diagrams of currents and the state of the inner switches during a first switching state. <FIG> shows the on-state of T2 and <FIG> the on-state of T3 corresponding to step S1 <NUM> of the method <NUM> described herein and depicted in <FIG>. <FIG> shows the flow of the current from the positive pole of the battery through inductance <NUM>, the inner switches <NUM> and <NUM>, and the inductance <NUM> to the minus pole <NUM> of the battery <NUM>. By this flow of the current, the inductivities <NUM> and <NUM> are loaded. <FIG> shows the rise of the current in the inductance <NUM> connected to the battery <NUM>. Since T2 and T3 are conductive, the voltage between the connection points of the inner and outer switches, i.e. between switches <NUM> and <NUM>, and between switches <NUM> and <NUM>, is zero, such that no current is flows to inductance <NUM> between the midpoints, as shown in <FIG>.

<FIG> show the flow of the current in voltage balancing circuit <NUM> and diagrams of currents and the state of the inner switches during a second switching state. <FIG> shows the on-state of T2 and <FIG> the off-state of T3 corresponding to step S2 <NUM> of the method described herein. Switching off T3 results in driving the current stored in inductance <NUM> over the freewheeling diode of switch <NUM> to the capacitor <NUM> on one hand and via the closed transistor to inductance <NUM>. As shown in <FIG>, the current of inductance <NUM> decreases, whereas the amount of current of inductance <NUM> increases. Further, the current keeps flowing also through the freewheeling diode of T4 <NUM> and inductor <NUM>.

<FIG> show the flow of the current in voltage balancing circuit <NUM> and diagrams of currents and the state of the inner switches during a third switching state. <FIG> shows the off-state of T2 and <FIG> the off-state of T3 corresponding to step S3 <NUM> of the method described herein. In this phase, inductance <NUM> continues driving the flow of the current over DC capacitor <NUM> and freewheeling diode <NUM> and freewheeling diode of switch <NUM> back to the inductance <NUM>. Thus, the stored current in <NUM> flows through the freewheeling diode of switch <NUM> and decreases therefore further as shown in <FIG>, and, which is the point, the current is decreased until zero before the next cycle starts. That is, by ensuring that twice the time extension, i.e. twice int2, which is the time between switching off T2 and T3, fully fits within the freewheeling phase there will be not CM voltage impressed on the battery terminals. This can also be expressed as Int3 has to be greater than Int2. In addition, current keeps flowing through inductors <NUM>, <NUM> and freewheeling diode of T1 <NUM>.

Thus, the proposed topology and the delay between switching off T3 and T2 avoid that a voltage peak caused by continued conduction through free wheel diodes of outer switches when both inner switches are closed, that would result in common mode (CM) voltage impressed on the battery terminals.

DC voltage balancing may also be obtained by operating the BCB as a 4th Leg converter, when idle that is, driving ON T1 and T2 simultaneously, or T3 and T4 simultaneously, loading the L4 inductor, and leaving it to discharge through the FWD of the opposite pair.

<FIG> shows the differential voltage impressed by voltage balancing circuit bridge. The voltage is <NUM> V if one of T2 or T3 are open and <NUM> V if T2 and T3 are closed.

<FIG> shows the CM voltage on battery terminals and it can be seen that the CM voltage remains approximately zero as desired.

<FIG> shows the DC link voltage, which has only a slight ripple. The ripple is linked to the ratio of the battery voltage to the DC-link voltage. The voltage ripple may be reduced by increasing the DC-link capacitance and/or the converter switching frequency.

<FIG> shows a diagram of the method <NUM> comprising the following steps: In a first step S1, <NUM>: the first and the second switch are switched on. In a second step S2, <NUM>, after a first time interval, the second switch is switched off. In a third step S3, <NUM>, after a further second time interval, the first switch is switched off. In a fourth step, S4, <NUM>, after a further third time interval, it is jumped to S1 <NUM> and the previous steps are repeated.

Claim 1:
Double-boost DC/DC converter (<NUM>), operable in a voltage balancing mode; wherein the double-boost DC/DC converter comprises:
a split DC-link (<NUM>, <NUM>) with midpoint (<NUM>);
outer switches (<NUM>, <NUM>) and inner switches (<NUM>, <NUM>) parallel-connected to the DC-link (<NUM>, <NUM>), wherein the outer switches are connected to capacitors of the split DC-link and to the inner switches (<NUM>, <NUM>), and the inner (<NUM>, <NUM>) switches are connected to each other at a midpoint (<NUM>); and
a DC source terminal to which a DC source (<NUM>) is connectable in parallel to the inner switches (<NUM>, <NUM>) over inductances (<NUM>, <NUM>);
the inductances (<NUM>, <NUM>) by which the DC source (<NUM>) is connectable in parallel to the inner switches (<NUM>, <NUM>);
an inductance (<NUM>) connected to the midpoint (<NUM>) of the DC link and to the midpoint (<NUM>) of the inner switches (<NUM>, <NUM>); and
a controller configured to perform the following steps:
in a first operation mode, which is a double-boost converter mode wherein current is exchanged with the DC source:
S1 (<NUM>): the first (<NUM>) inner and the second (<NUM>) inner switch are switched on;
S2 (<NUM>): after a first time interval int1, the second (<NUM>) inner switch is switched off;
S3 (<NUM>): after a further second time interval int2, the first (<NUM>) inner switch is switched off;
S4 (<NUM>): after a further third time interval int3, jump to S1 (<NUM>);
wherein the sequence of switching of the first and the second inner switch may be reversed such that S3 is performed prior to S2;
or, in a second operation which is a capacitor voltage balancing mode wherein no current is exchanged with the DC source,
switch on the first outer switch (<NUM>) and the first inner switch (<NUM>) contemporarily with switching off the second outer switch (<NUM>) and the second inner switch (<NUM>);
switch off the first outer switch (<NUM>) and the first inner switch (<NUM>) contemporarily with switching on the second outer switch (<NUM>) and the second inner switch (<NUM>).