DE102020106660A1 - Method for operating an electronic power converter and electronic power converter - Google Patents

Abstract

A method is used to operate an electronic power converter (1), the converter (1) comprising: • a primary bridge (23) between a primary intermediate circuit voltage V and a primary side of a transformer (3), • and a secondary bridge (43) between a secondary intermediate circuit voltage V and a secondary side of the transformer (3). In order to control a flow of electrical power from the primary side to the secondary side of the converter (1), the following steps are carried out: • Select, with regard to an operating point of the converter (1) , a line mode for the operation of the converter (1) and a corresponding duty cycle d and a corresponding phase shift g, • controlling the primary bridge (23) to supply voltage pulses with a duty cycle d to the transformer (3), • controlling the secondary bridge (43) by actively switching only two switching units in order to supply voltage pulses with a phase shift g to the transformer (3).

Description

The invention relates to the field of power electronics, in particular power converters, and more particularly power converters used in on-board chargers for electric vehicles. It relates to a method for operating an electronic power converter and an electronic power converter as described in the preamble of the corresponding independent claims.

The requirements for on-board chargers (OBCs) of electric vehicles (EV) often specify bidirectional power transfer capability. The main application for this is vehicle-to-vehicle (V2V) charging, which enables the empty battery of one EV to be charged from the full battery of another EV. Other uses are vehicle-to-load (V2L), which means that the vehicle provides alternating power to electrical devices connected to a plug in the EV, or vehicle-to-grid (V2G), with the EV providing power to the utility grid at peak times.

OBCs typically consist of a power factor compensation (PFC) rectifier followed by an isolated DCDC converter. For unidirectional OBCs according to the prior art, the well-known resonant converter LLC topology is often used for the DCDC converter stage. Since the LLC is a unidirectional topology, it must be extended to the CLLC for bidirectional power transmission. The CLLC typically only provides 50% of the charging power in the discharge mode, since the secondary side runs in half-bridge mode, i.e. H. the lower side IGBT is constantly in discharge mode. In addition, the CLLC takes over the disadvantages of the LLC, namely variable switching frequency and the necessary resonance capacitors.

An alternative to the CLLC is the dual active bridge (DAB) converter ( 1 ), which offers full power transfer in both directions. Other advantages of DAB is that it does not require any resonance capacitors and that it runs without a constant switching frequency.

The DAB converter comprises two full bridges, one on the primary and one on the secondary side of a transformer, with defined integrated leakage and magnetizing inductance. There are three control variables: the primary duty cycle, the secondary duty cycle and the phase shift between the primary and secondary sides. Since all three control variables affect the output power, there are two degrees of freedom that can be used to ensure that zero voltage switching (ZVS) is achieved on the semiconductor switches, typically MOSFETs, and that conduction losses are minimized. However, ZVS is only achieved if the switches are controlled in exactly the right way. It was shown how this is theoretically possible, but real conditions such as component tolerances, measurement errors, parasitic capacitances and body diode recovery time are not taken into account and can lead to operating conditions under which the ZVS is lost.

• • Krismer F. et al: Performance Optimization of a High Current Dual Active Bridge with a Wide Operating Voltage Range, POWER ELECTRONICS SPECIALISTS CONFERENCE, 2006. IEEE June 18, 2006.
• • Florian Krismer: Modeling and optimization of bidirectional dual active bridge DC-DC converter topologies, DISS. ETH NO. 19177 (2010) - Chapter 3.1.
• • Nikolas Schibli: Symmetrical multilevel converters with two quadrant DC-DC feeding. École Polytechnique Fédérale de Lausanne, Dissertation N° 2200 (2000) - Chapter 4.

• • Krismer F. et al: A comparative evaluation of isolated bi-directional DC/DC converters with wide input and output voltage ranges, Conference Record of the 2005 IEEE Industry Applications Conference Fortieth LAS Annual Meeting, IEEE CAT., Bd. 1, 2. Oktober 2005 (2005-10-02).
• • US 2015/365005 A1
• • CH 707 553 A2

The following publications disclose the use of dual active bridge circuits:

• • Krismer F. et al: Performance Optimization of a High Current Dual Active Bridge with a Wide Operating Voltage Range, POWER ELECTRONICS SPECIALISTS CONFERENCE, 2006. IEEE June 18, 2006.
• • Florian Krismer: Modeling and optimization of bidirectional dual active bridge DC-DC converter topologies, DISS. ETH NO. 19177 (2010) - Chapter 3.1.
• • Nikolas Schibli: Symmetrical multilevel converters with two quadrant DC-DC feeding. École Polytechnique Fédérale de Lausanne, Dissertation N ° 2200 (2000) - Chapter 4.

• • Krismer F. et al: A comparative evaluation of isolated bi-directional DC / DC converters with wide input and output voltage ranges, Conference Record of the 2005 IEEE Industry Applications Conference Fortieth LAS Annual Meeting, IEEE CAT., Vol. 1, 2. October 2005 (2005-10-02).
• • US 2015/365005 A1
• • CH 707 553 A2

However, all of the circuits and control methods presented therein have the aforementioned problems.

It is therefore an object of the invention to provide a method for operating an electronic power converter and an electronic power converter of the type mentioned at the outset which overcome the disadvantages mentioned above. In particular, the goal can be to reduce switching losses To reduce circumstances under which parameters affecting the operation of the circuit are not fully known or controllable. The aim may be to provide an alternative method of reducing switching losses in a converter, particularly in a dual active bridge converter.

These objects are achieved by methods of operating an electronic power converter and an electronic power converter according to the respective independent claims.

• • eine primäre Brückenschaltung, die angeordnet ist, um zumindest eine primäre Zwischenkreisspannung V_p (oder Eingangsgleichspannung) von einer Primärseite des Wandlers, oder die invertierte primäre Zwischenkreisspannung ―V_p , zu einer Primärseite eines Transformators zuzuführen,
• • und eine sekundäre Brückenschaltung, die angeordnet ist, um zumindest eine sekundäre Zwischenkreisspannung V_s (Ausgangsgleichspannung) von einer Sekundärseite des Wandlers, oder die invertierte sekundäre Zwischenkreisspannung ―V_s , zu einer Sekundärseite des Transformators zuzuführen, wobei die sekundäre Brückenschaltung zwei Halbbrücken umfasst, wobei jede Halbbrücke eine obere und eine untere Schalteinheit umfasst,

The method is used to operate an electronic power converter, the converter comprising:

• • a primary bridge circuit which is arranged to supply at least one primary intermediate circuit voltage V _p (or input DC voltage) from a primary side of the converter, or the inverted primary intermediate circuit voltage ―V _p , to a primary side of a transformer,
• • and a secondary bridge circuit which is arranged to supply at least one secondary intermediate circuit voltage V _s (DC output voltage) from a secondary side of the converter, or the inverted secondary intermediate circuit voltage ―V _s , to a secondary side of the transformer, the secondary bridge circuit comprising two half bridges, each half bridge comprising an upper and a lower switching unit,

• • Bestimmen eines Arbeitspunkts des Wandlers, wobei der Arbeitspunkt eine Funktion der primären Zwischenkreisspannung V_p , der sekundären Zwischenkreisspannung V_s und der elektrischen Leistung P, die von der Primärseite zu der Sekundärseite übertragen werden soll, ist,
• • Bestimmen, aus dem Arbeitspunkt, eines ausgewählten Leitungsmodus für den Betrieb des Wandlers, wobei der ausgewählte Leitungsmodus einer von zumindest drei Leitungsmodi ist,
• • für den ausgewählten Leitungsmodus, Bestimmen eines Tastgradwerts d und eines Phasenverschiebungswerts g,
• • für den ausgewählten Leitungsmodus, Steuern der primären Brückenschaltung, um abwechselnd positive und negative Spannungsimpulse zur Primärseite des Transformators zuzuführen, wobei die Spannungsimpulse in Bezug auf eine Schaltperiode einen Tastgrad gemäß dem Tastgradwert d aufweisen,
• • für den ausgewählten Leitungsmodus, Steuern der sekundären Brückenschaltung durch aktives Schalten von nur zwei der Schalteinheiten der sekundären Brückenschaltung, um abwechselnd positive und negative Spannungsimpulse zur Sekundärseite des Transformators zuzuführen, wobei die Spannungsimpulse eine Phasenverschiebung in Bezug auf die Spannungsimpulse, die zur Primärseite des Transformators zugeführt werden, gemäß dem Phasenverschiebungswert g aufweisen.

The method comprises the following steps to control a flow of electrical power from the primary side to the secondary side of the converter:

• • Determination of an operating point of the converter, the operating point being a function of the primary intermediate circuit voltage V _p , the secondary intermediate circuit voltage V _s and the electrical power P that is to be transmitted from the primary side to the secondary side,
• • Determination, from the operating point, of a selected line mode for the operation of the converter, the selected line mode being one of at least three line modes,
• • for the selected line mode, determining a duty cycle value d and a phase shift value g,
• • for the selected line mode, controlling the primary bridge circuit in order to alternately supply positive and negative voltage pulses to the primary side of the transformer, the voltage pulses having a duty cycle in relation to a switching period according to the duty cycle value d,
• • for the selected line mode, controlling the secondary bridge circuit by actively switching only two of the switching units of the secondary bridge circuit in order to alternately supply positive and negative voltage pulses to the secondary side of the transformer, the voltage pulses being a phase shift with respect to the voltage pulses leading to the primary side of the transformer are supplied, according to the phase shift value g.

This enables the power flow through the converter to be controlled using only two variables - the duty cycle value d and a phase shift value g - and a reduction in switching losses in the secondary bridge circuit, since its remaining switching units, typically two, are not actively switched. They are only switched to passive.

Active switching or active control of a switch or a switching unit means that an active switch is controlled by a switching signal or gate signal in order to switch the switch on and off. A corresponding gate connection of the switch is separate from connections through which a current flows and is switched. In contrast, passive switching takes place when a voltage across a diode changes polarity, which causes the diode to block the current or to pass the current through the diode. A switching unit that is switched to active comprises an active switch and typically a parallel freewheeling diode. A switching unit that is only switched passively can only include one diode in a certain operating mode. It can also include an active switch in parallel with the diode, which switch, however, is not operated in the specific operating mode. Thus, a switching unit that only works as a diode can be implemented by a diode alone or by a diode with an active switch in parallel therewith, the active switch not being operated.

It goes without saying that when only two of the switching units of the secondary bridge circuit are actively switched and the remaining switching units are only switched passively, this takes place for the selected line mode, ie over a large number of switching periods. In other words, for the duration of a large number of switching periods, the case applies that only two of the switching units of the secondary bridge circuit are active can be switched, while the remaining two are switched passively. Such a plurality of switching periods may include, for example, ten or fifty or one hundred or more switching periods.

The switching period is the shortest period of time after which the pattern or the sequence of switching processes of the converter repeats itself.

The phase shift between positive voltage pulses is defined as the time between the rising edges of the pulses divided by the switching period. The time between the falling edges can be different.

The phase shift between negative voltage pulses is defined as the time between the falling edges of the pulses divided by the switching period. The time between the rising edges can be different.

• • umfasst der Schritt des Steuerns der sekundären Brückenschaltung, dass bei einer ersten und einer zweiten der aktiv geschalteten zwei Schalteinheiten der sekundären Brückenschaltung in jeder Schaltperiode nach der Phasenverschiebung die erste eingeschaltet und die zweite ausgeschaltet wird und dann,
• • nach einer Zeitdauer, die bis zur Hälfte der Schaltperiode beträgt, die erste ausgeschaltet und die zweite eingeschaltet wird.

In embodiments,

• • the step of controlling the secondary bridge circuit comprises that in a first and a second of the two actively switched switching units of the secondary bridge circuit in each switching period after the phase shift, the first is switched on and the second switched off and then,
• • after a period of up to half the switching period, the first is switched off and the second switched on.

In embodiments, the time period is equal to half the switching period.

• • entweder aktives Schalten von nur unteren Schalteinheiten der sekundären Brückenschaltung, wobei obere Schalteinheiten nur als Dioden arbeiten,
• • oder aktives Schalten von nur oberen Schalteinheiten der sekundären Brückenschaltung, wobei untere Schalteinheiten nur als Dioden arbeiten.

In embodiments, the step of controlling the secondary bridge circuit comprises:

• • either active switching of only lower switching units of the secondary bridge circuit, whereby upper switching units only work as diodes,
• • or active switching of only upper switching units of the secondary bridge circuit, with lower switching units only working as diodes.

Typically, the lower switching units are arranged between the bridge centers of the secondary bridge circuit and the negative output terminal, and the upper switching units are arranged between the bridge centers and the positive output terminal.

• • aktives Schalten von nur oberen und unteren Schalteinheiten von einer der Halbbrücken, wobei obere und untere Schalteinheiten der anderen Halbbrücke nur als Dioden arbeiten.

In embodiments, the step of controlling the secondary bridge circuit comprises:

• • Active switching of only upper and lower switching units of one of the half bridges, whereby the upper and lower switching units of the other half bridge only work as diodes.

Typically, a dead time is inserted between the switching of the upper and lower switching units.

In embodiments,
a beginning of a positive or negative voltage pulse, which is applied to the secondary side of the transformer, is determined by the two switching units which are switched to active;
one end of a positive or negative voltage pulse that is applied to the secondary side of the transformer is determined by one of the switching units, which only work as diodes and switch to a blocking state due to a current flowing through the corresponding switching unit, thereby changing its direction is reversed.

• • einen kontinuierlichen Leitungsmodus (CCM), bei dem
  • ◯ der Tastgrad der Primärseite gemäß dem Tastgradwert d abhängig vom Arbeitspunkt variiert wird, wenn V_s' < V_p, und konstant bei 50 % bleibt, wenn V_s' > V_p; wobei $V_s^{\text{'}}=\frac{N_p}{N_s}{V}_s$
    
    die sekundäre Zwischenkreisspannung ist, die auf die Primärseite bezogen ist,
• • einen ersten diskontinuierlichen Leitungsmodus (DCM1), bei dem
  • ◯ der Tastgrad der Primärseite gemäß dem Tastgradwert d abhängig vom Arbeitspunkt variiert wird,
• • einen zweiten diskontinuierlichen Leitungsmodus (DCM2), bei dem
  • ◯ der Tastgrad der Primärseite gemäß dem Tastgradwert d konstant bei 50 % bleibt.

In embodiments, the at least three conduction modes include:

• • a continuous line mode (CCM) in which
  • ◯ the duty cycle of the primary side is varied according to the duty cycle value d depending on the operating point if V _s '<V _p , and remains constant at 50% if V _s '> V _p ; in which $V_s^{\text{'}}=\frac{N_p}{N_s}{V}_s$
    
    is the secondary intermediate circuit voltage that is related to the primary side,
• • a first discontinuous conduction mode (DCM1) in which
  • ◯ the duty cycle of the primary side is varied according to the duty cycle value d depending on the operating point,
• • a second discontinuous conduction mode (DCM2) in which
  • ◯ the duty cycle of the primary side remains constant at 50% according to the duty cycle value d.

• • wenn V_s' < V_p und die Leistung P kleiner als $$P_{\text{max},\text{<figure-callout id="1" label="dcm" filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png" state="{{state}}">dcm</figure-callout>}1}=\frac{{\text{V}}_{\text{s}}^{\text{'}}\left({\text{V}}_{\text{s}}^{\text{'}}-\frac{{\text{V}}_{\text{s}}^{\text{'}}{}^2}{{\text{V}}_{\text{p}}}\right)}{4{\text{L}}_{\text{s}}f}$$
  
  ist, dann wird DCM1 angewandt;
• • wenn V_s' > V_p und die Leistung P kleiner als $$P_{\text{max},\text{<figure-callout id="2" label="dcm" filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png" state="{{state}}">dcm</figure-callout>}2}=\frac{{\text{V}}_{\text{p}}^2\left({\text{V}}_{\text{s}}^{\text{'}}-{\text{V}}_{\text{p}}\right)}{4L_s{\text{V}}_{\text{s}}^{\text{'}}f}$$
  
  ist, dann wird DCM2 angewandt; ansonsten wird CCM angewandt;

wobei V_p die primäre Zwischenkreisspannung ist, V_s' die sekundäre Zwischenkreisspannung ist, die auf die Primärseite bezogen ist, $V_s^{\text{'}}=\frac{N_p}{N_s}{V}_s$

ist, wobei Np die Anzahl von Windungen der Primärwicklung ist, Ns die Anzahl der Windungen der Sekundärwicklung ist, f die Schaltfrequenz ist und Ls die Streuinduktivität ist, die auf die Primärseite bezogen ist, und absolute Werte für P verwendet werden.In embodiments, limits between the conduction modes are determined by the following rules depending on the operating point:

• • if V _s '<V _p and the power P less than $$P_{\text{Max},\text{<figure-callout id="1" label="dcm" filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png" state="{{state}}">dcm</figure-callout>}1}=\frac{{\text{V}}_{\text{s}}^{\text{'}}\left({\text{V}}_{\text{s}}^{\text{'}}-\frac{{\text{V}}_{\text{s}}^{\text{'}}{}^2}{{\text{V}}_{\text{p}}}\right)}{\mathrm{4th}{\text{L.}}_{\text{s}}f}$$
  
  then DCM1 is applied;
• • if V _s '> V _p and the power P less than $$P_{\text{Max},\text{<figure-callout id="2" label="dcm" filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png" state="{{state}}">dcm</figure-callout>}2}=\frac{{\text{V}}_{\text{p}}^2\left({\text{V}}_{\text{s}}^{\text{'}}-{\text{V}}_{\text{p}}\right)}{\mathrm{4th}{\mathrm{L.}}_s{\text{V}}_{\text{s}}^{\text{'}}f}$$
  
  then DCM2 is applied; otherwise CCM is used;

where V _{p is} the primary intermediate circuit voltage, V _s ' is the secondary intermediate circuit voltage, which is related to the primary side, $V_s^{\text{'}}=\frac{N_p}{N_s}{V}_s$

where Np is the number of turns of the primary winding, Ns is the number of turns of the secondary winding, f is the switching frequency and Ls is the leakage inductance related to the primary and absolute values for P are used.

The boundaries between conduction modes as expressed above can be replaced by other expressions using other variables of the transducer that are mathematically or physically equivalent.

If power is to be transferred from the secondary side, the power P is negative. In this case, the definition of the primary and secondary side is switched, just like the number of turns on the transformers.

The working point can be determined by measuring the two intermediate circuit voltages and using the power P as an input variable, which is generated, for example, by a monitoring control unit. The operating point changes over time, but can be regarded as stationary for the duration of a switching period.

Absolute values of P are used because P can be positive or negative depending on the direction of power transfer.

• • Berechnen, für jeden Leitungsmodus, der Parameter d und g, um eine Zielfunktion zu minimieren, wobei die Zielfunktion insbesondere ein Transformator-Spitzenstrom ist oder wobei die Zielfunktion ein Transformator-Effektivstrom ist.

In embodiments, the method comprises the following step:

• • Calculate, for each line mode, the parameters d and g in order to minimize an objective function, wherein the objective function is in particular a transformer peak current or the objective function is a transformer effective current.

The transformer peak current can be determined as the maximum of the absolute values of the transformer current over a switching period. The transformer effective current (RMS effective current) can be determined as an effective value (RMS value) of the transformer current over a switching period.

In embodiments, the parameters d and g in each of the switching modes as a function of the power P, which is transferred from the primary side to the secondary side, the primary intermediate circuit voltage V _p , and the secondary intermediate circuit voltage V _s , which may be related to the V _s ' of the primary side is determined.

In embodiments, the converter is a dual active bridge converter, that is, both the primary circuit and the secondary circuit 4th are full bridge inverters.

In a full-bridge inverter, each half-bridge comprises two switching units, each switching unit comprising an active switch and a freewheeling diode in parallel.

The electronic power converter comprises a control unit, wherein the control unit comprises analog and / or digital signal processing units which are configured to carry out a method described herein. The control unit typically also includes sensors for measuring the primary intermediate circuit voltage V _p and the secondary intermediate circuit voltage V _s and / or input channels or input means for such measured voltages and / or an input channel or an input means for a desired power flow from the primary to the secondary side or vice versa.

• eine primäre Brückenschaltung, die angeordnet ist, um zumindest eine primäre Zwischenkreisspannung V_p (oder Eingangsgleichspannung) von einer Primärseite des Wandlers, oder die invertierte primäre Zwischenkreisspannung ―V_p, zu einer Primärseite eines Transformators zuzuführen,
• und eine sekundäre Brückenschaltung, die angeordnet ist, um zumindest eine sekundäre Zwischenkreisspannung V_s (Ausgangsgleichspannung) von einer Sekundärseite des Wandlers, oder die invertierte sekundäre Zwischenkreisspannung ―V_s, zu einer Sekundärseite des Transformators zuzuführen. Der sekundäre Schaltkreis umfasst:
  • • entweder, dass aktive Schalteinheiten der sekundären Brückenschaltung nur in der unteren Hälfte jeder Halbbrücke vorhanden sind und jeweils einen entsprechenden Brückenmittelpunkt mit dem negativen Ausgangsanschluss (14) verbinden, wobei obere Schalteinheiten den entsprechenden Brückenmittelpunkt mit dem positiven Ausgangsanschluss verbinden, der nur Dioden umfasst,
  • • oder dass aktive Schalteinheiten der sekundären Brückenschaltung nur in der oberen Hälfte jeder Halbbrücke vorhanden sind und jeweils einen entsprechenden Brückenmittelpunkt mit dem positiven Ausgangsanschluss verbinden, wobei untere Schalteinheiten den entsprechenden Brückenmittelpunkt mit dem negativen Ausgangsanschluss (14) verbinden, der nur Dioden umfasst.

In embodiments, an electronic power converter is provided, comprising:

• a primary bridge circuit which is arranged to supply at least one primary intermediate circuit voltage V _p (or input DC voltage) from a primary side of the converter, or the inverted primary intermediate circuit voltage ―V _p , to a primary side of a transformer,
• and a secondary bridge circuit which is arranged to supply at least one secondary intermediate circuit voltage V _s (DC output voltage) from a secondary side of the converter, or the inverted secondary intermediate circuit voltage ―V _s , to a secondary side of the transformer. The secondary circuit includes:
  • • Either that active switching units of the secondary bridge circuit are only available in the lower half of each half bridge and each have a corresponding bridge center point with the negative output connection ( 14th ), with the upper switching units connecting the corresponding bridge center point to the positive output terminal, which only includes diodes,
  • • or that active switching units of the secondary bridge circuit are only present in the upper half of each half bridge and each connect a corresponding bridge center point to the positive output connection, with lower switching units the corresponding bridge center point to the negative output connection ( 14th ) which only includes diodes.

In summary, the control method described above allows ZVS to be achieved, minimizes the transformer peak current and is stable with respect to the influence of parasitic switching elements. At high output power levels, the converter, particularly a DAB converter, is operated in continuous conduction mode (CCM).

At lower output power levels, it operates in discontinuous conduction mode (DCM). Depending on the ratio of the input and output DC link voltages and the power level, one of the two different DCMs is selected.

Further embodiments emerge from the dependent patent claims. Features of the method claims can be combined with features of the device claims and vice versa.

• 1 einen dualen aktiven Brücken- (DAB-) Wandler;
• 2 idealisierte Wellenformen im kontinuierlichen Leitungsmodus (CCM);
• 3 Wellenformen im CCM mit Vs'<Vp und Totzeit Td, wobei T_d eine Totzeit oder Verzugszeit ist, die auf den Halbbrücken der Primärseite verwendet wird, um Durchschießen, d. h. Kurzschließen des primären Zwischenkreises, zu verhindern;
• 4 Wellenformen im CCM mit Vs'>Vp und Totzeit Td;
• 5 Leitungsmodi der DAB als Funktion des Leistungspegels P/P0 und Spannungsverhältnisses Vs'/Vp;
• 6 Schaltintervalle, Spannungen und Ströme für den CCM;
• 7 das Gleiche für den diskontinuierlichen Leitungsmodus 1 (DCM1);
• 8 das Gleiche für den diskontinuierlichen Leitungsmodus 2 (DCM2);
• 9 Wellenformen im DCM1 mit Vs'<Vp und Totzeit Td;
• 10 Wellenformen im DCM2 mit Vs'>Vp und Totzeit Td;
• 11 idealisierte Wellenformen im CCM für Rückleistungsfluss;
• 12 eine einfachgerichtete Variante eines DAB-Wandlers; und
• 13 ein Flussdiagramm eines Verfahrens zum Steuern des Wandlers.

The object of the invention is explained in more detail below with reference to exemplary embodiments which are illustrated in the accompanying drawings, which schematically show:

• 1 a dual active bridge (DAB) converter;
• 2 idealized waveforms in continuous conduction mode (CCM);
• 3 Waveforms in the CCM with Vs'<Vp and dead time Td, where T_d is a dead time or delay time that is used on the half bridges on the primary side in order to prevent shoot-through, ie short-circuiting, of the primary intermediate circuit;
• 4th Waveforms in the CCM with Vs'> Vp and dead time Td;
• 5 Conduction modes of the DAB as a function of the power level P / P0 and the voltage ratio Vs' / Vp;
• 6th Switching intervals, voltages and currents for the CCM;
• 7th the same for the discontinuous conduction mode 1 (DCM1);
• 8th the same for the discontinuous conduction mode 2 (DCM2);
• 9 Waveforms in DCM1 with Vs'<Vp and dead time Td;
• 10 Waveforms in DCM2 with Vs'> Vp and dead time Td;
• 11 idealized waveforms in the CCM for reverse power flow;
• 12th a single-directional variant of a DAB converter; and
• 13 a flow diagram of a method for controlling the converter.

The reference symbols used in the drawings and their meanings are given in summarized form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

1 Figure 3 shows schematically a dual active bridge (DAB) converter. It can be controlled in such a way that it transfers electrical power from a primary side with a primary intermediate circuit voltage V _p , also written as V_p or Vp, to a secondary side with a secondary intermediate circuit voltage V _s (V_s, Vs) or from the secondary side to the primary side.

The converter comprises on the primary side or input side 1 a primary circuit 2 with a positive input terminal 11 and a negative input terminal 12th that can be connected to an intermediate circuit of the PFC of the OBC. The primary circuit 2 includes a primary bridge circuit 23 with two half bridges 21st , 22nd and a primary intermediate circuit capacitance 24 . Each of the two primary connections of the primary winding of the transformer is with a corresponding bridge center point 27 , 28 one of the half bridges 21st , 22nd connected. The primary bridge circuit 23 can therefore apply a primary transformer voltage V _Tp (V_Tp) to the primary terminals of the transformer 3 respectively. In particular, this voltage can be a square wave with a variable duty cycle.

The transformer 3 can be represented as having Np turns in the primary winding, Ns turns in the secondary winding, a series inductance Ls in series and a magnetizing inductance Lm parallel to one of the windings.

The converter comprises on the secondary or output side 1 a secondary circuit 4th with a positive output terminal 13 and a negative output terminal 14th that can be connected to a battery. The secondary circuit 4th includes a secondary bridge circuit 43 with two half bridges 41 , 42 and a secondary intermediate circuit capacitance 44 . Each of the two terminals of the transformer's secondary winding is connected to a corresponding bridge center point 47 , 48 one of the half bridges 41 , 42 connected. The secondary bridge circuit 43 can therefore apply a secondary transformer voltage V _Ts (V_Ts) to the secondary terminals of the transformer 3 respectively. In particular, this voltage can be a variable duty cycle square wave.

In the primary bridge circuit 23 is every half bridge 21st , 22nd arranged around the associated bridge centers 27 , 28 either to the positive input terminal 11 or the negative input terminal 12th connect to. This is done using switching units 29a , 29b , especially the upper switching units 29a and the lower switching units 29b , reached. The upper switching units 29a are between the corresponding bridge centers 27 , 28 and the positive input terminal 11 arranged. The lower switching units 29b are between the corresponding bridge centers 27 , 28 and the negative input terminal 12th arranged.

In the secondary bridge circuits 43 is every half bridge 41 , 42 arranged around the associated bridge centers 47 , 48 either to the positive output terminal 13 or the negative output terminal 14th connect to. This is done using switching units 49a , 49b , especially the upper switching units 49a and the lower switching units 49b , reached. The upper switching units 49a are between the corresponding bridge centers 47 , 48 and the positive output terminal 13 arranged. The lower switching units 49b are between the corresponding bridge centers 47 , 48 and the negative output terminal 14th arranged.

In embodiments, the switching units can be active switching units which are implemented in parallel with a diode by a semiconductor switch S1, S2, S3, S4, S5, S6, S6, S8, for example a MOSFET. In embodiments, one or more of the switching units can be a passive switching unit that is triggered by a diode 49 is implemented alone, while the other switching units are active switching units.

At high output power levels, the DAB operates in continuous line mode (CCM). Typical waveforms for this mode are primary transformer voltage (V_Tp), secondary transformer voltage (V_Ts), and primary transformer current (I_Lp) in 2 shown together with the gate signals S1-S4 for the primary side and S5-S8 for the secondary side.

Only two control variables are used in the control method: the primary duty cycle d and the phase shift g, as in FIG 2 specify the times in relation to the switching period T_s = 1 / f. Since both control variables affect the amount of power transfer, the available degree of freedom is used to reduce the transformer peak current.

Continuous line mode

The continuous conduction mode uses the switching sequence based on that 2 , but with a dead time T _d between switching operations on the half bridges on the primary side in order to avoid shooting through.

Exemplary waveforms for CCM with dead time and gate signal generation are shown in FIG 3 for Vs'<Vp and in 4th for Vs'> Vp.

1. a) S2, S4, S6 sind eingeschaltet. Der Primärstrom ist negativ und zirkuliert durch S2 und S4, die primäre Transformatorspannung V_Tp ist null. Der Sekundärstrom ist negativ und strömt durch S6 und die Diode von S7, die sekundäre Transformatorspannung V_Ts ist die invertierte sekundäre Zwischenkreisspannung V_s . Der Primär- und Sekundärstrom beginnen zuzunehmen.
2. b) S2 wird abgeschaltet und nach einer Verzögerungszeit Td wird S1 eingeschaltet. Der Primärstrom ist negativ und strömt durch S1 und S4, die primäre Transformatorspannung V_Tp ist die primäre Zwischenkreisspannung V_p . Der Tastgrad für den positiven Spannungsimpuls in V_Tp beginnt. Der Primär- und Sekundärstrom nehmen weiterhin zu.
3. c) Der Primär- und Sekundärstrom ändern ihre Vorzeichen, von negativ auf positiv. Die Diode von S7 blockiert, ihr Strom wird von S8 übernommen. Der Sekundärstrom ist positiv und zirkuliert durch S6 und S8, die sekundäre Transformatorspannung V_Ts ist null.
4. d) Nach der Phasenverschiebung g, relativ zum Abschalten von S2, wird S6 abgeschaltet und S8 gleichzeitig eingeschaltet. Der Sekundärstrom ist positiv und strömt durch S8 und S5, die sekundäre Transformatorspannung V_Ts ist die sekundäre Zwischenkreisspannung V_s . Der Primär- und Sekundärstrom nehmen weiterhin zu.
5. e) S4 wird abgeschaltet und nach einer Verzögerungszeit Td wird S3 eingeschaltet. Der Primärstrom ist positiv und zirkuliert durch S1 und S3, die primäre Transformatorspannung V_Tp ist null. Der Tastgrad für den positiven Spannungsimpuls in V_Tp endet. Der Primär- und Sekundärstrom beginnen abzunehmen.
6. f) S1 wird abgeschaltet und nach einer Verzögerungszeit Td wird S2 eingeschaltet. Der Primärstrom ist positiv und strömt durch S2 und S3, die primäre Transformatorspannung V_Tp ist die invertierte primäre Zwischenkreisspannung V_p . Der Tastgrad für den negativen Spannungsimpuls in V_Tp beginnt. Der Primär- und Sekundärstrom nehmen weiter ab.
7. g) Der Primär- und Sekundärstrom ändern ihre Vorzeichen, von positiv auf negativ. Die Diode von S5 blockiert, ihr Strom wird von S6 übernommen. Der Sekundärstrom ist negativ und zirkuliert durch S6 und S8, die sekundäre Transformatorspannung V_Ts ist null.
8. h) Nachdem er die Hälfte der Schaltperiode lang eingeschaltet war, wird S8 abgeschaltet und S6 gleichzeitig eingeschaltet. Der Sekundärstrom ist negativ und strömt durch S6 und S7, die sekundäre Transformatorspannung V_Ts ist die invertierte sekundäre Zwischenkreisspannung V_s . Der Primär- und Sekundärstrom nehmen weiter ab.

In the waveform according to 3 the converter passes the following states in each switching period (only the active switches S1-S8 that are switched on, i.e. conducting, are labeled; the other switches are switched off):

1. a) S2, S4, S6 are switched on. The primary current is negative and circulates through S2 and S4, the primary transformer _voltage V _Tp is zero. The secondary current is negative and flows through S6 and the diode of S7, the secondary transformer voltage V _Ts is the inverted secondary intermediate circuit voltage V _s . The primary and secondary currents begin to increase.
2. b) S2 is switched off and after a delay time Td S1 is switched on. The primary current is negative and flows through S1 and S4, the primary transformer _voltage V _Tp is the primary intermediate _{circuit voltage} V _p . The duty cycle for the positive voltage pulse in V _Tp begins. The primary and secondary currents continue to increase.
3. c) The primary and secondary currents change their signs, from negative to positive. The diode of S7 blocks, its current is taken over by S8. The secondary current is positive and circulates through S6 and S8, the secondary transformer voltage V _Ts is zero.
4. d) After the phase shift g, relative to the switching off of S2, S6 is switched off and S8 is switched on at the same time. The secondary current is positive and flows through S8 and S5, the secondary transformer voltage V _Ts is the secondary intermediate _{circuit voltage} V _s . The primary and secondary currents continue to increase.
5. e) S4 is switched off and after a delay time Td S3 is switched on. The primary current is positive and circulates through S1 and S3, the primary transformer _voltage V _Tp is zero. The duty cycle for the positive voltage pulse in V _Tp ends. The primary and secondary currents begin to decrease.
6. f) S1 is switched off and after a delay time Td S2 is switched on. The primary current is positive and flows through S2 and S3, the primary transformer _voltage V _Tp is the inverted primary intermediate _{circuit voltage} V _p . The duty cycle for the negative voltage pulse in V _Tp begins. The primary and secondary currents continue to decrease.
7. g) The primary and secondary currents change their signs, from positive to negative. The diode of S5 blocks, its current is taken over by S6. The secondary current is negative and circulates through S6 and S8, the secondary transformer voltage V _Ts is zero.
8. h) After it has been switched on for half the switching period, S8 is switched off and S6 is switched on at the same time. The secondary current is negative and flows through S6 and S7, the secondary transformer voltage V _Ts is the inverted secondary intermediate circuit voltage V _s . The primary and secondary currents continue to decrease.

State a) is reached again in that S3 is switched off and S4 is switched on after a delay time Td.

Of course, zero voltage switching takes place when a switch is switched on and this switch is in parallel with a diode which is already conducting the current through the switch unit. Zero voltage switching also takes place at the beginning of state e), similar to a phase-shifted full bridge operation: When S4 is switched off, the current charges the parasitic capacitance of S4 and simultaneously discharges S3 until the body diode of S3 is activated. From this point in time, S3 can be safely switched on using zero voltage switching. The same applies if S4 is switched on again later at the beginning of state a).

The sequence of states of the converter in a switching period can be made up of the following 4th , 6-8 can be derived in the same way as the above states.

In 4th for Vs'> Vp the waveforms are analogous to those off 3 The difference being that the duty cycle d for the voltage pulses on the primary side is 50%, which means that the positive and negative pulses of the primary transformer voltage V _{Tp have} a length that corresponds to half the switching period. In other words, S2 and S3 are turned on and off simultaneously, and S1 and S4 are also turned on and off simultaneously. This causes the primary transformer _voltage V _Tp to jump from the primary intermediate _{circuit voltage} V _p directly to the inverted primary intermediate circuit voltage V _p and back without a phase in which it is zero.

In any case, the end of the positive and negative voltage pulse on the secondary side, the secondary transformer voltage V _Ts , is determined by the fact that the secondary current changes its sign and blocks the diode from S5 or S7. The blocked current is then driven through the diode of the respective opposite switch unit of the same half-bridge. This causes the secondary current to circulate and the secondary transformer voltage V _{Ts to become} zero. This ends the duty cycle of the secondary transformer voltage V _Ts . The same or an analogous sequence of events takes place in the other conduction modes when the secondary current changes its sign.

The switches of the secondary bridge circuit 43 which are used, the lower switches S6 and S8 in the present examples, can then be toggled at zero voltage (from on to off or from off to on) in order to start the duty cycle of the next voltage pulse. Therefore, these switches have a constant duty cycle of 50% (half of the switching period), whereas the secondary voltage has a shorter duty cycle which, as explained in the previous paragraph, is terminated by the passive switching of corresponding diodes.

Boundary Between Continuous and Discontinuous Line Mode At lower power output levels, the DAB operates in discontinuous line mode (DCM). There are two different DCMs (DCM1 and DCM2), depending on the ratio of the intermediate circuit voltages and the power levels. The line modes are in 5 as a function of the transferred power P (the vertical axis is normalized, relative to the base power $$P_{}$$$${}_0=\frac{{\text{V}}_{\text{p}}{\text{V}}_{\text{s}}^{\text{'}}}{\mathrm{8th}f{\mathrm{L.}}_s}$$

The maximum power that can be transferred to CCM is given as $$P_{\text{Max},\text{ccm}}=\frac{{\text{V}}_{\text{p}}{\text{V}}_{\text{s}}^{\text{'}}}{\mathrm{8th}{\mathrm{L.}}_{s}f}\left(1-\frac{1}{\frac{2{\text{V}}_{\text{p}}}{{\text{V}}_{\text{s}}^{\text{'}}}+\frac{2V_p^2}{{\text{V}}_{\text{s}}^{\text{'}}{}^2}+1}\right)$$

dann wird DCM1 angewandt. Wenn V_s' > V_p und die Leistung P kleiner ist als P_max,dcm2 = $\frac{{\text{V}}_{\text{p}}^2\left({\text{V}}_{\text{s}}^{\text{'}}-{\text{V}}_{\text{p}}\right)}{4L_s{\text{V}}_{\text{s}}^{\text{'}}f},$

dann wird DCM2 angewandt.If V _s '<V _p and the power P is less than $P_{\text{Max},\text{dcm}1}=\frac{{\text{V}}_{\text{s}}^{\text{'}}\left({\text{V}}_{\text{s}}^{\text{'}}-\frac{{\text{V}}_{\text{s}}{\text{'}}^2}{{\text{V}}_{\text{p}}}\right)}{\mathrm{4th}{\text{L.}}_{\text{s}}f},$

then DCM1 is applied. If V _s '> V _p and the power P is less than P _{max, dcm2} = $\frac{{\text{V}}_{\text{p}}^2\left({\text{V}}_{\text{s}}^{\text{'}}-{\text{V}}_{\text{p}}\right)}{\mathrm{4th}{\mathrm{L.}}_s{\text{V}}_{\text{s}}^{\text{'}}f},$

then DCM2 is applied.

Determination of the control variables d and g

und die zu transferierende Leistung P zu bestimmen.In each of the switching modes, the same method can be used to set the parameters d and g as a function of the primary intermediate circuit _voltage V _p , the secondary intermediate circuit voltage, which is related to the primary side, $V_s^{\text{'}}=\frac{N_p}{N_s}{V}_s,$

and to determine the power P to be transferred.

).(Note: As is well known in a circuit with a transformer, an equivalent circuit can be used to map its behavior, with switching elements and electrical quantities on one side of the transformer related to the other side based on the transformer ratio $\frac{N_p}{N_s}.$

).

In each switching mode, the power that is transferred can be calculated from the point in time at which the switching operation takes place. Each switching operation defines a new state of the converter, ie the path of the current that flows in the converter. The time intervals between switching operations in the two bridges are referred to as switching intervals T _n , where n = 1 ... number of switching intervals. The sum of all switching intervals corresponds to a fraction of the switching period, usually half of the switching period. In this case, the switching intervals are the same for two half periods, with the voltages and the current changing polarity in every half period.

The switching intervals are functions of parameters d and g. Some of the switching intervals depend directly on d and g, others are functions of the course of the current and can be determined by calculation when the current flowing through a diode crosses zero.

In each state, a change in the current is a function of its switching interval T _n and the voltages and inductances that influence the current in this state.

The 6th to 8th show for each of the switching modes the sequence of the states and thus the general form of the voltages and currents, as well as the timing of the switching operations between the states, ie the switching intervals T ₁ , T ₂ , ..., which are a function of the parameters d and g are. The effects due to dead time and parasitic capacitances can be neglected when calculating the switching intervals and the optimal parameters d and g.

$T_1=\frac{V_p\left(g{T}_s+T_3\right)-V_s\left(T_3+T_4\right)}{V_s+2V_p}$

$\mathrm{\Delta }{I}_2=\frac{1}{L}{V}_p{T}_2$

T₂ = gT_s - T₁ $\mathrm{\Delta }{I}_3=\frac{1}{L}\left(V_p-V_s\right)T_3$

T₃ = (d - g)T_s $\mathrm{\Delta }{I}_4=-\frac{1}{L}{V}_s{T}_4$

$T_4=\frac{T_s}{2}-d{T}_s$

6th shows the shape of voltages and currents in continuous conduction mode (CCM). Each half of a switching period comprises four switching intervals with the respective time periods T ₁ , T ₂ , T ₃ , T ₄ , which each correspond to a different state of the converter. The corresponding values for the current change ΔI _n in each interval and the duration of the switching intervals are as follows (for the sake of simplicity, the formulas are shown here and for the other conduction modes for the case in which the transformer ratio 1 amounts. In other cases, the secondary voltage V _s must be replaced by the secondary voltage which is related to the primary side, V _s '). Current change Interval duration $\mathrm{\Delta }{\mathrm{I.}}_1=\frac{1}{\mathrm{L.}}\left(V_p+V_s\right){\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png"\; state="\{\{state\}\}">T</figure-callout>}}_1$

${\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png"\; state="\{\{state\}\}">T</figure-callout>}}_1=\frac{V_p\left(G{T}_s+T_3\right)-V_s\left({\mathrm{<figure-callout\; id="3"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_3+T_{\mathrm{4th}}\right)}{V_s+2V_p}$

$\mathrm{\Delta }{\mathrm{I.}}_2=\frac{1}{\mathrm{L.}}{\mathrm{<figure-callout\; id="2"\; label="V\; p\; T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">V</figure-callout>}}_p{T}_{}{}_2$

T ₂ = gT _s - T ₁ $\mathrm{\Delta }{\mathrm{I.}}_3=\frac{1}{\mathrm{L.}}\left(V_p-V_s\right){\mathrm{<figure-callout\; id="3"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_3$

T ₃ = (d - g) T _s $\mathrm{\Delta }{\mathrm{I.}}_{\mathrm{4th}}=-\frac{1}{\mathrm{L.}}{V}_s{T}_{\mathrm{4th}}$

$T_{\mathrm{4th}}=\frac{T_s}{2}-d{T}_s$

T₁ = gT_s $\mathrm{\Delta }{I}_2=\frac{1}{L}\left(V_p-V_s\right)T_2$

T₂= (d - g)T_s $\mathrm{\Delta }{I}_3=-\frac{1}{L}{V}_s{T}_3$

$T_3=\frac{V_p}{V_s}\left(T_1+T_2\right)-T_2$

7th shows the shape of voltages and currents in the first discontinuous conduction mode (DCM1). Each half of a switching period comprises three switching intervals with the respective time periods T ₁ , T ₂ , T ₃ . The corresponding values for the current change ΔI _n in each interval and the duration of the switching intervals are: Current change Interval duration $\mathrm{\Delta }{\mathrm{I.}}_1=\frac{1}{\mathrm{L.}}{\mathrm{<figure-callout\; id="1"\; label="V\; p\; T"\; filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png"\; state="\{\{state\}\}">V</figure-callout>}}_p{T}_{}{}_1$

T ₁ = gT _s $\mathrm{\Delta }{\mathrm{I.}}_2=\frac{1}{\mathrm{L.}}\left(V_p-V_s\right){\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_2$

T ₂ = (d - g) T _s $\mathrm{\Delta }{\mathrm{I.}}_3=-\frac{1}{\mathrm{L.}}{V}_s{\mathrm{<figure-callout\; id="3"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_3$

${\mathrm{<figure-callout\; id="3"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_3=\frac{V_p}{V_s}\left({\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png"\; state="\{\{state\}\}">T</figure-callout>}}_1+T_2\right)-{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_2$

T₁ = gT_s $\mathrm{\Delta }{I}_2=\frac{1}{L}\left(V_p-V_s\right)T_2$

$T_2=\frac{V_p}{V_s-V_p}{T}_1$

8th shows the shape of voltages and currents in the second discontinuous conduction mode (DCM2). Each half of a switching period comprises two switching intervals with the respective time periods T ₁ , T ₂ . The corresponding values for the change in current ΔI _n in each interval and the duration of the switching intervals are: Current change Interval duration $\mathrm{\Delta }{\mathrm{I.}}_1=\frac{1}{\mathrm{L.}}{\mathrm{<figure-callout\; id="1"\; label="V\; p\; T"\; filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png"\; state="\{\{state\}\}">V</figure-callout>}}_p{T}_{}{}_1$

T ₁ = gT _s $\mathrm{\Delta }{\mathrm{I.}}_2=\frac{1}{\mathrm{L.}}\left(V_p-V_s\right){\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_2$

${\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="DE102020106660A1\_0033.png,DE102020106660A1\_0034.png"\; state="\{\{state\}\}">T</figure-callout>}}_2=\frac{V_p}{V_s-{\mathrm{<figure-callout\; id="1"\; label="V\; p\; T"\; filenames="DE102020106660A1\_0037.png,DE102020106660A1\_0039.png"\; state="\{\{state\}\}">V</figure-callout>}}_p}{T}_{}{}_1$

To summarize the above: In any conduction mode, the switching intervals and current can be expressed as functions of the parameters d and g. The current changes in each state or the switching interval can be added, which gives the current at each point in time. The current curves (I _LS ) from the 6th - 8th are based on the individual current and the current changes ΔI _n in the respective time periods.

Based on this, the charge transferred in each state is a function of the duration of that state and the current flowing in that state.

worin f die Schaltfrequenz ist.Adding the charge transferred in each state gives the total charge Q _p transferred from the primary to the secondary side in one switching period. The power P transferred from the primary side to the secondary side is a function of this charge and the primary voltage, where $$P=2f{Q}_p{V}_p$$

where f is the switching frequency.

Therefore, the transmitted power P can be expressed and calculated as a function of the parameters d and g. The course of the transformer current can also be calculated. In addition, the peak current and / or the effective current and / or another characteristic value of the operation of the converter can be calculated and used as a target function to be optimized. In contrast, assuming the desired power P to be transmitted and assuming the objective function, the two parameters allow two degrees of deviation and enable the determination of a solution comprising values for d and g which achieve the desired power and minimize the objective function.

With reference to 6-8 the peak of the corresponding current curve (I _LS ) can be minimized, with the average current remaining the same over time.

Determining the solution, including values for d and g, can be achieved by means of an optimization process. If the functions mentioned above, which make the function of P dependent on d and g, can be expressed algebraically, an analytical solution can be determined. In other situations, numerical optimization can be used to determine the solution.

Results of an optimization can e.g. in the form of formulas for calculating the parameters d and g from the working point or in the form of value tables.

For the two discontinuous conduction modes, the optimization that minimizes the peak current can be expressed by the following equations for the control variables:

Control variables in discontinuous line mode 1

$$g=\frac{V_{p}d}{V{\text{'}}_{s}2}-\frac{1}{4}$$

In the DCM1, the parameters d and g are calculated according to the following equations: $$d=\frac{\sqrt{\left(V_p{}^2-V_{p}V{\text{'}}_s\right)f{\mathrm{L.}}_{s}P}}{V_p{}^2-V_{p}V{\text{'}}_s}$$

$$G=\frac{V_{p}d}{V{\text{'}}_{s}2}-\frac{1}{\mathrm{4th}}$$

The value g is negative in DCM1, which means that the phase of gate signal S6 matches the phase of gate signal S2 as in FIG 9 shown. This is exactly the opposite as in CCM, in which g is positive and the gate signal S6 is located behind the gate signal S2.

The main part of the curves of voltages and currents emerges directly from the timing of the switching processes. The following explains what happens when the transformer voltages are zero on both sides (phase A). During a phase immediately before this phase, S6 is turned OFF and S8 is turned ON. The current decreases on both the primary and the secondary side, and the current on the secondary side changes direction (indicated by an asterisk *). After the secondary current has changed direction, the current flows through the parasitic capacitances of S5 and S6. After the parasitic capacitance has been derived from S6, the body diode of S6 is activated, i.e. it becomes leading.

During phase A. the current on the secondary side is negative, which means that the capacities of S2 on the primary side were not completely discharged. To solve this problem, S8 is turned OFF and S6 is turned ON with a phase shift relative to switching operations of the primary side. The switch unit, including S6, was already conducting through its body diode in parallel with S6. In this way, switch S6 is turned ON in a more or less zero voltage state.

The phase shift is assumed to be negative, since the switching to ON and OFF of S6 and S8 takes place when at least one of the switches on the primary side is set to ON (ie in 9 left of D). In other words, while switch S1 is ON on the primary side, switches S6 and S8 are respectively ON and OFF before S1 is switched OFF.

During the phase B. the parasitic capacitance of S8 is charged and the voltage rises (negative) until the current on the secondary side exceeds zero. At this point, phase C.

During phase C. the secondary current continues to rise, now in positive direction, and the parasitic capacitance of S8 is diverted until the body diode of S8 becomes conductive, at which point phase D. begins. When the diode of S8 conducts, the secondary current flows in a direction that supports zero voltage switching on the primary side by increasing the primary current.

In principle, all secondary MOSFETs in DCM2 could be constantly switched off. However, as suggested, switching S6 and S8 prevents the output capacitance of the secondary MOSFET from overshooting with the transformer leakage inductance.

Control variables in discontinuous line mode 2

$$g=\frac{\sqrt{\left(1-\frac{V_p}{V{\text{'}}_s}\right)f{L}_{s}P}}{V_p}$$

In the DCM2 the parameters d and g are calculated according to the following equations: $$d=0.5$$

$$G=\frac{\sqrt{\left(1-\frac{V_p}{V{\text{'}}_s}\right)f{\mathrm{L.}}_{s}P}}{V_p}$$

Typical waveforms of transformer voltages and currents and gate signals in DCM2 are in 10 shown. It should be noted that the secondary transformer voltage V_Ts (dashed line) is shown in such a way that it has lower values than the primary transformer voltage V_Tp. However, the secondary voltage which is passed on to the primary side is higher than the primary voltage, which is why, if the switch units S6 and S8 are switched after the phase shift g, the currents are driven in the direction of zero.

Synchronous rectification

In order to reduce conduction losses, the switch units, which are operated as diodes if they are active switch units, as well as switches S5 and S7 in the present embodiment, can be switched to active in order to operate the secondary bridge circuit as a synchronous rectifier.

Inverse power flow

Due to the symmetry of the circuit, the same control method is used for the inverse power transfer, but with the primary and secondary quantities exchanged. This means that Vp must be swapped with Vs', and the control signals from S1 with S5, S2 with S6, S3 with S7 and S4 with S8 must be swapped. An example of an inverse power transfer in CCM is in 11 shown.

If power flows from the primary to the secondary side, switches S5 and S7 are always off. Therefore, if only one-way operation is required, switches S5 and S7 can be replaced with diodes, as in FIG 12th shown. This circuit can also be controlled using the method described above.

Selection of switch units for active switching

The previous examples showed the control of the secondary bridge circuit 43 by actively switching only the lower switch units 49b or just the upper switch units 49a , 49c and wherein the other switch units (upper and lower respectively) only work as diodes. Alternatively, there may be embodiments in which the secondary bridge circuit 43 by actively switching only the upper and lower switch units of one of the half bridges 41 ; 42 is operated, the upper and the lower switch unit of the other of the half bridges 42 ; 41 only works as a diode. In such embodiments, substantially the same timing of the switching operations can be used. Face of the switching sequence 3 , in which S6 and S8 are shown as to be switched, S5 can be switched instead of S8, for example. In order to prevent penetration, there must be a delay time between switching S5 and S6.

13 shows a flowchart that summarizes a method for controlling the converter based on the steps described above: The steps shown in the flowchart are repeated for each operating point depending on the primary and secondary voltage and the power to be transmitted. The series inductance Ls of the transformer and the switching frequency f usually remain constant. These steps result in performance mode being applied, and parameters d and g being used in that performance mode. These steps can be performed once, and the resulting parameters can be applied repeatedly, regardless of whether the operating point has changed or not. In certain embodiments, these steps are performed once per switching period.

• • Identifizieren der Richtung des Leistungsflusses, d.h. von der Primär- zur Sekundärseite oder umgekehrt. Üblicherweise wird dies abhängig von einem ausgewählten Arbeitsmodus des Wandlers, z.B. zum Aufladen einer Batterie oder durch Zuführen von Leistung an das Netz oder an ein weiteres EV, durch eine Überwachungssteuereinheit vorgegeben.
• • Bestimmen des Leitungsmodus abhängig von dem Leistungspegel.
• • Berechnen der Parameter d und g für den bestimmten Leitungsmodus.
• • Bestimmen der Schaltsignale ausgehend von dem Arbeitsmodus und den Parametern d und g und Steuern der Schaltereinheiten des Wandlers mit diesen Schaltsignalen.

Determining the line mode to apply and parameters d and g to be used involves the following steps:

• • Identify the direction of the power flow, ie from the primary to the secondary side or vice versa. This is usually specified by a monitoring control unit as a function of a selected working mode of the converter, for example for charging a battery or by supplying power to the network or to another EV.
• • Determining the conduction mode depending on the power level.
• • Calculate parameters d and g for the particular line mode.
• • Determination of the switching signals based on the working mode and the parameters d and g and controlling the switch units of the converter with these switching signals.

While the invention has been described in the present embodiments, it should be expressly understood that the invention is not limited thereto, but can otherwise be embodied and practiced in various ways within the scope of the claims.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 2015365005 A1 [0006]
• CH 707553 A2 [0006]

Non-patent literature cited

• Krismer F. et al: Performance Optimization of a High Current Dual Active Bridge with a Wide Operating Voltage Range, POWER ELECTRONICS SPECIALISTS CONFERENCE, 2006. IEEE June 18, 2006. [0006]
• Florian Krismer: Modeling and optimization of bidirectional dual active bridge DC-DC converter topologies, DISS. ETH NO. 19177 (2010) [0006]
• Krismer F. et al: A comparative evaluation of isolated bi-directional DC / DC converters with wide input and output voltage ranges, Conference Record of the 2005 IEEE Industry Applications Conference Fortieth LAS Annual Meeting, IEEE CAT., Vol. 1, 2. October 2005 (2005-10-02). [0006]

Claims (13)

A method for operating an electronic power converter (1), the converter (1) comprising: a primary bridge circuit (23) which is arranged to at least one primary intermediate circuit voltage V _p from a primary side of the converter (1), or the inverted primary intermediate circuit voltage ―V _p , to be fed to a primary side of a transformer (3), and a secondary bridge circuit (43) which is arranged to supply at least one secondary intermediate circuit voltage V _s from a secondary side of the converter (1), or the inverted secondary intermediate circuit voltage ―V _s , to a secondary side of the transformer (3), wherein the secondary bridge circuit (43) comprises two half-bridges, each half-bridge comprising an upper and a lower switching unit, characterized in that the method comprises the following steps to flow electrical power from the primary side to the secondary side of the converter (1) to control: • Determine an operating point of the Wan dlers (1), where the operating point is a function of the primary intermediate circuit voltage V _p , the secondary intermediate circuit voltage V _s and the electrical power P to be transferred from the primary side to the secondary side, • Determining, from the operating point, a selected line mode for the operation of the converter (1), wherein the selected line mode is one of at least three line modes, • for the selected line mode, determining a duty cycle value d and a phase shift value g, • for the selected line mode, controlling the primary bridge circuit (23) to alternately supply positive and negative voltage pulses to the primary side of the transformer (3), the voltage pulses having a duty cycle in relation to a switching period according to the duty cycle value d, • for the selected line mode, controlling the secondary bridge circuit (43) by actively switching only two of the Switching units of the secondary bridge circuit (43), to alternately supply positive and negative voltage pulses to the secondary side of the transformer (3), the voltage pulses having a phase shift in relation to the voltage pulses which are supplied to the primary side of the transformer (3) according to the phase shift value g. Procedure according to Claim 1 wherein • the step of controlling the secondary bridge circuit (43) comprises that in a first and a second of the two actively switched switching units of the secondary bridge circuit (43) in each switching period after the phase shift, the first is switched on and the second switched off and then, • after a period of up to half the switching period, the first is switched off and the second is switched on (43). Procedure according to Claim 2 , the time period being equal to half the switching period. Procedure according to Claim 1 or Claim 2 or Claim 3 , wherein the step of controlling the secondary bridge circuit (43) comprises: • either active switching of only lower switching units (49b) of the secondary bridge circuit (43), with upper switching units (49a, 49c) only working as diodes, • or active switching of only upper switching units (49a) of the secondary bridge circuit (43), lower switching units (49b) only working as diodes. Procedure according to Claim 1 or Claim 2 or Claim 3 wherein the step of controlling the secondary bridge circuit (43) comprises: • active switching of only upper and lower switching units of one of the half bridges (41; 42), wherein upper and lower switching units of the other half bridge (42; 41) only operate as diodes . Method according to one of the preceding claims, wherein a start of a positive or negative voltage pulse, which is applied to the secondary side of the transformer (3), is determined by the two switching units which are switched to active; an end of a positive or negative voltage pulse, which is applied to the secondary side of the transformer (3), is determined by one of the switching units, which only work as diodes and switch to a blocking state due to a current flowing through the corresponding switching unit, whereby its direction is reversed. The method according to any one of the preceding claims, wherein the at least three conduction modes comprise: • a continuous conduction mode (CCM), in which ◯ the duty cycle of the primary side is varied according to the duty cycle value d depending on the operating point if V _s '<V _p , and remains constant at 50% if V _s '> V _p ; in which $V_s^{\text{'}}=\frac{N_p}{N_s}{V}_s$

is the secondary intermediate circuit voltage, which is related to the primary side, • a first discontinuous conduction mode (DCM1) in which ◯ the duty cycle of the primary side is varied according to the duty cycle value d depending on the operating point, • a second discontinuous conduction mode (DCM2) in which ◯ the duty cycle of the primary side remains constant at 50% according to the duty cycle value d. Procedure according to Claim 7 , whereby the limits between the line modes are determined by the following rules depending on the operating point: • if V _s '<V _p and the power P is less than $P_{\text{Max},\text{dcm}1}=\frac{{\text{V}}_{\text{s}}^{\text{'}}\left({\text{V}}_{\text{s}}^{\text{'}}-\frac{{\text{V}}_{\text{s}}{\text{'}}^2}{{\text{V}}_{\text{p}}}\right)}{\mathrm{4th}{\text{L.}}_{\text{s}}f}$

then the first discontinuous conduction mode (DCM1) is applied; • if V _s '> V _p and the power P less than $P_{\text{Max},\text{dcm}2}=\frac{{\text{V}}_{\text{p}}^2\left({\text{V}}_{\text{s}}^{\text{'}}-{\text{V}}_{\text{p}}\right)}{\mathrm{4th}{\mathrm{L.}}_s{\text{V}}_{\text{s}}^{\text{'}}f}$

then the second discontinuous conduction mode (DCM2) is applied; otherwise continuous conduction mode (CCM) is used; where V _{p is} the primary intermediate circuit voltage, V _s ' is the secondary intermediate circuit voltage, which is related to the primary side, $V_s^{\text{'}}=\frac{N_p}{N_s}{V}_s$

where Np is the number of turns of the primary winding, Ns is the number of turns of the secondary winding, f is the switching frequency and Ls is the leakage inductance related to the primary and absolute values for P are used. Procedure according to Claim 7 or 8th , comprising the following step: calculating, for each conduction mode, the parameters d and g in order to minimize an objective function, wherein the objective function is in particular a transformer peak current or wherein the objective function is a transformer effective current (RMS current). Method according to one of the preceding claims, wherein in each of the switching modes the parameters d and g are determined as a function of the power P which is transferred from the primary side to the secondary side, the primary intermediate circuit voltage V _p being the secondary intermediate circuit voltage V _s , possibly related to the primary side V _s '. A method according to any one of the preceding claims, wherein the converter (1) is a dual active bridge converter, i. H. both the primary circuit (2) and the secondary circuit (4) are full bridge inverters. Electronic power converter (1), wherein the converter (1) comprises a control unit (5), wherein the control unit (5) comprises analog and / or digital signal processing units which are configured to carry out the method according to one of the preceding claims. Electronic power converter (1), the converter (1) comprising: a primary bridge circuit (23) which is arranged to at least one primary intermediate circuit voltage V _p (or input DC voltage) from a primary side of the converter (1), or the inverted primary intermediate circuit voltage ―V _p , to be fed to a primary side of a transformer (3), and a secondary bridge circuit (43) which is arranged to at least one secondary intermediate circuit voltage V _s (DC output voltage) from a secondary side of the converter (1), or the inverted secondary intermediate circuit voltage ―V _s to be fed to a secondary side of the transformer (3), the secondary circuit (4) comprising: • either that active switching units (49b) of the secondary bridge circuit (43) are only present in the lower half of each half bridge and one each corresponding bridge center with the negative Connect output terminal (14), with upper switching units (49c) connecting the corresponding bridge center point to the positive output terminal (13), which only comprises diodes, • or that active switching units (49a) of the secondary bridge circuit (43) only in the upper half of each half bridge are present and each connect a corresponding bridge center point to the positive output connection (13), with lower switching units connecting the corresponding bridge center point to the negative output connection (14), which only comprises diodes.

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