DE102018221195A1 - Bidirectional DC / DC converter and method for operating the DC / DC converter - Google Patents

Abstract

Bidirectional DC / DC converter for energy transmission between a primary side (HV) and a secondary side (LV) with connections for a primary energy storage (UHV) and a secondary energy storage (UNV), with one or more transformers (1) for galvanic isolation of the primary side (HV ) from the secondary side (LV), with switching elements (D1 to D4, M1 to M4), for connecting and reversing the polarity of the windings of the transformer (1) on the primary and secondary side, with a control device (2) for controlling the switching elements (D1 to D4, M1 to M4); and with a secondary series inductance (W1); and with a blocking switching element (S1) which is connected in parallel with the secondary-side series inductance (W1).

Description

The invention relates to a bidirectional DC / DC converter which has a supplement which enables improved operation of the DC / DC converter, and also comprises a method for operating the DC / DC converter.

State of the art

DC / DC converters are powered by a DC voltage source and provide electrical energy to a consumer as DC voltage at a different voltage level. For example, electrical energy is transferred from a high-voltage network to a low-voltage network and converted to a voltage level of the low-voltage network.

For example, in electric or hybrid vehicles, the drive motor is operated from the high-voltage network with a voltage of several 100 volts, while the low-voltage on-board network has a voltage of mostly 12 volts, occasionally also 24 or 48 volts. Both networks each have a battery and are connected to each other via a DC / DC converter, which contributes to the stability of the overall system. The low-voltage battery is regularly charged from the high-voltage network via the DC / DC converter, similar to the battery in a car with a combustion engine using an alternator. The high-voltage battery, on the other hand, has to be regularly recharged at petrol stations or possibly replaced.

In certain repair and maintenance situations, but also when the vehicle is in the normal off state, the high-voltage battery must be disconnected and the high-voltage network must be voltage-free; for this, the intermediate circuit capacitor, which is connected in parallel to the high-voltage battery, must be discharged.

If the high-voltage battery were later suddenly reconnected to the high-voltage network, potentially high and rapidly increasing currents would flow, in particular by recharging the intermediate circuit capacitor. A charging device for the intermediate circuit capacitor with energy from the battery to be connected is therefore provided in the prior art. This charging device comprises an ohmic charging resistor and a mechanical switch. After switching on the switch, a charging current flows through the charging resistor to the intermediate circuit capacitor, and only when this is charged can the main connection of the battery to the intermediate circuit capacitor be switched, which bridges the charging resistor.

From the WO 2017/125204 A1 a DC-DC converter is known which, under certain boundary conditions, can also be used for power transfer from the secondary to the primary side if active switching elements are used on the secondary side. For low voltages, additional power electronic components are required for the power transfer in the reverse direction. For this purpose, an additional magnetic component with circuitry is preferably used.

Disclosure of the Invention and Advantages of the Invention

A first aspect of the invention is directed to the hardware circuitry of the converter. The DC / DC converter according to the invention for energy transmission between a primary side and a secondary side has connections for a primary energy store and a secondary energy store. The DC / DC converter preferably comprises on the primary side an intermediate circuit capacitor which is connected on the primary side to the connections for the primary energy store. One or more transformers ensure the galvanic separation of the primary side from the secondary side in such a way that an energy transfer takes place only via the inductive coupling between the transformer coils. The transformer coils can be supplied with current pulses from the corresponding energy store, primary energy store or secondary energy store on the energy-emitting side in that diodes designed as switching elements connect them to the energy store by means of a control device with high frequency (a few kHz) and reverse the polarity. On the energy-receiving side, diodes act as rectifiers for the transmitted current pulses (synchronous rectifiers), but if the DC / DC converter is operated bidirectionally, diodes must be used that can also be switched if necessary.

A secondary series inductance is used in normal operation, i.e. in the case of energy transmission from the primary side to the secondary side, to smooth the current pulses on the secondary side.

In order to be able to control a primary-side intermediate circuit capacitor from the secondary energy store and charge it in a current-limited manner, a blocking switching element is provided, which is connected in parallel with the secondary-side series inductance. The term blocking switching element refers to the ability of the blocking switching element to be able to block bidirectionally. For this purpose, a blocking switching element is provided, which is connected in parallel to the series inductance on the secondary side and short-circuits the series inductance on the secondary side in the closed state.

A DC / DC converter supplemented in this way has the advantage that improved reverse operation is made possible and, for example, no longer a charging circuit for charging an intermediate circuit capacitor on the primary side from a primary energy store is required, which in particular requires an additional complex mechanical switch on the primary side; rather, the intermediate circuit capacitor can be charged to its setpoint value from zero voltage by suitably controlling the switching elements of the converter on the secondary side, before the primary energy store is connected. The losses that otherwise occur in the charging resistor of a charging circuit according to the prior art are also eliminated, which increases the efficiency.

But also for other applications, for example for certain function tests, a DC / DC converter modified in this way can charge a primary-side intermediate circuit capacitor to any desired voltage from the secondary-side secondary energy store when the primary energy store is disconnected.

Types of the converter bring further advantages.

The DC / DC converter with a blocking switching element, which is connected in parallel to the series inductance on the secondary side, can be a forward converter with electrical isolation of the primary and secondary side and with a current-supplied secondary side. For example, it can be a single-phase or multi-phase phase-shifted full-bridge (PSFB) converter, a push-pull converter, resonance converter or a multilevel converter. In particular, the transformer, the switching elements and the control device can also be connected and operated in such a way that the converter is designed as a single-phase phase-shifted full bridge (PSFB) DC / DC converter, preferably for hybrid and electric vehicles.

It is an advantage of the invention that it can be used universally in all of these types of converters.

In another embodiment of the invention, the blocking switching element is a bidirectionally blocking switching element. The blocking switching element can thus prevent current flow in both directions. When the blocking switching element is opened in this way, reverse operation of the DC / DC converter, i.e. energy transfer from the secondary side to the primary side, is only possible if the primary-side voltage is greater than a specific voltage value, which is the product of the voltage of the secondary energy storage and the quotient from the turns ratio of the primary winding to the secondary winding of the transformer. With the blocking switching element closed, reverse operation of the DC / DC converter is also possible if the primary-side voltage is less than this specific voltage value. A DC / DC converter is advantageously provided, which can be operated in reverse mode even with the smallest primary-side voltages and also at 0 volts.

In another embodiment of the invention, the blocking switching element comprises two semiconductor switches, whose gate connections are connected and form a first connection of the blocking switching element and whose source connections are connected and form a second connection of the blocking switching element. The bidirectionally lockable blocking switching element is formed from two semiconductor switches arranged in such a way that the two intrinsic free-wheeling diodes are aligned with one another. A topology for a bidirectionally lockable switching element is advantageously provided, which can be implemented in the converter circuit using available components.

In another embodiment of the invention, one or more damping capacitors are provided which are connected in parallel to the connections for the secondary energy store. Or there is a series connection of a damping resistor and a damping capacitor, which is connected in parallel to the series connection of series inductance and to the connections for the secondary energy store. These circuit supplements are used for the advantageous smoothing of voltage peaks in the secondary switching operations.

A second aspect of the invention relates to the method for operating a bidirectional DC / DC converter in reverse operation, the blocking switching element being closed and thus the secondary series inductor being short-circuited.

The advantage here is that when the blocking switching element is closed, reverse operation of the DC / DC converter is made possible, even with the smallest primary-side voltages and also at 0 volts. In this boost mode, a primary-side intermediate circuit capacitor can preferably be charged to practically any desired voltage.

• - Schließen des Sperr-Schaltelement, solange die primärseitige Spannung einen vorgebbaren ersten Spannungsgrenzwert unterschreitet;
• - Öffnen des Sperr-Schaltelementes, falls die die primärseitige Spannung einen vorgebbaren zweiten Spannungsgrenzwert nicht unterschreitet.

Es wird ein Verfahren bereitestellt, welches den Boostbetrieb bei kleinsten primärseitigen Spannungen und auch bei 0 Volt ermöglicht. Der erste und der zweite Spannungsgrenzwert korrelieren mit dem oben beschriebenen spezifischen Spannungswert. Der erste und der zweite Spannungsgrenzwert wird dem Verfahren entweder vorgegeben oder online während des Betriebs des DC/DC Wandlers ermittelt und in Abhängigkeit der sekundärseitigen Spannung vorgegeben. Inbesondere gibt es einen ersten und einen zweiten Spannungsgrenzwert um gegebenenfalls eine Hysterese vorzusehen, dass ein häufiges Umschalten vermieden wird. Hierzu ist bevorzugt der erste Spannungsgrenzwert kleiner als der zweite Spannungsgrenzwert. Bevorzugt kann der erste und der zweite Spannungsgrenzwert auch identisch sein.In another embodiment, the method for operating a bidirectional DC / DC converter in reverse operation comprises the following steps:

• - Closing the blocking switching element as long as the primary-side voltage falls below a predeterminable first voltage limit value;
• - Opening the blocking switching element if the primary voltage does not fall below a predeterminable second voltage limit.

A method is provided which enables the boost operation at the smallest primary-side voltages and also at 0 volts. The first and second voltage thresholds correlate with the specific voltage value described above. The first and second voltage limit values are either predefined for the method or determined online during the operation of the DC / DC converter and predefined as a function of the secondary-side voltage. In particular, there are a first and a second voltage limit value in order, if necessary, to provide a hysteresis that frequent switching is avoided. For this purpose, the first voltage limit is preferably less than the second voltage limit. The first and second voltage limit values can preferably also be identical.

When the blocking switching element is closed, a DC / DC converter results, which can transmit power bidirectionally regardless of the primary and secondary voltage, so that the power transfer in the reverse direction is possible. At least two of the four half bridges are actively controlled to transmit the power. To optimize the RMS values of the switch and transformer currents, more complex controls such as the "three-level" or "triple-phase shift" control can be used, which are derived from the control of dual active bridge DC / DC Is known to walkers. While the high-frequency alternating current component of the secondary-side full bridge switch currents is not conducted by the secondary-side inductance short-circuited by means of the blocking switch, since the impedance is very much greater than that of the closed blocking switching element, the direct-current component can in part be taken over by the short-circuited secondary-side inductor. As a result, the blocking switching element does not have to be designed for the effective value of the secondary-side current. The blocking switching element is preferably opened in all other operating areas of the converter, so that the influence on the circuit is negligible. The smoothing of the output current by the secondary inductance reduces the effective values of the converter currents (transformer and switch currents) in these operating ranges. This improves efficiency and increases the maximum output power.

It is also advantageous if the control device can work with pulses of a fixed frequency.

Figure list

• 1 shows the circuit of a single-phase phase-shifted full bridge (PSFB) DC / DC converter with the additions provided for operating the bidirectional DC / DC converter;
• 2nd shows a schematic representation of a blocking switching element;
• 3rd schematically represents a process flow diagram for the operation of the bidirectional DC / DC converter.

1 shows the circuit of a single-phase phase-shifted full-bridge (PSFB) DC / DC converter, which is a possible converter type, in which, by means of modifications, which are described in detail below, an operation of the bidirectional DC / DC converter and so that a boost operation from the primary voltage zero volts and an energy transfer from the secondary energy storage is possible. However, the modifications can be used on any flow converter with galvanic isolation and current-fed primary intermediate circuit, for example in push-pull converters or multilevel converters.

The in 1 PSFB converter shown has a transformer 1 on that in normal operation from that on the primary side connection HV connected primary energy storage UHV , preferably a high-voltage battery, is fed. Two half bridges with the switching elements arranged on the primary side M1 to M4 switch this voltage to the primary winding of the transformer with an alternating sign with a clock frequency of a few kHz 1 , whereby its core, periodically alternating, is magnetically charged. By shifting the switch-on times of the second half-bridge with the switching elements M2 and M4 compared to those of the first half bridge with the switching elements M1 and M3 the relative duration of the alternating voltage pulses is changed. The duration of the required voltage pulses is essentially determined by the ratio of the voltage of the primary energy store and that of the secondary energy store. A resonance coil LRES preferably provides the primary and / or secondary side of the transformer 1 for smooth switching of the switching elements so that their switching power loss is minimized.

Secondary side of the transformer 1 , preferably in the low-voltage range, an induction voltage is generated by the passive diodes D1 to D4 is rectified. The induction shock is about the series inductance W1 preferably on the capacitor C2 and preferably the secondary connections LV to which the Secondary energy storage UNV and preferably the secondary-side consumers of the low-voltage circuit are connected. The series inductance W1 is used to smooth the output current. In normal operation, the PSFB converter works as a buck converter.

It should be noted here that the switching elements M1 to M4 on the primary side and on the secondary side D1 to D4 on the low voltage side are referred to as "diodes" as well as "switches", depending on whether it is in the foreground that in the current function of the circuit the transition between the conductive and the non-conductive state is passively unsigned the applied voltage is determined, or that this transition by active switching at certain times by the control device 2nd is specified. However, the same terms are always to be understood under the two terms.

The primary energy store must be preferred in certain repair and maintenance situations, but also in the normal off state of the vehicle, for example in the case of an electric or hybrid vehicle UHV , in particular a high-voltage battery, are disconnected, and the primary side HV must be free of tension; the DC link capacitor must be used for this CZK be discharged.

Would later become the primary energy store UHV , in particular the high-voltage battery, are suddenly reconnected to the primary side, preferably the high-voltage network, in this case, in particular by recharging the intermediate circuit capacitor CZK , so high and rapidly increasing currents flow that at least some components exceed the permissible values and these components are at risk.

The converter described so far, which is essentially symmetrical in terms of input and output, is now to be modified in such a way that it works bidirectionally and enables boost operation at the smallest primary-side voltages and also at 0 volts. The step-up converter is preferably capable of the intermediate circuit capacitor CZK with secondary-side energy from the low-voltage battery UNV charge. This eliminates the need for a special charging device that uses energy from the high-voltage battery to be connected in the prior art for this purpose UHV to the intermediate circuit capacitor CZK transmits. For this purpose, the converter described is by means of the blocking switching element

For this purpose, the DC / DC converter is operated like a dual active bridge DC / DC converter when the blocking switching element is closed.

The series inductance W1 can be implemented using conventional technology, i.e. wired or integrated in a printed circuit board using planar technology.

Shorting the series inductance W1 is only necessary for low primary voltages. Therefore, the lock switch element S1 parallel to the series inductance on the secondary side W1 provided that by the control device 2nd this first phase is only closed for as long as the primary-side voltage is less than a predeterminable first voltage limit value; in all other operating states of the converter, the blocking switching element is preferred S1 opened, and then the modification of the circuit by the blocking switching element S1 ineffective.

In the 1 The circuit shown shows further modifications for the safe operation of the DC / DC converter. Parallel to the series connection of series inductance W1 and the connections for a secondary energy storage device are preferably the series connection of a damping capacitor CS and a damping resistor RS intended. These components smooth out voltage peaks during the switching processes with the switching elements on the secondary side D1 to D4 . More smoothing capacitors CFB are parallel to the connections for the secondary energy storage UNV switched.

2nd shows a schematic representation of a blocking switching element S1 , preferably a bidirectionally lockable switching element. For example, there are two semiconductor switches 210 , 220 arranged so that their gate connections 212 , 222 are connected and a first connection 230 of the blocking switching element S1 form. The source connections 214 , 224 of the two semiconductor switches 210 , 220 are also connected and form a second connection 240 of the blocking switching element S1 out. This creates the intrinsic body diodes of the semiconductor switches 210 , 220 opposite so that current flows in both directions through the blocking switching element S1 can be prevented.

3rd schematically illustrates a process flow diagram for operating the bidirectional DC / DC converter. The individual steps of the method 100 are used for the operation of the DC / DC converter by the control device 2nd executed. With step 10th starts the process 100 . In step 20th becomes the primary-side voltage, which is preferred at the connections for the primary energy storage UHV is present, recorded, for example by means of a voltage measurement or by reading physical quantities already recorded in the system, from which the voltage can be derived. In step 30th the voltage is compared with a first and / or a second voltage limit. If the primary voltage is one The method branches to step below the predeterminable first voltage limit value 40 . In step 40 becomes the blocking switching element S1 closed. If the voltage on the primary side does not fall below a predeterminable second voltage limit value, the method branches to step 50 . In step 50 becomes the blocking switching element S1 open. For this purpose, the first voltage limit is preferably less than the second voltage limit. The first and second voltage limit values can preferably also be identical. After completing the step 40 or 50 the method branches back to step 20th by detecting the primary side voltage.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included 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

• WO 2017/125204 A1 [0006]

Claims (10)

Bidirectional DC / DC converter for energy transmission between a primary side (HV) and a secondary side (LV) with connections for a primary energy store (UHV) and a secondary energy store (UNV), With one or more transformers (1) for galvanically separating the primary side (HV) from the secondary side (LV), With switching elements (D1 to D4, M1 to M4), for connecting and reversing the polarity of the windings of the transformer (1) on the primary and secondary sides, • with a control device (2) for controlling the switching elements (D1 to D4, M1 to M4); • and with a secondary series inductance (W1); • and with a blocking switching element (S1), which is connected in parallel to the secondary series inductance (W1). Bidirectional DC / DC converter after Claim 1 , wherein the converter is a flow converter with galvanic isolation and powered secondary side. Bidirectional DC / DC converter after Claim 1 or 2nd , wherein the converter is designed as a single-phase or multi-phase phase-shifted full bridge (PSFB) converter, as a push-pull converter or as a multilevel converter. Bidirectional DC / DC converter after Claim 1 , in which the transformer (1), the switching elements (D1 to D4, M1 to M4) and the control device (2) are designed as a single-phase phase-shifted full bridge (PSFB) DC / DC converter for hybrid and electric vehicles . Bidirectional DC / DC converter according to one of the Claims 1 to 4th , wherein the blocking switching element (S1) is a bidirectionally blocking switching element. Bidirectional DC / DC converter according to one of the Claims 1 to 5 , wherein the blocking switching element (S1) comprises two semiconductor switches, whose gate connections are connected and form a first connection of the blocking switching element (S1) and whose source connections are connected and form a second connection of the blocking switching element (S1). Bidirectional DC / DC converter according to one of the Claims 1 to 6 , with one or more parallel smoothing capacitors (CFB), which are connected in parallel to the connections for the secondary energy storage (UNV). Bidirectional DC / DC converter according to one of the Claims 1 to 7 , with a series connection of a damping resistor (RS) and a damping capacitor (CS), which is connected in parallel to the series connection of series inductance (W1) and the connections for the secondary energy storage (UNV). Method (100) for operating a bidirectional DC / DC converter according to one of the Claims 1 to 8th in reverse operation, the blocking switching element (S1) being closed and thus the secondary series inductance (W1) being short-circuited. Method (100) for operating a bidirectional DC / DC converter Claim 9 , wherein the blocking switching element (S1) is closed as long as the primary-side voltage falls below a predeterminable first voltage limit value and is opened if the primary-side voltage does not fall below a predeterminable second voltage limit value.

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