WO2020114649A1

WO2020114649A1 - Bidirectional dc/dc converter and method for operating the dc/dc converter

Info

Publication number: WO2020114649A1
Application number: PCT/EP2019/076995
Authority: WO
Inventors: Jan Riedel; David CELLO; Christoph Kienzler; Christian Winter
Original assignee: Robert Bosch Gmbh
Priority date: 2018-12-07
Filing date: 2019-10-07
Publication date: 2020-06-11
Also published as: DE102018221195A1

Abstract

The invention relates to a bidirectional DC/DC converter for transmitting energy between a primary side (HV) and a secondary side (LV), comprising connections for a primary energy accumulator (UHV) and a secondary energy accumulator (UNV), one or more transformers (1) for galvanically isolating the primary side (HV) from the secondary side (LV), switching elements (D1 to D4, M1 to M4) for connecting and reversing the polarity of the windings of the transformer (1) on the primary side and on the secondary side, a controller (2) for controlling the switching elements (D1 to D4, M1 to M4), a secondary-side series inductor (W1), and a blocking switching element (S1) which is connected parallel to the secondary-side series inductor (W1).

Description

description

title

Bidirectional DC / DC converter and method for operating the DC / DC

Converter

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 an internal combustion engine

Alternator. The high-voltage battery, on the other hand, must be regularly recharged or possibly replaced at petrol stations.

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 that High-voltage network must be voltage-free; for this in particular the

The intermediate circuit capacitor, which is connected in parallel to the high-voltage battery, can 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. In the prior art is therefore a charging device for the

DC link capacitor provided with energy from the battery to be connected. 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.

A DC-DC converter is known from WO 2017/125204 A1, 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 one on the primary side

DC link 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 only takes place 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 at a high frequency (a few kHz) and reverse the polarity. On the energy-receiving side, diodes act as rectifiers for the transmitted current pulses

(Synchronous rectifier), however, if the DC / DC converter is operated bidirectionally, diodes must be used that can also be switched if required.

A secondary series inductance is used in normal operation, i.e. at

Energy transfer from the primary to the secondary, to that

Smooth current impulses on the secondary side.

In order to be able to control and charge a current-limited intermediate circuit capacitor from the secondary energy store, a blocking switching element is provided which is parallel to the secondary side

Series inductance is switched. 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 charging circuit for charging an intermediate circuit capacitor on the primary side from one

Primary energy storage is more needed, which in particular requires an additional complex mechanical switch on the primary side; rather, the intermediate circuit capacitor can be controlled by suitably controlling the

secondary switching elements of the converter, starting from the voltage zero to be charged to its setpoint before the primary energy storage device 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, such a modified DC / DC converter can

primary intermediate circuit capacitor with disconnected

Charge the primary energy store from the secondary-side secondary energy store to any desired voltage.

Types of the converter bring further advantages.

The DC / DC converter with a blocking switching element, which is connected in parallel with 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 wired 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. With the blocking switching element opened in this way, reverse operation of the DC / DC converter is one

Energy transfer from the secondary side to the primary side, 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-side winding to the secondary-side winding of the transformer results. 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, even with the smallest primary voltages and can also be operated at 0 volts in reverse mode.

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 are

Damping capacitors are provided, which are connected in parallel to the connections for the secondary energy store. Or it is a series connection of a damping resistor and a damping capacitor, which is parallel to the series connection of series inductance and to the connections for the

Secondary energy storage is provided. These

Circuit additions are used for the advantageous smoothing of voltage peaks in the switching operations on the secondary side.

A second aspect of the invention relates to the method for operating a bidirectional DC / DC converter in reverse operation, the

Locking switching element is closed and thus the secondary side

Series inductance is 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 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.

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 first and second

Voltage limit to provide a hysteresis, if necessary, that frequent switching is avoided. The first is preferred

Voltage limit is 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 switches and transformer currents, more complex controls such as the "three-level" or "triple-phase shift" control can be used, which results from the control of dual active bridge DC / DC Is known to walkers. During the high-frequency AC component of the secondary side

Full bridge switch currents from that by means of the lock switch

short-circuited secondary-side inductance is not conducted, since the impedance is much larger than that of the closed blocking switching element, the DC component can partially from the short-circuited

secondary inductance are taken over. 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

becomes negligible. The smoothing of the output current by the

secondary-side inductance reduces the in these operating areas RMS values of the converter currents (transformer and switch currents). 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.

Brief description of the drawings

FIG. 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;

FIG. 2 shows a schematic illustration of a blocking switching element;

FIG. 3 schematically represents a process flow diagram for operating the bidirectional DC / DC converter.

Figure 1 shows the circuit of a single-phase phase-shifted full-bridge (PSFB) DC / DC converter, which is a possible converter type, in which

Modifications, which are described in detail below, an operation of the bidirectional DC / DC converter and thus a boost operation from the primary-side voltage zero volts and an energy transfer from the secondary energy store are possible. However, the modifications can be used on any flow converter with galvanic isolation and current-fed primary intermediate circuit, e.g. with push-pull converters or multilevel converters.

The PSFB converter shown in FIG. 1 has a transformer 1 which is fed in normal operation from the primary energy store UHV, preferably a high-voltage battery, connected to the primary-side connection HV. Two half bridges with the switching elements M1 to M4 arranged on the primary side switch this voltage with a clock frequency of a few kHz with alternating sign to the primary winding of the transformer 1, as a result of which the 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 Ml 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 storage and that of the secondary energy storage determined. A resonance coil LR ES on the primary and / or secondary side of the transformer 1 preferably ensures a smooth switching of the switching elements, so that their switching power loss is minimized.

An induction voltage is generated on the secondary side of the transformer 1, preferably in the low-voltage range, which is rectified by the passive diodes D1 to D4. The induction pulse is preferably applied to the capacitor C2 and preferably the secondary side via the series inductance W1

Connected LV lines to which the secondary energy storage UNV and preferably the secondary-side consumers of the low-voltage circuit are connected. The series inductance W1 serves 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 Ml to M4 on the primary side and Dl to D4 on the secondary side on the low voltage side are referred to both as "diodes" and "switches", depending on whether the focus is on the current function of the circuit the transition between the conductive and the non-conductive state is determined passively by the sign of the applied voltage, or that this transition is predetermined by the control device 2 by active switching at certain times. However, the same terms are always to be understood under the two terms.

Preferred for certain repair and maintenance situations, but also when the vehicle is normally off, e.g. at an electrical or

Hybrid vehicle, the primary energy storage UHV, in particular a high-voltage battery, must be disconnected, and the primary side HV

be stress-free; the intermediate circuit capacitor CZK must be discharged for this. Would the UHV primary energy storage, especially the high-voltage battery, suddenly return to the primary side, preferably the high-voltage network,

would be connected, especially through the

Recharging the intermediate circuit capacitor CZK, so high and so rapidly rising currents flow that at least some components exceed the permissible values and these components are at risk.

The converter described so far, the in terms of input and output in

Is essentially symmetrical, should now be modified so that it works bidirectionally and a boost operation with the smallest primary

Voltages and even at 0 volts. The step-up converter is preferably able to charge the intermediate circuit capacitor CZK with secondary-side energy from the low-voltage battery UNV. It will be a special one

Charging device superfluous, which in the prior art for this purpose energy from the high-voltage battery to be connected to the UHV

DC link 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 realized using conventional technology, i.e. wired or integrated in a printed circuit board using planar technology.

Short-circuiting the series inductance W1 is only necessary for low primary voltages. Therefore, the blocking switching element S1 is provided in parallel to the secondary-side series inductance W1, which by the

Control device 2 of this first phase is closed only 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 S1 is preferably open, and the modification of the circuit by the blocking switching element S1 is then ineffective.

The circuit shown in Figure 1 shows still 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 store are preferably provided with a series connection of a damping capacitor CS and a damping resistor RS. Smooth these components

Voltage peaks in the switching operations with the secondary side

Switching elements Dl to D4. Further smoothing capacitors CFB are connected in parallel to the connections for the secondary energy storage unit UNV.

Figure 2 shows a schematic representation of a blocking switching element S1, preferably a bidirectionally blocking switching element. For example, two semiconductor switches 210, 220 are arranged such that their gate connections 212, 222 are connected and form a first connection 230 of the blocking switching element S1. The source connections 214, 224 of the two semiconductor switches 210, 220 are also connected and form a second connection 240 of the

Lock switching element Sl off. As a result, the intrinsic body diodes of the semiconductor switches 210, 220 face each other, so that a current flow in both directions through the blocking switching element S1 can be prevented.

FIG. 3 schematically represents a process flow diagram for operating the bidirectional DC / DC converter. The individual steps of the method 100 are described in FIG. 3 for the operation of the DC / DC converter

Control device 2 executed. The method 100 starts with step 10. In step 20, the primary-side voltage, which is preferably applied to the

Connections for the primary energy storage UHV are present, recorded, for example by means of a voltage measurement or by reading out physical quantities already recorded in the system, from which the voltage can be derived. In step 30, the voltage is compared to a first and / or a second voltage limit. If the voltage on the primary side falls below a predeterminable first voltage limit value, the method branches to step 40. In step 40, the blocking switching element S1 is closed. If the voltage on the primary side does not fall below a predeterminable second voltage limit value, the method branches to step 50. The first is preferred

Voltage limit is less than the second voltage limit. The first and second voltage limit values can preferably also be identical. To Executing step 40 or 50 branches the method back to step 20 by detecting the primary-side voltage.

Claims

Expectations

1. 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 (Dl to D4, Ml 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 (Dl to D4, Ml to M4);

• and with a secondary series inductance (Wl);

• and with a blocking switching element (Sl), which is parallel to

secondary series inductance (Wl) is switched.

2. Bidirectional DC / DC converter according to claim 1, wherein the converter is a flow converter with electrical isolation and current-fed

Is secondary.

3. Bidirectional DC / DC converter according to claim 1 or 2, 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.

4. Bidirectional DC / DC converter according to claim 1, wherein the transformer (1), the switching elements (Dl to D4, Ml to M4) and the control device (2) as a single-phase phase-shifted full bridge (PSFB) DC / DC converters are designed for hybrid and electric vehicles.

5. Bidirectional DC / DC converter according to one of claims 1 to 4, wherein the blocking switching element (Sl) is a bidirectionally blocking switching element.

6. Bidirectional DC / DC converter according to one of claims 1 to 5, wherein the blocking switching element (Sl) comprises two semiconductor switches, the

Gate connections are connected and a first connection of the

Form blocking switching element (Sl) and their source connections are connected and form a second connection of the blocking switching element (Sl).

7. Bidirectional DC / DC converter according to one of 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).

8. Bidirectional DC / DC converter according to one of claims 1 to 7, with a series connection of a damping resistor (RS) and one

Damping capacitor (CS) in parallel with the series connection of

Series inductance (Wl) and the connections for the

Secondary energy storage (UNV) is switched.

9. The method (100) for operating a bidirectional DC / DC converter according to one of claims 1 to 8 in reverse operation, wherein the

Locking switching element (Sl) is closed and thus the secondary series inductance (Wl) is short-circuited.

10. The method (100) for operating a bidirectional DC / DC converter according to 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.