Inverter

Abstract

An inverter (1) with galvanic separation including a resonant converter (2) and an upstream mounted boost chopper (3) is intended to provide galvanic separation in the context of a variable input and output voltage as it exists in photovoltaic systems, with the efficiency being intended to be optimized over the entire input voltage range. This is achieved in that a boost chopper (3) or a buck chopper (4) is mounted upstream of the resonant converter (2).

Description

The invention relates to an inverter as set forth in the preamble of claim 1.

An input converter for railbound vehicles is known from DE 198 27 872. It consists of a resonant converter with galvanic separation. Input/output voltage ratio is fixed. In loss optimized operation, the fixed input/output voltage ratio is determined by the transformation ratio of the transformer.

Due to varying ambiant conditions (solarization, temperature, . . . ) and different generator designs, inverters in photovoltaic systems must be designed for a wide input voltage range and at the same time be highly efficient over this entire range.

It is the object of the invention to provide an inverter which, in the context of a wide input voltage range as it is given in photovoltaic systems and of variable mains voltage, provides galvanic separation, with the efficiency being intended to be optimized over the entire input voltage range.

This object is solved by an inverter having the characterizing features of claim 1 in connection with the features recited in the preamble.

The invention enables optimum efficiency in operation of the inverters in the case of varying solarization and temperature of the photovoltaic modules or of different photovoltaic module configurations. The switching losses are minimized by the fact that the resonant converter operates independently of the input voltage in a constant operating point, this operating point being set over a wide input voltage range by the boost chopper. Provided the voltage at the resonant converter is stabilized, a boost chopper is more beneficial than a buck chopper insofar as the resonant converter achieves higher efficiency at higher stabilized input voltages.

The inverter of the invention combines advantages with respect to voltage adaptation and operation of a HF circuit without the tradeoff of adverse power and voltage ratings, high repetition rate of the adaptation stage, switching overvoltages and limited range of operation.

In an advantageous developed implementation of the converter of the invention, there is provided that the inverter is configured to be a single-phase inverter, with a power electronic half-bridge circuit, a series resonant capacitance and a high frequency transformer being provided. Whilst the resonant converter operates in a constant operating point so that its input/output voltage ratio, which is dictated by the transformation ratio of the transformer, is fixed, the input voltage at the boost chopper may vary.

The boost chopper may advantageously be configured for permanent interval operation so that the free-wheeling diode of the boost chopper will never experience hard switching commutation at turn-off. EMC transients are thus reduced and efficiency is increased as turn-on power losses are avoided.

The boost chopper needs not be activated if input voltage U₀ is high enough for the voltage at the capacitor C_mains to be sufficient for mains electricity supply. As a result, it is activated only if the actual mains voltage is so high that the actual PV voltage is not sufficient to set the voltage at the C_mains.

Efficiency is significantly improved using such a method since the losses are low in operating points without boost chopper operation. This is particularly efficient if the boost chopper reactors are particularly small due to the interval mode design, thus comprising very low ohmic resistances.

Appropriately, the repetition rate of the half-bridge circuit is lower than the resonance frequency. Said resonance frequency is obtained from the leakage inductance of the transformer and the series resonant capacitance. As a result, the semiconductors are zero-current switched, both on and off.

According to another advantageous developed implementation of the invention, there is provided a synchronous activation of the boost chopper and of the resonant converter. Synchronous activation has the advantage to minimize the effective current load in the capacitors (C3, C4).

A particular effect of benefit is obtained if the transformer is configured to be a planar transformer having two printed circuit boards, with the primary winding being disposed on the one printed circuit board and the secondary winding on the other printed circuit board. The advantage thereof is that the printed circuit boards are coupled via a magnetic flux, which allows the expensive plug-and-socket connectors to be eliminated altogether. A cast housing, which is manufacturable at a lower cost than a corresponding sheet metal housing, comprises projections for good integration of such a planar transformer having two overlapping printed circuit boards with regard to cooling.

The leakage inductance of the planar transformer may be beneficially increased by introducing an additional reactor core with an air gap (FIG. 3).

Other advantageous embodiments of the invention are characterized in the dependent claims.

One exemplary embodiment is discussed in closer detail with reference to the drawing, with further advantageous developed implementations of the invention and their advantages being described.

In the drawing:

FIG. 1 shows a schematic diagram of an inverter of the invention,

FIG. 2 shows a voltage/current diagram of the resonant converter,

FIG. 3 shows an illustration of a planar transformer,

FIG. 4 shows a sectional view of the planar transformer; and

FIG. 5 shows an advantageous switching variant of FIG. 1.

In the FIGS., like elements bear the same reference numerals.

FIG. 1 shows an inverter 1 of the invention for photovoltaic systems. It comprises a boost chopper 3, a resonant converter 2 and a mains bridge MB. The resonant converter consists of a half-bridge circuit with the semiconductor switches S1, S2 and a HF transformer HFT, which is provided with a series resonant capacitance or rather a capacitor C1, and of a semiconductor bridge D. Input/output ratio of the resonant converter 2 is fixed and dictated by the transformation ratio of the transformer. Together with the rectifier bridge D, the resonant converter forms an HF circuit.

In accordance with the invention, a boost chopper 3 is mounted upstream of the resonant converter 2. Alternatively, a buck chopper may be mounted upstream of the resonant converter.

The boost chopper serves to adapt the voltage to voltage variations of the photovoltaic generator that may occur as a result of diverse operating conditions (solarization, temperature, . . . ), different generator designs or dynamic adaptations to different mains voltage levels.

The boost chopper consists of a series reactor L, a switching element T3 and a diode D3 that are arranged parallel to the half-bridge circuit and to two series-mounted capacitors C₃, C₄. The capacity of the capacitors C₃, C₄ is thereby greater than the series resonant capacitance (capacitor C₁).

The boost chopper operates in an input voltage range U₀ of 150 to 350 volt. The voltage U₁ applied downstream of the boost chopper is about 350 volt. The voltage U₂ at the output of the resonant converter also is 350 volt. If input voltage is in excess of 350 volt, the boost chopper is not clocked and the voltages U₁ and U₂ increase proportionally with U₀.

The resonant DC/DC converter 2 is operated unidirectionally with the circuit shown in FIG. 1. Other circuit arrangements for bidirectional operation are also possible, though.

The boost chopper 3 and the resonant converter 2 are activated synchronously in order to reduce the effective current load in the capacitors.

FIG. 2 shows the transformer voltage 5 and the transformer current 6 with the cycle duration of T=1/f_switch, wherein f_switch is the switching frequency of the HF circuit.

It is preferred that the repetition rate of the half-bridge circuit be lower than the resonance frequency, which is obtained from the leakage inductance of the transformer and the series resonant capacitance.

The transformer HFT is configured to be a planar transformer 10, as illustrated in the FIGS. 3 and 4. Said transformer comprises a primary winding 11 and a secondary winding 12 that are disposed on one printed circuit board 13 or on two printed circuit boards 14 and 15 (FIG. 4). The printed circuit board is connected to an aluminium cast housing 16 via two insulating heat conducting foils. The line semiconductors are mounted on the printed circuit boards 14 and 15, more specifically as SMD components.

FIG. 5 shows a circuit variant in which the load current conducting capacitor C1 has been eliminated and the resonant capacitance is drawn into the DC circuit in the form of C5 and C6. The capacitors C3 and C4 with quite high capacitance are replaced by the sole capacitor C₁₂. Two smaller resonance capacitors C5, C6 with small capacitance are mounted in parallel thereto. The advantage of this circuit is that on the one side the sum of the capacitor currents is reduced so that the costs of the capacitors are lowered and that on the other side the switching losses are significantly reduced.

In the circuit shown in FIG. 5, the switching losses are reduced over the circuit shown in FIG. 1 since the magnetizing current of the transformer causes the parasitic switch capacitances to be discharged during the recovery time in which the two switches are open with the switch voltage being reduced to the value of the voltage of the respective resonance capacitor minus half the intermediate circuit voltage, as a result thereof, before S1 or S2 are switched on again,.

In the circuit shown in FIG. 1, by contrast, the parasitic switch capacitances of S1 and S2 can only discharge to the value of the resonance capacitance C1 during the recovery time before being switched on again so that the switching losses are accordingly higher.

LIST OF NUMERALS

1 inverter

2 resonant converter

3 boost chopper

4 buck chopper

5 transformer voltage

6 transformer current

10 planar transformer

11 primary winding

12 secondary winding

13-15 printed circuit boards

HFT HF transformer

S1, S2 semiconductor switch

D semiconductor bridge

C_mains capacitor at the input of the mains

bridge

MB mains bridge

Claims

1. An inverter with galvanic separation and with a DC-input, characterized in that, at the DC-input, the inverter comprises a resonant converter (2) having an upstream mounted boost chopper (3) or buck chopper.

2. The inverter as set forth in claim 1, characterized in that one boost chopper (3) only is mounted upstream of the resonant converter.

3. The inverter as set forth in claim 1, characterized in that the inverter is configured to be a single-phase inverter.

4. The inverter as set forth in claim 1, characterized in that the resonant converter comprises a power electronic half-bridge circuit (S1, S2), a series resonant capacitance (C1) and a high frequency transformer (HFT).

5. The inverter as set forth in claim 4, characterized in that the repetition rate of the half-bridge circuit is lower than the resonance frequency which is obtained from the leakage inductance of the transformer and the series resonant capacitance (C1).

6. The inverter as set forth in claim 1, characterized by synchronous activation of the boost chopper (3) and of the resonant converter (2).

7. The inverter as set forth in claim 1, characterized in that the boost chopper (3) is operated in the interval mode.

8. The inverter as set forth in claim 1, characterized in that the boost chopper (3) is operated at a variable repetition rate.

9. The inverter as set forth in claim 1, characterized in that the transformer is configured to be a planar transformer (10).

10. The inverter as set forth in claim 9, characterized in that leakage inductance of the planar transformer is increased by introducing a reactor core with an air gap that is wound either with the primary winding only or with the secondary winding only.

11. The inverter as set forth in claim 1, characterized in that it includes a cast housing (16) comprising contact surfaces for heat coupling that are stepped in such a manner that the planar transformer and two overlapping printed circuit boards can be coupled.

12. The inverter as set forth in claim 9, characterized in that the planar transformer is provided with two overlapping printed circuit boards (14, 15).

13. The inverter as set forth claim 1, characterized by an activation behavior that only activates the boost chopper when the intermediate circuit voltage at the capacitor Cmains drops below the voltage needed for mains electricity supply in the actual operating point.

14. The inverter as set forth in claim 1, characterized in that the resonant capacitances C5 and C6 are disposed in the DC-circuit.

15. The inverter as set forth in claim 14, characterized in that a large DC filter capacitor (C12) is disposed parallel to C5 and C6.

16. The inverter as set forth in claim 1, characterized in that power from a PV generator is fed into the mains supply.