Method of converting a direct current voltage from a source of direct current voltage, more specifically from a photovoltaic source of direct current voltage, into a alternating current voltage

1. Field of the Invention

The invention relates to a method of converting a direct current voltage of a source of direct current voltage, more specifically of a photovoltaic source of direct current voltage, into an alternating current voltage at a frequency through a bridge circuit comprising switching elements and free-wheeling elements.

2. Description of the Prior Art

The document DE 197 32 218 C1 describes a transformerless inverter circuit arrangement with an inverting buck-boost converter circuit and a non-inverting converter circuit. Such type inverters are used for coupling photovoltaic systems. The circuit comprises a direct connection that is steady in terms of potential, that is, a conductor connection that remains at a fixed potential. At need, it is possible to use this direction connection as the neutral of the system between one of the two direct current connection terminals and one of the alternating current connection terminals and to couple thereto the negative terminal of the source of direct current for example, which has the great advantage of avoiding EMC problems. The circuit is intended to provide a lightweight transformerless inverter of a small construction that also ensures high personal safety and causes little EMC problems.

An inverter circuit having a semiconductor bridge, a solar generator and a switching element mounted between the solar generator and the semiconductor bridge is known from DE 103 12 921 A1. This circuit arrangement is configured such that the switching element opens when a defined direct current voltage of the solar generator is exceeded and the switching element is switched on when the voltage remains below the generator voltage so that the input voltage range, which is possible for the inverter circuit or the consumer, is increased. The circuit as configured does not avoid high-frequency voltages and does not eliminate EMC problems.

The document DE 102 21 592 A1 discloses a transformerless inverter with a bridge circuit. In this circuit, there are provided two separate electrical connection paths in each of which there are provided one switch and rectifier diodes connected in series. They are connected in the forward direction in the various connection paths. As contrasted with symmetric clocking, this circuit permits to clearly lower a current ripple in the output current. The additional free-wheeling diodes allow for ohmic decoupling between a solar generator and the alternating current voltage connection terminals. This permits to avoid high-frequency voltage transients at the connecting lines of the generator and improves EMC behaviour as a result thereof.

It is moreover known to utilize for transformerless photovoltaic inverters an H-bridge circuit with fourth semiconductor switches that are alternately connected in such a manner that, from the direct current voltage applied, an alternating current voltage is obtained between the branch connections of the bridge halves. What is more, the switching elements are provided with symmetric clocking. An upper switch of a bridge half is hereby clocked and pulse-width modulated at a high clock rate in synchronism with another lower switch of the other bridge half.

To reduce harmonics of the thus produced alternating current voltage, reactors are utilized. In order to keep harmonics in the alternating current voltage low, the reactors must be of quite large dimensions. This solution results in quite high hysteresis losses in the reactors so that the efficiency of the circuit is reduced.

Further losses also occur because two switching elements are switched on and off concurrently and because, at free-wheeling state, the current flows into the direct current voltage intermediate circuit through two free-wheeling diodes. At free-wheeling state, the direct current voltage in the intermediate circuit acts as a reverse voltage, which results in an increased current ripple and in an increased power loss.

In order to reduce these losses, it is known to asymmetrically clock the bridge. This means that while the upper switches are gated with mains frequency, the lower switches are gated at the high clock rate. As a result, at free-wheeling state, the reverse voltage from the intermediate circuit is eliminated as the current only commutates through a diode and a switch. This results in a lower current ripple and in reduced losses. This asymmetric gating however generates high-frequency potential fluctuations at the terminals of the photovoltaic generator which worsen the EMC behaviour of the generator.

A provision preventing the drawbacks of the two solutions is shown and described in the printed document DE 102 21 592 A1. There is hereby provided that two connection paths are additionally provided between the outputs of a bridge circuit or of an H-bridge. Four semiconductor components are located in the connection paths, namely each being another switching element with an associated driver stage and a diode connected in series.

Accordingly, the two drawbacks are avoided at the price of a more complex circuitry due to the much greater number of components which translates into a less reliable circuit and in increased material cost.

It is the object of the present invention to indicate a method of the generic type by which high-frequency voltage fractions at the connection terminals of the generator are avoided on the one side and which, on the other side, allows for simple low-loss circuitry and, as a result thereof, for high efficiency. It furthermore aims at reducing the material cost and at increasing reliability.

This object is solved in that the alternating current voltage circuit is decoupled from the direct current voltage circuit during the free-wheeling phases by means of a switching element additionally disposed in the direct current voltage circuit. In accordance with the invention, a free-wheeling current flows through the inner elements of the bridge in the bridge circuit so that, thanks to the decoupled state of the two circuits achieved by means of the additional opened switch, no high-frequency disturbances generated by clock operations occur in the direct current voltage circuit.

The method of the invention permits to readily avoid high-frequency voltage fractions at the connection terminals of the direct current voltage intermediate circuit using therefore but one additional component (as compared to a simple H-bridge having four switching elements and four free-wheeling diodes). The complexity of the circuit is reduced over an H-bridge having additional connection paths (according to DE 102 21 592 A1 with six switching elements and six free-wheeling diodes), as but one semiconductor switching element with associated gating unit is needed instead of two additional semiconductor switching elements including gating units and two additional diodes (overall, only five switching elements with five free-wheeling diodes). Additional costs and the probability of failure are minimized as a result thereof.

A gating signal for the additional switching element may moreover be obtained for a gating circuit without additional expense thanks to the solution of the invention by logical interconnection of the gating signals of the lower switching elements of the H-bridge for example.

The method of the invention makes it possible to have an asymmetric operation without high-frequency disturbances, more specifically if, according to an advantageous embodiment of the invention, the bridge circuit is gated in such a manner that the additional switching element located in the direct current voltage circuit is clocked in synchronism with a lower switch of the one bridge half during one half-wave while being clocked in synchronism with a lower switch of the other bridge half during the other half wave. Accordingly, the additional switch is clocked simultaneously with only one single switch.

Asymmetric operation permits to minimize losses and to improve the efficiency of the circuit. In accordance with this advantageous embodiment, the method of the invention combines at minimum component expense the advantages of a simple H-bridge circuit with asymmetric clocking exhibiting good efficiency with the advantages of a simple H-bridge circuit with symmetric clocking that avoids high-frequency signals at the terminals of the generator.

As contrasted with the symmetric clocking, a zero voltage state occurs like with asymmetric clocking since the voltage at the output terminals of the inverter are commutated between +U, zero, −U. As a result thereof, the hysteresis losses in the line reactors are reduced and the partial load efficiency in particular is improved as well.

If the corresponding switching elements of the bridge circuits are clocked in the kHz range, the forward losses are less important than the switching losses so that, thanks to the additional switching element, the efficiency is almost the same as with the H-bridge with additional connection paths between the outputs of the bridge circuit.

Since, as a result of the invention, a voltage at the alternating current voltage terminals virtually switches back and forth between a positive voltage potential, zero and a negative voltage potential on the one side and since, on the other side, load can be matched by pulse-width modulated clocking, it is possible to use the invention in a transformerless photovoltaic inverter for example.

By providing an anti-parallel diode of the additional switch that does not act as a free-wheeling diode or that is not engaged, like the lower free-wheeling diodes of the clocked switches, which are not engaged either in the normal case, the semiconductors used can be optimized. As a result thereof, semiconductor components can be used which, although they have unfavourable diode properties, exhibit good switching properties and low forward losses. As a result, the latest MOSFET-semiconductor components can be utilized for the switching elements to be clocked, said components permitting to further optimize efficiency and reduce cost.

Further developed implementations of the invention are recited in the subordinate claims.

The invention as well as the advantages thereof will be explained with reference to the drawings.

In the drawings:

FIG. 1 shows a circuit arrangement of an inverter in accordance with the invention

FIG. 2 is a schematic with a current path during a positive half wave

FIG. 3 is a schematic with a current path during a negative half wave

FIG. 4 is a schematic with a current path in the decoupled state.

FIG. 1 shows an inverter 1 of the invention with a solar generator SG or with a photovoltaic generator. This circuit allows for a method of converting a direct electric current of a photovoltaic source of direct current into an alternating current at a frequency of 50 Hz for example.

At the input terminals 2 of the inverter 1 , a filter capacitor C or storage capacitor is connected in parallel to the solar generator SG. Together with the capacitor C, the solar generator SG forms a direct current voltage intermediate circuit or a DC-circuit. The inverter has an H-bridge 3 with four semiconductor switching elements V 1 -V 4 and one additional switch V 5 . Free-wheeling diodes D 1 -D 5 are connected in parallel to the switching elements V 1 -V 5 . Two reactors L 1 and L 2 are located at the bridge branch in the alternating current voltage part.

The upper switching elements V 1 and V 3 are gated at a mains frequency of 50 Hz for example whereas the lower switching elements V 2 and V 4 are provided with a pulse-width modulated clocking at a high clock rate in the kHz range, for example at 16 kHz.

The additional semiconductor switching element V 5 , which could more specifically also be implemented as a MOSFET component, is clocked together with the lower switching elements V 2 and V 4 respectively at the high clock rate of for example 16 kHz. Accordingly, during the one half wave of the mains voltage in which the switching element V 1 is switched on, the switching element V 5 is clocked and pulse-width modulated in synchronism with the switching element V 4 . The load current is then built up through the switching elements V 5 , V 1 and V 4 . If the semiconductor switches V 5 and V 4 provided with a high-frequency clocking are switched off in synchronism, the load current commutates into a free-wheeling path consisting of V 1 and of the diode D 3 that is anti-parallel to V 3 .

As illustrated in FIG. 3 , the switching element V 5 is clocked and pulse-width modulated in synchronism with the switching element V 2 during the other half wave of the mains voltage (negative half wave) in which the switching element V 3 is switched on. Then the load current is built up through the switching elements V 5 , V 3 and V 2 . If the semiconductor switches V 5 and V 2 provided with a high-frequency clocking are switched off in synchronism, the load current commutates to the parallel free-wheeling diode D 1 of the switching element V 1 , as can be seen from FIG. 4 .

As a result thereof, the load circuit is decoupled, in accordance with the invention, from the terminals of the generator by means of a switching element V 5 disposed in the direct current voltage circuit, high-frequency voltage components on these connecting lines being avoided as a result thereof. Accordingly, the switching element V 5 serves as an additional decoupling switch of the one direct current voltage connection terminal with respect to the alternating current voltage circuit. The other direct current voltage connection terminal is also decoupled from the alternating current voltage circuit through a respective one of the switches V 2 and V 4 .

It is thereby important that the voltage be distributed symmetrically to the switching elements V 5 and V 2 or V 4 respectively. It is therefore preferred to utilize switching elements or diode elements exhibiting the same properties.

Additionally, this freewheel permits to reduce the current ripple and, as a result thereof, the hysteresis losses in the line reactors L 1 and L 2 .

This invention readily provides a low-loss, transformerless and cost-optimized inverter that does not cause high-frequency disturbances to occur in the direct current voltage circuit.

1 inverter

2 input terminals

3 bridge

SG solar generator

V 1 -V 4 switching elements of the bridge circuit

D 1 -D 4 free-wheeling elements of the bridge circuit

D 5 diode

V 5 decoupling switch (switching element)

C filter capacitor

L 1 -L 2 line reactors

R LAST load resistance

L LAST load inductance