Inverter protected in respect of the rates of increase of current and voltage

A single-phase or multi-phase inverter protected with respect to the rates of increase of current and voltage, the said inverter comprising a filter capacitor and a choke protecting with respect to the rate of increase of current, as well as, members protecting with respect to the voltage increase rate. In order for it to be possible to discharge the energy stored in the magnetic field of the choke and in the capacitor, between the change-over steps of the inverter substantially without losses, for each phase, there is a series connection of the capacitor and a diode connected in parallel with only one semiconductor switch and a diode is connected between the connection point between the capacitor and the diode, and one terminal of a storage capacitor. The energy in this way transferred to the separate storage capacitor can then be transferred to the filter capacitor by use of a breaker.

FIELD OF THE INVENTION 
The present invention is directed to a single-phase or multi-phase inverter 
protected with respect to surges or the rates of increase of current and 
voltage. The inverter comprises a filter capacitor and a choke for 
protecting with respect to the current, increase rate connected at the 
DC-side of the inverter. Also provided, for each phase, are at least two 
controlled semiconductor switches and, connected in parallel with these, 
members protecting with respect to the voltage increase rate and a 
separate storage capacitor whose first terminal is connected to either 
terminal of the filter capacitor, whereas the other terminal is connected 
to means by which energy is transferred from the storage capacitor to the 
filter capacitor, as well as to means for transferring energy from the 
members protecting in respect of the rates of increase of current and 
voltage to the storage capacitor. The inverter in accordance with the 
invention is suitable for use as an inverter component of a 
pulse-width-modulated frequency transformer, by means of which an 
AC-voltage of desired frequency is formed out of a fixed DC-voltage, or as 
a step-down transformer of DC-voltage. The controlled semiconductor switch 
of the inverter may consist of any semiconductor component whatsoever that 
can be controlled conductive and nonconductive from a control electrode 
and which requires external limitation of the rate of increase of current 
and voltage. Such components are, e.g., a bipolar transistor, a power-FET, 
and a GTO-thyristor. 
BACKGROUND OF THE INVENTION 
An inverter in accordance with the conventional technology, which is 
provided with connections limiting the current-increase rate (di/dt) and 
the voltage-increase rate (du/dt), is illustrated with respect to one 
phase in FIG. 1. Therein, U.sub.DC is the supplying DC-voltage, C.sub.DC 
is the filter capacitor of the DC-voltage, S.sub.1 and S.sub.2 are 
semiconductor switches, D.sub.1 and D.sub.2 are so-called zero diodes, L 
is the choke limiting the rate of increase of the current, whereas the 
diode D.sub.5 and the resistor R.sub.3 constitute an alternative route for 
the current of the choke, and C.sub.1 and C.sub.2 are the capacitors 
limiting the rate of increase of the voltage, whereat diodes D.sub.3 and 
D.sub.4 as well as resistors R.sub.1 and R.sub.2, together with C.sub.1 
and C.sub.2, form so-called polarized RC-shields. 
The arrangement of FIG. 1 constitutes a change-over switch by which the 
poles of the input voltage can be alternately connected to the output 
pole. The change-over of the switch, e.g., from the lower branch to the 
upper branch takes place by opening S.sub.2, which had been conductive, 
and by closing S.sub.1. Thus, C.sub.1, which had been charged to a voltage 
U.sub.DC, is discharged through the resistor R.sub.1. C.sub.2, whose 
voltage has been zero, is charged along the path U.sub.DC -L-S.sub.1 
-D.sub.4 -C.sub.2. After C.sub.2 has been charged, excess current of the 
choke L is turned so as to pass vid D.sub.5 and R.sub.3, being gradually 
reduced to zero, at which time a stable condition has been reached. The 
change-over of the switch in the opposite direction takes place in a 
corresponding way. 
The operation of the inverter in accordance with FIG. 1, however, involves 
the drawback that the energy 1/2CU.sup.2 of the capacitor protecting with 
respect to the voltage increase rate, discharged on each change-over of 
the switch, as well as the energy 1/2LI.sup.2 .sub.max charged in the 
choke protecting with respect to the current increase rate on swinging of 
the resonance circuit are converted to heat in the resistors R.sub.1, 
R.sub.2 and R.sub.3. The power loss is proportional to the number of 
change-overs of the switches per unit of time, which makes the efficiency 
of the inverter poor at high (higher than 1 kHz) frequencies. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an inverter in which the 
energies of the di/dt protective choke and of the du/dt protective 
capacitor are not dissipated as heat, but they are returned to the filter 
capacitor of the DC-voltage side of the inverter, so that the efficiency 
of the arrangement is improved remarkably. According to the invention, 
this has been accomplished so that the members protecting with respect to 
the voltage increase rate comprise, for each phase, a series connection of 
a capacitor and a diode connected in parallel with only one of the 
controlled semiconductor switches and that the energy transfer means 
comprises a diode connected between the connection point between the 
series connected protective capacitor and diode, and the other terminal of 
the energy transfer diode is and one a terminal of a storage capacitor. 
In the case of a three-phase inverter, the inverter is characterized in 
that the means for transferring energy to a storage capacitor common for 
all phases comprises diodes connected between the connection points 
between the series connected protective capacitor and diode for each 
phase, and the terminal of the storage capacitor.

DETAILED DESCRIPTION OF THE INVENTION 
In FIGS. 2 to 4, the same reference denotations have been used for 
corresponding components as were used in FIG. 1, already described above. 
The arrangement of FIG. 2 differs from that of FIG. 1 in the respect that 
the protective capacitor C.sub.2, the diode D.sub.4 and the resistor 
R.sub.2 of S.sub.2 have been omitted, so that the capacitor C forms a 
joint du/dt protective capacitor both for the upper branch and for the 
lower branch. Moreover, it has been possible to omit the resistor R.sub.3 
and the diode D.sub.5 of the di/dt protective choke L, because the energy 
of the choke can be discharged through the diode D.sub.4 connected to the 
connecting point between the capacitor C and the diode D.sub.3 to a 
separate storage capacitor C.sub.A. Therefore, it has also been possible 
to omit the resistor R.sub.1 of FIG. 1. In the arrangement of FIG. 2, the 
energies of both the di/dt protective choke L and the du/dt protective 
capacitor C are, thus, fed through the diode D.sub.4 to a separate storage 
capacitor C.sub.A , from which the energy is supplied by means of a 
separate breaker back to the filter capacitor C.sub.DC of the main 
voltage. Thus, the arrangement includes no resistive components, so that, 
in principle, it operates without losses. Thus, especially at high 
change-over frequencies, the overall efficiency of the inverter is 
essentially improved. 
Below, the operation of the inverter is accordance with the invention will 
be examined on the basis of the exemplifying embodiment shown in FIG. 2. 
To begin with, it is assumed that S.sub.2 is conductive, U.sub.C =U.sub.DC 
+U.sub.A, and i.sub.L =0. The changing-over of the switch starts with the 
opening of S.sub.2 and closing of S.sub.1. Accordingly, the voltage 
U.sub.1 effective over the connectors S.sub.1 and S.sub.2 is reduced to 
zero, because the voltage of C cannot change suddenly. A voltage U.sub.C 
-U.sub.A is effective over the choke L, so that its current starts 
increasing. Accordingly a resonance circuit L-S.sub.1 -C-D.sub.4 -C.sub.A 
is formed, whose current discharges C and charges C.sub.A. The voltage 
effective over S.sub.2 increases at the same rate at which C is 
discharged. When U.sub.C reaches the value zero, the current i.sub.L is 
turned so as to pass along the route L-D.sub.3 -D.sub.4 -C.sub.A, so that 
it is reduced at the rate di/dt=U.sub.A /L. When i.sub.L reaches the value 
zero, the situation is stable and the switch is in the upper position. 
From the above it is seen that, during the change-over of the switch, the 
energies of both C and L were charged in the storage capacitor C.sub.A. 
The changing-over of the switch from the upper position to the lower 
position takes place in a corresponding way by opening S.sub.1 and by 
closing S.sub.2. Then, also, the voltage U.sub.1 collapses to zero, 
because U.sub.C remained at the value zero after the preceding 
change-over. Correspondingly, now a resonance circuit L-D.sub.3 -C-S.sub.2 
-C.sub.DC is formed, which charges C. When U.sub.C =U.sub.DC +U.sub.A, 
i.sub.L is turned so as to pass along the path L-D.sub.3 -D.sub.4 
-C.sub.A, the current being reduced to zero. In this way the original 
situation has been reached. It should be stated that it is a prerequisite 
for operation of the type described above that U.sub.A is lower than 
U.sub.DC, because, in the contrary case, the voltage of C, U.sub.C, would 
never reach the value U.sub.DC +U.sub.A. 
In order for the energy charged in the storage capacitor C.sub.A to be 
transferred to the filter capacitor U.sub.DC, circuit components have been 
included in the arrangement for shifting the charge of C.sub.A to 
C.sub.DC. This operation can be accomplished by means of a number of 
prior-art circuit constructions, one of which is shown in FIGS. 2 and 3. 
It consists of a semiconductor switch S.sub.3, a diode D.sub.5, and a 
choke L.sub.A, which form a breaker by which it is possible to transfer 
energy from C.sub.A to C.sub.DC to an extent necessary in order to keept 
the voltage U.sub.A substantially constant. 
The operation of the breaker shown in FIGS. 2 and 3 and consisting of the 
semiconductor connector S.sub.3, choke L.sub.A, and diode D.sub.5 is 
described in the following. When S.sub.3 is conductive, the current of the 
inductance L.sub.A increases at a rate in accordance with the formula 
di/dt=U.sub.A /L.sub.A, i.e. energy is transferred from the capacitor 
C.sub.A into the magnetic field of the choke L.sub.A. After the current 
has increased to a certain maximum value, the connector S.sub.3 is opened. 
Then, the current of the choke L.sub.A is shifted to the diode D.sub.5 and 
starts charging the filter capacitor C.sub.DC. The current of choke 
L.sub.A is reduced at the rate di/dt=U.sub.DC /L.sub.A. When the current 
reaches the value zero, S.sub.3 may again be closed if the voltage U.sub.A 
is higher than the desired value. 
In connection with FIG. 2, only one embodiment of the invention has been 
described. The basic idea of the invention may also be accomplished, e.g., 
by means of the mirror-image arrangement shown in FIG. 3, which is similar 
to the arrangement shown in FIG. 2, but therein the di/dt- and 
du/dt-shields have been shifted to the lower branch of the inverter, so 
that the directions of the diodes D.sub.3, D.sub.4 and D.sub.5 have been 
reversed. 
The arrangement of FIG. 2 may also be changed so that, the second terminal 
of the capacitor C.sub.A is connected to the negative terminal of the 
filter capacitor instead of to the positive terminal of the filter 
capacitor, as is shown in the FIG. 2 by the broken line. 
FIGS. 2 and 3 show single-phase inverters formed in accordance with the 
invention. The invention can also be applied to a three-phase inverter, as 
is shown in FIG. 4. From FIG. 4 it is seen that for all phases it is 
possible to use a common di/dt protective choke L and a common storage 
capacitor C.sub.A. In FIG. 4, the breaker for transferring the energy of 
the capacitor C.sub.A to the capacitor C.sub.DC is denoted with reference 
denotation S. The breaker S may be similar to the breaker shown in FIGS. 2 
and 3, consisting of switch S.sub.3, choke L.sub.A and diode D.sub.5. 
As was already mentioned above, the arrangement in accordance with the 
present invention is also suitable for use as a step-down transformer, 
i.e. as a so-called DC/DC-transformer. In such a case, however, e.g., the 
arrangement of FIG. 2 must be completed by, to the point denoted as the 
AC-voltage outlet in FIG. 2, connecting a series choke and a parallel 
capacitor for the filtration of the DC-voltage formed, which is lower than 
the input voltage. It should be stated that in such a DC/DC-transformer 
the direction of flow of the current may also be reversed, in which case 
the arrangement functions as a step-up transformer of DC-voltage. 
When the preceding description and the attached patent claims and drawing 
are being interpreted, it should be taken into account additionally that 
in this connection, the components denoted as diodes are not to be 
understood as meaning a conventional diode alone, but they mean any 
component whatsoever conductive in one direction only, such as, e.g., a 
transistor connected as a diode.