GB1604142A

GB1604142A - Self-regulated push-pull dc/ac inverter

Info

Publication number: GB1604142A
Application number: GB22280/78A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-05-28
Filing date: 1978-05-25
Publication date: 1981-12-02
Also published as: IT7823842A0; DE2724400A1; FR2392532A1; IT1096337B

Description

(54) SELF-REGULATED PUSH-PULL DC/AC INVERTER (71) I, ING. WILFRIED STRAUSS, a citizen of the Federal Republic of Germany, of An St. Albertus Magnus 23, 4300 Essen 1, Germany, do hereby declare the invention for which I pray that a patent may be granted to me and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a self-regulated push-pull DC/AC inverter, particularly for use in an AC/DC/AC converter, with two push-pull transistors, a transformer installation fed from the push-pull transistors and if necessary with an oscillation build-up circuit; in which the transformer installation comprises a main winding consisting of two partial windings, a control winding consisting of two partial windings, and a load winding.

Alternating current transformers have been known for a long time in the most varied forms of construction. With normal alternating current transformers a large alternating voltage is transformed to a small alternating voltage - or the reverse - in the ratio of the numbers of turns on the primary and secondary windings, whilst the frequency of the alternating voltage remains the same. If considerable powers are to be transformed then such an alternating current transformer becomes large and heavy as the power that can be transformed depends upon the frequency of the alternating voltage, and this is comparatively low with a mains supply of 50 hertz.

In order to make use of the known fact that the maximum power that can be transformed by a transformer of given size increases with the frequency of the alternating voltage, alternating current transformers are known in which the large low-frequency alternating voltage to be fed to the transformer is first rectified and is then transformed by means of a self-regulated push-pull DC/ AC inverter, into a low alternating voltage of higher frequency before being fed to the transformer primary.

Push-pull DC/AC inverters of the previously-described kind are used in the above-described AC/DC/AC converter, but not there alone. For example, such selfregulated push-pull DC/AC inverters are known from the AEG Handbook, volume 3, "Transistors in Industry", Berlin 1961, particularly pages 52 to 64. In these known self-regulated push-pull DC/AC inverters two push-pull transistors are provided, each of their collectors being connected to the direct current supply through a part of the main winding of a transformer installation whilst each of their bases is connected to earth through a part of the control winding.

A resistance-capacity link is arranged at a suitable point as oscillation build-up circuit.

After oscillations have built up this selfregulated push-pull DC/AC inverter oscillates at a frequency determined by the components provided in the circuit, which usually lies between 20 and 50 kilohertz.

The use of this known self-regulating pushpull DC/AC inverter certainly leads to a considerable reduction of expenditure, and particularly to a considerable reduction of the size and weight of the alternating current transformer. However the energy loss (and the heat production associated with it, which makes cooling members or the like necessary) is still considerable in normal running, and also under no-load. Also the known self-regulated push-pull DC/AC inverters, particularly when they are used in an AC/DC/AC converter, are relatively susceptible to interference under no-load and short-circuit conditions.

The power losses of self-regulated pushpull DC/AC inverters naturally depend on the primary current, i.e. the current flowing through the push-pull transistors. During the switching-over process from one pushpull transistor to the other considerable loss currents always occur as the collector current of each push-pull transistor drops in relation to its base current to the residual current value after a considerable delay, namely only after the expiry of the so-called storage time of the push-pull transistor in question. The reason for this is that the push-pull transistors are driven to extreme saturation so that, particularly with the high-voltage transistors generally used, storage charges of considerable magnitude occur. These storage charges act in exactly the same way as an externally-applied base current, so that it is only after their decay that the collector current can drop. The result of this is that during each switchingover process both push-pull transistors are conductive for a short period, so that considerable collector currents can flow to earth as loss currents. The losses of energy thereby occurring are very considerable. To reduce the above-explained loss currents and the energy losses associated with these loss currents it is already known that the base current supplied to the individual push-pull transistor can be reduced by a resistance. Apart from the fact that no very drastic reduction of loss currents or the power losses associated with them is possible by series resistance of this kind alone, a drawback is associated with this feature of the circuit, that when running the push- ull DC/AC inverter unloaded on the secondary side, at the commencement of each hal - cycle the push-pull transistor concerned is driven inversely. Then the larger part of the collector current flows out from the base and brings about an additional voltage drop, thus causing an increase of the blocking voltage on the push-pull transistor. The resistance values of the series resistances are therefore to be specified as low as possible, so that in the event of no-load working the push-pull transistors are not destroyed.

However during normal working series resistances of low resistance value have in turn the drawback that the loss currents and the power losses associated with them as previously explained, are only reduced to a small extent.

However when operating the known selfregulated push-pull DC/AC inverters under no-load it is not only the above-described problem of possible destruction of the pushpull transistors that arises, but in addition the power losses arising under this condition are also quite considerable, as the push-pull DC/AC inverter also oscillates in the noload case. This is a particular problem when push-pull DC/AC inverters are installed in AC/DC/AC converters for no-load conditions are very often present there, for example when a low-voltage halogen lamp connected to the secondary side of the converter transformer has burnt out and at first is not changed, and then is changed without switching off the AC/DC/AC converter.

The known self-regulated push-pull DC/ AC inverters of the above-described type are as a whole still capable of a considerable degree of improvement in respect of the power losses associated with them as well as in respect of their reliability.

According to the present invention, a self-regulated push-pull DC/AC inverter, comprises two push-pull transistors, and a transformer installation fed from the pushpull transistors, the transformer installation comprising a main winding, a control winding consisting of two partial windings, and a load winding, the transformer installation being divided into a main transformer and a control transformer in which the main transformer consists of the main winding and the load winding, and the control transformer consists of the control winding and a feedback winding, the load winding of the main transformer and the feedback winding of the control transformer being connected in series, the control transformer being so designed that it becomes saturated before the main transformer.

Thus, the base current must be "switched off' before the end of each half-cyle in order that at the end of each half-cycle the collector current becomes zero, because in fact after the base current is "switched off" the collector current still flows during the storage period. Expressed differently, the base current must be "switched oft" at the right moment such that the base current time (the time during which the collector current flows on account of the base current) and the storage period (the time during which the collector current still flows on account of the storage charges present) together make up the duration of a half cycle. According to the invention the "switching off" of the base current before the end of each half cycle takes place because the control voltage, which drives the base current and is generated in the partial windings of the control winding of the control transformer, becomes zero before the end of each half-cycle. This in turn is accomplished because the control transformer becomes saturated relatively early, so that further change of flux with time and consequently the induction of a control voltage is no longer possible. In terms of the circuit this result is obtained in that the control winding, separated from the main winding and the load winding, is combined with a feedback winding connected in series with the load winding, to form a control transformer. This control transformer can then be so constructed that without being affected by the main transformer consisting of the main winding and the load winding, its magnetic saturation is obtained sufficiently early.

Preferably at least one series resistance is connected in series with the control winding of the push-pull DC/AC inverter according to the invention, as is in itself known. The series resistance is of use for fine adjustment and for balancing of the circuit, but on account of the design of the push-pull DC/AC inverter according to the invention it can be made extremely small. Also it is advantageous if a tuning resistance is arranged in parallel with at least one partial winding of the control winding. Such a tuning resistance permits the frequency of the push-pull DC/AC inverter to be adjusted within a certain range.

According to a further aspect of the invention, to which particular importance is attributed, particularly when the push-pull transistors are driven in a common emitter configuration, a Schottky diode is arranged in parallel with the base/collector path of each push-pull transistor. Naturally a Schottky diode can also be provided and connected at the physically appropriate position when the push-pull transistors are driven in the base circuit or in the collector circuit. A Schottky diode is a metal/semiconductor diode which, by suitable choice of materials, has a breakdown voltage which is slightly below that of a normal semiconductor/semiconductor junction. This means in the particularly important case when the push-pull transistors are driven in the emitter circuit, that the Schottky diode connected in parallel with the base/collector path of each push-pull transistor becomes conductive before the base/collector path of that push-pull transistor itself becomes conductive. This ensures that the base of the push-pull transistor can never be more positive in relation to its collector than the breakdown voltage of the Schottky diode, i.e. can never work in the conducting direction. This however means the same thing as a fundamental prevention of saturation of the push-pull transistors. This in turn guarantees that the collector currents subside practically simultaneously with the base currents, as the storage time becomes practically zero. By appropriate specification of the control transformer and of the series resistance that may be connected to the base, the branch current flowing through the Schottky diode can be limited to a minimum value so that no significant source of additional energy loss arises here.

According to a still further aspect of the invention, to which particular importance is also attributed, the oscillation build-up circuit comprises a storage condenser, a charging resistance connected in series with the storage condenser, and a series circuit consisting of an oscillation build-up winding and an electronic component with threshold value characteristics connected in parallel with the storage condenser, and the oscillation build-up winding is inductively coupled to the control winding of the transformer installation. By this oscillation build-up circuit, which is itself directly or indirectly connected to the direct current supply feeding the push-pull DC/AC inverter, spike impulses are continually produced, their duration and time-separation being determined by the storage condenser, the charging resistance, the oscillation build-up winding and the electronic component with threshold value characteristics. The spike impulses produced by the oscillation buildup circuit produce control impulses in the partial windings of the control winding of the transformer installation, which can cause the push-pull DC/AC inverter to be set into oscillation.

In a preferred form of construction of the last-described self-regulated push-pull DC/ AC inverter the transformer installation is divided into a main transformer and a control transformer, the main transformer consists of the main winding and the load winding, and the control transformer consists of the control winding, a feedback winding, and the oscillation build-up winding, and the load winding of the main transformer and the feedback winding of the control transformer are connected in series.

With this form of construction the main winding and the control winding are thus not directly coupled together magnetically, instead the coupling of the main winding to the control winding only takes place via the load winding and the feedback winding, wherein the load winding and the feedback winding in their turn are only coupled by the load current flowing through the load winding and the feedback winding. Consequently in this form of construction the spike impulses generated by the oscillation build-up circuit and the control impulses generated from the spike impulses only lead to oscillation of the push-pull DC/AC inverter if the push-pull DC/AC inverter is loaded on the secondary side, that is, does not run unloaded, for it is only then, as previously stated, that the main winding is coupled to the control winding, but this coupling is the necessary condition that the push-pull DC/ AC inverter should come into oscillation.

This form of construction thus has the advantage that when running unloaded the push-pull DC/AC inverter does not operate, so no energy loss occurs, but the push-pull DC/AC inverter is nevertheless always ready to be brought into oscillation.

According to a still further aspect of the invention a series resistance is associated with the transformer installation and a short-circuit protection circuit is provided in parallel with the series resistance. In this form of construction of the push-pull DC/ AC inverter according to the invention the voltage drop developed across the series resistance as a result of the current flowing through the transformer device and through the series resistance is compared with a reference voltage, and the oscillation of the push-pull DC/AC inverter is interrupted when the voltage drop across the series resistance exceeds this reference voltage.

In the last-described form of construction of the push-pull DC/AC inverter according to the invention the short-circuit protection circuit is expediently so built that it possesses an electronic switch, preferably a transistor and especially two transistors connected as a Darlington pair, a rectifier bridge connected to the output of the electronic switch, and a protective winding connected at the output of the rectifier bridge, that the protective winding is inductively coupled to the control winding of the transformer installation, and that when there is a shortcircuit (or at a pre-determined overload) the protective winding is short-circuited by the electronic switch. Thus, with this form of construction, if the electronic switch operates the protective winding is short-circuited by the electronic switch, so control voltages can no longer be induced in the partial windings of the control winding, accordingly the oscillation of the push-pull DC/AC inverter is interrupted. By this means, destruction of the push-pull transistors by the occurrence of currents overloading them is avoided.

Finally it is desirable in the form of construction of the push-pull DC/AC inverter according to the invention in which a short-circuit protection circuit is provided, that the short-circuit protection circuit should be equipped with a delay condenser.

This ensures that short-period voltage transients do not cause the short-circuit protection circuit to operate.

According to which of the abovedescribed features of the invention are implemented individually or in association with one another, the power loss is reduced to a minimum value in the cases of normal working, of no-load running, of normal working and no-load running, of normal working and short-circuit, of no-load running and short-circuit or normal working, or of no-load running and short-circuit, whilst at the same time the greatest possible safety of the push-pull transistors from destruction is guaranteed under all conditions. From this considerable advantages result relating to the size and weight of the self-regulated push-pull DC/AC inverter according to the invention compared with previously-known push-pull DC/AC inverters, or to AC/DC/ AC converters equipped with the push-pull DC/AC inverter according to the invention compared with previously-known AC/DC/ AC converters. As the disadvantages associated with the known push-pull DC/AC inverters are very substantially avoided by the invention, in many cases 50Hz/50 Hz AC/DC/AC converters that have long been installed can now be replaced by 50Hz/ approx. 20-50 kHz AC/DC/AC converters.

Two embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure I is a circuit diagram of an alternating current transformer using a first form of construction of the push-pull DC/AC inverter according to the invention, and Figure 2 is a partial representation of a second form of construction of the push-pull DC/AC inverter according to the invention.

The circuit diagram of an AC/DC/AC converter 1 shown in Figure 1 has a rectifier bridge 2 on the input side, to which a push-pull DC/AC inverter 5 is connected by supply leads 3, 4. The push-pull DC/AC inverter 5 features two push-pull transistors 6, a transformer installation 7 supplied from the push-pull transistors 6 and an oscillation build-up circuit 8. The transformer installation 7 comprises a main winding 9 consisting of two partial windings 9a and 9b, a control winding 10 consisting of two partial windings 10a and 10b, and a load winding 11.

As Figure 1 shows, the transformer installation 7 is subdivided into a main transformer 12 and a control transformer 13, in which main transformer 12 consists of the main winding 9 and the load winding 11, whilst the control transformer consists of the control winding 10 and a feed-back winding 14. Load winding 11 of main transformer 12 and feedback winding 14 of control transformer 13 are connected in series. Control transformer 13 is so designed that it becomes saturated sooner than main transformer 12.

As is to be seen in Figure 1, in the form of construction represented series resistances 15, 16, 17 are provided in series with control windings 10, i.e. with corresponding partial windings 10a and 10b. A tuning resistance 18 is also connected in parallel with partial winding 10a.

Furthermore Figure 1 shows an especially preferred form of construction of the pushpull DC/AC inverter according to the invention in respect of the oscillation build-up circuit 8. That is to say, the oscillation build-up circuit here consists of a storage condenser 19, a charging resistance 20 connected in series with the storage condenser 19, and in parallel with storage condenser 19 the series circuit of an oscillation build-up winding 21 and an electronic component 22 with threshold value characteristics. The whole oscillation build-up circuit 8 is connected to supply leads 3, 4, and thereby to the output of rectifier bridge 2, whilst in addition a smoothing condenser 23 is connected in parallel with the output of rectifier bridge 2. Oscillation build-up winding 21 is inductively coupled to control winding 10 of the transformer installation 7.

Finally Figure 1 shows a preferred form of construction of push-pull DC/AC inverter 5 according to the invention in the respect that a series resistance 24 is associated with the transformer installation 7 and that a short-circuit protection circuit 25 is provided in parallel with series resistance 24. The short-circuit protection circuit 25 features an electronic switch 26, a rectifier bridge 27 connected to the output of electronic switch 26, and a protective winding 28 connected to the output of rectifier bridge 27. Protective winding 28 is inductively coupled to control winding 10. When there is a short-circuit (or at a predetermined overload) protective winding 28 is short-circuited by electronic switch 26. In addition a delay condenser 29 is connected in parallel with series resistance 24 and therewith to the input of short-circuit protection circuit 25.

In the modification shown in Figure 2 of the push-pull DC/AC inverter according to the invention represented in Figure 1, pushpull transistors 6 are connected in series as is in itself known. A series circuit of two bridge condensers 30, 31 is connected between supply leads 3 and 4. Main winding 9 of main transformer 12 lies between the junction point the two push-pull transistors 6 and the junction point of the two bridge condensers 30, 31.

WHAT WE CLAIM IS: 1. A self-regulated push-pull DC/AC inverter, comprising two push-pull transistors, and a transformer installation fed from the push-pull transistors, the transformer installation comprising a main winding, a control winding consisting of two partial windings, and a load winding, the transformer installation being divided into a main transformer and a control transformer in which the main transformer consists of the main winding and the load winding, and the control transformer consists of the control winding and a feed-back winding, the load winding of the main transformer and the feed-back winding of the control transformer being connected in series, the control transformer being so designed that it becomes saturated before the main transformer.

2. A self-regulated push-pull DC/AC inverter according to Claim 1, wherein at least one series resistance is connected in series with the control winding.

3. A self-regulated push-pull DC/AC inverter according to Claim 1 or Claim 2, wherein a tuning resistance is arranged in parallel with at least one partial winding of the control winding.

4. A self-regulated push-pull DC/AC inverter according to any one of Claims 1 to 3, wherein the push-pull transistors are arranged in the common emitter configuration, a Schottky-diode being arranged in parallel with the base/collector path of each push-pull transistor.

5. A self-regulated push-pull DC/AC inverter according to any one of Claims 1 to 4, further comprising an oscillation build-up circuit, wherein the oscillation build-up circuit comprises a storage condenser, a charging resistance connected in series with the storage condenser, and a series circuit consisting of an oscillation build-up winding and an electronic component with threshold value characteristics connected in parallel with the storage condenser, and the oscillation build-up winding being inductively coupled to the control winding of the transformer installation.

6. A self-regulated push-pull DC/AC inverter according to any one of Claims 1 to 5, wherein a series resistance is associated with the transformer installation and a short-circuit protection circuit is provided in parallel with the series resistance.

7. A self-regulated push-pull DC/AC inverter according to Claim 6, wherein the short-circuit protection circuit has a electronic switch, a rectifier bridge connected to the output of the electronic switch and a protective winding connected at the output of the rectifier bridge, the protective winding being inductively coupled to the control winding of the transformer installation, whereby with a short-circuit, or at a predetermined overload, the protective winding is short-circuited by the electronic switch.

8. A self-regulated push-pull DC/AC inverter according to Claim 6 or Claim 7, wherein the short-circuit protection circuit has a delay condenser.

9. A self-regulated push-pull DC/AC inverter substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. whole oscillation build-up circuit 8 is connected to supply leads 3, 4, and thereby to the output of rectifier bridge 2, whilst in addition a smoothing condenser 23 is connected in parallel with the output of rectifier bridge 2. Oscillation build-up winding 21 is inductively coupled to control winding 10 of the transformer installation 7. Finally Figure 1 shows a preferred form of construction of push-pull DC/AC inverter 5 according to the invention in the respect that a series resistance 24 is associated with the transformer installation 7 and that a short-circuit protection circuit 25 is provided in parallel with series resistance 24. The short-circuit protection circuit 25 features an electronic switch 26, a rectifier bridge 27 connected to the output of electronic switch 26, and a protective winding 28 connected to the output of rectifier bridge 27. Protective winding 28 is inductively coupled to control winding 10. When there is a short-circuit (or at a predetermined overload) protective winding 28 is short-circuited by electronic switch 26. In addition a delay condenser 29 is connected in parallel with series resistance 24 and therewith to the input of short-circuit protection circuit 25. In the modification shown in Figure 2 of the push-pull DC/AC inverter according to the invention represented in Figure 1, pushpull transistors 6 are connected in series as is in itself known. A series circuit of two bridge condensers 30, 31 is connected between supply leads 3 and 4. Main winding 9 of main transformer 12 lies between the junction point the two push-pull transistors 6 and the junction point of the two bridge condensers 30, 31. WHAT WE CLAIM IS:

1. A self-regulated push-pull DC/AC inverter, comprising two push-pull transistors, and a transformer installation fed from the push-pull transistors, the transformer installation comprising a main winding, a control winding consisting of two partial windings, and a load winding, the transformer installation being divided into a main transformer and a control transformer in which the main transformer consists of the main winding and the load winding, and the control transformer consists of the control winding and a feed-back winding, the load winding of the main transformer and the feed-back winding of the control transformer being connected in series, the control transformer being so designed that it becomes saturated before the main transformer.