DE2724400A1 - PROCEDURE AND CIRCUIT ARRANGEMENT FOR MAINS OPERATION OF INVERTERS - Google Patents

Description

The invention relates to a method and a circuit arrangement for network operation of self-controlled transistor inverters. Transistor inverters that are to be operated directly with rectified mains voltage must have a number of special properties in order to be able to replace a mains transformer.

It is known to let transistor inverters operate with a frequency that exceeds the normal line frequency of 50 Hz by a multiple, since the transformer transformers can be smaller. It is also known to use oscillating aids for inverters by means of RC circuits which generate an oscillating pulse when the supply voltage is switched on. Likewise, the tendency to oscillate is increased via a continuous current through the base-emitter sections. Furthermore, the property of voltage feedback push-pull inverters is known to stop operating in the event of a short circuit. A battery-operated circuit of an inverter has become known, which feeds an LC series resonant circuit, via whose L or C a fluorescent lamp is operated as a load, in which the bases of the push-pull transistors are fed directly through a separate current transformer, which is fed by the current from the series resonant circuit, controlled and kept in series resonance.

However, none of these measures are suitable for achieving the object on which the invention is based.

There is no known inverter system which, on the one hand, ceases to vibrate when idling and, on the other hand, does the same in the event of an overload, one for each load case precisely adapted small but sufficient control power can be fed back. Furthermore, in the previously known methods, the change takes place in rhythm, not by prematurely switching off the base control, but by the formation of a gap between the collector and base current, which leads to high power losses in the switching transition and, in particular in the case of high-voltage transistors, to an early reaching of the permissible limit value .

Temporal overlaps of the switching times of the push-pull transistors occur due to the storage times and lead to high short-circuit-like current peaks. As a result, there are considerable power losses.

The task of replacing mains transformers for 50 Hz with inverter arrangements, e.g. for operating halogen lamps, in order to save both weight and volume, is ultimately determined by the entire cooling surface. However, this also roughly defines the required volume. Ultimately, assuming the same average temperature of the mains transformer and inverter arrangement, a reduction in size will only be possible if the total losses of the inverter arrangement are significantly lower than the already relatively low losses of conventional mains transformers.

Furthermore, the RC oscillation aids used for transistor inverters are not suitable for ensuring reliable automatic oscillation of the inverter after shutdown due to idling or short circuit after the cause of the shutdown has been eliminated.

Voltage controlled inverters have relatively high no-load losses. The shutdown in the event of a short circuit only takes place in the event of significant overloads. Another disadvantage of the known operating methods for transistor inverters is the overloading of the transistors when operating incandescent lamps, since the inrush current peak can reach fifteen times the operating current. In addition, the control power to be used is to be used regardless of the load for the maximum permissible case and must also be applied for the lowest loads.

In the case of the inverter with a series resonant circuit, which is fed with the payload via a separate current transformer, the series resonant circuit is no longer damped when idling and, due to an increase in resonance voltage and resonance current, it receives very high no-load losses, which overload the inverter transistors. In addition, the output voltage is heavily load-dependent.

The invention has set itself the task of creating a method for operating inverters which is suitable for surpassing the essential properties and characteristic values of network transformers. This applies in particular to weight, volume, power loss and short-circuit and no-load behavior.

Other features such as galvanic isolation and the option of multiple voltage outputs are fully retained.

The object is achieved according to the invention in that the current I [low] B (FIG. 1) fed back to the bases of the transistors is so designed in terms of its size and curve shape that it is switched off earlier than the transistor storage time t [low] s at least Collector current I [low] K and an automatic size proportionality between the base and collector current is established during the base current flow time, which goes so far that the inverter is shut down when idling, while on the other hand an overload (short circuit) also leads to shutdown, which when canceled the cause of the shutdown leads to the inverter starting to oscillate again.

In the further embodiment of the invention, the method can advantageously be carried out in that a current proportional to the load current is passed through the input winding (1, Fig. 2) of a separate current transformer, in whose output windings (2 and 3) resistors (3 and 4) are introduced, which are dimensioned so that the current transformer works in the magnetic saturation range, in which the ampere turns on the feed side (1) are significantly larger than the ampere turns on the take-off side.

The short-circuit protection is achieved in the further embodiment of the invention that a load current-dependent voltage queried via the resistor (10) controls a transistor (11) when the maximum value is exceeded, via its collector-emitter path the voltage of the current transformer winding rectified by the rectifier (18) (9) is wholly or partially short-circuited.

The advantages that can be achieved are, in particular, that 50 Hz mains transformers can be replaced by the inverter arrangement described in many areas of application. The favorable overall efficiency of up to over 95% achieved by the process allows small surfaces for heat dissipation. In connection with the shock load capacity, short-circuit and no-load security, as well as the slight no-load losses, arrangements of very low weight result at the usual frequencies of 20-50 kHz.

In addition, the shape of these inverter arrangements - in contrast to the mains transformer - must be largely adapted to the intended use.

In an embodiment for the supply of a low-voltage halogen lamp of 12 volts and 55 watts, the weight was reduced from 1700 g to 60 g, the volume from 478 ccm to 40 ccm and the power loss from 9 W to 1.7 W, in each case in Compared to the transformer, achieved.

The method is illustrated by FIG. 1. There the base current is switched off well before the end of the collector current. This only switches off after the storage time t [low] s has elapsed. Furthermore, it is shown how a corresponding increase in the base current is achieved by increasing the collector current I [low] K (load increase) (see dashed curve). A circuit arrangement by means of which this behavior can be achieved is shown in FIG. 2. The windings 1, 2, 3 as well as 8 and 9 are arranged on a separate core. The winding 8 is fed by the running pulses from diac 7 and generates needle-shaped oscillation pulses in windings 2 and 3, which result in a collector current through one of the push-pull transistors 20 or 21, which is load-dependent via winding 1 to windings 2 and 3 is fed back.

Without further circuit measures, the load current via the consumer 19, which is proportional to the collector current at the beginning of the cycle, is transformed via the winding 1 to the winding halves 2 or 3 in each case. There is an equilibrium of ampere turns. Towards the end of the cycle, in addition to the transformed load current component, there is a considerable current peak, which is added to the collector current by the magnetizing current of the transformer transformer 25, which goes into magnetic saturation. A corresponding increase in the base current does not take place, however, since this magnetizing current does not flow proportionally via the winding 1 and can therefore be fed back. Due to the gap between the strongly increasing collector current and an approximately constant base current, the lossy shutdown process at the transistor is initiated and delayed by the storage time of the high-voltage transistors so that the cycle times still overlap and thus short-circuit-like currents occur across both transistors.

If, as an additional circuit measure, the resistors 3 and 4 (Fig. 2) and / or 23 are introduced and dimensioned in such a way that the current converter transformer in turn enters the magnetic saturation state due to the voltage time area that now occurs long before the saturation time for the transformer 25 is reached The base current I [low] B is switched off at the transistors so early that, despite the switch-off delay t [low] s of the collector current, a lossy switch-off and an overlapping of the cycle times are in principle avoided.

The resistor 26 is used to set the desired frequency. It is of course possible to transfer this circuit measure to other arrangements of push-pull converters. For example, winding 1 (FIG. 2) can now also be introduced directly into the collector path of the transistors as a half-winding, since the saturation current region of the transmission transformer 25 is now in principle avoided.

Furthermore, it can be expedient to connect the inverter transistors in series and to connect them to a capacitor half-bridge, in whose shunt arm the transformer transformer is arranged. The center tap on the primary side can be omitted. The use of a full transistor bridge is of course also possible. In this case, a feedback winding is to be arranged galvanically separated from one another for each transistor and to provide the base circuits with resistors.

A load current-dependent voltage is generated for permanent short-circuit protection, e.g. via the resistor 10. This voltage controls the transistors 11 and 12 via the voltage divider 14, 15 and 16, with a winding 9 of the current transformer via the rectifier 18 and the collector-emitter path of the transistor 11 is short-circuited and thus establishes the ampere winding equilibrium at the lowest voltage, which in practice leads to a blocking of the push-pull transistors 20 and 21. The capacitor 17 delays this blocking, which can be desirable, for example, with incandescent lamps because of the inrush current peak. 24 is the power rectifier, 22 a smoothing capacitor.

Blank page

Claims (4)

1.) Method for operating self-controlled transistor inverters, characterized in that the current I [low] B (Fig. 1) fed back to the bases of the transistors is designed in terms of its size and curve shape so that it is at least by the transistor storage time t [low ] s is switched off earlier than the collector current I [low] K and an automatic size proportionality between base and collector current is established during the base current flow time, which goes so far that the inverter is switched off when idling, while on the other hand an overload (short circuit) also occurs a shutdown which, when the cause of the shutdown is eliminated, leads to the inverter starting to oscillate again automatically. 2.) Circuit arrangement for carrying out the method according to claim 1, characterized in that a current proportional to the load current is passed through the input winding (1, Fig. 2) of a separate current transformer, in whose output windings (2 and 3) resistors (3, 4 and / or 23) are introduced, which are dimensioned so that the current transformer works in the magnetic saturation range, in which the ampere turns of the feed winding (1) are significantly larger than the ampere turns of the take-off side. 3.) Circuit arrangement for performing the method according to claim 1, characterized in that a flip-flop circuit consisting of a series resistor (5), capacitor (6) and diac (7) operates on a winding (8) of the current transformer. 4.) Circuit arrangement for performing the method according to claim 1, characterized in that a load current-dependent voltage queried via the resistor (10) controls a transistor (11) when a maximum value is exceeded, via its collector-emitter path the rectified by the rectifier (18) Voltage of the winding 9 is wholly or partially short-circuited.