GB2028018A

GB2028018A - Protecting inverter circuits

Info

Publication number: GB2028018A
Application number: GB7927192A
Authority: GB
Original assignee: Siemens Corp
Current assignee: Siemens Corp
Priority date: 1978-08-07
Filing date: 1979-08-03
Publication date: 1980-02-27
Also published as: SE7906601L; JPS5526098A; DE2834561A1; FR2433260A1

Abstract

An inverter with alternately ignitable thyristors (2 to 5) which connect a load (10, 11) alternately to the positive and negative poles of a direct voltage supply has an LC oscillatory circuit (12, 13), connected as low-pass filter, in a parallel manner to each of two series connections of two thyristors (2, 3 and 4, 5) which are to be fired alternately such that, if both thyristors (2, 3 or 4, 5) of a series circuit are fired simultaneously, these thyristors (2, 3 or 4, 5) are quenched during a reverse charging phase after the charge reversal of a capacitor (12) of this LC oscillatory circuit (12, 13) as a result of a fall in voltage at a respective one of parallel- connected free-running diodes (6 to 9) in each case. Furthermore, there is monitoring ciruitry (19) for the inverter input voltage or the inverter current which prevents the firing of the thyristors (2 to 5) during a pre-determined period of time when this voltage or this current assumes a characteristic value for a short circuit. <IMAGE>

Description

SPECIFICATION Inverters The present invention relates to an inverter including alternately triggerable switch means, such as thyristors, for connecting a load alternately to the positive and negative poles of a direct-current supply.

An inverter of this kind can be used for example in a generator for diagnostic radiography apparatus for supplying a high-tension transformer. In this case, the supply frequency of the high-tension transformer can be set in the kHz region, i.e. be chosen essentially higher than the mains frequency. On account of this high supply frequency, the hightension transformer can have an essentially smaller and more simple construction than in the case of a generator which is operated at mains frequency.

It can happen that two switch means in series with each other, which are to be fired alternately and to the junction of which a load is connected, are triggered simultaneuously on account of interferences. This results in a short circu it which is ended in known inverters through the response of a preconnected fuse protection.

According to the present invention there is provided an inverter including alternately triggerable switch means for connecting a load alternately to the positive and negative poles of a direct-current source, in which inverter an LC oscillatory circuit is connected in a parallel manner to a series connection of two such switch means which are to be triggered alternately, this LC oscillatory circuit being such that, if both switch means of the series connection are triggered simultaneously, the LC oscillatory circuit oscillates with a reverse oscillation as a result of the short circuit current and, as a result of the reverse oscillation, the switch means are quenched through a fall in voltage at a parallel connected free-running diode in each case.

There could be monitoring circuitry for the inverter input voltage or current, which prevents the triggering of the switch means during a predetermined period of time when this input voltage or the inverter current assumes a characteristic value for a short circuit. Thus the inverter can be cut out momentarily and then brought back to its normal mode of operation.

Each of the switch means could be a thyristor.

The invention will now be described by way of example with reference to the single figure of the accompanying drawing.

The figure shows an inverter 1 which comprises a bridge of four thyristors 2 to 5, which are bridged by free-running diodes 6 to 9 respectively. An LC oscillatory circuit comprising a capacitor 10 and a winding 11 is connected in a diagonal of this bridge, in the example the winding 11 being the primary winding of a load comprising a high-tension transformer. An inductor 13, two capacitors 12, 14, an inductor 15 and fuse protection 16 are connected before the inverter. Thyristor pairs 2, 5 and 3, 4 respectively are fired alternately by means of control circuitry 17 in order to produce an a-c voltage.

Misfirings can occur during the firing of the next pair of thyristors on account of interference pulses or if the releasing time is not reached. Thus, In this way for example the thyristors 2 and 3 or 4 and 5 may be fired simultaneously. In this case, the inverter 1 is a short circuit. The capacitor 12 discharges thereby by way of a respective conductive thyristor branch, for example the branch 2, 3. The discharging of the capacitor 14 is limited by means of the inductor 15 to a value which is smaller then the reverse oscillation current of the LC oscillatory circuit 12, 13.After the change in charge of the capacitor 12 has ended, the simultaneously fired thyristors 2, 3 or 4, 5 are quenched during the reverse oscillation phase by way of the free-running diodes 6,7 or 8, 9.Consequently, a short circuit only lasts a few lis. The fuse protection 16 therefore does not respond. After the short circuit has been cleared, the inverter 1 operates again in a normal manner. A diode 18 connected in a parallel manner to the inductor 15 prevents an over-voltage at the capacitor 12 after the short circuit has ended.

Afurther provision for increasing the short circuit fuse protection consists in that monitoring circuitry 19 is provided for the inverter input voltage, which controls the control circuitry 17 for the thyristors 2 to 5 and prevents the firing of these thyristors when the voltage at it has a characteristic value for a short circuit. The supply of additional firing pulses is blocked therefore in this case for a pre-determined time, e.g. a few lis. The monitoring circuitry 19 can also be used to monitorthe input current of the inverter 1, which increases in the case of a short circuit.

The transformer 11 could for example be a high-tension transformer which supplies an X-ray tube by way of a high-tension rectifier.

The invention could also be applied to any inverter in which a short circuit occurs with the faulty simultaneous firing of two thyristors which are to be fired alternately, therefore, for example, also in the case of a rectangular inverter or an inverter in a semi-bridge form.

1. An inverter including alternately triggerable switch means for connecting a load alternately to the positive and negative poles of a direcS,:-current source, in which inverter an LC oscillatory circuit is connected in a parallel manner to a series connection of two such switch means which are to be triggered alernately, this LC oscillatory circuit being such that, if both switch means of the series connection are triggered simultaneously, the LC oscillatory circuit oscillates with a reverse oscillation as a result of the short circuit current and, as a result of the reverse oscillation, the switch means are quenched through a fall in voltage at a parallelconnected free-running diode in each case.

2. An inverter according to claim 1, including an inductor for limiting supply current to a value which is smaller than the reverse oscillation current of the LC-oscillatory circuit.

3. An inverter according to claim 2, including a

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Inverters The present invention relates to an inverter including alternately triggerable switch means, such as thyristors, for connecting a load alternately to the positive and negative poles of a direct-current supply. An inverter of this kind can be used for example in a generator for diagnostic radiography apparatus for supplying a high-tension transformer. In this case, the supply frequency of the high-tension transformer can be set in the kHz region, i.e. be chosen essentially higher than the mains frequency. On account of this high supply frequency, the hightension transformer can have an essentially smaller and more simple construction than in the case of a generator which is operated at mains frequency. It can happen that two switch means in series with each other, which are to be fired alternately and to the junction of which a load is connected, are triggered simultaneuously on account of interferences. This results in a short circu it which is ended in known inverters through the response of a preconnected fuse protection. According to the present invention there is provided an inverter including alternately triggerable switch means for connecting a load alternately to the positive and negative poles of a direct-current source, in which inverter an LC oscillatory circuit is connected in a parallel manner to a series connection of two such switch means which are to be triggered alternately, this LC oscillatory circuit being such that, if both switch means of the series connection are triggered simultaneously, the LC oscillatory circuit oscillates with a reverse oscillation as a result of the short circuit current and, as a result of the reverse oscillation, the switch means are quenched through a fall in voltage at a parallel connected free-running diode in each case. There could be monitoring circuitry for the inverter input voltage or current, which prevents the triggering of the switch means during a predetermined period of time when this input voltage or the inverter current assumes a characteristic value for a short circuit. Thus the inverter can be cut out momentarily and then brought back to its normal mode of operation. Each of the switch means could be a thyristor. The invention will now be described by way of example with reference to the single figure of the accompanying drawing. The figure shows an inverter 1 which comprises a bridge of four thyristors 2 to 5, which are bridged by free-running diodes 6 to 9 respectively. An LC oscillatory circuit comprising a capacitor 10 and a winding 11 is connected in a diagonal of this bridge, in the example the winding 11 being the primary winding of a load comprising a high-tension transformer. An inductor 13, two capacitors 12, 14, an inductor 15 and fuse protection 16 are connected before the inverter. Thyristor pairs 2, 5 and 3, 4 respectively are fired alternately by means of control circuitry 17 in order to produce an a-c voltage. Misfirings can occur during the firing of the next pair of thyristors on account of interference pulses or if the releasing time is not reached. Thus, In this way for example the thyristors 2 and 3 or 4 and 5 may be fired simultaneously. In this case, the inverter 1 is a short circuit. The capacitor 12 discharges thereby by way of a respective conductive thyristor branch, for example the branch 2, 3. The discharging of the capacitor 14 is limited by means of the inductor 15 to a value which is smaller then the reverse oscillation current of the LC oscillatory circuit 12, 13.After the change in charge of the capacitor 12 has ended, the simultaneously fired thyristors 2, 3 or 4, 5 are quenched during the reverse oscillation phase by way of the free-running diodes 6,7 or 8, 9.Consequently, a short circuit only lasts a few lis. The fuse protection 16 therefore does not respond. After the short circuit has been cleared, the inverter 1 operates again in a normal manner. A diode 18 connected in a parallel manner to the inductor 15 prevents an over-voltage at the capacitor 12 after the short circuit has ended. Afurther provision for increasing the short circuit fuse protection consists in that monitoring circuitry 19 is provided for the inverter input voltage, which controls the control circuitry 17 for the thyristors 2 to 5 and prevents the firing of these thyristors when the voltage at it has a characteristic value for a short circuit. The supply of additional firing pulses is blocked therefore in this case for a pre-determined time, e.g. a few lis. The monitoring circuitry 19 can also be used to monitorthe input current of the inverter 1, which increases in the case of a short circuit. The transformer 11 could for example be a high-tension transformer which supplies an X-ray tube by way of a high-tension rectifier. The invention could also be applied to any inverter in which a short circuit occurs with the faulty simultaneous firing of two thyristors which are to be fired alternately, therefore, for example, also in the case of a rectangular inverter or an inverter in a semi-bridge form. CLAIMS

3. An inverter according to claim 2, including a smoothing capacitor connected before the inductor, the latter being connected between this capacitor and a capacitor of the oscillatory circuit, the inductor being bridged by a diode for preventing an overvoltage after termination of a short circuit as a result of the reverse oscillation of the LC oscillatory circuit.

4. An inverter according to any of claims 1 to 3, including monitoring circuitry for the inverter-input voltage or the inverter current, for preventing the triggering of the switch means during a predetermined period of time, if this voltage or this current assumes a characteristic value for a short circuit.

5. An inverter according to any preceding claim, wherein each of the switch means is a thyristor.

6. An inverter, substantially as herein described with reference to the single figure of the accompanying drawing.