GB2138977A

GB2138977A - Constant current source

Info

Publication number: GB2138977A
Application number: GB08408483A
Authority: GB
Inventors: Georg Dorfmeister
Original assignee: Schrack Elektronik AG
Current assignee: Schrack Elektronik AG
Priority date: 1983-03-31
Filing date: 1984-04-02
Publication date: 1984-10-31
Also published as: ATA115883A; DE3411904A1; CH665506A5; GB2138977B; GB8408483D0; AT387875B

Abstract

A circuit for operating a load, for example, for charging an accumulator 7, from a blocking-oscillator-type converter comprising a transformer 2 with a primary winding 4 in series with a transistor 1, a secondary winding 6 supplying the load via a rectifier 3 and a transistor 12 for turning off the transistor 1, the base of transistor 12 receiving signals from current sensing resistors 5,8 respectively located in the primary circuit of the transformer 2 and in the load circuit. The mean load current is thereby maintained approximately constant with variation in the input voltage to the converter, eg. for inputs in the range of about 100 to 250 V. A motor 21 may be energised by the accumulator 7, or the converter may simultaneously supply the accumulator 7 and the motor 21, current sensing resistors 8,23 then determining the magnitude of the constant charging current and constant motor current. A network 9,10,11 provides feedback from the secondary 6 to the base of transistor 1. Where a low voltage accumulator 7 is used, the network 9,10,11 may alternatively be connected to an additional secondary (24), (Fig. 2), of transformer 2. <IMAGE>

Description

SPECIFICATION A circuit arrangement for operating a load The invention relates to a circuit arrangement for operating a load such as, for example, for charging an accumulator, from an electric energy source, this energy source being either a direct current energy source or an alternating-current energy source which is followed in the circuit by a rectifier and in which arrangement in parallel with the energy source a series circuit is provided which contains the primary winding of a blocking-oscillator-type converter, a first electronic switch having a control electrode and a resistance, that furthermore a second electronic switch is provided, one main connection of which is connected to the control electrode of the first electronic switch, and that the blocking-osxillator-type converter is provided with a secondary winding which is connected via a rectifying element to the load, for example to the accumulatorto be charged.

Circuit arrangements of the type described above are provided especially for small electrical devices which can be connected to different mains voltages having mains-voltage-values of different magnitudes. A problem existing in this connection consists in the fact that, regardless of the limits within which the mains voltage is fluctuating, it must always be guaranteed that the maximum permissible charging current of the small accumulators which are provided for operating the small electrical devices must not be exceeded as otherwise these accumulators will be destroyed. For this reason, it has been proposed to compensate the oscillating condition of the blocking-oscillator-type converter as a function of the input voltage fluctuations in such a manner that the frequency of oscillation of the converter is affected as a function of the input voltage fluctuations.Such compensation, however, can, due to its very nature, have only incomplete effects and is limited to a relatively small range of input voltage fluctuations. It is incomplete, that is to say fluctuations will continue to occur in the charging current so that it is not possible to keep the charging current constant within small limits.

The invention has, therefore, the object of developing the circuit arrangement initially mentioned in such a manner that it is guaranteed that the load current and in particular the charging current of the accumulator can be kept constant within narrow limits with input voltage fluctuations of about 100 to 250 V. According to the invention, this is achieved by the fact that the control electrode of the second electronic switch has applied to it simultaneously both an electric control value which is proportional to the current flowing through the primary winding of the blocking-oscillator-type converter and an electric control value which is proportional to the load current, in particular to the charging current of the accumulator.By these measures, therefore, the load current, and in particular the accumulator current, is regulated, and this guarantees the load current remaining highly constant.

In the text which follows, the invention is described by way of example by referring to the drawings, in which: Figure land Figure2 in each case show circuit diagrams of possible embodiments of the circuit arrangement according to the invention.

Figure 1 shows an illustrative embodiment of an accumulator charging circuit according to the invention. The switching transistor 1 forms in conjunction with the convertertransformer2 and the rectifier diode 3 the power section of a blocking-oscillatortype converter. In this arrangement, a series circuit consisting of the primary winding 4 of the transformer, the main conducting path of the transistor 1 and a current sensing resistance 5 is connected to the input terminals AB of the blocking-oscillator-type converter and forms the primary circuit of the blocking-oscillator-type converter. The secondary circuit of the blocking-oscillator-type converter comprises a series circuit consisting of the secondary winding 6 of the transformer 2, the secondary rectifier diode 3, the accumulator 7 as the load and a second current sensing resistance 8.The feedback branch of the blocking-oscillator-type converter consists, on the one hand, of a series/parallel circuit of capacitors 9, 10 and the resistance 11 of the control electrode of the first switch transistor 1 to the junction of the secondary winding 6 ofthetransfor- mer and the secondary rectifier diode 3 and, on the other hand, of a second switch transistor 12, the main conducting path of which is arranged in parallel with the series circuit of the main conducting path of the transistor 1 and the current sensing resistance 5. The control input of the transistor 1 is also fed via the resistance 20 to the junction of the main conducting path of this transistor and the primary winding 4 of the transformer 2. The main conducting path of the transistor 12 is connected in anti-parallel within diode 13.The control input of the transistor 12 is fed both via a resistance 14 to the end which is connected to the main conducting path of the transistor 1 of the current sensing resistance 5 and, via a resistance 15, to the end connected to the secondary winding 6 of the parallel circuit of the current sensing resistance 8 and the capacitor 16.

The operating voltage applied to the input terminals A,B of the blocking-oscillator-type converter can be direct voltage with high ripple. If the circuit is to be supplied from the alternating-current mains, a rectifier circuit 17 having input terminals A', B' and consisting of a rectifier bridge circuit 18 and a low capacity filter capacitor 19 can be connected in front of the blocking-oscillator-type converter. In addition to the accumulator 7 connected to the output of the blocking-oscillator-type converter, a motor 21 can be connected in parallel with the accumulator7 via a two-pole closing switch 22, in which arrangement a resistance 23 can be connected via the second closing contact of the switch 22 in parallel with the parallel circuit consisting of resistance 8 and the capacitor 16.

In the text which follows, the operation of the circuit according to Figure 1 will be described in simplified form. As soon as a direct voltage of sufficient magnitude is present at the input terminals A, B, where A is the positive terminal, a small control current flows via the resistance 20 into the base of transistor 1 and causes the transistor 1 to become conductive. This reduces the collector/emitter vol tage of this transistor and the voltage difference between the input voltage at the terminals A, B and the collector/emitter voltage of the transistor 1 will be present at the primary winding 4 of the transfor mer 2. This induces a voltage of the same polarity by transformer action in the secondary winding 6 (the end marked with a dot of the winding 6 being positive).Via the feedback network 9 or 10 and 11, respectively, which represents the connection be twen the positively oriented end of the secondary winding 6 and the control electrode of the transistor 1, an additional control current is fed in this way into the base of transistor 1. This finally causes this transistor to be driven into saturation. Correspond ing to the linear rise of current through an induct ance to which a constant voltage is applied, a triangularly increasing current occurs in the primary circuit of the converter which current also flows through the current sensing resistance 5 and there produces a voltage drop. This voltage is fed via the resistance 11 to the base of the braking transistor 12.

As soon as the threshold determined by the input characteristic of the bipolar transistor 12 is ex ceeded, the transistor 12 begins to conduct and takes over the current which hitherto has flowed as control current to the transistor 1 from the feedback network 9 or 10, 11, respectively. This causes the transistor 1 to be turned off.As soon as the current in the primary circuit of the blocking-oscillator-type converter decreases, the polarity of the voltages at the windings of the transformer are inverted and the switch transistor 1 is fully switched off via the current which now flows in the feedback network 9 or 10, 11, respectively, in the reverse direction and is held switched off until the voltage has collapsed at the windings of the transformer 2, that is to say the flux in the core of the transformer has been completely removed; only then is current through the resistance 20 able to switch the transistor 1 partially on again and the oscillating cycle recommences.

During the switching-off process, the diode 13 prevents the negative base/emitter voltage at transistor 1 from becoming too great, making it possible for the two capacitors 9 and 10 to reverse their charges.

During the phase of oscillation now following, during which the magnetic energy built up during the preceding end phase of conduction of the transistor 1 in the core of the transformer is reduced again, charging current for the accumulator 7 flows in the secondary circuit of the converter consisting of the series circuit of the secondary winding 6 of the transformer 2, the diode 3, the accumulator 7 and the current sensing resistance 8. The voltage occurring at the current sensing resistance 8 and which is proportional to the charging current of the accumulator, is integrated by the capacitor 16 and applied via the resistance 15 to the base of the switching-off transistor 12.By this means, a change, corresponding to the charging current of the accumulator, in the primary currentthreshold for switching offthe transistor 1 is achieved during its conducting phase.

By the fact that the charging current of the accumula tor is kept approximately constant even with a greatly fluctuating input voltage of the converter.

The charging current of the accumulator can be determined in a simple manner by means of the magnitude of the resistance 8. By this means also, for example, as indicated in Figure 1 by the motor 21, the switch 22 and the additional resistance 23, a motor connected to the output can be operated either directly from the accumulator when the accumulator has been adequately charged and no supply voltage is applied to the input terminals of the converter or can be operated via the secondary current of the converter in the case of mains supply.

This secondary current can be selected within wide limits in accordance with the operating current of the motor via the resistance 23 which is connected in parallel with the current sensing resistance 8. Thus, for example, it is possible to supply the accumulator with a charging current of 60 mA and, on the other hand, to supply 1 A operating current for the connected motor. This charging current for the accumulator or operating current for the motor, respectively, is kept approximately constant, for example for an input alternating-voltage range (at terminals A', B') of between 100 and 250 V.

Figure 2 shows another circuit arrangement according to the invention for charging an accumulator or for supplying a motor with a positive output voltage. Due to the fact that in the circuit according to Figure 2, the polarity of the secondary winding 6 is inverted in comparison with Figure 1, the voltage induced in it cannot be used as feedback voltage for the switch transistor 1. For this reason, an additional secondary winding 24 of the transformer 2 exists which, in conjunction with the feedback network 9, 10, 11, takes over the job of the winding 6 of Figure 1 for controlling the converter.Corresponding to the polarity of the charging current, which is inverted in comparison with the polarity of the charging current in Figure 1, the charging current sensing resistance 8 is in Figure 2 arranged in the connecting line of the accumulator to the reference potential of the circuit (terminal B). In addition, the two-pole closing switch is arranged in Figure 2 in such a manner that simultaneously the motor 21 can be connected in parallel with the accumulator 7 and a resistance 25 can be connected in parallel with the current sensing resistance 8. Apart from the structural differences, the operation of the circuit according to Figure 2 is analogous to the circuit described in Figure 1.

Components corresponding to each other have the same reference symbol in each case. Here also the same advantageous behaviour as regards stability of the charging current of the accumulator with the addition of a stability of the total load current in motor operation, with respect to fluctuations of the input voltage and selectable and greatly different charging currents of the accumulator or total load currents and in particular also operating currents of the motor are achieved. The particular advantage of this circuit, compared to the circuit described in Figure 1, consists mainly in that the voltage effective for the feedback can here be adjusted by means of the ratio of the number of turns of the windings 24 and 6 of the transformer 2, independently of the charging voltage of the accumulator, whereas the voltage available for feedback in the circuit according to Figure 1 is equal to the charging voltage of the accumulator, whereas the voltage available for feedback in the circuit according to Figure 1 is equal to the charging voltage of the accumulator. For ths reason, the circuit according to Figure 2 is particularly suitable as a charging circuit for accumulators having few series-connective cells, that is to say a low accumulator voltage.

Claims

1. A circuit arrangement for operating a load such as, for example, for charging an accumulator, from an electric energy source, this energy source being either a direct current energy source or an alternating-current energy source which is followed in the circuit by a rectifier and in which arrangement in parallel with the energy source a series circuit is provided which contains the primary winding of a blocking-oscillator-type converter, a first electronic switch having a control electrode and a resistance, that furthermore a second electronic switch is provided, one main connection of which is connected to the control electrode of the first electronic switch, and that the blocking-oscillator-type converter is provided with a secondary winding which is connected via a rectifying element to the load, for example to the accumulator to be charged, characterised in that the control electrode of the second electronic switch (12) has applied to it simultaneously both an electric control value which is proportional to the current flowing through the primary winding (4) of the blocking-oscillator-type converter (6) and an electric control value which is proportional to the load current, for example the charging current (12) of the accumulator.

2. A circuit arrangement according to Claim 1, characterised in that the control electrode of the second electronic switch is connected both to a first current sensor which is connected in series in the load circuit and to a second current sensor which is connected in series with the primary winding.

3. A circuit arrangement according to Claim 1 or 2, charactrisd in that the first current sensor is formed by an RC section, the time constant of which is greater than the duration of the period of the oscillating frequency of the electromagnetic converter.

4. A circuit arrangement according to Claim 1,2 or 3, characterisd in that the second current sensor is formed by an Ohmic resistance.

5. A circuit arrangement according to one of Claims 1 to 4, characterised in that the control electrode of the second electronic switch is connected to the current sensors via in each case one weighting resistance.

6. A circuit arrangement according to one of Claims 1 to 5, characterised in that the main current path of the second electronic switch is connected in parallel with a second secondary winding of the blocking-oscillator-type converter, if necessary via a feedback network.

7. A circuit arrangement according to one of Claims 1 to 6, characterised in that the first current sensor is constructed to be variable for the operation of a motor by the accumulator and/or the circuit arrangement.