GB2347523A

GB2347523A - Battery charger with current control

Info

Publication number: GB2347523A
Application number: GB0004300A
Authority: GB
Inventors: Guenter Lohr
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1999-03-02
Filing date: 2000-02-23
Publication date: 2000-09-06
Anticipated expiration: 2020-02-23
Also published as: DE19908991A1; GB2347523B; DE19908991B4; GB0004300D0

Abstract

A battery charging circuit has AC input terminals 1,2 and DC output terminals 5,6 connected through a transformer 7 and rectifier bridge 8. Current in the transformer primary winding is controlled by a triac 23 in series with a shunt resistor 32 which controls a transistor switch 24 and phase-determining circuit 11,12,13,31,41,42 so as to limit the current flow. A further transistor may be connected in anti-parallel with transistor 24 so as to control current flowing in each direction.

Description

Battery charger The invention concerns a current-regulated charger for

batteries, having an input for a primary alternating voltage, a transformer and an output for a secondary voltage produced by the transformer. Such current- regulated chargers can be used for charging batteries having different operating voltages. The required charging voltage is set automafically_byimpressing a predetermined charging current onto this voltage by means of the charger, Known chargers realise current regulation at the secondary side of the transformer with the aid of an in-phase regulator or a phase control circuit.

Advantages of the invention In contrast the invention proposes to arrange a phase control circuit to limit the output current value at the primary side of the transformer. The steep edges of the transformer input current resulting from the phase control at the primary are transformed very effectively and for partial load operation, which takes up the largest portion of the operating time of the transformer while batteries are being charged, enables a compact, cost-effective transformer to be used.

A switching element of the phase control circuit preferably contains a 6ac, since this, once triggered, remains conducting until, at the end of a half-cycle of the phase of the primary alternating voltage, the current flowing exceeds a limiting value. A diac can be provided for reliable triggering of the triac; this diac receives a trigger voltage and drives the control input of the triac with a steeply rising voltage pulse after the trigger voltage of the diac is exceeded.

in order to sensitively control the trigger point of the switching element and to generate the trigger voltage, an RC element is preferably used which contains a first capacitor and, parallel to this, a storage element having at least a second capacitor and a first diode. As long as the voltage which is increasing at the first capacitor at the start of a half-cycle does not exceed the sum of the junction voltage of the diode and the voltage stored in the second capacitor by an earlier half-cycle, only the first capacitor is charged up. If this sum is exceeded, both are charged up in parallel, which takes place more slowly because of the increased capacitance. Changes in the voltage stored in the second capacitor cause a change in the time delay between the start of a halfcycle of the primary alternating voltage and the time at which the voltage at the capacitors exceeds the trigger voltage of the switching element, On the one hand, in order for the voltage stored in the second capacitor to change sufficiently slowly to facilitate regulation with a time constant of several cycles of the mains voltage, but on the other hand not slow down the rise in the voltage at the first capacitor too much when this capacitor is connected in parallel with the storage element, it is useful to fit the storage element with a third capacitor. This third capacitor can be connected in series with the first diode and the second capacitor and one of its electrodes can be connected to the switching element, so that the potential at this electrode represents a control potential for the switching element. Another possibility is to arrange the third capacitor in parallel with the second capacitor and connect a resistor in series with the last capacitor, so that a charging current flowing through the storage element is distributed between the two capacitors and voltage changes at the second capacitor are slowed down by the resistor. In both cases, the third capacitor preferably has a smaller capacitance than the second one.

The phase control circuit preferably contains a second switching element that discharges the second capacitor if the current through the primary winding of the I transformer exceeds a limiting value. The residual charge remaining in this capacitor at the end of a half-cycle of the primary alternating voltage depends on the time at which the current through the primary winding has exceeded the limiting value, and thus also on the secondary current to be limited. Due to the action of the first diode, this residual charge remains stored with the same sign until the following half-cycle and influences the charging characteristic of the RC element in this following half-cycle.

Accordmig to a first variant of the charger, a second diode is connected in antiparallel to the first diode and to the second capacitor. For the second halfcycle, this diode, together with the third capacitor, causes, with opposite sign, a slowing down of the rise in the charging voltage at the first capacitor, exactly as the first diode did with the third capacitor for the first half-cycle.

In a second variant, provision is made for the RC element to contain, parallel to the first capacitor, a second storage element having at least a fourth capacitor and a diode connected in inverse parallel to the first diode. For the second half-cycle, this second storage element fulfils the same regulating fimction as does the first storage element for the first half-cycle.

A third switching element discharges the fourth capacitor if the current through the primary winding of the transformer exceeds the limiting value with an opposite sign.

Further features and advantages of the invention are revealed in the following description of exemplary embodiments with reference to the figures.

Figures Figure 1 shows a circuit diagram of a charger according to a first development of the invention; Figure 2 shows voltage waveforms at various points of the circuit of Figure I as a function of time; Figures 3 and 4 show circuit diagrams of a phase control circuit of a charger according to further developments of the invention.

Description of the exemplary embodiments

Figure I shows a circuit diagram of a charger according to a first development of the invention. The charger has input terminals 1,2 for connection to a mains power supply. A phase control circuit extends between the input terminals 1,2 and primary terminals 3 and 4 of a transformer 7. A rectifier 8, for example a Graetz full-wave bridge rectifier, rectifies the secondary current of the transformer 7 and outputs it to output terminals 5,6 to which the battery to be charged can be connected.

In order to simplify the further description, the potential at the terminal 2 is assumed to be ground potential. This deterinination has no effect whatsoever on the construction and mode of operation of the circuit.

A primary current path runs from the input terminal I to the ground terminal 2 via the primary winding of the transformer 7 located between the terminals 3 and 4, a first switching element 21 and a shunt 32. The switching element 21 is constructed from a triac 23 and a diac 22 arranged in the current path, which receives an input signal from the switching element 21 if the voltage difference at the diac 22 exceeds a limiting value and as a result biases the triac, 23 into conduction.

An RC element is used to bias the switching element 21 into conduction. This PC element contains a first resistor 31 and a first capacitor 11, which, together with the shunt 32, is connected 'in series between the terminals 4 and 2. The control input of the switching element 21 is connected to the interconnecting point 51 between the first resistor 3 1 and the first capacitor 11.

Furthermore, the point 51 is connected to the ground terminal 2 via a storage element consisting of a second capacitor 13, a first diode 41 and a third capacitor 12. The second capacitor 13 is an electrolytic capacitor and has a considerably higher capacitance than the third capacitor 12. The polarity of the diode 41 is such that it is conducting for a positive half-cycle applied to the terminal 1. A second diode 42 of opposite polarity to the first diode 41 is arranged between the second electrode of the third capacitor 12 at a point 54 and the ground terminal 2.

A transistor 24 forms a second switching element. The collector of the transistor 24 is connected to a point 52 at the connection between the first diode 41 and the second capacitor 13, the base is connected via a resistor 33 and the emitter is directly connected in each case to a terminal of the shunt 32.

A further capacitor 16 is connected between the input terminals I and 2 to suppress radio interference which is caused by the switching operations in the circuit.

The mode of operation of the circuit is explained with reference to Figure 2. Figure 2 contains three curves A, B, C, showing the waveforms of the voltages with respect to time at the input terminal 1, the point 51 and the shunt 32, respectively. A primazy alternating or mains voltage A "in the form of a sinusoidal oscillation appears at the input terminal 1.

6 - At the start of the positive half-cycle of the mains voltage A at time t. the triac 23 is still conducting since a magnetisation current of the transformer 7 is still flowing from the previous negative half-cycle of the mains. The potential B at the point 51 goes towards ground potential since the capacitor I I discharges via the resistor 31 and the triac 23. After the transformer is demagnefised, the triac 23 is turned off and via the components 31 and I I of the RC element the voltage rises to the original value of the mains voltage (Q. The voltage drop at the shunt 32 can be ignored since the resistance value of the resistor 31 is considerably higher than that of the shunt 32. As a result of this voltage rise the first capacitor I I charges up rapidly and the potential B increases. The potential D at the point 54 likewise increases since both diodes 41 and 42 are blocking and the voltage at the third capacitor 12 therefore remains constant.

If the potential at the point 54 exceeds the potential at the point 52 by the forward voltage of the diode, the diode 41 becomes conducting (t2). A portion of the charging current from the resistor 3 1, that previously flowed into the first capacitor 11, now flows into the third capacitor 12, the diode 41 and the second capacitor 13. This slows down the charging up of the first capacitor 11. If, at time t3l the potential B at the point 51 reaches the positive diac trigger voltage, the voltage at the third capacitor 12 is the difference between the positive diac trigger voltage and the control voltage 52, reduced by the forward voltage of the diode 4 1. The diac 22 and the triac 23 are triggered and the potentials B and D fall instantaneously to lower values. Both diodes 41 and 42 are therefore again blocking. The first capacitor I I discharges via the first resistor 3 1 and the triac 23, whereby the potentials B and D slowly decrease.

After the triac is triggered, the mains voltage A appears at the primary winding of the transformer. The transformed voltage drives the charging current into the battery via the rectifier 8. This charging current is transformed at the primary side of the transformer 7 and causes a voltage drop across the shunt resistor 32, which is represented as potential C in Figure 2.

If the potential C at point 53 exceeds the forward base-emitter voltage of the transistor 24, this transistor is biased on and it discharges the second capacitor 13 to a small extent. In the stationary state, this discharge again cancels out the previous charging between t2 and The resistor 33 protects the transistor 24 against excessively high base currents.

After the charging current pulse has ended shortly before the mains voltage zero crossing, the magnetisation current again flows into the transformer 7 and continues to maintain the triac 23 'in a conducting state. After the magnetisation current decays at time t, the triac 23 turns off and the negative half-cycle of the mains appears at the resistor 3 1 and the capacitor I I of the RC element. The potentials 51 and 54 therefore go rapidly to negative values.

If the potential 54 exceeds the ground potential by the forward voltage of a diode (Q, the diode 42 becomes conducting. The third capacitor 12 is again recharged and the rise in potential at the point 51 slows down. On reaching the negative diac trigger voltage, the voltage at the third capacitor 12 reduces the negative diac trigger voltage by the forward voltage of the diode 42 (t7). The potential at the point 52 does not change in the negative half-cycle of the mains.

In the positive half-cycle of the mains the potential B reaches the positive diac trigger voltage and the negative diac trigger voltage in the negative half-cycle of the mains; it is therefore boosted by twice the diac trigger voltage. On the other hand, the potential D goes between ground and the control potential at the point 52, if the forward voltages of the diodes are ignored.

The recharging of the third capacitor 12 in the two half-cycles of the mains is thus influenced by the control potential at the point 52. If the control potential is twice, the diac trigger potential, the third capacitor is not recharged. The maximum recharge is produced when the control potential is at ground potential. The greater the recharging of the third capacitor 12, the greater the delay to the triac triggering. Late triggering of the triac in the mains half-cycle means a smaller charging current thus less discharge of the second capacitor 13 in the positive mains halfcycle. This results in a rise in the potential 52 and earlier triggering of the triac. A regulated charging current is therefore produced.

The value of a current injected into a battery by the charger depends on the resistance value of the shunt 32. This is typically rated so that a charger designed to charge batteries with a capacity of approximately I to 1.4 ampere hours injects a charging current of about 250 milliamperes.

Only a slightly fluctuating positive potential at the point 52 appears during the entire operation of the circuit. The value of this positive potential determines at which instant a half-cycle begins to charge the first capacitor 11, as.well as the storage element consisting of the capacitors 12, 13 and the diodes 41, 42. The higher the voltage at the point 52, the later this instant occurs and the earlier the trigger voltage of the switching element 21 is reached. Therefore, with a rising charging state of the battery the instantaneous value of the mains voltage rises at the trigger point of the diac 22 or the triac 23, respectively; the secondary voltage rises and the charging current remains substantially constant over a greater part of the charging cycle.

Figure 3 shows a variant of the phase control circuit of Figure 1. In this variant, a diode 46 is connected between base and emitter of the transistor 24 as protection against voltage spikes.

I The mode of operation of this circuit is essentially the same as described above with reference to Figure 2.

Figure 4 shows a further-developed variant of the phase control circuit of Figure 1. A capacitor 15 and a second transistor 25, with collector and emitter parallel to the capacitor 15, are added between the diode 42 and the ground terminal 2. The second transistor is a p-n-p transistor, compared to the first n-p-n transistor 24. The bases of both transistors 24, 25 are connected via a resistor 33 to the point 5'). The second transistor 25 is biased on during the negative half-cycle of the mains voltage A in the same way as the first transistor 24 is during the positive half-cycle. A negative control potential, analogous to the positive control potential at the point 52, is thus obtained at the point 55 connected to the collector of the second transistor 25.

The difference of these two control potentials is definitive for the recharging of the third capacitor 12 and therefore for the trigger angle of the diac 22. The remaining functions of the circuit are the same as those described with reference to Figure 2.

A dynamic regulation of the charging current is possible by evaluating the charging current in the positive and negative half-cycles.

Claims

1. Current-regulated charger for batteries, having input terminals (1,2) for a primary alternating voltage, a transformer (7) and output terminals (5,6) for a secondary voltage produced by the transformer (7), characterised in that a phase control circuit for limiting a current value appearing at the output terminals (5,6) is arranged at the primary side of the transformer (7).

2. Charger according to Claim 1, characterised in that a switching element (2 1) of the phase control circuit contains a triac (23).

3. Charger according to Claim 2, characterised in that the switching element (2 1) contains a triac (22) that receives a trigger voltage and drives the control input of the triac (23).

4. Charger according to one of Claims 2 to 4, characterised in that the phase control circuit contains an RC element having a first capacitor (11) and parallel to said first capacitor (11) a storage element having at least a second capacitor (13) and a first diode (41) connected in series.

5. Charger according to Claim 4, characterised in that the storage element contains a third capacitor (12) connected in series, of which one electrode is connected to the switching element (2 1).

6. Charger according to Claim 5, characterised in that the third capacitor (12) has a smaller capacitance than the second capacitor (13).

7. Charger according to Claim 5 or 6, characterised in that a second diode (42) is connected in antiparallel to the first diode (4 1) and to the second capacitor (13).

8. Charger according to one of Claims 5 to 6, characterised in that the RC element contains, parallel to the first capacitor (11), a second storage element having at least a fourth capacitor (15) and a diode (42) connected in miverse parallel to the first diode (41).

9. Charger according to one of Claims 4 to 8, characterised in that the phase control circuit contains a second switching element (24), that discharges the second capacitor (13) if the current through the primary winding of the transformer exceeds a limiting value.

10. Charger according to Claim 8 and Claim 9, characterised in that the phase control circuit contains a third switching element (25), that discharges the fourth capacitor (15) if the current through the primary winding of the transformer (7) exceeds the limiting value with an opposite sign.

11. Any of the chargers as hereiribefore described with reference to the accompanying drawings.