GB2137441A - Capacitor charging circuit - Google Patents

Abstract

A circuit for charging a capacitor from a direct-current supply includes a transformer 10 having a centre-tapped primary winding and a secondary winding 16. The centre tap of the primary winding is connected to one side of the supply, whilst each end of the primary winding is connected through a separate switch 14, 15 to the other side of the supply. The two switches are arranged to be operated alternately. A capacitor 21 is connected to the secondary winding by an arrangement of diodes 17-20 such that the capacitor may be charged to a voltage greater than that of the supply. In another embodiment (Fig. 3) instead of a bridge of diodes and a single capacitor, a pair of parallel capacitors each in series with a respective diode, the diodes being oppositely poled, are provided. <IMAGE>

Description

SPECIFICATION Capacitor charging circuit This invention relates to a capacitor charging circuit intended for charging a capacitor which is to be used to supply power to a load.

Many forms of circuit are known for charging capacitors, the two common types being the forward converter circuit and the flyback converter circuit.

Each of these has disadvantages, the forward converter circuit for example being unable to charge the capacitor above a value equal to the supply voltage multiplied by the transformer turns ratio. The flyback converter circuit, on the other hand, is able to charge to a higher voltage than that of the supply but is slow in operation when the capacitor is in an almost completely discharged state.

It is an object of the invention to provide a capacitor charging which overcomes the above disaduantages.

According to the present invention there is provided a capacitor charging circuit which includes a transformer having a centre-tapped primary winding and a secondary winding, first and second switching means connected respectively one to each end of the primary winding and operable in turn to connect a direct-current supply voltage between the centre tap and each end of the winding, and a capacitor connected to the secondary winding by an arrangement of diodes such that the capacitor may be charged to a voltage exceeding that of the supply.

The invention will now be described with reference to the accompanying drawings, in which Figure 1 is a schematic diagram of a circuit according to a first embodiment: Figure 2 is a series of waveforms illustrating the operation of the circuit of Figure 1; Figure 3 is a schematic diagram of a circuit according to a second embodiment, and Figure 4 iliustrates the operation of the circuit of Figure 3.

Referring now to Figure 1, the circuit includes a transformer 10 having a primary winding in two halves 11 and 12 connected together at a centre tap 13. A first switch 14 is connected between the end of one half 11 of the primary winding and one side of a d.c. supply vpltage, whilst a second switch 15 is connected between the end of the other half 12 of the winding and the same side of the supply voltage.

The other side of the supply voltage is connected to the centre tap 13. The transformer secondary winding 16 is connected to a bridge rectifier circuit comprising diodes 17, 18, 19 and 20, poled as shown. A capacitor 21 is connected to the output of the bridge and to a load.

The switches 14 and 15 are operated alternately by means of a switch control mechanism 22 when the circuit is in operation.

The operation of the circuit of Figure 1 will now be described with reference also to Figure 2. Each time that the first switch 14 is closed a voltage appears across the half 11 of the primary winding as shown at a) in Figure 2. The time for which the switch 14 remains closed is arranged such that the waveform of the current i1 is approximately sinusoidal, as shown at b) in Figure 2. A similar current waveform (c) in Figure 20 is induced in the secondary winding 16, the magnitude of this current 3 being dependent upon the turns ratio N of the transformer secondary winding relative to each half of the primary winding.

The current i3 flows through diode 17, capacitor 21, and diode 20, causing the voltage across the capacitor 21 to increase, as shown at d) in Figure 2.

After switch 14 has opened, the second switch 15 is closed for a similar period of time. The voltage and current waveforms in the other half 12 of the primary winding are shown ate) and f) in Figure 2. The current i4 shown at g) resulting from the voltage induced in the secondary winding flows diode 19, capacitor 21 and diode 18, causing the voltage across capacitor 21 to increase again in the same sense as shown at h) in Figure 2.

The capacitor charges in the manner described above until the voltage across each is equal to N times the supply voltage, after which it cannot increase further by forward converter action. However, at this point, shown by the line B In Figure 2, flyback converter action takes over. The closing of switch 14 now has no effect on the voltage across the capacitor, but causes a current ii to flow at a rate determined by the inductance of the transformer half-winding 11, which result$ in energy being stored in the inductance of the transformer secon dary winding 18. When the switch 14 is opened therefore, this energy is released in the form of current due to a voltage greater than N times the supply voltage.This voltage causes a current to flow, but in the opposite direction to that due to forward converter action However, the action of the diode bridge causes the resulting current to flow in the same sense, to charge capacitor 21 further. Similarly, operation of switch 15 results in the charge on capacitor 21 being further increased. As with a conventional flyback converter circuit, there is no theoretical limit to the output voltage except that imposed by the non-perfect nature of the switching device and the transformer.

Figure 3 illustrates a circuit using a pair of storage capacitors to obtain a split output voltage.

The arrangement of the power supply, switches and transformer 10 is as already described. A first capacitor 31 is connected in series with a diode 32, poled as shown, across the secondary winding 16 of the transformer. A second capacitor 33 is connected in series with a second diode 34, poled oppositely to diode 32, across the same secondary winding. An output to a load is taken from the two capacitor diode junctions, resulting in a series connection of the two capacitors.

In operation the two capacitors 31 and 32 charge alternately. Initially with the charge on each capacitor at a low value, the current i3 resulting from the closing of switch 14, passes through diode 32 and capacitor 31, whilst the current i4 resulting from the closing of switch 15 passes through diode 34 and capacitor 33, thus increasing the voltage across the capacitors in opposite senses as shown at d) and h) in Figure 4. As in the previous embodiment, the above process continues until the voltages across the capacitors 31 and 33 are equal to N times the supply voltage. The flyback mode action takers over, again with the voltages across the two capacitors increasing alternately. For example, the opening of switch 14 after energy has been stored in the -transformer inductance causes cu current i4 to flow through capacitor 33, rather than current i3 through capacitor 31. Hence the voltages across the two capacitors continue to increase in opposite senses.

As already indicated, the load voltage is the sum of the voltages across the individual capacitors.

Claims (5)

1. A capacitor charging circuit which includes a transformer having a centre-tapped primary'winding and a secondary winding, first and second switching means connected respectively one to each end ofthe primary winding and operable in turn to connect a direct-current supply voltage between the centre tap and each end of the winding, and a capacitor connected to the secondary winding by an arrange ment of diodes such that the capacitor may be charged to a voltage exceeding that of the supply.

2. A circuit as claimed in Claim 1 in which a single capacitor is connected across the transformer secondary winding by means of a bridge rectifier circuit.

3. A circuit as claimed in Claims1 in which a first capacitor is connected in series with a first diode across the transformer secondary winding, and a second capacitor is connected in series with an oppositely-poled diode across the same transformer secondary winding.

4. A circuit as claimed in any one of Claims 1 to 3 which includes switch control means operable to -open and close the first and second switching means in a predetermined sequence.

5. A capacitor charging circuit substantially as herein described with reference to the accompany ing drawings.