GB1589662A - Power supply circuits - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO POWER SUPPLY CIRCUITS (71) We, COULD ADVANCE IdMITED, a British Company of Roebuck Road, Hainault, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to starter circuits for power supplies and to power supplies incorporating such starter circuits.

In power supplies including a rectifier.

capacitor combination large switch-on surges may occur if the switch-on occurs at a time when the a.c. supply is at or near its peak value.

Such surges may also occur in transformer input circuits when the magnetic core is left magnetised in one direction at switch-off. At the subsequent switch-on, and in particular if the first half cycle of the mains is magnetising the core in the same direction as its remanent magnetisation partial saturation of the core occurs and a large surge current flows. Such surges can also occur at switch-on with nonlinear loads such as tungsten filament lamps.

It is an abject of the present invention to provide circuit arrangements in which this disadvantage may be overcome or reduced.

According to the present invention there is provided a starter circuit for supplying current to a load, the circuit comprising a controlled switching device for controlling the flow of current to the load from a current source arranged to provide either a.c. or full wave rectified undirectional current, and means for controlling operation of the switching device so that an initial switch-on current only flows to the load near the end of a half cycle of current from the current source, and means for advancing the instant of operation of the switching device within a said half cycle, wherein the controlling means comprises means for sensing the instantaneous voltage of the current source, and means for sensing the direction of change of instantaneous voltage.

Features and advantages of the invention will become apparent from the following description of embodiments thereof, when taken in conjunction with the accompanying drawings, in. which; Figure 1 is a graph illustrative of the operation of the circuits shown in Figures 2 and 3; Figure 2 shows a power supply arrangement which is controlled in response to a voltage in the circuit; Figure 3 shows a modification of the power supply arrangement of Figure 2; Figure 4 shows a full wave power supply arrangement based on Figure 3; Figure 5 shows a voltage stabiliser based on the arrangement of Figure 3, and Figure 6 shows the voltage stabiliser of Figure 5 combined with a conventional voltage stabiliser.

The graph of Figure 1 shows one half cycle of an applied alternating voltage, in which the ordinate V represents voltage and the abscissa t represents time. Iri starter circuits embodying the invention it is arranged that a thyristor in series with the load is only allowed to become conducting when the voltage across the thyristor is less than a predetermined voltage Vc. In this way it is arranged that the voltage initially applied to a reservoir capacitor is never too large to cause excessive current to flow into said capacitor. It will be appreciated that this condition will ensure that switching can occur only during the parts of the cycle represented on the graph of Figure 2 by the line from the origin 0 to the point P or between the point Q and the point R where the curve subsequently crosses the abscissa.

Switching at any time during the time OP is not acceptable since the voltage applied sub sequently increases to its maximum value, and consequently it must be ensured that switching can only occur during the time corresponding to the position QR. The conditions then for switching are that both the voltage across the thyristor is less than Vc, and also that the rate of change of the voltage is negative. This is achieved in the circuit shown in Figure 2, in which a source 61 is connected to an input terminal 62 which is connected to an output terminal 63 through a thyristor 65. This source 61 can either be an alternating current source or a full wave rectified unidirectional current source. The output terminal 63 is shunted to a common line by the reservoir capacitor 64.The trigger electrode of thyristor 65 is shunted to the output terminal al 63 by two transistors 66 and 67 connected in parallel. If either of these transistors is conductive, then the thyristor 65 is prevented from conducting. Current is supplied to the two transistors 66 and 67 from the terminal 62 through a diode 68 and a load resistance 69. The base of transistor 66 is connected to the junction of diode 68 and resistor 69 through a zener diode 70. When the voltage between terminals 62 and 63 is greater than a voltage determined by the zener diode 70, transistor 66 is rendered conducting. This represents the condition that the voltage between terminals 62 and 63 is greater than Vc.In addition the junction of diode 68 and 69 is connected through a capacitor 71 and a diode 72 to the terminal 63, the junction of capacitor 71 and diode 72 being connected to the base of transistor 67, The differentiation produced by capacitor 71 and diode 72 ensures that whenever the rate of change of voltage between terminals 62 and 63 is positive transistor 67 is rendered conducting. Thus the two transistors 66 and 67 ensure that when the voltage across terminals 66 and 63 has a rate of change which is positive and the voltage is greater than Vc the thyristor 65 is prevented from conducting. Accordingly the thyristor can only fire during times corresponding to an applied voltage in the region QR of the graph of Figure 1. As the capacitor 64 charges the level Vc rises thus advancing the firing of the thyristor.The maximum level of Vc is the maximum amplitude of the a.c. power supply and so the maximum advance can only be to 90 .

The circuit shown in Figure 3 is similar to that of Figure 2 but the functions of the two transistors 66 and 67 are combined in a single transistor 73 the voltage circuit 70 and the differentiating circuit 71, 72 being connected across it. The operation is similar to that of the circuit of Figure 2.

The circuits of Figure 2 and Figure 3 are both half wave rectifier circuits but they can of course be used in a full wave manner as described previously, and the circuit shown in Figure 4 is similar to that of Figure 3 but adapted for full wave operation. The alternating current is supplied to a full wave rectifier bridge which includes the thyristor 65 and a further thyristor 65 connected as shown. Other portions of the circuit are similar to those of Figure 3 and have been provided with the same reference numerals as in Figure 3.

A circuit similar to that of Figure 3 may be used, as shown in Figure 5 as a voltage regulator.

The circuit only differs from that shown in Figure 3 by the fact that the emitter of the transistor 73 instead of being connected to the terminal 63 is connected to the common line.

In a further development, the regulator of Figure 5 may be used as a pre-regulator in association with a conventional voltage regulat or as shown in Figure 6. In the path From the input terminal 62 to the output terminal 63, a conventional series transistor stabiliser 75 is connected in series with the thyristor 65 and the emitter of transistor 73 is connected to the output terminal 63. The series stabiliser 75 is controlled in conventional fashion through a control circuit 76 which samples the output voltage at the output terminal 63.

Similarly the circuits shown in Figures 5 and 6 can also be connected in a full wave manner.

In the circuits shown in Figures 2 to 6 a current limiting resistor may be provided in series with the zener diode 70 and capacitor 71, and a base emitter resistor may be provided to take up the leakage for transistor 66 in Figure 2 or transistor 73 in Figures 3 to 6.

With an a.c. source, the thyristor may be replaced by a triac and the a.cO 'current control led accordingly, the load current path of the circuit being arranged in a similar manner to that disclosed in our co-pending Application No. 7934002 (Serial No. 1589664) divided from the present application.

WHAT WE CLAIM IS: 1. A starter circuit for supplying current to a load, the circuit comprising a controlled switching device for controlling flow of current to the load from a current source arranged to provide either a.c. or full wave rectified unidir ectional current, and means for controlling operation of the switching device so that an initial switch-on current only flows to the load near the end of the half cycle of current from the current source, and means for ad vancing the instant of operation of the switch ing device within a said half cycle, wherein the controlling means comprises means for sensing the instantaneous voltage of the current source, and means for sensing the direction of change of instantaneous voltage.

2. A starter circuit according to Claim 1, wherein the voltage sensing means and the direction sensing means are arranged to sense voltage and direction respectively for both half cycles of the current source.

3. A starter circuit according to either of the preceding claims, wherein the switching device is a silicon controlled rectifier or trial, 4. A starter circuit substantially as herein before described with reference to any one of Figures 2, 3 or 4.

5. A voltage regulator including a starter circuit according to any one of the preceding

Claims (1)

1. claims.

   6. A voltage regulator as claimed in Claim 5 and as described with reference to Figures 5 or 6