GB2041669A - DC-AC-DC converter - Google Patents

Abstract

The converter includes a chopping transistor (7) in series with a transformer primary (11), and a thyristor (61) for short-circuiting the base of transistor (7) under control of the positive output voltage of a regulating circuit (80). The gate of thyristor (61) in biased by a voltage of negative polarity and developed using voltage pulses originating from the charge on the transformer when a chopping transistor (7) is saturated. The converter is used to supply a TV receiver at a constant d.c. voltage and starts under a rated load without trip- out. <IMAGE>

Description

SPECIFICATION An electrical supply system having a chopping facility The invention relates to electrical supply systems having a chopping facility and adapted to deliver considerable power on starting and to chop a d.c.

input voltage to obtain an output voltage of controlled amplitude over a wide variation in the input voltage and a wide variation in the power output.

Chopping is used so as to limit losses to the absolute minimum. This saves considerable energy, which is valuable in applications such as the power supply to television sets.

Supply systems of this kind are known and have been described in particular in French Patent Application No: 76 08933 filed by the present Applicants on 26 March 1976 under the title: "A supply system comprising a chopping facility and voltage regulation".

Figure lisa simplified diagram of the supply system described in the Application. The following are disposed in series between the positive and negative terminals of a d.c. voltage source: A primary winding 11, a damping cell comprising a resistor 21 and a diode 22 in parallel, a chopping transistor 7 and a measuring resistor 8. A protective cell is connected between the emitter and the collector of transistor 7 and comprises a capacitor 31 in series with a diode 33 shunted by a resistor 32.

A control circuit 90 sends release signals to the base transistor 7. The signals begin with a release signal supplied by a generator inside circuit 90. The pulse releases transistor 7 and the current flowing in the primary winding 11 induces a feedback voltage in a first secondary winding 12, which is connected to the control circuit 90. The feedback voltage takes over from the release pulse supplied by the internal generator, so that the current flowing through the primary winding 11 increases in substantially linear manner.

Thyristor 61 becomes conductive when the current in the primary winding 11 reaches a value determined by a regulating circuit 80 in dependence on the supply voltage and the load supplied by the device.

The thyristor connects the base of transistor 7 to the negative terminal of the d.c. supply source via an inductance 62 connected in parallel with a resistor 63 and a voltage generator 40 whose negative terminal is connected to the base of transistor 7. This unsaturates transistor 7, which becomes nonconductive, thus cutting off the current in the primary winding 11.

Energy thus accumulates in the transformer during the flow of current in primary winding 11 and is then discharged in the operating circuit via a second secondary winding 14 and a rectifying diode 100.

The direction of winding 14 and the direction in which diode 100 is connected are such that the operating circuit is efficiently supplied during the discharging of the transformer. The thyristor is triggered as follows: The reference potential is taken from the emitter of the chopping transistor 7, so that the voltage measured at the terminals of the measuring resistor 8 varies negatively with respect to the reference potential. The voltage is applied to the cathode of thyristor 61, which is triggered when the cathode voltage is 0.6 V below its gate voltage. In order to ensure that the voltage at the terminals of resistor 8 and consequently the current in the primary winding 11 vary in a manner compatible with a given rated output power, the gate of thyristor 61 is polarized negatively with respect to the emitter of transistor 7.The negative biasing is obtained by a bias circuit 50 via a resistor 65. The bias circuit is supplied via the secondary winding 12.

The negative voltage supplied by the bias circuit 50 is thus obtained by rectifying the voltage pulses present at the secondary winding 12 during the discharging of the transformer. At starting, therefore, the d.c. voltage is first zero and then progressively increases. Circuit 80 measures the voltage obtained from a third secondary winding 13 connected to the emitter of transistor 7 and to circuit 80, in a direction such that it supplies circuit 80 with positive pulses during the discharging ofthetrans- former. The voltage measured by circuit 80, therefore, is substantially the voltage supplied by the supply system to its load, depending on the value of the load and on the mains voltage.Circuit 80, using the voltage measured at the terminals of winding 13, delivers a substantially constant positive voltage which varies in the same direction as the voltage supplied by the system to its load. The positive voltage supplied by circuit 80 is added by resistor 88 to the negative voltage supplied by the bias circuit 50, but the sum of the two remains negative relative to the emitter of transistor 7.

Thus, as a result of the negative biasing by circuit 50, the regulating circuit 80 is used to vary the biasing of the gate of thyristor 61 relative to the emitter of transistor 7, thus acting on the range of variation of the current flowing through the primary winding 11 and the conduction time of transistor 7 so as to maintain a constant voltage supply irrespective of variations in the load and the mains voltage.

On starting, the voltage measured by circuit 80 is zero. As we have seen, however, the negative voltage supplied by circuit 50 is likewise zero. The gate of transistor 61 is thus brought to zero potential relative to the emitter of transistor 7. The cathode gate voltage of thyristor 61 is then the voltage generated at the terminals of resistor 8. The limit to the possible variation in the voltage at the terminals of resistor 8 relative to the emitter of transistor 7 is therefore the thyristor triggering threshold, i.e., substantially -0.6 V.

The variation in the current in the primary winding 11 is thus very limited, like the conduction time of the transistor. Consequently, the energy supplied by the supply system to its output load will be very small. If the load on starting is too large, even though acceptable during normal operation, the supply system cannot give the required output energy. For this reason, no negative bias voltage will be supplied by circuit 50, and it will be even more difficult for the regulating circuit 80 to supply a regulating voltage. The result is the equivalent of a trip-out on starting.

To obviate this disadvantage, it is proposed according to the invention to rectify the negative pulses supplied by a secondary winding during the pulse charging the transformer. The system can thus supply power equal to its rated power, directly on starting.

Other features and advantages of the invention will be clear from the following description, given by way of non-limitative example, with reference to the accompanying drawings in which: Figure lisa simplified diagram of a known supply system; Figure 2 is a simplified diagram of a supply system according to the invention, and Figure 3 is a diagram of component 51 of Figure 2.

In the embodiment of the invention shown in Figure 2, a fourth secondary winding 15 is connected to the emitter of transistor 7 and to the negative bias circuit 51, which corresponds to circuit 50 of Figure 1, but is in an embodiment adapted to the invention.

As Figure 2 shows, secondary winding 15 is connected in a direction such that pulses applied to circuit 51 are negative when transistor 7 conducts.

The voltage of these negative pulses is therefore related to the mains voltage and not to the output voltage of the system. On starting therefore, even when the mains voltage is very small, the negative voltage supplied by the secondary winding 15 is considerable and not zero, like the voltage supplied by the secondary winding 12. Circuit 51 rectifies the negative voltage pulses and on starting, after a few half-waves, delivers a negative d.c. voltage sufficient to bias the gate of thyrister 61 properly via resistor 65.

In the embodiment of circuit 51 shown in Figure 3, negative pulses supplied by winding 15 during the charging of the transformer are rectified by diode 52.

The rectified pulses charge a capacitor 54 via a protective resistor 53. The d.c. voltage supplied by capacitor 54 is regulated via a zener diode 56 and a resistor 55. Accordingly, the negative voltage required for biasing the gate of thyristor 61 is obtained at the common point of resistor 35 and zener diode 56, and is applied to the gate via resistor 65. The other end of secondary winding 15 is connected to capacitor 54 and to zener diode 56 at a connecting point 57 corresponding in Figure 2 to the emitter of transistor 7.

In an embodiment, the following values were used: resistor 53,470 ohms; capacitor 54, 22 microfarads; resistor 55, 1 kohm; zenor diode 56, 12 V; and resistor 65, 10 kohms. The secondary winding 15 was dimensioned so as to deliver pulses adequate in all cases to charge capacitor 54 to a voltage higher than that of zener diode 56. It was found that under these conditions, the supply system could start-at the rated load in all cases.

As shown in Figure 2, the secondary windings 13 and 15 are wound in the same direction starting from the common point 57. If, therefore, the regulating circuit 80 and the secondary winding 13 are dimensioned so that negative voltage pulses delivered by the secondary winding 13 during the charging of the transformer are of sufficient value, after rectification, to deliver the negative voltage for biasing the thyristor gate, the secondary winding 15 can be replaced by winding 13 by connecting the latter simultaneously to the regulating circuit 80 and to the negative bias circuit 51. In that case, negative pulses will be used when charging the transformer in circuit 51 and positive pulses when discharging the transformer in circuit 80.

Claims (5)

1. An electrical supply system having a chopping facility and adapted to deliver considerable power on starting, of the kind comprising a d.c. voltage source supplying in series a transformer primary winding and a chopping transistor, a control circuit for supplying the base of the chopping transistor with recurrent release pulses, a first secondary winding of the transformer for supplying the base of the chopping transistor, after each release pulse, with a feedback voltage adapted to prolong the action of the release pulse, a thyristor adapted, under the action of a regulating voltage delivered by a regulating circuit, to short-circuit the base of the chopping transistor so as to inactivate it, a second secondary winding of the transformer for supplying the regulating circuit, each time after the chopping transistor has been inactivated, with a first voltage for measuring the voltage output of the supply device, and means for supplying the thyristor with a bias voltage of opposite polarity to the regulating voltage, characterised in that the thyristor-biasing means comprise means for generating the bias voltage from a second voltage which has the opposite polarity from the first voltage and is induced in the transformer during the inactivation of the chopping transistor.

2. A supply system according to claim 1, characterised in that the biasing means comprise a third secondary winding of the transformer connected in the opposite direction from the first secondary winding and delivering the second voltage.

3. A supply system according to claim 1, characterised in that the secondary winding can also supply the second voltage, the first and second voltages being isolated by diodes in the regulating circuit and in the biasing means respectively.

4. A supply system according to claim 2 or 3, characterised in that the biasing means comprise a diode for charging a capacitor with the second voltage and a voltage regulator connected to the capacitor and adapted to deliver a given bias voltage.

5. An electrical supply system substantially as hereinbefore described with reference to, and as illustrated in Figures 2 and 3 of the accompanying drawings.