GB2056794A - Power supply converter - Google Patents

Abstract

The converter comprises an electronic switching device (SCR1) coupled in series with the primary winding (N1) of a transformer (M) across a rectified AC supply for producing in the primary winding (N1) a primary current pulsating at a desired frequency. Coupled across a secondary winding (N2) of the transformer (M) is a rectifier (D3) and capacitor (C2) arranged, by selection of the value of the capacitor (C2), as a tuned rectifier such that the current in the secondary winding (N2) is cut off by the rectifier action at a low value of the primary current before the switching device (SCR1) is commutated. Such a converter may be used in a television receiver with the transformer (M) comprising the television receiver line transformer and with one secondary winding (N2) supplying the sound circuits of the receiver and a further secondary winding (N3) supplying the picture circuits and, in such application, has the advantage of minimizing the effect of sound load variations upon the size and/or brightness of the picture. <IMAGE>

Description

SPECIFICATION Source of current The present invention relates to a source of current comprising a transformer with primary and secondary windings, there being, con nected in series with the primary winding, an electronic switch which is opened and closed in order to produce a primary current pulsating at a certain frequency and, connected in series with the secondary winding, at least one rectifier means for producing a rectified output voltage.

For example, in a television receiver or some other similar apparatus which has both a picture display unit and a sound output stage, the operating voltage of the sound output stage is usually taken, for economic reasons, directly from the line output stage or from the source of current of the line output stage. As a result, variations in the load of the sound output stage easily affect the size and/ or brightness of the picture.

Attempts have been made to eliminate this disadvantage by various means: 1. An A-class sound output stage; disadvantage: continuous consumption of energy.

2. Separate source of current for the sound output stage; disadvantage: expensive construction.

3. Loose coupling to the source of current of the picture display unit; disadvantage: high output impedance, in which case the sound power obtained remains low.

4. Compensation of change in picture size; in compensation circuits there appear time constants which with sound loads of certain types may even worsen the changes in picture size.

Furthermore, compensation of change in width often increases change in brightness, and vice versa.

Disadvantages also appear in other current source couplings which have the switch means mentioned in the ingress, primarily a transistor or a thyristor. When a transistor is used, the disadvantage mainly consists of losses at the time of connecting, which are manifested in heating of the transistor, and when a thyristor is used with high currents, the problem consists of providing a sufficiently effective commutation current.

For example, in couplings in accordance with USP 4,017,784 and SEP 208,927, the current of the primary switch is disconnected by means of a resonance circuit on the primary side. Also, the terminal voltage of the primary winding of the transformer deviates from rectangular and is therefore not suitable for a circuitry which in itself causes the commutation of the current of the primary switch.

During commutation, the current of the primary switch must not be decreased, and therefore it is necessary to develop a circuitry which loads the primary switch only during its positive half cycle.

The object of the present invention is to solve these problems, and in order to achieve this object the invention is characterized in that the secondary circuit is formed as a tuned rectifier by means of a capacitor connected to the secondary winding, the value of the capacitor having been selected in such a way that the current of the secondary winding is cut off by the effect of the rectifier means at a time when the current of the primary winding is low.

The invention is described below in greater detail in the form of examples and with reference to the accompanying drawings, in which Figure 1 depicts diagrammatically the line output stage of a television receiver, with its sources of current for picture and sound, Figure 2 depicts curve forms relating to the description of Fig. 1.

Figure 3 depicts a similar source of current, with only one load circuit, Figure 4 depicts a case corresponding to Fig. 3, when the connecting means is a transistor, and Figure 5 depicts the coupling of the source of current having two switches which operate during alternate phases.

The circuitry according to Fig. 1 includes the following components: M = differential transformer; D, = network rectifier; C, = filter capacitor; SCR,-D2 = primary switch, N, = primary winding of the transformer; N2 = secondary winding of transformer, from which the operating voltage of the sound output stage is rectified by means of a diode D3 to the capacitor C2; N2 = input winding of picture display unit; L, = filter coil; C3 = filter capacitor.

FIP 55278 discloses one television receiver circuitry, in which tha switch has been implemented using a transistor. In Fig. 2, 11a represents the current of the primary switch consisting of SCR,-D2 without a sound load, and l,b represents the same current with a sound load in the circuitry according to the said Patent 55278, when the switch has been implemented using a transistor.

U, is the voltage of the above switch, U2 is the secondary voltage of the transformer, 12 is the current of the secondary winding N2, with a sound load, and U3 is the voltage of the capacitor C2.

The operation is as follows: When the primary switch SCR, is in the current-conducting state, the terminals of the winding N, have a rectified direct voltage from the network of C, In this case the terminals of N2 have a voltage in accordance with the N,-N2 transformation ratio in such a manner that the anode of D3 has a positive voltage. If U2 is greater than U3, the diode D3 begins to conduct current.

Thereby C2 is connected, by transmission of N2, as the load of N,. Since there is always a coupling coefficient of some value, which is < 1, between N, and N2, N, "sees" as its load the diffusion inductance formed by the coupling coefficient and the coupling in series of the capacitance of C2, reduced by the transformation ratio. Thus a step voltage is fed in this case into the L-C chain connected in series, whereby a sinusoidal current, 12, is formed in this chain. Since the coupling takes place by transmission of the diode Ds, the circuit is capable of oscillating only one half cycle, i.e. power is transferred from the primary N, to the capacitor C2.

Thereby the terminal voltage of the capacitor C2 increases in accordance with the curve Us of Fig. 2 within the period t,-t3. This ripple of U3 is filtered by low-pass L,C3.

Curve Ila in Fig. 2 shows that the primary switch is in the current-conducting state during the period t,-t2, whereafter the current transfers to the parallel diode D2 for the time interval t3-t4, whereafter, at time t4, the primary switch opens. Since the primary switch in this case is a thyristor, its current must be disconnected for the release period required by the thyristor.

This is the period t3-t4. In order that the operation continue unperturbed, the current of the sound winding N2 must be disconnected at the time t3 at the latest. This can be achieved by selecting a suitable value for C2, i.e. by forming a secondary circuit tuned with the aid of N2 and C2 in such a manner that 12 can be disconnected at the time t3 or before.

Since the charging of the picture load power also takes place while the primary switch is conducting current into the diffusion inductance between N, and N3, this charge remains constant if the terminal voltage of the primary remains constant. The terminal voltage of the primary depends on the network voltage, on the resistance loss of N,, and on the saturation voltage of SCR,.

The network voltage remains substantially unchanged regardless of the variations in the sound load in question. The loss of voltage of the resistance of N, must be kept low in any case just to prevent heat formation, and therefore it is not significant. The saturation voltage of SCR, is constant since this is a thyristor.

Thus the current of N2 can vary during the period t,-t3 without the picture size changing.

In practice, values have been measured in which the sound load has varied between zero and twice the value of the picture load without a disturbing effect on the picture.

The solution according to the invention can also be applied to other sources of current than that of television output stage.

In the case described above, the winding N3 induces the commutating current of the thyristor SCR, to the primary of the transformer. If there is only one load circuit, as shown in Fig.

3, a separate commutating circuit is constructed by a known.method; in the figure, this commutating current consists of another thyristor SCR2, the inductance L2 and the capacitor C4. The tuned rectifier according to the invention offers even in this case a considerable advantage in that the thyrstor can be disconnected at low values of current, and therefore the commutation circuit is simple and inexpensive.

Fig. 4 depicts a corresponding situation when the switch is a controlled transistor T1, i.e. a so-called chopper current source is involved. The connecting losses can now be minimized by again tuning the connecting to take place at the time of low current of the transistor The connecting time can, when so desired, be for example determined by means of back coupling taken from the secondary circuit by means of logic, the transistor current being zero or close to zero at the time of connecting.

Finally, Fig. 5 depicts a source of current operating according to the alternate-phase principle, intended for higher voltage, for example, as a source of welding current. The coupling is the same as above, but on the primary side there are two thyristors, SCR, and SCR3, controlled to operate during alternate phases, and on the secondary side there are respectively two tuned rectifiers, having a common capacitor C2 which determines the resonance frequency.

Claims (8)

1. A source of current comprising a transformer (M) with primary and secondary windings (N1, N2, N3), there being connected in series with the primary winding (N1) an electronic switch (SCR1; T) which is opened and closed in order to produce a primary current pulsating at a certain frequency and coupled in series with the secondary winding (N2) at least one rectifier means (D3) for producing a rectified output voltage (Us), characterized in that the secondary circuit has been formed as a tuned rectifier by means of a capacitor (C2) connected to the secondary winding (N2), the value of the capacitor having been selected so that, under the effect of the rectifier means (D3), the current of the secondary winding (N2) is disconnected at a time when the current of the primary winding (N1) is low.

2. A source of current according to Claim 1, wherein the switch of the primary side has been implemented using a thyristor (SCR1), characterized in that the capacitor (C2) of the secondary side has been selected so that the current of the secondary winding (N2) is zeroed slightly before the thryistor receives its commutating current.

3. A source of current according to Claim 2 for a television receiver, wherein the transformer in question (M) consists of a line transformer, characterized in that the picture load and the sound load of the receiver have been connected to two different secondary windings, (N2, N3) of the line transformer and that the tuned rectifier has been formed using the secondary winding (N2) of the sound load.

4. A source of current according to Claim 1, wherein the switch of the primary side has been implemented using a transistor (T), characterized in that, by means of back coupling taken from the secondary circuit, the transistor is regulated to open at a time when the current of the secondary winding (N2) is zero or close to zero.

5. A source of current according to any of the above claims, wherein the primary circuit has two electronic switches (SCR1, SCR3) operating according to the alternate-phase principle, characterized in that the corresponding two secondary circuits of the transformer have been formed as tuned rectifiers and their rectified outputs have been coupled together.

6. A source of current according to Claim 5, characterized in that the capacitor (C2) which tunes the secondary circuit is common to the two secondary windings.

7. A current source substantially as herein described with reference to any of the accompanying drawings.

8. A television receiver incorporating a current source as claimed in any of the preceding claims.