MEDIATOR (PHILIPS) 66K365/16Z MULTISTANDARD CHASSIS KM2 SWITCH MODE POWER SUPPLY CIRCUIT.

Control circuit for a switched-mode power supply, particularly for a television receiver: PHILIPS CHASSIS CTV KM2 SWITCH MODE POWER SUPPLY.



A switched-mode power supply provided with a control stage and a switching stage coupled by means of a transformer. The collector of an additional transistor is connected to the transformer. In this manner the ratio of the collector current to the base current of the switching transistor can assume a predetermined value, for example a constant value whatever the value of the mains voltage applied to the power supply.

What is claimed is:

1. A control circuit for a switched-mode power supply, said power supply comprising a non-regulated rectified DC voltage source, a driver transistor, a first transformer having primary and secondary windings, an end of said primary being coupled to the collector-emitter path of said driver transistor, a switching transistor having a base coupled to said secondary, a second transformer having a primary winding coupled in series with said switching transistor, and a plurality of secondary windings, said control circuit comprising a first additional transistor having a collector coupled to the remaining end of the primary winding of the first transformer not connected to the driver-transistor and an emitter coupled to the non-regulated direct voltage source.

2. A control circuit as claimed in claim 1, further comprising a constant voltage source coupled to the base of the additional transistor.

3. A control circuit as claimed in claim 1, further comprising a constant current source, and a resistor coupled between the emitter of the additional transistor and the constant current source.

4. A control circuit as claimed in claim 3, wherein the constant current source comprises a second additional transistor, the two additional transistors being of complementary conductivity and their emitters being connected with each other through said resistor, the collector of the second additional transistor being coupled to the non-regulated rectified direct voltage source and the collector of the first additional transistor being coupled to the end of the primary winding of the first transformer not connected to the driver transistor.

5. A control circuit as claimed in claim 4, further comprising a resistor coupled in series with the collector circuit of said second additional transistor and the non-regulated rectified direct voltage source.

6. A control circuit as claimed in claim 5, further comprising a zener diode coupled between the base of the second additional transistor and the non-regulated voltage source.

7. A control circuit as claimed in claim 6, further comprising a resistance bridge coupled to the base of the first additional transistor and arranged between the two electrodes of the zener diode.

8. A control circuit as claimed in claim 7, wherein the driver transistor and the switching transistor do not conduct simultaneously, and the voltage between the two electrodes of the zener diode as well as the values of the resistors arranged between the said electrodes and of the resistor arranged between the emitters of the two additional transistors are chosen so that the first additional transistor is in the saturated state at the lowest value of the non-regulated voltage while it operates in the linear state at a higher value of said non-regulated voltage.

Description:

The present invention relates to a control circuit for a switched-mode power supply, particularly in a television receiver, said power supply comprising a rectified, non-regulated rectified DC voltage source, a driver transistor whose collector-emitter path is arranged in series with a primary winding of a first transformer, a secondary winding of the latter being coupled to the base of a switching transistor which is arranged in series with a primary winding of a second transformer having a plurality of secondary windings.

This type of switched-mode power supply is used more and more because of the numerous advantages it presents as regards energy efficiency, reliability, compactness, etc. However, as for the majority of the other types of power supplies, its operation on mains supplies of different voltages imposes the use of either a transformer with taps or switch-over from full wave rectification at the highest mains voltage to a voltage doubler rectification for the lowest mains voltage.

It is known that the specific qualities of a switched-mode power supply depend for a large part on the switching speed of the switching transistor at the moment at which the latter passes periodically from the conductive state to the blocking state; this speed is at its maximum when the switching transistor presents, at the turn-off moment, a certain ratio between the collector current and the base current IC/IB: if this ratio is too low, the delay in the recombination of the charges stored in the base increases the switching time; if it is too high there is the risk that the transistor is brought out of saturation before it is blocked, which results in its substantially immediate destruction. For the known switched-mode power supplies it is not possible to maintain a suitable IC/IB ratio in the presence of large variations of the non-regulated rectified DC voltage which result from the connection to the nominal mains voltages of, for example, 110 or 220 V; actually, if the variations in IB are substantially proportional to the variations in the non-regulated voltage, the same does not happen for those of the IC whose amplitude is less.

However, the importance of having a power supply which can operate without any switching on mains supplies of 110 or 220 V is evident: for the manufacturer it is cheaper to produce and the reliability is increased; while the user does not run the risk of incorrect manipulations, particularly when the power supply is destined for use in portable television sets.

One of the objects of the invention is to realize a control circuit which permits the switched-mode power supply to operate without switching in conditions which are substantially optimum and in the presence of mains voltage variations in the range of 90 to 250 Volts.

A further object of the invention is to ensure that said IC/IB ratio of the switching transistor has a predetermined and, more particularly a constant value at the turn-off moment whatever the value of the mains voltage applied to the power supply.

The control circuit according to the invention is characterized in that the end of the primary winding of the first transformer not connected to the driver transistor is connected to the collector of an additional transistor whose emitter is coupled with the non-regulated direct voltage source. Advantageously it is characterized in that the emitter of the additional transistor is connected to one end of a resistor, the other end of this resistor being connected to a constant current source, and that the constant current source is constituted by a second additional transistor, the two additional transistors being of complementary conductivity and their emitters being connected with each other through a resistor, whilst the collector of the second additional transistor is connected to one of the poles of the non-regulated rectified direct voltage source and the collector of the first additional transistor is connected to the end of the primary winding of the first transformer not connected to the driver transistor.

Whilst combining the action of a ballast transistor with that of a variable current generator, the circuit according to the invention thus maintains automatically a desired IC/IB ratio of the switching transistor whatever the value of the mains voltage applied to the power supply.

PHILIPS CHASSIS KM2:Two-state switched-mode power supply:

A switched-mode power supply having at least two output terminals for supplying at least two different voltages and capable of operating in an alternate state in which some of the voltages are reduced. The power supply includes at least two controllable voltage sources coupled, respectively, to the output terminals and at least one voltage regulating circuit coupled to control inputs of the voltage sources for reducing the output voltages of both voltage sources proportionately when the voltage on the output of a first one of the voltage sources exceeds a preset reference level. The power supply further includes a circuit for selectively coupling a second one of the voltages sources to the regulating circuit. If the output of the second voltage source exceeds that of the first voltage source, when the second voltage source is coupled to the regulating circuit, the output voltages of all the voltage sources are reduced proportionately, while the voltages on the power supply output terminals are also reduced except for the output terminal coupled to the regulating circuit.

1. A switched-mode power supply capable of operating optionally in two states, comprising at least two controllable voltage sources having respective control inputs, at least two power supply output terminals and first regulating means coupled to the control input on each of said voltage sources for supplying a signal which acts, on the switching operation, to reduce the values of the voltages supplied by all of the voltage sources when one voltage exceeds a first reference value, characterized in that said switched-mode power supply further comprises second regulating means also coupled to said control inputs for supplying a signal which acts, on the switching operation, to reduce the values of the voltages supplied by all the voltage sources when a voltage exceeds a second reference voltage,

first change-over elements for selectively connecting an output of one of the voltage sources to the first regulating means in a first state or to the second regulating means in a second state,

second change-over elements for selectively connecting one of said output terminals to one voltage source in the first state or to the other voltage source in the second state,

and a device which simultaneously controls the first and second change-over elements for selecting the desired state.

2. A switched-mode power supply capable of operating optionally in two states, said power supply comprising a first voltage source having an output for supplying a first voltage and a control input, a second voltage source having an output for supplying a second voltage and a control input, said second voltage being higher than said first voltage, first and second output terminals coupled, respectively, to the outputs of said first and second voltage sources, and regulating means coupled to the control input of each of said voltage sources for supplying thereto a control signal when a voltage applied to said regulating means exceeds a reference level, characterized in that said regulating means is coupled to said first output terminal and said power supply further comprises a change-over element for selectively coupling said first output terminal to the output of said second voltage source, whereby, in a first of said two states, the outputs of said first and second voltage sources are connected, respectively, to said first and second output terminals, said regulating means generating said control signal based on the output of said first voltage source, and, in a second of said two states, the output of said second voltage source is connected to both said first and second output terminals, said regulating means generating said control signal based on the output of said second voltage source, said control signal thereby causing said first and second voltage sources to reduce said first and second voltages proportionately. 3. A switched-mode power supply as claimed in claim 2, characterized in that said power supply further comprises additional voltage sources connected to additional output terminals, said additional voltage sources having control inputs coupled to receive said control signal, whereby in said second state, voltages at the outputs of said additional voltage sources are reduced proportionately with said first and second voltages. 4. A switched-mode power supply as claimed in claim 3, characterized in that said power supply further comprises further regulating means coupled to at least the control input of one of said additional voltage sources for supplying a control signal when the voltage at the output of said additional voltage source applied thereto exceeds a further reference level, said further reference level being higher than said reference level. 5. A switched-mode power supply as claimed in claim 2, characterized in that said power supply further comprises further regulating means coupled to the control input of said second voltage source for supplying a control signal when said second voltage applied thereto exceed a further reference level, said further reference level being higher than said reference level. 6. A switched-mode power supply as claimed in claim 2 or 5, characterized in that said power supply further comprises a current limiter arranged in series with said change-over element. 7. A switched-mode power supply as claimed in claim 2 or 5, characterized in that said power supply further comprises a voltage-limiting circuit coupled between said change-over element and said first terminal.

Description:

BACKGROUND OF THE INVENTION

The invention relates to a switched-mode power supply capable of operating optionally in two states, comprising at least two voltage sources and having at least two supply output terminals and comprising first means for supplying a signal which acts, on the switching operation, to reduce all the voltages supplied by the sources when one voltage exceeds a first reference value.

The invention also relates more specifically to a circuit capable of operating optionally in the following states:

either in a first state in which a first voltage source connected to a first output terminal applies thereto a first voltage, and at least a second voltage source connected to at least a second output terminal applies thereto a second voltage which is higher than the first voltage,

or in a second state in which a voltage which is higher than or equal to the voltage applied to the first terminal in the first state is applied to it, while a voltage is applied to the second terminal which is less than the voltage applied to it in the first state.

The circuits of the type according to the invention are used particularly in television receivers having a remote control. They may be in two states corresponding to either normal operation of the television receiver or to the stand-by state, in which the receiving circuits of the remote control are normally fed, while the other circuits are supplied with a reduced voltage or are not fed at all.

In widely circuits, two separate supply sections are used, one for the remote control receiving circuit, the other one for the remaining portion of the television receiver. This system is expensive as it requires two separate power supplies.

In another widely used circuit, one single switched-mode power supply is used, and the voltages, other than those feeding the remote-controlled receiver, are cut-off, by means of change-over switches, during the stand-by state. It is then necessary to provide the possibility to cut-off, during operation, several high voltages and currents, which necessitates the use of a plurality of highly reliable and therefore expensive interruptors.

German Patent Specification No. DE 2,620,191 discloses a more satisfactory circuit, in which one of the windings of a switched-mode supply transformer is wound in a sense opposite to the winding sense of the other windings. Consequently, the voltages from this winding do not depend on the duty cycle. By varying the duty cycle, it is possible to reduce the other voltages without modifying the voltage from the said winding, which is used to feed the remote control receiving circuit. This system has, however, the disadvantage that the voltage from the inverse winding is not stabilized by the overall stabilizing system and that it is then necessary to provide an additional series-arranged stabilizing circuit for the usual case in which the line voltage may vary considerably, and especially with the now prevailing tendency according to which the television sets may be connected without change-over to all the 90 v. to 250 v. lines voltages. This stabilizing circuit, apart from its price, has the disadvantage that it increases the power consumption in the stand-by state.

SUMMARY OF THE INVENTION

The circuit according to the invention has for its object to provide a power supply which does not have any of the above-mentioned disadvantages.

The circuit according to the invention is therefore characterized in that all the voltages from the sources are modified simultaneously in the same proportion, and, at the same time, at least one connection between an output terminal and a source is replaced by a connection between this output terminal and another source.

In an advantageous variation, two of these voltage sources are interconnected, while all the voltages from these sources are modified simultaneously in the same proportion.

A circuit according to the invention is characterized in that second means are furthermore provided for supplying a signal which acts on the switching operation to reduce together the values of the voltage supplied when a voltage exceeds a second reference value, and in that it also comprises:

first change-over elements for connecting one of the voltage sources optionally either to the first or to the second means,

second change-over elements for connecting an output terminal either to the one or the other voltage source,

and a device which simultaneously controls the first and second change-over elements for selecting the desired state.

In an advantageous variation, a circuit according to the invention is characterized in that the first means are connected to the first terminal and the circuit comprises change-over elements for adjusting the circuit to the second state by optionally connecting the first terminal to the second voltage source.

An advantageous embodiment of the invention is characterized in that a current limiter is arranged in series with the change-over elements, or the change-over elements comprise a controllable current limiter.

The advantage of the invention resides in the fact that it makes it possible to reduce all the voltages, except one, by means of one single interrupter through which no high currents flow, all the voltages remaining permanently stabilized by the single stabilizing circuit.