SWITCHED-MODE POWER SUPPLY

Abstract

The present invention relates to a switched-mode power supply that comprises a detection circuit for detecting a coupling of an electrical device to the switched-mode power supply and a decoupling of the electrical device from the switched-mode power supply, a switch for controlling the supply of electric power to the switched-mode power supply, a control circuit for controlling the switch, the control circuit being configured to turn on the switch when the coupling of the electrical device has been detected, and to turn off the switch when the decoupling of the electrical device has been detected, and a supply circuit for providing a supply voltage to the detection circuit and the control circuit.

Claims

1. A switched-mode power supply, comprising: a primary side, a secondary side, a transformer between the primary side and the secondary side, a detection circuit for detecting a coupling of an electrical device to the switched-mode power supply and a decoupling of the electrical device from the switched-mode power supply, a switch for controlling the supply of electric power to the switched-mode power supply, a control circuit for controlling the switch, the control circuit being configured to turn on the switch when the coupling of the electrical device has been detected, and to turn off the switch when the decoupling of the electrical device has been detected, a supply circuit for providing a supply voltage to the detection circuit and the control circuit, a rectifier having two input and two output terminals, the input terminals being coupled by interference suppression capacitors to the input terminals of the switched-mode power supply, and a charging capacitor coupled between the output terminals of the rectifier for storing an electric energy to operate the detection circuit and the control circuit.

2. The switched-mode power supply according to claim 1, wherein the supply circuit comprises a zener diode connected in parallel with the charging capacitor for limiting the maximum voltage of the charging capacitor.

3. The switched-mode power supply according to claim 1, wherein the positive output terminal of the switched-mode power supply is connected through a diode to the positive terminal of the charging capacitor in order to provide a supply voltage to the detection circuit and the control circuit during an active state of the switched-mode power supply.

4. The switched-mode power supply according to claim 1, wherein: the switched-mode power supply comprises a USB interface having interconnected D+ and D− data lines, through which USB interface electric power is to be supplied to the electrical device, and the detection circuit is configured to detect the coupling and the decoupling of the electrical device based on a voltage at the interconnected D+ and D− data lines.

5. The switched-mode power supply according to claim 4, wherein the detection circuit comprises a first bipolar transistor, the base of which is connected through a first resistor to the interconnected D+ and D− data lines.

6. The switched-mode power supply according to claim 5, wherein the emitter of the first bipolar transistor is connected to the positive terminal of the charging capacitor.

7. The switched-mode power supply according to claim 1, wherein the control circuit comprises: an opto-triac comprising a light emitting diode at its input side and a photosensitive triac at its output side, the photosensitive triac being connected to the switch, and a field-effect transistor connected to the light emitting diode for controlling the current flow through the light emitting diode.

8. The switched-mode power supply according to claim 7, wherein the anode of the light emitting diode is connected to the positive terminal of the charging capacitor, and the cathode of the light emitting diode is connected through a second resistor to the drain of the field-effect transistor.

9. The switched-mode power supply according to claim 7, wherein the control circuit comprises a second bipolar transistor for switching the field-effect transistor, the collector of the second bipolar transistor being connected to the gate of the field-effect transistor, and the emitter of the second bipolar transistor being connected through a third resistor to the collector of the first bipolar transistor.

10. The switched-mode power supply according to claim 1, wherein the switch is connected between an input terminal of the switched-mode power supply and an input terminal of a rectifier of the switched-mode power supply.

11. The switched-mode power supply according to claim 10, wherein the switch is a triac.

12. The switched-mode power supply according to claim 1, wherein the switch is connected between an output terminal of a rectifier of the switched-mode power supply and a terminal of a reservoir capacitor of the switched-mode power supply.

13. The switched-mode power supply according to claim 12, wherein the switch is a thyristor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 illustrates the known principle of a switched-mode power supply,

[0033] FIG. 2 illustrates a switched-mode power supply according to a first embodiment of the invention, and

[0034] FIG. 3 illustrates a switched-mode power supply according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0035] Embodiments of the invention will now be described with reference to FIGS. 2 and 3.

[0036] FIG. 2 illustrates a switched-mode power supply according to a first embodiment of the invention. As in FIG. 1, the power supply comprises a primary side 101 and a secondary side 102, which are linked together through a transformer 103 having a primary winding 106 and a secondary winding 109. An AC input to the primary side 101 is rectified in a rectifier 104, the output of which is smoothed by a reservoir capacitor 105. The primary current through the primary winding 106 is regularly chopped with a primary switch 107 that is driven by an oscillator circuit 108. Intercepting the current in the primary winding 106 causes the energy that was temporarily stored in the magnetic field of the transformer 103 to discharge in the form of a current through the secondary winding 109. A diode 110 on the secondary side 102 rectifies the secondary current and causes a secondary voltage to be maintained across a reservoir capacitor 111.

[0037] The switched-mode power supply of FIG. 2 comprises a detection circuit 201 for detecting whether or not an electrical device (not shown) is coupled to the output of the power supply. The coupling/decoupling of the electrical device is determined depending upon a voltage applied at a detection terminal 202 to which the electrical device is connected when the electrical device is coupled to the power supply. The detection circuit is connected to a control circuit 203 that controls, based on the coupling information, a switch 204 that is located on the primary side 101, between an output terminal of the rectifier 104 and one terminal of the reservoir capacitor 105. The switch 204 controls the supply of electric power from the mains to the primary side 101. The control circuit 203 keeps the switch 204 in a conducting state when an electrical device is coupled to the power supply and in a non-conducting state when no electrical device is coupled to the power supply. A supply circuit 205 provides a supply voltage to the detection circuit 201 and the control circuit 203. The supply circuit 205 is coupled to the input terminals of the power supply in order to provide the required supply voltage during the standby state of the power supply. The supply circuit 205 is also coupled to the secondary side of the power supply so that the detection circuit 201 and the control circuit 203 can be supplied with the required supply voltage during the active state of the power supply.

[0038] FIG. 3 illustrates a switched-mode power supply according to a second embodiment of the invention. The circuit diagram of FIG. 3 will now be explained focusing on the detection circuit, the switch, the control circuit and the supply circuit.

[0039] The supply circuit comprises a bridge rectifier that consists of diodes D60, D61, D62 and D63. The input of the bridge rectifier is coupled through interference suppression capacitors CY2 and CY3 to the input of the power supply. The supply circuit also comprises a charging capacitor C57 for storing an electric energy to operate the detection circuit and the control circuit during the standby state of the power supply. The charging capacitor C57 is connected via resistors R54 and R64 between the output terminals of the bridge rectifier. The maximum voltage of the charging capacitor C57 is limited with a zener diode D55 that is connected in parallel with the charging capacitor C57. The supply circuit further comprises a diode D58 that is connected between the positive output terminal of the power supply and the positive terminal of the charging capacitor C57 so that electrical energy can be provided to the detection circuit and the control circuit during the active state of the power supply.

[0040] The switch used in the circuit diagram of FIG. 3 is a thyristor D2 that is connected between the negative output terminal of a rectifier D10 and the negative terminal of a reservoir capacitor C1. The thyristor D2 that controls the supply of electric energy from the mains to the primary side of the power supply is controlled by the control circuit.

[0041] The control circuit comprises an opto-triac IC51 having a light emitting diode at its input side and a photosensitive triac at its output side. The output terminal 3 of the opto-triac IC51 is connected to the gate of the thyristor D2. The input terminal 1 of the opto-triac IC51 is connected to the positive terminal of the charging capacitor C57. The control circuit comprises a field-effect transistor T51 that controls the operation of the opto-triac IC51. The drain of the field-effect transistor T51 is connected through a resistor R63 to the input terminal 2 of the opto-triac IC51. The gate of the field-effect transistor T51 is connected to the collector of a bipolar transistor T52B. The bipolar transistor T52B prevents the field-effect transistor T51 from conducting when the supply voltage provided by the charging capacitor C57 is so low that the photosensitive triac of the opto-triac IC51 cannot be triggered into a conducting state. Resistors R60, R61 and R62 are used to define the voltage level at which the bipolar transistor T52B changes its state.

[0042] The detection circuit comprises a bipolar transistor T52A, the base of which is connected through a resistor R58 to interconnected D+ and D− data lines of a USB interface. Electric power is to be supplied to an electrical device through the USB interface. The emitter of the bipolar transistor T52A is connected to the positive terminal of the charging capacitor C57, and the collector of the bipolar transistor T52A is connected through a resistor R59 to the emitter of the bipolar transistor T52B.

[0043] When an electrical device is coupled to the power supply, the resistor R58 is connected through pull-down resistors of the D+ and D− data lines to the ground. As a result of this the bipolar transistor T52A becomes conducting and its collector voltage rises. Since the collector of the bipolar transistor T52A is connected through the resistor R59 to the emitter of the bipolar transistor T52B, the bipolar transistor T52B becomes conducting and its collector voltage rises so that the field-effect transistor T51 becomes conducting and current starts to flow through the light-emitting diode of the opto-triac IC51. The light-emitting diode triggers the photo-sensitive triac of the opto-triac IC51 into a conducting state, as a result of which the thyristor D2 is turned into a conducting state, whereby the power supply switches into the active state.

[0044] When the electrical device is decoupled from the power supply, the base voltage of the bipolar transistor T52A rises. As a result of this the bipolar transistor T52A switches into a non-conducting state. This results in that the gate voltage of the field-effect transistor T51 drops and therefore the field-effect transistor T51 switches into a non-conducting state. As a result of this the thyristor D2 is turned off, whereby the power supply switches into the standby state.

[0045] Only advantageous exemplary embodiments of the invention are described in the figures. It is clear to a person skilled in the art that the invention is not restricted only to the examples presented above, but the invention may vary within the limits of the claims presented hereafter. Some possible embodiments of the invention are described in the dependent claims, and they are not to be considered to restrict the scope of protection of the invention as such.