GB2203003A

GB2203003A - Power supply circuit

Info

Publication number: GB2203003A
Application number: GB08708101A
Authority: GB
Inventors: David George Sanderson
Original assignee: Spectrol Reliance Ltd
Current assignee: Spectrol Reliance Ltd
Priority date: 1987-04-04
Filing date: 1987-04-04
Publication date: 1988-10-05
Also published as: GB8708101D0

Abstract

A power supply circuit, preferably a miniature power supply circuit, for providing from an AC source an unregulated low voltage DC power supply comprises a reservoir capacitor (6), electronic switch (5) for regulating the voltage from the AC source to charge the reservoir capacitor (6), and means (1) for applying only forward biasing voltages to the switch (5). The voltage developed across the capacitor (6) is an unregulated DC voltage which is used as the power supply. Voltage and current limiting devices may be included. <IMAGE>

Description

POWER SUPPLY CIRCUIT This invention relates to a power supply circuit and more particularly to a power supply circuit for providing from an AC source an unregulated DC power supply.

A DC power supply is usually derived from an AC power source by the use of large or dissapative devices such as transformers, power resistors and large coupling capacitors and there is a need for a DC power supply which avoids the use of such devices and is more suited to miniaturisation. The present invention has as its object to fulfil this need.

The invention provides a power supply circuit for providing from an AC source an unregulated low voltage DC power supply, comprising reservoir capacitor means1 electronic switch means for regulating the voltage from the AC source to charge the reservoir capacitor means, and means for applying only forward biasing voltages to the electronic switch means, the voltage developed across the reservoir capacitor means being an unregulated DC voltage which is used as the power supply.

The electronic switch means is preferably a power FET (field effect transistor) or other voltage driven electronic switch, although with suitable modification of the circuit to provide the additional current requirements a current driven electronic switch, such as a bipolar transistor, could be used if required.

The means for applying only forward biasing voltages to the electronic switch means may comprise a rectifier.

Current limit means may be provided for limiting the current to the electronic switch means to within the operating window of the electronic switch means. Such current limit means may comprise a current limit circuit in series with the electronic switch means.

Disabling means may be provided for disabling the electronic switch means when the voltage differential across the electronic switch means exceeds a predetermined maximum. Said disabling means may comprise input voltage monitoring means for monitoring the input voltages to the electronic switch means. Output voltage monitoring means may also be provided for monitoring the output voltages from the electronic switch means.

The invention will be more particularly described with the reference to the accompanying drawings in which: Figure 1 is a diagram of a power supply circuit according to a first embodiment of the present invention, and Figure 2 is a diagram of a power supply circuit according to a second embodiment of the present invention Referring to Figure 1 of the drawings it will be seen that the circuit illustrated therein comprises a rectifier 1 which is connected by electric conductors 2, 3 to an AC source (not shown) such as a source of mains electricity. The rectifier 1 is connected by an electric conductor 4 to an electronic switch which in the embodiment illustrated comprises an FET 5, the rectifier 1 applying forward biasing voltages only to the FET 5.

The FET 5 acts as a switch to limit the voltage across a reservoir capacitor 6 to which it is connected by means of an electric conductor 7. The voltage developed across the reservoir capacitor 6 is an unregulated DC voltage which is used as the power supply by way of electric conductors 8 and 9. An output voltage monitor 10 is provided for monitoring the output voltages from the FET 5 and limiting these to a predetermined maximum.

With the circuit of Figure 1, very large currents can be present when the FET switches on so that there is a substantial voltage difference between the input voltage amplitude and the voltage across the reservoir capacitor 6. This high rate of change of current (di/dt) generates undesirable RFI noise. In addition, due to the high current passing through the FET 5 at peak line voltages, very large instantaneous power dissipation levels can be achieved in the FET 5 which might cause damage thereto. These difficulties can be overcome as shown in Figure 2.

The circuit of Figure 2 is substantially the same as that of Figure 1 and like parts have been given like reference numerals. However, in the circuit of Figure 2 current limit means 11 is provided in series with the FET 5 to limit currents to within the operating window of the FET 5 and an input voltage monitor 12 is provided for disabling the FET 5 when the voltage differential across the FET 5 exceeds a predetermined maximum. A major advantage of the power supply circuit of Figure 2 has been found to be its tolerance to line input variation (over a range of 4:1).

The applications of the power supply circuit of Figure 2 are limited only by the characteristics of the FET 5, the three most important characteristics being its breakdown voltage rating, its pulse current rating and its pulse power dissipation capability.

The power supply circuit described may be a miniature circuit, i.e., may be smaller in size than the known large or dissapative devices presently in use.

Claims

1. A power supply circuit for providing from an AC source an unregulated low voltage DC power supply, comprising reservoir capacitor means, electronic switch means for regulating the voltage from the AC source to charge the reservoir capacitor means, and means for applying only forward biasing voltages to the electronic switch means, the voltage developed across the reservoir capacitor means being an unregulated DC voltage which is used as the power supply.

2. A power supply circuit according to claim 1, wherein the electronic switch means comprises a power FET.

3. A power supply circuit according to claim 1 or 2, wherein said means for applying only forward biasing voltages to the electronic switch means comprises a rectifier.

4. A power supply circuit according to claim 1, 2 or 3, wherein current limit means is provided for limiting the current to the electronic switch means to within the operating window of the electronic switch means.

5. A power supply circuit according to claim 4, wherein the current limit means comprises a current limit circuit in series with the electronic switch means.

6. A power supply circuit according to any one of the preceding claims, comprising disabling means for disabling the electronic switch means when the voltage differential across the electronic switch means exceeds a predetermined maximum.

7. A power supply circuit according to claim 6, wherein said disabling means comprises input voltage monitoring means for monitoring the input voltages to the electronic switch means.

8. A power supply circuit according to any one of the preceding claims, comprising output voltage monitoring means for monitoring the output voltages from the electronic switch means.

9. A power supply circuit substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.

10. A miniature power supply circuit according to any one of the preceding claims.