GB2056799A

GB2056799A - Power supply unit

Info

Publication number: GB2056799A
Application number: GB8024372A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1979-08-07
Filing date: 1980-07-25
Publication date: 1981-03-18
Also published as: DE2932048B1

Abstract

A regulated power supply unit has an electronic switch (ST) which is periodically opened and closed under the control of a pulse generator (MD) which controls the relationship between opening and closing times of the switch, as a function of the output voltage (or current). To protect against suddenly-occurring short circuits, either the positive gradient of the output current is measured, or the negative gradient of the output voltage, by means of a gradient discriminator (V). When a predetermined value is exceeded, the electronic switch (ST) is opened. <IMAGE>

Description

SPECIFICATION Power supply unit This invention relates to power supply units.

Power supply units with electronic safety devices for preventing overload are known, for example, from the book "Switches with semiconductor structural units" by Gelder/Hirschmann, Vol. 3, pages 240 to 255. Such units may comprise an electronic switch connected to an unregulated voltage source, which switch Is periodically opened and closed by a control pulse generator which controls the relationship between opening and closing times of the switch as a direct function of the output voltage or the output current. A filter section stabilises the output pulses of the switch.

To ensure against overloads in general, it is known to connect a current measuring resistance into a load current circuit, and to monitor the voltage drop across this. If the voltage drop exceeds a predetermined value, then the current is limited by the load current circuit or completely disconnected. From German Patent Specification 22 13 921 it is also known to minitor directly an output voltage, rather than to use a current measuring resistance.

Electronic safety devices of this type may work satisfactorily when the current regulation or disconnection works without delay. However, frequently delay and dead times are contained in the current regulation circuit, e.g.

they are unavoidable with so-called pulsed power supply units, with which an electronic switch connected to an unregulated voltage source is periodically opened and closed and the resultant pulses are stabilised by means of a filter section. Above all, with pulsed power supply units of this type, known electronic safety devices, with which current is used directly as a control value, are not sufficiently safe against a suddenly-occurring short circuit at the output, particularly if the current is not measured at the output but before the filter section, if necessary even before a transformer, as is frequently necessary for reasons of power loss.

The present invention aims to provide a pulsed power supply unit in which safety against suddenly-occurring short circuits at the output may be improved.

According to the present invention, there is provided a pulsed power supply unit comprising an electronic switch adapted to open and close periodically a current path between input terminals and output terminals of the unit, a control pulse generator adapted to control opening and closing of said switch, and a gradient discriminator which is arranged to cause opening of said switch when the negative gradient of the magnitude of the timevarying output voltage or the positive gradient of the magnitude of the time-varying output current, at said output terminals, exceeds a predetermined value.

In contrast to the known power supply units for safety against overload, according to the invention it is not the absolute magnitude of the output current but its positive gradient or, what amounts to the same thing, the negative gradient of the output voltage, that is monitored. In practice, changes in voltage will also occur during normal operation. Negative voltage gradients thus occurring can be reckoned, for example, from the value of the capacitance at the output terminals and the maximum admissible current, to which a current regulation circuit (largely known per se) may be adjusted. In the case of a suddenly occurring short circuit, however, the voltage gradient is clearly greater than the voltage gradients occurring during normal operation and can therefore be distinguished.With such a power supply unit, no current measuring resistance causing power loss needs to be connected into the load current circuit.

As a gradient discriminator there may be used a high pass filter, to which a bistable device, which opens the switch when set, responds. It is set during a drop in voltage or increase in current at the output of the power supply unit. It is preferable that the output signal of the high pass filter is conveyed to a threshold value stage whose threshold value gives the value of the gradients at which disconnection of the power supply unit should occur.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will be made, by way of example only, to the accompanying diagrammatic drawing, which is a simplified circuit diagram of a pulsed power supply unit.

Input terminals to which an unregulated direct voltage is supplied are referenced E.

Connected in series to the terminals E are the primary winding of a power transformer LU and the collector-emitter path of a switching transistor ST. This is periodically opened and closed. The resultant pulses in the secondary winding of the transformer LU are rectified by a rectifier GL. The resulting direct voltage is stabilised in a filter section SG.

The control pulses for the switching transistor ST are produced in a control pulse generator MD, to which the output voltage of the filter section SG is fed as a control voltage and which adjusts the mark-space ratio of its output pulses to regulate the output voltage of the filter section SG. The direct voltage occurring at the output terminals A is therefore stabilised.

The alternating voltage component occurring during change of the output voltage reaches the inverting input of an amplifier V via a capacitor C. The greater the change of the output voltage and the steeper the path of the output voltage as a function of the time, the greater is the impulse which reaches the amplifier V via the capacitor C. Particularly large negative impulses result when a short circuit occurs at the output terminals A.The amplifier V forms together with resistances R1, R2, R3, R4 a threshold value switch which is so adjusted that with a constant voltage at the output terminals A the voltage at the inverting input of the amplifier V is so much the greater than that at the non-inverting input, that normal changes in the output voltage do not change the switching state of the amplifier. (The resistances R1, R2 together with the capacitor C also form a highpass filter). On the other hand, if the negative gradient of the output voltage reaches the value occurring in the case of a short circuit, then at the inverting input of the amplifier V there appears a negative impulse of such amplitude that the voltage at the inverting input becomes smaller than that at the noninverting input. The amplifier V emits a positive pulse which is conveyed via a diode D to a voltage divider R5, R6 at whose tapping points the ignition electrode of a thyristor TH is connected. This ignites and holds a blocking input of the control pulse generator MD at zero potential so that further pulse transmission is interrupted and the transistor ST is held in the open state. The power supply unit it thus disconnected.

Claims

1. A pulsed power supply unit comprising an electronic switch adapted to open and close periodically a current path between input terminals and output terminals of the unit, a control pulse generator adapted to control opening and closing of said switch, and a gradient discriminator which is arranged to cause opening of said switch when the negative gradient of the magnitude of the timevarying output voltage or the positive gradient of the magnitude of the time-varying output current, at said output terminals, exceeds a predetermined value.

2. A unit according to claim 1, including a substantially unregulated energy source connected to said input terminals.

3. A unit according to claim 1 or 2, wherein the control pulse generator is operable to control opening and closing of said switch as a function of said output voltage or output current.

4. A unit according to claim -1, 2 or 3, comprising a filter section for stabilising output pulses generated by said switch, the gradient discriminator being connected to the output of the filter section.

5. A unit according to any preceding claim, wherein said gradient discriminator comprises a high pass filter, and a bistable device which is arranged to respond to the output of the high pass filter such that, in the event of said predetermined value being exceeded, the bistable device is set, thereby to open said switch.

6. A unit according to any preceding claim, wherein said gradient discriminator comprises a threshold value stage connected between said high pass filter and said bistable device.

7. A unit according to claim 5 or 6, wherein said bistable device comprises a thyristor.

8. A'unit according to any preceding claim, wherein the gradient discriminator is arranged to control the control pulse generator.

9. A power supply unit substantially as hereinbefore described with reference to the accompanying drawing.