GB2363213A

GB2363213A - Maintaining power supply during loss of input

Info

Publication number: GB2363213A
Application number: GB0013889A
Authority: GB
Inventors: David Tadeusz Kowalczyk; Robert James Rushton
Original assignee: Lucent Technologies Inc
Current assignee: Nokia of America Corp
Priority date: 2000-06-07
Filing date: 2000-06-07
Publication date: 2001-12-12
Also published as: GB0013889D0

Abstract

An inexpensive means of ensuring continuous power output in the event of a momentary loss of power input, for example, in the base station of a mobile telecommunications system, has a power supply which receives a DC input voltage V<SB>in</SB> and provides a desired DC output voltage V<SB>out</SB>. The DC input voltage V<SB>in</SB> is provided to the input winding 8 of a transformer T1, and the DC output voltage V<SB>out</SB> is derived from the secondary winding 16 and rectifying arrangement 18, 20,22. A switch 10 is switched by a square wave source 12 to provide an alternating voltage which is rectified by diode 24 to charge a holding capacitance 28 to a value substantially higher than the input voltage V<SB>in</SB>. Loss of input power is detected by an undervoltage detector 32, the output of which switches on a MOSFET switch 40, whose main current path is coupled between holding capacitance 28 and the transformer 8. A resistive capacitive network 42, 44 controls the discharge of capacitance 28 through MOSFET 40 for maintaining the power supply for the period of power loss.

Description

2363213 POWER SUPPLY 5 The present invention relates to a power supply

with provision for maintaining output power during momentary loss of input power.

The problem of momentary loss of input power to a voltage regulating circuit which provides a controlled output voltage and/or current is known in a variety of situations. For example, 10 US-A-3, 697, 856 discloses a voltage regulating circuit having an AC input power supply which may be interrupted for example, by lightning strikes. An energy- storage network is provided including a transformer and rectifier for charging a capacitor rapidly to a voltage value above the input voltage. If AC power loss occurs, the capacitor discharges through a switch to supplement the output of the conventional capacitor to maintain steady DC power 15 for a longer period of time than would otherwise be possible.

A particular application for maintaining output power during momentary loss of input power occurs with base stations of mobile telecommunications systems where the ETSI standard requires a break in input power supply for a period of four milliseconds to permit fuse 20 clearing. Base equipment is frequently powered by a 48 volt DC supply which provides an output of, for example, 200 watts. Thus, a means is required of detecting loss of DC input power, and an inexpensive means of maintaining power at least for a period of 4 milliseconds. The arrangement disclosed in US-A-3, 697, 856 would not be suitable for this application, firstly because it is responsive to an AC power input, and secondly because the construction, 25 including a transformer, is relatively expensive.

Summary of the Invention

It is an object of the present invention to provide a power supply circuit with improved means 30 for preventing momentary loss of power.

The invention provides in a first aspect, a power supply, comprising regulating means for converting a DC input voltage to a DC output of predetermined value of voltage and/or current, means for charging a power holding capacitance to a value substantially higher than the magnitude of the DC input voltage, means for detecting an undesirably low value of DC 5 input voltage, coupled to control a switch means, and a main current path of the switch means being connected to permit the power holding capacitance to provide controlled current flow, through the main current path, to the regulating means in the event of detection of an undesirably low value of DC input voltage.

10 Since the holding capacitance is charged to a higher value than the input voltage, the amount of capacitance required to store a given amount of charge is reduced. Hence the volume of the capacitance, taken up on a circuit card for example, may be reduced, and the cost of the capacitance required is reduced.

15 As preferred, the switch means comprises a MOSFET switch, and a resistive-capacitive network is connected between the gate and source of the MOSIFET switch to control the turning on of the MOSFET switch so that it turns on gradually and limits the peak current in the switch, ensuring that the power supply remains within its safe operating area and turn on losses of the switch are minimised. A resistive-capacitive network is preferable to 20 alternatives including inductors, which are relatively high cost.

The power supply according to the invention may be adapted to receive an AC input voltage, instead of or in addition to the DC input voltage so that in some modes of operation it is powered solely by AC voltage. Where the power supply is powered by DC input, a converter 25 means is provided for switching the input DC voltage to an alternating value, and supplying the alternating voltage to the primary winding of a transformer, the output winding of which is coupled to a rectification arrangement so as to provide an output having a desired value of DC voltage and/or current. A switch means is provided for switching the input voltage, preferably coupled to a network for scaling up the value of the switched voltage for 30 application to a primary transformer winding. This scaled up version of the input voltage may be used for charging said power holding capacitance by providing a suitable rectification means.

In a further aspect, the invention provides a power supply, comprising regulating means for converting an input voltage to a DC output of predetermined value of voltage and/or current, means for charging a power holding capacitance to a value substantially higher than the magnitude of the input voltage, means for detecting an undesirably low value of input voltage, 5 coupled to control a MOSFET switch, and the main current path of the MOSFET switch being connected to permit the power holding capacitance to provide controlled current flow, through the main current path, to the regulating means m the event of detection of an undesirably low value of input voltage.

10 Brief Description of the Drawings

A preferred embodiment of the invention will now be described with reference to the drawings wherein: - Figure 1 is a circuit diagram of a power supply with protection against momentary loss of input power in accordance with the invention; and Figure 2a is a detail of the circuit of Figure 1, and Figure 2b is a waveform diagram of the waveforms generated in Figure 2a.

Description of the Preferred Embodiment

A method of maintaining voltage at the output of a power supply during momentary loss of input power is described. The method described minimises the volume of capacitance required so that a smaller value of capacitance is required to achieve a given hold-up time. This results in a lower cost and a smaller volume for the power supply. A method of 25 controlling switch current, through a MOSFET switch which is used to connect the hold-up capacitance to the input of the power supply, is also described.

Referring to Figure 1, an input DC voltage Vffi of 48 volts is applied to terminals 2 and is provided to a voltage stabilising arrangement including a diode 4 and capacitor CiP 6. The 30 prima ry winding 8 of a transformer T I is coupled to diode 4 and capacitor 6 and is connected in series with a MOSFET switch 10 forming part of a converter circuit. Switch 10 is switched rapidly by a square wave source 12, and a network 14 connected across the primary winding 8 is operative to increase the voltage across the primary winding. The output of the secondary 4 winding 16 of transformer TI is fully rectified by a rectification circuit comprising diodes 18, choke 20 and output capacitor 22. A DC output V,,,t is provided from across capacitor 22.

Referring now to Figures 2a and 2b the converting circuit shown in Figure 1 is shown in 5 more detail, wherein similar parts are denoted by the same reference numeral. The network 14 connected across winding 8 comprises a diode 60, resistor 62 and capacitor 64, with capacitor 64 and resistor 62 connected in parallel to the anode of diode 60, the cathode of the diode being connected to the primary winding of transformer 8. This passive network provides at node 66 a voltage waveform as shown in Figure 2b as 70. The waveform 10 provided by square wave source 12 is indicated at 72. It may be seen that the peak voltage at node 66 is effectively doubled from the input voltage Vin.

Referring back to Figure 1, charging circuit is connected to node 66, the charging circuit comprising diode 24 and resistor 26, for charging a holding capacitance Ch,,1d 28. Capacitance is 28 is implemented as several capacitors connected in parallel to give a desired value. An optional voltage clamping device (zener diode), D2, 30 may be used to limit the peak voltage on Chld. An undervoltage detector 32 is provided connected across terminals 2, 4 to monitor input voltage Vffi and provide a signal on line 34 if Vi. fall beneath a predetermined value.

Line 34 is connected via voltage dividing resistors 36, 38 to the gate of a p-type MOSFET 20 switch 40. The main current path of switch 40 is connected between holding capacitor 28 and input capacitor Cip 6. A resistive capacitive network comprising resistor 42 and capacitor 44 is connected between the gate and drain of switch 40.

In operation, capacitor Ch.1d 28 is charged from voltage node 66 to a higher voltage than the 25 average value of Vi,, Undervoltage detection circuit 32 monitors the input voltage Vi When the input voltage falls below a predetermined value MOSIFET switch 40 is closed, which allows charge stored On Chold 28 to be transferred to Cip 8. A circuit RI Cl controls the turn on of MOSFET switch 40, which limits the peak current in switch 40, ensuring that the switch remains within its safe operating area and the turn on losses of the switch are minimised.

Since the energy stored in Chold is at a higher voltage than the normal input voltage the physical volume of the capacitor, required to a meet a given hold-up time, is minimised- Although one particularly advantageous method has been described of charging capacitor Chold 28, other methods could be employed. For example, an extra winding may be added to the transformer T1 to further increase the charging voltage..

5 The present invention provides the following advantages:

1. The invention minimises the amount of capacitance required for hold-up.

2. Current through the MOSFET switch which is used to connect the hold-up capacitors is controlled.

10 The present invention is applicable to linear or switched mode power supply units.

Claims

1. A power supply, comprising regulating means for converting a DC input voltage to a DC output of predetermined value of voltage and/or current, means for charging a power holding capacitance to a value substantially higher than the magnitude of the DC input voltage, means for detecting an undesirably low value of DC input voltage, 10 coupled to control a switch means, and a main current path of the switch means being connected to permit the power holding capacitance to provide controlled current flow, through the main current path, to the regulating means in the event of detection of an undesirably low value of DC input voltage.

15

2. A power supply according to claim 1, wherein said switch means comprises a MOSFET switch, with the gate thereof coupled to said means for detecting an undesirably low voltage.

3. A power supply according to claim 2, including a resistive-capacitive network 20 connected between the gate and source of the MOSFET switch to control the turning on of the MOSFET switch.

4. A power supply according to claim 2 or 3, wherein the means for detecting an undesirably low power input voltage is coupled to a resistive dividing network for 25 establishing a desired voltage between the drain and gate of the MOSFET switch.

5. A power supply according to any preceding claim, including conversion means for converting the DC input voltage to an alternating voltage and for applying the alternating voltage to the primary winding of a transformer, and means coupled to the 30 secondary winding of the transformer for providing said DC output, said means for charging the power holding capacitance being responsive to the alternating voltage applied to said primary winding, and including diode means for rectifying the alternating voltage for providing a voltage for charging the holding capacitance.

6. A power supply according to claim 5, wherein said conversion means includes switch means, controlled by an alternating voltage source, for providing said alternating voltage, and including a network for raising the value of the alternating voltage 5 applied to the primary winding of the transformer.

7. A power supply according to claim 5 or 6 and wherein the means for charging the power holding capacitance includes a further transformer winding of said transformer.

10

8. A power supply according to any preceding claim, including zener diode means connected across the holding capacitance for limiting the charging voltage.

9. A power supply according to any preceding claim, arranged for receiving an AC input voltage.

10. A power supply, comprising regulating means for converting an input voltage to a DC output of predetermined value of voltage and/or current, means for charging a power holding capacitance to a value substantially higher than the magnitude of the input voltage, means for detecting an undesirably low value of input voltage, coupled to 20 control a MOSFET switch, and a main current path of the MOSFET switch being connected to permit the power holding capacitance to provide controlled current flow, through the main current path, to the regulating means in the event of detection of an undesirably low value of input voltage.

25.

11. A power supply according to claim 10, including a resistivecapacitive network connected between the gate and source of the MOSFET switch to control the turning on of the MOSFET switch.

12. A power supply substantially as described herein with reference to the accompanying 30 drawings.