GB2254204A - Regulated power supply with controlled output power failure mode - Google Patents

Abstract

A voltage regulation facility uses the voltage of a higher output rail 31 to sustain momentarily the voltage decay of a relatively lower voltage rail 32 using a transfer circuit incorporating a series voltage regulator 41 coupled to a diode 43 itself forward-biassed into conductance upon the relative falls of the higher and lower voltage rails to discharge a capacitance 37 into the lower rail 32, capacitance 37 being charged from the higher rail 31. A 5 volt supply, derived from rail 32 via a regulator 48, may be used to power a data memory. A power fail logic signal may be derived and applied to a processor to transfer data into a static memory after mains failure but before the 5 volt supply falls out of specification. <IMAGE>

Description

Regulated (Multi-level) Power Supply with Controlled Output Power Failure Mede This invention relates to power supplies, and is particularly, but not exclusively, concerned with secondary power supplies exhibiting prescribed output decay or 'failure fall-off' characteristics, upon the interruption or failure of a primary power supply source.

The term (secondary) power supply (unit), or 'p.s.u.'(psu), used herein embraces any form of controlled or regulated voltage, transformer-coupled device.

The term 'regulation' used herein embraces any form of support, alteration, modification, limitation, enhancement, control or other operational behaviourial characteristic discipline or correction.

When operational devices incorporating subsidiary electronic operating circuitry, in particular those employing digital data logic devices, rely upon a tightly specified (secondary) power supply, peremptory failure of that supply may cause unacceptable device behaviour - eg irrecoverable data mode failure.

Thus, if the operational device is being relied upon to perform some important role, whether on its own account, or in a chain of other devices, 'controlled shut-down' may be desirable.

One such output performance criterion for the operational device's power supply is a prescribed decay pattern of the power supply output voltage. In that regard too abrupt or sudden a collapse in voltage would be operationally unacceptable.

According to one aspect of the invention there is provided a power supply incorporating a transformer with a primary winding for connection to a primary input power supply source, and a plurality of secondary windings respectively connected to a corresponding plurality of independent voltage rails, at least one of those voltage rails being used to support - or at least momentarily sustain - the voltage of a relatively lower voltage rail upon a failure in the input power supply source.

In practice the input power supply could be the mains, with all its inherent vulnerability to erratic voltage fluctuation and failure.

Conveniently, the higher voltage (rail) is fed to the lower voltage (rail) via a (coupling) series voltage regulator having a set voltage equal to the lowest unregulated voltage. This inhibits the lower (supported) voltage rail from drawing energy from the higher voltage rail under normal operating conditions and levels.

Desirably, the coupling series voltage regulator is coupled through a steering diode fed from the associated regulated voltage.

This diode is normally reverse-biassed into non-conducting mode with the input power supply maintained and thus the two voltage rails isolated, and forward-biassed into conductance upon failure of the input power supply, to allow discharge of the capacitance in the higher voltage rail and thereby temporarily sustain the lower output voltage.

There now follows a description of a particular embodiment of the invention, by way of example only, with reference to the accompanying schematic and diagrammatic drawings, in which: Figure 1 snows the principal circuitry components of a conventional (linear) power supply; Figure 2 snows graphically the input and (deficient) output performance characteristics of the conventional power supply circuitry of Figure 1; Figure 3 snows the principal circuitry components of a power supply according to the invention, set out by way of a comparison with the conventional circuitry of Figure 1; Figure 4 shows graphically the performance characteristics of the inventive circuit of Figure 3, and by way of contrast with Figure 2.

Figure 5 shows a detailed exemplary circuit diagram embodying the principles of Figure 3.

Referring to the drawings, and in particular Figure 1, a conventional power supply unit incorporates a voltage transformer 16 with a primary winding 14 connected to a primary power source, such as the mains supply of 240V (volts) AC (alternating current) and a secondary winding 15 connected across a smoothing capacitance stage 18 and through a diode rectification stage 17 to an output voltage rail 21.

A series voltage regulator 22 in the output voltage rail controls the prescribed output voltage V out, in this case some 5 V (volts) DC (direct current).

Figure 2 shows graphically a plot of voltage against time for the conventional circuitry configuration of Figure 1.

Specifically, the voltage V cap across the smoothing capacitance 18 is represented by the upper line 19A for a sustained mains supply and reflects a generally even rectified DC voltage level, subject to minor fluctuating DC voltage cycles. The latter may be 'clipped' by the subsequent series regulator 22 to achieve a move even, albeit lower, regulated output voltage V out.

At an input mains failure transition point 'A', the plot line 19A changes to a progressively (exponential) decaying voltage 19B.

The corresponding output voltage V out is represented initially by a smooth, steady continuous DC voltage line 20A followed, only at a somewhat marginally later transition point 'B', by a progressively (again generally exponential in character) decaying voltage line 20B.

The interval between the transition points A-B is approximately 3 mS (milliseconds).

The output voltage moves out of specification when the series regulating device 22 saturates; ie the voltage across it falls below a minimum voltage necessary for it to operate in a controlled linear mode.

In contrast to Figures 1 and 2, the corresponding power supply unit circuitry configuration and attendant performance characteristics according to the invention are depicted in Figures 3 and 4 respectively.

Thus, referring to Figure 3, a transformer 26 has a primary winding 23 for connection to an input power supply, such as 240 Volt AC mains, and a pair of secondary windings 24, 25 feeding respective relatively high and low DC voltage rails 31, 32 through associated diode rectification paths 35, 36.

Clamped to the voltage rails 31, 32 are respective capacitances 37, 38, each serving as a modest temporary or transitional reservoir of stored energy - ready for - and to resist, inhibit or suppress, any voltage fluctuation or transition.

Essentially, the relatively lower voltage rail 32 is supported or backed up by the relatively higher voltage rail 31, at the 'sacrifice' of the voltage decay 'performance' of that rail 31, as described elsewhere and depicted graphically in Figure 4.

The two rails 31, 32 are coupled by a series regulated transfer circuit 41, feeding the lower voltage rail 32, through a series diode 43.

This arrangement enables the capacitance 37 (C1) to discharge through the transfer circuit 41, maintaining temporarily the voltage level of the lower voltage rail 32, for a transitional period sufficient to meet the prescribed output decay voltage characteristics desired.

For example, the lower voltage output could be sustained through a complete (but absent or failed) mains cycle, equivalent to a duration of some approximately 30 mS.

Without this transfer arrangement, a standard power supply unit would sustain its prescribed output voltage for only 2 to 3 mS upon an input mains failure. Indeed this, or even worse, is the (accepted) fate of the higher voltage output.

In the particular example, the higher voltage rail 31 is set at 14 V through a series regulator 47, as an auxiliary output, whereas the primary and lower output voltage rail 32 is set at 5 V through a series regulator 48.

The series regulated transfer circuit 41 is set at an intermediate 7 V in order to arbitrate between the 14 V and 5 V levels, as described below.

In fact, in order to maintain output stability, the minimum input voltage applied to the 5 V regulator 48 is about 6.5 V The voltage at the anode of diode 43 (D1) is held at 7 V, by the series regulated transfer circuit 41.

During normal operation, the cathode of diode 43 (D1) is held at the normal charge voltage of capacitor 38 (C2), ie just over 7 V, and the diode 43 is thus reverse-biassed, thereby isolating the (series regulated) transfer circuit 41 and the rails 31, 32.

At mains failure, the input to the series regulator 48 falls to 6.5 V in 2-3 mS and this falling voltage is then caught by the 7 V regulated level from the series regulated transfer circuit 41 and energy stored in 37 (C1), equivalent to about 20 V, is discharged therethrough, to maintain the 5 V output of the lower voltage rail 32 for some 30 mS.

These performance characteristics are depicted in chart form in Figure 2, which should be generally self-explanatory.

Thus, plot lines 55 and 56 respectively plot the two output rail voltages V vr and V cap across the diode 43 (D1), along with the prescribed lower output voltage V (of sane 5 V in this example) plot line 57.

The higher output voltage V aux, of sane 14 V in this example, is not so critical in the 'mains failure' condition, and is 'sacrificed' to preserving temporarily the more critical lower voltage, as described elsewhere.

Rather, any critical data handling element, such as memory, of an operational device reliant upon such a psu is typically driven fran the lower (5 V) rail and so only this output is critical in retaining data.

In a practical case, a 'power fail' (P.F.) warning logic signal may be derived within the psu, and applied to trigger a processor to transfer data into a static permanent memory file, only after the mains supply fails, but before the 5 V output falls out of specification, ie. during the 30 mS 'hold-up' period.

Reverting to Figure 4, input power supply failure at instant 'C' is not reflected in the (lower) output voltage until an instant 'D', the interval C-D being some 30 mS - ie some 10 times that prevailing in the standard circuit of Figure 1.

Specifically, the lower output voltage V out is preserved at 5 V until belated instant 'D', whereupon an exponential character delay prevails.

The 'support' of the voltage V cap across the capacitance 38 (C2) at a marginally higher level than of the output is reflected in plot line 57.

Figure 5 shows a more detailed circuit, (using standard legends and insignia, which will not be described in detail), incorporating additional linear power supply operating refinements, such as temperature stability control.

Proprietary integrated circuit elements not themselves part of the present invention are used to provide the series regulation facility referred to.

me dual supply voltage (capability) switch coupling SW1 on the primary side of the voltage transformer TRN1 is used to activate polarity cumulative applied voltage control of the two elements of the primary winding associated with the dual voltage secondary windings. That is, to achieve given (lower) output voltages, the mains supply may be connected across the primary windings in series, for a higher mains voltage, or in parallel for a lower mains voltage. In some applications, this may be automated by a relay driven mains level sensor.

Claims (1)

1. Claims

   I. A voltage regulation facility comprising an automatically operable voltage transfer connection (41) between a plurality of secondary voltage lines (31, 32), operable upon failure of a primary input power supply to transfer a higher level decay of a relatively higher voltage line (31) to a lower voltage line (32), and thereby sustain that lower voltage mamentarily.

   II. A voltage regulation facility as claimed in Claim 1, wherein the transfer connection incorporates a diode, reverse-biassed into non- conductive mode during normal operation with the secondary voltages sustained to their normal prescribed level, a capacitance charged by the higher voltage level, and discharged to the lower voltage line upon falling of the lower voltage, enabling forwarding biassing into conductance of the diode.

   III. A voltage regulation facility, as claimed in Claim 2, incorporating series voltage regulation in one or more of said voltage lines or transfer connection.

   IV. A voltage regulation facility, substantially as hereinbefore described, with reference to and as shown in the acccmpanying drawings.

   V. A power supply incorporating a voltage regulation facility as claimed in any of the preceding claims.