GB2198558A

GB2198558A - Regulated power supply

Info

Publication number: GB2198558A
Application number: GB08629294A
Authority: GB
Inventors: John Slater
Original assignee: Farnell Instruments Ltd
Current assignee: Farnell Instruments Ltd
Priority date: 1986-12-08
Filing date: 1986-12-08
Publication date: 1988-06-15
Anticipated expiration: 2006-12-08
Also published as: GB2198558B; GB8629294D0

Abstract

A constant voltage network consisting in its basic form of a small step-up transformer 10 having a diode 19 and a resistor 20 in parallel fed by its secondary winding is added in series with an output secondary winding 7 of a multi output power supply, of which the output secondary winding 7 has a control loop 12, 13, 14 associated with it to provide a stabilised output voltage. The inclusion of the constant voltage network enables the output voltages at terminals 15, 16, 17 to be increased by a small amount which cannot be easily done by varying the transformer turns ratio. <IMAGE>

Description

"REGULATED POWER SUPPLY" The present invention relates to power supplies and more particularly to a power supply which includes a transformer having a plurality of secondary windings to provide a plurality of output voltages of which one is automatically stabilised in closed loop by means of variation in the energisation of a primary winding for the transformer.

In such an arrangement, the minimum alteration or adjustment in an output voltage other than the main stabilised output voltage is determined by a unity increase in the number of effective turns on a respective secondary winding. The present invention is concerned with the problem of producing a change in the output voltage or output voltages other than the main stabilised voltage by less than the change that could be achieved as aforesaid.

A preferred form of the present invention is based on the provision of a close approximation to an ideal constant voltage drop in series with the main stabilised output circuit. Preferably this voltage drop is provided by a step-up transformer of which the secondary circuit includes, for example, a diode and a resistor in parallel. The low, but substantially constant voltage drop in the main stabilised output circuit causes the voltage per turn of the transforier winding to be increased by the action of the control loop. This increase causes a proportionate increase in the absolute level of output voltage from all the outputs other than the main stabilised output terminals, the main stabilised output being maintained at its original level by the action of the control loop.

In a modification of the invention the constant voltage drop is provided in series with an unstabilised output of the main transformer.

BRIEF DESCRIPTION OF THE DRAWING Figures 1 to 4 illustrate various embodiments of the invention; and Figure 5 illustrates a modification of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT A preferred embodiment of the invention includes a power supply in which a d.c. output voltage is stabilised by means of a control loop which varies the power and accordingly the mean voltage across the primary winding of the transformer by means of a switch of which the conductive periods are varied.

In the example shown in Figure 1, an a.c.

voltage source 1 provides input power which is rectified by a rectifier bridge 2 and smoothed by a capacitor 3 to provide a d.c. voltage between the voltage rails 3a and 3b. A transformer 4 has a first primary winding 5 in series with a transistor switch 6 connected between the rails 3a and 3b.

In a manner to be described, the switch 6 is turned on and off to control the mean voltage applied to the primary winding. A further winding 5a is connected in a recovery loop with the winding 5 and a diode 5b.

The transformer 4 has secondary windings 7, 8, 9 and 10 each of which is coupled in circuit with a respective diode 7a and capacitor 7b for the winding 7, a respective diode 8a and capacitor 8b for the winding 8 and so on so as to provide respective output voltages on the output rails 11, 15, 16 and 17.

In the present embodiment the main output is (for example) a five volt output which is stabilised by means of a control circuit 12 which may include a comparator and an inverter so as to provide an a.c.

control signal which is fed to an isolating transformer 13, the output of which drives a pulse width modulator 14 controlling the switch 6, the "on" periods of the switch 6 being increased or decreased as required to stabilize the main output voltage.

The circuit as thus far described is in generally known form. The outputs on the rails 15, 16 and 17 are, where the main output voltage on the rail 11 is stabilised, determined by the respective turns ratio between the primary winding and the respective secondary winding of the transformer 4.

In order to increase the voltage at the output rails 15 to 17 by some small increment, for example of the order of 19, which cannot conveniently be achieved by adjustment of the turns ratio, there is provided in series with the xain stabilised secondary circuit a primary 21 of a transformer 18 of which the secondary circuit includes a diode 19 in parallel with a resistor 20. The transformer 18 may have a turns ratio of any convenient size, for example 1 to 14 so that the voltage drop across the primary winding is 1/14 of the almost constant voltage drop (in this embodiment about 1 volt) across the diode 19 and resistor 20.The resultant voltage across the primary winding 21 causes, by the action of the control loop, the voltage per secondary turn on the transformer to increase such that the output voltages on the rails 15 to 17 increase by the desired factor.

The resistor 20 provides a current path when the diode is off (reversed biased), whereby the magnetic energy stored in the transformer 18 can be reduced to near zero. Otherwise the transformer core (usually ferrite) would saturate and action of the transformer 18 would cease.

Preferably, as shown, the transformer 18 has its primary in a branch connected to the junction between two (similarly wound) secondaries of the transformer 4.

The secondary circuit of the transformer 18 preferably employs a single diode so that the voltage across the secondary winding is a minimum; thus the flux density in the core and hence the size of the core may be minimised.

However, in place of the parallel combination of diode and resistor as shown in Figure 1, it is feasible to employ for example two diodes in series in a branch parallel to a resistor; a zener or avalanche diode in parallel with a resistor or a diode in parallel with a branch comprising a diode in series opposition with a zener or avalanche diode.

Figure 2 illustrates another embodiment of the invention. This embodiment is generally similar to that of Figure 1, but the regulator 12 responds to the outputs from windings 5 and 9; otherwise the circuit may be as previously described.

Figure 3 illustrates an embodiment similar to that of Figure 1 but including means for adjusting the effective turns ratio of the transformer 18. In this embodiment the resistor 20 is connected to the ends of the secondary winding 22 which has taps 23 the diode. 19 being connected between, for example, one end of the winding 22 and one of the taps 23.

Figure 4 illustrates another embodiment.

In this embodiment there are two controlled switches 6a and 6b each connected to one of two halves 5a, 5b of a centre-tapped primary of the main transformer.

In this embodiment the stabilised output is developed from a centre-tapped secondary of which the two halves are the windings 7 and 8. The transformer 18 has two primaries 21a and 21b, each in series with a respective one of the windings 7 and 8. The secondary circuit of this transformer comprises two anti-parallel diodes 19a and 19b.

Also, in this embodiment, the windings 9 and 10 are connected to constitute a centre-tapped secondary connected to provide positive and negative outputs, relative to a zero volts line 16, on lines 17 and 15 respectively.

Figure 5 illustrates a odificiation in which the transformer 18 is disposed in series with a winding 9 the output of which is not directly sensed by the regulator. This modification enables the absolute voltage level of the output from winding 9 to be adjusted with respect to the main regulated output level. Otherwise this embodiment may be modified as previously described.

Claims

CLAIMS:

1. A power supply comprising a transformer having a plurality of secondary windings connected to provide a corresponding plurality of output voltages, means responsive to at least one of these output voltages to control the power supply to a primary winding of the transformer whereby to stabilise at least the said one voltage, and means including a step-up transformer arranged to provide a substantially constant voltage drop in the circuit in which the stabilised output voltage is developed.

2. A power supply according to claim 1 in which the said step-up winding comprises a secondary circuit including means providing rectifying and energy recovery action.

3. A power supply comprising a transformer having a plurality of secondary windings connected to provide a corresponding plurality of output voltages, means responsive to at least one of these output voltages to control the power supply to a primary winding of the transformer whereby to stabilise the said one voltage, and means including a step-up transformer having a secondary circuit including means providing rectifying and energy recovery action, arranged to provide a substantially constant voltage drop in a circuit in which one of the output voltages is developed.

4. A power supply according to claim 2 or claim 3, in which the said secondary circuit comprises at least one diode in parallel with a resistor or two anti-parallel diodes, or at least one zener or avalanche diode either alone or in combination with at least one other diode.

5. A power supply according to any foregoing claim further comprising means for adjusting the transformer ratio of the said step-up transformer.

6. A power supply according to claim 4 in which the step--up transformer has a secondary winding with selectable tape.

7. A power supply according to any foregoing claim in which the first-mentioned transformer has two primary windings and centre-tapped secondary windings.