POWER SUPPLY

Abstract: A power supply receives and rectifies mains voltage using bridge D1-D4. This is used by a pair of transistors Q1 and Q2 to alternate the supply to a transformer T via capacitor C8. Outputs are available from secondary windings T2, T6 and T7. Further windings T4 and T5 provide an internal power source for system circuitry which includes optical regulation via IC1 and optical switching control using IC2.

Description: The invention relates to a power supply system. Traditionally power supplies for powering electronic circuits consisted of a mains transformer with a step-down secondary winding and a rectifier followed by capacitor smoothing to provide the low voltage d. c. supply. As a need for more precise regulation arose, such as required for transistor or logic chip circuits, semiconductor regulators were also employed. An alternative form of power supply is the 'switch mode' system which employs a fixed frequency oscillator control running at several kHz, and controlled by varying the mark : space ratio of this frequency. Such an arrangement is disclosed in GB 2131209A. However the fixed frequency utilised has been increased to 100kHz. The present invention is concerned with improving on such known power supplies. According to the invention there is provided a power supply system including: first and second series connected switching means (Q1, Q2), connected to receive a d. c. voltage; transformer means (T) having a first winding connected to the switching means and having a second winding configured to provide a power supply output at a desired voltage; first rectification means (D9, D10) for converting this voltage into a substantially d. c. voltage; sensing means (ZD3, Q3) for detecting the output voltage errors; and characterised by; variable frequency generator means (IC3) for determining the switching rate of the switching means, and optical feedback means (IC1) for providing a control path from said sensing means to said switching means to control the output frequency of the generator means to modify the switching frequency of the switching means to compensate for detected output errors. The invention will now be described by way of examples with reference to the accompanying drawing which shows an embodiment of the power supply. A mains voltage bridge rectifier is connected to the mains supply inputs 10,11 and its d. c. output is connected across a capacitor C1. One side of the capacitor C1 is connected to the ground rail 12, and its other side is connected to relay contact RL1 switchable between positions A or B. The A position connects the capacitor C1 via a resistor R2 and diode D7 to a first positive rail 13 and in the B position to a second positive rail 14. The connection node of diode D7 is also connected to diode D6 which is series connected with a secondary winding T5 of the supply transformer T. In parallel there is the relay coil RL1 with capacitor C2 connected across it. Diode D5 connects between the capacitor C5 and the capacitor C1. Between rails 12 and 13 there is connected a timer integrated circuit IC3. In this example, a type 555 is employed which is an 8 pin device. Pin 1 is connected to ground 12 and to capacitor C6 which has its other lead connected to rail 13. Trigger pin 2 is directly connected to pin 6 and is connected to capacitor C4 which is then connected to rail 12. Pin 6 is also connected to rail 13 via resistors R3, R1 and R4. Pin 4 is the reset input connected to zener diode ZD1 and to rail 13, (the other end of ZD1 being connected to rail 12). Pin 5 is the control pin connected to capacitor C3 which is then connected to rail 12. Discharge pin 7 is connected to resistor R4 which is then connected to rail 13. In this configuration the timer operates as an oscillator. The oscillator output from pin 3 is connected to the interconnection point of the light-emitting-diodes (LEDs) within IC2 (eg type ILD74). The diode portions of IC2a,b are associated with photo-transistor portions IC2a,b. One photo-transistor IC2a is connected to rails 12 and 13 (via resistor R6) and the other IC2b is connected to rails 14 and 15 (via resistor R9). The series connected LEDs of IC2 are connected across the rails 12 and 13 via current limiting resistors R7 and R8. An opto-coupled FET (eg type H11F1) device IC1 has its transistor portion connected in parallel to R1. The LED portion of IC1 is connected in series with a resistor R10 across the power unit outputs 16,17 via collector/emitter electrodes of transistor Q3. In parallel with these is a capacitor C9 and these are connected to receive the output of centre tapped secondary windings T2 and T3 of transformer T via rectifier diodes D9 and D10. A zener diode ZD3 is series connected with a resistor R11 to rail 17. The junction of ZD3 and R11 is connected to the base of the transistor Q3. Further windings T6 and T7 of the transformer provide rectified outputs to terminals 18-20 via diodes D11 and D12. Smoothing is provided by capacitors C10, C11. No other regulation is provided for these outputs. The primary winding T1 of the transformer is connected via capacitor C8 across the source and drain of a power FET (eg VMOS type 2SK310), Q2 its gate being connected to the collector of IC2a. In series with this FET there is a second FET Q1 (eg VMOS type 25J117) which series devices are connected between rails 12 and 14. The gate of Q1 is connected to the common connection of a zener diode ZD2, the emitter of opto-transistor IC2b, and resistor R9. A capacitor C7 is connected between rails 14 and 15 and a diode D8 is series connected from rail 15 to the rail 14 via a secondary winding T4 of transformer T. A resistor R5 from the rail 15 is connected in series with capacitor C5 to the rail 13. The system operates as follows. On connection of the 240 a.c. supply to terminals 10 and 11, the rectified output from D1-D4 charges C1 up to approximately 339 volts d.c. The relay contact RL1, at switch on, will be in position A as shown in the Figure and this will allow the d.c. voltage from the bridge to charge up C6 (via R2 and D7) and charge up C2 (via R2) until the potential is sufficient to energise relay coil RL1 (eg 12V). Until RL1 is energised, C6 will provide power to IC2 and 3, this being regulated by zener ZD1. The oscillator output from IC3 (at pin 3) will alternately switch on LEDs IC2a,b at a rate determined by the oscillator frequency. The switching action will actuate the photo-transistor portions of IC2a,b in sympathy. This will alternately gate the FETs Q1 and Q2. Power to the photo-transistor portion of IC2a is available via limiting resistor R6 (to prevent excess currents when IC2a is switched on). Once RL1 is energised the contact of RL1 will move to position B and so apply the 339V between rails 14 and 12. C5 will begin to charge, initially providing a 15 volt supply to photo-transistor IC2b, as regulated by ZD2, via limiting resistor R9. The oscillator action will alternately switch Q1 and Q2 (via the opto-isolators IC2a,b) so that when Q1 is conducting, the 339V will be applied to the winding T1 connected to the common connection of Q1 and Q2, and when Q2 is alternatively conducting, OV will be applied to T1. This alternating action will cause an alternating current to flow in T1 and charge C8 to an average voltage of half supply, and induce a stepped down voltage in the secondary windings T2-T5. Once this transformer action has started, winding T5 can now provide the source of power via rectifier D6 to rail 13. Similarly, winding T4 will provide power via D8 to photo-transistor IC2b. The capacitor C5 will now be able to become fully charged. The centre tapped secondaries T2,T3 provide the power supply output via rectifiers D9,10 and smoothed by reservoir capacitor C9 to terminals 16 and 17. A similar rectification and smoothing system is provided by the additional outputs to terminals 18, 19 and 20. Regulation of the supply output at terminals 16 and 17 is however additionally provided and is achieved by a rising output voltage being detected by zener ZD3 which will cause increased conduction of the LED portion of IC1 driven via transistor Q3. This causes the opto-coupled FET portion to decrease its resistence, thereby modifying the resistance relationship between R3, R1 and R4 (as the FET is parallel with R1). This relationship change affects the CR ratio between pins 6 and 7 of IC3 and so raises the frequency of oscillation of this device. This increase in frequency causes an increase in the switching rate of IC2a,b and Q1 and 2 and hence a reduction in output voltage from the transformer secondaries so adjusting automatically the power supply output. The detection by ZD3 and the driving of ICI via transistor Q3 ensures regulation over a wide range of output currents, due to the high gain of the feedback circuit. The provision of capacitor C8 in the primary winding allows charging and discharging through the transformer to provide the source of alternating current for use by this transformer and is more cost effective than providing four transistor switches in combination to drive the transformer circuit. In addition C8 (eg 2.2 µF) will provide some overload protection to the circuit. When the power supply is no longer required and is switched off, the relay contact will return to position A and C5 can be discharged through D7 and R2 via D5. Although transformer T may be toroidal, in the present configuration the transformer can be of minimal size compared to normal power supply transformers. Thus a Mullard CX3920 ferroxcube based transformer for example with 150 turn primary and 12 turn secondaries will be capable of handling at least 30W. This is for a dimension of only 24mm square (and containing all the secondary windings T2-T6). The configuration shown with optical control with only a single transformer and dual power transistor switching provision can be accommodated on board size of only 65 × 75 mm, so being only about one third size of typical commercially available units. Tests on the power supply based on this configuration have shown that this can easily provide an output of 30W. IC3 can typically run at 12 kHz with a regulation band of 5-30 KHz dependent on output power. As the regulating frequency increases the impedence in t1 also increases which makes the current therethrough decrease. This in turn causes a reduced induced voltage in the secondaries and thus will regulate the output by reducing this voltage. As the control frequency goes down the impedence reduces and the induced voltage in T increases. The power supply gives a good degree of regulation eg better than ± 2 percent and the efficiency is of the order of 85 percent. This gives a supply suitable for many applications with electronic circuits and without excessive heat generation problems. The transistors run almost cold at this rating so eliminating the need for forced draught cooling or large heatsinks. With a larger transformer and uprated transistors Q1 and Q2 larger power requirements could easily be handled.

Claims: A power supply system including: first and second series connected switching means (Q1, Q2), connected to receive a d. c. voltage; transformer means (T) having a first winding connected to the switching means and having a second winding configured to provide a power supply output at a desired voltage; first rectification means (D9, D10) for converting this voltage into a substantially d. c. voltage; sensing means (ZD3, Q3) for detecting the output voltage errors; and characterised by; variable frequency generator means (IC3) for determining the switching rate of the switching means, and optical feedback means (IC1) for providing a control path from said sensing means to said switching means to control the output frequency of the generator means to modify the switching frequency of the switching means to compensate for detected output errors. A system as claimed in claim 1, wherein the first and second switching means each comprise at least one solid state switch (Q1, Q2), said generator means comprises an oscillator (IC3), and said optical feedback means includes a first optoemitting device (IC1) having a variable output dependent on output variations detected by said sensing means and includes a first opto-receiving FET device (IC1) associated with the oscillator for providing a variable frequency therefrom so as to adjust the switching rate of said solid state switches. A system as claimed in claim 2, including a second opto-emitting and receiving device (IC2), said second opto-emitting device receiving the oscillator output and said second opto-receiving device being associated with the solid state switches to provide optical control of the switching thereof. A system as claimed in claim 3, wherein the second opto-device includes at least two transmitters and receivers (IC2a, IC2b) each associated with a respective solid state switch. A system as claimed in claim 2, 3 or 4, wherein second rectification means (D6, D8) are provided to rectify an a. c. voltage for use by the first and second switching means. A system as claimed in any one of claims 2 to 5, wherein each solid state switch comprises a power FET. A system as claimed in any one of claims 2 to 6, wherein the regulating means includes a zener diode (ZD3) connected to a control transistor (Q3) for driving the first opto-emitting device. A system as claimed in claim 1 or 2 wherein tranformerless driving means (IC2) are provided to drive the switching means at a rate determined by the variable frequency generator means. A system as claimed in claim 8, wherein the transformerless driving means comprises first and second optical driving means (IC2a, IC2b) for driving the first and second switching means respectively. A system as claimed in any one of claims 1 to 9, wherein the optical feedback means (IC1) are configured to vary the capacitive/resistive (CR) ratio of the variable frequency generator means. A system as claimed in any one of claims 1 to 10, wherein the variable frequency generator means (IC3) is configured to vary the switching frequency so as to increase or decrease the impedance of the first winding of the transformer means to induce a lower or higher voltage therein without changing the voltage applied to the switching means. A system as claimed in any one of claims 1 to 11, wherein the first and second series connected switching means are connected at their node to one side of the first winding of said transformer means and the second side of this transformer winding is connected in series with a capacitor (C8) to allow charging and discharging thereof through the transformer to provide the source of alternating current therefore. A system as claimed in any one of claims 1 to 12, wherein only a single transformer means is provided for all circuit requirements. A system as claimed in claim 13, wherein the transformer means includes a plurality of windings for providing both output power and internal power requirements. A system as claimed in claim 14, wherein the transformer means consists of a single transformer configuration (T1) constructed from a ferrite based material with windings associated therewith. A system as claimed in claim 14 or 15, wherein diverting means (RL1, C6) are included to provide an alternative power source for internal power requirements when unavailable from said transformer means. A system as claimed in claim 16 wherein the diverting means are connected so as to be powered by an output derived from the d. c. voltage intended for the switching means. A system as claimed in claim 16 or 17, wherein the diverting means includes a relay (RL1) and the alternative power source includes a capacitor (C6) chargeable or dischargable through the path provided by the diverting means.

Inventors: LAIDLER, GRAHAM ANTHONY
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1988
Main classification: H02M3
IPC codes: H02M3