Switch mode power supplies are commonly available in the market place and are often required to deliver a fairly large output power typically of the order of 50 W. Such power supplies generally include Schottky diodes to effect half-wave rectification and these diodes generally require forced cooling to prevent thermal destruction. Further, due to dead time in the half-wave rectification cycle, a toroid core of substantial size, with its concomitant costs, is used on the secondary side of such conventional power supplies to reduce output voltage fluctuations. It is an object of this invention to provide a switch mode power supply with enhanced operating characteristics.

According to the invention, there is provided a switch mode power supply which includes a planar transformer including a core with a primary winding and a plurality of secondary windings for operatively providing a corresponding number of secondary output voltages; pre-regulation circuitry drivingly connected to the primary winding and operable to control voltage applied to the primary winding

independent of fluctuations in voltage fed to the pre-regulation circuitry; and at least two low loss switches each connected to a secondary winding for effecting at least half-wave rectification, the switches each being switched by an independent secondary winding which is on the core which is common to all the secondary windings.

Further in accordance with the invention, there is provided a method of controlling switches for effecting at least half-wave rectification in a switch mode power supply including a planar transformer driven by pre-regulation circuitry, the method including switching the switches by feeding power directly sourced from a secondary winding of the planar transformer and which shares a common core with current to be switched by the switch, to gates of the switches; and limiting power fed from a mains source to a primary winding of the planar transformer by means of the pre-regulation circuitry thereby to limit a maximum voltage fed from the secondary winding to the switch.

As the gates of the switches are fed directly from the secondary windings on the common core, no elaborate switching circuitry or protection circuitry is required and limitation of the secondary voltage by the pre-regulation circuitry inhibits damage to the switches from fluctuations in the mains supply which could otherwise cause damage.

The low loss switches are typically low RDS on MOSFETS such as IRL 2205 configured for synchronous rectification. Accordingly,

as the maximum voltage fed to the planar transformer is controllable by the pre-regulation circuitry, possible destruction of the switches due to increased voltage on the secondary winding driving the switches from fluctuations in power from the source is minimised.

The secondary windings on the common core may define switching windings connected to the switches, and power windings for supplying power for half-wave rectification by the switches and which supply power to apparatus or circuitry connected to the power supply.

Typically, the switching windings are configured so that the power supply is suitable for use in a personal computer requiring supply voltages of about 3.3 V, 5 V, 12 V,-5 V, and/or-12 V. Accordingly, the invention extends to a computer power supply including a power supply as hereinbefore described. Preferably, the switching windings and the power windings are provided on a common board of the planar transformer. The common board may also include the various electronic components of the power supply.

The pre-regulation circuitry may include a feedback path connected to an output, typically the 5 V output, of the power supply.

Accordingly, the method may include monitoring an output voltage on the secondary side of the transformer and varying voltage fed to the primary winding in response to variations in the secondary voltage.

The power supply may include an input stage including a EMI core/filter operatively connected to the mains power source and rectification circuitry connected to the core/filter for rectifying the AC mains source and feeding DC to the pre-regulation circuitry.

The pre-regulation circuitry may include a conventional control integrated circuit, e. g. a UC3843 available from MOTOROLA.

The planartransformer may form part of step-down circuitry of the power. The step-down circuitry may include an oscillator, e. g. a conventional IR 2151 integrated circuit (from International Rectifier), which drive switches operatively feeding power to the primary winding.

The oscillator typically has a 50% duty cycle at a substantially stable frequency. The oscillator in the step-down circuitry typically runs at about half the operational frequency of the pre-regulation circuitry.

The invention extends to electronic apparatus which includes a switch mode power supply as hereinbefore described.

The invention is now described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings, Figure 1 shows a schematic circuit diagram of a switch mode power supply in accordance with the invention; and Figures 2 to 4 show schematic circuit diagrams of output rectification circuitry of the power supply of Figure 1.

Referring to the drawings, reference numeral 10 generally indicates a switch mode power supply in accordance with the invention.

The power supply 10 includes an input stage 12, stand-by circuitry 14, pre-regulator circuitry 15, step-down circuitry 16, and half-wave output rectification circuitry 18,20,22 (see Figures 2 to 4). The power supply

10 forms part of electronic apparatus, typically a personal computer or the like, also in accordance with the invention.

The input stage 12 is coupled via a fuse 24 to a conventional AC mains power source 26, and via an EMI core/filter 28, to a conventional rectification bridge 30. The bridge 30 together with its associated smoothing circuitry 32 feeds DC power to the stand-by circuitry 14 and to the pre-regulator circuitry 15. Both the stand-by circuitry 14 and the pre-regulator circuitry 15 use conventional integrated circuits 34,36 which are UC 3843 from MOTOROLA and IR 51H420 from International Rectifier respectively. A switch 38 of the input stage 12 allows the power supply 10 to be selectively driven by both 240 V AC as well as 110 V AC in a conventional manner.

The step-down circuitry 16 receives a limited DC voltage from the pre-regulator circuitry 15 via lines 40 which is fed to switches 42 for effecting half-wave rectification. Thereafter power is fed from the switches 42 via line 44 and a capacitor 46 to a primary winding 48 of a planar transformer 50. The planar transformer 50 includes twelve secondary windings 52 to 74 which share a common core with the primary winding 48.

The secondary windings 52,58,60,66,68, and 74 define switching windings which are connected directly to switches 78 to 88 for selectively switching the switches 78 to 88 to effect half-wave rectification. Power for half-wave rectification is sourced from secondary windings 54,56,62,64,70,72 which define power windings which supply power to electronics circuitry to be driven by the power supply 10.

Referring in particular to Figure 2 of the drawings, the secondary windings 52 and 54, and 56 and 58 are arranged so that the switching winding 52 enables the switch 78 at an appropriate instance thereby to feed power from the power winding 54 to an inductor 90 which, in turn, feeds regulated 3.3 V output power via terminal 92 to the electronic circuitry. Likewise, secondary windings 56 and 58 are arranged so that an appropriate time, switching winding 58 activates the switch 80 to feed power from the power winding 56 via line 94 to the inductor 90 thereby to effect half-wave rectification.

Referring in particular to Figure 3 of the drawings, the rectification circuitry 20 functions in a substantially similar fashion to the rectification circuitry 18 (see Figure 2) but is arranged to provide a positive 12 V output at terminal 96 and a negative 12 V output voltage at terminal 98 via inductors 100,110 respectively. Referring in particular to Figure 4 of the drawings, the rectification circuitry 22 substantially resembles the rectification circuitry 20 but is arranged to provide a positive 5 V output at terminal 112 and a negative 5 V output at terminal 114.

Unlike conventional switch mode power supplies which generally include schottky diodes to effect half-wave rectification, the switch mode power supply 10, in accordance with the invention, includes the switches 42, which are typically low loss MOSFETS IRF 740L available from International Rectifier, configured for synchronous rectification. The pre-regulator circuitry 15 has its integrated circuit 36 arranged to switch power to the planar transformer which operates with a 50% duty cycle and, accordingly, the switching windings 52,58,60, 66,68 and 74 switch the switches 78 to 88 with a 50% duty cycle.

The oscillator 41 in the step-down circuitry 16 operates at half the

frequency of the oscillator 36 in the pre-regulator circuitry 15. The switches 78 to 88 are low loss switches as mentioned above which typically have a negative temperature co-efficient and tend to be suitable to be placed in parallel (see Figure 4) further to reduce 12R losses.

Further, as the step-down circuitry 16 operates with a 50% duty cycle, losses usually associated with dead time are reduced in the power supply 10. Accordingly, forced cooling of the power supply 10 by a fan or the like is no longer required.

The integrated circuit 36 of the pre-regulator circuitry 15 has a feedback input 160 which is connected to the terminal 96 of the rectification circuitry 20 to provide a feed back facility to monitor the output voltage. In particular, when the output voltage at terminal 96 decreases due to specific load conditions, the integrated circuit 36 compensates for the decrease by increasing the voltage applied to the step-down circuitry 14. The maximum voltage fed to the step-down circuitry 16 limits the maximum voltage present on the switching windings 52,58,60,66,68, and 74 thereby to reduce the risk of any damage being caused to the switches 78 to 88. The planar transformer is typically designed so that a maximum voltage of about 15 volts is present on the switching winding 52,58,60,66,68, and 74.

The inventors believe that it is an advantage of the invention that with the inclusion of the pre-regulator circuitry 15 and the switches 78 to 88, the inductors required in the secondary stage, namely 90,100, 110 may be of reduced size thereby reducing the cost of the switch mode power supply 10. Further, the inclusion of the pre-regulator circuitry 15 which limits the maximum voltage applied to the planar transformer facilitates and simplifies the requirements of the planar

transformer. As the step-down circuitry 16 operates with a 50% duty cycle synchronous rectification is more efficient since freewheeling diodes conduct less current and dead time is minimized.

As the switch mode power supply 10 includes a planar transformer typically including an appropriately etched printed circuit board for defining the secondary windings, the various components of the supply 10 may be mounted on the same board thereby facilitating manufacture.