Appliances such as audio amplifiers, televisions, monitors, computers and the like operate from standard Alternating Current (AC) mains power supplies, and therefore require internal power supply units incorporating step-down transformers and rectifiers of a known type to step the voltage down to levels suitable for operating the appliance, and rectify to DC.

Typically, the incoming AC mains voltage is converted to unregulated DC, and individual circuits are powered by separate DC-to-DC converters, most commonly switch mode converters, which may be regulated or unregulated.

A SMPS operates at a high frequency, which may commonly

be in the range of 60-250 kHz, and thus is liable to generate electromagnetic noise. One particular source of unwanted"noise"arises from the parasitic interwinding capacitance between the primary and secondary windings of a transformer (also referred to as the interwinding capacitance). The efficiency of a transformer is increased by improving the flux linkage between the transformer windings. The flux linkage can be increased by increasing the proximity of the primary and secondary windings. However, this also has the effect of increasing the interwinding capacitance.

In a SMPS, where a generally square wave is applied to the primary winding of a transformer, the interwinding capacitance has the effect of introducing noise to the output from the secondary winding, in the form of spikes at the edges of the square waveform. In conventional SMPSs, this noise is reduced by filtering the output from the secondary winding; e. g. by the use of a capacitor connected between the secondary winding and the primary DC supply.

This creates problems when it is desired to filter lower frequency noise as the physical size and current leakage from mains to the capacitor used to filter out the noise increases as the frequency of the noise which it is desired to filter decreases. Thus, in order to filter relatively low frequency noise, relatively large capacitors are needed. This creates problems in modern electrical equipment such as audio amplifiers and computers, where space is often important and thus the size of components vital. Furthermore, the mains leakage current permitted by law is regulated, therefore there is a limit to the size of the capacitors (and therefore the degree of filtering) which is possible. Moreover, capacitors used in this

way must comply with applicable safety regulations, thereby increasing the cost of components.

It is an object of the present invention to provide improved means for suppressing noise arising from interwinding capacitance in a SMPS.

According to a first aspect of the present invention, there is provided a switch mode power supply having a noise suppression system; the power supply including a transformer having primary and secondary windings and means for switching an input voltage across the primary winding; the noise suppressing system comprising a feed-forward system arranged to supply a feed-forward voltage to the transformer secondary, the feed-forward voltage being an inverted proportion of the voltage applied to the primary and the feed forward system including a capacitor such that the output from the feed-forward system substantially cancels noise arising in the transformer secondary as a consequence of the interwinding capacitance between the transformer primary and secondary windings.

Preferably, said capacitor has a capacitance Cl9 = X. Cw where Cw is the interwinding capacitance of the transformer, and x is the value of said proportion of the voltage applied to the primary.

Preferably, the feed-forward system comprises an inverting operational amplifier having an output connected to the secondary via said capacitor.

Preferably, said proportion of the voltage applied to

the primary is determined by a voltage divider resistance pair connected in the primary.

Preferably also, the power supply is a half-bridge arrangement, the switching means being a pair of MOSFETs, and the resistance pair is connected across one side of the primary.

Preferably also, the noise suppression system further includes a second capacitor connected between the transformer secondary and the primary DC supply.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig 1 is a circuit diagram of a switch mode power supply incorporating conventional means for suppressing noise arising from interwinding capacitance; Fig 2 is a circuit diagram of a switch mode power supply incorporating improved means for suppressing noise arising from interwinding capacitance, in accordance with the present invention; and Figs 3A, 3B and 3C are waveforms in particular portions of the circuits of Fig 1 and Fig 2.

Referring to the drawings, in Fig. 1 there is shown generally at 1 an example of a circuit for a conventional switch mode power supply (SMPS).

The SMPS includes a transformer shown generally at 2 having a primary coil 3 and a secondary coil 4. The

SMPS is driven by a DC power supply across lines 5 and 6. Two Metal Oxide Silicon Field Effect Transistors (MOSFETs) 7 and 8 are provided between the power lines 5 and 6, and the MOSFETs 7 and 8 are switched alternately (by a conventional driving circuit not shown) to generate a substantially square wave voltage waveform Vs which is applied to the transformer primary 3. The transformer primary 3 is connected between the voltage Vs and the power supply lines 5 and 6 via coupling capacitors 9 and 10. The waveform of the voltage Vs is shown generally in Fig 3A. It will be understood that the waveform Vs applied to the primary winding 3 could be generated by any other suitable switching network.

The output from the secondary winding 4 is applied to a voltage rectifying and/or regulating circuit or the like, generally designated at 22, to produce the required final output from the SMPS.

Noise is introduced into the output from the secondary winding 4 as a result of the interwinding capacitance Cw which exists between the primary and secondary windings 3 and 4. When a generally square wave such as that illustrated in Fig. 3A is applied to the primary winding 3, the interwinding capacitance Cw results in spikes being introduced into the output from the secondary winding 4 at locations corresponding to the edges of the square wave, as indicated in Fig. 3B.

In the conventional circuit illustrated in Fig. 1, this noise is filtered by means of a capacitor 20 connected between the secondary winding 4 and the primary DC supply. The effectiveness of such passive filtering is limited by the size and cost of the capacitor 20 required to compensate for the interwinding capacitance

Cw. In such a known system, the noise generated would have the waveform V,, as shown in Fig 3B, having a peak to peak Voltage of the order of 1V.

Referring now to Fig 2, there is shown a circuit for a SMPS incorporating a noise suppression system according to a preferred embodiment of the invention, in which parts equivalent in function to the parts shown in Fig 1 have the same reference numerals.

In the circuit of Fig 2, a voltage divider shown generally at 11 is connected between Vs and the primary ground 6, the voltage divider 11 having first and second resistors 12 and 13. The centre voltage of the voltage divider 11 is supplied via line 14 to the inverting input 16 of an inverting operational amplifier 15, the non-inverting input 17 of the amplifier 15 being connected to the primary ground line 6. The amplifier 15 is arranged in an inverting feedback implementation, with a feedback resistor 21.

In this way, the waveform of the output of the inverting amplifier 15 is a fraction of the switching voltage waveform Vs applied to the transformer, its value being set by the values of the resistors 12,13 and 21.

The inverting operational amplifier 15 inverts the waveform of the supply entering the amplifier 15 to 180° out of phase with the supply voltage Vs, having a peak to peak voltage equivalent to a fraction of that of Vs, as stated above.

This supply from the output 18 of the operational amplifier 15 is fed to the secondary side of the circuit at the output of the secondary winding 4 of the

transformer 2. The arrangement is therefore a feed-forward voltage system.

The output from the operational amplifier 15 is fed to the secondary side of the circuit via a capacitor 19, the value of which is selected on the basis of the interwinding capacitance Cw and the fraction of the switching voltage waveform determined by the voltage divider 11, as shall be discussed further below. The capacitor 19 modifies the output from the operational amplifier 15 in the same way that the interwinding capacitance Cw modifies the waveform Vs. Since the output from the operational amplifier 15 is 180° out of phase with the waveform Vs, the resultant signal is substantially equal and opposite to that generated by Cw so that the two signals substantially cancel one another.

Optionally, and preferably, the circuit also includes capacitor 20A, corresponding to capacitor 20 of Fig. 1.

The value of capacitor 20A can be substantially less than that of capacitor 20. This provides a well balanced system wherein noise suppression is provided in the event of failure of one or both of the capacitors 19 or 20A or the amplifier 15.

The waveform after cancellation is shown at Fig 3C, showing that the peak to peak voltage without cancellation is of the order of 1 Volt (as in Fig 3B), and with cancellation is of the order of 0.1 Volts.

The capacitance of filter capacitor 19 is preferably equivalent to Cw (the interwinding capacitance of the transformer 2) multiplied by a factor x which, is equivalent to that factor by which the supply Voltage Vs is reduced at the output 18 of the amplifier 15. Thus

Cl9 = x. Cw and the voltage at output 18 of the amplifier 15 is equal to Vs/x.

Thus the cancelled or partially suppressed noise (the waveform of which is shown at Fig 3C) is reduced to acceptable levels which can be dissipated in known ways, for example leakage through earth wires.

The invention thus provides improved means for suppressing noise arising from the interwinding capacitance Cw by generating a substantially equal and opposite signal which substantially cancels the noise at the output from the secondary winding 4. It will be understood that a similar noise-cancelling signal could be generated by means other than that described in the present embodiment. However, the present arrangement is preferred in view of its relative simplicity and low cost.

The invention can readily be implemented with an existing SMPS and could also be incorporated into an integrated SMPS unit.

Modifications and improvements may be incorporated herein without departing from the scope of the invention.