Abstract:
In an on-chip transformer, external electromagnetic field influences are reduced by providing an isolation transformer having primary and secondary windings with a  figure 8  configuration so that current induced by an external magnetic field is nulled.

Description:
FIELD 
       [0001]    The present invention relates to transformer designs. In particular it relates to on-chip transformers as are commonly used for transient voltage rejection. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is common to provide on-chip transformers to provide transient voltage rejection. As the rate of change of the voltage (dV/dt) in the primary winding of the transformer increases the back electromagnetic field (EMF) increases. Thus, with increasing frequency, as is the case with voltage transients, the reactance increases and the windings act as low pass filters thereby rejecting the transient voltage. However, these on-chip transformers remain susceptible to external magnetic fields. As shown in  FIG. 1 , as the component of any external magnetic flux (H)  100  passes parallel to the coil axis it generates a current  102  in the coil  104 . This effect is evident in both the primary and secondary windings of a transformer. One such prior art on-chip transformer is shown in  FIG. 2 , which shows a primary winding  200  formed by etching a metal layer to define a helical configuration. The transformer further includes a secondary winding  202 , again defined by a helically etched metal layer. Typically these helical coils  200 ,  202  are formed by photolithographic techniques as known in the art. As will be appreciated from the discussion above, an external magnetic field  210  will generate current in both the secondary winding  202  and the primary  200 . Thus, current will flow in the secondary winding  202  due not only to the current directly induced in the secondary winding by the magnetic field but also due to the magnetic coupling between the primary and secondary windings  200 ,  202 , which causes the current induced in the primary winding  200  to be transferred to the secondary  202 . It will therefore be appreciated that any external magnetic fields that pass through the transformer windings perpendicular to the windings (parallel to the axes of the windings, which in the transformer shown in  FIG. 2  coincide with one another) will cause substantial interference. 
         [0003]    In the field of electric guitars the vibration of the guitar strings is translated into sound by making use of pick-ups. These comprise coils formed around one or more permanent magnets. The permanent magnets define a magnetic field, which if altered, produces a current in the coils. By placing a pick-up underneath each of the guitar strings, the vibration of the string, which passes through the magnetic field of the magnet causes the reluctance to change, which produces a current in the coil that is then coupled though an amplifier to a speaker to produce sound. In U.S. Pat. Ser. No. 3,962,946 to Rickard, the use of two coils wound in opposite directions is described, with permanent magnet cores oriented in opposite directions (north-south in the one and south-north in the adjacent one). Thus the magnetic fields generated by the two adjacent magnets have flux lines flowing in opposite directions to produce currents flowing in opposite directions in the coils when the guitar string is plucked. However, since the coils themselves are oppositely wound and connected in series, the currents combine. On the other hand any external magnetic fields passing through the windings will cause currents generated in the two coils to cancel each other out to eliminate noise or hum due to interference from external magnetic fields such as the 60 Hz mains interference. The present invention seeks to reduce interference from external magnetic fields in an on-chip de-coupling transformer. 
       SUMMARY OF THE INVENTION 
       [0004]    According to the invention, there is provided a transformer comprising a primary winding and a secondary winding, the primary and secondary windings each including a first winding and a second winding wound in opposite directions and connected in series. The primary and secondary windings of the transformer may be formed as part of an integrated circuit. The primary and secondary windings may include a core, which may comprise a ferromagnetic core. Thus the windings may include an air core or a ferromagnetic core. 
         [0005]    The first and second windings of both the primary and the secondary windings may be configured as traces substantially in the form of a figure 8 to define a clockwise first winding and a counter-clockwise second winding. 
         [0006]    Further, according to the invention, there is provided a method in a semiconductor device of increasing signal immunity to external magnetic fields, comprising providing a voltage isolation on-chip transformer that includes a primary and a secondary winding each of which has a topology arranged to provide for canceling or nulling of current in the winding that is induced by an external magnetic field. Each of the windings may have a figure 8 topology, which may be formed by known photolithographic techniques. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a three dimensional view of a coil showing current generation due to an external magnetic field, 
           [0008]      FIG. 2 , shows the effect of an external magnetic field on a transformer with helical primary and secondary windings, and 
           [0009]      FIG. 3  is a top view of one embodiment of a primary or secondary winding of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    In the above discussion of the prior art, the use of pickups in electric guitars was discussed and the solution proposed by Rickard to reduce noise from external magnetic fields. Rickard deals with wire coils formed around permanent magnet cores that pre-define an electric field passing through the coils under normal operation. A single figure-8 coil is provided for each pickup to provide for current canceling during external magnetic field influences. It does not deal with voltage conversion using transformers or with the use of isolation transformers and does not contemplate any on-chip applications. 
         [0011]    The present invention, on the other hand deals with voltage isolation transformers that are provided in integrated circuits (ICs) and the problem of noise due to external electromagnetic interference that has plagued engineers and other people skilled in the art relating to circuit design and more specifically isolation devices used in ICs. 
         [0012]    The present invention proposes forming a primary winding and a secondary winding of a transformer each in the shape of a figure-8. The windings are etched during processing of the IC using photolithographic techniques, as is known in the art. In the embodiment shown in  FIG. 3 , a primary winding  300  and a secondary winding  302  are formed to define traces running substantially parallel to each other and each defining a first clockwise helical coil and a second counter-clockwise helical coil. The first coil of the primary winding is defined in  FIG. 3  by reference numeral  310 , the second coil of the primary winding by reference numeral  312 , first coil of the secondary winding by reference numeral  320  and the second coil of the secondary winding by reference numeral  322 . 
         [0013]    When a signal passes through the primary winding  300  it produces a magnetic field which couples with the secondary winding  302  to produce a current in the secondary winding. On the other hand, any external magnetic field or component thereof passing parallel to the axes of the windings (passing into or out of the page in  FIG. 3 ), e.g. electromagnetic radiation due to 60 Hz mains AC will cause current to flow either clockwise in both or counter-clockwise in both the first and secondary coils of the secondary winding  302  and in both the first and secondary coils of the primary winding  300 . The result is that the induced currents in the secondary winding  302  cancel each other out, and the induced currents in the primary winding  300  also cancel each other out. Consequently no noise is produced on the secondary side due to the external magnetic field. 
         [0014]    While the above embodiment made use of planar helical coils, the invention could be implemented in an IC using different configurations provided that both the primary and secondary windings of the resultant transformer include two serially connected coils that produced currents that cancel each other out when magnetic flux lines pass through the coils in the same direction.