GB2307353A - Radio frequency transformer and method of assembly - Google Patents

Abstract

A radio frequency transformer (35) includes a primary winding element (25) containing a secondary winding arrangement (22), wherein the primary winding element (25) is formed by coupling a first winding element (30) to a second winding element (31) and a ferrite element (24) is formed around the primary winding element (25). The winding elements (30, 31) may be identical and may include a centre tap arrangement (27, 28). The input and output connections (20, 21 and 26-29) of the transformer (35) may be arranged for surface mounting and the transformer (35) may include a heat sink arrangement. A method of constructing the above transformer is also disclosed.

Description

RADIO FREQUENCY TRANSFORMER AND METHOD OF ASSEMBLY Field of the Invention This invention relates to radio frequency transformers. The invention is applicable to, but not limited to, high power, surface mount, radio frequency transformers and method of assembly.

Background of the Invention One option for connecting radio frequency circuits together is by use of radio frequency transformers. To ensure maximum power transfer between radio frequency circuits, the input and output impedance of the circuits need to be carefully designed. The careful design of such impedance in such circuits is termed 'matching', with the output of one circuit being matched to the input of the next circuit, often at an impedance value of 50 ohms. A broadband radio frequency transformer is often needed to achieve accurate matching of radio frequency circuits at high frequencies across a wide frequency range.

A radio frequency transformer includes primary and secondary windings. The ratio between the primary and secondary windings dictate the ratio of power that is transformed. A disadvantage associated with prior art radio frequency transformers is that losses occur when connecting the windings of the transformer to external circuitry.

Consequently, the efficiency of the radio frequency transformer decreases.

Such transformer losses at high operating powers appear as heat generated within the transformer. A further disadvantage associated with such prior art radio frequency transformers is that they are difficult to assemble due to their complex construction, particularly for surface mount applications. In addition, losses due to the length of the high-impedance primary winding are undesirable.

This invention provides an improved broadband radio frequency transformer which mitigates at least some of the above mentioned disadvantages associated with broadband radio frequency transformer designs.

Summarv of the Invention In a first aspect of the present invention, a radio frequency transformer for transforming radio frequency power is provided. The radio frequency transformer comprises a primary winding element containing a secondary winding arrangement, wherein the primary winding element is formed by coupling a first winding element to a second winding element. A ferrite element is formed around the primary winding element.

In this manner, the primary winding element is formed and, when contained within the ferrite element and coupled to the secondary winding arrangement, facilitates the transforming of radio frequency power.

Preferably, the first winding element is identical to the second winding element and the first winding element and second winding element are arranged in a complimentary manner to form the primary winding element. In a preferred feature of the present invention the primary winding element is formed to include a centre tap connection for coupling portions of transformed radio frequency power.

In a preferred embodiment of the present invention, the radio frequency transformer is a high power radio frequency transformer operating at a high frequency and includes an input connected to the primary winding element and an output connected to the secondary winding arrangement in a surface mount configuration to form a radio frequency transformer circuit. The radio frequency transformer further comprising a heat sink operably coupled to the ferrite element via a thermal conductor for dissemination of heat from the radio frequency transformer at high powers.

In a second aspect of the present invention, a method of assembly of a radio frequency transformer is provided. The method includes the steps of connecting a first winding element to a second winding element to form a primary winding element and forming a ferrite element around the primary winding element to provide a conductor for electrical currents.

The secondary winding arrangement is routed within the primary winding element to facilitate a transforming of radio frequency power.

In a preferred embodiment of the second aspect of the invention, the method further includes the steps of attaching the ferrite element to a heat sink using a thermal conductor and encasing the ferrite element and primary winding element in a case having an input port operably coupled to the primary winding element, the input port being isolated from an output port which is operably coupled to the secondary winding arrangement to form a high power surface mount radio frequency transformer.

In this manner, a high power radio frequency transformer is easily and efficiently assembled.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Description of the Drawings FIG. 1 shows a top view representation of a prior art radio frequency (RF) transformer.

FIG. 2 shows a side view representation of the prior art radio frequency (RF) transformer.

FIG. 3 shows a top view of a radio frequency transformer according to the preferred embodiment of a first aspect the invention.

FIG. 4 shows a perspective view of a first winding element and a second winding element to form a primary winding element of a radio frequency transformer according to the preferred embodiment of the invention.

FIG. 5 shows a side view of the radio frequency transformer coupled to a heat sink in a surface mount topology according to the preferred embodiment of the invention.

FIG. 6 shows an equivalent circuit of the radio frequency transformer according to the preferred embodiment of the invention.

FIG. 7 is a flow chart showing a method for assembly of a radio frequency transformer according to a second aspect of the invention.

Detailed Descrintion of the Drawings Referring first to FIGs. 1 and 2, a top view and side view representation of a prior art radio frequency (RF) transformer are shown.

Primary impedance to secondary impedance ratios of n2:1 in the transformation of radio frequency power, e.g. 1:1, 4:1, 9:1, are typically used with this construction. A low impedance primary winding element consists of metal tubes 14, through which an appropriate number of 'n' turns of a conductive material are threaded to form a high-impedance secondary winding arrangement 12. To improve the broadband characteristics, of the radio frequency transformer, a high primary winding inductance is used. This is achieved by using a magnetic material, e.g. ferrite rings 10, in the construction of the transformer. The metal tubes 14 are connected to a metal plate 15 to improve the ground contact, with the metal plate 15 being connected to a ground point on a transformer circuit by the attachment 13.Typically in this type of construction, the contact 11, between the metal tubes 14 and the metal plate 15, creates undesirable losses in the radio frequency transformer resulting in inefficient power transformation. In high power applications, the transformer losses appear as heat, generated within the transformer.

A significant disadvantage of such a prior art radio frequency transformer design is that it is difficult to assemble due to its complex construction, particularly for surface mount applications. In particular, when the surface mount transformer is connected to a printed circuit board (PCB) by a "pick and place" soldering machine. In addition, there are losses associated with the high-impedance primary winding conductive material.

Referring now to FIG. 3, a top view of a radio frequency transformer, according to the preferred embodiment of a first aspect the invention, is shown. The radio frequency transformer transforms radio frequency power, and comprises a primary winding element containing a secondary winding arrangement, wherein the primary winding element is formed by coupling a first winding element to a second winding element; and a ferrite element formed around the primary winding element.

The primary winding element 25 is formed by coupling a first winding element 30 to a second winding element 31. Preferably, the first winding element 30 and the second winding element 31 are substantially identical and made of a split copper shell as shown in FIG.4. They are arranged in a complimentary manner such that the first winding element 30, e.g. a first half of the copper shell, is placed substantially on top of the second winding element 31, e.g. a second half of the copper shell, to form the primary winding element 25. Two holes on one side of the copper shell and one hole on the opposite side of the copper shell are designed in such a way as to connect the first and second winding elements together to form a "U" shape copper tube. In addition once arranged in a complimentary manner, the holes allow the primary winding element 25 to be attached to the PCB or heat sink as shown in FIG. 3. The primary winding element 25 of the preferred embodiment contains only one secondary winding arrangement, hence forming a "1:n" RF transformer.

Referring now to FIG. 5, a side view of the radio frequency transformer according to the preferred embodiment of the invention is shown. The radio frequency transformer preferably includes an input connected to the primary winding element and an output connected to the secondary winding arrangement in a surface mount configuration to form a radio frequency transformer circuit, the radio frequency transformer further comprising a heat sink operably coupled to the ferrite element via a thermal conductor for dissemination of heat from the radio frequency transformer at high powers.

In the preferred embodiment of the invention the radio frequency transformer 35 is coupled to a heat sink 33 in a surface mount topology.

The radio frequency (RF) transformer 35 includes a primary winding element 25, a ferrite element 24, e.g. square ferrite, and a secondary winding arrangement 22. The ferrite element 24 is formed around the primary winding element 25 and the secondary winding arrangement 22 is contained within the primary winding element 25. The primary winding element 25 has a centre tap connection legs 27 and 28 for coupling portions of the transformed RF power. The secondary winding arrangement 22 is preferably made of wire, e.g. Teflon wire, and is enclosed inside the copper shell of the primary winding element 25. The RF transformer 35 in FIG. 5 is connected to a transformer circuit 32 in a surface mount arrangement with a heat sink 33 operably coupled to the ferrite element 24 via a thermal conductor 34.

The RF transformer 35 of FIGs. 3 and 5 includes primary winding leads 20 and 21, a case 23, a ferrite element 24, a primary winding element 25, a secondary winding arrangement 22, secondary winding legs 26, 29 and centre tap connection legs 27, 28. The primary winding leads 20 and 21 of the primary winding element 25 form the input and the secondary winding legs 26 and 29 of the secondary winding arrangement 22 form the output of the RF transformer in a surface mount arrangement within a RF transformer circuit. In the preferred embodiment of the invention, the RF transformer 35 is a high power RF "1:n" transformer operating at a high radio frequency.

Referring now to FIG. 6, an equivalent circuit of the RF transformer according to the preferred embodiment of the invention is shown. The equivalent circuit represents the electrical elements of a broadband RF transformer. The equivalent circuit includes a radio frequency generator 40, connected to a series source resistor (Rs) 41, and coupled to a primary winding element (L1) 46 via a primary series resistor R1 42. A secondary winding element (L2) 47, is operably coupled to the primary winding element (L1) 46, a secondary series resistor (R2) 44 and a load resistor (R3) 45.

In operation, the series source resistor (Rs) 41 and the primary series resistor (R1) 42 represent the primary winding element 25 and associated losses due to the primary winding leads 20, 21. The secondary series resistor (R2) 44 represents the secondary winding arrangement 22 and associated losses. The primary winding element (L1) 46 is the inductance of the primary winding element 25, with secondary winding element (L2) 47 being the inductance of the secondary winding arrangement 22.

The efficiency of the RF transformer t7 is given by Rs R3 Rs+Rl R3+R2 (1) Equation (1) shows that, with resistance being directly proportional to the length of the primary and secondary winding leads, shorter leads reduce RF transformer losses, e.g. losses associated with the primary series resistor (R1) 42 and secondary series resistor (R2) 44. Hence, the efficiency 1 of the RF transformer will increase, providing optimal power transformation of RF signals to the load resistance (R3) 45.

Referring now to FIG. 7, a flow chart of a second aspect of the invention is shown. The flow chart illustrates a method of assembly of a radio frequency transformer in accordance with the preferred embodiment of the invention. The method of assembly of a radio frequency transformer, the method comprises the steps of connecting a first winding element and a second winding element to form a primary winding element; forming a ferrite element around the primary winding element to provide a conductor for electrical currents; and routing a secondary winding arrangement within the primary winding element to facilitate a transforming of radio frequency power.Preferably, the method includes the steps of attaching the ferrite element to a heat sink using a thermal conductor; and encasing the ferrite element and primary winding element in a case having an input port operably coupled to the primary winding element, the input port being isolated from an output port which is operably coupled to the secondary winding arrangement to form a high power surface mount radio frequency transformer.

The assembly of the RF transformer, shown in FIG. 7, starts by connecting a first winding element 30 to a second winding element 31 to form a primary winding element 25, as shown in step 100. A ferrite element 24 is formed around the primary winding element 25, as in step 102. A secondary winding arrangement 22 is routed within the primary winding element 25 to form a transformer, as in step 104. The transformer may be connected to a transformer circuit and encased to provide isolated input and output ports for the primary winding element 25 and secondary winding arrangement 22, as shown in step 106. The encased ferrite element may be attached to a heat sink 33 using a thermal conductor 34, as shown in step 107.It will be appreciated by a person skilled in the art that it is possible for these steps to be perfomed in a different order in the building of the RF transformer, i.e. the secondary winding arrangement being routed in the primary element before the ferrite element is formed.

The method described is a preferred embodiment of the second aspect of the invention for manufacturing such a transformer.

Advantageously, the split copper shell in the preferred embodiment of the invention has a very low resistance and the short leads used to attach the transformer to the PCB reduce the parasitic inductance and minimise broadband RF transformer losses. In the preferred embodiment of the invention, the split copper shell is made from two identical parts to reduce manufacturing costs. As an alternative to using a heat sink, the ferrite is mounted directly on a flat chassis for heat dissipation, eliminating the need for mechanical adapters that are normally needed for high power RF transformers.

Thus an improved broadband radio frequency transformer, and method of assembly, are provided mitigate some of the disadvantages associated with broadband radio frequency transformer designs.

Claims (8)

Claims

1. A radio frequency transformer for transforming radio frequency power, the radio frequency transformer comprising: a primary winding element containing a secondary winding arrangement, wherein the primary winding element is formed by coupling a first winding element to a second winding element; and a ferrite element formed around the primary winding element.

2. The radio frequency transformer according to claim 1, wherein the first winding element is identical to the second winding elements and the first winding element and second winding element are arranged in a complimentary manner to form the primary winding element.

3. The radio frequency transformer according to any of the preceding claims, wherein the primary winding element includes a centre tap connection for coupling portions of transformed radio frequency power.

4. The radio frequency transformer according to any of the preceding claims, wherein the radio frequency transformer is a high power radio frequency transformer for operation at a high radio frequency.

5. The radio frequency transformer according to any of the preceding claims, wherein the radio frequency transformer further includes an input connected to the primary winding element and an output connected to the secondary winding arrangement in a surface mount configuration to form a radio frequency transformer circuit, the radio frequency transformer further comprising a heat sink operably coupled to the ferrite element via a thermal conductor for dissemination of heat from the radio frequency transformer at high powers.

6. A method of assembly of a radio frequency transformer, the method comprising the steps of: connecting a first winding element to a second winding element to form a primary winding element; forming a ferrite element around the primary winding element to provide a conductor for electrical currents; and routing a secondary winding arrangement within the primary winding element to facilitate a transforming of radio frequency power.

7. The method of assembly of a radio frequency transformer according to claim 6, the method further comprising the steps of: attaching the ferrite element to a heat sink using a thermal conductor; and encasing the ferrite element and primary winding element in a case having an input port operably coupled to the primary winding element, the input port being isolated from an output port which is operably coupled to the secondary winding arrangement to form a high power surface mount radio frequency transformer.

8. A radio frequency transformer substantially as described herein with respect to FIG. 3.