GB2307795A - Isolation transformer with plural magnetic circuits coupled by a winding - Google Patents

Abstract

An isolation transformer comprises two or more independent magnetic paths formed by high permeability magnetic cores 7, 8. Each magnetic core 7, 8 includes one or more independent windings 1, 4 and at least one coupling winding 6. Each core 7, 8 and its independent associated winding(s) 1, 4 is enclosed by an electrostatic screen 2, 5 whilst the coupling winding 6 is located outside the electrostatic screen 2, 5. The coupling winding 6 may be formed by interconnecting separate windings associated with individual cores and may be arranged to couple with more than two cores. Alternatively a core with a primary winding may be linked by plural coupling windings to respective secondary cores to drive plural secondary output windings.

Description

HIGH ISOLATION POWER TRANSFORMER WITH MORE THAN ONE MAGNETIC CIRCUIT COUPLED BY A SEPARATE WINDING.

This invention concerns the design of power transformers to facilitate the construction of isolated power supplies that exhibit very low AC leakage currents between the primary and secondary sides. The term Power Transformer is intended to mean a transformer utilised to transfer power in a power supply circuit. In particular it operates with primary and secondary voltages on its windings, normally greater than 1Volt. This statement is intended to distinguish the application from that of Current Transformers which are used for measurement purposes and are intended to operate with very low winding voltages.

Power supplies are used in many applications and are usually concerned with the conversion of voltage from an input level such as mains voltage or a DC voltage such as 12Volts commonly present in motor vehicles, to an output level required to operate the circuits or equipment to be powered. In many cases the output is required to be electrically isolated from the input for safety or other reasons related to measurement and these types incorporate a transformer, commonly referred to as the power transformer, to provide both the neccessary voltage and current conversion and the isolation. In some cases a plurality of outputs is required where each is isolated from the other and the invention is easily extended to this application.

High isolation and very low leakage current is often required in power supplies used to operate sensitive measuring equipment or safety critical equipment such as that used in patient monitoring.

Most transformers and isolated power supplies exhibit a leakage current that flows through the capacitance between primary and secondary sides and is driven by the AC voltages present at the primary and secondary windings. For well constructed small ( < 10 Watts) line frequency transformers this is typically in the region of .1 to 100 microamperes. For switched mode applications where higher frequencies are used this leakage current is often several milliamperes at the switching frequency and its harmonics. Such high levels of leakage current can interfere with sensitive measurements and in some cases can even constitute a safety hazard.

It is possible to minimise the leakage current by careful construction of the transformer, in particular by utilising electrostatic screens around the windings. However, with conventional transformer construction it is difficult and expensive to achieve near perfect (fully enclosing) screens.

Furthermore, because of space limitations and efficiency considerations the screens tend to be separated by a thin layer of insulation which results in quite high capacitance between primary and secondary.

This is often a disadvantage where the transformer is used to supply power to measurement circuits because the capacitance between each screen also provides a low impedance leakage current path for any voltages induced onto the screens from external connections.

One way often used to minimise some of these problems is to use two transformers in series as shown in the schematic of Fig. 2.

This allows the secondary side transformer to be driven with a lower voltage than otherwise if the primary side is of the stepdown type. However, with normal construction, there is fairly high capacitance between the coupling winding and the secondary and there will be voltage present on the coupling winding, depending on the turns ratio. This induced voltage is also electrostatically coupled to the windings and screens and therefore itself causes leakage currents to flow. By reducing to a single turn a minimum non zero voltage will be present in the coupling winding but consequently high currents will flow, perhaps resulting in excessive stray magnetic fields.

It is a prime purpose of the invention to provide a practical, low cost means for constructing power transformers and hence power supplies that exhibit very low levels of leakage current induced by the winding voltages. Using this method of transformer construction reduction of leakage currents to a few picoamperes is possible in line frequency transformers (50Hz60Hz) and to a few nanoamperes in transformers used in switching supplies operating at 3kHz or above. The method by which this is done is to use independantly wound magnetic cores, each typically enclosed in an electrostatic screen, and with a coupling winding wound around both cores such that a minimum voltage is induced in the coupling winding, virtually regardless of the number of turns.The arrangement allows almost ideal electrostatic screens to enclose each winding and the windings to be physically separated by a far greater distance than if they were both on one similarly sized core.

The invention resembles the schematic of figure 2 but differs in that the coupling winding is a single or multiplicity of conductive turns or paths that are closed around two or more separate magnetic cores as symbolically indicated in the schematic of Fig. 3. Since this winding encloses both magnetic cores and is a short circuit, that is its start is electrically connected to its finish, it tends to force the magnetic field in the secondary core to counterbalance that in the primary core such that the current in the coupling winding is minimised.

Thus if a primary winding is around one core and covered with an enclosing electrostatic screen and driven by a voltage that causes an alternating magnetic field to be present in that core then the coupling winding will cause an opposing field in the secondary core which will induce a voltage in the secondary winding. The coupling winding is closed and therefore has no net voltage induced regardless of the number of turns, which means that minimal currents flow into the primary or secondary screens or windings due to voltages induced in the coupling winding electrostatically coupling through the capacitance between coupling winding and either of the screens.

Since the action of the coupling winding is to maintain a net null magnetic field within itself it does so regardless of the number of turns. The transformer action is thus between primary and secondary in the manner normal for conventional transformers. Therefore the turns ratio is simply the ratio of the number of turns of the primary to the number of turns of the secondary regardless of the number in the coupling winding.

Descristion of' Drawings.

Figure 1 is a cross sectional drawing of a power transformer in accordance with one embodiment of the invention that utilises two toroidal cores each with its own winding and screen and the two coupled by an additional winding around both cores.

Fig. 2 is a schematic representation of two conventional power transformers connected in series.

Fig. 3 is a schematic representation of an arrangement in accordance with the invention.

Fig. 4 is a schematic representation of three conventional power transformers connected in series, and Fig 5 is a schematic representation of a further arrangement in accordance with the invention.

Referring to Fig.1 of the drawings, there is shown a cross section depicting the construction of a power transformer according to the invention. It utilises two independent toroidal cores 7,8 of the type commonly used in transformer manufacturing. Each core is surrounded by one or more windings 1,4 in the manner well known to those versed in the art of transformer or inductor manufacture. Henceforth we will refer to core 7 as the primary core carrying the primary windings 1 and core 8 as the secondary core carrying the secondary windings 4. The object of the construction is to provide near normal transformer behaviour between primary connections 9 and secondary connections 12 with minimal electrostatic coupling of the primary and secondary voltages to each other.

Insulation 3 is placed on top of the windings 1,4 and a conductive screen 2,5 placed on top of the insulation 3. This screen can be of very low resistivity material such as Copper but will then need to be broken around its circumference and ideally overlapped with intervening insulation such that it does not constitute a shorted turn. Alternatively a material such as conductive paint can be used such that it has sufficiently high resistance to be a negligible load on the transformer when it encloses the core to give a shorted turn around the single core, but sufficiently low to be an effective electrostatic screen. A connection 10,11 is made to each screen and can be brought out for connection to an external circuit as shown or can be connected internally to a point on a winding. Normally another layer of insulation 3 is placed over the screens 2,5 to protect them.

The two wound, screened cores are then stacked, as shown in Fig.

1, with insulation 3 between them. This can be fairly thick to minimise the capacitance between the screens of the two cores.

A coupling winding 6 is then wound around the two stacked cores, as if they were a single core as shown in Fig. 1, and the end of the coupling winding is connected to its start regardless of the number of turns. The current in the coupling winding is inversely proportional to the number of turns and so its resistivity should be chosen to minimise losses caused by that current dissipating power in the resistance of the coupling winding.

In order to minimise the amount of insulation required on top of each screen the coupling winding may be of the type having an insulating coating or sleeve, preferably of a very high voltage breakdown and low dielectric constant material such as Polytetraflourethylene (PTFE or "Teflon") or Polythene.

It will be appreciated that many other versions of the basic construction are possible including side by side placing of the cores and other (non-toroidal) shapes of core. It is also possible to omit one or both of the screens in some types of construction whilst retaining most of the isolation benefits. In some cases it may be beneficial to utilise a plurality of coupling windings.

It may also be useful to use an intermediate core such that the primary core is coupled to the intermediate core with one coupling winding and the secondary core to the intermediate core with another coupling winding. It then becomes useful to make connections to the coupling windings such that they may be used for electrostatic screening in place of or in addition to the aforementioned electrostatic screens. Figure 5 shows the schematic of such an arrangement utilising the invention whereas Figure 4 shows an inferior arrangement which uses an additional transformer in known manner. The latter arrangement has a higher voltage induced in the intermediate windings for a similar number of turns which then leads to additional electrostatically coupled current flow between primary and secondary.

Claims (9)

Claims

1. A power transformer intended for power coupling wherein at least two independent magnetic paths provided by relatively high permeability magnetic cores each surrounded by windings and screens are coupled by an electrically conductive closed path of a single or multiplicity of turns of a coupling winding such that one or more of the turns encloses all of the relatively high magnetic permeability cores.

2. A power transformer according to claim 1 wherein the magnetic paths are provided by cores of the type commonly known as an "E" core.

3. A power transformer according to claim 1 wherein the magnetic paths are provided by cores of the type commonly known as a "C" core.

4. A power transformer according to claim 1 wherein the magnetic paths are provided by magnetic cores of which the shapes are the same or different.

5. A power transformer according to any one of claims 1 to 4 wherein more than 2 cores are enclosed by the coupling winding to provide a multiplicity of secondary, independently isolated, windings.

6. A power transformer according to any one of claims 1 to 5 wherein an additional core is used together with two coupling windings such that a primary winding and core is coupled with one coupling winding or circuit to the additional core and the additional core is further coupled to one or more secondary wound cores with an additional, but not necessarily electrically isolated, coupling winding.

7. A power transformer according to any of the preceding claims where different winding and screening arrangements are chosen.

8. A power transformer according to any of the preceding claims wherein the term coupling winding comprises a plurality of separate windings, each of at least one turn and with each start electrically connected to each finish.

9. A power transformer according to any of the preceding claims wherein a plurality of coupling windings each surround a primary core and one of a plurality of secondary cores to give a plurality of independent secondaries driven from one primary.