Isolation transformer for use in isolated DC-to-DC switching power supply

An isolated DC-to-DC switching power supply includes an isolation transformer having a magnetic core, a first winding around the magnetic core, a first winding-shield around the magnetic core, a second winding-shield within the first winding-shield, and a second winding within the second winding-shield. There is no direct coupling between the first winding and the second winding since the second winding is enclosed within the second winding-shield and the second winding-shield is enclosed within the first winding-shield.

TECHNICAL FIELD

This disclosure relates to enhancing isolated DC-to-DC switching power supplies by using an isolation transformer with low leakage inductance and low isolation capacitance to reduce the noise in the isolated DC-to-DC switching power supplies.

BACKGROUND

Switching power supplies are notorious for generating electrical noise. Isolated switching power supplies, however, have added electrical noise via displacement-current across an isolation barrier of the isolated switching power supply. Normally, isolation transformers are used to provide the isolation barrier between the input and output of the switching power supply. The design of the isolation transformer, however, can greatly impact the level of electrical noise within the switching power supply.

Most isolated switching DC-to-DC power supplies use isolation transformers that contain electrostatic winding-shielding between primary and secondary windings. The goal of these transformers is to have the voltage swings of the primary winding couple only to a primary winding-shield and the voltage swings of the secondary winding couple only to a secondary winding-shield; however, the transformers currently being used still create a large amount of electrical noise across the isolation barrier. The separation of the primary windings and the secondary windings results in high leakage inductance in the transformer. High leakage inductance often increases the electrical noise of a switching power supply.

A low leakage inductance transformer construction method is to wind a transformer using a bifilar winding technique in which two wires are wound next to each other at the same time. As the wire pair is repeatedly wound around a magnetic core, each turn of the wire pair couples to other turns that then lay upon previous turns. This additional coupling changes the leakage inductance and isolation capacitance. Small changes in the winding process can cause changes to these couplings. Thus the electrostatic coupling is not well controlled causing displacement current across the isolation barrier.

Therefore, there remains a need for improved isolation transformers. In an ideal transformer, the electrostatic coupling between the primary and secondary windings is only between the primary and secondary winding-shields. The two winding-shields voltage swings are the same and thus there is no displacement current between them. The voltage swings of the primary winding couple only to a primary winding-shield and the voltage swings of a secondary winding couple only to a secondary winding-shield. Also in an ideal transformer, the leakage inductance should remain low as in a bifilar wound primary/secondary transformer.

SUMMARY

Certain embodiments of the disclosed technology include an isolation transformer for an isolated switching DC-to-DC power supply, where an electrostatic coupling between the primary and secondary windings occurs only between the primary winding-winding-shield and secondary winding-winding-shields.

Certain embodiments include an isolated DC-to-DC switching power supply including an isolation power transformer that has a magnetic core, a first winding around the magnetic core, a first winding-shield around the magnetic core, a second winding-shield within the first winding-shield, and a second winding within the second winding-shield.

DETAILED DESCRIPTION

In the drawings, which are not necessarily to scale, like or corresponding elements of the disclosed systems and methods are denoted by the same reference numerals.

In order to reduce the noise in an isolated switching DC-to-DC power supply, the disclosed isolation transformers reduce the noise across the isolation barrier of the power supply. In the disclosed transformers, the coupling between the primary winding and the secondary winding is only between the first winding-shield and the second winding-shield. That is, the primary winding and the secondary winding are completely isolated from each other.

FIG. 1is an example of an isolation transformer100with two first wires102covered by insulation layers104wrapped around a magnetic core106, acting as a first winding. The isolation transformer100also includes a double-shielded cable wrapped around the magnetic core106. The first two wires are external to the double-shielded cable. The double-shielded cable includes an outside insulation layer108. The outside insulation layer108encloses an outer electrostatic winding-shield110, a first insulation layer112, an inner electrostatic winding-shield114, two second insulation layers116, and two second wires118. “Electrostatic winding-shield” will also be referred to as a “winding-shield” throughout the specification. The two second wires118are each covered with second insulation layers116. The two second wires118and the second insulation layers116are located inside the inner winding-shield114, which is located inside the outer winding-shield110. Each of the first wires102, second wires118, outer winding-shield110and inner winding-shield114are soldered at soldering points120to a circuit board. In the double-shielded cable, the inner winding-shield114and the outer winding-shield110are composed of copper braided sheaths.

A cross-section of the layers of the double-shielded cable along with the first wires102external to the double-shielded cable are shown inFIG. 2. The first wires102and the second wires118can both be either the primary or secondary windings of the transformer, as will be readily understood by one of ordinary skill in the art. For ease of discussion, the second wires118will be referred together as the primary winding, and the first wires102will be referred together as the secondary winding. In this arrangement, the inner winding-shield114acts as the primary winding-shield and the outer winding-shield110acts as the secondary winding-shield. Since the primary winding is fully encased within the primary winding-shield and the primary winding-shield is fully encased in the secondary winding-shield, there is no direct coupling between the primary winding and the secondary winding. Therefore, the voltage swing of the primary winding couples only to the primary winding-shield and the voltage swing of the secondary winding couples only to the secondary winding-shield. The capacitance between the two winding-shields is not charged or discharged. Therefore, there is no charge flow from the primary winding-shield to the secondary winding-shield.

In this embodiment shown inFIG. 1, the center windings of the transformer of both the first wires102and the second wires118have the same voltage as the outer winding-shield110and the inner winding-shield114. Due to this feature, the outer winding-shield110, or secondary winding-shield, can be used as a center turn of the secondary winding of the transformer, and the inner winding-shield114, or primary winding-shield, can be used as a center turn of the primary winding of the transformer.

Rather than a double-shield cable, this embodiment includes a coaxial cable. If a coaxial cable is used, the first wires102would still be external to the coaxial cable. It may be desirable to use a single first wire102or more wires that the two first wires shows inFIG. 1. The coaxial cable includes a center conductor, an inner insulation layer, an outer conductor, and an outer insulation layer. Within the coaxial cable, the center conductor would function as both the second winding-shield and the second winding. The outer conductor would function as the first winding shield.

FIG. 3shows an embodiment similar to that ofFIG. 1. However, in this embodiment, three first wires102are used in an isolation transformer200, along with three second wires118. Each of the three first wires102and each of the three second wires118has its own insulation layer104and116, respectively. The remainder of this embodiment is the same as the embodiment discussed above with regard toFIG. 1. A cross-section of the double-shielded cable of this embodiment, along with the three external first wires, is shown inFIG. 4. In this embodiment, for ease of discussion, the three second wires118again comprise the primary winding and the three first wires102comprise the secondary winding. Each turn of the primary winding is a second wire118. Therefore, the three turns of the primary winding (the three second wires118) are completely enclosed within the inner winding-shield114, also referred to as the primary winding-shield, and the outer winding-shield110, also referred to as the secondary winding-shield.

The configuration of the transformer inFIG. 3provides that the coupling between the primary winding and the secondary winding occurs only between the primary winding-shield and the secondary winding-shield. Again, there is no direct coupling between the primary and secondary windings and the voltage swing of the primary winding couples only to the primary winding-shield and the voltage swing of the secondary winding couples only to the secondary winding-shield.

Another embodiment is shown inFIG. 5. In this embodiment, an isolation transformer300is built using a single triaxial cable around the magnetic core106. The triaxial cable is wound around the magnetic core106a single time. The isolation transformer300has two isolated secondary windings. The triaxial cable includes a conductor202, a braided sheath204, and another braided sheath206. This configuration can be seen in the cross-section of the triaxial cable inFIG. 6. Further, as shown inFIG. 5, each of these layers includes an insulation layer208and212between them, with a final insulation layer210on the outside. Each conductor of the triaxial cable, including conductor202and braided sheaths204and206can be independent windings of the transformer and winding-shields. Therefore, the isolation transformer300, for example, can have a primary winding (conductor202) and two secondary windings (braided sheaths204and206, referred to herein as first secondary winding and second secondary winding, respectively). No wires are external to the triaxial cable in this embodiment. The two braided sheaths204and206, however, still act as a primary and secondary winding-shield, respectively, as well. As will be readily understood by one of ordinary skill in the art, the conductor202could be the secondary winding and the two braided sheaths204and206could be the two primary windings.

Both the primary winding of conductor202and the secondary winding of braided sheath206are independent due to the braided sheath204. The second secondary winding, braided sheath206, has no direct coupling to the primary winding, conductor202. Again, in this embodiment there will be capacitance between the braided sheath204and conductor206. This capacitance, however, is not charged or discharged.

In an alternative to this embodiment (not shown), external wires may be provided outside the triaxial cable, providing more turns to the primary winding, similar to that shown inFIGS. 1 and 3. These external wires of the primary winding will still be shielded from the conductor202due to the braided sheath206.

In another alternative to this embodiment, a coaxial cable may be used in place of the triaxial cable. In this configuration, the inner conductor of the coaxial cable would act both as a second winding-shield and the second winding. The outer conductor would act as both a first winding-shield and the first winding.

In another embodiment, the triaxial cable shown inFIG. 5can be wound around the magnetic core106two times, as shown inFIG. 7. Therefore, each primary winding and the two secondary windings have two turns in the isolation transformer400. As can be seen inFIG. 7, the insulation layer210is removed around the braided sheath206in three spots. The braided sheath206acts as the primary winding and is soldered to soldering points120at these three spots. Finally, as inFIG. 5, the first secondary winding (braided sheath204) and the second secondary winding (conductor202) are also soldered to the circuit board at soldering points120.

In another embodiment, shown inFIGS. 8 and 9, an isolation transformer is shown using two triaxial cables wound in parallel around a magnetic core. One triaxial cable is placed on the top of a circuit board and the other triaxial cable is placed on the bottom of the circuit board. For ease, the triaxial cables are referred to as the top cable and the bottom cable. The two cables are wound around a magnetic core106. Further, four wires are provided external to the triaxial cable. Two wires302, referred to herein as the top wires, are provided on the top of the circuit board, and two of the wires304, referred to herein as the bottom wires, are provided on the bottom of the circuit board. The circuit diagram for such a configuration is shown inFIG. 8. An isolation transformer500on a top side of the circuit board is shown inFIG. 9. The isolation transformer on the bottom side of the circuit board would contain the same configuration as that shown inFIG. 9.

As can be seen in the circuit diagram ofFIG. 8, the primary winding is composed of four turns including the bottom conductor306, the top inner winding-shield308, the bottom inner winding-shield310, and the top conductor312. The secondary winding includes six turns comprising the second top wire302, the first top wire302, the top outer winding-shield314, the bottom outer winding-shield316, the first bottom wire304, and the second bottom wire304. The bottom conductor306and the top conductor312of the primary winding are each connected to MOSFET switches318. The top inner winding-shield308and the bottom inner winding-shield310are both connected to a primary ground (earth ground)320. In the secondary winding, the top outer winding-shield314and the bottom outer winding-shield316are connected to a secondary floating ground322. Each of these components is shown inFIG. 9. Further, soldering points120and insulation layers324are shown. In this configuration, the inner winding-shields308and310and the outer winding-shields314and316act as both a turn of the windings and winding-shields, as discussed above with respect toFIG. 4.

In an alternative to this embodiment, more than two transformers can be wound in parallel around the magnetic core106. Further, in another alternative to this embodiment, coaxial cables may be used in place of the triaxial cables.

Each of the isolation transformers described above in the various embodiments, wherein the primary winding and the secondary winding have no direct coupling, have provided noise across the isolation barrier magnitudes lower than previously used isolation transformers.

In each of these embodiments, the magnetic core106may be ferrite for example. However, any type of magnetic core known in the art may be used. Further, the braided sheaths of the triaxial cables and the double-shielded cables should be of the highest quality. If the braided sheaths are not of the highest quality, the primary winding and the secondary windings may be able to couple directly through the winding-shields and provide electrical noise. The better the quality of the braided sheaths, the less electrical noise provided through the isolation transformer.

Having described and illustrated the principles of the disclosed technology in a preferred embodiment thereof, it should be apparent that the disclosed technology can be modified in arrangement and detail without departing from such principles. We claim all modifications and variations coming within the spirit and scope of the following claims.