The conditions under which this invention was made are such as to entitle the Government of the United States under paragraph I(a) of Executive Order 10096, as represented by the Secretary of the Air Force, to the entire right, title and interest therein, including foreign rights.
This invention relates to low-voltage, high-current inverter transformers, and more particularly relates to an improved winding configuration for such transformers.
High frequency switching circuits have been used to advantage in power conditioning apparatus for many decades. A major advantage comes from the fact that high-frequency transformers are much smaller, lighter, cheaper and more efficient than low-frequency units of the same power rating. In the particular range of applications involving rectified outputs at low voltage and high current, the efficiency of the rectifiers and rectifier circuit are also important considerations. The two commonest rectifier circuits are the full-wave bridge rectifier (FWBR) and the full wave circuit center-tapped rectifier (FWCTR), shown in
The function of both circuits shown is to rectify the AC (alternating current) provided from the transformer, by means of the rectifiers or diodes, to DC (direct current) which is then delivered to the load. In the case of the bridge rectifier, the AC current from the transformer passes through two diodes in series in order to flow to the load. In the case of the center-tapped circuit the AC current from the transformer passes through only one diode to flow to the load. In either case, the peak and average current is the same. Given that the forward drop of each diode is the same, the power loss in the diodes as a whole in the bridge circuit is twice that of the center-tapped circuit. Typically, the forward drop of a diode is on the order of 1 to 2 volts. If the output voltage of the rectifier is low, this has a significant impact on the efficiency. The impact is much more for the bridge rectifier than for the center-tapped circuit. For example, if the output voltage is 5 volts and the diode drop is 1 volt, the rectifier efficiency for the bridge is 5/(5+2)=71%; and for the center-tapped circuit is 5/(5+1)=83%.
There are other differences between the two circuits. The peak inverse voltage rating of the diodes in the full wave center-tapped circuit must be about twice that of the diodes in the bridge circuit. However, there are only two diodes in the FWCTR opposed to four in the FWBR. The transformer winding for the FWBR is half the voltage rating of the center-tapped winding and has a better utilization factor. However, considering all factors, the center-tapped circuit is the better choice for low-voltage high-current applications. Another consideration is the fact that rectifiers or diodes for high frequency applications are limited in the available maximum current rating to a few tens of amperes. This requires the use of parallel rectifier circuits in high current applications. The parallel circuits must match closely in electrical characteristics to insure that current will share equally, hence the transformer secondary windings of each parallel circuit must be matched. The best way to accomplish this is to wind identical secondary sections (Sy) side-by-side over the primary (Py) as shown in
From the coil diagram in
The present invention is an improved secondary winding technique using simplified coils with multiple secondaries for inverter transformers used in low-voltage high-current rectifier applications. High-frequency fast recovery rectifiers have limited maximum current ratings requiring parallel circuits for high-current high-frequency applications. To insure good current sharing, it is advantageous to use separate transformer windings for each rectifier set and each section must be as closely matched as possible. The coil design of this invention is applicable to both full-wave bridge rectifiers and full wave center-tapped rectifiers. It uses foil conductors and provides multiple parallel secondary windings for the purpose of controlling skin effect losses, improving the winding space factor, and improving heat transfer. In addition, the configuration provides matched and balanced coupling and leakage inductance for each secondary section. Coil designs are disclosed that are both simple and economical to manufacture.
a shows a typical full wave bridge rectifier circuit diagram.
b shows a typical full wave center-tapped rectifier circuit diagram.
a is a FWCTR circuit.
b is the electrical equivalent circuit shown in
High-frequency high-current windings require consideration of the skin effect. This is the well-known behavior of current distribution in a conductor at high frequency. With increasing frequency the depth of current penetration into a conductor diminishes, this depth is know as the skin depth and is given approximately, for copper, by:
Where:
δ is skin depth in cm
f=frequency in Hertz
The most detrimental effect is the increase in apparent resistance and corresponding loss. This is conventionally dealt with by the use of Litz wire or thin foil conductors. These approaches work because the dimensions of the Litz wire individual conductors or the thickness of the foil are selected to be less then or comparable to current skin depth. Thus the current distribution within the conductor is approximately uniform and the apparent resistivity is not significantly increased above the DC value. Litz wire has a much lower space factor than foil because it consists of woven bundles of fine wire. High power inverters usually operate at a frequency less than 100 kHz. The skin depth at 100 kHz is about 0.2 mm or 0.008 inches. This is a reasonable foil thickness and when used with 0.002 inch layer insulation and results in a space factor of about 80%. The comparable space factor for Litz wire is typically less than about 40%. The use of Litz wire will result in a larger winding size because of the lower space factor and will also have a lower thermal conduction that complicates the heat transfer and maximum operating temperature limitation. The use of foil is advantageous for these reasons and is also instrumental in the implementation of the present invention, as will become apparent in the subsequent discussion.
There are two implementations of the invention, the simpler for non-center-tapped windings and a more involved for center-tapped windings. The non-center-tapped embodiment will be considered first. The non-center-tapped winding is compatible with the FWBR. The FWBR connects between the two leads of each winding section. It is permissible to have one side of each winding common to all other corresponding sides of the other windings as shown schematically by 1 in
Connecting one end of each winding, as shown in
The manner of implementing this pre-construction arrangement is illustrated in
Next consider the implementation of the present invention with respect to center-tapped windings. The circuit schematic of a typical four secondary center-tapped configuration is shown in
The phasing or polarity of this pair of windings must be properly maintained when connected to the diodes and circuit. The direction of rotation of the winding of the section determines the polarity. That is, if both sections of the side-by-side windings as shown in
Another possibility is to reverse wind the second half of the secondary 41 in
Both of these two options will accomplish the proper circuit function; however, the winding process is complicated by the inter connections and/or the need to reverse wind. The result is that each section must be individually wound and interconnected in the process, making the winding task laborious, inconvenient and, expensive.
It is an objective of the present invention to disclose a procedure that both accomplishes the electrical requirements and simplifies the winding procedure such that only a single winding operation is required given proper preparation of the materials. To this end consider the diagrams in
The right circuit (
The pre-arrangement of the conductors and insulation for the center-tapped winding implementation of the invention is shown in
The tab lead 80 in
The major advantage of the invention is the winding method that provides a simple, economic means for producing a multi-secondary transformer with superior electrical performance and suitable for either FWBR or FWCTR applications. In addition the use of foil windings improves the space factor and reduces the over-all size of the transformer. Further, the foil winding, having a superior space factor, also has a superior thermal conductivity and therefore an improved heat transfer capability. This results in a cooler running unit or equivalently a higher power rating for the same maximum operating temperature.
Two alternative implementations of the invention have been presented: the implementation for single winding sections for FWBR applications and the side-by-side coil pairs for FWCTR applications. These implementations were based on a single coil structure per transformer. Alternatively, two coil structures 90, 91 per transformer can be used, one on each leg of a single loop core 92 as indicated in
The scope of the invention includes all modification, design variations, combinations, and equivalents that would be apparent to persons skilled in the art, and the preceding description of the invention and its preferred embodiments is not to be construed as exclusive of such.
Number | Name | Date | Kind |
---|---|---|---|
3745440 | Lord | Jul 1973 | A |
5579202 | Tolfsen et al. | Nov 1996 | A |
5627482 | Lamatsch | May 1997 | A |
6417592 | Nakamura et al. | Jul 2002 | B2 |