WO2016046826A1

WO2016046826A1 - Resonant transformers and their applications

Info

Publication number: WO2016046826A1
Application number: PCT/IL2015/050967
Authority: WO
Inventors: Alexander JARONKIN; Zeev Shpiro
Original assignee: Advanced Magnetic Solutions, Limited
Priority date: 2014-09-23
Filing date: 2015-09-22
Publication date: 2016-03-31

Abstract

An isolating transformer is provided comprising a primary- winding having multiple turns, two secondary windings each consists of at least one turn both having the same number of turns, and a core with an air gap on which the primary and secondary windings are wound. The two secondary windings are disposed symmetrically with respect to the air gap and the winding plane of the two secondary windings and of the air gap have a substantially common axis of symmetry. Also a DC to DC and AC to DC converters are provided that comprise the above transformer.

Description

RESONANT TRANSFORMERS AND THEIR APPLICATIONS

PARENT CASE DATA

This application is based on U.S. Provisional Application Ser. No. 62/053,841 filed on September 23 2014, which is incorporated herein by reference in its entirety, and claims the benefit thereof for priority purposes.

FIELD OF THE INVENTION

The present invention relates generally to resonant transformers and their applications to power supplies, and more particularly, to resonant based switching power supplies.

BACKGROUND OF THE INVENTION

As part of the world becoming more aware to energy waste, and the growing need in the market for smaller and lighter consumer products, there is a continuing effort to reduce dimensions and weight of Power Adapters, also known as Power Supplies (PS) while improving their energy conversion efficiency. For example, the current market specifications for AC to DC switching power supplies include target efficiency of about 90% over a wide range of AC input voltages (90V to 264V) and loading conditions. The demand for smaller size PS intensifies the requirement for high efficiency due to the necessity to dissipate the heat resulting from the PS losses through the small PS package surface area.

It is known in the art that resonant (e.g. LLC) based PS (e.g. DC to DC Converter, AC to DC Converter, etc.) can achieve high efficiency. It is also known in the art that the Transformer is a key element of the Power Adapter circuit, affecting its dimensions weight and efficiency. An innovative transformer structure is thus needed for enabling small, light and efficient resonant based Power Adapters.

In an LLC AC to DC Converter for up to 70W, there is no need for a Power Factor Correction (PFC) to precede the LLC stage. In this case the LLC stage shall support a wide DC input voltage dynamic range. This implies the implementa ion of a high quality factor (Q) resonant circuit comprising the primary winding of an isolating power transformer and a coupling capacitor. This high Q is normally associated with high copper and magnetic losses at the transformer. An innovative design of an LLC transformer is thus critical for achieving the above requirements.

Different transformer structures were offered to address the above requirements, e.g. an article by Peng Xu et al, entitled "Single Magnetic Push-Pull Forward Converter Featuring Built-in Input Filter and Coupled- Inductor Current Doubler for 48V VRM", US Patent 7940534 "Resonant Transformer systems and methods of use", and more. However, the overall structure of the demonstrated transformers cannot provide the desired efficiency and performance that are necessary for implementing small light and efficient PS .

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention to provide improved techniques for implementing efficient and small size power transformers, DC to DC converters as well as AC to DC converters including AC to DC converters without PFC stage that support wide input voltage dynamic range.

Thus, in accordance with an embodiment of the present invention, there is provided an isolating transformer comprising a primary winding having multiple turns, two secondary windings each consists of at least one turn both having the same number of turns, and a core with an air gap on which the primary and secondary windings are wound. The two secondary windings are disposed symmetrically with respect to the air gap and the winding plane of the two secondary windings and of the air gap have a substantially common axis of symmetry. In some embodiments, each of the two secondary windings consists of a single turn.

In some embodiments, the substantially common axis of symmetry also relates to the winding plane of the primary winding.

In some embodiments, the air gap is located in the middle of a central leg of the core.

In accordance with an embodiment of the present invention, there is also provided a DC to DC converter comprising a switching stage configured to convert a DC input voltage to a substantially rectangular voltage signal, the above transformer operatively coupled to the switching stage for forwarding the substantially rectangular voltage signal in an isolating manner, and an output rectifying circuit coupled to the two secondary windings for producing a DC output voltage.

In some embodiments of the DC to DC converter the output rectifying circuit is configured to forward power from each of the two secondary windings during different half cycle of the substantially rectangular voltage signal.

In some embodiments, the DC to DC converter further includes at least one capacitive element operatively coupled to the primary winding for causing resonance to affect the current flowing through the primary winding while forwarding the substantially rectangular voltage signal.

In typical embodiments, the DC to DC converter operates as an LLC type DC to DC converter.

In typical embodiments, the DC to DC converter further comprising a control arrangement configured to affect the substantially rectangular voltage signal frequency, based on operatively sensing the DC output voltage, so as to maintain the DC output voltage within a predetermined DC voltage range. In some embodiments the control arrangement is configured to affect the frequency of the substantially rectangular voltage signal continuously. In some embodiments of the DC to DC converter the control arrangement is configured to affect the substantially rectangular voltage signal frequency based on operatively sensing both the DC input voltage and the DC output voltage, so as to maintain the DC output voltage within a predetermined DC voltage range .

In some embodiments of the DC to DC converter, each of the two secondary windings consists of less than 4 turns, the transfer ratio of the transformer is greater than 30, and the control arrangement is configured to determine a desired value of the DC output voltage so as to maintain the DC output voltage within the predetermined DC voltage range while the DC input voltage is variable from a minimum DC input voltage to at least twice the minimum DC input voltage.

In some embodiments of the DC to DC converter the control arrangement is further configured to detect the amount of loading of the DC output voltage and to cause reduction of the overall capacitance of the at least one capacitive element in response to detecting that the loading of the DC output voltage is, smaller than a predetermined value. The substantially rectangular voltage signal frequency may be maintained above or below F0 - the resonance frequency of the primary current resonance .

In accordance with an embodiment of the present invention, there is also provided a AC to DC converter comprising an input rectifying circuit and the above DC to DC converter coupled to the input rectifying circuit.

In some embodiments of the AC to DC converter the input rectifying circuit is adapted to receive an AC voltage that is variable in the range of at least 100-240Vac.

These and other features and benefits of the invention disclosed herein will be more fully understood upon consideration of the following description and the accompanying drawings . BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention with regard to the embodiments thereof, reference is made to the accompanying drawings, in which like numerals designate corresponding elements or sections throughout, and in which:

Fig. 1 is a block diagram that schematically illustrates a DC to DC and an AC to DC converter, in accordance with an embodiment of the present invention; and

Figs. 2A and 2B are mechanical diagrams that schematically illustrate transformer configurations, in accordance with embodiments of the present invention,-

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention that are described below relate to efficient resonant transformers, efficient DC to DC converters, and efficient AC to DC converters that include the above DC to DC converters .

A comprehensive example of a highly efficient AC to DC converter that supports wide AC input voltage dynamic range is described in detail . The AC to DC converter includes an LLC type DC to DC converter which includes an efficient high transfer ratio isolating transformer.

Referring to Fig. 1, there is provided a block diagram 100 that schematically illustrates an AC to DC converter, in accordance with an embodiment of the present invention. Typical flow of electrical power through converter 100 starts with an AC line voltage applied to an input line filter 104. The AC voltage may vary in the range of at least 100V to 240V RMS, while, in an embodiment, a safety margin of +/-10% is supported. Input filter 104 comprises inductive and capacitive elements for suppressing interference from the line as well as switching noise produced by converter 100. There are multiple known ways to implement such filter. Next, rectifying circuit 106 rectifies the AC voltage by means of a diode bridge 112. A storage capacitor 114 then smooths the rectified voltage thus producing a DC voltage associated with some ripple at frequency of twice the line frequency. The resulting DC input voltage, denoted Vin, is provided to a DC to DC converter 108 for powering an external load 110 with a regulated DC output voltage denoted Vout.

Within DC to DC converter 108, a switching stage 116 converts Vin to a rectangular voltage signal of frequency F by means of fast power switches 118 and 120. In an embodiment, MOSFET switches are used, however, in other embodiments other switch types may be used. A coupling capacitor 122 transfers the rectangular voltage signal to an isolating transformer 124. The transformer comprises a primary winding 126 of about 60 turns, and two secondary windings 128 and 130 of a single turn each. Consequently, the transfer ratio Vs/Vp is about 1/60. In other embodiments, other turn numbers and ratios may be implemented and additional secondary windings and associated circuits may be employed. All windings are wound on a core 132. Winding 126 and capacitor 122 constitute a resonant circuit with a resonant frequency F0 of about 50 Hz. In other embodiments, other techniques and schemes and resonance frequencies are employed to construct the resonance circuit.

An output rectifying circuit 134 that follows transformer 124 rectifies the transformer output for supplying DC power to external load 110, once connected to converter 100. In rectifying circuit 134, switch 136 rectifies winding 128 output to charge output capacitor 138, whereas switch 140 rectifies winding 130 output to charge output capacitor 142. In other embodiments switches 136 and 140 may be implemented by different components such as diodes and transistors. Switches 136 and 140 are connected to inverse polarities of windings 128 and 130 respectively so that each of the two secondary windings charges its corresponding output capacitor during different half cycle of the rectangular voltage signal. The voltages on capacitors 138 and 142 then sum up, by virtue of their serial interconnection, to produce Vout of about 10V. Control circuit 144 serves for controlling all the aforementioned switches. It comprises a primary part 148 for controlling switches 118 and 120, and a secondary part 146 for controlling switches 136 and 140. Both parts are interconnected by an isolating optical coupling unit 150. In the following description no distinction is made between the two parts of control circuit 144. In an embodiment, control circuit 144 comprises a general purpose micro-processor for control and management operations as well as an ASIC for fast control of the switches as well as for driving thereof.

Control circuit 144 determines Vout while attempting to maintain it within a predetermined permitted range by continuously determining an appropriate switching rate of switching stage 116, thereby the frequency F of the rectangular voltage signal at its output, which is normally higher than F0. For this sake, control circuit 144 continuously senses Vout as illustrated in Fig. 1. Upon sensing a small reduction of Vout, due to a positive change in the load current and/or a negative change in Vin, it slightly decreases F to bring it closer to F0, thus increasing the gain of the resonant circuit. Correspondingly, an increase in Vout would entail increased F, hence decreased gain of the resonant circuit. When F is close to F0, the resonant gain is high, which allows for low AC input voltage. When F is quite higher than F0, the resonant gain is low, which allows for high AC input voltage. A wide input voltage range thus necessitates high quality factor (Q) of the resonant circuit, which is achieved by high magnetizing current through primary winding 126. In an embodiment, the transformer loss, which is normally implied by high magnetizing current, is minimized due to the specific structure of transformer 124, as described below.

In some embodiments, control circuit 144 senses also Vin so as to improve the dynamics of the above described control loop. In some embodiments, circuit 152 is included in converter 108, comprising a capacitor 156 which is switchable by a switch 154. When control circuit 144 detects, typically based on Vin and F, that the load current is quite low, e.g. below half of its rated value, it sets switc 154 off, thereby decreasing the overall capacitance in series with winding 126. This increases the resonant impedance (Zo) of the resonant circuit, thereby decreases the magnetizing current and thus the loss in the converter. As FO increases as well, in some embodiments F is not changed meaningfully respectively and becomes lower than FO .

Fig. 2A is a mechanical diagram, presented in a crosssectional view, that schematically illustrates transformer 116 configuration, in accordance with an embodiment of the present invention. The transformer includes magnetic core 132 having an air gap 204 in the middle of a central leg 210 of the core. In an embodiment, core 132 is an EQ structured soft ferrite made of MnZn. In other embodiments other core structures and materials can be employed. Primary winding 126 is disposed in a single section bobbin 208. On both sides of the bobbin there are secondary windings 128 and 130, each consisting of a single turn. In an embodiment, the secondary windings are of Helical Coil inding type. In other embodiments other winding types can be implemented such as Litz wires, flattened wires, winding wires and any suitable combination thereof. A dashed line 212 depicts the axis of symmetry of air gape 204, i.e. it is perpendicular to the air gap plane and crosses its center.

There are two symmetry related properties of transformer 124 that are manifested in Fig. 2A:

(a) Secondary windings 128 and 130 are identical, and disposed symmetrically with respect to air gap 204.

(b) The winding planes of secondary windings 128 and 130 have the same axis of symmetry as air gap 204, i.e. line 212.

In some embodiments also the winding plane of primary winding 126 has the same axis of symmetry as air gap 204.

Fig. 2B shows a mechanical diagram, presented in a crosssectional view, that schematically illustrates transformer 116 configuration, in accordance with an alternative embodiment of the present invention. Fig. 2B differs from Fig. 2A in that secondary windings 128a and 130a are wound over primary winding 126 and are separated therefrom by an isolation tape 216.

The above description has focused on the specific elements of AC to DC converter 100 that are essential for understanding certain features of the disclosed techniques. Conventional elements of the converter that are not needed for this understanding have been omitted from Figs . 1 , 2A and 2B for the sake of simplicity but will be apparent to persons of ordinary skill in the art. The configurations shown in the figures are example configurations, which are chosen purely for the sake of conceptual clarity. In alternative embodiments, any other suitable configurations can also be used.

It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims

1 . A transformer comprising:

a primary winding having multiple turns; two secondary windings, each consists of at least one turn, both having the same number of turns; and a core on which the primary and secondary windings are wound, the core having an air gap,

wherein the two secondary windings are disposed symmetrically with respect to the air gap and the winding plane of the two secondary windings and of the air gap have a substantially common axis of symmetry.

2 . The transformer of claim 1, wherein each of the two secondary windings consists of a single turn.

3 . The transformer of claim 1, wherein said substantially common axis of symmetry also relates to the winding plane of the primary winding.

4. The transformer of claim 1, wherein the air gap is located in the middle of a leg of the core.

5 . The transformer of claim Error! Reference source not found., wherein said leg is a central leg of the core.

6. A DC to DC converter comprising:

a switching stage configured to convert a DC input voltage to a substantially rectangular voltage signal; a transformer as recited in claim 1, operatively coupled to the switching stage for forwarding the substantially rectangular voltage signal in an isolating manner; and

an output rectifying circuit coupled to the two secondary windings for producing a DC output voltage.

7 . The DC to DC converter of claim Error ί Reference source not found. , wherein the output rectifying circuit is configured to forward power from each of the two secondary windings during different half cycle of the substantially rectangular voltage signal.

8. The DC to DC converter of claim Error ! Reference source not found. , further including at least one capacitive element operatively coupled to the primary winding for causing resonance to affect the current flowing through the primary winding while forwarding the substantially rectangular voltage signal.

9. The DC to DC converter of claim Error ! Reference source not found. , wherein the at least one capacitive element is such operatively coupled to the primary winding so as to cause the DC to DC converter to operate as an LLC type DC to DC converter.

10. The DC to DC converter of claim Error ! Reference source not found. , further comprising a control arrangement configured to affect the substantially rectangular voltage signal frequency, based on operatively sensing the DC output voltage, so as to maintain the DC output voltage within a predetermined DC voltage range.

11. A DC to DC converter comprising:

a switching stage configured to convert a DC input voltage to a substantially rectangular voltage signal;

a transformer, having a primary winding and a secondary winding, operatively coupled to the switching stage for forwarding the substantially rectangular voltage signal in an isolating manner;

at least one capacitive element operatively coupled to the primary winding for causing resonance to affect the current flowing through the primary winding while forwarding the substantially rectangular voltage signal; an output rectifying circuit coupled to the two secondary windings for producing a DC output voltage; and

a control arrangement configured to affect the substantially rectangular voltage signal frequency, based on operatively sensing the DC output voltage, so as to maintain the DC output voltage within a predetermined DC voltage range ,

wherein the control arrangement is configured to affect the frequency of the substantially rectangular voltage signal continuously.

. A DC to DC converter comprising: a switching stage configured to convert a DC input voltage to a substantially rectangular voltage signal;

a transformer, having a primary winding and a secondary winding, operatively coupled to the switching stage for forwarding the substantially rectangular voltage signal in an isolating manner

a control arrangement configured to affect the substantially rectangular voltage signal frequency, based on operatively sensing both the DC input voltage and the DC output voltage, so as to maintain the DC output voltage within a predetermined DC voltage range.

. The DC to DC converter of claim 11, wherein each of the two secondary windings consists of less than 4 turns, the transfer ratio of the transformer is greater than 30, and the control arrangement is configured to determine a desired value of the DC output voltage so as to maintain the DC output voltage within the predetermined DC voltage range while the DC input voltage is variable from a minimum DC input voltage to at least twice the minimum DC input voltage.

The DC to DC converter of claim Error I Reference source not found. , wherein the control arrangement is further configured to detect the amount of loading of the DC output voltage and to cause reduction of the overall capacitance of the at least one capacitive element in response to detecting that the loading of the DC output voltage is smaller than a predetermined value .

The DC to DC converter of claim Error! Reference source not found. , wherein, with FO being the resonance frequency of the primary current resonance, the control arrangement is configured to affect the substantially rectangular voltage signal frequency while maintaining the substantially rectangular voltage signal frequency above FO when the loading of the DC output voltage is greater than the predetermined loading value and below FO when the loading of the DC output voltage is smaller than the predetermined loading value.

An AC to DC converter comprising:

an input rectifying circuit; and a DC to DC converter, as recited in claim Error! Reference source not found. , coupled to the input rectifying circuit .

. An AC to DC converter comprising:

an input rectifying circuit; and a DC to DC converter, as recited in claim Error! Reference source not found. , coupled to the input rectifying circuit, wherein the input rectifying circuit is adapted to receive an AC voltage that is variable in the range of at least 100-240Vac.

An AC to DC converter comprising:

an input rectifying circuit; and

a DC to DC converter, as recited in claim Error ! Reference source not found. , coupled to the input rectifying circuit, wherein the input rectifying circuit is adapted to receive an AC voltage that is variable in the range of at least 100-240Vac.