Patent Publication Number: US-2007115700-A1

Title: Transformer with current sensing means

Description:
FIELD OF THE INVENTION  
      The invention relates to a main transformer for a power supply having at least one transformer winding and means for sensing a current in said at least one transformer winding. The invention further relates to a power supply with such a main transformer, a controllable switching device and a control circuit that is coupled to said controllable switching device for controlling the switching device, where said means for sensing a current in said at least one transformer winding is coupled to said control circuit.  
     BACKGROUND  
      In power circuitry, power switches are among the key contributors to overall power loss. It is therefore an aim of power supply designers to minimize the losses arising from non-zero currents and voltages across the switching devices while the switching action is performed. Among the switching devices used are rectifier diodes and synchronous rectifiers.  
      Rectifier diodes have the advantage that they are simple to design into a power supply. They are inserted into the power circuit and the voltage across the power train windings will force the diode to commutate when it is appropriate. Because of their simplicity, diode rectifiers are cheap and easy to incorporate into a given circuit. While offering a simple rectifier design, diode rectifiers have several downsides, e.g. diodes have a fixed forward voltage drop that is independent of the current. This results in high levels of power dissipation, especially at high current levels. When the efficiency of diode rectifiers is calculated for various output voltages, they are found to be efficient at high output voltages, but as the output voltage drops, the efficiency of diode rectifiers drops precipitously.  
      For high current applications, synchronous rectification is preferred over rectification by discrete diodes due to the high conduction losses associated. Synchronous rectifiers are e.g. MOSFETs (metal oxide semiconductor field effect transistor), bipolar transistors or other semiconductor switches driven in such a way as to perform a rectifying function. Synchronous rectifiers, however, have the disadvantage that the switching action of the synchronous rectifiers needs to be actively controlled by an additional circuit. In power circuits, synchronous rectifiers are therefore often complicated to use because of timing issues. Whereas some methods use hard-switching techniques, soft switching or zero voltage/current switching has become widely spread due to the lower switching losses.  
      In order to predict the correct timing by the latter method, current sensing devices are needed to provide the controlling circuit of the switches with accurate information about the current in the circuit. Thereby, it is very important that the current is determined with a high accuracy in order to minimize losses.  
      In the case of transformers with a push-pull output stage, which are typically used in high power applications, two current transformers, each connected to one of the two serially connected, secondary windings of the transformer, are typically needed to properly control the synchronous rectifiers. But two current transformers not only raise the manufacturing costs of such a converter, for example due to the increased number of components, in high current applications there are also significant losses that occur in the wiring to and from the current transformers. Furthermore, the current sensing may be incorrect due to the magnetizing current in the current transformers and, in addition, the two current transformers need a lot of space.  
      Another possibility for current sensing is a sense resistor (also called current shunt). However, such resistors cause additional losses due to their voltage drop. Furthermore, they are too inaccurate in their current sensing. In order to improve the accuracy of such a resistive device, for example in a push-pull output stage, where such a sense resistor is provided in the common path of the two secondary windings, the resistor is combined with a precision OPAMP (operational amplifier). But these devices are rather expensive.  
     BRIEF DESCRIPTION  
      It is an object of the invention to create a main transformer assembly (hereinafter also referred to as transformer) of a power supply as well as a power supply pertaining to the technical field initially mentioned that enables to overcome or reduce the disadvantages of the prior art and particularly to enable the design of low loss, small and cheap transformer arrangements with current sensing device.  
      According to the invention the main transformer for a power supply has at least one transformer winding as well as means for sensing the current in this at least one transformer winding. The invention is wherein the means for sensing the current in the at least one transformer winding include a single current sensing device that is integrated into the main transformer.  
      In addition to such a main transformer, a power supply according to the invention further includes a controllable switching device and a control circuit that is coupled to the controllable switching device for controlling the switching of the switching device. In order to provide the result of the current sensing to the control circuit which may then generate a control signal for controlling the switching device, the means for sensing the current in the transformer winding are coupled to the control circuit.  
      By integrating the current sensing device into the main transformer there is no wiring to and from the current sensing device as in the prior art converters. Accordingly, in high current applications the losses engendered can be virtually eliminated which means that the overall losses can be significantly reduced. While a transformer with a current sensing device could generally be used in low power applications, high power applications are therefore preferred applications of a power supply according to the invention. In this connection, high power means power levels of some dozens of watts and above.  
      The invention further provides the possibility to sense the current with a high accuracy, for example, with an accuracy in the range of 1% to 2% or even lower than 1% over the whole load range.  
      Furthermore, the price for a power supply or a main transformer according to the invention is much lower than a comparable prior art design because a single current sensing device costs significantly less than two current sensing devices such as current transformer or a sense resistor with a precision OPAMP as needed in the prior art. Also the space requirements are reduced since one single current sensing device needs only about half of the space that two current transformers need.  
      The means of the term “integrated” in connection with the current sensing device is that the current transformer is not a separate element that is manufactured independently of the main transformer and connected to the main transformer at a later stage. It means that the current sensing device and the main transformer form a single unit. Preferably they are manufactured at the same time in a common process such that the current sensing device forms an integral, built-in or embedded part of the main transformer.  
      While the current sensing device could be a resistive device in combination with an OPAMP or any other device that enables a current measurement, the current sensing device preferably includes a current transformer. In this case, a primary winding of the current transformer is formed by a section of the at least one transformer winding of the main transformer and not by a junction wire that interconnects the transformer winding with another (preceding or subsequent) circuit of the power supply. In other words, the same portion of a conductor forms a section of the transformer winding as well as the primary winding of the current transformer. The primary current transformer winding and the winding of the main transformer have therefore at least one common (full or fractional) turn.  
      Such a current transformer shows low losses and is easy and therefore inexpensive to manufacture.  
      The current transformer further preferably includes a secondary winding and a magnetic core, particularly a ring-type core, which means that the core forms a closed loop such that the magnetic flux can circulate therein. By choosing an appropriate winding ratio—the number of windings of the secondary winding is, for example, much greater than the winding number of the primary winding—the current transformer can measure the high current flowing in the transformer winding by producing a much lower but proportional current in its secondary winding.  
      While said transformer winding can also be a primary winding of the main transformer, it is in a preferred embodiment of the invention a secondary winding of the main transformer. This is, for example, advantageous in a transformer with two serially connected secondary windings in a push-pull configuration, where the current in both secondary windings has to be measured in order to ensure an adequate current sensing.  
      During current sensing, the magnetizing inductance of the current transformer does not affect the average measured signal, which considerably improves the accuracy of the measurements.  
      In another preferred embodiment of the invention, the main transformer includes not only one but at least two secondary windings. In this case, the secondary windings are connected in series and a section of at least one of the secondary windings forms a primary winding of said current transformer. That is a section of only one secondary winding, a section of several secondary windings or a section of each secondary winding forms a primary winding of the current transformer.  
      While it would be possible that a center section of the at least one secondary transformer winding forms a primary winding of the current transformer, it is preferred that a primary winding of the current transformer is formed by an end portion of a secondary winding, which facilitates the winding process during manufacturing of the transformer.  
      It is also possible to apply the invention in a main transformer assembly that includes two or more transformer outputs. In this case, each transformer output includes at least one secondary transformer winding and each output includes current sensing means for sensing a current in this secondary winding of each transformer output. Again, these current sensing means include a single current sensing device that is integrated into the main transformer assembly.  
      The invention is preferably applied in a transformer with a push-pull output stage with two secondary windings connected in series. Accordingly, a section of the first secondary winding forms a primary winding of the current transformer and a section of the second secondary winding also forms a primary winding of the current transformer. In such a configuration the invention is of particular benefit, since it allows a considerable reduction of costs, losses and size in comparison with a conventional transformer having a push-pull output stage with two separate current transformers each connected to one secondary winding.  
      Nevertheless, the invention can also be used in other standard converter topologies such as, for example, forward, flyback, full-bridge, half-bridge, current or voltage fed and further push-pull converters as well as other converter topologies.  
      In a preferred embodiment of the power supply according to the invention, the current sensing device includes, as mentioned before, a secondary winding. This secondary winding is built for producing a current sense signal that depends on and is representative of the current in the transformer winding that forms the primary winding of the current transformer. The current transformer is further coupled to said control circuit such that the sense signal produced by the secondary current transformer winding is provided to the control circuit.  
      The controllable switching device can principally be of any known type of switch that can be controlled by applying a suitable control signal to a corresponding control input of the switching device. Preferably, the switching device includes a semiconductor switching device such as, for example, a MOSFET or a bipolar transistor where the control input is the gate of the transistor.  
      Although it would be possible that the switching device is a primary switch of a switched mode power supply, the switching device is preferably configured as a synchronous rectifier on a secondary side of the main transformer, which means it is a part of the rectifier of the power supply.  
      In order to control a switching of the controllable switching device, the control circuit preferably includes means for producing a control signal in dependency of the sense signal provided by the current transformer. Accordingly, the control signal is produced by the control circuit in dependency of the current flowing in the transformer winding a part of which forms the primary winding of the current transformer. The control circuit further includes means for providing the generated control signal to a control input of the switching device.  
      The invention is particularly suitable for power supplies with a resonant converter. So in a preferred embodiment of the invention, the power supply includes a resonant circuit. Advantageously, an inductance of the main transformer forms an inductance of this resonant circuit. In a non-resonant power supply with a continuous current flow it is not possible to apply the invention.  
      In an even more preferred embodiment, the resonant circuit is an output circuit of the power supply, particularly a LLC-type output circuit.  
      A winding of the main transformer includes, for example, a wire or a litze wire wound around a core of the main transformer.  
      In principle, it would also be possible to provide the main transformer with more than one integrated current transformer, for example, an additional current transformer on the primary side of the transformer. However, the space and cost requirements would be increased as well thereby nullifying some of the advantages of the invention.  
      It is further to note that the invention is not only suited for generating the input signals of a control circuit for controlling switching devices, it is well suited in every application where current (either the instantaneous current, the integrated current or other current properties) has to be sensed with a high accuracy, for example, in current output limiting devices, for status reporting of converter arrangements or for controlling the current sharing in converter arrangement with multiple converters connected in parallel.  
      The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic circuit diagram of a half-bridge converter with a push-pull output stage and an integrated current transformer according to the invention  
       FIG. 2  is a schematic, perspective view of a transformer with an integrated current transformer according to the invention;  
       FIG. 3  is a schematic, detailed, perspective view of an integrated current transformer according to the invention;  
       FIG. 4  is a schematic circuit diagram of a further embodiment of the invention where the current transformer is integrated at another position; and  
       FIG. 5  is a schematic circuit diagram of a transformer arrangement with two outputs where each output includes an integrated current transformer according to the invention. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows a schematic circuit diagram of a half-bridge converter  1  with a push-pull output stage, an input circuit  10 , a switching circuit  20 , a transformer stage  30 , a rectifier circuit  40  and an output circuit  50 . The input circuit  10  includes a voltage source  11  that is connected to the single primary winding  32  of the main power transformer  31 . The switching circuit  20  includes two controllable semiconductor switches  21 ,  22  and two capacitors  23 ,  24  in a half-bridge configuration. That is, a serial circuit of the two semiconductor switches  21 ,  22  is connected across the voltage source  11  and in parallel to a series circuit of the two capacitors  23 ,  24 . Accordingly, the positive terminal of the voltage source  11  is connected to the first terminal of switch  21  as well as to the first terminal of capacitor  23 . The negative terminal of the voltage source  11  is connected to the second terminal of switch  22  as well as to the second terminal of capacitor  24 . The second terminal of switch  21  is connected to a first terminal of switch  22 , and the second terminal of capacitor  23  is connected to the first terminal of capacitor  24 . Then, this switching circuit  20  is connected to the primary winding  32  of the transformer  31  such that the common terminals of the capacitors  23 ,  24  are connected to the first terminal of the primary winding  32 , and such that the common terminals of the switches  21 ,  22  are connected to the second terminal of the primary winding  32 .  
      The transformer stage  30  includes in its secondary two serially connected secondary windings  33 ,  34  with a center tap  35 . The rectifier circuit  40  includes two synchronous rectifiers  41 ,  42  and the output circuit  50  includes an output capacitor  51 . The first terminal of the first secondary winding  33  is connected to the first terminal of the output capacitor  51  via the first synchronous rectifier  41  and the second terminal of the second secondary winding  34  is also connected to the first terminal of the output capacitor  51  via the second synchronous rectifier  42 . The second terminal of the first secondary winding  33  as well as the first terminal of the second secondary winding  34  are connected to the center tap  35  which is connected to the second terminal of the output capacitor  51 .  
      The converter further includes a current sensing device in its secondary. In the example shown, the current sensing device is a current transformer  60  that is integrated into the main transformer  31 . Accordingly, an end portion  33 . 3  of the first secondary winding  33  of the main transformer  31  forms a primary winding of the current transformer  60 , and an end portion  34 . 3  of the second secondary winding  34  of the main transformer  31  also forms a primary winding of the current transformer  60 . The current transformer  60  further includes a secondary winding  63 , both terminals of which are connected to an input of a control circuit  70 . The control circuit  70  includes several outputs where each output provides a control signal  71 . 1 ,  71 . 2  to the control inputs of the synchronous rectifiers  41 ,  42  respectively.  
      With such a control circuit  70  or a similar control circuit, the converter  1  may, for example, be controlled in a resonant mode where zero current and/or zero voltage switching of the synchronous rectifiers  41 ,  42  can be achieved.  
      In the prior art, a power converter with a push-pull output stage included two current transformer, one in the connection between the first secondary winding and the first synchronous rectifier and one in the connection between the second secondary winding and the second synchronous rectifier. Accordingly, the invention cuts the number of necessary current transformers in half which results in lower costs, lower space requirements and lower losses. The losses can be lowered twice. First, there are no connecting lines necessary between the main transformer  31  and the current transformer  60 , because the current transformer  60  is integrated into the main transformer  31  and therefore the losses in the wiring from and to the current transformer (this wiring makes up impedances that falsify the current sensings) is eliminated. Second, there is no extra leakage inductance introduced between the secondary windings as in the prior art. Furthermore, the current sensing accuracy is improved, since errors due to the magnetizing currents in the current transformers are eliminated or at least reduced. Measurements have shown that the errors in the current sensing are below one percent which means that the current can be measured with a high accuracy.  
      The overall result is a considerable increase of the converter&#39;s efficiency.  
      It is to note that the converter  1  may include further components and circuits as known in the art. However, for a better clarity, these components and circuits have not been shown in the drawings.  
      It would, for example, be possible to feed the converter  1  with an AC current or voltage and to provide a rectifier for rectification of such an AC input. A current transformer according to the invention could also be employed in other converter topologies as mentioned before. In other embodiments of the invention, the rectifier circuit could also include a single synchronous rectifier, a full bridge rectifier or other known rectifier circuits with synchronous rectifiers.  
      Furthermore, it is self-evident that the control circuit  70  can not only control a switching of the synchronous rectifiers  41 ,  42  in the secondary, but also the switching of the switches  21 ,  22  or other controllable switching devices as desired.  
       FIG. 2  shows a schematic, perspective view of a transformer  131  according to the invention. The transformer  131  includes a core that is made up of two E-type core halves  137 . 1 ,  137 . 2  that are, for example, clamped together by clamps  139 . A primary winding  132  is wound around the middle leg  138 . A first secondary winding  133  and a second secondary winding  134  are also wound around the middle leg  138 , for example, on top of the primary winding  132 . The first secondary winding  133  includes a first end portion  133 . 1  which is connected to a terminal (not shown), a center portion  133 . 2  which is wound directly around the middle leg  138  and a second end portion  133 . 3  which is fed through a ring-type core  164  and is then connected to a center tap  135 . Accordingly the second secondary winding  134  includes a first end portion  134 . 1  which is connected to a terminal (not shown), a center portion  134 . 2  which is wound directly around the middle leg  138  and a second end portion  134 . 3  which is fed through the ring-type core  164  and is then connected to the center tap  135  too. Around the ring-type core  164  is wound a further winding, the secondary winding  163 . The ring-type core  164 , the primary windings  133 . 3  and  134 . 3  and the secondary winding  163  form the current transformer  160 .  
      Hence, the end portions  133 . 3  and  134 . 3  of the first and second secondary windings  133 ,  134  form a turn of the secondary windings  133 ,  134  of the main transformer  131  as well as at the same time a turn of the primary windings of the current transformer  160 . This means that the current transformer  160  is an integrated part of the main transformer  131 .  
      A simple and therefore preferred way of providing the secondary windings (not shown in the drawings) is by winding a bifilar wire, for example, a litz wire, several times around the middle leg and finally once through the ring-type core  164  and then connecting the beginning of one of the conductors of the bifilar wire and the end of the other conductor of the bifilar wire to the center tap.  
       FIG. 3  shows a schematic, perspective view of another current transformer  260  integrated into one of the legs  238  of a main transformer  231 . Again, the first secondary winding  233  includes a first end portion  233 . 1 , a center portion  233 . 2  which is wound around the leg  238  and a second end portion  233 . 3  which is fed through the ring-type core  264  of the integrated current transformer  260  and forms a further turn of the secondary winding  233  as well as a turn of the primary winding of the current transformer  260 . In the same manner, the second secondary winding  234  includes a first end portion  234 . 1 , a center portion  234 . 2  which is wound around the leg  238  and a second end portion  234 . 3  which is fed through the ring-type core  264  and forms a further turn of the secondary winding  234  as well as a turn of the primary winding of the current transformer  260 .  
      Again, the end portions  233 . 1 ,  234 . 1  are connected together to form the center tap (not shown). One of the differences to the transformer shown in  FIG. 2  is that the primary winding of this transformer  231  is not wound around the same, but around another leg (not shown) of the transformer core.  
      While in both examples of  FIGS. 2 and 3  only one turn of the secondary windings  133 ,  134 ,  233 ,  234  is shown to be fed through the ring-type core  164 ,  264 , it is self evident that it is possible to feed also two or more turns of the secondary windings  133 ,  134 ,  233 ,  234  through the ring-type core  164 ,  264 . However, since the current transformers  160 ,  260  are used for current measurements, it is desired that the current in the secondary winding  163 ,  263  is not too high. Therefore, the ratio of the number of turns of the primary windings  133 ,  134 ,  233 ,  234  to the number of turns of the secondary windings  163 ,  263  should be rather small, which means that the number of turns of the primary windings  133 ,  134 ,  233 ,  234  is chosen to be low (such as for example one turn as shown in the drawings) or that the number of turns of the secondary winding  163 ,  263  is chosen to be high.  
      It would further be possible to feed the end portions  133 . 1 ,  134 . 1  or  233 . 1 ,  234 . 1  through the ring-type core  164 ,  264  instead of the end portions  133 . 3 ,  134 . 3  or  233 . 3 ,  234 . 3  as shown in  FIGS. 2 and 3 . Another possibility to integrate the current transformer  160 ,  260  into the main transformer  131 ,  231  would be to feed the end portions  133 . 1 ,  233 . 1  and  134 . 3 ,  243 . 3  or  133 . 3 ,  233 . 3  and  134 . 1 ,  243 . 1  through the ring-type core  164 . It would even be possible to feed one of the turns of the center portion  133 . 2 ,  233 . 2  through the ring-type core  164 ,  264 , but this would be more complex to manufacture.  
       FIG. 4  shows the schematic electrical diagram of a further power transformer  331  according to the invention where not the end portions  333 . 3 ,  334 . 3 , but the end portions  333 . 1  and  334 . 1  are fed through the ring-type core  364  of the current transformer  360 . The end portions  333 . 3 ,  334 . 3  are connected together and form the center tap  335 .  
       FIG. 5  shows a schematic circuit diagram of a transformer arrangement of a power supply with two outputs. The arrangement includes a primary side with an input circuit  10  and a switching circuit  20  that corresponds to the primary side of the converter  1  shown in  FIG. 1 . The transformer stage  30  includes a transformer  431  having four secondary windings  33 ,  34 ,  433 ,  434  where the secondary windings  33 ,  34  provide the secondary voltages for the first output of the transformer arrangement and the secondary windings  433 ,  434  provide the secondary voltages for the second output. The first output with rectifier circuit  40 , output circuit  50  and current transformer  60  correspond to the secondary side of the converter  1  as shown in  FIG. 1 . No control circuit for controlling the switching of the switches of this transformer arrangement is shown.  
      The second output is of the push-pull type as well and is connected in parallel to the first output. It is similar or even identical to the first output and includes a rectifier circuit  440  with synchronous rectifiers  441 ,  442  and an output circuit  450  with output capacitor  451 . The second output includes a further current transformer  460  that is integrated into the main transformer  431 . An end portion  433 . 3  of the first secondary winding  433  of the main transformer  431  forms a primary winding of the current transformer  460  and an end portion  434 . 3  of the second secondary winding  434  also forms a primary winding of the current transformer  460 . The current transformer  460  further includes a secondary winding  463  for providing a current signal representing the sensed current in the secondary windings  433 ,  434 . Again, no control circuit for controlling the switching of the switches of this transformer arrangement is shown.  
      Control circuits that generate one of more control signals for controlling the synchronous rectifiers or other switches of such a transformer arrangement in dependency of the output current flowing in the secondary windings are well known in the art and are therefore not described here.  
      In summary, it is to be noted that the invention enables not only the design of cost effective and less space-demanding power transformers and power supplies but as well as transformers and power supplies with an improved efficiency, since several inaccuracies in the current measurement are eliminated or at least reduced.  
      Although preferred embodiments of the invention have been described in detail, it will be readily appreciated by those skilled in the art that further modifications, alterations and additions to the invention embodiments disclosed may be made without departure from the spirit and scope of the invention as set forth in the appended claims.