Patent Application
20120119676
1. Field of the Disclosure
The present invention relates generally to energy transfer. More specifically, the present invention relates to a divided energy transfer element for use in a flyback power converter.
2. Background
A wide variety of ac-dc and dc-dc power supplies are used in a variety of applications ranging for example from industrial equipment to household appliances. Flyback power converters are often an attractive design choice because of their desired features as well as the isolation that they provide. A known flyback power converter operates by storing energy in a magnetic field of an energy transfer element when a power switch is switched on and transmitting the energy to an output load when the power switch is switched off. An energy transfer element, such as a mutual inductor or coupled inductors, in an isolated flyback power converter behaves similar to a transformer with the input and output windings having opposite polarities. It is appreciated that energy transfer elements are often illustrated in electrical schematic diagrams with opposite dot positions and are often also referred to as flyback transformers.
In the recent years, the utilization of light emitting diodes (LEDs) has become very popular in a variety of applications including for example providing backlighting for large flat screen monitors and television screens. The brightness of an LED as well as the color of light that is emitted from an LED are sensitive to the current through the LED. As a result of these characteristics, as well as the LED's behavior as a forward biased diode, tight current controls are often necessary when driving LEDs. Accordingly, ac-dc off-line flyback converters with tight current controls have often been used to drive LEDs that are used to provide backlighting for large flat screen monitors and television screens. However, due to the high output voltage and high power requirements, long strings of LEDs are separated into shorter multiple strings of LEDs. Each of the shorter multiple strings of LEDs are then individually powered with separate current controllers.
The use of multiple strings of LEDs with separate current controllers to drive each string creates a number of complexities. For instance, there is the added complexity of providing balanced current distribution for all the individual strings of LEDs. Unbalanced currents can result in undesired uneven output brightness and color from the multiple strings of LEDs. In addition, by having multiple separate current controllers to drive each of the separate multiple strings of LEDs, additional components are required, which drives up the costs to power the multiple strings of LEDs.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Methods and apparatuses for implementing a flyback power converter with an energy transfer element having a divided structure are described. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.
Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.
As will be discussed, a flyback power converter including a divided energy transfer element that may be utilized in a flyback power converter to provide high power and high output voltage with a low profile is disclosed. Examples of the disclosed flyback power converter with a divided energy transfer element may be utilized to drive long single strings of light emitting diodes (LEDs), such as those that are utilized to provide backlighting for large flat screen monitors and television screens. Examples of the disclosed flyback power converter design enjoy a low profile structure while providing high power and high output voltage with reduced loss, higher efficiency and lower heat dissipation. In addition, examples of the disclosed flyback power converter design provide a single current control for a long single string of LEDs, which therefore provides homogeneous output brightness and color for all of the LEDs in the long single string of LEDs.
To illustrate,
In one example, divided energy transfer element 140 includes a plurality of input windings, including a first input winding 142 wound around first magnetic core 145 and a second input winding 152 wound around second magnetic core 155. It is noted that input windings 142 and 152 may also be referred to as primary windings. As shown in the example, first input winding 142 is coupled in parallel with second input winding 152. In one example, a clamp circuit 130 is also coupled across first input winding 142 and second input winding 152 as shown. In the example depicted in
In one example, a power switch S1170 is also coupled to first and second inputs windings 142 and 152 at node 124, and to the input 110 of flyback power converter 100. In one example, a switch drive signal 199 is coupled to be received by power switch S1170 to control the switching of power switch S1170. In one example, capacitor 120 provides a low impedance path for bypassing the switching current ripples. In one example, a clamp circuit 130 is coupled across the first and second input windings 142 and 152 to protect the power switch S1170 from the high amplitude oscillations due to the leakage that can occur when power switch S1170 is turned off. Depending on design requirements, clamp circuit 130 may include a known resistor-capacitor-diode arrangement, a known zener-diode arrangement, or any other suitable type of clamp circuitry in accordance with the teachings of the present invention.
As shown in the depicted example, divided energy transfer element 140 also includes a plurality of output windings, including a first output winding 143 wound around first magnetic core 145 and a second output winding 153 wound around second magnetic core 155. It is noted that output windings 143 and 153 may also be referred to as secondary windings. As shown in the example, a rectified output of first output winding 143 is coupled in series with a rectified output of second output winding 153. In one example, first output winding 143 includes a plurality of sections having rectified outputs coupled in series, including first section 144 and second section 146 as shown. Similarly, second output winding 153 includes a plurality of sections having rectified outputs coupled in series, including first section 154 and second section 156 as shown. As shown in
In the example shown in
Continuing with the illustrated example, an inductor 180 is coupled between the output windings 143 and 153 of energy transfer element 140 and an output 185 of flyback power converter 100. In addition, capacitors 182 and 183 are stacked and coupled across output windings 143 and 153 at the output 185 as shown. As shown in the illustrated example, the node between stacked capacitors 182 and 183 at the output 185 is labeled as node 186. As shown, a load 189 is to be coupled to the output 185 of flyback power converter 100. In one example, load 189 is a long single string of LEDs to be powered by flyback power converter 100 in accordance with the teachings of the present invention.
In operation, when the power switch S1170 is switched on in response to switch drive signal 199, current flows through power switch S1170, input diodes 134 and 138 and input windings 142 and 152 in parallel. However, because of the opposite polarities corresponding to the winding directions of output windings 144, 146, 154 and 156 (note the opposite dot sign of input and output windings) and the reverse directions of the output diodes 162, 164, 166 and 168, energy is not transferred to the output windings 143 and 153 and to the load 189 while power switch S1170 is switched on. Instead, energy is stored in the air gap of magnetic cores 145 and 155 while power switch S1170 is switched on.
However, when power switch S1170 is switched off in response to switch drive signal 199, the direction of current at output windings 143 and 153 reverses, and the energy that was stored in the air gaps of each magnetic core 145 and 155 while power switch S1170 was previously switched on, is then transferred through the output windings 143 and 153 to the load 189 at the output 185 of the flyback power converter 100. The current through each output winding is rectified by the output diodes 162, 164, 166 and 168. In the illustrated example, ripple currents are filtered by the output bulk capacitors 172, 174, 176 and 178. By series coupling of the output bulk capacitors 172, 174, 178 and 178 as shown and thereby stacking the output voltages across all of the output windings 143 and 153, the total required high voltage output across load 189 is achieved in accordance with the teachings of the present invention. In the example, inductive filter 180 and the bulk electrolytic capacitors 182, 183 further smooth the dc output across the load 189, which in one example is the long single string of LEDs.
Thus, it is appreciated that the example illustrated energy transfer element 140 of the flyback power converter 100 is divided into a plurality of transformers. In the illustrated example, each divided transformer has two sections of rectified outputs of output windings on the same magnetic core that are coupled in series with each other as well as in series with the two sections of rectified outputs of the other output windings of the second transformer. As a result, the output voltages are added together. The combined output voltages of the four output winding sections result in a very high total output voltage across the output 185 of flyback power converter 100. Since the high output voltage is distributed across the multiple windings and multiple stacked capacitors, a cost effective design is achieved since smaller lower profile components may be utilized because of the lower voltage requirements. Furthermore, a cost effective design is achieved with a low profile transformer with reasonable number of layers and safe isolation of each winding plus the lower rating of each output electrolytic capacitor. The series coupling of capacitors 182, 183 as the final output capacitive filter stage also provides a cost effective lower voltage rating of electrolytic capacitors as well as access to a fraction of total output voltage at node 186 in accordance with the teachings of the present invention.
Indeed, when comparing example flyback power converters 100 with divided energy transfer elements 140 as shown in
Examples of flyback power converters in accordance with the teachings of the present invention utilize a low profile divided energy transfer element structure that provide high output voltage, such as for example in the range of 500 volts, and high power, such as for example in the range of 60 watts. Such high voltage high power applications may include driving loads of long single strings of LEDs in applications such as large flat screen monitors and television screens.
To illustrate, the example of
In the example depicted in
As shown in the example depicted in
In one example, a power switch S1270 is coupled to first and second input windings 242 and 252 at node 224, and to the input 210 of flyback power converter 200. In one example, a switch drive signal 299 is coupled to be received by power switch S1270 from a controller 298 to control the switching of power switch S1270. In one example, controller 298 receives a feedback signal 295 from feedback/supply circuit 291 from feedback/supply winding 290 to generate switch drive signal 299. Feedback signal 295 is a signal representative of an output value at an output 285 of the flyback power converter 200. In the illustrated example, feedback/supply winding 290 generates feedback signal 295 by sensing the flux in the magnetic core 255 to provide feedback signal 295 to the controller 298 to generate switch drive signal 299 to control the transfer of energy from the input 210 to the output 285 of flyback power converter 200. In the illustrated example, feedback/supply supply circuit 291 also provides the supply voltage 296 for the controller 298 across a bypass capacitor 297.
In one example, controller 298 and power switch S1270 are both included in an integrated circuit. In one example, the integrated circuit including both controller 298 and power switch S1270 is a monolithic integrated circuit. In another example, the integrated circuit including both controller 298 and power switch S1270 is a hybrid integrated circuit. In yet another example, controller 298 and power switch S1270 are not included in the same integrated circuit.
In an example with universal input flyback, the controller 298 may also receive an input level signal 294 through the input voltage level detection circuitry 293 coupled to a dc bus at node 292 across input capacitor 220 after the rectifier 211. In the illustrated example, capacitor 220 provides a low impedance path for bypassing the switching current ripples. In one example, a clamp circuit 230 across the first and second input windings 242 and 252 protects the power switch S1270 from the high amplitude oscillations due to the leakage that may occur when power switch S1270 is switched off. In the illustrated example, clamp circuit 230 includes a resistor-capacitor-diode plus zener circuit as shown.
As shown in the depicted example, divided energy transfer element 240 also includes a plurality of output windings, including a first output winding 243 wound around first magnetic core 245 and a second output winding 253 wound around second magnetic core 255. As shown in the example, a rectified output of first output winding 243 is coupled in series with a rectified output of second output winding 253. In one example, first output winding 243 includes a plurality of sections having rectified outputs coupled in series, including first section 244 and second section 246 as shown. Similarly, second output winding 253 includes a plurality of sections having rectified outputs coupled in series, including first section 254 and second section 256 as shown.
As shown in
In the example shown in
Continuing with the illustrated example, an inductor 280 is coupled between the output windings 243 and 253 of energy transfer element 240 and the output 285 of flyback power converter 200. In addition, capacitors 282 and 283 are stacked and coupled across output windings 243 and 253 at the output 285 as shown. As shown in the illustrated example, the node between stacked capacitors 282 and 283 at the output 285 is labeled as node 286. As shown, a load 289 is to be coupled to the output 285 of flyback power converter 200. In one example, load 289 is a long single string of LEDs to be powered by flyback power converter 200 in accordance with the teachings of the present invention. It is noted that load 289 is also coupled as shown to the reference terminal 287 at the output 285 of flyback power converter 200. In the example, reference terminal 287 on the output side of energy transfer element 240 is galvanically isolated from the reference terminal 288 on the input side of energy transfer element 240. Accordingly, energy transfer element 240 provides isolation between the input and output sides of the energy transfer element 240 in accordance with the teachings of the present invention.
In operation, the output current from each output winding section 244, 246, 254 and 256 is individually rectified by output diodes 262, 264, 266 and 268, respectively. In the example, the bulk electrolytic filter capacitors 272, 274, 276 and 278 are coupled across external terminals of each rectified output winding help to filter the output. As shown in the depicted example, the negative terminal of the rectified and filtered dc voltage of each output winding section 244, 246, 254 and 256 is coupled to the corresponding positive terminal of the next output winding section 244, 246, 254 and 256. In this way, the dc outputs of all output winding sections either on the same magnetic core or on other magnetic cores, are coupled in series, thus combining the output voltages to a total dc output voltage that is at a much higher level and yet with a reasonable number of isolation layers. Energy transfer element 240 therefore has a lower profile with a reduced size and height compared to an energy transfer element with just a single core with single input and output windings.
As shown in the depicted example, divided energy transfer element 340 also includes a plurality of output windings, including a first output winding 343 wound around first magnetic core 345 and a second output winding 353 wound around second magnetic core 355. As shown in the example, a rectified output of first output winding 343 is coupled in series with a rectified output of second output winding 353. In one example, first output winding 343 includes a plurality of sections having rectified outputs coupled in series and wound around magnetic core 345, including first section 344 and second section 346 as shown. Similarly, second output winding 353 includes a plurality of sections having rectified outputs coupled in series and wound around magnetic core 355, including first section 354 and second section 356 as shown.
As shown in the example depicted in
Referring back to
In operation with a flyback power converter, the divided core and winding structure of energy transfer element 340 distributes the transfer of energy from the plurality of input windings 342 and 352 to the plurality of output windings 343 and 353 among the plurality of magnetic cores 345 and 355. By sharing the distribution of the transfer of energy among the plurality of cores and windings and lower the power rating requirements as discussed, it is appreciated that each of the cores can have lower profile and have a smaller size and smaller height than known energy transfer elements that have the same power rating and utilize a single magnetic core with single input and output windings.
In one example, the first and second sections 341 and 343 of input winding 342 are coupled in series on a bobbin on magnetic core 345, and the first and second sections 351 and 352 of input winding 352 are coupled in series on a bobbin on magnetic core 355. In one example, the ends of input windings 342 and 352 are coupled in parallel at nodes 322 and 324 through printed circuit board traces. In the example, there is an equal distribution current through the input windings 342 and 352 with the parallel coupling of the input windings on the different magnetic cores. By distributing the current among the plurality of input windings as discussed, it is appreciated that relative size of conductors or wires utilized for each magnetic core 342 and 352 is smaller and less bulky than the sizes of the conductors or wires utilized in known energy transfer elements that have the same power rating and utilize a single magnetic core.
In order to reduce the risk of circulating current between the parallel input windings 342 and 352 due to unbalanced input windings, diode 326 is coupled between node 322 and input winding 342 and diode 328 is coupled between node 322 and input 352 as shown. In one example, diodes 326 and 328 are fast diodes and are coupled in a direction such that they conduct current to transfer energy from the input side of energy transfer element 340 to the output side of energy transfer element 340, but prevent any current reverse direction, which could occur due to possibility of an unbalanced winding structure resulting in extra losses and lower efficiency.
In one example, the ends of each section 344, 346, 354 and 356 of output windings 343 and 353 are brought out on the bobbins of the respective magnetic cores 345 and 355 such that the rectified outputs of each section are coupled together in series as shown through bobbin pins coupled to respective printed circuit board traces. In the output section 391 illustrated in
As shown in the example of
As shown in the depicted example, the dc outputs across bulk capacitors 372, 374, 376 and 378 of the section windings 344, 346, 354, and 356, respectively, are externally stacked through the printed circuit board traces. In other words, the negative terminal of each output bulk capacitor is connected to the corresponding positive terminal of the next bulk capacitor. For instance, as shown in
To illustrate,
In the example, the two sections 441 and 443 of the input winding are coupled in series by coupling their terminals of different polarity, which are illustrated in
Referring now to
Similar to the winding structure 445 illustrated in
In the example, the two sections 451 and 453 of the input winding are coupled in series by coupling their terminals of different polarity, which are illustrated in
The above description of illustrated examples of the present invention, including what is described in the Abstract, are not intended to be exhaustive or to be limitation to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present invention. Indeed, it is appreciated that the specific voltages, currents, frequencies, power range values, times, etc., are provided for explanation purposes and that other values may also be employed in other embodiments and examples in accordance with the teachings of the present invention.
These modifications can be made to examples of the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.
