Patent Application
20110122658
1. Field of the Invention
The present invention relates generally to a power converter, and more specifically, to a power converter using energy stored in the leakage inductance of a transformer to power a switch controller of the power converter.
2. Description of the Related Art
The leakage inductance of the transformer in a power converter is often a large factor in degrading the performance of the power converter. The leakage inductance of the transformer slows down switching transitions and steals a significant amount of energy that is to be delivered to an output of the power converter. Therefore, it is generally preferable to decrease the leakage inductance in the power converter. The leakage inductance, however, cannot be eliminated due to non-ideal properties of the transformer.
The leakage inductance can also cause damage to a switch (typically a bipolar junction transistor or a field effect transistor) in the power converter because the energy stored in the leakage inductance causes spikes in the voltage across the primary winding of the transformer. The voltage spike causes excessive current through the switch that may damage the switch. Therefore, a clamp is generally coupled between the primary winding of the transformer and ground to prevent the voltage spike across the primary winding from damaging the switch. The clamp protects the switch by diverting the excessive current away from the switch. The energy in the form of diverted current, however, is wasted and not put to any use by the power converter. The waste of the diverted current reduces the overall efficiency of the power converter.
The energy stored in the leakage inductance increases as the load coupled to the power converter increases. The energy stored in the leakage inductance of the primary winding of the transformer is represented as
where Lk is the leakage inductance and Ip is the current in the primary winding of the transformer. An increased load of the power converter is accompanied by increased current Ip in the primary winding. The increased current in the primary winding Ip in turn results in increase of energy E stored in the leakage inductance Lk. Therefore, more energy is lost from the leakage inductance Lk when the load of the power converter is increased.
Another issue with the leakage inductance is the electromagnetic interference (EMI). The transformer has parasitic capacitance between the windings. The parasitic capacitance in conjunction with the leakage inductance Lk of the transformer causes EMI emission from the power converter. In order to reduce EMI, an RC snubber is generally placed across the primary winding of the transformer. The RC snubber, however, decreases the efficiency of the power converter because the RC snubber slows down switching transition. Specifically, when a switch in the power converter is turned off, energy stored in the RC snubber manifests itself as added voltage across the primary winding. Also, when the switch is turned on, the RC snubber increases the initial current spike. The increase in the voltage and the current spike increases switching loss in the power converter. Further, the current spike caused by the RC snubber may also distort the current waveform across the primary winding, causing faulty detection of the current across the primary winding. Therefore, it is necessary to implement measures to reduce EMI generated by the power converter without using the RC snubber across the primary winding of the transformer.
Therefore, there is a need for a power converter that can utilize the energy stored in the leakage inductance to increase efficiency. There is also a need for a power converter that obviates a clamp for diverting an excessive current away from the primary winding. Moreover, there is a need for a power converter that reduces EMI emission without decreasing the efficiency of the power converter.
One embodiment of the present invention includes a power converter including a transformer that uses the energy stored in the leakage inductance of the first primary winding of the transformer to provide at least part of the power needed for operating a switch controller. The switch controller controls on-times and off-times of a switch that couples or decouples the transformer to or from a power source of the power converter through a transformer to regulate an output voltage of the power converter to the load.
In one embodiment of the present invention, the transformer includes a secondary primary winding that receives at least part of the energy stored in the leakage inductance of the first primary winding. The secondary primary winding is coupled to the switch controller to provide the energy received from the first primary winding to the switch controller.
In another embodiment of the present invention, the first primary winding and the second primary winding are wound around the core of the transformer in an alternating manner.
In one embodiment, the power converter includes a third primary winding coupled to sense an output voltage of the power converter to the load from the secondary winding. The second primary winding may have a greater number of turns than the third primary winding. The third primary winding is wound adjacent to the secondary winding.
The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings.
The Figures (FIG.) and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention.
Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.
In one embodiment, the energy stored in the leakage inductance of a transformer of a power converter is utilized to provide at least part of the power needed to operate a switch controller of the power converter. By using the energy stored in the leakage inductance (otherwise dissipated or wasted), the overall efficiency of the power converter is increased because less power is drawn from the input of the power converter to power the switch controller. Moreover, by using the energy stored in the leakage inductance, the electromagnetic interference (EMI) emission of the power converter may also be reduced.
The transformer T1 includes, among other components, a first primary winding P1, a second primary winding P2, a third primary winding P3, and a secondary winding S1. The primary side (coupled to the power source) of the transformer T1 includes the first primary winding P1, the second primary winding P2 and the third primary winding P3. The secondary side (coupled to the load) of the transformer T1 includes the secondary winding S1.
Referring to
When the switch Q1 is turned off, the energy stored in the leakage inductance Lk and the mutual inductance LM of the first primary winding P1 is released. The energy released from the mutual inductance LM of the first primary winding P1 is received by the secondary winding S1 because the diode D9 becomes forward biased when the switch Q1 is turned-off. The secondary side diode D9 rectifies a current is to provide an output (OUTPUT) voltage at the output of the power converter.
Likewise, the energy released from the leakage inductance Lk of the first primary winding P1 is coupled to and received by the second primary winding P2 when the switch Q1 is turned off because the diode D6 is forward biased. The energy received by the second primary winding P2 is provided as a current i3 to a node 124 via a path 134 that includes the capacitor C6 and a resistor R4. To lower the impedance of the path 134, the capacitor C6 with a large capacitance is used. The current i3 from the second primary winding P2 merges with a current iin from the EMI filtering circuit 110 via resistors R6, R7 at node 124 to form a supply current icc. A capacitor C4 is placed between the node 124 and ground to help increase common mode rejection ratio (CMRR) of supply voltage (Vcc) at pin 4 of the switch controller 100. The supply current icc is provided to node 4 of the switch controller 100 to power the switch controller 100. Note that part of the energy stored in the leakage inductance Lk of the first primary winding P1 is converted to the current i3 to provide part of the power necessary to operate the switch controller 100. Therefore, the switch controller 100 can draw less current iin (i.e., energy) from the input (INPUT) of the power converter. By drawing less current iin from the input (INPUT) of the power converter to power the switch controller 100, the overall efficiency of the power converter is increased.
In one embodiment, the switch controller 100 receives at node 1 a divided-down version (Vsense) of voltage across the third primary winding P3 via a network of resistors (R8, R9, R10 and R11) and a capacitor C2. A diode D8 is coupled across the third primary winding P3. The switch controller 100 also receives at node 5 an input voltage (Vin) which is a scaled down version of the output voltage of the bridge rectifier 104 as passed through the EMI filtering circuit 110. Based on Vsense and Vin, the switch controller 100 determines the on-times and off-times of an output signal 128 from its output node (node 3). The output signal 128 to the switch Q1 via the resistor R13 turns on and turns off the switch Q1.
In another embodiment, the third primary winding P3 is omitted and its function is replaced with the second primary winding P2. In this embodiment, a voltage across the second primary winding P2 is divided down to obtain Vsense. Vsense is fed into the switch controller 100 to determine the on-times and off-times of the output signal 128.
In one embodiment, an RC snubber 108 is placed in parallel with the switch Q1 to protect the switch Q1 from the voltage spike at node 152 caused by energy in the leakage inductance Lk that is not received by the second primary winding P2. Although it would be ideal to have the secondary primary winding P2 receive all the energy in the leakage inductance Lk, some energy (“leftover energy”) in the leakage inductance Lk is not received by the second primary winding P2 due to, among other reasons, imperfect coupling of the first primary winding P1 and the second primary winding P2. Such leftover energy causes voltage spike at node 152. The voltage spike across the switch Q1, however, is smaller when the second primary winding P2 is used because the second primary winding P2 receives at least part of the energy stored in the leakage inductance Lk. Because the voltage spike at node 152 is decreased, a smaller RC snubber with smaller RC time constant can be placed across the switch Q1 compared to a power converter that does not use the second primary winding P3. The smaller RC snubber leads to less switching loss. Therefore, by using the second primary winding P2 to receive the energy stored in the leakage inductance Lk of the first primary winding P1 and by using that energy to power the switch controller 100, the overall efficiency of the power converter is increased and EMI emission of the power converter is reduced. In one or more embodiments, the RC snubber 108 may be omitted if the energy spike at node 152 is low enough so that the switch Q1 is not damaged.
Referring back to
Then the third primary winding P3 is wound around the first primary winding P1, and the secondary winding S1 is wound around the third primary winding P3. Winding the secondary winding S1 adjacent to the third primary winding P3 is advantageous because the third primary winding P3 can receive the energy from the secondary winding S1 most effectively. Further, placing the third primary winding P3 between the first primary winding P1 and the secondary winding S1 reduces common mode EMI noise.
A first insulation layer 314 is placed between the second primary winding P2 and the first primary winding P1 to provide insulation between the second primary winding P2 and the first primary winding P1. A second insulation layer 316 is placed between the first primary winding P1 and the third primary winding P3 to provide insulation between the first primary winding P1 and the third primary winding P3. A third insulation layer 318 is placed between the third primary winding P3 and the secondary winding S1 to provide insulation between the third primary winding P3 and the secondary winding S1. The secondary winding S1 is then covered with a fourth insulation layer 320 and a fifth insulation layer 322. The directions of the turns in the primary windings P1, P2, P3 and the secondary winding S1 are as explained above with reference to
In one embodiment, the first primary winding P1 has the most number of turns among all of the windings of the transformer T1. The second primary winding P2 has more number of turns than the third primary winding P3. For example, the first primary winding P1 has fifty-three (53) turns, the second primary winding P2 has sixteen (16) turns, the third primary winding P3 has thirteen (13) turns, and the secondary winding S1 has seventeen (17) turns. It is advantageous for the second primary winding P2 to have more turns than the third primary winding P3 because more energy stored in the leakage inductance Lk may be received from the first primary winding P1.
The connection of transistor pins to the windings in the embodiment of
The functions of the windings P1, P2, P3 and S1 of the embodiment of
As in the embodiment of
When the switch Q1 is turned off, the first primary winding P1 is decoupled 530 from the power source of the power converter. As a result, the energy stored in the leakage inductance Lk and the mutual inductance LM of the first primary winding P1 is released 540 from the first primary winding P1. The energy stored in the mutual inductance LM of the first primary winding P1 is received 550 by the secondary winding S1. The energy stored in the leakage inductance Lk of the first primary winding P1 is received 560 by the second primary winding P2. The energy received by the second primary winding P2 is then used 570 to power the switch controller 100. By using the energy received from the leakage inductance Lk of the first primary winding P1, the overall efficiency of the power converter is increased. Then the output voltage of the power converter is sensed 580 by the third primary winding P3 to control the on-times and off-times of the switch Q1. The process then returns to step 510.
Although the present invention has been described above with respect to several embodiments, various modifications can be made within the scope of the present invention. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CN2008/001429 | 8/6/2008 | WO | 00 | 1/25/2011 |
