Patent Publication Number: US-9837920-B2

Title: Commutation current steering method in a zero volt switching power converter using a synchronous rectifier

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 61/883,294, filed Sep. 27, 2013, which is/are hereby incorporated by reference. 
    
    
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of electrical power conversion, and more particularly to soft switched power converters equipped with synchronous rectification. Even more particularly, the present invention relates to zero voltage switching (ZVS) power conversion implementing a commutation current boosting method. 
     Soft switched power converters, such as those configured for switching at zero voltage (ZVS), are popular for their relatively low switching losses and smooth switching waveforms, providing high efficiency and good electromagnetic compatibility (EMC). One of the parameters to be calculated during design of such converters is a level of commutation energy required to maintain zero volt switching. This minimum level of commutation energy can be calculated based on parasitic elements in the circuit design. From this calculation, a value for the minimum amplitude of the commutation current can be obtained. This current introduces an internal reactive power constantly circulating throughout the converter, and is responsible for a remarkable degree of power losses. These losses further result in lower efficiency, especially at the lower half of the load range. 
     Therefore, designers try to minimize the level of the commutation current, so as to offer a smaller reactive energy circulating in power circuits of the converter and also resulting in a smaller permanent power loss and higher efficiency. Unfortunately, a smaller commutation current also notably reduces ZVS reliability. 
     One previously known method for addressing this problem involves implementing a variable dead time in combination with a variable phase shift between primary and secondary control signals generated by a controller, thereby ensuring that power switching elements are not exposed to cross conduction or non-ZVS operation across the range of operating conditions. This approach, unfortunately, brings about additional demands on the controller, ultimately resulting in any or all of a more expensive, more complex or lower performing control solution. 
     Conventional power converters further typically enable or disable synchronous rectifier switches as a function of the load current. One problem with this technique is that it may cause dips on the output voltage, which can be critical for applications requiring tight output voltage regulation. The need for current sensing in order to provide proper synchronous rectifier control further requires a more demanding design. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, a zero volt switching LLC-type power converter and method are provided for converting power from an input DC voltage source to a variable load. A first plurality of switches operate under ZVS conditions, a second plurality of switches operate as a synchronous rectifier, and a resonant tank is further provided, wherein one or more components of the converter at least partially define a minimum current outside ZVS commutation periods and one or more parasitic components at least partially define a maximum current during ZVS commutation periods while converter is operated under light or zero load conditions. 
     A boosting resonance is induced by controlling each of the synchronous rectifier switches to turn off at a time prior to a turn-off time for a corresponding one of the switches operating under ZVS conditions. The boosting resonance derives a commutation current boosting effect, which yields to faster and more reliable ZVS conditions for the ZVS operated switches. 
     In one exemplary aspect of a power converter as described herein, the synchronous rectifier switches are operated across an entire output current range. In another aspect, the synchronous rectifier switches are operated across an entire input voltage range. 
     In another aspect, the switches operating under ZVS conditions are driven with a constant dead time. 
     In another aspect, the synchronous rectifier switches are driven to provide a constant phase shift between the switches operating under ZVS conditions and the synchronous rectifier switches. 
     In yet another aspect, the LLC-type converter includes a resonant capacitor, and primary and secondary leakage inductances of an isolation transformer, and may further include a resonant choke. A ratio between a primary magnetizing inductance of the isolation transformer and an inductance of the resonant choke is substantially increased because of a reduced commutation current, and may preferably be greater than seven. The smaller commutation current offers a smaller reactive energy circulating in the power circuits of the converter, which may generally result in a smaller permanent power loss and higher overall efficiency. 
     In still another aspect, the switches operating under ZVS conditions are primary-side switches with respect to the isolation transformer and the synchronous rectifier switches are secondary-side switches with respect to the isolation transformer. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit diagram representing a circuit model of a power converter used to describe an embodiment of a method of the present invention. 
         FIG. 2  is a graphical diagram representing exemplary voltage waveforms for operation according to an embodiment of a method of the present invention. 
         FIG. 3  is a graphical diagram representing exemplary current waveforms for operation according to an embodiment of a method of the present invention. 
         FIG. 4  is a timing diagram representing control pulses driving an exemplary power converter model of  FIG. 1 , further in accordance with a method as related in  FIGS. 2 and 3 . 
         FIG. 5  is an exemplary full bridge converter according to an embodiment of the present invention 
         FIG. 6  is an exemplary half-bridge converter according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. 
     The terms “switching element” and “switch” may be used interchangeably and may refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, JFET, etc.), a switching diode, a silicon controlled rectifier (SCR), a diode for alternating current (DIAC), a triode for alternating current (TRIAC), a mechanical single pole/double pole switch (SPDT), or electrical, solid state or reed relays. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa. 
     The terms “power converter” and “converter” unless otherwise defined with respect to a particular element may be used interchangeably herein and with reference to at least DC-DC, DC-AC, AC-DC, buck, buck-boost, boost, half-bridge, full-bridge, H-bridge, series resonant converter, parallel resonant converter, LLC converter or various other forms of power conversion or inversion as known to one of skill in the art. 
     The terms “controller,” “control circuit” and “control circuitry” as used herein may refer to processing circuitry including one or more of a general microprocessor, an application specific integrated circuit (ASIC), a digital signal processor (DSP), a digital signal controller (DSC), a microcontroller, a field programmable gate array (FPGA), and/or various alternative blocks of discrete processing circuitry, and any pre-processing modules or other such circuitry as may be designed as is known in the art to perform functions as further defined herein. In an embodiment of the present invention the controller may be formed of processing circuitry and program instructions or firmware which is integrally embodied therewith. In other embodiments, the processing circuitry may be separately embodied but functionally linked to a processor-readable medium having program instructions or firmware residing thereon and which is executable by the processor to perform functions as further defined herein. 
     Conditional language used herein, such as, among others, “can,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     Referring generally to  FIGS. 1-6 , a power converter and associated methods may now be described. Where the various figures may describe embodiments sharing various common elements and features with other embodiments, similar elements and features are given the same reference numerals and redundant description thereof may be omitted below. 
     Briefly stated, an exemplary commutation current steering method according to embodiments of the present disclosure is provided in the context of a zero volt switched (ZVS) power converter. The commutation current is forced to flow through at least a primary circuit branch and a secondary circuit branch with respect to an isolation transformer (and consequently through switches operating in ZVS conditions and switches operating as synchronous rectifiers) by keeping the operation of the synchronous rectifier switches enabled eventually down to zero load. By doing this, the power losses associated with the commutation current are reduced since both of the switches operating under ZVS conditions and the switches operating as synchronous rectifiers share the commutation current in their respective on-states. 
     A modification in the timing of the synchronous rectifier control signals further essentially steers the commutation current from the secondary side to the primary side of the isolation transformer, providing more commutation energy for proper ZVS operation of the switches operating in ZVS conditions. This is achieved by inducing a boosting resonance which is triggered by an early turn-off of the synchronous rectifier switch with respect to the otherwise corresponding turn-off instant for the switches operating in ZVS conditions. 
     The above-referenced method may be more particularly demonstrated in accordance with a simplified LLC converter simulation model  10  as shown in  FIG. 1 . The simulation model of  FIG. 1  may represent certain elements as discrete components, although one of skill in the art may appreciate that such elements in practice may in the alternative be intrinsically embedded in another component or element. Elements Q 1 /D 1  and Q 2 /D 2  represent first and second primary-side switching devices operating in ZVS conditions, C 1  represents a parasitic capacitance of the switching elements Q 1  and Q 2 , L 1  is a resonant choke and capacitor C 4  is a resonant capacitor. L 2 -L 4  represent primary and secondary-side magnetizing inductances for a transformer T 1 , and L 5 -L 7  represent primary and secondary-side isolation transformer leakage inductances. A resonant tank of the LLC converter is formed by C 4 , L 1  and includes also L 5 -L 6  or L 5 -L 7 , respectively. In case L 1  is not present, only L 5 -L 6  or L 5 -L 7  respectively forms inductive part of the resonant tank. Q 3 /D 3 /C 3  and Q 4 /D 4 /C 2  represent secondary-side switching devices as synchronous rectifier switches. An exemplary load is represented by voltage source V 2  while V 1  represents an input voltage source. 
     Referring also now to  FIGS. 2 and 3 , in an embodiment of the method a magnetizing current generated by magnetizing inductance L 2 -L 3 -L 4  is simultaneously also the commutation current providing ZVS for the primary switches. Whenever operation of the synchronous rectifier switches is enabled, the commutation current is shared between the primary switches and the synchronous rectifier switches. Therefore, at the time instant  12  the commutation current flows through switching elements Q 1  and Q 4 . Before the switching element Q 1  commutates to switching element Q 2 , the synchronous rectifier switch Q 4  is first turned off at point  13 , forcing a parasitic capacitance C 2 +C 3  of the switching elements Q 3  and Q 4  to resonate with the total transformer leakage (L 5 +L 6 ∥L 7 ) and the resonant choke L 1 . Therefore a boosting resonant circuit is formed by L 1 +L 5 +(L 6 ∥L 7 ) and C 2 +C 3 . In case the resonant choke L 1  is not present, L 1 =0. The magnetizing inductances do not play a role in this resonance because the high impedance acts as a current source. 
     During this time, the voltage on the synchronous rectifier switch Q 4  rises and falls to form a resonant period  14 . At the same time, the current through secondary leakage inductance L 7  falls to zero, reverses to negative value, and returns back to original value. The current through secondary leakage inductance L 7  is fully reflected to the primary side and forms a boosting resonant wave  20  that increases the primary commutation current. During the time of resonance the primary switching element Q 1  is turned off at point  16  and while the commutation current is boosted as described above, the commutation of switches Q 1 , Q 2  forms a fast and reliable ZVS transition  18 . As a result, ZVS commutation of the primary switches Q 1 , Q 2  can further be maintained during no-load conditions while the converter can be designed with larger magnetizing inductance and/or constant dead times. Power losses associated with the commutation current are substantially reduced, as all of the aforementioned switching elements Q 1 , Q 2 , Q 3 , Q 4  share the commutation current during most of the time. 
     Referring now to  FIG. 4 , primary and secondary control pulses are illustrated driving the power stage according to the simplified model of a LLC-type resonant converter of  FIG. 1 . A controller for generating the control pulses may in various embodiments be either of an analog controller or a digital controller without altering the scope of the present invention. 
     Pulses PA and PB control the respective switching elements Q 1  and Q 2  operating in ZVS conditions. Pulses SA and SB drive the respective synchronous rectifier switches Q 3  and Q 4 . The synchronous rectifier switching elements Q 3 , Q 4  are switched off in advance of the switch-off time for corresponding ZVS switching elements Q 1 , Q 2  by an advance time  22 . Alternatively stated, the switches Q 1 , Q 2  are controlled to be turned off after a delay time  22  with respect to the respective turn-off time for corresponding switches Q 3 , Q 4 . This advance or delay  22  results in the aforementioned resonance condition and causes the boost  20  in the primary-side current Iprim flowing through switching elements Q 1 , Q 2  as described above. Note that the position of leading pulse edges  24  are here aligned for each of pulse signals PA and PB, respectively, but the exact position depends on how the synchronous rectifier drivers are designed. Therefore, any positive or negative time shift of the leading pulse edges  24  may fall within the scope of the present invention. 
     In an embodiment of the apparatus and method, FETs with highly non-linear Coss may be implemented as switching elements for the converter topology  10 , combined with relatively large magnetizing inductance of the isolation transformer and still achieving ZVS across the entire load and input voltage range, resulting in a relatively small commutation current being reflected to the primary side with respect to the boost wave  20  induced by the corresponding resonance condition. As a result, the primary switch commutation is relatively short despite the strong Superjunction Coss nonlinearity which is remarkable only at the end of the transition. Therefore, the boost wave  20  assures that the voltage across the switch can go down to zero volt despite of the large capacitance increase of Coss in this region and hence enables operation of the converter with constant dead time and constant phase shift in the primary-to-secondary control signals across the entire load and input voltage range. 
     Because various embodiments of a method as described herein allow for operation of a power converter  10  with small commutation current, it may further be noted that a high ratio between the primary magnetizing inductance and the inductance of the resonant choke can be achieved, thereby increasing the efficiency of the converter. 
       FIG. 5  shows a full bridge and  FIG. 6  shows a half bridge LLC-type of converter ( 10   b  and  10   c , respectively) operated according to an embodiment of the invention and characterized with ratio between magnetizing inductance of isolation transformer T and resonant inductance Lr larger than 7. Both converters  10   b ,  10   c  are supplied from input voltage source V 1  and supplies converted power to a load RL. 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of an invention herein, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.