Abstract:
An auxiliary quasi-resonant dc tank (AQRDCT) power converter with fast current charging, voltage balancing (or charging), and voltage clamping circuits is provided for achieving soft-switched power conversion. The present invention is an improvement of the invention taught in U.S. Pat. No. 6,111,770, herein incorporated by reference. The present invention provides faster current charging to the resonant inductor, thus minimizing delay time of the pulse width modulation (PWM) due to the soft-switching process. The new AQRDCT converter includes three tank capacitors or power supplies to achieve the faster current charging and minimize the soft-switching time delay. The new AQRDCT converter further includes a voltage balancing circuit to charge and discharge the three tank capacitors so that additional isolated power supplies from the utility line are not needed. A voltage clamping circuit is also included for clamping voltage surge due to the reverse recovery of diodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional Patent Application 60/630,840 filed Nov. 24, 2004, and is related to U.S. Pat. No. 6,111,770, issued Aug. 29, 2000, both herein incorporated by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   This invention was made with United States Government support under Contract No. DE-AC05-00OR22725 between the United States Department of Energy and U.T. Battelle, LLC. The United States Government has certain rights in this invention. 

   FIELD OF THE INVENTION 
   This invention relates to soft-switching converters, and more particularly to an auxiliary quasi-resonant dc tank based converter to achieve soft and essentially lossless switching for power conversion. 
   BACKGROUND OF THE INVENTION 
   Soft switching technique has been used in power converters to reduce switching losses and alleviate electromagnetic interference (EMI). At present, there are two main topologies of soft switching inverters, resonant dc link and resonant snubber. The active clamped resonant dc link (ACRDCL) converter in U.S. Pat. No. 4,864,483 of Divan et al, issued Sep. 5, 1989, the auxiliary quasi-resonant dc link (AQRDCL) converter in U.S. Pat. No. 5,172,309 of De Donker, issued Dec. 15, 1992, and the voltage clamped parallel resonant (VCPR) converter in U.S. Pat. No. 5,559,685 of Lauw, issued Sep. 24, 1996, are examples of resonant dc link inverters. They are hereby incorporated by reference. Auxiliary resonant snubber inverters (in other words, the auxiliary resonant commutated pole (ARCP) or resonant snubber inverters (RSI)), represented in U.S. Pat. No. 5,047,913 of De Donker et al, issued Sep. 10, 1991, U.S. Pat. No. 5,710,698 of Lai et al, issued Jan. 20, 1998, U.S. Pat. No. 5,572,418 of Kimura et al, issued Nov. 5, 1996, and U.S. Pat. No. 5,574,636 of Lee et al, issued Nov. 12, 1996, belong to the second category, resonant snubber inverters. 
   The resonant snubber inverters ( FIGS. 1–4 ) employ two resonant capacitors and one resonant inductor for each phase leg to achieve soft switching. In spite of their advantages of lower EMI, dv/dt, and switching losses, these soft switching inverters have common problems compared to the resonant link inverters and traditional hard-switching inverters: (1) excessive number of additional active and passive components; (2) high current stress on the main switching devices, and (3) poor reliability. Accordingly, soft switching technology is expected to be used for medium or high power (&gt;100 kW) applications and special load/environment requirements, such as EMI-sensitive equipment, etc. 
   The resonant link inverters have advantages over the resonant snubber inverters in terms of less component count and low cost. In the ACRDCL converter ( FIG. 5 ), a resonant circuit, incorporated with an active clamping switch and clamping capacitor, is used as an interface between a dc power supply and the dc bus of an inverter. The ACRDCL resonates periodically, bringing the dc bus voltage to zero once each resonant cycle. The inverter switching devices are switched on and off at zero voltage instants of the resonant dc link, thus achieving lossless switching. However, the ACRDCL converter has some disadvantages, such as, high voltage stress across the inverter switches and continuous resonant operation of the dc link. To overcome the disadvantages of the ACRDCL converter, the auxiliary quasi-resonant dc link (AQRDCL,  FIG. 6 ) converter has been developed. The AQRDCL converter is employed to achieve soft-switching in an inverter coupled to a dc power supply via a resonant dc link circuit. The resonant dc link circuit includes a clamping switch limiting the dc bus voltage across the inverter to the positive rail voltage of the dc supply and auxiliary switching device(s) assisting resonant operation of the resonant bus to zero voltage in order to provide a zero-voltage switching opportunity for the inverter switching devices as the inverter changes state. The AQRDCT converter embraces its own problems such as high current stress because the clamping switch Sc and diode Dc ( FIG. 6 ) have to carry the full dc current, although it does not have the high voltage stress problem of the ACRDCL. The VCPR converter ( FIG. 7 ) was developed to reduce the current stress of the link switches (Sc 1  and Sc 2 ). However, the dc current that delivers dc power to the inverter has to still flow partly through the switches and partially through the resonant inductor (LR). 
   Despite their advantages and advances, the ACRDCL converter, the AQRDCL converter, and the VCPR converter have the following common disadvantages: (1) The resonant dc link circuit acts as an interface (i.e., a dc-to-dc converter) between the dc power supply and the inverter and needs to transmit power and to carry dc current from the dc power supply to the inverter or from the inverter back to the dc power supply via switch(es) and/or resonant component(s), which can lead to significant power losses; (2) The voltage clamping, voltage control, and charge balancing become difficult due to the dc power transmission; (3) The current stress on the auxiliary switch(es), clamping switch(es), and resonant inductor is high (at least as high as that on the inverter main switches); and (4) Two resonant dc link circuits are needed for an ac-to-dc-to-ac converter/inverter system to implement soft-switching at both ac-to-dc power conversion stage and dc-to-ac power conversion stage. 
   BRIEF DESCRIPTION OF THE INVENTION 
   An auxiliary quasi-resonant DC tank (AQRDCT) power converter with fast current charging, voltage balancing (or charging), and voltage clamping circuits is provided for achieving soft-switched power conversion. The present invention is an improvement of the invention taught in U.S. Pat. No. 6,111,770, herein incorporated by reference. The present invention provides faster current charging to the resonant inductor, thus minimizing delay time of the pulse width modulation (PWM) due to the soft-switching process. The new AQRDCT converter includes three tank capacitors or power supplies to achieve the faster current charging and minimize the soft-switching time delay. The new AQRDCT converter further includes a voltage balancing circuit to charge and discharge the three tank capacitors so that additional isolated power supplies from the utility line are not needed. A voltage clamping circuit is also included for clamping voltage surge due to the reverse recovery of diodes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an auxiliary resonant commutated pole (ARCP) circuit. (prior art) 
       FIG. 2  shows a delta configured resonant snubber inverter (RSI) circuit. (prior art) 
       FIG. 3  shows a resonant snubber inverter phase leg circuit. (prior art) 
       FIG. 4  shows a zero-voltage transition inverter circuit. (prior art) 
       FIG. 5  shows an ACRDCL inverter circuit. (prior art) 
       FIG. 6  shows an auxiliary quasi-resonant DC link inverter circuit. (prior art) 
       FIG. 7  shows a VCPR circuit. (prior art) 
       FIG. 8  shows an embodiment of the auxiliary resonant DC tank inverter (ARDCT) circuit. (prior art) 
       FIG. 9  shows the waveforms of the ARDCT circuit in  FIG. 8 . (prior art) 
       FIG. 10  shows a preferred embodiment of the AQRDCT circuit. 
       FIG. 11  shows the waveforms of the AQRDCT circuit in  FIG. 10 . 
       FIG. 12  shows another embodiment of the AQRDCT circuit with the charging circuit omitted. 
       FIG. 13  shows another embodiment of the present invention, in which the resonant inductor is omitted. 
       FIG. 14  shows another embodiment of the present invention, where an AQRDCT circuit is directly applied to a main inverter phase leg to provide soft-switching transition to the phase leg. 
       FIG. 15  shows a preferred clamping circuit of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An auxiliary quasi-resonant dc tank (AQRDCT) circuit is taught as an improvement of U.S. Pat. No. 6,111,770 as shown in  FIG. 8 . The AQRDCT is employed to provide a quasi-resonant or resonant dc bus across the converter without transmitting real power and carrying dc current. Moreover, such an AQRDCT circuit has no problems of voltage clamping and balancing and is capable of providing opportunity for soft-switching at both ac-to-dc power conversion stage and dc-to-ac conversion stage of an ac-to-ac converter, thus making the converter circuit more compact and efficient. In addition, the AQRDCT inverter has minimum component count and minimum changes to the traditional hard-switching inverters. 
   The AQRDCT inverter provides a novel alternative to the existing soft-switching topologies, has been proven of concept in a 10 kW prototype, and has been put into practical use in electric bus drives (100 kW). The AQRDCT inverter includes an auxiliary resonant tank circuit which provides a quasi-resonant dc bus across the inverter without transmitting or carrying load power and carrying dc current, and has less current and voltage stresses on the switches. The AQRDCT circuit is an add-on part to the traditional PWM inverters and will not affect the normal PWM operation of the inverter. This feature makes the ART inverter much more reliable than today&#39;s soft-switching inverters. Moreover, the AQRDCT inverter has no problems of voltage clamping and balancing, thus making control simpler. Experimental results demonstrate tremendous reduction of EMI, dv/dt, and switching losses. The AQRDCT inverter is a promising alternative that can alleviate the problems of today&#39;s soft-switching inverters. 
   An advantage of this invention is a AQRDCT circuit that shortens charging time of the resonant inductor, thus delay time of PWM operation can be minimized. A soft-switching process (t 1 –t 8  in  FIG. 9 ) can take 5 to 10 microseconds to complete, which affects the normal PWM operation of the inverter and causes time delay of PWM operation. The most undesired time consuming process is charging the resonant inductor. In  FIG. 9 , t 1 –t 2  and t 5 –t 6  are the positive and negative charging intervals of the resonant inductor, respectively. Time t 3 –t 4  and t 7 –t 8  are the positive and negative discharging intervals of the resonant inductor, respectively. Time t 2 –t 3  and t 6 –t 7  are the intervals for the voltage to change, usually the slower the better in order to reduce dV/dt. In order to minimize the time delay caused by a soft-switching process, it is desirable to shorten the charging time of the inductor. 
   Moreover, in order to ensure that the inverter bus voltage Vb resonates from the dc tank voltage (Vt) to zero and from zero back to Vt, the resonant inductor has to be charged to a pre-determined level before gating off the clamping switch Scl and changing switching state of the main inverter. This pre-determined current level is dependent on the resonant circuit losses (compensated by lb), dc supply current ld, and inverter dc bus current li, which is difficult to detect. As a result, the control is complicated and difficult to implement because of time-variant load current and uncertainty of the losses. Therefore, another advantage of the present invention is to provide a modified AQRDCT circuit that does not require a pre-determined resonant current level for the bus voltage Vb to resonate from the dc tank voltage (Vt) to zero and from zero back to Vt. 
   In addition, another advantage of the present invention is to provide a modified AQRDCT circuit that can charge and balance the dc tank capacitors. 
   Yet another advantage of the present invention is to provide voltage clamping for the auxiliary switches due to reverse recovery and noise related mal-gating. 
     FIG. 10  shows a preferred embodiment of the present invention. The AQRDCT  20  is connected in parallel across the dc bus of the dc power supply  10  that directly feeds the main inverter  30 . The AQRDCT includes three tank capacitors, Ct 1 , Ct 2 , and Ct 3 , to provide three voltage levels V 1 , V 2 , and V 3 , a resonant inductor Lr, a first auxiliary resonant switch Sap and diode Dap to provide positive resonant current through the resonant inductor Lr, a second auxiliary resonant switch San and diode Dan to provide negative resonant current through the resonant inductor Lr, a charge circuit to provide three stable voltage levels V 1 , V 2  and V 3 , a first clamping means  55  having switch Sc and a clamping diode Dc to clamp the bus voltage Vb to the tank voltage Vt, a pair of resonant capacitors Cr 1  and Cr 2 . The charge circuit  40  consists of a transformer Tr coupled across the resonant inductor Lr, feeding a diode bridge that charges the tank capacitor Ct 2 . The main inverter  30 , as an example in the figure, is a three-phase bridge consisting of 6 main switches S 1 –S 6  and 6 anti-parallel diodes D 1 –D 6 . 
     FIG. 11  shows example waveforms and control timing during a switching transient. Compared with the original AQRDCT inverter ( FIG. 8  and its waveforms and control timing  FIG. 9 ), the control is much simpler. The waveforms and control sequence can be explained as follows. 
   Before t 1 , the clamping switch Sc is already gated on, whether Sc or Dc carries the current it depends on the direction of the resonant tank current lo (lo=ld−li). In  FIG. 11 , it is assumed that the tank current lo is positive Dc is conducting. At t 1 , when the main inverter desires to switch, Sc is gated off and Sap is gated on at the same time. Sc is zero-voltage turnoff and Sap is zero-current turnon. The resonant current Ir through the resonant inductor Lr increases linearly and rapidly since voltage V 1 +V 2  is applied across the inductor during t 1 –t 2 . At t 2 , Ir reaches the resonant tank current lo and the clamping diode Dc&#39;s current becomes zero, i.e. Dc turns off. From t 2  to t 3 , a resonant circuit forms via Cr 1 , Cr 2 , Lr, Dap, and Sap. The tank capacitors Ct 1 , Ct 2 , and Ct 3  are much larger than Cr 1  and Cr 2 . As results, V 1 , V 2 , and V 3  are assumed constant during the switching transient (t 1 –t 8 ). The resonant current Ir charges Cr 1  and discharges Cr 2 . The bus voltage Vb decreases to zero at t 3 . When the resonant current Ir attempts to negatively charge the capacitor Cr 2 , the diodes of the main inverter phase legs, D 1  and D 2 , D 3  and D 4 , and D 5  and D 6  clamps the voltage to zero. Right after t 3  (Vb reaches zero), all main devices are gated on to clamp the zero voltage level. The resonant current decreases linearly and slowly since only V 3  is applied to the inductor. At t 4 , the resonant current reaches zero and stay zero till t 5  when San is gated on and all main switches change to the desired state at zero voltage. At this time, the inverter dc current li step-changes to a new level due to the main devices&#39; switching. If (Id−li)&lt;0, the main diodes D 1 –D 6  take over the main switches&#39; clamping function. As a result, the resonant tank current lo becomes to zero. The resonant current Ir through the resonant inductor Lr starts negatively charging and increases linearly and rapidly since voltage V 2 +V 3  is applied negatively across the inductor. At t 6 , Ir reaches the new current level (ld−li) and the main diodes (D 1 –D 6 )′ clamping ends. From t 6  to t 7 , a resonant circuit forms via Cr 1 , Cr 2 , Lr, Dan, and San. The resonant current Ir charges Cr 2  and discharges Cr 1 . The bus voltage Vb increases to the tank voltage Vt at t 7 . When the resonant current Ir attempts to over charge the capacitor Cr 2  and to negatively charge Cr 1 , the clamping diode Dc clamps the voltage Vb to Vt. Right after t 7 , Sc is gated on at zero voltage and zero current. The resonant current Ir decreases linearly and slowly because V 1  is applied across the inductor. At t 8 , Ir reaches zero and San is gated off right after t 8  at zero-current turnoff. A switching cycle completes. 
   In  FIG. 11 , the charging current waveform is not shown. During t 1 –t 8 , there are two charging intervals, t 1 –t 2  and t 5 –t 6 . During t 1 –t 2 , voltage (V 1 +V 2 ) is applied to the primary of the transformer Tr, inducing a voltage (V 1 +V 2 )*(n 2 /n 1 ) on the secondary which charges Ct 2  through the diode bridge  40 . Similarly during t 5 –t 6 , voltage (V 2 +V 3 ) is applied to the primary of the transformer Tr, inducing a voltage (V 2 +V 3 )*(n 2 /n 1 ) on the secondary which charges Ct 2  through the diode bridge  40 . The secondary-over-primary turns ration of the transformer Tr, (n 2 /n 1 ), is designed so that a desired V 2  can be obtained. The desired voltage level, V 2 , is dependent on the resonant circuit losses, desired charging rate of the resonant current Ir, etc. A preferred voltage level of V 2  is 10˜30% of the tank voltage Vt. An equal voltage level for V 1  and V 3  is desirable. As results, the charging circuit can be designed so that V 1 =V 3 =45˜35% of Vt and V 2 =10˜30% of Vt. 
     FIG. 12  shows another embodiment of the present invention, where the charging circuit of Ct 2  is omitted. In this case, an outside dc power supply is needed to maintain Ct 2 &#39;s voltage level V 2 . 
     FIG. 13  shows another embodiment of the present invention, in which the resonant inductor is omitted. The primary of the transformer Tr is employed as the resonant inductance so that the transformer serves as dual purposes. 
     FIG. 14  shows another embodiment of the present invention, where an AQRDCT circuit is directly applied to a main inverter phase leg to provide soft-switching transition to the phase leg. 
   In  FIGS. 10 , and  12 – 14 , the auxiliary switches Sap and San are gated off at zero current. However, in real applications, a voltage surge may occur due to the auxiliary diodes (Dan and Dan)&#39;s reverse recovery or noise-related mal-gating or mistiming.  FIG. 15  shows a preferred clamping circuit of the present invention, applied to  FIG. 10  to clamp such voltage surges. The diodes Dc 1  and Dc 2  clamp the voltage to the dc tank so that the voltage across the auxiliary switches Sap and San never exceed the tank voltage Vt. Similarly, this clamping circuit can be applied to all other inverter circuits. 
   While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope.