Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of a priority under 35 USC 119(a)-(d) to French Patent Application No. 02 06443 filed May 27, 2002, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a double resonance electronic converter and a method operating such a converter, which may be used to obtain very high dc voltages, on the order of 100 kilovolts and more, for providing power, e.g., to an X-ray tube in a radiological imaging apparatus. 
     There exist many types of electronic converters including the type known as the “double resonance”. The “double resonance” uses a circuit having two resonance frequencies. 
     A double resonance electronic converter generally comprises four parts: a switching circuit; a double resonance circuit; a rectifying and filtering circuit; and a control circuit for the switching circuit. The switching circuit comprises two switches whose switching on (conduction) or off (break) are controlled by the control circuit. A dc voltage is applied to the terminals switches so that the potential of the positive pole is applied to the resonance circuit when only one switch is on, while the potential of the negative pole is applied when only the other switch is on. 
     The resonance circuit comprises: a parallel resonant circuit comprising an inductor and a capacitor; a series resonant circuit comprising inductors, as well as a capacitor; and a transformer. 
     The rectifying and filtering circuit comprises two rectifier diodes, two filtering capacitors and a load resistor. The output voltage of the converter is taken from the terminals of resistor. 
     The control circuit controls the conduction and blockage of the switches as a function the following three parameters: the series current in one inductor; the parallel current in another inductor; and the measured output voltage. 
     The operating characteristics of the resonant circuit provides a curve of the modulus of the frequency response between an input voltage at the common node of the switches, and an output voltage at the terminals of capacitor. This curve shows two resonance frequencies and zero transmission at an intermediate frequency. The conduction of one of the switches establishes a direct current in the resonant circuit that unbalances the series current and deactivates the parallel current. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention and embodiments thereof is a method and circuit for controlling the switches of a double resonance converter so as to obtain a balanced start up condition. 
     In an embodiment of the invention the start of the conduction of the switches is synchronized with the value of the parallel current. More particularly, the start of the first conduction of one of the switches is carried out at the maximum positive value of the parallel current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention and embodiments thereof will become more apparent from reading the following description together with the appended drawings in which: 
     FIG. 1 is a simplified diagram of a known double resonance converter; 
     FIG. 2 is a frequency response curve for the output voltage of the converter with respect to its input voltage in a known double resonance converter; 
     FIG. 3 is a diagram illustrating the operation of a known double resonance converter; 
     FIGS. 4 a  to  4   e  are diagrams illustrating the starting up of a double resonance converter according to the prior art, 
     FIGS. 5 a  to  5   e  are diagrams illustrating the starting up of a double resonance converter in an embodiment of the invention; and 
     FIGS. 6 and 7 are diagrams illustrating two state circuit arrangements in an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a simplified circuit diagram of a known double resonance electronic converter, while FIG. 2 is a curve showing the modulus of the frequency response of the resonant circuit between an input voltage Ve and an output voltage Vs. 
     As shown in FIG. 1, a double resonance electronic converter comprises four parts: a switching circuit  10 ; a double resonance circuit  12 ; a rectifying and filtering circuit  14 ; and a control circuit  16  for the switching circuit  10 . 
     The switching circuit  10  comprises two switches T 1  and T 2  whose switching on (conduction) or off (break) are controlled by the control circuit  16 , the control being represented by a broken line  18 . The two switches T 1  and T 2  can be formed, e.g. of IGBT (insulated gate bipolar transistor) type transistors, and their switching drive circuitry comprises an inductor L 1  (or L 2 ) in series with the transistor/switch, a serial circuit R 1 C 1  (or R 2 C 2 ) and a diode D 1  (or D 2 ) in parallel. 
     A dc voltage E is applied to the terminals of both switches T 1  and T 2  so that the potential of the positive pole is applied to the resonance circuit  12  when only switch T 1  is on, while the potential of the negative pole is applied when only switch T 2  is on. 
     A known resonance circuit  12  comprises: a parallel resonant circuit comprising an inductor Lp and a capacitor Cp; a series resonant circuit comprising inductors L 1 , L 2 , Lr and Lm, as well as a capacitor Cr and a transformer TR having a transformer ratio equal to m. 
     A known rectifying and filtering circuit  14  comprises: two rectifier diodes Dr 1  and Dr 2 , two filtering capacitors Cf 1  and Cf 2  and a load resistor Rc. 
     The output voltage V of the converter is taken from the terminals of resistor Rc. 
     The control circuit  16  controls the conduction and blockage of switches T 1  and T 2  as a function the following three parameters: the series current Is in inductor Lr, as measured by a device  20 ; the parallel current Ip in inductor Lp, as measured by a device  22 ; the output voltage V measured by a device  24 . 
     The operating characteristics of the resonant circuit  12  provides a curve  26 , shown in FIG. 2, of the modulus of the frequency response between an input voltage Ve at the common node  24  of switches T 1  and T 2 , and an output voltage Vs at the terminals of capacitor Cp. This curve  26  shows two resonance frequencies f 0  and f 2 , and zero transmission at an intermediate frequency f 1 . As an example, the values can be as follows: f 0 =18.16 KHz, f 1 =19.37 KHz and f 2 =138.59 KHz. 
     The converter in an embodiment of the invention may be used between frequencies f 1  and f 2 , between which the gain varies from 40 to 80 decibels, to enabling varying the voltage and power at the converter output. The operation is shown in FIG. 3, where the series current Is as a function of time, Is being measured by device  20 , as follows. The instant when switch T 1  or T 2  begins to conduct is defined with respect to the instant X k , X k+1 , X k+2  or X k+3  of end of conduction of the preceding switch, which corresponds to a passage to zero of the series current Is, along one direction or the other, by counting respectively a period T(k−1), T(k) or T(k+1). 
     Accordingly, the start of conduction of switch T 2  is determined by counting down a duration T(k) that was calculated at the time of the preceding cycle during the conduction time of diode D 2 , referred to as a freewheeling diode. 
     This aspect of calculating durations T is shown in the diagrams of FIG.  4 . Thus, FIG. 4 a  shows curves for the series current Is (full lines) and parallel current Ip (broken lines) upon starting a supply sequence. After starting, for example, a radiological (X-ray) image acquisition, and in a stable mode, switch T 1 , for instance is conducting, a calculation of duration T( 4 ) is carried out during interval ( 4 ) and its value is counted down from the instant X 4  when the series current Is an switch T 2  passes to zero. At the end of the interval T( 4 ), switch T 1  is conducting between the instants X 4  and X 5 . T( 5 ) is calculated during interval ( 5 ), and its value is counted down from the instant X 5  when the series current Is in switch T 1  passes to zero, and so on for the values T( 6 ) and T( 7 ). 
     When starting, for example, the radiological image acquisition, one of switches T 1  or T 2  is systematically caused to be conducting, for instance T 1  In the example of FIG. 4 (curve  40 ). Also, there is carried out a first calculation of the countdown duration T( 1 ), immediately from the starting instant, during the interval ( 1 ). T 1  being counted down from the instant X 1  when the series current Is in switch T 1  passes to zero. 
     From the start of counting down T( 1 ), T( 2 ) is calculated during the interval ( 2 ), which corresponds substantially to the conduction time of diode D 1 . Duration T( 2 ) is counted down from the instant X 2  when the series current Is shown by curve  42  passes to zero. When the countdown of T( 2 ) ends, switch T 1  becomes conducting again. 
     Meanwhile, the countdown duration T( 3 ) is calculated during interval ( 3 ), the countdown of T( 3 ) taking place from the instant X 3  when the series current Is through switch T 1  passes to zero. 
     FIG. 4 d  shows the intervals during which switch T 1  is conducting while FIG. 4 e  shows the intervals during which switch T 2  is conducting. The diagrams of FIG. 4 show that the start sequence of the converter leads to very asymmetrical series currents is in going from one switch to the other. The conduction of the first switch establishes a direct current in the resonant circuit, which unbalances the series current Is and deactivates the parallel current Ip. 
     The drawings of FIGS. 5 a  to  5   e  are analogous to those of FIGS. 4 a  to  4   e , but correspond to an embodiment of the invention. The embodiment of the invention comprises making, e.g., switch T 1  conducting, which yields the curves  40  and  42  for the series current is as well as curve  44  for the parallel current Ip as in FIG. 4 a . However, there is no calculation of the duration of times T( 1 ) and T( 2 ) to switch on switches T 2  and T 1 , respectively. 
     In an embodiment, switch T 2  is set to the on state when the parallel current Ip reaches a maximum positive value MAX at peak  46 . During the time interval ( 1 ′), the time period T( 1 ′) is calculated for the count down starting from the point of passage to zero X′ 1  of the start of switch T 1  being conducting. During the time interval ( 2 ′), the time period T( 2 ′) is calculated for the count down starting from the point of passage to zero X′ 2  of the start of switch T 2  being conducting. During the time interval ( 3 ′), the time period T( 3 ′) is calculated for the count dawn starting front the point of passage to zero X′ 3  of the start of switch T 1  being conducting (curve not shown). 
     To obtain the above-described operation, the control circuit  16  comprises two state arrangements  50  and  60  that are shown schematically in FIGS. 6 and 7, respectively. 
     When off, the two state arrangements  50  and  60  are at the rest state REP for arrangement  50  and LIB for arrangement  60 . 
     Before starting, for example, a radiological image acquisition, the operator carries out a number of settings according to the type of image to be acquired by inputting the corresponding parameters, and then starts the image acquisition by pressing a button. This button triggers the two state arrangements  50  and  60  by a reset to zero signal RAZ that bring them to an initial state. In, for example, a radiological acquisition, there is produced the logic signal P=1 so that arrangement  50  passes to a state R 1  of counting down the time period T(p) while arrangement  60  passes to a state M. 
     When the countdown in completed. T(p)=0, switch T 1  is conducting, so bringing arrangement  60  to a RUN state corresponding to logic signal SQ=1. 
     When the series current is becomes equal to zero, logic signal Ispos=1 while SQ=1, arrangement  50  returns to the rest state REP. The signal of that rest state REP causes arrangement  60  to pass to state SYNIp, referred to as the state of synchronization with the parallel current Ip. 
     This state SYNIp enables the definition of the instant of the maximum value of the parallel current Ip, for instance by counting down a time period DIp corresponding to one quarter of the time period of current Ip. When DIp=0 and SQ=1, arrangement  50  passes to state R 2 . 
     In the general case of a starting sequence; there is no countdown of the time period T(p), i.e. T(p)=0 or that period is fixed, so that switch T 2  is conducting. Arrangement  60  then passes to a DONE state, which signifies the end of the starting sequence. Arrangement  60  then passes to the LIB state at the end of the acquisition when the arrangement  50  returns to the rest state REP at the end of the radiological image acquisition, i.e., when the logic signal P=0 appears, that signal P=0 occurring during the countdown states R 1  or R 2 . 
     An embodiment has been described for a control circuit  16  which first triggers the conducting state of switch T 1 , then that of switch T 2  when the parallel current Ip attains the maximum positive value MAX. However, the embodiment can be implemented with a control circuit that first triggers the conducting state of switch T 2  and then that of switch T 1  but, in this case, the maximum value that is taken into account is the negative value of the parallel current Ip. 
     The invention and embodiments thereof is therefore directed to a double resonance electronic converter comprising: a switching circuit comprising a first switch and a second switch; a double resonance resonant circuit comprising a series resonant circuit and a parallel resonant circuit; a rectifying and filtering circuit, and a control circuit for controlling the switching circuit comprising two arrangements: a first arrangement for controlling states of the switching circuit and a second arrangement for controlling the start of the first arrangement. 
     In an embodiment of the invention, the second arrangement comprises: means for measuring a current Ip in the parallel resonant circuit; and means for triggering first the conduction of the first switch and for triggering thereafter the conduction of the second switch when the parallel current reaches a maximum value. 
     In an embodiment of the invention the instant of the maximum value of the parallel current is determined by a countdown of the duration of one quarter of the time period of the parallel current starting from the passage to zero of the parallel current. 
     In an embodiment of the maximum value of the parallel current is of the same polarity as the series current flowing in the series resonant circuit. 
     One skilled in the art may make or propose various modifications to the function and/or way and/or result of the disclosed embodiments without departing from the scope and extent of protection.

Technology Category: h