Abstract:
The configurations of a bridgeless PFC circuit system and a controlling method thereof are provided. The proposed system includes a bridgeless PFC circuit including a first bridge arm having a first and a second terminals and a first middle point, a second bridge arm having a first and a second terminals and a second middle point, and a bidirectional switch coupled between the first middle point and the second middle point, and an inductor coupled between the first middle point and an AC power source coupled to the second middle point, and a current sensing circuit including a first current transformer sensing a first current flowing through the bidirectional switch, which having a primary side winding coupled to the bidirectional switch and a first and a second secondary side windings, and a switching device coupled to the two secondary side windings.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a current sensing circuit of a bridgeless power factor correction (PFC) circuit system and a controlling method thereof. More particularly, it relates to a current sensing circuit of an H-PFC circuit system and a controlling method thereof. 
     BACKGROUND OF THE INVENTION 
     In the conventional boost PFC circuits, the bridge rectifier loss becomes one of the main losses of the switching power. Following the requirements regarding the convention efficiency are improved, the bridgeless boost topology deriving from the conventional boost PFC circuit has gradually become the focus of the R&amp;D. The bridgeless boost topology omits the bridge rectifier of the boost PFC circuit so that there&#39;re only two diodes in the operational loop, which reduces a diode conducting loss, and increases the efficiency. H-PFC circuit belongs to a kind of bridgeless boost topologies suitable for the medium power and large power applications (please refer to  FIG. 1 ). In  FIG. 1 , the bridgeless PFC circuit includes an inductor L, four diodes D 1 -D 4 , a bidirectional switch comprising two switches Q 1 -Q 2 , and an output capacitor C B , receives the AC input voltage Vin, and generates the DC output voltage Vo. 
       FIGS. 2(   a )- 2 ( b ) are schematic circuit diagrams respectively showing the operational statuses of the positive half-cycle and the negative half-cycle of a period of operational frequency of the H-PFC circuit as shown in  FIG. 1 . The elements included in  FIGS. 2(   a )- 2 ( b ) are the same as those of  FIG. 1 , wherein the driving signals of the switches Q 1  and Q 2  have the same phase. In  FIG. 2(   a ), L, D 1 , D 4 , Q  1 , Q 2  and C B  form a boost circuit when the input voltage Vin is in its positive half-cycle, wherein D 4  and Q 2  are turned on continuously. The current flows through L, Q 1  and Q 2  and then returns when Q 1  turns on. The current flows through L, D 1 , C B , and D 4  and then returns when Q 1  turns off. In  FIG. 2(   b ), L, D 2 , Q 1 , Q 2 , D 3  and C B  form another boost circuit when the input voltage Vin is in its negative half-cycle, wherein Q 1  and D 3  are turned on continuously. The current flows through L, Q 1  and Q 2  and then returns when Q 2  turns on. The current flows through L, D 3 , C B  and D 2  and then returns when Q 2  turns off. The currents flow through L, Q 1  and Q 2  are respectively in reverse directions when they are in the positive and the negative half-cycles. Due to that D 3  and D 4  clamp the AC power source to the output capacitor C B  of the boost circuit, a common mode noise, which is the same as the conventional boost PFC circuit, could be obtained. Since the current flows through only two semiconductor elements during any time in a switch period, the conduction loss is decreased. 
     Keeping the drawbacks of the related arts in mind, and employing experiments and research full-heartily and persistently, the applicants finally conceived a bridgeless power factor correction circuit system having a current sensing circuit and a controlling method thereof. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a bridgeless power factor correction circuit system having a current sensing circuit sensing a current flowing through switches and an inductor of the PFC circuit. The current sensing circuit has the advantages of having simpler electrical circuit configuration and method, lower cost and sensing accurately, can be used in occasions requiring either stronger anti-interference capabilities, or larger power, or higher requirements regarding current signal sampling. 
     According to the first aspect of the present invention, a bridgeless power factor correction (PFC) circuit system comprises a bridgeless PFC circuit comprising a first bridge arm having a first and a second terminals and a first middle point, a second bridge arm having a first and a second terminals and a second middle point, and a bidirectional switch coupled between the first middle point and the second middle point, and an inductor coupled between the first middle point and an AC power source, wherein the AC power source is coupled to the second middle point, and a current sensing circuit comprising a first current transformer (CT) for sensing a first current flowing through the bidirectional switch coupled to the bidirectional switch in series, which having a primary side winding coupled to the bidirectional switch and a first and a second secondary side windings, and a switching device coupled to the first and the second secondary side windings. 
     Preferably, the bridgeless PFC circuit is an H-PFC circuit which comprising: a first and a second input terminals, a first and a second output terminals, the first bridge arm with a first and a second switches and the first middle point, the second bridge arm with a third and a fourth switches and the second middle point; the bidirectional switch comprising a fifth and a sixth switches, each of the first to the sixth switches and the inductor has a first and a second terminals; wherein the first terminal of the first switch and the second terminal of the second switch are coupled to the first middle point, the second terminal of the first switch and the second terminal of the third switch are coupled to the first output terminal, the first terminal of the third switch and the second terminal of the fourth switch are coupled to the second middle point, the first terminal of the second switch and the first terminal of the fourth switch are coupled to the second output terminal, the second terminal of the fifth switch is coupled to the second terminal of the sixth switch, the primary side winding of the first CT is selected to be coupled to the first terminal of the fifth switch or the first terminal of the sixth switch, the first terminal of the inductor is coupled to the first input terminal, the second terminal of the inductor is coupled to the first middle point, and the second middle point is coupled to the second input terminal, and an output capacitor having a first terminal and a second terminal, wherein the first terminal of the output capacitor is coupled to the first output terminal and the second terminal of the output capacitor is coupled to the second output terminal. 
     Preferably, each of the first and the second secondary windings has a first and a second terminals, a first terminal of a first resistor is coupled to the first terminal of the first secondary winding, a second terminal of the first resistor is coupled to the second terminal of the first secondary winding and the first terminal of the second secondary winding, a first terminal of a second resistor is coupled to the first terminal of the second secondary winding, and a second terminal of the second resistor is coupled to the second terminal of the second secondary winding. 
     Preferably, the current sensing circuit further comprises a third resistor, a eighth and a ninth switches, the switching device comprises a seventh switch, each of the third resistor and the seventh to a ninth switches has a first and a second terminals, the first terminal of the first secondary winding is coupled to the second terminal of the eighth switch, the second terminal of the first secondary winding is coupled to the first terminal of the seventh switch, the second terminal of the second secondary winding is coupled to the second terminal of the ninth switch, the first terminal of the eighth switch is coupled to the first terminal of the ninth switch and the first terminal of the third resistor, and the second terminal of the seventh switch is coupled to the second terminal of the third resistor and is grounded. 
     Preferably, each of the first to the fourth and the eighth to the ninth switches is a diode having an anode being the first terminal and a cathode being the second terminal, the fifth to the seventh switches are MOSFETs, the first and the second currents are accumulated to form a current signal flowing through the inductor, and the current signal is a saw wave having a sinusoidal enveloping line. 
     Preferably, the current sensing circuit further comprises a second CT having a primary side winding coupled to the bridgeless PFC circuit for sensing a second current flowing through the inductor. 
     Preferably, the bridgeless PFC circuit is an H-PFC circuit which comprising: a first and a second input terminals, a first and a second output terminals, the first bridge arm with a first and a second switches and the first middle point, the second bridge arm with a third and a fourth switches and the second middle point; the bidirectional switch comprising a fifth and a sixth switches, each of the first to the sixth switches and the inductor has a first and a second terminals; wherein the first terminal of the first switch and the second terminal of the second switch are coupled to the first middle point, the second terminal of the first switch is coupled to the second terminal of the third switch, the first terminal of the third switch and the second terminal of the fourth switch are coupled to the second middle point, the first terminal of the second switch and the first terminal of the fourth switch are coupled to the second output terminal, the second terminal of the fifth switch is coupled to the second terminal of the sixth switch, the primary side winding of the first CT is selected to be coupled to the first terminal of the fifth switch or the first terminal of the sixth switch, the first terminal of the inductor is coupled to the first input terminal, the second terminal of the inductor is coupled to the first middle point, and the second middle point is coupled to the second input terminal; and an output capacitor having a first terminal and a second terminal, wherein the first terminal of the output capacitor is coupled to the first output terminal and the second terminal of the output capacitor is coupled to the second output terminal. 
     Preferably, the primary side winding of the second CT has a first terminal and a second terminal, the first terminal of the primary side winding of the second CT is coupled to the first terminal of the output capacitor and the first output terminal, the second terminal of the primary side winding of the second CT is coupled to the second terminal of the first switch and the second terminal of the third switch; and wherein each of the first and the second secondary windings of the first CT has a first and a second terminals, a first terminal of a first resistor is coupled to the first terminal of the first secondary winding of the first CT, a second terminal of the first resistor is coupled to the second terminal of the first secondary winding of the first CT and the first terminal of the second secondary winding of the first CT, a first terminal of the a resistor is coupled to the first terminal of the second secondary winding of the first CT, and a second terminal of the second resistor is coupled to the second terminal of the second secondary winding of the first CT, 
     Preferably, the current sensing circuit further comprises a third resistor, a eighth and a ninth switches, the switching device comprises a seventh switch, each of the third resistor and the seventh to a ninth switches has a first and a second terminals, the first terminal of the first secondary winding of the first CT is coupled to the second terminal of the eighth switch, the second terminal of the first secondary winding is coupled to the first terminal of the seventh switch, the second terminal of the second secondary winding of the first CT is coupled to the second terminal of the ninth switch, the first terminal of the eighth switch is coupled to the first terminal of the ninth switch and the first terminal of the third resistor, the second terminal of the seventh switch is coupled to the second terminal of the third resistor and is grounded, the second CT further comprises a secondary side winding having a first and a second terminals, the first terminal of the secondary side winding of the second CT is coupled to the second terminal of the third resistor, the current sensing circuit further comprises a tenth switch having a first and a second terminals, the second terminal of the secondary side winding of the second CT is coupled to the second terminal e of the tenth switch, and the first terminal of the tenth switch is coupled to the first terminal of the third resistor. 
     Preferably, each of the first to the fourth and the eighth to the tenth switches is a diode having an anode being the first terminal and a cathode being the second terminal, the fifth to the seventh switches are MOSFETs, the first and the second currents are accumulated to form a current signal flowing through the inductor, and the current signal is a triangle wave having a sinusoidal enveloping line. 
     Preferably, the current sensing circuit further comprises a third resistor, a ninth and a tenth switches, the switching device comprising a seventh and a eighth switches, each of the third resistor and the seventh to the tenth switches has a first and a second terminals, the first terminal of the first secondary winding of the first CT is coupled to the first terminal of the ninth switch, the second terminal of the first secondary winding of the first CT is coupled to the second terminal of the third resistor, the second terminal of the second secondary winding of the first CT is coupled to the first terminal of the tenth switch, the second terminal of the ninth switch is coupled to the first terminal of the seventh switch, the second terminal of the tenth switch is coupled to the first terminal of the eighth switch, the second terminal of the seventh switch is coupled to the second terminal of the eighth switch and the first terminal of the third resistor, and the second terminal of the third resistor is grounded. 
     Preferably, each of the first to the fourth and the ninth to the tenth switches is a diode having an anode being the first terminal and a cathode being the second terminal, and the fifth to the eighth switches are MOSFETs. 
     According to the second aspect of the present invention, a controlling method for sensing current flowing through a bidirectional switch and an inductor of a bridgeless power factor correction (PFC) circuit system comprises the steps of (a) sensing a first current flowing through a first loop, wherein the first loop is formed from a first current transformer (CT), the bidirectional switch and the inductor connected in series when the bidirectional switch is turned on; and (b) sensing a second current flowing through the inductor when the bidirectional switch is turned off. 
     Preferably, the bridgeless power factor correction (PFC) circuit system further comprises a first to a fourth switches and an output capacitor, wherein the controlling method further comprises a step of (c) providing an AC input voltage at a positive half-cycle; and wherein sensing the second current flowing through the inductor of step (b) is accomplished by sensing a current flowing through a second loop, wherein the second loop is formed from the inductor, the first switch, the output capacitor and the fourth switch connected in series when the bidirectional switch is turned off. 
     Preferably, the second loop further comprises a second current transformer connected in series for sensing the second current flowing through the second loop. 
     Preferably, the controlling method comprises the steps of (a) providing an AC input voltage at a positive half-cycle; (b) sensing a first current flowing through a first loop, wherein the first loop is formed from a first current transformer (CT), the bidirectional switch and the inductor connected in series when the bidirectional switch is turned on; and (c) sensing a second current flowing through the inductor when the bidirectional switch is turned off; and wherein the step (b) comprises: (b1) providing a duty ratio of a secondary side of the first CT over a primary side of the first CT being n, wherein the third resistor has a resistance of R 3 ; (b2) causing a sampling current flowing through the third resistor to be equal to the first current*(−n)*R 3  when the bidirectional switch is turned on; and (b3) obtaining the first inductor current via the sampling current flowing through the third resistor. 
     Preferably, sensing the second current flowing through the inductor of step (c) is accomplished by sensing a current flowing through a second loop, wherein the second loop is formed from a second current transformer, the inductor, the first switch, the output capacitor and the fourth switch connected in series when the bidirectional switch is turned off, the first and the second currents are accumulated to form a current signal flowing through the inductor, and the current signal is a saw wave having a sinusoidal enveloping line. 
     Preferably, the controlling method comprises the steps of (a) providing an AC input voltage at a positive half-cycle; (b) sensing a first current flowing through a first loop, wherein the first loop is formed from a first current transformer (CT), the bidirectional switch and the inductor connected in series when the bidirectional switch is turned on; and (c) sensing a second current flowing through the inductor when the bidirectional switch is turned off; and wherein the step (b) comprises: (b1) providing a duty ratio of a secondary side of the first CT over a primary side of the first CT being n, wherein the third resistor has a resistance of R 3 ; (b2) causing a sampling current flowing through the third resistor to be equal to the first current*(−n)*R 3  when the bidirectional switch are turned on; and (b3) obtaining the first inductor current via the sampling current flowing through the third resistor. 
     Preferably, sensing the second current flowing through the inductor of step (c) is accomplished by sensing a current flowing through a second loop, wherein the second loop is formed from a second current transformer, the inductor, the first switch, the output capacitor and the fourth switch connected in series when the bidirectional switch is turned off, the first and the second currents are accumulated to form a current signal flowing through the inductor, and the current signal is a saw wave having a sinusoidal enveloping line. 
     The present invention may best be understood through the following descriptions with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit diagram of an H-PFC circuit in the related art; 
         FIGS. 2(   a )- 2 ( b ) are schematic circuit diagrams respectively showing the operational statuses of the positive half-cycle and the negative half-cycle of a period of operational frequency of the input voltage of the H-PFC circuit as shown in  FIG. 1 ; 
         FIGS. 3(   a )- 3 ( b ) are circuit diagrams respectively showing the main circuit and the current sensing circuit of the H-PFC circuit system according to the first preferred embodiment of the present invention; 
         FIG. 3(   c ) is a waveform diagram of a saw wave having a sinusoidal enveloping line when a current sensing circuit as shown in  FIG. 3(   b ) is used to sense a current flowing through Q 1  and Q 2  of the H-PFC circuit as shown in  FIG. 3(   a ); 
         FIG. 4  is an operational waveform diagram of a positive half-cycle of a period of operational frequency of an AC input voltage of the main circuit and the current sensing circuit of H-PFC circuit according to the first preferred embodiment of the present invention; 
         FIGS. 5(   a ) and  5 ( b ) are respectively showing schematic circuit diagrams of the main circuit and the current sensing circuit as shown in  FIG. 3(   a ) in a first operational stage of positive half-cycle of a period of operational frequency of an AC input voltage; 
         FIGS. 5(   c ) and  5 ( d ) are respectively showing schematic circuit diagrams of the main circuit and the current sensing circuit as shown in  FIG. 3(   b ) in a second operational stage of positive half-cycle of a period of operational frequency of an AC input voltage; 
         FIGS. 6(   a )- 6 ( b ) are circuit diagrams respectively showing the main circuit and the current sensing circuit of the H-PFC circuit system according to the second preferred embodiment of the present invention; 
         FIG. 6(   c ) is a waveform diagram of a triangle wave having a sinusoidal enveloping line when a current sensing circuit as shown in  FIG. 6(   b ) is used to sense a current flowing through Q 1  and Q 2  of the H-PFC circuit as shown in  FIG. 6(   a ); 
         FIG. 7  is an operational waveform diagram of a positive half-cycle of a period of operational frequency of an AC input voltage of the main circuit and the current sensing circuit of H-PFC circuit according to the second preferred embodiment of the present invention; 
         FIGS. 8(   a )- 8 ( b ) are respectively showing schematic circuit diagrams of the main circuit and the current sensing circuit of H-PFC circuit in a second operational stage of positive half-cycle of a period of operational frequency of an AC input voltage according to the second preferred embodiment; and 
         FIG. 9  is a circuit diagram of the current sensing circuit of the H-PFC circuit system according to the third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 3(   a )- 3 ( b ) are circuit diagrams respectively showing the main circuit and the current sensing circuit of the H-PFC circuit system according to the first preferred embodiment of the present invention. The main circuit employing this current sensing circuit is shown in  FIG. 3(   a ). The difference between  FIG. 3(   a ) and  FIG. 1  is that a current transformer (CT) CT 1  is added. This first preferred embodiment employs a current sensing circuit as shown in  FIG. 3(   b ) to sense a current flowing through the bidirectional switch comprising two switches Q 1  and Q 2  of the H-PFC circuit as shown in  FIG. 3(   a ). In other embodiments, other types of bidirectional switch which can be controlled to achieve bidirectional switching function may be used, for example, a bridge type bidirectional switch. The current sensing circuit as shown in  FIG. 3(   b ) is employed to sense a saw wave having a sinusoidal enveloping line, and the saw wave is shown in  FIG. 3(   c ). Except for the first secondary winding CT 1 _ 1  and the second secondary winding CT 1 _ 2  of the secondary side of the CT CT 1 , this current sensing circuit further includes R 1 , R 2 , D 5 , D 6 , R 3  and Q 3 , and the driving signal of Q 3  is the same as the driving signals of Q 1  and Q 2 .  FIG. 4  shows an operational waveform diagram of a positive half-cycle of a period of operational frequency of an AC input voltage of the main circuit and the current sensing circuit of H-PFC circuit according to the first preferred embodiment of the present invention. The gate driving signals of Q 1 , Q 2  and Q 3  are the waveforms of gate driving signals of the switches Q 1 , Q 2  and Q 3  (they are all MOSFETs or IGBTs). iL is the current flowing through the inductor, and I 1  is the current flowing through switches Q 1  and Q 2 , i.e. the current flowing through the primary side of CT 1 . 
       FIGS. 5(   a ) and  5 ( b ) respectively show schematic circuit diagrams of the main circuit and the current sensing circuit as shown in  FIG. 3(   a ) and  FIG. 3(   b ) in a first operational stage of positive half-cycle of a period of operational frequency of the input voltage.  FIGS. 5(   c ) and  5 ( d ) are respectively showing schematic circuit diagrams of the main circuit and the current sensing circuit as shown in  FIG. 3(   a ) and  FIG. 3(   b ) in a second operational stage of positive half-cycle of a period of operational frequency of an AC input voltage.  FIGS. 5(   a ) to  5 ( d ) show concrete analyses of operational statuses of circuits as shown in  FIGS. 3(   a ) and  3 ( b ). Analyses of operational statuses of circuits in the negative half-cycle of the input voltage etc. are similar to those of the positive half-cycle of the input voltage. n is the turns ratio of the secondary side over the primary side of the CT 1  (the primary side of the CT is located on the main circuit side, and the secondary side is located on the sensing circuit side). 
     As shown in  FIGS. 5(   a )- 5 ( b ), in the first operational stage of the circuit, three switches Q 1 , Q 2  and Q 3  are turned on simultaneously, the current flows through L, Q 1 , Q 2  and CT 1 , and then returns. During this period of time, the primary side current of CT 1  is I L , the secondary side current of CT 1  is nI L , the secondary side current of CT 1 _ 1  flows through Q 3 , R 3  and D 5  and then returns, the secondary side current of CT 1 _ 2  flows through R 2  and then returns, therefore, the voltage across the sampling resistor R 3  is −nI L R 3 . 
     As shown in  FIGS. 5(   c )- 5 ( d ), it is the second operational stage of the H-PFC circuit. After switches Q  1 , Q 2  and Q 3  are turned off simultaneously, the current flows through L, D 1 , CB and D 4 , and then returns. During this period of time, due to Q 3  is turned off, the secondary windings of CT 1 : the first secondary winding CT 1 _ 1  and the second secondary winding CT 1 _ 2  are separated from the sampling resistor R 3 , and CT 1  has no effect in the circuit. Thus, the current flows through the main power elements Q 1  and Q 2  could be sensed, and could be accurately sensed without the phase detection. This method uses just one CT 1  and has relatively simpler configuration of circuit. 
     For obtaining the waveforms of current flowing through the inductor, another CT (CT 2 ) is required to be added. CT 2  forms another current sampling circuit between the rectifying bridge D 1 , D 2 , D 3  and D 4 , and the capacitor C B  as shown in  FIG. 6(   b ), which is the second preferred embodiment of the present invention, and the main circuit of which is shown in  FIG. 6(   a ). This current signal sampling circuit samples the triangle wave having a sinusoidal enveloping line, and the triangle wave current signal is shown in  FIG. 6(   c ). When switches Q 1 , Q 2  and Q 3  are turned on simultaneously as shown in  FIGS. 5(   a )- 5 ( b ), it is the same as the first preferred embodiment. After the MOSFETs Q 1 , Q 2  and Q 3  are turned off simultaneously, the current flows through L, D 1 , CT 2 , CB and D 4 , and then returns. During this period of time, the current of CT 2  is I L , the current of secondary side of CT 2  is −nI L , and the current flowing through the secondary side of CT 2  flows through R 3  and D 7  and then returns, thus the voltage across the sampling resistor is −nI L R 3  as shown in  FIG. 6(   b ). The current signals during the two time periods, wherein switches Q 1 , Q 2  and Q 3  are turned on and turned off respectively, are accumulated to form the current signal flowing through the inductor. This way, the current flowing through the inductor could be sensed by only using the two CTs CT 1  and CT 2 . 
       FIG. 7  shows an operational waveform diagram of a positive half-cycle of a period of operational frequency of an AC input voltage of the main circuit and the current sensing circuit of H-PFC circuit according to the second preferred embodiment of the present invention.  FIGS. 8(   a )- 8 ( b ) are respectively showing schematic circuit diagrams including current paths of the main circuit and the current sensing circuit of H-PFC circuit in a second operational stage of positive half-cycle of a period of operational frequency of an AC input voltage according to the second preferred embodiment. In  FIG. 7 , I 1 +I 2  is the waveform showing the accumulated current signals during the two time periods, wherein switches Q 1 , Q 2  and Q 3  are turned on and turned off respectively. 
     The switch Q 3  as shown in  FIG. 3(   b ) of the present invention could be replaced by two switches Q 4  and Q 5  as configured and shown in  FIG. 9 .  FIG. 9  is a circuit diagram of the current sensing circuit of the H-PFC circuit system according to the third preferred embodiment of the present invention, wherein the anode of the diode D 5  is coupled to the first terminal of the resistor R 1 , the cathode of the diode D 5  is coupled to the first terminal of the switch Q 4 , the anode of the diode D 6  is coupled to the second terminal of the resistor R 2 , the cathode of the diode D 6  is coupled to the first terminal of the switch Q 5 , the second terminals of Q 4  and Q 5  are both coupled to the first terminal of the resistor R 3 , and these are the differences between  FIG. 9  and  FIG. 3(   b ). In this way, the anti-interference capabilities are much stronger, can be used in occasions requiring either larger power, or higher requirements regarding current signal sampling (e.g., digital control), but phase detection is required. 
     The present invention employs the above-mentioned current sensing circuits, there is no need to increase the control circuit, the triangle wave and saw wave both having the sinusoidal enveloping lines can be sampled. This method is simple, has low cost and is capable of sensing accurately. 
     According to the aforementioned descriptions, the present invention provides a bridgeless power factor correction circuit system having a current sensing circuit sensing a current flowing through switches and an inductor of the PFC circuit. The current sensing circuit has the advantages of having simpler electrical circuit configuration and method, having lower cost and being capable of sensing accurately, can be used in occasions either requiring stronger anti-interference capabilities, or requiring larger power, or having higher requirements regarding current signal sampling. 
     While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the present invention which is defined by the appended claims.