Patent Document

PRIORITY CLAIM 
       [0001]    This application claims the priority benefit of French Patent application number 1360660, filed on Oct. 31, 2013, the contents of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
       TECHNICAL FIELD 
       [0002]    The present disclosure generally relates to electronic circuits and, more specifically, to a control circuit for field-effect diodes assembled in a half-bridge. The present disclosure more specifically applies to the forming of a converter in the form of a switched-mode power supply. 
       BACKGROUND 
       [0003]    Switched-mode power supplies use, in particular at the secondary, free wheel diodes which are most often assembled with a common electrode (anode or cathode), and thus in a half-bridge. 
         [0004]    In the forming of a power converter, it is often difficult to find an acceptable compromise between the voltage across the diodes in the on state and the off-state leakage current. 
       SUMMARY 
       [0005]    An embodiment overcomes all or part of the disadvantages of assemblies using diodes assembled in a half-bridge. 
         [0006]    Another embodiment provides a half-bridge diode assembly with an improved compromise between the forward voltage drop and the leakage current. 
         [0007]    Another embodiment uses field-effect diodes comprising a diffusion pocket in the substrate. 
         [0008]    An embodiment provides a circuit comprises: a first field-effect transistor assembled as a first diode and provided with drain, source, and gate electrodes, as well as with an additional electrode; a second field-effect transistor assembled as a second diode and provided with drain, source, and gate electrodes as well as with an additional electrode; a first switch connecting the gate of the first transistor to its drain; a second switch connecting the gate of the second transistor to its drain; and a circuit for controlling the first and second switches. 
         [0009]    According to an embodiment: the additional electrode of the first transistor is further directly connected to the gate of the second transistor; and the additional electrode of the second transistor is further directly connected to the gate of the first transistor. 
         [0010]    According to an embodiment, the two switches are controlled to be simultaneously off in a phase where one of the diode risks, under the effect of the connection of its gate to the additional electrode of the other diode, being on while it is reverse biased. 
         [0011]    According to an embodiment, the diodes are interconnected by their anodes. 
         [0012]    According to an embodiment, the diodes are interconnected by their cathodes. 
         [0013]    According to an embodiment, each additional electrode contacts a diffusion pocket in a substrate. 
         [0014]    According to an embodiment, each additional electrode contacts an insulating layer. 
         [0015]    An embodiment also provides a power converter of switched-mode power supply type comprising, at the secondary of a transformer, at least one inductive element and one capacitive element, and a circuit such as described hereabove. 
         [0016]    According to an embodiment, the first diode connects a reference terminal of a voltage provided by the converter to an electrode of a winding of the secondary of the transformer, the second diode connecting this reference terminal to the other terminal of this winding of the secondary. 
         [0017]    According to an embodiment, said control circuit is formed of a flip-flop having an inverted output controlling said switches formed of MOS transistors, and having its set and reset terminals receiving control signals from a circuit for controlling the power converter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
           [0019]      FIG. 1  is a simplified representation of a power converter formed of a switched-mode power supply of voltage-step down type (forward type); 
           [0020]      FIG. 2  is a simplified representation of an example of field-effect diodes with a diffusion pocket in the substrate, assembled in a half-bridge; 
           [0021]      FIG. 3  is a simplified cross-section view of a structure reproducing the diagram of  FIG. 2 ; 
           [0022]      FIG. 3A  is a partial simplified cross-section view of another structure reproducing the diagram of  FIG. 2 ; 
           [0023]      FIG. 4  is a simplified representation of the pair of field-effect diodes of  FIG. 3 ; 
           [0024]      FIG. 5  shows an embodiment of a circuit for controlling the pair of diodes of  FIG. 4 ; 
           [0025]      FIG. 6  illustrates an example of assembly of the circuit of  FIG. 5  at the secondary of a forward-type power converter; and 
           [0026]      FIGS. 7A ,  7 B,  7 C,  7 D,  7 E, and  7 F illustrate, in the form of timing diagrams, the operation of the circuit of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those steps and elements which are useful to the understanding of the embodiments which will be described have been shown and will be detailed. In particular, the control of a power converter of switched-mode power supply type has not been detailed, the described embodiments being compatible with the usual forming of such power converters. Further, the forming of field-effect diodes with a diffusion pocket in the substrate has not been detailed. To form such diodes, reference is made to U.S. Pat. Nos. 8,148,748 and 8,421,118, and also to United States Patent Application Publication Nos. 2009/0267111, 2010/0271851 and 2011/0051305, the contents of which are hereby incorporated by reference to the extent allowable by the law, may be used as a guideline. Further, the applications for which the control circuit and the power converter may be intended have not been detailed, the described embodiments being here again compatible with usual applications of such converters. 
         [0028]      FIG. 1  very schematically shows an example of a power converter of forward switched-mode power supply type, for example, of voltage step-down type. It should however be noted that this type of converter may also be used as a voltage step-up converter. Its operation is based on a galvanic isolation (transformer  12 ) between a primary  2  and a secondary  3  and a switching of a D.C. voltage Vdc applied between two terminals  21  and  23  for powering the primary. In the shown example, an inductive element L 1  forming the primary of transformer  12  is connected, via switches, respectively K 25 , K 27 , to terminals  21  and  23 . Switches K 25  and K 27  have the function, under control of a circuit  24  (CTRL) of switching D.C. voltage Vdc, such a switching being transmitted by transformer  12  to its secondary  3 . Two diodes D 26  and D 28  connect respective junction nodes  25  and  27  of winding L 1  and switches K 25  and K 27  to terminals  23  and  21 , the anode of diode D 26  being on the side of terminal  23  and the cathode of diode D 28  being on the side of node  21 . The function of circuit  24  is to cause the turning-on of switches K 25  and K 27  at a relatively high frequency (in the range from some ten to some hundred kilohertz). At the secondary of transformer  12 , an inductive winding L′ associated with a capacitive element C and with a rectifying half-bridge formed of diodes D 32  and D 34  is used to rectify and to smooth the voltage recovered across inductive winding L 2  forming the transformer secondary, to provide a D.C. output voltage Vout between terminals  31  and  33 . Element L′ connects a first terminal  35  of secondary winding L 2  to terminal  31 . Capacitive element C connects terminals  31  and  33 . Diode D 32  connects a second terminal  37  of winding L 2  to terminal  33 , its anode being on the side of terminal  33 . Finally, diode D 34  connects first terminal  35  of winding L 2  to terminal  33 , its anode being on the side of terminal  33 . In  FIG. 1 , a load  4  (Q) intended to be connected to terminals  31  and  33  in order to be powered by voltage Vout has been illustrated. 
         [0029]    The operation of a switched-mode power supply such as illustrated in  FIG. 1  is usual. Most often, circuit  24  adapts the duty cycle of the on periods of switches K 25  and K 27  according to the power needs of load  4 . 
         [0030]    A problem associated with the use of diodes is that they generate conduction losses due to their on-state voltage drop. 
         [0031]    A type of field-effect diodes based on a vertical MOS transistor structure has recently been developed, which enables to improve the compromise between the forward voltage drop (due to the on-state drain-source resistance and to a potential barrier created by the gate-source connection) and the reverse leakage current. Ideally, it is desired for these two quantities to be minimal. 
         [0032]    This type of field-effect diodes comprises, in addition to the usual source, drain, gate, and substrate electrodes (in the MOS sense of the term), an additional electrode, called pocket electrode, connected either to a diffusion pocket taking up a portion of the space separating two MOS transistor channels, or to a polysilicon layer deposited on the gate oxide above this same space, to form a capacitance therewith. 
         [0033]    In the diode mode operation, the source, gate, bulk (in the MOS sense of the term), and pocket electrodes are interconnected, and there thus only remain two separate electrodes, the source and drain electrodes. 
         [0034]    In the case of a half-bridge assembly, only the source and substrate electrodes are interconnected. The gate electrode of one of the diodes of half-bridge is connected to the pocket electrode of the other diode, and conversely. 
         [0035]      FIG. 2  is a simplified representation of an embodiment of diodes D 32  and D 34  using a technology of field-effect diodes with diffusion pockets connected to the substrate. 
         [0036]      FIG. 3  is a simplified example of structure of diodes D 32  and D 34  according to the diagram of  FIG. 2 . 
         [0037]      FIGS. 2 and 3  are inspired from the embodiments described in document U.S. Pat. No. 8,421,118. 
         [0038]    In this example, the diodes are formed by using a vertical MOS transistor technology in an N-type wafer  40  (N−). Regions of wafer  40  each defining diodes are separated, generally by sawing of the wafer where they are formed, the resin of the packages (not shown) isolating the diodes from one another. 
         [0039]    Each transistor forming one of the diodes comprises four electrodes: 
         [0040]    a source electrode S 1 , respectively S 2 , contacting an N-doped source region S 1 ′, respectively S 2 ′, (N++), as well as a P-doped so-called bulk region (in the MOS sense of the term)  43  (P+); 
         [0041]    a drain electrode D 1 , respectively D 2 , contacting a drain region D 1 ′, respectively D 2 ′, of type N (N++); 
         [0042]    a gate electrode G 1 , respectively G 2 , contacting a polysilicon layer  47  deposited on an insulating layer  45  (gate oxide) and controlling the conductivity of a narrow channel  41 , respectively  42 , in a P-type region (P+); and 
         [0043]    an additional pocket electrode P 1 , respectively P 2 , contacting a diffusion pocket P 1 ′, respectively P 2 ′, in this example, of type P (P++), in wafer  40 . 
         [0044]    A function of the pocket electrode is to extract a signal which may be used as a gate control signal for the neighboring transistor forming the other diode. 
         [0045]    In the example of  FIG. 3 , ring-shaped concentric gates and sources are considered, the pocket electrodes being at the center. 
         [0046]      FIG. 3A  illustrates an embodiment where the diffusion pocket is replaced with a capacitor.  FIG. 3A  only illustrates the left-hand portion of the structure (diode D 32 ). The same modification is performed on the other diode (D 34 ). 
         [0047]    As compared with the embodiment of  FIG. 3 , insulating layer  45  (gate oxide) here extends under additional electrode P 1 , which contacts this insulating layer with a pad in polysilicon layer  47 . 
         [0048]    When the structure, for example, D 32 , is in the conductive state, the voltage drop between source S 1 ′ and drain D 1 ′ is small, generating a low signal at the level of pocket P 1 ′. In the non-conductive state, the depleted area changes with the voltage applied between the source and the drain. For a low applied voltage, the depletion area is located between pocket P 1 ′ and the source and typically ends under the gate. In this state, pocket P 1 ′ can be considered as shorted with the drain, and the voltage of pocket electrode P 1  follows that of drain electrode D 1 . However, for higher applied voltages, the depletion area extends under pocket P 1 ′. The pocket voltage is then substantially constant and independent from the drain voltage. The signal present on pocket electrode P 1  can thus be used as a signal indicative of the conductive or non-conductive state of structure D 32 . 
         [0049]    To form a diode half-bridge, gate G 1  of the transistor forming diode D 32  is connected to pocket P 2  of structure D 34  while gate G 2  thereof is connected to pocket P 1  of structure D 32 . 
         [0050]    In the example of an embodiment based on N-channel transistors such as illustrated in  FIG. 3 , sources S 1  and S 2  are interconnected to form the common anode of diodes D 32  and D 34  (A) while respective drains D 1  and D 2  define the respective cathodes K 1  and K 2  of diodes D 32  and D 34 . Such an interconnection enables to take advantage of the electric characteristics of structures with diffusion pockets and in particular to make the compromise between the forward voltage and the leakage current easier. In other words, for a given leakage current, the forward voltage of a diode of the half-bridge, when its gate is biased by the pocket electrode of the other half-diode of the half-bridge, itself reverse-biased, will be significantly lower than that of a usual diode, including when it is formed from the same structure operating in simple diode mode (shorted source, gate, and diffusion pocket). 
         [0051]      FIG. 4  is a simplified representation of diodes D 32  and D 34 , connected in the same way as in  FIGS. 2 and 3 . The diodes have been specifically symbolized in a way showing their respective pocket electrodes P 1  and P 2  and gate electrodes G 1  and G 2  and their cross-connections. 
         [0052]    In an assembly of the type in  FIG. 1 , the respective conductive and non-conductive states of the diodes only depend on the voltages applied thereacross (anode and cathode). 
         [0053]    There thus are four possible states for these two diodes. A state where cathode-anode voltages V 1  and V 2  are both positive. A state where cathode-anode voltages V 1  and V 2  are both negative. A state where voltage V 1  is positive and voltage V 2  is negative. A state where voltage V 1  is negative and voltage V 2  is positive. 
         [0054]    Assuming a positive voltage V 1  and a negative voltage V 2 , diode D 32  is on and diode D 34  is off. The fact for pocket electrode P 2  to be connected to gate G 1  improves (decreases) the forward voltage drop. The voltage of pocket P 1  is very low since its barrier height has been lowered by the biasing transferred from the pocket of the other half-diode. Accordingly, the reverse diode will have a gate voltage substantially equal to its source voltage and the leakage current will thus be equivalent to that of a diode having its gate and source electrodes shorted (which corresponds to the conventional structure). 
         [0055]    The reverse state corresponds to a positive voltage V 2  across diode D 34  and a negative voltage V 1  across diode D 32 . Diode D 34  is then conductive and diode D 32  is non-conductive. Like for the above case, the non-conductive diode has a pocket voltage which lowers the barrier height of the conductive diode, which thus has a lower forward voltage. 
         [0056]    In the case where the two diodes are forward biased (voltages V 1  and V 2  positive), their pocket voltage remains close to their source voltage, which provides again the configuration where the gate and source are at the same voltage (which corresponds to a conventional structure). Forward voltages are thus not improved in this embodiment. 
         [0057]    In the case where both diodes are reverse biased (voltages V 1  and V 2  negative), they both have high pocket voltages, which will strongly lower the barrier height of the diodes. Said diodes will thus have a significant leakage current, or even a conductive-type behavior although the voltages are reverse. This case is a problem since the diodes may lose their rectifying function. 
         [0058]      FIG. 5  is an electric diagram of an embodiment of a half-bridge assembly of two diodes D 32  and D 34  of the type illustrated in  FIGS. 2 to 4 . 
         [0059]    It is provided to interpose, on the one hand between gate G 1  belonging to diode D 32  and the ground (terminal  33 ), and on the other hand between gate G 2  belonging to diode D 34  and this same ground  33 , switches  54  and  56  controlled by a circuit  52  (CTR) which will be described hereafter in one of its embodiments. Circuit  52  has the function of causing the turning-on of switches  54  and  56  to ground the gates and the diffusion pockets of the MOS transistors forming the diodes. Switches  54  and  56  are turned on at the same time since an issue is raised in the circuit of  FIG. 4  when both diodes are to be non-conductive. 
         [0060]    For example, circuit  52  receives one or a plurality of control signals CT from the outside, typically from power converter control circuit  24 . A galvanic isolation then has to be provided. 
         [0061]    According to another example, circuit  52  integrates circuits for measuring voltages V 1  and V 2  capable of generating its own control signals. Circuit  52  then comprises two input terminals connected to cathodes K 1  and K 2 . 
         [0062]      FIG. 6  shows an example of assembly of controlled half-bridge  5  of  FIG. 5  at the secondary of a converter of the type in  FIG. 1 . For simplification, the elements connected upstream of primary L 1  of transformer  12  have not been illustrated in  FIG. 6 . At the secondary, one can find elements C, L′, D 32 , and D 34  and, according to this embodiment, a control circuit  52  associated with switches  54  and  56  such as described in relation with  FIG. 5 . 
         [0063]    In the example of  FIG. 6 , circuit  52  is formed of an RS flip-flop having its inverted output NQ connected to the control electrodes of switches  54  and  56  and having its respective set (1) and reset (0) inputs receiving signals SET, RESET, generated from a measurement of voltages V 1  and V 2 . As a specific embodiment, switches  54  and  56  are made in the form of N-channel MOS transistors. 
         [0064]      FIGS. 7A ,  7 B,  7 C,  7 D,  7 E, and  7 F are timing diagrams illustrating the operation of half-bridge  5  of  FIG. 6  when it operates in discontinuous conduction mode, and respectively show examples of shapes of signals SET and RESET as well as corre-sponding examples of shapes of voltages V 1  and V 2  across diodes D 32  and D 34  and currents I 1  and I 2  crossing these diodes (from anode to cathode). 
         [0065]    A first period T 1  starting at a time t0, during which diode D 34  is non-conductive while diode D 32  is conductive, is assumed (voltage V 2  positive and voltage V 1  equal to zero). In this situation, signal SET is activated (high state), which forces the turning-off of switches  54  and  56 . This is the normal operation of field-effect diodes with a diffusion pocket such as illustrated in relation with  FIGS. 2 to 4 . Current I 1  in diode D 1  increases until a time t 1  when the primary of transformer  12  is switched (turning-off of switches K 25  and K 27 ,  FIG. 1 ). 
         [0066]    At time t 1 , this turning on of the primary inverts the flow direction at the secondary (free wheel operation) and causes the flowing of a current through diode D 34 . Diode D 32  is non-conductive (its voltage V 1  being at a maximum level). At the end of a period T 2 , starting at time t 1  and stopping at a time t 2 , signal RESET is switched to state  1  to force the turning-on of switches  54  and  56  and to then take back to ground the gates of the transistors of diodes D 32  and D 34 . 
         [0067]    Between times t 1  and t 2 , signal SET and signal RESET are in the low state. Switches  54  and  56  however remain off. 
         [0068]    The turning-on of switches  54  and  56  ascertains that between times t 3  and t′ 0  (of beginning of the next cycle), while the current in diode D 34  has disappeared, both diodes D 32  and D 34  are effectively non-conductive. During period T 3 , the voltage thereacross strongly depends on the forward converter operating mode. In a discontinuous operating mode, which generally occurs for a small load, current I 2  becomes zero before time t 0 ′ and voltages V 1  and V 2  oscillate around voltage Vout if the diodes are non-conductive. In a continuous operating mode, which generally takes place for a high load, current I 2  does not become zero before time t 0 ′ and there thus is no reverse voltage across the diodes, since they are conducting. The same operation is repeated from time t 0 ′ for a next cycle. 
         [0069]    An advantage of the described embodiments is that the use of diffusion pocket diodes provides a gain in performance over conventional diodes, as well as a gain in bulk. 
         [0070]    Another advantage is that the provided switching is particularly simple to achieve and avoids the parasitic conduction phenomenon. Advantage is thus taken from this new type of field-effect diodes while preserving the operation of a switched-mode power supply. 
         [0071]    The generation of signals SET and RESET and their respective synchronizations depends on the type of application having the half-bridge inserted therein. In practice, according to the application, the voltages present and their expected variations area are analyzed to provide an adapted generation of signals SET and RESET in a design phase. 
         [0072]    Various embodiments and variations have been described. Such embodiments and variation may of course be combined. In particular, although the embodiments have been described in relation with an example of diodes having common anodes (formed based on N-channel field-effect transistors), a diode bridge with common cathodes may also be formed by forming the diodes by means of P-channel transistors. Further, the sizing of the components depends on the application as well as on the generation of signals SET and RESET. Finally, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the functional indications given hereabove. 
         [0073]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.

Technology Category: h