Abstract:
An electrical network configured to suppress voltage transients includes a capacitor and an electrical impedance in parallel with a diode. The capacitor is in series with the parallel connected diode and electrical impedance. The electrical network is configured to suppress voltage transients occurring across the series combination of the capacitor and the parallel connected diode and electrical impedance.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. Patent Application No. 14/169,555, filed Jan. 31, 2014, the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to Z source networks, and more particularly to circuits for snubbing transients in Z source networks. 
       BACKGROUND OF THE INVENTION 
       [0003]      FIG. 1  is a block diagram of an electronic system  10 . Electronic system  10  includes a DC source  2  which provides power to load  6  through Z source network  4  and converter/inverter  5 . Converter/inverter  5  is controlled by controller  8 . 
         [0004]    DC (Direct Current) source  2  may be a current or a voltage source. For example, DC source  2  may include one or more of a battery, a diode rectifier, a thyristor converter, a fuel cell, an inductor, a capacitor, a transistor, and a current source. Other DC sources may additionally or alternatively be used. 
         [0005]    Converter/inverter  5  may be comprised to perform any of DC to AC power conversion, AC to DC power conversion, AC to AC power conversion, and DC to DC power conversion. For example, converter/inverter  5  may comprise a three-phase inverter configured to receive a DC power voltage and to provide power in  3  phases to an AC motor load. Converter/inverter  5  includes switches which are controlled by signals from a controller  8 . 
         [0006]    The Z source network  4  receives DC power from DC source  2  and provides power to converter/inverter  5 . The Z source network for maybe configured to provide power, for example, outside of the DC range of the output of DC power source  2 . 
         [0007]    Because of the switching operation of the converter/inverter  5 , the Z source network  4  experiences a switched load, which may cause voltage and current spikes. In some circumstances, the transient spikes may damage circuitry. To ensure reliability, components are oversized, which causes current systems to be expensive, slow, and power inefficient. 
       SUMMARY OF THE INVENTION 
       [0008]    One inventive aspect is an electrical network configured to suppress voltage transients. The network includes a capacitor and an electrical impedance in parallel with a diode. The capacitor is in series with the parallel connected diode and electrical impedance, and the electrical network is configured to suppress voltage transients occurring across the series combination of the capacitor and the parallel connected diode and electrical impedance. 
         [0009]    Another inventive aspect is a Z source network, including positive and negative input terminals, a shunt capacitor connected to the positive and negative input terminals, and a diode having an anode connected to the positive input terminal and a cathode connected to an internal node of the Z source network. The Z source network also includes positive and negative output terminals, a first inductor connected to the inner node and to the positive output terminal, and a first capacitor connected to the inner node and to the negative output terminal. The Z source network also includes a second inductor connected to the negative input terminal and to the negative output terminal, a second capacitor connected to the negative input terminal and the positive output terminal, and an electrical network configured to suppress voltage transients. The network is includes a capacitor and an electrical impedance in parallel with a diode. The capacitor is in series with the parallel connected diode and electrical impedance, and the electrical network is configured to suppress voltage transients occurring across the positive and negative input terminals. 
         [0010]    Another inventive aspect is an electrical network configured to suppress voltage transients. The network includes a first capacitor and an electrical impedance in parallel with a diode, where the first capacitor is in series with the parallel connected diode and electrical impedance. The network also includes second and third capacitors, where the second and third capacitors are in series with the first capacitor and the parallel connected diode and electrical impedance. The electrical network is configured to suppress voltage transients occurring across the series combination of the first capacitor, the parallel connected diode and electrical impedance, the second capacitor and the third capacitor. 
         [0011]    Another inventive aspect is a Z source network, including positive and negative input terminals, a shunt capacitor connected to the positive and negative input terminals, and a diode having an anode connected to the positive input terminal and a cathode connected to an internal node of the Z source network. The Z source network also includes positive and negative output terminals, a first inductor connected to the inner node and to the positive output terminal, and a first capacitor connected to the inner node and to the negative output terminal. The Z source network also includes a second inductor connected to the negative input terminal and to the negative output terminal, a second capacitor connected to the negative input terminal and the positive output terminal, and an electrical network configured to suppress voltage transients. The electrical network includes a first capacitor and an electrical impedance in parallel with a diode, where the first capacitor is in series with the parallel connected diode and electrical impedance. The electrical network also includes second and third capacitors, where the second and third capacitors are in series with the first capacitor and the parallel connected diode and electrical impedance. The electrical network is configured to suppress voltage transients occurring across the positive and negative output terminals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate implementations of inventive concepts and, together with the description, serve to explain various advantages and principles of the invention. 
           [0013]      FIG. 1  is a block diagram of an electronic system. 
           [0014]      FIG. 2  is a schematic diagram of a Z source network, which can be used in the system of  FIG. 1 . 
           [0015]      FIGS. 3-5  show current flow within the Z-Source network of  FIG. 2  under various load conditions. 
           [0016]      FIG. 6  is a schematic diagram of the Z source network of  FIG. 2 , showing parasitic inductances. 
           [0017]      FIG. 7  is a schematic diagram of an alternative Z source network, which can be used in the system of  FIG. 1 . 
           [0018]      FIG. 8  is a schematic diagram of an alternative Z source network, which can be used in the system of  FIG. 1 . 
           [0019]      FIG. 9  is an illustration of a simulation result. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Reference is made to implementations illustrated in the accompanying drawings. The same reference numbers are generally used throughout the drawings and the following description to refer to the same or like elements. 
         [0021]    The electrical power conversion field is generally understood to have three fundamental conversion topologies: voltage source converters, current source converters, and impedance (or Z) source converters. The three types are defined by the network topology used to store energy. 
         [0022]    A voltage source converter (VSC) utilizes an energy storage network that stores energy as a change in a voltage, such as the voltage across a capacitor. The energy storage element, storing and releasing energy at a rate based on the difference in voltage, is capable of sourcing and sinking a large amount of current and is not able to attain a voltage higher than the source voltage. A voltage source converter may provide unlimited fault current, limited fault voltage, and may have a bucked voltage output. Due to this behavior, the network of a voltage source converter is not protected from a short circuit at the output. 
         [0023]    A current source converter (CSC) relies on an energy storage network that stores energy as a change in a current, such as the current flowing through an inductor. Since the energy storage element stores and releases energy at a rate based on the difference in current, the converter can provide very large changes in voltage and can provide a voltage higher than the source voltage. A current source converter may provide limited fault current, unlimited fault voltage, and may have a boosted voltage output. Due to this behavior, the network of a current source converter is not protected from an open circuit at the output. 
         [0024]    An impedance source (ZSC or Z-Source Converter) relies on an energy storage network that exhibits properties of both voltage and current source converter types. A Z-Source network stores energy, for example, in both inductive and capacitive storage elements where the elements are in series, and the effects of either the inductive or the capacitive dominate at different frequencies or energy storage levels. The series combination allows both the fault current (short circuit) and fault voltage (open circuit) to be limited while also allowing the network to buck or boost the output voltage relative to the input. In response to a short circuit at the output terminals, the Z-Source network stores energy in the inductive elements. In response to an open-circuit applied to the output terminals, the Z-Source network stores energy in the capacitive elements. 
         [0025]      FIG. 2  is a schematic diagram of a Z source network  20 , which can be used in the system of  FIG. 1 . Z source network  20  includes shunt capacitor  21 , rectifier diode  22 , cross coupled capacitors  23  and  24 , inductors  25  and  26 , positive and negative input terminals  27 , and positive and negative output terminals  28 . As discussed in more detail below, Z source network  20  performs well even when being open or short circuited. 
         [0026]      FIG. 3  shows the current flow within the Z-Source network as the output is short-circuited. While short-circuited, current flows from the positive input terminal  27  through rectifier diode  22 , through inductor  25 , and out positive output terminal  28 . The current then flows through the short, into negative output terminal  28 , and through inductor  26  to negative input terminal  27 . As the current through the inductors  25  and  26  increases, energy is stored in the inductors  25  and  26 . 
         [0027]    In response to the low impedance short-circuit being opened, the current flowing through the inductors  25  and  26  is forced by the stored energy in inductors  25  and  26  to flow to cross coupled capacitors  23  and  24 . As a result, cross coupled capacitors  23  and  24  are charged. As shown in  FIG. 4 , current from the inductor  25  charges the capacitor  23 , and current in the inductor  26  charges the capacitor  24  to a voltage below the negative input voltage at the negative input terminal  27 . 
         [0028]    Following the decay of the current, the rectifier diode  22  blocks reverse current that would otherwise drain the cross coupled capacitors  23  and  24 . Once the cross coupled capacitors  23  and  24  are thus charged, the Z-Source network can provide a boosted input voltage for use at the output terminals  28 . Current flow during this condition is shown in  FIG. 5 . 
         [0029]    There are numerous configurations of Z-Source networks. One embodiment is shown in  FIGS. 2-5 . The various aspects and principles discussed herein, while discussed as applied to the configuration shown in  FIGS. 2-5 , may also be applied to various other configurations and variations of Z-Source networks. In addition, the various aspects and principles discussed herein may be applied to quasi-Z-Source networks. Similarly, the various aspects and principles discussed herein are applicable to all configurations of sources and loads. For example, the source or load may be any of a voltage source, a current source, an n-level load, a breaker configuration, IGBTs, MOSFETs, SCRs, etc. 
         [0030]    Parasitic inductances  29  of the Z-Source network  20  are shown in  FIG. 6 . The Z-Source network  20  provides current snubbing because the relatively large impedance network inductance from inductors  25  and  26  is in series with the capacitive discharge path of the network. This provides excellent protection for semiconductor devices during turn-on and current limiting during diode reverse recovery. However, as  FIG. 6  illustrates, there are many parasitic inductance paths that contribute to voltage spikes, for example, during turn-off. When the output terminals  28  are opened with current flowing through the inductors  25  and  26 , as previously discussed with reference to  FIG. 3 , the cross coupled capacitors  23  and  24  provide a path for the inductor current. However, parasitic inductance or equivalent series inductance (ESL), for example, in the capacitors  23  and  24 , prevents the capacitors  23  and  24  from instantaneously conducting current. This delay causes a voltage spike which is seen at the output terminals  28 . This is exacerbated due to the capacitors  23  and  24  being cross coupled such that the wires of the capacitors  23  and  24  are relatively long, and due to the physical size of the capacitors, which are sized to handle very high ripple currents (full output current) while providing energy storage for the impedance network. 
         [0031]    Compounding the effect of the ESL, under typical operating conditions, the capacitors  23  and  24  can source current to the output terminals  28  when changing from a boosting state to an open circuit state. The ESL of the shunt capacitor  21  is also added to the total current path, but note that this ESL is in parallel with the input source. Note also that the shunt capacitor  21  may, in some embodiments, be part of the input source driving the network. Similarly, the ESL of the diode and lead inductance can be added to the total parasitic inductance, but the effect of this parasitic inductance is dependent on the mode of operation as current to may be flowing preceding a transient. 
         [0032]      FIG. 7  is a schematic diagram of an alternative Z source network  50 , which can be used in the system of  FIG. 1 . Z source network  50  includes shunt capacitor  51 , rectifier diode  52 , cross coupled capacitors  53  and  54 , inductors  55  and  56 , positive and negative input terminals  57 , and positive and negative output terminals  58 . Z source network  50  also includes voltage snubbing circuit  60 . As discussed in more detail below, snubbing circuit  60  suppresses voltage spikes which result from parasitic inductances. 
         [0033]    Voltage snubbing circuit  60  includes capacitor  62 , diode  64 , and impedance element  66 . In some embodiments, capacitor  62  may be much smaller than shunt capacitor  51 . In such embodiments, capacitor  62  contributes an insignificant amount to the total energy storage capability of the system, and hence carries negligible current. However, the capacitor  62  can be physically much smaller, and have a lower ESL than shunt capacitor  51 . The small size of capacitor  62  similarly results in a lower peak current in snubbing circuit  60 , such that diode  64  may be relatively small and fast. For example diode  64  may be significantly smaller and faster than rectifier diode  52 . As a result, diode  64  may have a lower ESL and faster recovery characteristics than rectifier diode  52 , which is sized for carrying the full impedance network current. Voltage snubbing circuit  60  provides snubbing for the rectifier diode  52  by reducing the dv/dt of the voltage transitions at the cathode of rectifier diode  52 . 
         [0034]    Diode  64  and impedance element  66  collectively allow rapid voltage increase at the cathode of rectifier diode  52 . Diode  64  and impedance element  66  also prevent capacitor  62  from forming a path which would allow current to flow through the stacked network of capacitors  62 ,  54 , and  53  when the output terminals  58  are shorted. The impedance element  66  is sized such that the RC time constant formed by impedance element  66  and capacitor  62  is very large compared to the switching frequency of the system. This allows the capacitor  62  to be substantially permanently pre-charged (for inrush) and prevents the voltage of the capacitor  62  from charging up to the boosted voltage of the Z-Source network  50 . Capacitor  62  remains charged to approximately the input voltage with very little ripple current flowing through it. In some embodiments, substantially the only ripple current flowing through capacitor  62  cancels the effect of the larger ESL of the energy-carrying network and supports recovery of diode  64 . Accordingly, in such embodiments, voltage snubbing circuit  60  has no impact on the functionality of the Z-Source network  50 , and substantially only serves to reduce voltage spikes. 
         [0035]    As shown in  FIG. 7 , impedance element  66  includes a resistor. In some embodiments, impedance element  66  includes another impedance network. For example impedance element  66  may include one or more resistors, capacitors, or inductors. 
         [0036]    In some embodiments, input element  66  is omitted. 
         [0037]    As shown in  FIG. 7 , capacitor  62  is a single capacitor. In some embodiments, additional impedance elements are included in series or in parallel with capacitor  62 . 
         [0038]    In some embodiments, impedance element  66  is configured to be controlled so as to controllably remove energy from capacitor  62 . This allows for the controlled or active suppression of transient spikes. In some embodiments, impedance element  66  may include or be an energy storage element or an energy conversion element. 
         [0039]    In some embodiments, an energy storage element or an energy conversion element may be placed in parallel with capacitor  62 . This allows for the controlled or active removal of energy from capacitor  62 . 
         [0040]      FIG. 8  is a schematic diagram of an alternative Z source network  80 , which can be used in the system of  FIG. 1 . Z source network  80  includes shunt capacitor  81 , rectifier diode  82 , cross coupled capacitors  83  and  84 , inductors  85  and  86 , positive and negative input terminals  87 , and positive and negative output terminals  88 . Z source network  80  also includes voltage snubbing circuit  60 , discussed above, and voltage snubbing circuit  90 . As discussed in more detail below, snubbing circuit  90  suppresses voltage spikes which result from parasitic inductances. 
         [0041]      FIG. 8  shows snubbing circuit  90  in parallel with the remainder of Z source network  80 . As shown, snubbing circuit  90  includes cross coupled capacitors  91  and  92 , capacitor  93 , impedance element  94 , and diode  95 . 
         [0042]    The value of the cross coupled capacitors  91  and  92  is less than cross coupled capacitors  83  and  84 . In some embodiments, additional resistors (not shown) are placed in series with cross coupled capacitors  91  and  92 . As a result, snubbing circuit  90  carries negligible current. 
         [0043]    The operation of capacitor  93 , diode  95 , and impedance element  94  is similar to the operation of snubbing circuit  60  discussed above, and is not repeated. In some embodiments, substantially the only ripple current flowing through capacitor  93  cancels the effect of the larger ESL of the energy-carrying network and supports recovery of diode  95 . Accordingly, in such embodiments, voltage snubbing circuit  90  has no impact on the functionality of the Z-Source network  80 , and substantially only serves to reduce voltage spikes. 
         [0044]    Because they do not need to carry the network current, capacitor  93 , diode  95 , and impedance element  94  may have values and physical sizes which are small. For example, they may be smaller than the corresponding components of snubbing circuit  60 , discussed above. As a result, parasitic inductance of snubbing circuit  90  is negligible. In addition, the high-frequency current path of the Z source network  80  is physically very small, having short lengths, leading to a further reduction in inductance. 
         [0045]    In some embodiments, the components of snubbing circuit  90  are integrated into a single package. In such embodiments, the package of the snubbing circuit  90  has terminals which may be mounted to terminals of a load, such as an integrated circuit, of the Z source network  80 . In some embodiments the terminals of the package of the snubbing circuit  90  may be mounted so as to contact the terminals of the load. 
         [0046]    As shown in  FIG. 8 , impedance element  94  includes a resistor. In some embodiments, impedance element  94  includes another impedance network. For example impedance element  94  may include one or more resistors, capacitors, or inductors. 
         [0047]    In some embodiments, input element  94  is omitted. 
         [0048]    As shown in  FIG. 8 , capacitor  93  is a single capacitor. In some embodiments, additional impedance elements are included in series or in parallel with capacitor  93 . 
         [0049]      FIG. 8  shows a particular embodiment of a Z-Source network. Note that other embodiments may be implemented, for example, by rearranging the components of snubbing circuit  60  or by rearranging the diode  82  and shunt capacitor  81 . Similarly, other embodiments may be implemented, for example, by connecting the snubbing circuit  90  at other locations in Z source network  80 . 
         [0050]    In some embodiments, impedance element  94  is configured to be controlled so as to controllably remove energy from capacitor  93 . This allows for the controlled or active suppression of transient spikes. In some embodiments, impedance element  94  may include or be an energy storage element or an energy conversion element. 
         [0051]    In some embodiments, an energy storage element or an energy conversion element may be placed in parallel with capacitor  93 . This allows for the controlled or active removal of energy from capacitor  93 . 
         [0052]      FIG. 9  shows a simulation result indicating an example of the improvement achieved by using Z source network  80 . The simulation was run with a Z source network  80  providing power to a 3-phase load. The simulation result shows the voltage at the output of Z source network  80  while providing approximately 200A sourced from a 600VDC supply. No Z Source boost factor is used, and the IGBTs are run without deadtime, such that shoot-through events are allowed, causing the shown ringing. Parasitic inductances typical of this power level and known to those skilled in the art, were used. For the simulation, cross coupled capacitors  83  and  84  had 300 nH of equivalent series inductance (ESL). In addition, parasitic inductance from wiring and connectors to the switching devices were 100nH. The simulated Z-Source network snubbing circuit  90  used a 2.0 uF capacitor  93  having an ESL of 20 nH. As shown, the snubbing circuit  90  in this simulation limited the peak voltage overshoots at the load to approximately +/−200V, whereas the Z source network without snubbing circuit  90  has voltage overshoots at the load of approximately +/−600V. 
         [0053]    The present invention has been described above in terms of presently preferred embodiments so that an understanding of the present invention can be conveyed. There are, however, many configurations for network devices and management systems not specifically described herein but with which the present invention is applicable. The present invention should therefore not be seen as limited to the particular embodiments described herein, but rather, it should be understood that the present invention has wide applicability with respect to network devices and management systems generally.