Abstract:
A Low Forward Voltage Rectifier (LFVR) circuit includes a bipolar transistor, a parallel diode, and a capacitive current splitting network. The LFVR circuit, when it is performing a rectifying function, conducts the forward current from a first node to a second node provided that the voltage from the first node to the second node is adequately positive. The capacitive current splitting network causes a portion of the forward current to be a base current of the bipolar transistor, thereby biasing the transistor so that the forward current experiences a low forward voltage drop across the transistor. The LFVR circuit sees use in as a rectifier in many different types of switching power converters, including in flyback, Cuk, SEPIC, boost, buck-boost, PFC, half-bridge resonant, and full-bridge resonant converters. Due to the low forward voltage drop across the LFVR, converter efficiency is improved.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of, and claims the benefit under 35 U.S.C. §120 from, nonprovisional U.S. patent application Ser. No. 13/317,800, entitled “Low Forward Voltage Rectifier,” filed on Oct. 29, 2011. This application expressly incorporates by reference the entire content of U.S. patent application Ser. No. 13/317,800. 
    
    
     TECHNICAL FIELD 
     The described embodiments relate to rectifiers in switching power converters. 
     BACKGROUND INFORMATION 
     In a switching power converter, a substantial amount of power is dissipated across the rectifier in the output stage of the converter. In an example in which a charging current is made to flow through a diode rectifier on its way to charging an output capacitor, a voltage drop occurs across the diode. The instantaneous power lost is the product of the instantaneous voltage drop across the diode multiplied by the instantaneous current flow through the diode. This instantaneous power loss, integrated over time, represents an amount of energy lost. The energy is said to be lost due to the conversion of electrical energy into heat. Reducing the amount of energy lost in the output rectifier of a switching power converter, as a percentage of the total amount of energy delivered to the load, is desired. 
     SUMMARY 
     A Low Forward Voltage Rectifier (LFVR) circuit includes a bipolar transistor, a parallel diode, and a capacitive current splitting network. In one example the bipolar transistor is an NPN transistor, and the current splitting network involves a first capacitor, a second capacitor, a first inductor, and a second inductor. The forward current flowing through the LFVR is received onto a first node of the LFVR circuit. Some of the incoming forward current flows through the first capacitor to the base of the bipolar transistor. A base current is provided to the bipolar transistor through this first capacitor. The rest of the incoming forward current flows through the second capacitor to the collector of the bipolar transistor. The combined currents flowing into the base and into the collector merge and flow out of the emitter to the second node of the LFVR circuit. The LFVR circuit, when it is performing a rectifying function, conducts a forward current from the first node to the second node provided that the voltage from the first node to the second node is adequately positive. The LFVR circuit also has a third node. A first inductor is coupled between the third node and the collector of the bipolar transistor. A second inductor is coupled between the third node and the base of the bipolar transistor. The term “between” as it is used here means between in the electrical sense. 
     In addition to the first embodiment involving an NPN bipolar transistor, a second embodiment of the LFVR circuit involves a PNP bipolar transistor rather than an NPN bipolar transistor. The emitter of the PNP bipolar transistor is coupled to the first node. A first capacitor is coupled between the base of the transistor and a second node. A second capacitor is coupled between the collector of the transistor and the second node. The parallel diode has an anode coupled to the emitter of the transistor and a cathode coupled to the collector of the transistor. A first inductor is coupled between the collector of the transistor and a third node. A second inductor is coupled between the base of the transistor and the third node. When the PNP LFVR circuit is performing its rectifying function, a forward current is conducted from the first node, through the bipolar transistor, and to the second node, provided that the voltage from the first node to the second node is adequately positive. A base current is drawn out of the base of the transistor so that the emitter-to-collector voltage across the PNP transistor is low if a forward current is flowing. 
     The LFVR circuit is usable as a rectifier in a switching power converter. In an example of a flyback converter power supply, pulses of current from the secondary winding of the main transformer pass through the rectifier and charge the output capacitor. During times when the pulses are charging the output capacitor, the forward voltage drop across the LFVR circuit is, on average, substantially less than 1.0 volts. 
     Both the NPN and the PNP embodiments of the LFVR circuit see widespread use as rectifiers in other switching power converter circuits such as, for example, in a Cuk converter, in a SEPIC converter, in a boost converter, in a buck-boost converter, in a power factor correction circuit, in a half-bridge resonant converter, and in a full-bridge resonant converter. In each case, forward conduction losses of the circuit&#39;s rectifier are reduced as compared to a realization of the switching power converter than uses a conventional diode for rectification. 
     In some examples, the bipolar transistor is a Reverse Bipolar Junction Transistor (RBJT) and the parallel diode is a distributed diode. The RBJT and the distributed diode are integrated together onto the same semiconductor die. The RBJT has a V BE  reverse breakdown voltage of at least twenty volts. 
     Further details, embodiments, methods and techniques are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  is a diagram that shows the forward voltage voltage-to-current characteristics of a MOSFET, a diode, and a bipolar transistor. 
         FIG. 2  is a diagram that illustrates the forward voltage drop across a diode rectifier. 
         FIG. 3  is a diagram that illustrates the forward voltage drop across a bipolar transistor, having a parallel-connected diode. 
         FIG. 4  is a diagram that illustrates how a rectifier diode in a switching power converter can be replaced with a bipolar transistor and parallel diode. 
         FIG. 5  is a diagram that illustrates capacitive current splitting. 
         FIG. 6  is a diagram that illustrates how a rectifier diode circuit can be replaced with a first embodiment of a low forward voltage rectifier (LFVR) circuit. 
         FIG. 7  is a diagram that illustrates how a rectifier diode circuit can be replaced with a second embodiment of a low forward voltage rectifier (LFVR) circuit. 
         FIG. 8  is a diagram of a flyback switching power converter power supply. 
         FIG. 9  is a diagram that illustrates how the output capacitor of the flyback power supply of  FIG. 8  can be replaced with a pi filter. 
         FIG. 10  is a diagram that illustrates how rectifier circuitry in the modified circuit of  FIG. 9  can be replaced with a low forward voltage rectifier that uses capacitive current splitting. 
         FIG. 11  is a waveform diagram that illustrates voltages and currents present in the switching power converter of  FIG. 10 . 
         FIG. 12  is a waveform diagram that illustrates voltages and currents present in the switching power converter of  FIG. 10 . 
         FIG. 13  is a diagram of a Cuk converter that employs an LFVR circuit having capacitive current splitting. 
         FIG. 14  is a diagram of a SEPIC converter that employs an LFVR circuit having capacitive current splitting. 
         FIG. 15  is a diagram of a boost converter that employs an LFVR circuit having capacitive current splitting. 
         FIG. 16  is a diagram of a boost-type power factor correction (PFC) converter circuit that employs an LFVR circuit having capacitive current splitting. 
         FIG. 17  is a diagram of a half-bridge series loaded resonant converter that employs an LFVR circuit having capacitive current splitting. 
         FIG. 18  is a diagram of a full-bridge phase shift parallel loaded resonant converter that employs an LFVR circuit having capacitive current splitting. 
         FIG. 19  is a diagram of a buck-boost converter that employs an LFVR circuit having capacitive current splitting. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to background examples and some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 1  is a diagram that shows the forward voltage voltage-to-current characteristics of a metal oxide semiconductor field effect transistor (MOSFET), a diode, and a bipolar junction transistor (BJT). In the case of the diode, there is substantially no current flow through the diode for forward voltages less than a voltage V T . In the case of a PN junction bipolar diode, this voltage V T  at which a forward current starts to flow is about 0.65 volts. In the case of a unipolar diode (a Schottky diode), the voltage V T  is about 0.3 volts. In the case of a bipolar transistor, a collector-to-emitter current begins to flow for forward voltages greater than a V T  of about 0.02 volts, assuming that the bipolar transistor is supplied with an adequate amount of base current. In the case of a MOSFET having a high breakdown voltage, a source-to-drain current begins flowing at a zero voltage source-to-drain voltage, but at the same operating current the source-to-drain voltage drop across the MOSFET is larger than the collector-to-emitter voltage drop across the bipolar transistor at the same operating current. In  FIG. 1 , the operating current is represented by horizontal dashed line  1 . In a switching power converter, a diode is usually used as the rectifier in the output stage. If a bipolar transistor could be used in the place of the diode rectifier, then the forward voltage drop across the rectifier could be reduced, thereby resulting in less power loss. 
       FIG. 2  is a diagram that illustrates a diode rectifier  2 . At an operating current flow of ten amperes, there is a one volt drop across the diode. This corresponds to a power loss of ten watts. 
       FIG. 3  is a diagram that illustrates the forward voltage drop across a bipolar transistor  3 , having a parallel-connected diode  4 . Given an adequate current flow into the base, there is a 0.3 volt voltage drop between collector and emitter. Due to this low voltage, the voltage drop across the diode is less than the V T  of the diode  4 , and there is no forward current flow through the diode  4 . For the same ten amperes of current flow considered with respect to the diode of  FIG. 2 , one ampere of this current is supplied to the base, and the remaining nine amperes is supplied to the collector. The one ampere base current undergoes a one volt voltage drop, so this current represents a one watt loss of power. The nine amperes of collector current undergo a 0.3 volt voltage drop, so this current represents a 2.7 watt loss of power. The overall loss of power is therefore 3.7 watts. The 3.7 watt loss of power with the bipolar transistor of  FIG. 3  is substantially less than the ten watt loss of power with the ordinary rectifier diode of  FIG. 2 . Using the bipolar transistor as the rectifier in the output stage of the switching power converter is therefore desired. 
       FIG. 4  is a diagram that illustrates a rectifier diode in such a switching power converter. A current coming out of an existing part  5  of the power supply flows through the rectifying diode  6 . To replace the rectifier diode  6  with the low forward voltage bipolar transistor rectifier  8  and parallel diode  9 , the circuitry  5  of the power supply is modified. The modified circuit  7  splits the current in a rough  9  to  1  ratio such that about one tenth of the overall current is supplied to the base of the bipolar transistor  8 . 
       FIG. 5  is a simplified diagram that illustrates the capacitive current splitting manner of splitting a current flow into a collector current and a base current. The circuit involves a first capacitor C 1  and a second capacitor C 2 . In the circuit of  FIG. 4 , when the forward current is being conducted through the rectifier, the voltage on the collector is quite close to the voltage on the base. For conceptual purposes, the two voltages are the same, and therefore the two nodes are considered to operate as one node. The two left terminals of capacitors C 1  and C 2  are coupled together. Due to the collector voltage and the base voltage being roughly the same, the two right terminals of capacitors C 1  and C 2  are also coupled together. Accordingly, the ratio of current flow through the two capacitors is given by the ratio of their capacitances, for a voltage V 1 -V 2  between the two nodes  10  and  11 . This capacitive current splitting is employed to drive the bipolar transistor of the LFVR circuit in the switching power converter. 
       FIG. 6  illustrates a first circuit  12  on the left. This first circuit  12  involves a rectifier diode  13 . This first circuit  13 , if found in a switching power converter, can be replaced with the first low forward voltage rectifier (LFVR) circuit  14  shown to the right in  FIG. 6 . The first LFVR circuit  14  involves an NPN bipolar transistor  15 , a parallel diode  16 , two capacitors C 1  and C 2 , and two inductors L 1  and L 2 . Node N 3  is a node that carries a substantially constant DC voltage or ground potential. 
       FIG. 7  illustrates a second circuit  17  on the left. This second circuit  17  involves a rectifier diode  18 . This second circuit  17 , if found in a switching power converter, can be replaced with the second LFVR circuit  19  shown to the right in  FIG. 7 . The second LFVR circuit  19  involves a PNP bipolar transistor  20 , a parallel diode  21 , two capacitors C 1  and C 2 , and two inductors L 1  and L 2 . Node N 3  is a node that carries a substantially constant DC voltage or ground potential. 
     Although the ends of the inductors L 1  and L 2  opposite the transistor are coupled together at node N 3  in the examples of  FIG. 6  and  FIG. 7 , these ends of the inductors L 1  and L 2  are not connected together in all embodiments. For example, in some circuits these ends of the inductors L 1  and L 2  are coupled to two different nodes at two different DC voltages. In a typical example, the capacitance of capacitor C 2  is at least ten times greater than the capacitance of capacitor C 1 . 
       FIGS. 8 ,  9  and  10  are a sequence of diagrams that illustrates how the first LFVR circuit  14  of  FIG. 6  is incorporated into an example of a switching power converter circuit. In this example, the switching power converter circuit is a flyback isolated DC-DC converter  22 . 
       FIG. 8  is a diagram of the flyback isolated DC-DC converter  22 . The converter converts 110 VAC from a voltage source  23  into 5 VDC. The 5 VDC is provided onto output terminals T 1  and T 2 . The 110 VAC source, and the load  24 , are not parts of the actual switching power converter. Reference numerals  25  and  26  represent a connector and terminals by which the switching power converter is coupled to the AC voltage source  23 . A bridge rectifier involving diodes  27 ,  28 ,  29  and  30  full wave rectifies an 110 VAC signal such that 150 volts is present across input capacitor  31  between a VIN node and conductor  32  and a ground node and conductor  33 . A switch  41  is opened and closed as is known in the art such that pulses of current are drawn from node  32  and through the primary winding  34  of a transformer  35 . Stopping current flow in the primary winding results in a pulse of current flowing up through the secondary winding  36 , and through the rectifier diode  37 , to charge output capacitor  38 . The turns ratio of the transformer  35  is such that the output capacitor  38  is charged to 5 VDC. The 5 VDC is present between output supply node and conductor  39  and ground node and conductor  40 . The load  24  draws power through terminals T 1  and T 2 . 
       FIG. 9  shows a modification that can be made to the converter  22  of  FIG. 8 . The output capacitor  38  of the converter is replaced with a pi filter  42 . The pi filter  42  includes a first capacitor  43 , a second capacitor  44 , and an inductor  45 . How to carry out a pi filter substitution for a capacitor is known in the art. In addition to the pi filter substitution, the rectifier diode  37  of  FIG. 8  is moved to the position indicated in  FIG. 9 . 
       FIG. 10  shows how circuitry of the modified circuit of  FIG. 9  is replaced with the first LFVR circuit  14  of  FIG. 6 . The illustration of  FIG. 10  is a simplification. The AC voltage source, full wave rectifier, and input capacitor are represented in  FIG. 10  by the 150 VDC voltage source symbol  50 . The control and switch driving circuitry of the flyback converter is represented by a signal source symbol V 2   46 , and no output voltage monitoring circuitry is shown. There are many known ways to control and to monitor a flyback converter. How the flyback switching power supply is controlled and monitored is well known in the art and is not described here. 
     The primary output rectifier of the flyback converter is not the diode  37  of  FIG. 8  as is conventional, but rather is the first LFVR circuit  14  of  FIG. 6  that uses capacitive current splitting. The proportion of the rectifier forward current that is supplied to the collector of the bipolar transistor  47  via capacitor C 2  as compared to amount of rectifier forward current that is supplied to the base of the bipolar transistor via capacitor C 1  is determined by the relative capacitances of C 1  and C 2 . This proportion changes somewhat throughout the time period that the forward voltage is flowing for various reasons, but the proportion is roughly fixed and is set by the C 1  and C 2  values. 
     When the secondary current first starts flowing out of the secondary winding  36  and to node  39 , the bipolar transistor  47  may not start conducting and working as a rectifier instantaneously. The parallel diode  48  is provided to perform the rectification function during this time. Also later, when the secondary current stops flowing, the base current to the bipolar transistor  47  may be cut off before the collector current stops flowing. The bipolar transistor  47  may therefore be turned off too fast. The parallel diode  48  also may perform rectification during this time. Whether and how the parallel diode  48  performs rectification at the beginning of secondary current flow and at the ending of secondary current flow depends on details of the particular converter and how it is operating. The parallel diode  48  can be provided, and the operation of the converter monitored. The parallel diode  48  can then be removed, and the operation of the converter monitored. Operation of the converter with the parallel diode is compared to operation of the converter without the parallel diode. If circuit operation with the parallel diode removed is adequate, then the parallel diode is not required. 
       FIG. 11  is a waveform diagram that illustrates voltages and current present in the flyback converter of  FIG. 10  as the power supply operates. Additional waveforms are shown in  FIG. 12 . As seen in the waveform of  FIG. 12  labeled “collector to emitter voltage”, the voltage across the bipolar transistor between collector and emitter is substantially less than 1.0 volt for most of the time t 2  to t 3  when the rectifier is conducting a forward current. In the illustrated example, as indicated by the waveform “diode current”, the parallel diode  48  only conducts during the initial time when secondary current starts flowing between t 1  and t 2 . In this particular example, the parallel diode  48  does not conduct at the end of the cycle when the secondary current stops flowing. 
     In  FIG. 12  in the “collector to emitter voltage” waveform and in the “base to emitter voltage” waveform, the voltages before time t 1  and the voltages after time t 4  is about −25 volts. Because this low voltage is off the scale of the diagram, the low voltages during these times are not seen in  FIG. 12 . 
     Although an example of a switching power converter is set forth above where the first LFVR circuit  14  involving capacitive current splitting has an NPN transistor, the second LFVR circuit  19  involving capacitive current splitting having a PNP transistor may also be employed. The NPN and PNP circuits are not limited to use in flyback switching power converter circuits, but rather are generally usable in other switching power converter circuits. 
       FIG. 13  is a simplified diagram of another type of switching power converter circuit, a Cuk converter  100 . Cuk converter  100  includes an instance of the LFVR circuit  14  that has capacitive current splitting. Resistor R 2  represents the load. C 3  is the output capacitor. V 1  represents a DC voltage source which can be the combination of an AC voltage source, a rectifier bridge, and an input capacitor. The V 2  symbol represents the control circuit that drives the switch M 1 . 
       FIG. 14  is a simplified diagram of another type of switching power converter circuit, a SEPIC converter  200 . SPIC converter  200  includes an instance of the LFVR circuit  14  that has capacitive current splitting. Resistor R 2  represents the load. C 3  is the output capacitor. V 1  represents a DC voltage source which can be the combination of an AC voltage source, a rectifier bridge, and an input capacitor. The V 2  symbol represents the control circuit that drives the switch M 1 . 
       FIG. 15  is a simplified diagram of another type of switching power converter circuit, a boost converter  300 . This circuit can be used for power factor correction. Boost converter  300  includes an instance of the LFVR circuit  19  that has capacitive current splitting. Resistor R 2  represents the load. C 3  is the output capacitor. V 1  represents a DC voltage source which can be the combination of an AC voltage source, a rectifier bridge, and an input capacitor. The V 2  symbol represents the control circuit that drives the switch M 1 . 
       FIG. 16  is a simplified diagram of another type of switching power converter circuit, a boost-type power factor correction (PFC) circuit  400  which does not have rectifying bridge diodes. PFC circuit  400  includes an instance of the LFVR circuit  19  that has two PNP transistors and parallel diodes, as well as a capacitive current splitting network. Resistor R 2  represents the load. C 3  is the output capacitor. V 1  a DC voltage source which can be the combination of an AC voltage source, a rectifier bridge, and an input capacitor. The V 2  symbol represents the control circuit that drives the switches M 1  and M 2 . 
       FIG. 17  is a simplified diagram of another type of switching power converter circuit, a half-bridge series loaded resonant converter  500 . Converter  500  includes an instance of the LFVR circuit  14  that has two NPN transistors and parallel diodes, as well as a capacitive current splitting network. Resistor R 2  represents the load. C 3  is the output capacitor. V 2  represents a DC voltage source which can be the combination of an AC voltage source, a rectifier bridge, and an input capacitor. The V 1  symbol represents the control circuit that drives switch M 1 . The V 3  symbol represents the control circuit that drives switch M 2 . 
       FIG. 18  is a simplified diagram of another type of switching power converter circuit, a full-bridge phase shift parallel loaded resonant converter  600 . Converter  600  includes an instance of the LFVR circuit  14  that has two NPN transistors and parallel diodes, as well as a capacitive current splitting network. Resistor R 2  represents the load. C 3  is the output capacitor. V 3  represents the AC voltage source, the rectifier bridge, and the input capacitor. The V 1 , V 2 , V 4  and V 5  symbols represent control circuits that drive the switches M 1 , M 2 , M 3  and M 4 , respectively. 
       FIG. 19  is a simplified diagram of another type of switching power converter circuit, a buck-boost converter  700 . Converter  700  includes an instance of the LFVR circuit  14  that has an NPN transistor, as well as a capacitive current splitting network. Resistor R 2  represents the load. C 3  is the output capacitor. V 1  represents a DC voltage source which can be the combination of an AC voltage source, a rectifier bridge, and an input capacitor. The V 2  symbol represents the control circuit that drives the switch M 1 . 
     In some examples, the bipolar transistor of the LFVR circuit is a Reverse Bipolar Junction Transistor (RBJT) and the parallel diode is a distributed diode. The RBJT and the distributed diode are integrated together onto the same semiconductor die. The RBJT has a V BE  reverse breakdown voltage of at least twenty volts. For further details on one example of an integrated version of the bipolar transistor and the parallel diode, see: U.S. patent application Ser. No. 13/317,800, entitled “Low Forward Voltage Rectifier”, filed Oct. 29, 2011 (the entire subject matter of which is incorporated herein by reference). 
     A method of manufacture involves attaching (for example, by soldering) a first capacitor, a second capacitor, a bipolar transistor, a parallel diode, a first inductor, and a second inductor to a substrate (for example, a printed circuit board or direct metal bonded substrate) so as to realize the LFVR circuit  14  of  FIG. 6  or the LFVR circuit  19  of  FIG. 7 . In this way the components are provided on the substrate as part of a switching power converter circuit. 
     In another method of manufacture, a LFVR circuit is provided in a three-terminal package. The components of the LFVR circuit are disposed on a substrate, and an amount of encapsulant is made to overmold the components to form a package body. The three package terminals (for nodes N 1 , N 2  and N 3 ) extend from the package body. Either the LFVR circuit  14  of  FIG. 6  or the LFVR circuit  19  of  FIG. 7  can be provided in an easy-to-use three-terminal package in this way. 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. An LFVR circuit utilizing capacitive current splitting is not limited to use in power electronics, but rather sees general applicability. For example, where a capacitor is to be charged by current flow through a rectifying diode, the LFVR circuit can be applied by performing the pi filter substitution for the capacitor, and by placing the diode as appropriate for the type of LFRV circuit to be employed, and then by substituting the LFVR circuit for the diode and components of the pi filter as explained above in the example of  FIGS. 8-10 . Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.