Patent Publication Number: US-2015085544-A1

Title: Rectifying Circuit, Electronic Circuit, and Electronic Apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-198923, filed on Sep. 25, 2013; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a rectifying circuit, an electronic circuit, and an electronic apparatus. 
     BACKGROUND 
     It is desirable that rectifying circuits such as high breakdown voltage diodes used in a power source apparatus and the like have a low loss. In addition, for example, the diodes are required to have resistance to superimposed lightning surge currents in a power source apparatus connected to an AC power supply line. Elements made of a wide band gap compound semiconductor attract attention as such diodes. Among them, a diode (hereinafter, referred to as a GaN diode) formed using a nitride semiconductor such as GaN has a high saturation electron speed, and is currently being put into practical use as a high-speed device. 
     In the GaN diode, a forward voltage has a positive temperature coefficient with respect to a temperature. Under room temperature circumstances, the GaN diode has a forward voltage lower than a silicon diode and thus operates at a low loss, but, in a high temperature region, the GaN diode has a conduction loss greater than that of the silicon diode which has a negative temperature coefficient. In addition, the GaN diode has a forward voltage lower than that of the silicon diode in a small current region. However, in a large current region, the silicon diode showing an exponential current-voltage characteristic has a lower forward voltage, and the GaN diode has a greater conduction loss. 
     If an unintended excessive current such as a lightning surge or the like flows, a forward voltage rapidly increases and thus a great loss occurs in the GaN diode. Breakdown resistance is reduced further than that of the silicon diode. 
     The same problem also occurs in a normally-on type FET which uses GaN whose On-voltage has a positive temperature coefficient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram exemplifying a rectifying circuit according to a first embodiment; 
         FIG. 2  is a characteristic diagram illustrating temperature-dependency of forward voltage-forward current curves of silicon and GaN diodes; 
         FIG. 3  is a circuit diagram exemplifying a rectifying circuit according to a second embodiment; 
         FIG. 4  is a characteristic diagram illustrating dependency of a drain current of a normally-on type element on a potential of a control terminal; 
         FIG. 5  is a cross-sectional view exemplifying a rectifying circuit according to a third embodiment; 
         FIG. 6  is a cross-sectional view exemplifying a rectifying circuit according to a fourth embodiment; 
         FIGS. 7A and 7B  are circuit diagrams exemplifying a fifth embodiment; and 
         FIG. 8  is a circuit diagram exemplifying a sixth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment, a rectifying circuit includes a first rectifying portion and a second rectifying portion. The first rectifying portion has a positive temperature coefficient. The second rectifying portion has a negative temperature coefficient, is connected in parallel to the first rectifying portion, and has a forward voltage-forward current curve intersecting a forward voltage-forward current curve of the first rectifying portion. 
     According to another embodiment, there is provided an electronic circuit including a rectifying circuit. The rectifying circuit includes a first rectifying portion and a second rectifying portion. The first rectifying portion has a positive temperature coefficient. The second rectifying portion has a negative temperature coefficient, is connected in parallel to the first rectifying portion, and has a forward voltage-forward current curve intersecting a forward voltage-forward current curve of the first rectifying portion. 
     According to still another embodiment, there is provided an electronic apparatus including an electronic circuit. The electronic circuit includes a rectifying circuit. The rectifying circuit includes a first rectifying portion and a second rectifying portion. The first rectifying portion has a positive temperature coefficient. The second rectifying portion has a negative temperature coefficient, is connected in parallel to the first rectifying portion, and has a forward voltage-forward current curve intersecting a forward voltage-forward current curve of the first rectifying portion. 
     Various embodiments will be described hereinafter with reference to the accompanying drawings. In the following description, the same constituent elements are given the same reference numerals, and description of a constituent element described once will not be repeated. 
     First Embodiment 
       FIG. 1  is a circuit diagram exemplifying a rectifying circuit according to a first embodiment. 
     A rectifying circuit  1  includes a first rectifying portion  2  and a second rectifying portion  3 . The first rectifying portion  2  and the second rectifying portion  3  are connected in parallel to each other. 
     The first rectifying portion  2  has a positive temperature coefficient. The second rectifying portion  3  has a negative temperature coefficient. Forward voltage-forward current curves thereof intersect each other. 
     As the rectifying portion having a positive temperature coefficient, a diode which is made of a compound semiconductor or an oxide semiconductor may be used. The compound semiconductor is, for example, gallium nitride (GaN) or silicon carbide (SiC). The oxide semiconductor is, for example, zinc oxide (ZnO). As the rectifying portion having a negative temperature coefficient, for example, a diode (hereinafter, referred to as a silicon diode) made of silicon may be used. 
     Hereinafter, as an example, a description will be made of a case where a GaN diode is used as the first rectifying portion, and a silicon diode is used as the second rectifying portion. As will be described later, forward voltage-forward current curves of the diodes intersect each other. 
       FIG. 2  is a characteristic diagram illustrating temperature-dependency of forward voltage-forward current curves of the silicon and GaN diodes. 
     The transverse axis of  FIG. 2  expresses a forward voltage, and the longitudinal axis thereof expresses a forward current. If a temperature increases from 25° C. to 150° C., in the GaN diode, a slope of the forward voltage-forward current curve decreases. In other words, an On-resistance increases, and thus a forward voltage increases. In the silicon diode, the forward voltage-forward current curve transitions in a low voltage direction, and thus a forward voltage is reduced. 
     The GaN diode has a linear forward voltage-forward current curve, whereas the silicon diode has an exponential forward voltage-forward current curve. Even at the same temperature, in a small current region, the GaN diode has a low forward voltage, and, in a large current region, the silicon diode has a low forward voltage. As mentioned above, the forward voltage-forward current curves of both diodes intersect each other. 
     Therefore, if both diodes are connected in parallel to each other, so as to form the rectifying circuit  1 , the diode having a lower forward voltage performs a rectifying operation. In a low temperature region, the GaN diode performs a rectifying operation, and, in a high temperature region, the silicon diode performs a rectifying operation. Meanwhile, in a small current region, the GaN diode performs a rectifying operation, and, in a large current region, the silicon diode performs a rectifying operation. A forward voltage of the rectifying circuit  1  is the same as a lower forward voltage of the GaN diode and the silicon diode. 
     Effects of the First Embodiment Will be Described. 
     According to the embodiment, an effect can be achieved in which a loss is reduced since a forward voltage can be reduced in wider ranges of temperature and current than when the GaN diode or the silicon diode is used singly. An effect can also be achieved in which a surge resistance which is equivalent to a surge resistance of the silicon diode is obtained even if surge currents are superimposed. 
     Second Embodiment 
       FIG. 3  is a circuit diagram exemplifying a rectifying circuit according to a second embodiment. 
     A rectifying circuit  4  includes a first rectifying portion  5  and a second rectifying portion  3 . The first rectifying portion  5  and the second rectifying portion  3  are connected in parallel to each other. 
     The first rectifying portion  5  includes a diode  7  and a transistor  6  which is connected in series to a cathode of the diode  7  and has a gate which is a control terminal connected to an anode of the diode  7 . The second rectifying portion  3  is a silicon diode. 
     The transistor  6  is a normally-on type field effect transistor made of a compound semiconductor or an oxide semiconductor, and is, for example, a high electron mobility transistor (HEMT) made of GaN. The diode  7  is, for example, a silicon Schottky barrier diode. 
     An operation of the first rectifying portion  5  will be described. 
     When a forward voltage is applied, that is, a positive voltage is applied to the anode side of the diode  7 , the diode  7  is turned on, and thus the transistor  6  which is a normally-on type element is also turned on. For this reason, the rectifying portion is turned to an On state. When a backward voltage is applied, that is, a negative voltage is applied to the anode side of the diode  7 , the diode  7  is turned off. A gate-source voltage Vgs of the transistor  6  becomes a negative voltage, and thus the transistor  6  is also turned off. Therefore, the rectifying portion  5  is turned to an Off state. 
     When the backward voltage is applied, a reverse voltage applied to the diode  7  is Vgs of the transistor  6 , and thus a low breakdown voltage silicon Schottky barrier diode can be used as the diode  7 . Generally, the low breakdown voltage silicon Schottky barrier diode has a low forward voltage, and also has a low forward voltage during turned-on of the transistor  6 , and thus a forward voltage of the rectifying portion  5  is lower than that of a single GaN diode. 
     Characteristics of the Transistor  6  Will be Described with Reference to  FIG. 4 . 
       FIG. 4  is a characteristic diagram illustrating dependency of a drain current of the normally-on type transistor on a potential of the control terminal. The transverse axis of  FIG. 4  expresses a drain-source voltage, and the longitudinal axis expresses a drain current. 
     As is clear from  FIG. 4 , if a drain current Id reaches a predetermined current value, an On-resistance of the normally-on type transistor  6  increases. In other words, the normally-on type transistor  6  shows a constant current characteristic. The drain current Id in a state of showing the constant current characteristic depends on a gate-source voltage Vgs. As an absolute value of the gate-source voltage Vgs increases, a value of the drain current Id in the constant current characteristic decreases. 
     When a forward voltage is applied to the rectifying portion  5 , Vgs of the normally-on type transistor  6  becomes a voltage corresponding to a forward voltage of a silicon Schottky barrier diode, for example, 0.2 V. In this case, the drain current Id of the normally-on type transistor  6  in the constant current characteristics is equivalent to the maximal drain current Id in  FIG. 4 . If a current which is equal to or greater than the maximal current is to be made to flow, a forward voltage rapidly increases. 
     In addition, since GaN has a positive temperature coefficient, an On-resistance increases even under high temperature circumstances, and thus a forward voltage also increases. 
     Therefore, if a silicon diode which is the second rectifying portion is connected in parallel so as to form a rectifying circuit, the diode or the rectifying portion having a lower forward voltage performs a rectifying operation. In a low temperature region, the GaN diode performs a rectifying operation, and, in a high temperature region, the silicon diode performs a rectifying operation. Meanwhile, in a small current region, the GaN diode performs a rectifying operation, and, in a large current region, the silicon diode performs a rectifying operation. A forward voltage of the rectifying circuit  1  is the same as a lower forward voltage of the GaN diode and the silicon diode. 
     Effects of the Second Embodiment Will be Described. 
     An effect can be achieved in which a loss is reduced since a forward voltage can be reduced in wider ranges of temperature and current than when the rectifying portion  5  including the normally-on type field effect transistor made of a compound semiconductor or an oxide semiconductor and the silicon Schottky barrier diode is used singly. In the same manner as in the rectifying circuit according to the first embodiment, an effect can also be achieved in which a surge resistance which is equivalent to a surge resistance of the silicon diode is obtained. 
     Third Embodiment 
     The GaN diode and the silicon diode of the rectifying circuit illustrated in  FIG. 1  may be mounted in the same package. A cross-sectional view of the rectifying circuit in this case is illustrated in  FIG. 5 . 
       FIG. 5  is a cross-sectional view exemplifying a rectifying circuit according to a third embodiment. 
     A rectifying circuit  8  includes a GaN diode  9  and a silicon diode  10 . The GaN diode  9  and the silicon diode  10  are mounted on a metal support substrate  11 . The GaN diode  9  includes a cathode electrode  12 , a semiconductor substrate  13 , and an anode electrode  14 . The semiconductor substrate  13  has a silicon substrate, a buffer layer, an n type GaN semiconductor, and a p type GaN semiconductor. The silicon diode  10  includes a cathode electrode  15 , a semiconductor substrate  16 , and an anode electrode  17 . The semiconductor substrate  16  has an n type silicon semiconductor and a p type silicon semiconductor. 
     The anode electrode  14  is connected to the anode electrode  17  via a connection conductor  18 . A terminal  19  is connected to the anode electrode  14  or  17 , and a terminal  20  is connected to the metal support substrate  11 . 
     Effects of the Third Embodiment Will be Described. 
     According to the embodiment, an effect can be achieved in which a loss is reduced since a forward voltage can be reduced in wider ranges of temperature and current than when the GaN diode or the silicon diode is used singly. An effect can also be achieved in which a surge resistance which is equivalent to a surge resistance of the silicon diode is obtained even if surge currents are superimposed. The GaN diode and the silicon diode are mounted on the same metal support substrate so as to produce a single package, and thus an effect of miniaturizing the rectifying circuit can also be achieved. 
     Fourth Embodiment 
     The GaN diode and the silicon diode of the rectifying circuit illustrated in  FIG. 1  may be formed using a monolithic semiconductor structure. A cross-section of a rectifying circuit in this case illustrated in  FIG. 6 . 
       FIG. 6  is a schematic cross-sectional view exemplifying a rectifying circuit according to a fourth embodiment. 
     A rectifying circuit  21  includes a GaN diode  22  and a silicon diode  23 . The GaN diode  22  has a semiconductor substrate  26  and an anode electrode  14 . The semiconductor substrate  26  includes an n type GaN semiconductor and a p type GaN semiconductor. The silicon diode  23  has a diode region  27  which is provided inside a silicon substrate  25  as a diffusion layer, and an anode electrode  17 . The diode region  27  includes an n type silicon semiconductor and a p type silicon semiconductor. A cathode electrode  24  is shared by the GaN diode  22  and the silicon diode  23 , and is provided on a rear surface of the silicon substrate  25 . In addition, the silicon substrate  25  may be a growth substrate which crystal-grows the GaN diode  22 , or may be a support substrate obtained by appropriately removing a growth substrate on which a laminate structure of the GaN diode  22  is grown and by joining the laminate structure to a substrate. 
     Effects of the Fourth Embodiment Will be Described. 
     According to the embodiment, an effect can be achieved in which a loss is reduced since a forward voltage can be reduced in wider ranges of temperature and current than when the GaN diode or the silicon diode is used singly. An effect can also be achieved in which a surge resistance which is equivalent to a surge resistance of the silicon diode is obtained even if surge currents are superimposed. The GaN diode and the silicon diode are formed as a single package by using a monolithic semiconductor structure, and thus an effect of miniaturizing the rectifying circuit can also be achieved. 
     Fifth Embodiment 
     Next, a description will be made of an electronic circuit provided with the rectifying circuit according to the present embodiment. 
       FIGS. 7A and 7B  are circuit diagrams respectively exemplifying electronic circuits according to the fifth embodiment. 
     An electronic circuit  28  illustrated in  FIG. 7A  includes a bridge rectifier  30 , an inductor  31 , a switching element  32 , a rectifying element  33 , a capacitor  34 , and a control circuit (not illustrated). The switching element  32  is, for example, a metal-oxide-semiconductor field effect transistor (MOSFET). At least one of respective diodes forming the bridge rectifier  30  and the rectifying element  33  is the rectifying circuit according to the embodiments. 
     An AC power supply line  29  is connected to an input of the bridge rectifier  30 . One end of the inductor  31  is connected to a high potential output terminal  50  of the bridge rectifier  30 , and the other end of the inductor  31  is connected to a drain of the switching element  32 . A source of the switching element  32  is connected to a low potential output terminal  51  of the bridge rectifier  30 . An anode of the rectifying element  33  is connected to the drain of the switching element  32 , and a cathode of the rectifying element  33  is connected to one end of the capacitor  34 . The other end of the capacitor  34  is connected to the low potential output terminal  51  of the bridge rectifier  30 . 
     The electronic circuit  28  is a circuit which boosts a voltage by rectifying an AC voltage from the AC power supply line  29  and turning on and off the switching element  32 . 
     An electronic circuit  37  illustrated in  FIG. 7B  includes a rectifying element  39  and a capacitor  40 . The rectifying element  39  is a rectifying circuit according to the embodiments. 
     An anode of the rectifying element  39  is connected to a high potential terminal  52  of a DC power supply line  38 , and a cathode of the rectifying element  39  is connected to one end of the capacitor  40 . The other end of the capacitor  40  is connected to a low potential terminal  53  of the DC power supply line  38 . The rectifying circuit according to the embodiments is applied to the rectifying element  39 . 
     The electronic circuit  37  operates as a reverse connection protecting circuit of the DC power supply line. 
     Effects of the Fifth Embodiment Will be Described. 
     According to the embodiment, by employing the rectifying circuit according to the embodiments, an effect can be achieved in which a loss of the electronic circuit is reduced in wide ranges of temperature and current. Even if surge currents are superimposed, a surge resistance which is equivalent to a surge resistance of the silicon diode is obtained, and thus an effect can also be achieved in which an electronic circuit with higher reliability can be provided than when the GaN diode is used. 
     Sixth Embodiment 
       FIG. 8  is a circuit diagram exemplifying a sixth embodiment. 
     An electronic apparatus  41  illustrated in  FIG. 8  includes an electronic circuit  42  and a lighting load  49 . The lighting load  49  has a lighting light source such as, for example, a light emitting diode (LED). 
     The electronic circuit  42  includes a bridge rectifier  30 , a rectifying element  43 , a capacitor  44 , an inductor  45 , a switching element  46 , a rectifying element  47 , and a capacitor  48 . The switching element  46  is, for example, a MOSFET. At least one of respective diodes forming the bridge rectifier  30  and the rectifying elements  43  and  47  is the rectifying circuit according to the embodiments. 
     An AC power supply line  29  is connected to an input of the bridge rectifier  30 . An anode of the rectifying element  43  is connected to a high potential output terminal  50  of the bridge rectifier  30 , and a cathode of the rectifying element  43  is connected to one end of the capacitor  44 . The other end of the capacitor  44  is connected to a low potential output terminal  51  of the bridge rectifier  30 . One end of the inductor  45  is connected to the cathode of the rectifying element  43 , and the other end of the inductor  45  is connected to a drain of the switching element  46 . A source of the switching element  46  is connected to the low potential output terminal  51  of the bridge rectifier  30 . An anode of the rectifying element  47  is connected to one end of the capacitor  48 , and the other end of the capacitor  48  is connected to the low potential output terminal  51  of the bridge rectifier  30 . The lighting load  49  is connected in parallel to the capacitor  48 . 
     The electronic apparatus  41  is a lighting apparatus which rectifies and drops an AC voltage from the AC power supply line  29  and supplies the dropped voltage to the lighting load  49 . In this lighting apparatus, there is a probability that power consumption widely varies from turning-off thereof to complete turning-on thereof. For this reason, a magnitude of an input current also widely fluctuates. Since the lighting apparatus is connected to the AC power supply line, the lighting apparatus is required to have resistance to a surge current such as a lightning surge. By employing the rectifying circuit according to the embodiments, an effect can be achieved in which a loss is reduced in wide ranges of temperature and current since a forward voltage of the rectifying element can be reduced. Even if surge currents are superimposed, a surge resistance which is equivalent to a surge resistance in a case of using the silicon diode is obtained. 
     Effects of the Sixth Embodiment Will be Described. 
     According to the embodiment, by employing the rectifying circuit according to the embodiments, an effect can be achieved in which a loss of an electronic apparatus is reduced. Since the rectifying circuit according to the embodiment obtains a surge resistance which is equivalent to a surge resistance of the silicon diode, an effect can also be achieved in which an electronic apparatus with high reliability can be provided. 
     As mentioned above, the embodiments are described with reference to specific examples, but are not limited thereto and may have various modifications. 
     For example, the first and second rectifying portions are not limited to GaN. For example, a semiconductor element may be used which is formed on a semiconductor substrate by using a semiconductor (wide band gap semiconductor) having a wide band gap, such as, for example, silicon carbide (SiC), gallium nitride (GaN), or diamond. Here, the wide band gap semiconductor refers to a semiconductor having a wider band gap than that of gallium arsenide (GaAs), whose band gap is about 1.4 eV. The semiconductor includes semiconductors whose band gap is 1.5 eV or greater, for example, gallium phosphide (GaP whose band gap is about 2.3 eV), gallium nitride (GaN whose band gap is about 3.4 eV), diamond (C whose band gap is about 5.27 eV), aluminum nitride (AlN whose band gap is about 5.9 eV), silicon carbide (SiC), and the like. 
     In addition, the lighting load is not limited to an LED, and may be, for example, an organic electroluminescence (OEL) element, an organic light emitting diode (OLED), or the like. 
     As mentioned above, the embodiments are described with reference to the specific examples. However, the embodiments are not limited to the specific examples. In other words, ones obtained by those skilled in the art adding an appropriate design modification to the specific examples are included in an embodiment as long as they have features of the embodiment. The constituent elements of each of the above-described specific examples, the arrangements, the materials, the conditions, the shapes, the sizes, and the like thereof are not limited to the exemplified ones, and may be appropriately changed. 
     In addition, the respective constituent elements of each of the above-described embodiments may be combined with each other as long as the combination is technically possible, and the combination is also included in an embodiment as long as the combination includes features of the embodiment. Further, those skilled in the art can conceive of various changes and alterations in the scope of the spirit of the embodiments, and it is understood that the changes and alterations are included in the embodiments. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.