Abstract:
A series-regulator type of power supply circuit is provided. In the circuit, the emitter and collector of a transistor are connected to power input/output terminals. A control circuit controls a base current of the transistor based on the output voltage detected at the power output terminal and a given target voltage. A resistor circuit connects the base and the collector of the transistor. A bypass circuit connects the emitter and the base of the transistor and passes a bypass current. The accepting circuit connected to the power output terminal accepts (absorbs) current from an output current. An amount of the acceptance current is equal to or larger than an amount of the bypass current and a product of the bypass current and a resistance value of the resistance circuit is equal to or more than a difference between a voltage at the power input terminal and the target voltage.

Description:
CROSS REFERENCES TO RELATED DOCUMENT 
   The present application is a continuation-in-part application of Ser. No. 10/602,605 filed on Jun. 25, 2003 now abandoned and the entire disclosure of Japanese Patent Application No. 2002-186016 filed on Jun. 26, 2002 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The present invention relates to a power supply circuit with a series regulator. 
   2. Related Art 
   Power supply circuits, which are required by almost all electronic apparatuses, can be categorized into a large number of types, one of which is a series-regulator type of power supply circuit. 
     FIG. 1  exemplifies the electronic configuration of a series-regulator type of power supply circuit, which has typically been used by in-vehicle electronic equipment, such as ECU (Electronic Control Unit). 
   The power supply circuit  1  shown in  FIG. 1  has a supply circuit  4  (main power supply) to which a voltage VS is supplied from a battery  2  via an ignition (IG) switch  3  and a second supply circuit  5  (auxiliary power supply) to which the voltage VB is supplied directly from the battery  2 . Outputs of both supply circuits  4  and  5  are connected to a common output terminal  6  connected to a load circuit  7 . The input side of the supply circuit  4  is connected to a second load circuit  8 . The supply circuits  4  and  5  include main transistors  9  and  10 , respectively. The emitter and a base of each main transistor  9  ( 10 ) are connected to its input and output. These two-systemized supply circuits  4  and  5  individually compose series regulators that operate on mutually-different target output voltages. 
   This series-regulator type of power supply circuit  1  operates as follows. When the ignition switch  3  is in the on-state, the supply circuits  4  and  5  both work, so that the voltage Vo at the output terminal  6  is stabilized to either one, which is higher than the other, of the target output voltage of the supply circuit  4  or that of the supply circuit  5 . Meanwhile, when the ignition switch  3  is in the off-state, the supply circuit  5  operates alone, so that the voltage Vo at the output terminal  6  is stabilized to the target output voltage of the supply circuit  4 . 
   In the latter case, the base and collector of the PNP-type transistor  9  are inserted into the circuit in the forward direction. Therefore, though it depends on how the load circuit  8  is configured, it may happen that current flows in the backward direction from the supply circuit  5  to the load circuit  8  via the collector and base of the transistor  9  and the resistor  11 . 
   In order to avoid such a backward direction current, a conceivable countermeasure is to place a diode between the ignition switch  3  and the transistor  9 . However, placing the diode there gives rise to a decrease in the input voltage to the supply circuit  4  correspond to the forward voltage Vf of the diode, thus providing a swell in a minimum operating voltage to the battery voltage VB. 
   The problem of this flow of backward current is not always inherent to the configuration where the two-systemized supply circuits  4  and  5  use a common output terminal. Such a problem may arise even in one-system power supply circuits, as long as there is a possibility that the power supply circuit is subjected to an inverse application of voltage from the load circuit  7 . 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide, with due consideration to the drawbacks of the above conventional configuration, a series-regulator type of power supply circuit capable of preventing current flowing from an output terminal to an input terminal in the power supply circuit. 
   In order to accomplish the above object, the present invention provides a power supply circuit comprising: a transistor of which emitter and collector are connected to a power input terminal and a power output terminal, respectively; a voltage detection circuit configured to detect an output voltage at the power output terminal; a voltage control circuit connected to a base of the transistor and configured to control a base current of the transistor on the basis of both of the output voltage detected by the voltage detection circuit and a given target voltage; a resistor circuit placed to connect the base and the collector of the transistor; a current bypass circuit placed to connect the emitter and the base of the transistor and configured to bypass the transistor so that a bypass current flows through the current bypass circuit; and a current accepting circuit connected to the power output terminal and configured to accept a given amount of current from an output current passing the power output terminal by performing either absorption or discharge of the given amount of current, wherein the amount of current to be accepted is equal to or larger than an amount of the bypass current and a product of the amount of the bypass current and a resistance value of the resistance circuit is equal to or more than a difference between a voltage at the power input terminal and the target voltage. 
   That is, in this power supply circuit, the resistor circuit is inserted between the base and the collector (not between the emitter and the base) of the transistor arranged between the power input/output terminals. This resistor circuit is able to fix a potential at the base to an amount equal to a potential at the collector, thereby strengthening resistance to noise. 
   In addition, in the case of the circuitry of this power supply circuit, the emitter/base of the transistor provides a backward conjunction against the voltage applied to the power output terminal. And this circuitry provides no current path bypassing the emitter/base of the transistor. Accordingly, a backward current through the emitter/base of the transistor can be prevented, owing to the fact that the junction between the emitter/base of the transistor has a characteristic of cutting off the backward current. 
   Meanwhile, an input voltage is applied to the power input terminal, a base potential of the transistor rises up to a value near to the input voltage in reply to an emitter potential, so that the resistor circuit undergoes application of a voltage nearly equal to a difference between the input and output voltages. This voltage applied to the resistor circuit causes a current flowing therethrough. This current, however, flows as a bypass current supplied by the current bypass circuit placed between the emitter/base of the transistor, not supplied as a base current. Since a product of the bypass current and a resistance of the resistor circuit is equal to or more than a difference of “the input voltage−the target voltage,” all the current passing the resistor circuit in the condition where the output voltage is controlled to the target voltage can be supplied from the current bypass circuit. It is therefore possible to suppress the base current occurring due to the fact that the resistor circuit is added to the emitter/base of the transistor, thus preventing an unwanted swell in the output voltage on account of an excessive flow of the base current. 
   In cases where a load current decreases while the input voltage is applied to the power input terminal, it is difficult, if there is no current acceptance circuit configured according to the present invention, to give the resistor circuit the current necessary for suppressing the unwanted swell in the output voltage, which may bring about a situation where a voltage drop across the resistor circuit is reduced, resulting in an increase in the output voltage. 
   However, in the present embodiment, the current acceptance circuit is provided to avoid such an inconvenient situation. The current acceptance circuit has a capability of accepting current, the capability being equal to or higher than an amount of the bypass current. The current acceptance circuit thus absorbs or discharges the current that passes the resistance circuit. It is thus possible to make the current flow the resistance circuit even when there is no load, the current being required to suppress an unwanted swell in the output voltage. The output voltage can be controlled to the target voltage regardless of fluctuations in the amount of the load. 
   It is preferred that the current acceptance circuit is composed of a constant-current circuit. This makes it possible that, even when the output voltage fluctuates, the current acceptance circuit is able to steadily accept (practically, absorb or discharge) the current passing the resistor circuit from the current bypass circuit. The output voltage can be prevented from increasing beyond control. 
   It is still preferred that the current acceptance circuit is composed of a resistor. When giving the resistor an appropriately selected resistance value that is able to provide an amount of current equal to or higher than the bypass current, to an amount of the bypass current that flows under a condition where the output voltage is controlled to the target voltage, the output voltage can steadily be prevented from increasing beyond the target voltage. 
   Preferably, the current acceptance circuit is configured to absorb or discharge the acceptance current only when the current bypass circuit allows the bypass current to flow therethrough. Hence, in cases where the input voltage is not applied to the power input terminal so that the current bypass circuit is noting to do with the output of a bypass current, the current acceptance circuit is able to stop its current acceptance operation. An unnecessary output current will not therefore be stopped, thus saving a consumed power in the power supply circuit, thus increasing efficiency in energy saving. 
   Still, by way of example, it is preferred that the current bypass circuit is composed of a constant-current circuit. When the constant-current circuit is used, it is possible to provide a constant current that permits a product of the input voltage (which may fluctuate) and a resistance value of the resistor circuit to become an amount equal to or higher than a maximum difference between the input and output voltages. This prevents the output voltage from increasing over the target voltage in a reliable manner. 
   It is also preferred to, in addition to the main supply circuit, to comprise an auxiliary supply circuit configured to control the voltage at the power output terminal, independently of the voltage control performed by the main supply circuit. In this case, if the operation of the main supply circuit is stopped while one or more auxiliary supply circuits are in operation, a backward current circulating from the main supply circuit to the auxiliary supply circuits is eliminated. Without additional use of a backward-current preventing circuit such as a diode, there can be provided a plurality of supply circuit systems connected together to a common power output terminal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  shows the electrical configuration of a conventional power supply circuit applied to an in-vehicle ECU; 
       FIG. 2  shows the electrical configuration of a power supply circuit, which is applied to an in-vehicle ECU, according to an embodiment of the present invention; 
       FIGS. 3A and 3B  each show the electrical configurations of essential parts of the power supply circuits that were studied for achieving the power supply circuit according to the present invention; and 
       FIG. 4  shows an electrical configuration explaining a modification of the power supply circuit according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 2 to 3A  and  3 B, an embodiment of the present invention will now be described. 
     FIG. 2  shows in detail a power supply circuit, which is particularly taken from the electrical configuration of an ECU (Electrical Control Unit)  21  for use in vehicles (cars). 
   The ECU  21  has terminals  21   a  to  21   c , as shown therein. One of the terminals,  21   a , is electrically connected to a positive terminal of a battery  22  via an ignition (IG) switch  23 , while the other terminals  21   b  and  21   c  are electrically connected with the positive terminal and a negative terminal of the battery  22 , respectively. 
   The ECU  21  includes a frame (not shown), which incorporates a substrate (not shown). On the substrate, a power supply circuit  24  constructed in the form of an IC, a load circuit  25  that operates on power voltage supplied from the power supply circuit  24 , and a second load circuit  26  electrically connected with both the terminals  21   a  and  21   c  are provided. 
   Of these components, the load circuit  25 , which is configured in the form of an IC different from the power supply circuit  24 , includes a microcomputer serving as a main device therein. This microcomputer is formed to have both a normal operation mode and a low-power-consumption operation mode, and can selectively be switched one from the other. When the load circuit  25  is in low-power-consumption operation mode, consumed current is much lower to a large extent than that in the normal operation mode. 
   Meanwhile, the load circuit  26  includes a series circuit consisting of a switching element and a solenoid or relay coil, the switching element being subject to on/off control under a microcomputer. 
   The power supply circuit  24 , which has terminals  24   a  to  24   c  formed as IC terminals, is provided with a supply circuit  27  (serving as a main power supply) intervening between the terminals  24   a  and  24   c  and a second supply circuit  28  (serving as an auxiliary power supply) intervening between the terminals  24   b  and  24   c . The IC input terminals  24   a  and  24   b  are coupled with the terminals  21   a  and  21   b  of the ECU  21 , respectively, while the output terminal  24   c  and the ground terminal  24   d  are coupled with power input terminals of the load circuit  25 , respectively. 
   The supply circuits  27  and  28  are configured to have target output voltages of 5.0 [V] and 4.9 [V], respectively, and individually operate as a series regulator for controlling an output voltage Vo at the terminal  24   c  in a constant voltage control manner. One of the supply circuits,  27 , has a configuration described below. 
   Both the terminals  24   a  and  24   c  are connected to an emitter and a collector of a PNP-type transistor  29  functioning as a main transistor. The base and the collector of the transistor  29  are connected to both terminals of a resistor  30  (composing a resistor circuit), while the base of the transistor  29  is electrically connected to the ground via the collector and emitter of a driving NPN-type transistor  31 . 
   Further, the terminal  24   c  and the ground are connected to both terminals of a voltage dividing circuit  34  consisting of serially connected resistors  32  and  33  (composing a voltage detecting circuit). A resistor-connected point at which the voltage is divided is electrically connected to an inverting input terminal of the operational amplifier  35  that operates on the power from the terminal  24   a . The output terminal of this operational amplifier  35  is connected to the base of the foregoing driving transistor  31 , while a non-inverting input terminal of the operational amplifier  35  is connected to a reference voltage generating circuit  36  to output a reference voltage Vr 1  corresponding to a target output voltage (5.0 [V]). In this configuration, the transistor  31  and operational amplifier  35  compose a voltage control circuit. 
   Still further, the emitter and the base of the transistor  29  are connected to a transistor  38  (composing a current bypass circuit), and the terminal  24   c  and the ground are connected to a constant-current circuit  39  (composing a current accepting circuit). Each of the transistor  38  and the constant-current circuit  39  is driven by a bias voltage produced by a bias circuit  37 . The transistor  38 , a transistor  40  constructing the constant-current circuit  39 , and a transistor (not shown) constructing the bias circuit  37  have circuitry, in which all the bases thereof are connected together to a common base and all the emitters thereof are connected together to a common emitter. The constant-current circuit  39  is provided with a transistor  41  electrically inserted between the terminal  24   c  and the ground a further transistor  42  electrically inserted between the transistor  40  and the ground, both the transistors  41  and  42  composing a current mirror circuit. 
   This current mirror circuit configuration can be applied to both the transistors  38  and  40 . As a result, a current ratio between the current bypass circuit and the current accepting circuit can be fixed, thus making it possible to steadily set the current to be accepted to an amount equal to or more than the bypass current. 
   It is particularly preferred that, if both the transistors  41  and  4 Z are arranged close to each other to achieve the shortest wiring lengths therebetween so that a shift in the mirror ratio can be reduced. This arrangement for the shortest wiring length technique can also be applied to both the transistors  38  and  40 , which can reduce a shift in the mirror ratio as well. 
   In contrast, the remaining supply circuit  28  is configured in a similar way to the conventional. To be specific, a PNP-type transistor  43  is placed so that its emitter and collector are electrically connected to the terminals  24   b  and  24   c , while a resistor  44  intervenes between the emitter and the base of the transistor  43 . The base of the transistor  43  is grounded through the collector and emitter of a driving transistor  45 . 
   Furthermore, between the terminal  24   c  and the ground, there is connected a voltage-dividing circuit  48  consisting of serially connected resistors  46  and  47 . An intermediate point between the resistors  46  and  47 , at which the voltage is divided, is electrically connected to an inverting input terminal of an operational amplifier  49 . This operational amplifier  49 , which is driven on power supplied through the terminal  24   b , has an output terminal electrically connected to the base of the driving transistor  45  and a non-inverting input terminal electrically connected to a reference voltage generating circuit  50  outputting a reference voltage Vr 2  that corresponds to a further target output voltage (i.e., 4.9 [V]). Incidentally, each of the reference voltage generating circuits  36  and  50  is made with the use of, for example, a band-gap reference voltage circuit. 
   Referring to  FIGS. 2 ,  3 A and  3 B, the ECU  21  including the power supply circuit  24  will now be explained in terms of its operation. 
   When the ignition switch  23  in the on-state is turned off, the supply circuit  27  stops supplying the power, with the result that the other supply circuit  28  begins a constant-voltage operation, thus providing an output voltage Vo of 4.9 [V]. During this operation, a backward current from the collector of the transistor  29  to the emitter thereof will not flow, due to the reason described later. The microcomputer included in the load circuit  25  is able to sense an on/off operation of the ignition switch  23 . In response to a transition of the ignition switch  23  from its on-state to its off-state, the operation mode of the microcomputer will immediately shift from its normal operation mode to the low-power-consumption operation mode. Though the supply circuit  28  is set to a reduced current output capacity compared to that of the supply circuit  27  (whereby reducing power usage), it is still sufficient to supply power to the load circuit  25 . 
   In contrast, in response to a switchover of the ignition switch  23  from its off-state to its on-state, both of the supply circuits  27  and  28  are put into operation. Hence the output voltage Vo is stabilized to 5.0 [V], which is higher one of the target output voltage of the supply circuit  27  or that of the supply circuit  2 . In consequence, the supply circuit  28  of which target output voltage is 4.9 [V] turns the transistor  43  into its off-state, because the voltage error at the inputs of the operational amplifier  49  becomes a negative value. The microcomputer in the load circuit  25  shifts its operation mode from the low-power-consumption operation mode to the normal operation mode, so that the microcomputer is able to receive the power from the supply circuit  27 . 
     FIGS. 3A and 3B  each show the electrical configuration of essential parts of power supply circuits that were studied by the present inventors in the process for achieving the power supply circuit  24  ( FIG. 2 ) according to the present embodiment based on the conventional power supply circuit  1  ( FIG. 1 ). In  FIGS. 3A and 3B , the identical components to those in  FIG. 2  are represented by the same reference numbers.  FIGS. 3A and 3B  are not intended to show the formal power supply circuit, but introduced to explain only the significance of the presence of both the transistor  38  and constant-current circuit  39  in the power supply circuit  24 . 
   The power supply circuit shown in  FIG. 3A  has identical circuitry to that of the conventional power supply circuit  1  except that the resistor  30  is inserted between the base and collector of the transistor  29 , not the emitter and base thereof. In this configuration, if the ignition switch  23  is in its off-state, the constant voltage of 4.9 [V] outputted from the transistor  43  is applied as a backward voltage to the base/emitter junction of the transistor  29 . Thus a backward current is prevented from flowing into the load circuit  26  via the transistor  29 . In addition, the potential at the base of the transistor  29  is fixed to an amount that is the same as a potential at the collector thereof, thereby enhancing resistance to noise. 
   However the power supply circuit shown in  FIG. 3A  has a difficulty as follows. When the ignition switch  23  is switched to its off-state, the potential at the base of the transistor  29  becomes “VB−Vf (Vf: forward voltage),” so that a current proportional to “VB−Vf−Vo” flows through the resistor  30 . All of this current passing through the resistor  30  contributes to the base current of the transistor  29  independently of what state the transistor  31  takes. Because such base current will lead to a swell in the output voltage Vo, the output voltage Vo is obliged to exceed a target output voltage (i.e., 5.0 [V]). 
   On the other hand, the power supply circuit shown in  FIG. 3B  is configured such that the transistor  38  is added to the circuitry described in  FIG. 3A . This transistor  38  is able to output a constant current I 1  more than a current Ia defined by the following formula (1):
 
 I 1 ≧Ia =( VB−Vf− 5.0)/ Ra   (1),
 
wherein Ra is the resistance of the resistor  30 . This constant current I 1  corresponds to a bypass current according to the present invention.
 
   In cases where Vf is sufficiently smaller than “VB−5.0,” the formula can be approximated to the following formula (2):
 
 I 1 ≧Ia =( VB− 5.0)/ Ra   (2).
 
   In this circuitry, the current Ia passing through the resistor  30  under the on-state of the ignition switch  23  is supplied by the transistor  38 , not supplied as the base current of the transistor  29 . Accordingly, under conditions where a small amount of current flows into the load, the operational amplifier  35  is able to drive the transistor  31  so as to control the base current of the transistor  29 , with the result that the output voltage Vo can be limited to a constant voltage. During this control operation, any excessive amount of current “I 1 −Ia” is grounded via the transistor  31 . However, even this circuitry has a difficulty. In other words, when the output current from this power supply circuit becomes smaller than Ia, it is impossible to force the current to pass through the resistor  30 , thus causing a swell in the output voltage Vo. 
   In order to overcome such a difficulty, the power supply circuit  24  shown in  FIG. 2  according to the present embodiment has further been improved in that the constant-current circuit  39  is added to the circuit shown in  FIG. 3B . The constant-current circuit  39  is in charge of absorbing, from the output current of the transistor  29 , a constant amount of current I 2  which is equal to the current I 1  outputted by the transistor  38 . In the present embodiment, the relationship of I 1 =I 2  is kept, but this is not a definitive list. An alternative is that the current I 2  to be absorbed is higher than I 1 ; that is, the current I 2  is to satisfy the following formula (3):
 
I2≧I1  (3).
 
   In the present embodiment, the relationship of I 1 =I 2 ≧Ia is fulfilled, so that the constant-current circuit  39  is able to absorb all the current Ia flowing though the resistor  30 , while still limiting the output voltage Vo to 5.0 [V]. This absorption makes it possible to continue keeping the current Ia flowing through the resistor  30 , even when the current flowing from the power supply circuit  24  into the load circuit  25  is reduced. Accordingly, an unwanted swell in the output voltage Vo can be prevented reliably. 
   As described above, the power supply circuit  24  of the present embodiment includes the two supply circuits  27  and  28  of which outputs are supplied to a common load, wherein the one supply circuit  27  is configured such that an input voltage supplied to the supply circuit  27  including the transistor  29  is stopped by turning off the ignition switch  23 , wherein the resistor  30  is inserted to be connected to the base and collector of the transistor  29 , instead of being connected to the emitter and base thereof. Thus, when the ignition switch  23  is in its off-state, the emitter/base Junction of the transistor  29  prevents a backward current occurring on account of the output voltage Vo. Hence a current can be prevented from circulating from the supply circuit  28  to the load circuit  26 . In addition, the base potential of the transistor  29  is fixed to its collector potential, which enhances resistance to noise. 
   In contrast, in response to switching the ignition switch  23  to its on-state, the transistor  38  supplies the resistor  30  a current Ia, while the current-constant circuit  39  absorbs the current Ia from the output current of the transistor  29 . Thus, independent of the largeness of a load current, the output voltage Vo can be adjusted to a target output voltage (in this embodiment, 5.0 [V]) under constant-voltage control. 
   The ECU on a vehicle operates on the power from the battery  22 . Thus, whenever the vehicle is not in use and the ignition switch  23  is in its off-state, it is necessary to reduce consumed current (dark current) as much as possible through various countermeasures, such as a shift of the operation mode of the microcomputer to its low-power-consumption operation mode. Though both of the transistor  38  and the constant-current circuit  39  are added to the supply circuit  27 , such an addition will not increase the dark current, because both of the transistor  38  and the constant-current circuit  39  operate to output a constant current only when the ignition switch  23  is in its on-state. 
   For the sake of completeness, it should be mentioned that the various embodiments explained so far are not definitive lists of possible embodiments. The expert will appreciate that it is possible to combine the various construction details or to supplement or modify them by measures known from the prior art without departing from the basic inventive principle. 
   By way of example, the current acceptance circuit can be configured with the use of a resistor  50  (refer to  FIG. 4 ), in place of the foregoing constant-current circuit  39  that uses the current-constant circuit. The resistance Rb of the resistor  50  can be defined based on the following formula (4):
 
 Rb≦ 5.0 /I 1  (4).
 
In this circuitry, it is preferred that a switch circuit is connected to the resistor in series in such a manner that current is permitted to flow through the resistor only when the ignition switch  23  is in its on-state.
 
   Further, the current bypassing circuit to be connected to the emitter and base of the transistor  29  is sufficient if the circuit has the characteristics of preventing a backward current flowing from the base of the transistor  29  to the emitter thereof and of being able to supply the current I 1 , so that the current bypassing circuit is not limited to a configuration that uses a constant-current circuit. 
   Still further, the present invention can be applied to a series regulator that employs an NPN type of transistor  29  as the foregoing main transistor. 
   In addition, all the NPN and PNP type transistors adopted in the power supply circuit  21  can be replaced by PNP and NPN type transistors, respectively, for the negative-voltage specification. 
   It is also possible to use N-MOS and P-MOS type transistors instead of the NPN and PNP type transistors. 
   The present invention may be embodied in several other forms without departing from the spirit thereof. The embodiments and modifications described so far are therefore intended to be only illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them. All changes that fall within the metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the claims.