Abstract:
A power supply control circuit connectable with an input power monitor that monitors an input power value to the power supply, an output power monitor that monitors an output power value from the power supply, and an output power controller that varies the output power value from the power supply, the power supply control circuit having an output power determiner that determines a determined output power value depending on the input power value and a power adjuster that adjusts the output power value to the determined output power value by controlling the output power controller.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims priority of Japanese Patent Application No. 2000-270728 filed Sep. 6, 2000, the contents being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply control circuit, power supply and power supply control method for converting an existing power supply at a first voltage and power rating to a secondary voltage and power rating. 
     2. Description of the Related Art 
     In a portable electronic device, such as a notebook size personal computer, an AC adapter or automobile battery adapter, etc. can be used for an external power supply. The automobile battery adapter is called a power supply. The power supply provides an output power by adjusting the power from the car battery to the power required for the portable electronic device. 
     The capacity of the power supply and the AC adapter generally determine the maximum output voltage and the maximum output current. This maximum output voltage and the maximum output current are defined as the rated output. The power supply always operates to compensate to the rated output even if the input power varies. Therefore, when an input voltage is high, an input current is small. On the other hand, when an input voltage is low, an input current becomes large. 
     FIG. 1 is a schematic diagram of a power supply circuit of the related art. The circuit includes a noise eliminating filter section  10 , a voltage converting section  20  for converting an input power to an output power, a rectifying section  30  for rectifying an output of the secondary side, an output detecting section  40  for monitoring an output of the secondary side and a coupler  50  for transmitting the condition of an output detecting circuit in the secondary side to the voltage converting circuit in the primary side. 
     The filter section  10  is formed of a coil L 1  and a capacitor C 1 . The filter section  10  is a circuit for preventing propagation of noise generated in the voltage converting section  20  to the input side. 
     The voltage converting section  20  includes a transformer T 1  for voltage conversion, a transistor Tr 1  for shutting off a current flowing through the transformer T 1  and a control circuit  60  for controlling the transistor Tr 1 . 
     The rectifying section  30  includes a rectifying diode D 1  for rectifying a current outputted from the voltage converting section  20  and a capacitor C 2  for smoothing the rectified current. 
     The output detecting section  40  includes a sense resistor R 0  for detecting an output current value of the power supply and a sense circuit  70  for detecting a voltage value across both ends of the sense resistor R 0 . 
     The coupler  50  is a circuit for transmitting an output of the sense circuit  70  to the control circuit  60 . In the coupler  50 , a photo-coupler is used in general to electrically insulate the primary side and secondary side. 
     In FIG. 1, when the transistor Tr 1  is ON, an input current flows in the primary side coil of the transformer T 1 . When the transistor Tr 1  is OFF, an output current flows in the secondary side of the transformer T 1 . The circuit explained above is defined as an RCC type switching regulator. 
     In the RCC type switching regulator, when an output voltage value is Vout, input voltage value is Vin, the ON time of transistor Tr 1  is Ton and the OFF time of transistor Tr 1  is Toff, the relationship is defined by Vin×Ton=Vout×Toff. However, when the number of turns of the primary coil of the transformer T 1  is assumed to be identical to the number of turns of the secondary coil, this formula can be modified so that Vout=(Vin×Ton)/Toff. Moreover, it can be modified by the period of ON/OFF of the transistor Tr 1  replacing T, producing Vout=(Vin×Ton)/(T−Toff). 
     As indicated in the above formula, the input current can be adjusted by controlling the ON time of transistor Tr 1  while the output voltage is kept constant. Thus, even when the load connected to the output terminal of the power supply varies, the value of Vout can be maintained constant using the feed-back control that controls the ON time of the transistor Tr 1  by monitoring the output voltage Vout. 
     FIG. 2 is a schematic diagram of another power supply circuit of the related art. The circuit of FIG. 2 is different from the RCC type switching regulator in that the voltage converting and rectifying section  80  is formed by integrating the voltage converting section  20  and rectifying section  30  of FIG.  1 . Rectifying section  80  is therefore provided in place of individually providing a voltage converting section and a rectifying section. 
     In FIG. 2, when the transistor Tr 1  is ON, an input current flows through the primary side coil of the transformer T 1 . This causes the output current to flow through the secondary side coil of the transformer T 1 . This type of circuit is defined as a FORWARD type switching regulator. 
     In FIG. 2, the transformer T 1  operates as a switch circuit. The transformer T 1  does not operate as a voltage converting circuit. Therefore, a choke coil L 2  and a flywheel diode DO are required for voltage conversion in addition to the transformer T 1 . In the circuit of FIG. 2, the relationship of the voltage to the time the transformer T 1  is ON is Vout=(Vin×Ton)/(Ton+Toff)=(Vin×Ton)/T. 
     In addition, the current flowing through L 2  also flows in the output detecting section  40  and the noise eliminating filter section  10  while the transistor Tr 1  is ON. Moreover, current flowing through L 2  is supplied via D 1  while the transistor Tr 1  is OFF. Therefore, an average input current Iin to the power supply circuit becomes equal to a product of an output current lout and the ON time of transistor Trn. Accordingly, the relationship of current to the time transistor Tr 1  is ON is Iin=(Iout×Ton)/T. 
     As indicated in the above formula, controlling the ON time of the transistor Tr 1  can cover variation of the input voltage. Moreover, even when the capacity of the load connected to the output of the power supply is varied, Vout can be maintained constant by having the feedback control vary the ON time of the transistor Tr 1  in accordance with the output voltage Vout. 
     FIG. 3 is a schematic diagram illustrating details of the sense circuit  70  and control circuit  60  that monitor the output power in the circuit illustrated in FIG. 1 or FIG.  2 . The sense circuit  70  includes a voltage amplifier AMP 11 , a couple of error amplifiers ERA 11 , ERA 12  and reference voltage sources e11, e12. The control circuit  60  includes a triangular wave oscillator  66 , a PWM comparator  62  and a drive circuit  68 . 
     The reference voltage source e11 is the reference voltage used to determine the output current value. The reference voltage source e12 is the reference voltage used to determine the output voltage value. 
     The voltage amplifier AMP 11  measures a voltage drop generated by a current flowing through the sense resistor R 0 . The voltage amplifier AMP 11  outputs a voltage that is proportional to a current value flowing through the sense resistor R 0 . The error amplifier ERA 11  compares an output voltage value with the reference voltage value e11. The error amplifier ERA 11  outputs a low level when a large current flows through the sense resistor R 0  or a high level when a small current flows through the sense resistor R 0 . 
     Similarly, the error amplifier ERA 12  compares an output voltage value of the power supply with the reference voltage value e12. The error amplifier ERA 12  outputs a low level when the power supply outputs a high output voltage value or a high level when the power supply outputs a low output voltage value. 
     The PWM comparator  62  is a voltage comparator including one inverting input and a plurality of non-inverting inputs. Namely, the PWM comparator  62  illustrated in FIG. 3 is a voltage pulse width converter for controlling the ON time of an output pulse depending on an input voltage value. The PWM comparator  62  compares the minimum voltage value among a plurality of non-inverting inputs shown by a +, with the voltage value of an inverting input shown by a −. The PWM comparator  62  provides an output when the voltage value of inverting input is lower. An output signal from the triangular wave oscillator  66  is inputted to the inverting input of the PWM comparator  62 . Meanwhile, the output from the error amplifier ERA 11  and the output from the ERA 12  are inputted to the non-inverting input. 
     During the period where the triangular wave voltage value from the triangular wave oscillator  66  is lower than the output voltage of error amplifier ERA 11  and is also lower than the output voltage value of the error amplifier ERA 12 , an output voltage from the PWM comparator  62  is inputted to the drive circuit  68 . With this input, the drive circuit  68  is driven to drive the switching transistor Tr 1  of the power supply. Moreover, during the period where the triangular wave voltage value from the triangular wave oscillator  66  is higher than the output voltage value of the error amplifier ERA 11  or the triangular wave voltage value from the triangular wave oscillator  66  is higher than the output voltage value of the error amplifier ERA 12 , an output is not provided to the drive circuit  68  from the PWM comparator  62 . Thereby, drive of the drive circuit  68  stops and the switching transistor Tr 1  of the power supply turns OFF. 
     As explained above, the switching transistor Tr 1  is turned OFF depending on the output voltage value of the power supply that is detected with the sense circuit  70 . The power supply control circuit controls an output voltage and an output current of the power supply with the structure explained above. 
     In FIG. 3, an output from the sense circuit  70  for monitoring an output voltage and an output current is then inputted directly to the PWM circuit  62  of the control circuit  60 . However, if electrical isolation is required between the sense circuit  70  and the control circuit  60 , a photocoupler is connected to each input end of the PWM circuit  62 . Such electrical isolation can also be realized by attaching a photocoupler to the output of the sense circuit  60 . 
     In the circuit illustrated in FIG. 3, the PWM comparator  62  selects a lower voltage value among the outputs of the error amplifiers ERA 11  and ERA 12 . However, it is also possible that the sensor circuit  60  combines the output voltages of the error amplifiers ERA 11  and ERA 12  and transmits only the lower voltage value. 
     FIG. 4 is a schematic diagram illustrating a circuit to realize such modification. The circuit illustrated here is an analog circuit to transmit the lower voltage value among the outputs of the error amplifiers ERA 11  and ERA 12 . When a voltage of the error amplifier ERA 11  becomes high, the base potential of the transistor Tr 11  becomes high and a base current is reduced. Therefore, a collector resistance of Tr 11  becomes high and a constant current is supplied to the collector of Tr 1  from the constant current source i. Therefore, the collector voltage of Tr 11  becomes higher as the collector resistance of Tr 11  becomes large. 
     When the voltage of error amplifier ERA 11  becomes low, a base potential of Tr 11  becomes low. Therefore, since a base current increases, a collector resistance of Tr 11  becomes small. Since a constant current is supplied to the collector of Tr 11  from the constant current source i, a collector voltage of Tr 11  becomes low in proportion to reduction of the collector resistor of Tr 11 . 
     Similarly, when a voltage of the error amplifier ERA 12  becomes high, a base potential of Tr 12  becomes high. Thereby, since a base current is reduced, a collector resistance of Tr 12  becomes large. A constant current is supplied to the collector of Tr 12  from the constant current source i. Accordingly, a collector voltage of Tr 12  becomes high in proportion to increase of collector resistance of Tr 12 . 
     When a voltage of the error amplifier ERA 12  becomes lower, a base potential of Tr 12  becomes low. If so, since a base current increases, a collector resistance of Tr 12  becomes small. Since a constant current is supplied to the collector of Tr 12  from the constant current source i, a collector voltage of Tr 12  becomes low in proportion to reduction of collector resistance of Tr 12 . 
     The collectors of Tr 11  and Tr 12  are connected to the common constant current source i. Therefore, the collector voltage of Tr 11  and Tr 12  is fixed to the lower voltage. Accordingly, a lower voltage of the output voltage of error amplifier ERA 11  or the output voltage of error amplifier ERA 12  is outputted as the collector voltage of Tr 11  and Tr 12 . 
     FIG. 7 is a structural diagram illustrating a related art external power supply that is connected to an electronic device with the power supply. In this figure, a docking station  130  is connected as a new load between the external power supply  100  and electronic device  110 . 
     In FIG. 7, the power inputted from the external power supply  100  is supplied as the power source of the electronic device  110 . Moreover, when a secondary battery  111  is provided within the electronic device  110 , such power is also supplied as the charging power of the secondary battery  111 . The power supplied to the electronic device  110  is converted to the voltage value required by the device with a voltage converting circuit  112 . 
     The secondary battery  111  is a built-in battery to supply the power to the electronic device  110  when the power source from the external power supply  110  stops. Power supply  150  applies the power to charge the secondary battery  111  built in the electronic device. A microcomputer  113  detects start and end of charging at the time of charging the secondary battery  111 . The microcomputer  113  controls the ON/OFF condition of the power supply from a charger and also controls the power supply. 
     Voltage comparator COMP 101  detects that the power is supplied from the external power supply. COMP 101  sends a high level to the microcomputer when a voltage value measured with voltage dividing resistors R 103  and R 104  is higher than reference voltage value el. AMP 101  measures a current value supplied to the secondary battery  111  from the power supply  150 . A diode D 101  prevents leak of power of the secondary battery  111  to the external circuit. A diode D 102  prevents direct application of the power from the external power supply to the secondary battery  111  when the power is supplied from the external power supply. 
     The power supply  150  is a DC—DC converter to generate a constant voltage or a constant current or a constant voltage current. The power supply  150  includes a sense resistor R 101  for measuring an input current value from the external power supply, a sense resistor R 102  for measuring a charting current value of the secondary battery  111 , a main switch transistor FET 101 , a choke coil L 101 , a flywheel diode D 103 , a smoothing capacitor C 101  and a control circuit. Detailed operation of the power supply  150  is not explained here, because it is similar to the operation explained above. 
     The docking station  130  is provided to expand the functions of the electronic device  110 . This docking station  130  can expand the functions of the electronic device through connection with the electronic device. Portability is a very important factor in the portable type information device, such as a notebook size personal computer. 
     Therefore, the basic section of the notebook size personal computer must be given the required minimum functions through reduction in size and weight as much as possible. For this reason, the LAN connecting mechanism which cannot be used during the transportation and CD drive or DVD drive which is not used frequently are no longer loaded to the body of notebook size personal computer. Meanwhile, it is very convenient when various functions are provided in the notebook size personal computer for when it is used on the desk. Therefore, the docking station is provided with high level functions. Connecting a notebook size personal computer to the docking station can attain expandability for use of various devices. 
     In FIG. 7, the external power supply  100  sends the power to both electronic device  110  and docking station  130 . When the secondary battery  111  is charged in the side of electronic device  110 , the power supply  150  tries to extract the maximum current specified at the time of design from the external power supply  100 . However, the power of external power supply  100  is also supplied to the docking station  130 . Accordingly, the external power supply  100  enters the over-load condition and thereby shuts off the output. 
     Therefore, the external power supply  100  for docking station  130  shall have the capacity to cover the addition of the maximum current used for the docking station  130  to the power consumption of the electronic device  110 . 
     However, in this case, if a load current, for example, of the docking station  130  is reduced depending on the operating condition, an extra power cannot be used as the charging power of the secondary battery in the side of electronic device  110 . 
     As a method of overcoming such problem, there is proposed a charging control circuit (Japanese Published Unexamined Patent Application No. HEI 10-286586) for keeping constant the output voltage of external power supply and controlling the charging current by monitoring an output voltage value of the external power supply. However, this circuit cannot attain the result as expected when an output power value of external power supply does not conform to the theoretical value. 
     SUMMARY OF THE INVENTION 
     First, in accordance with the present invention, a power supply can attain a maximum output while preventing damage to an external power supply even when the external power supply needs to provide varying output voltage. Therefore, additional output can be provided for portable electronic terminals. Thus, the exemplary embodiments as noted herein help solve the tendency to increase power consumption. In addition, it is possible to provide a means for safely and efficiently consuming additional power when a new load is connected between the external power supply and the power supply. 
     For example, FIG. 15 shows that the exemplary external power supply drops the input voltage and increases the input current when overloaded and protects its output current as much as possible when overloaded. However, a power supply of lower reliability shown in FIG. 18 lowers the voltage when overloaded but sometimes provides an excessive output power. If this condition is continued for a long time, the external power supply will probably over-heat. 
     One exemplary embodiment is provided with a mechanism to monitor the input power value (input voltage and input current) of the charger and therefore it can determine, if the external power supply cannot supply the charging power to the receiving device, whether the cause is a limitation in the input current or in the input voltage. Therefore, the exemplary power supply does not become overloaded and can optimize the charging condition. 
     Moreover, it is possible to avoid erroneous recognition for the end of charging because of insufficient input power, even when a lithium ion (Li+) battery is used to complete the charging when the charging current value becomes the predetermined constant value or less. 
     The principles of the present invention have been explained above with an example of a car battery adapter in view of easily helping the understanding of the present invention. It is obvious that the present invention is useful not only for the car battery adapter but for many other similar power supply devices. The present invention can be used in any power supply circuit to convert an input voltage from a external power supply to a predetermined power. The present invention has particular effect in a condition where the voltage of external power supply is unstable. For example, in a certain region, the present invention can provide sufficient effect on the adapter connected to the commercial power supply. 
     Moreover, other exemplary embodiments can be used as the charging circuit for the battery within an electronic device and can provide the increased current for portable type electronic devices, assuring portability and sophisticated functions. For example, the present invention can provide sufficient effect to the portable type terminal that is used in connection with the so-called docking station and is also used, when it is carried, by separating only the body provided with the required minimum functions from the docking station. 
     FIG. 5 illustrates an output characteristic of the power supply illustrated in FIG.  1 . The rated output of this adapter is 16 volts/3.5 amperes. The output voltage of 16 volts is adjusted with the reference voltage e12 of the sense circuit  60 . An output current of 3.5 amperes is adjusted with the reference voltage e11 of the sense circuit  60 . 
     Here, it is assumed that a load connected to this adapter overloads its capacity by requiring an output current of 3.5 amperes or more. Even in such a case, the rated output of the adapter is 16 volts/3.5 amperes and therefore the output current of this adapter should not exceed 3.5 amperes under normal conditions. 
     FIG. 6 illustrates the relationship between an input current and an input voltage of an ideal power supply. Here, the voltage conversion efficiency of the power supply is assumed as 80%. This figure indicates the input current value and input voltage value when the rated output of 3.5 amperes is provided. When a battery voltage of a car is 12 volts or higher, the input current is 6.0 amperes or less. However, when the battery voltage of a car is lowered to 9 volts or less, the input current becomes 8.0 amperes. 
     When the related art power supply is connected to the car battery, such as through an automobile battery adapter, it is generally connected to the terminal of a cigarette lighter of a car. In the receptacle circuit of the cigarette lighter, a fuse of about 10A is inserted to prevent short-circuiting the battery. When a fuse is used in the rated condition, it is known that the fuse blows usually after two hours. In order to prevent blowout of the fuse, a current flowing through the fuse must be set 80% or less of the rated current. Therefore, when a related art power supply as shown in FIG. 1 is used, it is required to set the current input to the related art power supply to 8 amperes or less. 
     Moreover, it is also required to insert a protection fuse within the automobile battery adapter itself in order to avoid blow-out of the car fuse due to a short-circuit of the power supply. Namely, even when a short-circuit internal to the power supply is generated, the fuse within the power supply must blow out before the fuse of an automobile. Thus, most power supply adapters for use in automobiles use a fuse rated at 8 amperes. 
     However, as explained above, the fuse generally blows out after two hours when the current flows in its rated capacity. Therefore, it is required to limit an input current of the automobile battery adapter to 6A in order to provide adequate safety when the automobile adapter is used continuously. 
     On the other hand, when the performance of car battery is deteriorated or charging is insufficient, the output voltage of the battery is reduced, in some cases to 9 volts or less. The voltage value of about 9 volts is used because it is near to the limit value of the voltage required to start the engine. 
     Therefore, using the worst case, an input power to be inputted to the power supply becomes 9V/6A (6A×9V=54 watts (W)). When the conversion efficiency of automobile battery adapter is assumed as the ordinary value of 80%, an output voltage from the automobile battery adapter becomes equal to 54W×0.8=43.2W. 
     In recent years, power consumption by electronic devices has increased. For example, an exemplary hand-held terminal requires the occasional input of 70W. Therefore, the automobile battery adapter that can output only the power of about 43W or so has insufficient capacity. When the device therefore attempts to draw sufficient current to operate, an input current of the power supply increases and thereby blows-out the fuse. 
     The automobile battery adapter has been explained above as a simple example, but an additional exemplary problem lies in generating damage to the electric circuits due to excessive input current as well as in the automobile battery adapter in the environment that the input power becomes unstable. 
     In exemplary embodiments of the present invention as will be described below, a structure for monitoring an input power value to the power supply is added to the structure of the power supply control circuit of the related art. Thereby, it is possible to control the input current to a value lower than the fuse value without adjusting for variation of input voltage in the power supply control circuit. Therefore, the exemplary power supply using a power supply control circuit in accordance with the present invention can prevent over current being outputted to an external power supply. 
     Moreover, exemplary embodiments of the present invention as will be described below provide a power supply which outputs the rated power when an input voltage is normal. However, when an input voltage from the external power supply is low, the power outputted from the power supply is reduced. Therefore, the power supply can control an excessive current input from the external power supply. 
     Moreover, exemplary embodiments of the present invention as will be described below provide a power supply control circuit, which can output the rated power of the fuse for a constant period and thereafter reduce the output power when the input voltage from the external power supply is low. Therefore, the exemplary power supply using the power supply control circuit of the present invention can control input of excessive current from the external power supply. Moreover, even when a new load is connected between the external power supply and hand-held mobile terminal, the external power can be used effectively. 
     In order to achieve various advantages as noted and in accordance with embodiments of the present invention, there is provided a power supply control circuit connectable with an input power monitor that monitors an input power value to the power supply, an output power monitor that monitors an output power value from the power supply, an output power controller that varies the output power value from the power supply and a power supply control circuit connected to the input power monitor, the output power monitor and the output power controller, the power supply control circuit having an output power determiner that determines a determined output power value depending on the input power value and a power adjuster that adjusts the output power value to the determined output power value by controlling the output power controller. 
     As embodied herein, this circuit preferably has a first timer that measures passage of a first time a second timer that measures passage of a second time after the first timer measures the first time and an output power switch that instructs the first value to the output power determining section while the first timer measures passage of the first time, and instructs the second value to the output power determining section while the second timer measures passage of the second time. 
     In accordance with further embodiments of the present invention, there is provided a power supply, having an input power monitor that monitors an input power value, an output power monitor that monitors an output power value an output power controller that varies the output power value by controlling an input current value an output power determiner that determines a determined output power value depending on the input power value notified from the input power monitor and a power adjuster that maintains the output power value notified from the output power monitor as the determined output power by controlling the output power controller. 
     As embodied herein, this circuit preferably has a first timer that measures passage of a first time a second timer that measures passage of a second time after the first timer measures the first time and an output power switch that instructs the first value to the output power determining section while the first timer measures passage of the first time, and instructs the second value to the output power determining section while the second timer measures passage of the second time. 
     In accordance with further embodiments of the present invention, there is provided a power supply control method that includes outputting a first output power value in which an input current value becomes a first value for a first time period by controlling the input current value so that an output power is maintained as the first output power value and outputting a second output power value that is smaller than the first output power value for a second time period by controlling the input current value so that the output power is maintained as the second output power value. 
     In accordance with further embodiments of the present invention, there is provided an output power controller having an output power determiner that determines a determined output power value depending on an input power value and a power adjuster that adjusts the output power value to the determined output power value by controlling an input current value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a structural diagram illustrating a related art power supply; 
     FIG. 2 is a structural diagram illustrating another related art power supply; 
     FIG. 3 is a structural diagram illustrating a related art power supply control circuit; 
     FIG. 4 is a structural diagram illustrating another related art power supply control circuit; 
     FIG. 5 is a graph illustrating input characteristics of a related art power supply; 
     FIG. 6 is a graph illustrating output characteristics of an ideal power supply; 
     FIG. 7 is a diagram illustrating a structure of a related art power supply wherein a docking station is used; 
     FIG. 8 is a structural diagram illustrating a power supply in accordance with an embodiment of the present invention; 
     FIG. 9 is a diagram illustrating a power supply control circuit in accordance with an embodiment of the present invention; 
     FIG. 10 is a diagram illustrating a power supply control circuit in accordance with an embodiment of the present invention; 
     FIG. 11 is a diagram illustrating a power supply in accordance with an embodiment of the present invention; 
     FIG. 12 is a diagram illustrating a power supply control circuit in accordance with an embodiment of the present invention; 
     FIG. 13 is a diagram illustrating a power supply control circuit used in a power supply in accordance with an embodiment of the present invention; 
     FIG. 14 is a graph illustrating input characteristics of a power supply in accordance with an embodiment of the present invention; 
     FIG. 15 is a graph illustrating output characteristics of a power supply in accordance with an embodiment of the present invention; 
     FIG. 16 is a graph illustrating voltage control of a power supply control circuit in accordance with an embodiment of the present invention; 
     FIG. 17 is a graph illustrating current control of a power supply control circuit in accordance with an embodiment of the present invention; and 
     FIG. 18 is a graph illustrating output characteristics of an external power supply in lower performance. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 8 illustrates a first exemplary embodiment of a power supply circuit in accordance with the present invention. Voltage converting section  25  of the first exemplary power supply circuit is different from the circuit of the related art. In the first exemplary power supply, a current sense resistor R 1  measures an input current value that is added to the related art power supply. Both ends of sense resistor R 1  are connected to a control circuit  65 . Similar elements are designated with the same reference numerals of FIG.  1  and operate in a similar same manner are not explained here. 
     FIG. 9 illustrates details of the sense circuit  70  of the output detecting section  40  and the control circuit  65  of the voltage converting section  25 . Explanation of the part of sense circuit  70  of FIG. 9 that is similar to that in the related art is omitted here. 
     In the control circuit  65  of FIG. 9, e21 is the reference voltage used to determine an input current value. AMP 21  measures a voltage drop generated by a current flowing through the current sense resistor R 1 . AMP 21  outputs a voltage in proportion to a current value following through the sense resistor R 1 . ERA 21  compares an output voltage value of the voltage amplifier AMP 11  with the reference voltage value e21. When a current flowing through the sense resistor R 21  is high, the error amplifier ERA 21  outputs a low level signal. When the current flowing through the sense resistor R 1  is small, the error amplifier ERA 21  outputs a high level signal. 
     An output of ERA 21  is impressed to the non-inverting input of the PWM comparator  67 . Therefore, during the period where a triangular wave voltage value outputted from the triangular wave oscillator  66  is lower than any output voltage value of the error amplifier ERA 21  and sense circuit  70 , the drive circuit  68  is driven. Thereby, the switching transistor Tr 1  (FIG. 8) turns ON. Moreover, during the period where the triangular wave voltage value from the triangular wave oscillator is higher than any one of the output voltage value of the sense circuit  70  and error amplifier ERA 21 , the drive circuit  68  is driven. Thereby, the switching transistor Tr 1  (FIG. 8) turns OFF. 
     FIG. 15 illustrates how in the first exemplary power supply, even if an input voltage to the power supply is lowered, the output current value never exceeds the predetermined constant value. Therefore, the first exemplary power supply can prevent breakdown of the circuit due to over-current conditions. 
     FIG. 13 illustrates a second exemplary embodiment of a power supply circuit in accordance with the present invention. The second exemplary embodiment is capable of outputting the rated voltage and current when the input voltage value is higher and the input current value is equal to the specified value or less. However, when the input voltage value to the power supply is low and the input current value becomes equal to the specified value or more, an output current is lowered. 
     Even though the second exemplary embodiment does not protect the output power, it is still effective. Not all devices require full power all the time. In addition, in other devices, reduced power for short periods of time does not negatively impact operation. In addition, protecting the input power is advantageous given the additional power possible because of the properties of the fuse as noted above. 
     The power supply circuit illustrated in FIG. 9 measures a voltage drop generated with a current flowing through the resistor R 1  and thereby controls an output by utilizing a current sense resistor R 1 . The power supply circuit illustrated in FIG. 10 illustrates a structure to control the output by measuring an input voltage value in place of the power supply circuit explained above. 
     In the power supply control circuit  165  of FIG. 10, the voltage dividing resistors R 21  and R 22  for measuring input voltage are provided in place of the current sense resistor R 21  and voltage amplifier AMP 21 . An output of the voltage dividing resistor R 21  is inputted to the error amplifier ERA 121 . Thereby, an output of the voltage dividing resistor R 21  is compared with the reference voltage value e21. A voltage is outputted from the error amplifier ERA 121  depending on the output voltage value of the voltage dividing resistor R 21 . 
     When the input voltage value measured with the voltage dividing resistor sense resistors R 21  and R 22  is high, the error amplifier ERA 121  outputs a high level signal. When the input voltage value measured with the voltage dividing resistor sense resistors R 21  and R 22  is low, the error amplifier ERA 121  outputs a low level signal. 
     An output of ERA 121  is applied to the non-inverting input of the PWM comparator  167 . Therefore, the drive circuit  168  is driven during the period where the triangular wave voltage value from the triangular wave oscillator  169  is lower than the output voltage value from the sense circuit  70  and is lower than the output voltage value from the error amplifier ERA 121 . Thereby, the switching transistor Trn turns ON. Moreover, the drive of the drive circuit  168  is stopped during the period where the triangular wave voltage value from the triangular wave oscillator  169  is higher than the output voltage value from the sense circuit  70  or the triangular wave voltage value is higher than the output voltage value of the error amplifier ERA 121 . Thereby, the switching transistor Tr 1  turns OFF. 
     FIG. 14 shows characteristics of the fuse located prior to the above exemplary power supply circuit. As is already explained above, it is known that the fuse is generally blown out after two hours when it is used under the rated current condition. Therefore, it is designed, that a current exceeding 80% of the rated current does not flow into the fuse. However, in accordance with the present invention, the following information has been obtained as a result of detailed investigation of the characteristics of fuses. 
     Namely, a current of 8 amperes is applied continuously for 54 minutes to the fuse having the rated power of 8 amperes. Thereafter, a current of 6 amperes is applied for six minutes to such fuse. When such control is repeated, the fuse has not blown out even after the continuous operation of 1000 hours. Moreover, there are no changes observed in the resistance value of the fuse itself. 
     In other words, blow-out of the fuse can be prevented indefinitely by keeping, during continuous operation, the power level at the rated capacity for 90% of the operational time and also keeping the power level at 80% of rated capacity for 10% or more of the operational time. 
     FIG. 11 is an exemplary embodiment of a power supply in accordance with the invention based on the result explained above. The power supply comprising this power supply circuit is also provided with a 8 ampere-fuse in order to keep the input current from the battery within 8 ampere. However, since it is possible to apply the current of 8 ampere for the time of 90% of the usage time, an output of 72W can be attained during this period. Accordingly, this power supply can realize connection of a variety of electronic devices, such as a hand-held information terminals, requiring large consumption of power that will be used in the near future. 
     In FIG. 11, the voltage amplifier AMP 21 , error amplifier ERA 21  and PWM comparator  67 , etc. are similar to those of the power supply circuit illustrated in FIG.  8  and operate in a similar manner. 
     In the power supply circuit of FIG. 8, a kind of reference voltage value of the error amplifier ERA 21  is used for controlling an input current. However, in the circuit of the power supply of FIG. 11, two kinds of voltages, e31 and e32, are switched with the switch circuit SW 31 . Therefore, the current value inputted to the power supply may be maintained at any one of two values. As these two kinds of current values, the voltage value e31, for example, keeps the input voltage to 6 amperes, while the voltage value e32 keeps the input voltage to 8 amperes. 
     The voltage comparator COMP 31  detects that a current flowing into the current sense resistor R 1  is higher than the value corresponding to the first reference voltage value e31. The voltage comparator COMP 31  outputs a high level when the input current value is higher than the value corresponding to the first reference voltage value. The voltage comparator COMP 31  outputs a low level when the input current value is lower than the value corresponding to the first reference voltage value. 
     FET 31  is a switch circuit that is controlled for ON and OFF with an output of the voltage comparator COMP 31 . Variable i designates a constant current source to charge a capacitor C 31 . A resistor R 33  is a discharging resistor for discharging the capacitor C 31 . The voltage comparator COMP 32  compares a voltage value of the capacitor C 31  with the reference voltage value e33. 
     The voltage comparator COMP 32  outputs a high level when the voltage value of C 31  is higher than the reference voltage value e33. An output from the voltage comparator COMP 32  causes the error amplifier ERA 31  to select the reference voltage value e31 via the switch SW 31 . When the voltage value of C 31  is lower than the reference voltage e33, the voltage comparator COMP 22  outputs a low level signal. An output of the voltage comparator COMP 32  causes the error amplifier  31  to select the reference voltage value e32 via the switch SW 31 . 
     In the power supply circuit of FIG. 13, when an input current value to the power supply is lower than the value corresponding to the first reference voltage value e31, the voltage comparator COMP 31  continuously outputs a low level signal. Therefore, the voltage value of capacitor C 31  is kept low. Accordingly, the voltage comparator COMP 32  continuously outputs a low level signal. After all, the reference voltage value of the error amplifier ERA 31  is set to the second reference voltage value e32. An input current to the power supply is allowed to have the value corresponding to the second reference voltage value. 
     When an input current to the power supply increases and exceeds the current value corresponding to the first reference voltage value e31, the voltage comparator COMP 31  outputs a high level. An output of the voltage COMP 31  charges the capacitor C 31  by turning ON FET 31 . The capacitor C 31  is charged only when the input current value of the power supply is higher than the current value corresponding to the first reference value. When the input current value of the power supply is lower than the current value corresponding to the first reference value, the capacitor C 31  is never charged. 
     A voltage value of the capacitor C 31  rises therefore in proportion to the time where the input current value of the power supply is higher than the current value corresponding to the first reference voltage value. The voltage comparator COMP 32  compares the voltage value of capacitor C 31  with the reference voltage value e33. Therefore, when the total sum of the time where the input current value to the power supply is higher than the current value corresponding to the first reference voltage value becomes longer than the time designated with the reference voltage value e33, the voltage comparator COMP 32  outputs a high level. An output of the voltage comparator COMP 32  sets the reference voltage value of the error amplifier ERA 31  to the first reference voltage value e31. Since the reference voltage value of the error amplifier ERA 31  is varied to the first reference voltage value, an input current of the car adapter is limited to the value lower than the current value corresponding to the first reference voltage value. 
     While the voltage value of capacitor C 31  is higher than the reference voltage value e33, the voltage comparator COMP 32  continuously outputs a high level. However, the voltage value of output capacitor C 31  is discharged via a discharging resistor R 33 . Therefore, after the constant time determined with the capacitance value of the capacitor C 31  and a value of the discharging resistor R 33 , the voltage value of capacitor C 31  becomes lower than the reference voltage value e33. The voltage comparator COMP 32  outputs a low level to set the reference voltage value of the error amplifier ERA 31  to the second reference voltage value e32. 
     In the above explanation, the control circuit selectively uses two kinds of current value for simplifying the explanation. However, in actual use, these current values may be continuously varied between these two kinds of values. In this case, it is required to set in more detail the relationship between the current value and passage of time. For this purpose, the power supply circuit may be structured by combining a known power supply circuit structure with a power supply circuit in accordance with the present invention. In any case, when the current flows continuously to the fuse at the rated capacity for a maximum period of 108 minutes (90% of two hours) and the current flowing in the fuse is set to 80% of the rated capacity for the remaining 12 minutes of two hours. 
     It is preferable that the time when the current of the rated capacity flows into the fuse is set to the period within 54 minutes (90% of an hour) and the current flowing into the fuse is set to 80% of the rated capacity for the remaining six minutes of an hour. Thus, even when the power supply has been designed as explained above, assuming previously an output value of the external power supply, it is also possible to connect another load between the external power supply and power supply. 
     FIG. 11 is a circuit diagram illustrating the second exemplary embodiment of the present invention in such a case that a new load is connected between the external power supply and the portable type electronic device. 
     In the power supply of this embodiment, an input current notification line and an input power notification line to the control section  215  from the microcomputer  213  are newly added in comparison with the related art power supply circuit illustrated in FIG.  7 . An input current notification line to the microcomputer  213  from the control section  215  and the MASK signal line are also added. Content and procedure for these signals will be explained later. 
     Microcomputer  213  is a micro-controller to control the power supply, and is connected with power supply  250 . The microcomputer  213  can detect MASK 1  signal and MASK 2  signal from the power supply  250  and an input current value to the power supply  250 . 
     FIG. 12 is a diagram illustrating the control section  215  of the power supply circuit of FIG.  11 . The control section  215  comprises a reference voltage source e211 for setting the upper limit of the current value flowing into the sense resistor R 210  and a reference voltage source e221 for monitoring the voltage when the external power supply droops or drops the voltage under the overload condition. 
     R 211  and R 212  are divided resistors for measuring the voltage value of the external power supply  100  inputted to the power supply  250 . e212 is the reference voltage for comparing the input voltage value of the power supply  250 . ERA 212  amplifies a difference between the input voltage value of the power supply  250  obtained with the divided resistors R 211  and R 212  and the reference voltage value e212 and then outputs such difference to the PWM comparator. When the input voltage value of power supply  250  is lowered, difference from the reference voltage value e212 is reduced. Thereby, the error amplifier ERA 12  outputs a low level to reduce the charging power. Accordingly, a load of the external power supply  100  is reduced. When the input voltage value of power supply  250  becomes high, difference from the reference voltage value e212 becomes large. Thereby, the error amplifier ERA 212  outputs a high level. 
     This voltage comparator COMP 201  is the circuit to compare the output voltage value of the error amplifier ERA 211  for measuring a current to measure an output current value of the external power supply  100  with the reference voltage value e31. The reference voltage value e23 is identical to the maximum value of the output voltage value of the triangular wave oscillating circuit  220 . Similarly, the voltage comparator COMP 202  is the circuit to compare the output voltage value of the error amplifier ERA 12  for measuring a voltage to measure the output voltage value of the external power supply  100 . 
     Output signals of the voltage comparators COMP 201  and COMP 202  are OR&#39;ed with a logical sum circuit OR 201 . The MASK 1  signal becomes high level when the voltage comparator COMP 201  or COMP 202  outputs a high level. The MASK 1  signal becomes a low level when both voltage comparators COMP 202  and COMP 202  output low level. 
     Moreover, an output of the error amplifier ERA 211  for current measurement to measure the output current value of the external power supply is outputted to the outside of the control section  215 . Therefore, the microcomputer  213  can read the output current value of the external power supply. 
     The PWM comparator is a voltage comparator, as explained above, including one inverting input and a plurality of non-inverting inputs. The PWM comparator compares the lowest voltage value among a plurality of inverting inputs with the voltage value of the inverted input. This PWM comparator is a voltage pulse width converter for controlling the ON time of an output of the power supply depending on the input voltage value. 
     An output of the triangular wave oscillator is inputted to the inverting input of the PWM comparator. To the non-inverting inputs of the PWM comparator, outputs of the error amplifiers ERA 221 , ERA 222 , ERA 211  and ERA 212  are inputted. Therefore, the drive circuit  240  is driven during the period where the triangular wave voltage from the triangular oscillator  220  is lower than any voltage value of the output voltage values of the error amplifiers ERA 221 , ERA 222 , ERA 211  and ERA 212 . The drive circuit  240  drives the main switching FET 1  (FIG.  11 ). 
     Moreover, the driving of the drive circuit  240  stops during the period where the voltage value of the triangular wave from the triangular wave oscillator  220  is higher than any voltage value of the error amplifiers ERA 221 , ERA 222 , ERA 211  and ERA 212 . The drive circuit  240  turns OFF the main switching FET 1 . 
     The input voltage value is measured with the resistors R 221 , R 222 . The measured input voltage value is then amplified with the error amplifier ERA 222  and is then inputted to the PWM comparator. To the PWM comparator, the triangular wave from the triangular wave oscillator  220  is applied. When the output voltage value of the error amplifier ERA 222  becomes large, an output pulse width of the PWM comparator also becomes large. When the output voltage value of the error amplifier ERA 212  becomes smaller, an output pulse width of the PWM comparator also becomes small. Therefore, when the input voltage value becomes small, the difference from the reference voltage value e22 becomes small. An output voltage of the error amplifier ERA 222  also becomes small. Therefore, the output pulse width of the PWM comparator becomes wide. When the input voltage value becomes large, the difference from the reference voltage value e222 becomes large. An output voltage of the error amplifier ERA 222  also becomes high. Therefore, the output pulse width of the PWM comparator becomes narrow. 
     As explained above, the power supply of the PWM control system enables control of the output voltage by controlling the ON/OFF ratio (ratio of Ton and Toff) of the main switching FET 1 . 
     FIG. 16 is a graph illustrating the profile of the above described control for this power supply. The horizontal axis plots the time, while the vertical axis plots the voltage. In this figure, the output voltage value of the error amplifier ERA 221  is always lower than the output voltage value of the error amplifier ERA 222 . Accordingly, when an output of the PWM comparator is considered, it is enough to consider the relationship between the output voltage of the error amplifier ERA 222  and the voltage value of the triangular wave. After all, the PWM comparator becomes ON when the output voltage value of the error amplifier ERA 222  is higher than the voltage value of triangular wave or becomes OFF in other cases. As explained above, the PWM comparator adjusts the output voltage by adjusting the output time. 
     Moreover, a total current of the current consumed in the load side of the electronic device and a current consumed in the power supply flows into the sense resistor R 201  shown in FIG. 15 for measuring the consumed current value of the electronic device. In case the power supply  150  is replaced to power supply  250 , a total current of the current consumed in the load side of the electronic device and a current consumed in the power supply flows into the sense resistor R 201  for measuring the consumed current value of the electronic device. Voltage drop generated by the current flowing through the sense resistor R 201  is converted to a voltage with the amplifier AMP 211 . The voltage value is amplified with the error amplifier ERA 211  and is then inputted to the PWM comparator. To the PWM comparator, the triangular wave is applied from the triangular wave oscillator and when the output voltage value of the error amplifier ERA 211  becomes large, the output pulse width of the PWM comparator also becomes large. When the output voltage value of the error amplifier ERA 211  becomes small, the output pulse width of the PWM comparator also becomes small. 
     Therefore, a total input current of the current consumed in the load side of the electronic device and the current consumed in the charger is rather small, and the potential difference across the sense resistor R 201  becomes small. Thereby, the difference between the potential difference across the sense resistor R 201  and the reference voltage value e11 becomes large. The output voltage of the error amplifier ERA 211  becomes high. Therefore, the output pulse width of the PWM comparator becomes wide. 
     When an input current as a sum of the current consumed in the load side of the electronic device and the current consumed in the charger becomes large, the voltage drop by the sense resistor R 221  becomes large. Thereby, the difference between the voltage drop generated with the sense resistor R 221  and the reference voltage value e211 becomes small. Here, the output voltage of the error amplifier ERA 211  also becomes lower. Thereby, the output pulse width of the PWM comparator becomes narrow. 
     FIG. 17 illustrates how the PWM control system regulates output current by controlling the ON/OFF ratio (Ton to Toff ratio) of the main switching FET 1 . In the power supply control circuit operating as explained above, when a load of the electronic device is rather light or when an output of the external power supply is sufficiently large that is enough as the charging power of the secondary battery, the PWM comparator is controlled with either the output voltage value of the error amplifier ERA 221  or the output voltage value of the error amplifier ERA 222  to control the output of the charger. The output voltage value of the error amplifier ERA 211  for sensing the input of the external power supply and the output voltage value of the error amplifier ERA 212  do not take part in the control. 
     As a result, the output voltage value of the error amplifier ERA 211  and the output voltage value of the error amplifier ERA 212  become larger than the maximum value of the voltage value of the triangular wave and the voltage comparators COMP 201  and COMP 202  output low levels. Therefore, the MASK 1  and MASK 2  signals are kept in the low level, indicating that the output of the external power supply is sufficient. 
     On the other hand, when the power consumption in the electronic device side and thereby a current flowing into the sense resistor R 201  for measuring current dissipation value also increases and reaches the preset current value (set by the reference voltage source e211), the voltage drop generated with the sense resistor R 211  becomes large. The difference between the voltage drop generated with the sense resistor R 211  and the reference voltage value e211 becomes small and the output voltage of the error amplifier ERA 211  becomes lower. When the output voltage of the error amplifier ERA 211  becomes lower, the output pulse width of the PWM comparator becomes narrow. If this condition occurs, the output voltage value of the error amplifier ERA 211  becomes lower than the reference voltage value e23. As a result, the output of the voltage comparator COMP 201  becomes high level. An OR circuit  201  outputs the high level. 
     On the other hand, since the output voltage of the external power supply does not become lower, it does not take part in the control of output of the power supply. The output voltage value of the error amplifier ERA 212  is high and is larger than the maximum value of the voltage value of the triangular wave. Therefore, COMP 202  outputs a low level. As a result, the MASK 1  signal becomes high level. The MASK 2  signal stays in the low level. Therefore, it indicates that the charging current of the secondary battery is limited due to the limitation of the output current of the external power supply. 
     Next, in another exemplary embodiment, the load of the external power supply is equal to the total power of the power consumption of the docking station, power consumption of the electronic device and power consumption of the secondary battery. When the power consumption of the electronic device side increases under this condition, a current flowing into the sense resistor R 212  to measure the current dissipation in the electronic device side also increases. However, a load current in the docking station side does not flow into the sense resistor R 211 . 
     As a result, the current value of the sense resistor R 211  exceeds the capacity of the external power supply in some cases before such current value reaches the preset current value (set with the reference voltage source e211). In this case, the external power supply becomes overloaded. In this case, since the external power supply drops the output voltage and therefore the output voltage of the error amplifier ERA 212  becomes lower and the output pulse width of the PWM comparator becomes narrow. Under the condition that the PWM comparator is controlled with the output voltage value of the error amplifier ERA 212 , the output voltage value of the error amplifier ERA 212  becomes lower than the reference voltage value e231. As a result, the output of the voltage comparator COMP 202  becomes high level. 
     However, a current value flowing into the sense resistor R 211  does not reach the preset current value. The output voltage value of the error amplifier ERA 211  is kept higher than the reference voltage e31. The voltage comparator COMP 201  outputs a low level. Therefore, although the MASK 1  signal is kept in the low level, the MASK 2  signal becomes high level. This condition indicates that the charging current of the secondary battery is limited with limitation of the output voltage of the external power supply. 
     Thus, an output voltage of the charger can be controlled for the purpose that the output voltage of the external power supply does not drop. The power supply control circuit is capable of dynamically controlling the charging current depending on the capacity of the external power supply. 
     It is possible to know that the charging current of the secondary battery is limited with limitation of output capacity of the external power supply from the MASK 1  signal and MASK 2  signal. Moreover, it is also possible to identify that output capacity of the external power supply is limited with the output current or with the output voltage. 
     Moreover, the output current of the external power supply can be detected from an output of the voltage amplifier AMP 211 , since it flows into the sense resistor R 211 . The upper limit value of the current flowing into the sense resistor R 211  can be set with the reference voltage source e211. In addition, the upper limit value of the charging current of the secondary battery built in the electronic device can also be set with the reference voltage source e212. The monitoring voltage value when the external power supply droops or drops the voltage under the overload condition can also be set with the reference voltage source e221. 
     Although preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principle and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.