The present invention relates to a power supply circuit for electronic equipment and comprising of a main power supply and a plurality of auxiliary power supplies. More particularly, the present invention is concerned with a power supply circuit capable of guaranteeing power when momentary load variation occurs in electronic equipment or when the efficiency of a main power supply noticeably falls.
Various kinds of load variation occur in a personal computer or similar electronic equipment, depending on the operating condition of the equipment. For example, a rapid current load occurs in the event of an access to an HDD (Hard Disk Drive), CD-ROM (Compact Disk Read Only Memory) drive or similar motor-driven equipment. To cope with such a current load exceeding normal one, a power supply circuit must be provided with a capacity capable of dealing with the expected maximum current load. On the other hand, electronic equipment is sometimes driven in a mode needing only a relatively small current. For example, when a stop clock function available with a personal computer is active or when a video tape recorder or a television (TV) receiver is in a stand-by state, power which is only about 1/100 of the designed maximum capacity of a power supply suffices. The power supply circuit therefore must cope with such a low current load also.
The above conventional power supply circuit has some problems left unsolved in relation to load variation, as follows. The power supply circuit designed to deal with the maximum momentary load current is scaled up and increases the overall size and cost of the equipment. This problem is particularly serious with a portable telephone or similar portable equipment. When the power supply is implemented by a battery, the battery must deal even with the previously stated load variation. This increases a load on the battery and thereby reduces the life of the battery, i.e., the operation time of the equipment powered by the battery.
Another problem is that equipment of the type oscillating pulses causes the voltage of a battery to sharply drop. Specifically, assume that portable equipment powered by a secondary battery having a high internal impedance oscillates great current pulses continuously. Then, the battery reaches its end voltage before the amount of electricity stored therein is used up. As a result, the continuous operation time of the equipment is reduced. This problem will be discussed more specifically later.
A further problem is that when load variation is small for a given power supply capacity, efficient power conversion cannot be effected. Consequently, power far greater than the required current is simply wasted as heat, resulting in an extremely low energy conversion efficiency. Assume that a video tape recorder, for example, has the maximum power supply capacity of 80 W and consumes power of 0.1 W in its stand-by state. Then, power of 9.1 W is wasted as heat because the power supply can be lowered only up to about 10%, obstructing efficient use of energy to a critical degree.
Arranging a plurality of power supplies in a single electronic equipment is taught in, e.g., Japanese Patent Laid-Open Publication Nos. 7-225380 and 3-226029. Generally, when an LCD (Liquid Crystal Display) module is driven by portable equipment, a conventional battery cannot turn on a backlight because of its short capacity. In light of this, the above Laid-Open Publication No. 7-225380 teaches that a solar battery is positioned behind a secondary battery in order to charge the secondary battery with power generated by the solar battery. The secondary battery and solar battery in combination increase the total amount of power and allow the backlight of portable equipment to be turned on. However, such a combination is not directed toward efficient use of energy achievable with the distributed arrangement of a power supply and batteries capable of dealing with load variation. Laid-Open Publication No. 3-226029 proposes a radio pager including a secondary battery in addition to a power supply assigned to a radio section. This secondary battery is used to drive a vibrator, speaker or similar alerting means. With this configuration, it is possible to reduce the deterioration of sensitivity ascribable to noise produced by the alerting means and tending to reach the power supply of the radio section. Such a configuration, however, simply separates a circuit producing noise and a power supply in order to prevent the noise from entering the power supply and is not directed toward the efficient use of energy stated above.
In a power generation system using, e.g., sunrays or wind force, power generated thereby is not constant, but often varies with the elapse of time. Japanese Patent Laid-Open Publication Nos. 3-22829 and 5-328633, for example, each proposes to use a secondary battery or a commercially available power supply in combination with the above power generation system. The proposal, however, simply levels the power variation of the supply side and cannot cope with load variation occurring at the demand side. Japanese Patent Laid-Open Publication No. 6-150951, for example, teaches that a fuel battery and a gas engine for power generation are combined so as to absorb load variation including a sharp increase in load while generating power efficiently. However, the application of this proposal is limited to homes and buildings. It would be difficult to apply a gas turbine to a personal computer or similar electronic equipment, particularly portable electronic equipment, for configuration reasons and because such equipment needs a response time of the order of microsecond to millisecond.
Moreover, the conventional power supply circuit has a noise problem. How noise derived from a circuit or a system should be reduced is one of problems requiring urgent solutions in the design aspect. Today, various kinds of circuits, e.g., circuits for communication and calculation are arranged close to each other and operated in various ways, often interfering with each other. This is aggravated by the current trend toward miniaturization and multifunction arrangement, i.e., dense mounting of circuit devices and low drive voltage; dense mounting increases the probability of interference while low drive voltage reduces the threshold voltage of ICs (Integrated Circuits). The equipment is therefore caused to malfunction even by small noise. Generally, a noise path includes a noise source, a coupling channel transferring noise output from the noise source, and a receptor or circuit responding to the noise, as described in, e.g., Henry W. Ott "Noise Reduction Techniques in Electronic Systems", Second Edition, pp. 18-19. It is a common practice to devise the coupling channel against noise because measures applicable to the noise source and receptor are few. Conductors are the most obvious, but overlooked, constituent that conveys noise to circuitry. A conductor laid in a n environment involving noise picks up the noise and conveys it to another circuitry. Particularly, a feed line extending from a power supply cannot be provided with more than a certain length to a given circuit device because the power supply is located at a particular position within equipment.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 55-133768, 55-133772, 63-25984, 63-36319, and 8-308104.