Abstract:
The present invention is a method and system for actively limiting transient voltages across a voltage bus. Circuitry coupled to a bus may adjust the voltage level of a bus to a voltage within a desired range when transient voltages cause the voltage level on the bus to fall outside of a desired range. Circuitry of the present invention may include active elements that may adjust a voltage level of a bus in a rapid fashion. Additionally, use of the circuitry of the present invention reduces the cost and design limitations associated with stabilizing buses by solely using multiple low ESR/ESL capacitors.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to the field of voltage regulation, and more particularly to a method and system for actively limiting transient voltages. 
     BACKGROUND OF THE INVENTION 
     Modern high speed, high current drawing microcircuits such as processors, integrated memory circuits and peripheral interface circuits require a stable voltage for optimal performance. When multiple devices and circuits require a voltage supply, a bus is utilized which delivers a voltage generated by a conventional power supply. Microcircuits such as peripheral component interface (PCI) cards generate noise on the voltage bus. Power supplies are unable to react quickly enough to hold the voltage level of the bus within a desired range due to significant resistive and inductive parasitic elements between a power supply output capacitor and the microcircuits receiving an input voltage from the power supply. 
     In order to provide a stable voltage, the power noise sources may require isolation. Conventionally, buses have employed low equivalent series resistance (ESR) and low equivalent series inductance (ESL) capacitors near the control voltage inputs of the respective devices and circuits to attenuate voltage transients to an acceptable level. However, low ESR/ESL capacitors are passive elements that operate more slowly at limiting transients and limiting precision of a desirable voltage range. Low ESR/ESL capacitors must be placed close to the control voltage inputs of the microcircuits which causes additional design limitations. Additionally, low ESR/ESL capacitors are expensive and add significant cost to the voltage bus supplying microcircuits. Consequently, a method and system for actively limiting transients is necessary. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method and system for actively limiting transient voltages across a voltage bus. In an embodiment of the invention, circuitry coupled to a bus may adjust a voltage level of a bus to a voltage within a desired range when transient voltages cause the voltage level on the bus to fall outside of a desired range. Circuitry of the present invention may include active elements that may adjust a voltage level of a bus in a rapid fashion to ensure devices coupled to the bus operate optimally while reducing the cost and design limitations of low ESR/ESL capacitors. 
    
    
     It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
     FIG. 1 depicts a block diagram of a power supply system in accordance with an embodiment of the present invention; 
     FIG. 2 depicts an active transient limiter circuit in accordance with an embodiment of the present invention; 
     FIG. 3 depicts a process for providing a stable voltage across a voltage bus in accordance with an embodiment of the present invention; and 
     FIG. 4 depicts an exemplary diagram representing the operation of an active transient limiter circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     Referring to FIG. 1, a power supply system  100  in accordance with an embodiment of the present invention is shown. In an embodiment of the invention, power supply system  100  may include a power supply  110 , a bus  120 , an active transient limiter circuit  130 , and a first and second loads  140 - 150 . Power supply  110  may produce direct current (DC) power that is supplied to first and second loads  140 - 150  via bus  120 . In an embodiment of the invention, power supply  110  may be a switching power supply that may utilize power semiconductor switches in the on and off switching states. 
     Active transient limiter circuit  130  may ensure that the supply voltage provided by bus  120  remains within a desired range and accounts for voltage transients. Voltage transients may be momentary distortion in the form of a voltage waveform of relatively short duration. Typically, voltage transients may last for a period of one microsecond to several milliseconds, hence voltage regulation known to the art may not operate at a speed to fast enough to attenuate voltage transients within specified bus voltage limits. 
     Active transient limiter circuit  130  may provide isolation of the loads  140 ,  150  coupled to bus  120  and prevent loads from generating noise on the bus  120 . Through operation of the active transient limiter circuit  130  of the present invention, the voltage supplied to loads  140 ,  150  by bus  120  may be between a first voltage slightly higher than a nominal voltage and a second voltage slightly lower than the nominal voltage. 
     Referring now to FIG. 2, an active transient limiter circuit  130  in accordance with an embodiment of the present invention is shown. Active transient limiter circuit may include a first operational amplifier  210  and a second operational amplifier  220 . In a preferred embodiment, first operational amplifier and second operational amplifier may be high speed operational amplifiers. This may be advantageous as the active transient limiter circuit may operate fast enough to attenuate fast voltage transients. First operational amplifier  210  may drive transistor  225  while second operational amplifier may drive transistor  230 . Transistors  225 - 230  may be low “on-state resistance” metal oxide semiconductor field effect transistors (MOSFETs) in one embodiment of the invention. While transistors  225  and  230  are shown as field effect transistors, it should be understood by those with ordinary skill in the art that other types of transistors may be utilized in accordance with the present invention without departing from the scope and spirit of the present invention. 
     Active transient limiter circuit  130  may be controlled by two reference voltages: first reference voltage  240  and second reference voltage  245 . First reference voltage may be a voltage  240  that is slightly larger than the voltage output of the bus  120  of FIG. 1 (represented by Vo on FIG. 2) while second reference voltage  245  may be a voltage that is slightly less than the voltage output of bus  120 . Voltage  250  may be coupled to the transistor  230  and may preferably be a voltage of at least two volts greater than the output voltage of bus  120 . First and second reference voltages  240 ,  245  and voltage  250  may be produced through control voltage inputs, use of voltage dividing circuitry, and the like. Resistors  260 ,  265  and capacitors  270 ,  275  may provide a feedback loop compensation for stability of the amplifiers  210 ,  220 . 
     Turning now to the operation of active transient limiter circuit  130 , if the output voltage transient of bus  120  (Vo) should exceed the voltage of first reference voltage  240 , first operational amplifier  210  may turn transistor  225  on. This may load the bus  120  so that the transient is limited to a voltage approximately equivalent to the voltage of first reference voltage  240  or to a voltage level less than the voltage of the first voltage reference. If the voltage transient is below the voltage level of second reference voltage  245 , second operational amplifier may turn transistor  230  on. This may allow second transistor  230  to pull the voltage level of bus  120  up to approximately the voltage level of second reference voltage  245 , or pull the voltage level of the bus  120  greater than the voltage level of the second reference voltage. 
     In one embodiment of the invention, active transient limiter circuit  130  may limit voltage transients on a bus of a computer system. Voltage transients may be created on a bus by peripheral interface circuits such as peripheral component interconnect (PCI) cards, peripheral component interconnect extended (PCI-X) cards, and memory cards. The active transient limiter circuit  130  of the present invention may limit voltage transients to an acceptable level and may stabilize a bus quicker and at significantly less cost and may occupy less space than passive elements such as low ESR capacitors. 
     An advantageous aspect of the active transient limiter circuit of the present invention may be the reduction of capacitors needed to limit transients on a bus. The active transient limiter circuit  130  may occupy less space and may be manufactured at a lower cost than utilizing low ESR/ESL capacitors since low ESR/ESL capacitors are expensive. Also, use of the active transient limiter circuit of the present invention may reduce design complexity in power supply systems by eliminating the requirement that a low ESR/ESL capacitor be placed in close proximity to the input voltage of loads coupled to a voltage bus. Another advantage of the active transient limiter circuit of the present invention is the precision of voltage regulation provided by the circuitry  130 . The active transient limiter circuit  130  provides a more precise desirable voltage regulation range than what is provided by known passive transient limiter devices like zener diodes and metal oxide varistors. 
     Referring now to FIG. 3, a process  300  for providing a stable voltage across a voltage bus in accordance with an embodiment of the present invention is shown. In one embodiment of the invention, process  300  may be implemented by active transient limiter circuit  130  of FIG.  2 . Process  300  may begin by supplying at least two reference voltages to provide a desirable voltage range  310 . A first reference voltage may be slightly higher than a voltage level of a desired nominal voltage and a second reference voltage may be slightly lower than a voltage level of the desired nominal voltage. 
     A voltage level of the output voltage of a bus may be tracked  320 . A determination of whether the voltage level of the output voltage is within a desirable range may be completed  330 . If the voltage level of the output voltage is within a desirable range, the output voltage of a bus may supply the output voltage  340 . If the voltage level of the output voltage of a bus is not within a desirable range, one of the at least two reference voltages may be utilized to supply the output voltage for a load. For example, a high voltage level of an output voltage may be reduced to approximately below or equal to a first reference voltage  350 . A low voltage level of an output voltage may be increased to approximately above or equal to a second reference voltage  360 . 
     Referring now to FIG. 4, an exemplary diagram representing the operation of the active transient limiter circuit is shown. The voltage  410  may be utilized as an output voltage for a voltage bus when voltage  410  is between a first reference voltage  420  and a second reference voltage  430 . When a voltage level  410  is above a first reference voltage level  420 , (as shown in peak  440 ) the voltage of the circuit  130  may be reduced approximately below or equal to the level of the first reference voltage  420 . Additionally, when a voltage level  410  is below a second reference voltage level  430 , (as shown in valley  450 ) the output voltage may be increased approximately above or equal to the second reference voltage  430 , so the resulting output voltage on the voltage bus is forced to stay between reference voltage  420  and reference voltage  430 . 
     It is believed that the method and system of the present invention and many of its attendant advantages will be understood by the forgoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.