Abstract:
A circuit including bridge rectifier, switches across one or more of the diodes of the bridge rectifier, and a comparator providing control signals to the switch or switches can be constructed to apply a constant polarity voltage to an electrical load, regardless of the polarity of the input power applied to the circuit. The comparator produces a control signal depending upon a comparison of the input power voltages, and the control signal activates one or more of the switches to allow current flow through an appropriate path in the circuit to yield the constant polarity across the electrical load. Thus, the circuit can protect the electrical load from an inappropriately applied voltage by switching the applied voltage&#39;s polarity. Because an activated switch can short a diode in the bridge rectifier, power loss associated with current flow through the diode is reduced. Additionally, the circuit can provide constant polarity across the electrical load with either AC or DC input power.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to power supplies for electronic devices and particularly to circuits to maintain a constant polarity across an electrical load. 
     2. Description of the Related Art 
     Electronic devices, and particularly portable electronic devices such as portable computers, cellular phones, and personal digital assistants (PDAs) typically make use of alternating current to direct current adapters (“AC-DC adapters,” “AC adapters,” or simply “adapters”) either as a direct source of power, or as a source of power to charge on-board batteries. AC adapters can be built into such electronic devices, but given the size, weight, and cost constraints often imposed on such devices, AC adapters are more commonly provided as a separate module with a plug or cord for connecting the adapter to an AC outlet, and another cord for connecting the adapter to the electronic device through a connector. 
     Given the variety of electronic devices that use AC adapters, and the various output polarizations, voltage ratings, and current ratings of those adapters, an electronic device user is likely to have several, if not many, different adapters for different electronic devices. Consequently, matching the correct adapter to the intended device can be difficult because of similarity in appearance among adapters and similarity among the connectors associated with the adapters. Compounding this problem is the fact that adapters intended for different applications can be manufactured by the same company and look the same, yet have dissimilar electrical characteristics. Moreover, using the wrong adapter can damage expensive electronic equipment or even present a safety hazard. 
     Prior methods to prevent improper use of and/or mitigate the damage from improper use of an AC adapter generally fall into two categories: mechanical methods and electrical methods. The most common mechanical solution to the problem of improper adapter use is to provide the adapter and the electronic device using the adapter with unique connector keying such that the wrong connector cannot be inserted into the electronic device. One drawback to this method of preventing improper use of an adapter is that it prevents manufacturers from using standard connectors and adapters which allow the manufacturer to avoid the high costs of tooling, testing, and providing a custom part. 
     Electrical solutions typically include circuitry for clamping the improper input voltage with a dissapative device such as a zener diode, a metal oxide varistor (MOV), or a junction diode. These devices will only work if the input power source has power limiting within the capability of the dissapative device, and thus there ability to protect a device is limited. Such specialized circuits or components add cost and complexity to the electronic device. Additionally, operating conditions within the specification of the dissapative device are not always met, so damage to the electronic device can still result from using an improper adapter. 
     Accordingly, it is desirable to have a circuit that allows power input of various polarities, while supplying power with a constant polarity to an electrical load. Additionally, it is desirable to have such a circuit that can provide power having constant polarity given either AC or DC input power. 
     SUMMARY OF THE INVENTION 
     It has been discovered that a circuit including a bridge rectifier, switches across one or more of the diodes of the bridge rectifier, and a comparator to provide control signals to the switch or switches can be constructed to apply a constant polarity voltage to an electrical load, regardless of the polarity of the input power applied to the circuit. The comparator produces a control signal depending upon a comparison of the input power voltages, and the control signal activates one or more of the switches to allow current flow through an appropriate path in the circuit to yield the constant polarity across the electrical load. Thus, the circuit can protect the electrical load from an inappropriately applied voltage by switching the applied voltage&#39;s polarity. Because an activated switch can short a diode in the bridge rectifier, power loss associated with current flow through the diode is reduced. Additionally, the circuit can provide constant polarity across the electrical load with either AC or DC input power. 
     Accordingly, one aspect of the present invention provides a circuit including a first and a second input terminal, a first and a second output terminal, a bridge rectifier, a plurality of transistors, and a comparator. The bridge rectifier includes a plurality of diodes, and is coupled to the first and second input terminals and to the first and second output terminals. Ones of the plurality of transistors are coupled in parallel with ones of the plurality of diodes of the bridge rectifier. The comparator is coupled to the first and second input terminals, the first and second output terminals, and at least one of the plurality of transistors. The comparator is operable to provide a control signal to the at least one of the plurality of transistors depending upon a first signal received from the first input terminal and a second signal received from the second input terminal. 
     In another aspect of the invention, a computer system includes a processor, a memory coupled to the processor, and a circuit coupled to the processor and memory operable to deliver power to the processor and memory. The circuit includes a first and a second input terminal, a first and a second output terminal, a bridge rectifier, a plurality of transistors, and a comparator. The bridge rectifier includes a plurality of diodes, and is coupled to the first and second input terminals and to the first and second output terminals. Ones of the plurality of transistors are coupled in parallel with ones of the plurality of diodes of the bridge rectifier. The comparator is coupled to the first and second input terminals, the first and second output terminals, and at least one of the plurality of transistors. The comparator is operable to provide a control signal to the at least one of the plurality of transistors depending upon a first signal received from the first input terminal and a second signal received from the second input terminal. 
     In still another aspect of the invention, a method of maintaining a constant power supply polarity across an electrical load is disclosed. A first input voltage is compared with a second input voltage to identify which of the input voltages is more positive than the other. A first pair of switches in a circuit is activated when the first input voltage is more positive than the second input voltage. The switches are operable to receive the first and second input voltage so that activating the first pair of switches allows a voltage of a first polarity to develop across the electrical load. A second pair of switches in a circuit is activated when the second input voltage is more positive than the first input voltage. The switches are operable to receive the first and second input voltage so that activating the second pair of switches allows a voltage of the first polarity to develop across the electrical load. 
     In yet another aspect of the invention, a circuit includes a first and a second input terminal, a first and a second output terminal, a rectifying means, a first switching means, a second switching means, and a comparing means. The rectifying means is coupled to the first and second input terminals and coupled to the first and second output terminals for maintaining a constant polarity across an electrical load. The first switching means is for shorting a first portion of the rectifying means, and the second switching means is for shorting a second portion of the rectifying means. The comparing means is coupled to the first and second input terminals, the first and second output terminals, and at least one of the first and second switching means. The comparing means is for comparing a first voltage on the first input terminal with a second voltage on the second input terminal and providing a control signal to the at least one of the first and second switching means depending upon the comparison. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
     FIG. 1 shows a schematic diagram of a circuit that provides constant polarity across an electrical load. 
     FIGS. 2A and 2B show block diagrams of two computer systems including the circuit of FIG.  1 . In these figures, the electrical load is represented by the remaining elements of the computer system. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a circuit  100  which can receive input power signals at input terminals  1  and  2  and provides output power having a constant polarity across an electrical load (e.g. across resistor  110 ) at output terminals  101  and  102 . Considering only terminals  1 ,  2 ,  101 , and  102 , and diodes  10 ,  12 ,  14 , and  16 , one can recognize a full-wave bridge rectifier of conventional design. The cathodes of diodes  10  and  14  are coupled together, the anodes of diodes  12  and  16  are coupled together, the cathode of diode  16  is coupled to the anode of diode  14 , and the cathode of diode  12  is coupled to the anode of diode  10 . Input power is received by the bridge rectifier from terminals  1  and  2  at the junction between diodes  10  and  12 , and at the junction between diodes  14  and  16 , respectively. Output terminal  101  is coupled to the cathodes of diodes  10  and  14 , and output terminal  102  is coupled to the anodes of diodes  12  and  16 . In operation, a positive voltage at input terminal  1  and a less positive voltage at input terminal  2  forward biases diodes  10  and  16  and reverse biases diodes  12  and  14 . Consequently terminal  101  is held at a positive voltage with respect to terminal  102 , i.e. current flows through resistor  110  from  102  to  101 , which can be called a first electrical load polarity. When a positive voltage is applied at input terminal  2  and a less positive voltage at input terminal  1 , diodes  12  and  14  are forward biased, and  10  and  16  are reversed biased. Nevertheless, terminal  101  is still held at a positive voltage with respect to terminal  102 , thus maintaining the first electrical load polarity. 
     As mentioned above, one drawback of a conventional bridge rectifier is the power loss (e.g. resistive heating) associated with current flow through the forward biased diodes. That power loss can be reduced significantly by shorting the forward biased diodes after they have become forward biased. Transistors  20 ,  22 ,  24 , and  26  are coupled across diodes  10 ,  12 ,  14 , and  16 , respectively, so that when activated (e.g. when turned on) each transistor can short its associated diode. Although the diodes and the transistors can be implemented as discrete components, in a preferred embodiment each diode is a body diode of the transistor (in this case an insulated-gate field effect transistor (IGFET) or metal-oxide semiconductor field effect transistor (MOSFET)). Body diodes are the intrinsic diodes in a MOSFET (typically a power MOSFET) formed between the body (i.e. the substrate) of the MOSFET and the channel. Such body diodes are formed because it is common for the body of a power MOSFET to be connected internally to the source. In many applications, the body diode of a MOSFET is an unfortunate by-product, but in circuit  100 , the transistor/body diode pair is used to the circuit&#39;s advantage. 
     Control signals are applied to the gates of transistors  20 ,  22 ,  24 , and  26  to turn the various transistors on or off as appropriate. Comparator  40  produces the control signals, and drivers  30 ,  32 , and  34  invert the signal as needed. Comparator  40  can be implemented as a differential amplifier, an operational amplifier, or a specialized comparator circuit depending on the specific requirements of circuit  100 . In general, however, the non-inverting input of comparator  40  receives the input voltage applied to terminal  1  after it is divided by the voltage divider formed by resistors  42  and  44 . Zener diode  52  serves to clamp the input voltage into the non-inverting input of comparator  40 . Similarly the inverting input of comparator  40  receives the input voltage applied to terminal  2  after it is divided by the voltage divider formed by resistors  46  and  48 . Zener diode  54  serves to clamp the input voltage into the inverting input of comparator  40 . 
     Since the bridge rectifier formed by diodes  10 ,  12 ,  14 , and  16 , initially presents a DC voltage across load  110 , the null terminals of comparator  40  monitor the voltage across capacitor  90 . When that voltage has stabilized, comparator  40  is allowed to control transistors  20 ,  22 ,  24 , and  26 . Transistor  60 , in this case an n-channel MOSFET, allows the voltage at terminal  101  to be positive prior the transistor&#39;s enabling of drivers  30 ,  32 , and  34 . 
     When a positive voltage is applied at input terminal  1  and a less positive voltage at input terminal  2  is applied, the diodes are biased as described above. Comparator  40  receives the divided signals at its input terminals, and its output is driven high (e.g the output is saturated positively). Transistor  20  (in this case a p-channel MOSFET) is turned on because its gate is driven low by the control signal from comparator  40 . Note that the control signal is inverted by driver  30  between comparator  40  and the gate of transistor  20 . Transistor  26  (in this case a n-channel MOSFET) is turned on because its gate is driven high by the control signal from comparator  40 . Consequently, while diodes  10  and  16  are forward biased due to the polarity of the input voltages, the diodes&#39; associated transistors are turned on, thereby shorting the diodes and allowing current flow in the proper direction (i.e. yielding the desired polarity at  101  and  102 ) without the power losses associated with current flow through forward biased diodes. 
     When a positive voltage is applied at input terminal  2  and a less positive voltage at input terminal  1  is applied, the diodes are also biased as described above. Comparator  40  receives the divided signals at its input terminals, and its output is driven low (e.g. the output is saturated negatively). Transistor  24  (in this case a p-channel MOSFET) is turned on because its gate is driven low by the control signal from comparator  40 . Note that the control signal is inverted once by driver  32  and again by driver  34  between comparator  40  and the gate of transistor  24 . Transistor  22  (in this case a n-channel MOSFET) is turned on because its gate is driven high by the control signal from comparator  40 . The control signal is inverted by driver  32  between comparator  40  and the gate of transistor  22 . Consequently, while diodes  10  and  16  are forward biased due to the polarity of the input voltages, the diodes&#39; associated transistors are turned on, thereby shorting the diodes and allowing current flow in the proper direction (i.e. yielding the desired polarity at  101  and  102 ) without the power losses associated with current flow through forward biased diodes. 
     Depending upon the specific components used in circuit  100 , the circuit can produce a constant polarity across load  110  given either AC or DC power input at terminals  1  and  2 . Additionally, those having ordinary skill in the art will readily recognize that a variety of different rectifier architectures, transistor types, diode types, comparators, and drivers can be used in place of the components described above and illustrated in FIG.  1 . 
     Regarding terminology used herein, it will be appreciated by one skilled in the art that any of several expressions may be equally well used when describing the operation of a circuit including the various signals and nodes within the circuit. Any kind of signal, whether a logic signal or a more general analog signal, takes the physical form of a voltage level (or for some circuit technologies, a current level) of a node within the circuit. It may be correct to think of signals being conveyed on wires or buses. For example, one might describe a particular circuit operation as “the output of circuit  10  drives the voltage of node  11  toward VDD, thus asserting the signal OUT conveyed on node  11 .” This is an accurate, albeit somewhat cumbersome expression. Consequently, it is well known in the art to equally describe such a circuit operation as “circuit  10  drives node  11  high,” as well as “node  11  is brought high by circuit  10 ,” “circuit  10  pulls the OUT signal high” and “circuit  10  drives OUT high.” Such shorthand phrases for describing circuit operation are more efficient to communicate details of circuit operation, particularly because the schematic diagrams in the figures clearly associate various signal names with the corresponding circuit blocks and node names. Phrases such as “pull high,” “drive high,” and “charge” are generally synonymous unless otherwise distinguished, as are the phrases “pull low,” “drive low,” and “discharge.” It is to be appreciated by those skilled in the art that each of these and other similar phrases may be interchangeably used to describe common circuit operation, and no subtle inferences should be read into varied usage within this description. 
     It should also be noted that IGFET transistors are commonly referred to as MOSFET transistors (which literally is an acronym for “Metal-Oxide-Semiconductor Field Effect Transistor”), even though the gate material may be polysilicon or some material other than metal, and the dielectric may be oxynitride, nitride, or some material other than oxide. Use of such legacy terms as MOSFET should not necessarily be interpreted to literally specify a metal gate FET having an oxide dielectric. 
     While the invention has been described in light of the embodiments discussed above, one skilled in the art will recognize that certain substitutions may be easily made in the circuits without departing from the teachings of this disclosure. For example, many circuits using n-channel MOSFETs may be implemented using p-channel MOSFETs instead, as is well known in the art, provided the logic polarity and power supply potentials are reversed. 
     FIG. 2A illustrates a battery powered system  200  (in this case a portable computer system) that includes processor  210  with memory  220  and other computer system components  230  (e.g a hard disk drive, a graphics controller, a CD-ROM, a floppy disk drive, a network interface controller, a modem, etc.) coupled to processor  210 . System  200  also includes power supply  240  (e.g. a DC-DC regulator), rechargeable battery pack  250 , battery charger  260 , and AC-DC adapter  270 . Circuit  100  of FIG. 1 is implemented in system  200  as a polarity switch, where the electrical load of system  200  replaces resistor  110 . System  200  receives power from power supply  240  which in turn receives power from either rechargeable battery pack  250  or an electrical outlet (not shown) via AC-DC adapter  270  and polarity switch  100 . Battery charger  260  may charge rechargeable battery pack  100  if necessary. With polarity switch  100 , adapter  270  can provide DC input power of either polarity (e.g. terminal  1  can have a higher or lower voltage with respect to terminal  2 ), yet the remainder of the system will have power applied to it with a constant polarity, thereby protecting system components. 
     FIG. 2B illustrates a battery powered computer system  200 ′ similar to the battery powered system  200  of FIG.  2 A. In system  200 ′, the AC-DC adapter  270  has been replaced with AC-DC adapter  100 ′ which includes circuit  100  operating as a synchronous bridge rectifier, and additional components such as a transformer. 
     FIGS. 2A and 2B are merely illustrative of the electronic devices in which circuits  100  and  100 ′ can be used. Consequently, circuits  100  and  100 ′ need not be used in battery power electronic devices or in electronic devices that use rechargeable batteries, and are not limited to use in computer systems. Additionally, although adapters  270  and  100 ′ have been described as a component separate from and external to system  200 , the adapters can be physically incorporated into the system, as is the case in a portable computer that includes a built-in AC adapter. 
     The description of the invention set forth herein is illustrative and is not intended to limit the scope of the invention as set forth in the following claims. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.