Abstract:
A current sharing circuit provides redundant power supplies with current sharing through the use of multiple feedback loops and existing circuit diodes to generate shared load current within predefined load voltage regulation requirements. A method of sharing current allows adjustment of the feedback loops to accommodate characteristics of the circuit.

Description:
The present invention relates generally to current sharing, and more specifically to current sharing in redundant power supplies. 
   BACKGROUND 
   In high availability digital systems using an array of circuit cards with a common back plane interconnect, it is common practice to provide redundant powering of shared circuitry in order to reduce the frequency of system down time due to power supply failure. Power converter circuits providing this function are required to have fast response times so that in the event of a failure of one of the power sources, the others remaining respond to pick up the additional load without bus voltage varying outside of normal operation tolerances, thereby avoiding interruption of service. The sharing of load current has the advantages of reducing circuit reaction time to sudden load increases such as those occurring during a converter failure. Current sharing also reduces stress on individual power converters, thereby increasing power supply MTBF mean time before failure (MTBF) and system availability. 
   In circuits which do not have current sharing, there can be response time problems when a power supply or source fails or goes offline, since a backup power source or supply must pick up the load quickly. When there is not a current shared redundancy, the secondary source is typically in a saturated off condition. It takes time for the secondary power supply to respond to the increase in load demand. The response time required from a saturated off state to a full power operating state is greater than the response time from half power operating to that of full power operation. 
   Current sharing is typically accomplished in circuits by using an interconnection between power supplies, typically a current control signal which forces all of the power supplies to operate at approximately the same current level. Such a scheme provides excellent current sharing, but a fault on the current control line can cause the entire system to shut down. Techniques for preventing such a shutdown are very complicated, often including custom integrated circuits (ICs). Current sharing may also be accomplished by adding resistance in series with source outputs. Larger output resistances result in better current balance between sources but result in poorer voltage regulation and power loss, so there are trade offs between regulation and current balance between outputs. 
   There is a need in the art for current sharing which provides redundant power, balanced current sharing and low loss. 
   SUMMARY 
   In one embodiment, a current sharing circuit includes first and second power sources, and first and second feedback loops for each power source. Feedback loops are connected between outputs of their respective sources and the inverting inputs of their respective source. 
   In another embodiment, a circuit includes at least two power sources, each with a diode between its respective output and a common load, and each having a pair of differential inputs. The first, non inverting differential input is connectable to a precision voltage reference. The second, inverting differential input is connected to first and second feedback loops for each power source. The first feedback loop is connected from each power source output to its respective inverting differential input. Second feedback loops are connected from the common load to each power source inverting differential input. 
   In yet another embodiment, a method for current sharing in a redundant power supply system includes combining at least two power supply outputs at a common load node, and feeding back power supply output and diode filtered power supply output back to a differential amplifier for each power supply. Apparent source resistance is adjusted through the proportions of DC feedback provided by first and second loops. 
   In still another embodiment, a method of sharing current for multiple power sources includes diode oring the outputs from each of the power sources and feeding back each power supply output signal through first and second loops. The first loop is a direct raw output loop and the second loop is a diode filtered loop. 
   In yet another embodiment, a method of current sharing in a multiple source system includes combining outputs of each of the multiple sources, applying a load to the combined output, and measuring the effect of the load on current sharing and voltage regulation in the system. Each source output is fed back to a source input through a pair of feedback loops. The feedback loops are adjusted for an acceptable compromise between good current sharing and voltage regulation. 
   Other embodiments are described and claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a circuit according to one embodiment of the present invention; and 
       FIG. 2  is a flow chart diagram of a method according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In the following detailed description of the embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. 
     FIG. 1  is a block diagram view of a current sharing circuit  100  according to one embodiment of the present invention. Circuit  100  comprises two branches  102  and  104  having branch outputs connected commonly at node  132  to a common load  130 . In branch  102 , differential power source  112  has a precision voltage reference  110  connected to non inverting differential input  111 . The output  115  of power source  112  is supplied to feedback loop  116  which feeds back to the inverting differential input  113  of the source  112 . The output  115  is also supplied to diode  118  at its anode. The output from the cathode of the diode  118  is supplied to the common load  130  at node  132  and to second feedback loop  114  which also supplies inverting differential input  113  of source  112 . 
   In branch  104 , differential power source  122  has a precision voltage reference  120  connected non inverting differential input  121 . The output  125  of power source  122  is supplied to feedback loop  126  which supplies inverting differential input  123  of the source  122 . Source output  125  is also supplied to diode  128  at its anode. The output from the cathode of the diode  128  is supplied to the common load  130  at node  132  and to the second feedback loop  124  which also supplies the inverting differential input  123  of source  122 . 
   Power source  112  employs the two feedback loops  114  and  116  to sense the load voltage at node  132  and the source voltage  115  respectively. Power source  122  operates in a similar manner using feedback loops  124  and  126  which sense voltages at nodes  132  and  125  respectively. With only feedback loops  114  and  124  present, voltage regulation will be excellent at node  132  due to the high loop gain of sources  112  and  122 , but there will be no current sharing as small differences between reference voltages  110  and  120  as well as offset voltage differences between sources  112  and  122  will force one source into negative saturation, turning it off and forcing the other source to shoulder the entire load  130 . 
   Items  112  and  122  are power sources with differential inputs. Items  110  and  120  are precision voltage references. It is difficult if not impossible to exactly match precision voltage references. The outputs of power sources  112  and  122  are diode ored using diodes  118  and  128  to provide a combined output voltage at  132  which is unaffected by the non operation of one of the sources due to the isolation provided by reverse biased diodes  118  and  128 . 
   Conversely, if only feedback loops  116  and  126  are present, voltage regulation at nodes  115  and  125  will be excellent and resistances of diodes  118  and  128  will allow current sharing. However, regulation at node  132  will be poor for low voltage systems due to relatively large voltage variation across diodes  118  and  128  with varying load currents and temperature. Usage of only loops  116  and  126  would result in an unacceptable voltage variation at load  130  with changes in current. Even though supplies would do a good job of sharing current, load regulation would be unacceptably poor. This will occur because as more current is demanded at the output, diode drop voltages will increase, thereby reducing the voltage appearing at node  132 . As each diode has voltage drops which, in the present embodiments, are a substantial percentage of the load voltage, load regulation will suffer. In one embodiment, the diode has a voltage drop that is about half the load voltage so even very small variations in the diode are significant when compared with a 10% load voltage tolerance. 
   In contrast, using only loops  124  and  114 , which provide precise regulation at the load, will result in the one converter, with slightly higher output voltage, supplying all the load current while the other converter, with the slightly lower output voltage, will be forced into a saturated off condition. 
   Using both loops  114  and  116  in branch  102 , and both loops  124  and  126  in branch  104 , there is provided a system having good voltage regulation at the load  130 , while still maintaining good current sharing. 
   By proportioning the feedback of loop  114  to loop  116 , the apparent resistance of source  112  as seen by load  130  can be varied anywhere between the impedance of diode  118 , to values nearly zero so as to allow a compromise between acceptable load voltage regulation and current sharing. For relatively equal current sharing, the same proportioning is implemented between loop  124  and loop  126 . This will produce approximately equal output resistances in branch  102  and branch  104  which will result in essentially equal branch output currents for approximately equal branch output voltages. All sources are intended to share current as equally as possible with nearly identical output voltage and resistance parameters determined during a design phase. Proportioning of the two feedback loops in each branch acts to select a percentage of the diode resistance as the apparent source resistance. This percentage can be varied from zero (0) to one hundred (100) per cent of the diode resistance. 
   Additionally, control loop parameters of source  112  or  122  are more nearly constant with the addition of feedback loops  116  and  126  as opposed to only loops  114  and  124 . Both sources,  112  and  124 , always have minimum amounts of feedback guaranteed by loops  116  and  126 . In one embodiment, the variation in feedback is less than a three to one ratio when comparing a single source powering the load to two or more sources sharing the load. With feedback loops  114  and  124  only, feedback varies from nearly zero (open loop) for one source which is saturated off, to maximum feedback (unity gain) for the source taking total control of the load current. This is a variation of over one thousand to one. Where feedback is only by means of loops  114  and  124 , in essence, there is no feedback the for the one source which becomes saturated, with its output diode reversed biased. Closed loop gain becomes very high and the circuit is both sensitive to noise and prone to oscillation when both sources are very close to the same voltage. Sources may oscillate between on and an off states. When reverse biased, a diode acts as a switch, therefore loop  114  or  124  can not guarantee feedback from the source output back to the source input. 
   Further, proportioning the majority of high frequency feedback through loops  116  and  126  in one embodiment results in greater stability. Excessive phase shift in feedback loops  114  or  124  can occur due to the impedance of diodes  118  and  128  in conjunction with capacitance at load  130 . Proportioning most of the high frequency feedback through loops  116  and  126  stabilizes the circuit, because due to their much lower proportions, high frequency phase shift introduced by loops  114  and  124  becomes negligible after being combined with dominant high frequency feedback from loops  116  and  126  at inverting inputs  113  and  123  respectively. 
   When the high frequency feedback is proportioned primarily through loops  116  and  126 , the variation in overall high frequency loop characteristics is significantly less, so the required compensation is more nearly constant. 
   It is recognized that current sharing could also be incorporated by means of adding resistance between diodes  118  and  128  and load  130  using a single point of feedback from the junction of diode  118  cathode and the supplied resistance. As this would result in additional and unnecessary power loss, it is inferior to the present embodiment. 
   In a method according to one embodiment of the invention, the sources are made to look resistive by proportioning the feedback loops given known characteristics of the sources, the diodes, and the loops themselves. When sources look resistive, rather than their impedances looking low or near zero, multiple sources share current automatically without the need for a current control signal. In this embodiment, the sources are made to look resistive using existing components in the circuit, namely the diodes  118  and  128 . Specifically, the known voltage drop across the diodes  118  and  128  is used to assist in the provision of the signal used to make the output look resistive. This is accomplished in one embodiment by feeding the signals from the combined output at node  132  and from the source output at nodes  115  or  125  respectively through the two feedback loops,  114  and  116 , and  124  and  126 , respectively, to the inverting differential inputs  113  and  123  of the sources  112  and  122 . 
   In one embodiment, the existing diode in a circuit is used for its voltage drop across the diode to provide a signal which makes the output appear resistive. Since the diodes already appear in current sharing circuitry, there are no additional power components required to implement the various embodiments of the present invention. 
   The voltage drop across the diodes, for example diodes  118  and  128 , is used in one embodiment to provide the signal which makes the outputs  115  and  125  appear resistive. The two feedback loops for each branch are fed back to their respective sources. The two feedback networks are engineered to provide any desired amount of regulation. Regulation in various embodiments is from a maximum regulation to a minimum regulation, and allows great flexibility in fine adjustment in the trade off between output voltage regulation and balanced sharing between sources. 
     FIG. 2  is a flow chart diagram of a method  200  of current sharing in a redundant power supply system. The method  200  comprises combining at least two power supply outputs at a common load node in block  202 , and feeding back power supply output and diode filtered power supply output to a differential amplifier for each power supply in block  204 . During design, the source resistance is adjusted through proportioning the feedback loops as has been discussed above. In one embodiment, feeding back comprises combining raw power supply output and diode filtered power supply output at an inverting differential amplifier input, and supplying a reference voltage at another differential amplifier non-inverting input. 
   In still another embodiment, regulation is accomplished by adding resistance in series with the diodes. This solution, however, creates additional system loss which is undesirable. 
   In one embodiment, there is a desired compromise between current sharing and regulation. In this embodiment, the highest resistance value that allows the voltage regulation limits to be met is used. This provides the best current balance for the characteristics. A tradeoff is that higher resistance values result in poorer regulation. It is usually regulation limits that are imposed by a user or customer, so meeting the regulation specifications limits the maximum resistance value. As has been mentioned, in one embodiment, the highest resistance value that still allows the meeting of the regulation limits is used. 
   The embodiments of the present invention provide good current balance and load regulation within limits by providing two feedback loops which compensate for differences in sources, components, and reference voltages. Diodes already present in the circuits are used for making the sources look resistive without the need for further components. 
   Although two power supplies are shown in circuit  100 , it should be understood that other circuit embodiments of the present invention allow scalability of the number of power supplies. Such other circuits add additional branches to the circuit, and are within the scope of the invention. 
   It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.