# Patent

## Title
Sensing current of a DC-DC converter

## Abstract

A DC-DC converter includes a directly coupled inductor with coil elements and power-switching phases. Each phase includes a high-side and low-side switch, where the high-side switch couples a voltage source to a coil element and the low-side switch couples the coil element to a ground voltage. Each switch is configured to be alternately activated and no two switches are activated at the same time. A current sensor for the DC-DC converter includes a single current amplifier having inputs and an output. The output provides a current sensing signal. The current sensor also includes a single RC network coupled to one of the power-switching phases and a first input of the current amplifier. The current sensor also includes a resistive ladder. The ladder includes, for each of the other power-switching phases, a resistor coupled in parallel to the RC network resistor and to a second input of the current amplifier.

## Background
BACKGROUND OF THE INVENTION
Field of the Invention
The field of the invention is power conversion, or, more specifically, methods and apparatus for sensing current of a DC (‘Direct Current’)-DC converter.
Description of Related Art
Computer system technology is continually advancing. Data centers, for example, now include hundreds or thousands of servers. Given the number of servers in a data center, decreasing the physical size or ‘footprint’ of the servers is a top priority for server system and server component designers. One are of focus, for example, is in reducing the size of Direct Current (‘DC’)-DC converters that distribute DC power amongst components of servers and the like.
In current art, reducing the size of such DC-DC converters is limited, at least in part, by the need for a plurality output inductors and a filter capacitor. Some DC-DC converters of the prior art have implemented designs to somewhat reduce the physical footprint of the inductors and the capacitor by utilizing a single magnetic core for multiple inductors, or a multiple magnetic core coupled to behave as one single unit—an implementation of an indirectly coupled inductor. FIG. 1A, for example, sets forth a prior art DC-DC converter that includes an indirectly coupled inductor.
The example DC-DC converter (100) of FIG. 1A includes two power-switching phases (132, 134). Each phase includes two switches: a high-side switch (102, 106), and a low-side switch (104, 108). Each high-side switch (102, 106) includes a control input (110, 114) to activate the switch. Upon activation, each high-side switch (102, 106) couples a voltage source (VIN) to an indirectly coupled inductor (118). Each low-side switch (104, 108) also includes a control input (112, 116) to activate the switch. Upon activation, each low-side switch (104, 108) couples one coil of indirectly coupled inductor (118) to a ground voltage.
Coupled inductors come in two forms: indirectly coupled and directly coupled. The dots depicted in the example of FIG. 1A indicate the coupled inductor (118) is an indirectly coupled inductor. The dot convention specifies the flow of current in a coupled inductor as: when current flows ‘into’ one dot, current is induced in the alternate coil of the coupled inductor and flows ‘out of’ the other dot. Thus, in an indirectly coupled inductor, current generally flows in the same direction in both coils of the coupled inductor.
The example prior art DC-DC converter (100) of FIG. 1A also includes an output capacitor (120) that operates as a lowpass filter and a load, represented by a resistor (122).
FIG. 1B sets forth an example timing diagram (130) of activating the switches (102, 112, 106, 116) of the prior art DC-DC converter (100) of FIG. 1A. In the example timing diagram of FIG. 1B, switch (102) is activated between time T0 and T1, then deactivated from T1 through T3. Switch (112) is not activated from time T0 and T1, but is activated at time T1 through T3. Switch (114) is only activated between time T2 to T3. Switch (116) is activated from time T0 to T2 and activated again at time T3.
The timing diagram (130) in the example of FIG. 1B specifies that activation of the high-side switch and low-side switch in a single phase of the prior art DC-DC converter (100) of FIG. 1 is asynchronous. Further, during any one given time period, two of the switches are activated at the same time. Although the indirectly coupled inductor in the example prior art DC-DC converter (100) of FIG. 1A represents a reduction in size relative to two, discrete inductors, operating the indirectly coupled prior art DC-DC converter (100) in accordance with the timing diagram of FIG. 1B limits any further inductor and capacitance reduction due to many factors, including for example: efficiency, current ripple, and so on. Other similar circuits of the prior art also has several limitations including:

    
    
        Prior art circuits rely on an equal DC current to flow through windings of the inductor to gain flux canceling affects, which requires highly accurate current sensing;
        Because current flow through all legs of the inductor of the prior art occurs simultaneously no accurate current sensing can take place with industry standard DCR (DC resistance) sensing;
        Prior art circuits with indirectly coupled inductors employ loops to form the indirectly coupled inductors which creates additional series resistance that inversely affects regulator efficiently;
        In prior art circuits, the leakage inductance sets the current ripple of the design, so there is a minimum leakage inductance that must exist, bounding transient performance of the design, and requiring a higher switching frequency; and
        Adding additional phases in parallel in prior art circuits inversely affects the transient performance of design, where the slew rate the load can be supplied is bounded the voltage input, number of phases, and leakage inductance.
    
    


In systems that rely on such DC-DC converters of the prior art, sensing the output current of the DC-DC converter may be useful. Prior art current sensing techniques, however, are costly in terms of power consumption, space consumption, and are generally overly complex. Consider, for example, FIG. 5 which sets forth a prior art DC-DC converter with current sensing (500). The DC-DC converter of FIG. 5 is similar to that set forth in FIG. 1A. The only appreciable difference between the two DC-DC converters is that, in FIG. 5, each phase of the DC-DC converter includes a DCR current sensing circuit. That is, in the example of FIG. 5, each phase (132, 134) includes, in parallel to the indirectly coupled inductor (118), a DCR (direct current resistance) circuit. Each DCR circuit includes an RC network (506, 508). Each DCR circuit also includes a current amplifier (502m 504). The output (510, 512) of each current amplifier may be measured and provide an indication of output current of the phase to which the current amplifier is coupled.
Capacitors are larger than many other components and current amplifiers drain power in greater amounts than other components. Because each additional phase of the DC-DC converter of the prior art requires an additional DCR circuit (including an additional current amplifier and an additional capacitor) to sense the current of that additional phase, each additional phase increases power and space consumption.
SUMMARY OF THE INVENTION
Methods for current sensing in a DC-DC converter and current sensors for such DC-DC converters are described in this specification. Such a DC-DC converter includes: a directly coupled inductor comprising a plurality of coil elements; and a plurality of power-switching phases, each power-switching phase coupled to a different one of the plurality of coil elements, each phase including: a high-side switch and a low-side switch, where the high-side switch is configured, when activated, to couple a voltage source to one of the coil elements and the low-side switch is configured, when activated, to couple the coil element to a ground voltage, where each switch of the DC-DC converter is configured to be alternately activated and no two switches are activated at the same time.
Current sensing in such a DC-DC converter in accordance with embodiments of the present invention may be carried out by measuring a current sensing signal provided by a current sensor. Such a current sensor includes: a single current amplifier that includes a plurality of inputs and an output, where the output provides a current sensing signal. The current sensor also includes a single RC network (‘Resistor-Capacitor network’) coupled to one of the power-switching phases and a first input of the current amplifier, where the RC network includes a resistor in series with a capacitor. The current sensor also includes a resistive ladder that includes: for each of the other power-switching phases, a resistor coupled in parallel to the resistor of the RC network and coupled to a second input of the current amplifier.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular descriptions of exemplary embodiments of the invention as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts of exemplary embodiments of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A sets forth a prior art DC-DC converter that includes an indirectly coupled inductor.
FIG. 1B sets forth an example timing diagram of activating switches of the prior art DC-DC converter of FIG. 1A.
FIG. 2A sets forth sets forth an identity switching DC-DC converter that includes a directly coupled inductor, operated in accordance with embodiments of the present invention.
FIG. 2B sets forth an example timing diagram of activating switches of the identity switching DC-DC converter of FIG. 2A.
FIG. 3 depicts an identity switching DC-DC converter operated in accordance with embodiments of the present invention that includes a plurality of power-switching phases.
FIG. 4 sets forth a flow chart illustrating an example method of operation a DC-DC converter in accordance with embodiments of the present invention.
FIG. 5 sets forth a prior art DC-DC converter with current sensing.
FIG. 6 sets forth an example identity switching DC-DC converter that includes a directly coupled inductor and a current sensor in accordance with embodiments of the present invention.
FIG. 7 sets forth an example identity switching DC-DC converter that includes a directly coupled inductor, four power-switching phases, and a current sensor in accordance with embodiments of the present invention.
FIG. 8 sets forth a flow chart illustrating an example method of operating a DC-DC converter and sensing current in the DC-DC converter in accordance with embodiments of the present invention.



DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
Exemplary methods and apparatus for operating a DC-DC converter in accordance with embodiments of the present invention are described with reference to the accompanying drawings, beginning with FIG. 2A. FIG. 2A sets forth sets forth an identity switching DC-DC converter that includes a directly coupled inductor, operated in accordance with embodiments of the present invention.
The example identity switching DC-DC converter (200) of FIG. 2A includes a directly coupled inductor (218) that includes a first coil element and a second coil element. The first coil element and second coil element are coupled to an output filter—the capacitor (220)—and a load represented by a resistor (222). Unlike the prior art indirectly coupled inductor (118) of FIG. 1A, in the directly coupled inductor (218) in the example of FIG. 2A, current generally flows equal in magnitude and in the opposite direction in the coils of the coupled inductor. That is, when current enters one dot, current is induced to exit the other dot.
The example identity switching DC-DC converter (200) of FIG. 2A also includes a plurality of power-switching phases. More specifically, the example identity switching DC-DC converter (200) of FIG. 2A includes two power-switching phases (232, 234). Each power-switching phase is configured similarly: a first power-switching phase (232) includes a high-side switch (202) and a low-side switch (204). The high-side switch (202) is configured, when activated 180 degrees out of phase, by a control input (210), to couple a voltage source (VIN) to the first coil element of the directly coupled inductor (218). The low-side switch (204) is configured, when activated by a control input (212), to couple the first coil element to a ground voltage.
The second power-switching phase (234) of the example identity switching DC-DC converter (200) of FIG. 2A includes a high-side switch (206) and a low-side switch (208). The high-side switch (206) of the second power-switching phase (234) is configured, when activated by a control input (214), to couple the voltage source (VIN) to the second coil element of the directly coupled inductor (218). The low-side switch (208) of the second power-switching phase (234) is configured, when activated by a control input (216), to couple the second coil element to the ground voltage.
As will occur to readers of skill in the art, each of the switches (202, 204, 206, 208) in the example of FIG. 2A may be implemented as a Field Effect Transistor (‘FET’) or the like.
The identity switching DC-DC converter (200) of FIG. 2A is operated by alternatively activating each switch, where no two switches are activated at the same time. For further explanation, FIG. 2B sets forth an example timing diagram of activating switches of the identity switching DC-DC converter of FIG. 2A.
The DC-DC converter of FIG. 2A is described as an ‘identity switching’ converter due to the pattern of activating switches when viewed in a matrix or table. The example table below describes the timing of the switch activations as seen in the example timing diagram of FIG. 2B:







TABLE 1







Switch Activation Pattern For Identity


Switching DC-DC Converter (200) of FIG. 2A













Control Input, Switch
T0-T1
T1-T2
T2-T3
T3-T4







CI (210), HS Switch (202)
1
0
0
0



CI (212), LS Switch (204)
0
1
0
0



CI (214), HS Switch (206)
0
0
1
0



CI (216), LS Switch (208)
0
0
0
1










In the example Table 1 above, it can be seen that the control input and associated switches are alternatively activated (represented by a ‘1’ in the table) in a manner that forms an identity of the table. Further, no two switches are activated at the same time. As depicted in Table 1 and the example timing diagram (230) of FIG. 2B:
from time T0-T1, only the high-side switch (202) of the first power-switching phase (232) is activated; from time T1-T2, only the low-side switch (204) of the first power-switching phase (232) is activated; from time T2-T3, only the high-side switch (206) of the second power-switching phase (234) is activated; and from time T3-T4, only the low-side switch (208) of the second power-switching phase (234) is activated.
A ‘0’ in the table above represents that the switch is tri-stated, 0V, kept in the off position. That is, in embodiments in which the switches are implemented as FETs, no gate drive is applied to the silicon gate. In this way, when not activated, each switch may introduce a high impedance path to the system. As such, each loop coil element is alternatively coupled to the voltage source, the ground voltage, and the high impedance path.
Readers of skill in the art will recognize that the phrase “no two switches are activated at the same time” may be read literally in ideal conditions where the switches are implemented as unidirectional switches with little to no switching response time. In other, less ideal conditions, however—such as implementations in which the switches are implemented as FETs having a body diode—the phrase “no two switches are activated at the same time” means that no two switches are activated at nearly or approximately the same time. That is, the phrase “no two switches are activated at the same time” does not exclude minor overlap, but instead describes switch activation over a much longer time period—the switching period or duty cycle of the switches as a while. Two switches, for example, such as the low-side switch of the first phase and the high-side switch of the second phase may be activated at the same time, but for only for a very short amount of time, in order to fully discharge the body diode of the low-side switch. In such an example, immediately before the low-side switch of the first phase is deactivated, the high-side switch of the second phase may be activated in order to drain current in the body diode. The two switches in this implementation are ‘on’ concurrently for a very minimal amount of time, not representing an appreciable portion of the switching period of the switches. The phrase, “no two switches are activated at the same time,” then, may be thought of relative to switching schemes of the prior art in which two switches are activated concurrently for a very long time during the a switching period or for an entire duty cycle.
In this way, each phase is utilized at a 180 degree offset and each high-side switch for a period of time according to:




  
    D
    N
  
  ,




where D represents a duty cycle and N represents the number of power-switching phases. Each low-side switch is therefore activated for a period of time according to:





  
    
      (
      
        1
        -
        D
      
      )
    
    N
  
  .




In this way, the number of phases is inversely proportional to the duty cycle of activating the switches—that is, the ‘effective’ duty cycle—and thereby is inversely proportional to the inductance of the directly coupled inductor. Increasing the number of phases, therefore, decreases the inductance.
And the transfer function of the identity switching DC-DC converter (200) of FIG. 2A, when operated in accordance with the identity switching scheme in Table 1 and the timing diagram (230) of FIG. 2B is:




  
    
      V
      OUT
    
    
      V
      IN
    
  
  =
  
    D
    N
  




Operating the example identity switching DC-DC converter (200) of FIG. 2A in accordance with the identity switching scheme in Table 1 and the timing diagram of FIG. 2B enables energy to be stored between deactivating the low-side switch (212) of the first power-switching phase (232) and activation of the high-side switch of the second power-switching phase (234), thus increasing overall system efficiency and reducing current ripple. That is, current ripple experienced by the magnetic core of the directly coupled inductor (218) and the output capacitor (220) is reduced, relative to circuits of the prior art, due in part to the effective reduced duty cycle of the switch activations. The current ripple experienced by the output filter capacitor (220) and the load (222) may be calculated as:




  
    
      1
      
        
          f
          *
        
        ⁢
        
          L
          OL
        
      
    
    *
    
      (
      
        1
        -
        
          
            V
            OUT
          
          
            V
            IN
          
        
      
      )
    
    *
    
      
        V
        OUT
      
      N
    
  
  ,




where f represents the frequency of alternatively activating each switch, LOL represents the open loop inductance of the directly coupled inductor, N represents the number of power-switching phases, VIN represents the voltage of the voltage source and VOUT represents the voltage experienced at the output filter and load.

FIGS. 2A and 2B generally depict an identity switching DC-DC converter configured with two phases and operation thereof, but readers of skill in the art will recognize that an identity switching DC-DC converter operated in accordance with embodiments of the present invention may have any number of phases. For further explanation, therefore, FIG. 3 depicts an identity switching DC-DC converter (300) operated in accordance with embodiments of the present invention that includes a plurality of power-switching phases. The example DC-DC converter (300) of FIG. 3 includes four power-switching phases:

    
    
        a first power-switching phase that includes a high-side switch (302) and a low-side switch (304);
        a second power-switching phase that includes a high-side switch (306) and a low-side switch (308);
        a third power-switching phase that includes a high-side switch (310) and a low-side switch (312); and
        a fourth power-switching phase that includes a high-side switch (314) and a low-side switch (316).
    
    


Each high-side switch (302, 306, 310, 314) includes a control input (326, 330, 334, 338) for activating the switch. Each low-side switch (304, 308, 312, 316) includes a control input (328, 332, 336, 340) for activating the switch. Each pair of phases is connected to a directly coupled inductor (350, 352), an output filter capacitor (356), and a load (358).
The switches in the example identity switching DC-DC converter (300) of FIG. 3 are alternatively activated and no two switches are activated concurrently. The following table sets forth the timing of switch activations in the example DC_DC converter (300) of FIG. 3:







TABLE 2







Switch Activation Pattern For Identity


Switching DC-DC Converter (300) of FIG. 3
















0
0
180
180
90
90
270
270


Control Input, Switch
Deg.
Deg.
Deg
Deg.
Deg.
Deg.
Deg.
Deg.





CI (326), HS Switch (302)
1
0
0
0
0
0
0
0


CI (328), LS Switch (304)
0
1
0
0
0
0
0
0


CI (330), HS Switch (306)
0
0
1
0
0
0
0
0


CI (332), LS Switch (308)
0
0
0
1
0
0
0
0


CI (334), HS Switch (310)
0
0
0
0
1
0
0
0


CI (336), LS Switch (312)
0
0
0
0
0
1
0
0


CI (338), HS Switch (314)
0
0
0
0
0
0
1
0


CI (340), LS Switch (316)
0
0
0
0
0
0
0
1









In the example Table 2 above, no two switches are activated concurrently. The second power-switching phase operates an offset of 180 degrees from the first power-switching phase. The fourth power-switching phase operates at an offset of 180 degrees from the third power-switching phase.
For further explanation, FIG. 4 sets forth a flow chart illustrating an example method of operation a DC-DC converter in accordance with embodiments of the present invention. The DC-DC converter of FIG. 4 is similar to the DC-DC converter of FIG. 2A including as it does: a directly coupled inductor (218) that includes a first coil element and a second coil element, the first and second coil elements coupled to an output filter—a capacitor (220)—and a load (222); and a plurality of power-switching phases including a first and second power-switching phase (232), the first power-switching phase (232) including includes a high-side switch (202) and a low-side switch (204), the high-side switch (202) configured, when activated by a control input (210), to couple a voltage source (VIN) to the first coil element of the directly coupled inductor (218), the low-side switch (204) configured, when activated by a control input (212), to couple the first coil element to a ground voltage; the second power switching phase (234) also including a high-side switch (206) and a low-side switch (208), the high-side switch (206) configured, when activated by a control input (214), to couple the voltage source (VIN) to the second coil element of the directly coupled inductor (218), and the low-side switch (208) configured, when activated by a control input (216), to couple the second coil element to the ground voltage.
The method of FIG. 4 includes alternately activating (402) each switch, where no two switches are activated at the same time. In the method of FIG. 4, alternatively activating (402) each switch is carried out by: activating (404) the high-side switch of the first power-switching phase, including coupling the voltage source to the first coil element, energizing a magnetic core of the directly coupled inductor, and providing, via the first coil element, output current to the filter and load; activating (406) the low-side switch of the first power-switching phase, including coupling the first coil element to the ground voltage and providing, via the second coil element and the energized magnetic core, output current to the filter and load; activating (408) the high-side switch of the second power-switching phase, including coupling the voltage source to the second coil element, re-energizing the magnetic core of the directly coupled inductor, and providing, via the second coil element, output current to the filter and load; and activating (410) the low-side switch of the second power-switching phase, including coupling the second coil element to the ground voltage and providing, via the first coil element and the energized magnetic core, output current to the filter and load.
DC-DC converters configured to operate in accordance with embodiments of the present invention may be implemented in a variety of applications. One application, for example, in which a DC-DC converter configured to operate in accordance with embodiments of the present invention may be implemented, is a power supply for a computer.
In view of the explanations set forth above, readers will recognize that the benefits of operating a DC-DC converter in accordance with embodiments of the present invention include:

    
    
        reducing a physical footprint of a DC-DC converter without sacrificing efficiency or introducing an inordinate amount of current ripple;
        providing a DC-DC converter having current ripple characteristics independent of leakage inductance;
        providing a DC-DC converter that allows filter capacitance reductions, thereby reducing the need for large physical design layouts;
        providing a DC-DC converter with a coupled inductor that does not rely on flux cancelation of equal current flowing through loop coil elements to improve system performance; and
        providing a DC-DC converter having a coupled inductor in which current flow may be accurately and precisely measured through use of industry standard DCR current sensing.
    
    


Although industry standard DCR (Direct current sensing) may be utilized to sense current in the DC-DC converters of 2A and FIG. 3, as explained above, prior art current sensors inefficiently consume power, space, and require a large number of redundant components. To that end, FIG. 6 sets forth an example identity switching DC-DC converter that includes a directly coupled inductor and a current sensor in accordance with embodiments of the present invention.
The DC-DC converter (600) in the example of FIG. 6 is similar to the DC-DC converter in the example of FIG. 2A and includes similar components. Further, the DC-DC converter in the example of FIG. 6 is also operated in a similar manner to that of the DC-DC converter of FIG. 2A. That is, the DC-DC converter of FIG. 6 includes a directly coupled inductor (218) with a plurality of coil elements and a plurality of power-switching phases (232, 234). Each power-switching phase is coupled to a different one of the coil elements. Each phase (232, 234) includes a high-side switch (202, 206) and a low-side switch (204, 208). The high-side switch (202, 206) is configured, when activated through a signal at a control node (210, 214), to couple a voltage source to one of the coil elements and the low-side switch (204, 208) is configured, when activated through another control node (212, 216), to couple the coil element to a ground voltage. The switches of the DC-DC converter (600) are configured to be alternately activated in accordance with the identity switching scheme set forth above such that no two switches of the DC-DC converter (600) are activated at the same time.
The example current sensor of the DC-DC converter (600) includes a single current amplifier (604). That is, in contrast to the prior art DCR current sensors that require a separate current amplifier for each phase of the DC-DC converter, the current sensor in the example of FIG. 6 requires only a single current amplifier for any number of phases. The current amplifier (604) includes plurality of inputs and an output (606).
The output provides a current sensing signal that may be measured to indicate the output current of the DC-DC converter (600).
The example current sensor of the DC-DC converter (600) of FIG. 6 also includes a single RC network (‘Resistor-Capacitor network’) (602) coupled to one of the power-switching phases. Again, as opposed to the prior art DCR current sensors which require a separate RC network for each and every phase of the DC-DC converter, the current sensor of FIG. 6 requires only a single RC network for a single phase. The RCE network (602) in the example of FIG. 6 is coupled to a first input of the current amplifier (604) and includes a resistor in series with a capacitor.
The example current sensor of the DC-DC converter (600) of FIG. 6 also includes a resistive ladder. The resistive ladder includes: for each of the other power-switching phases (the phases other than the phase which is coupled to the RC network), a resistor coupled in parallel to the resistor of the RC network and coupled to a second input of the current amplifier. A resistive ladder as the term is used here refers to a plurality of resistors in parallel. In the example of FIG. 6, the resistive ladder is formed of the resistor coupled to phase (232) and the resistor of the RC network (602).
As mentioned above, the resistive ladder includes resistors for each of the phases of the current sensor. For further explanation, therefore, FIG. 7 sets forth an example identity switching DC-DC converter that includes a directly coupled inductor, four power-switching phases, and a current sensor in accordance with embodiments of the present invention. The example DC-DC converter of FIG. 7 is similar to that of FIG. 4 and includes many of the same components. Further, the example DC-DC converter is operated in a manner similar to that of the DC-DC converter of FIG. 4. That is, the DC-DC converter (700) of FIG. 7 is also operated in accordance with the identity switching schemes set forth above.
The only appreciable difference between the DC-DC converter (700) of FIG. 7 and that of FIG. 4 is that the DC-DC converter (700) of FIG. 7 also includes a current sensor. The current sensor of FIG. 7 includes a single current amplifier (704). The current amplifier includes a plurality of inputs and an output (706). The output provides a current sensing signal which may be measured to indicate current output of the DC-DC converter.
The current sensor in the example of FIG. 7 also includes a single RC network (702) that is coupled to one of the power-switching phases and a first input of the current amplifier (704). The RC network (702) includes a resistor in series with a capacitor.
The example current sensor of the DC-DC converter (700) of FIG. 7 also includes a resistive ladder. The resistive ladder in the example of FIG. 7 includes: for each of the other power-switching phases (the phases not coupled to RC network), a resistor is coupled in parallel to the resistor of the RC network and coupled to a second input of the current amplifier (704). Readers will appreciate that the resistive ladder enables the current sensor to include only a single RC network, a single current amplifier, and a single resistor for every phase (except one). By contrast, the current sensors of the prior art include a current amplifier for every phase and an RC network for every phase.
FIG. 8 sets forth a flow chart illustrating an example method of operating a DC-DC converter and sensing current in the DC-DC converter in accordance with embodiments of the present invention. The method of FIG. 8 may be carried out with a DC-DC converter similar to those set forth in FIG. 6 and FIG. 7. Such a DC-DC converter includes a directly coupled inductor comprising a plurality of coil elements. Such a DC-DC converter also includes a plurality of power-switching phases, with each power-switching phase coupled to a different one of the plurality of coil elements. Each phase also includes: a high-side switch and a low-side switch, where the high-side switch is configured, when activated, to couple a voltage source to one of the coil elements and the low-side switch is configured, when activated, to couple the coil element to a ground voltage.
In the method of FIG. 4, operating such a DC-DC converter is carried out by alternately activating (802) each switch, where no two switches are activated at the same time. That is, the DC-DC converter of FIG. 8 may be operated in accordance with the identity switching scheme set forth above. More specifically, in the method of FIG. 8, alternatively activating (402) each switch is carried out by: activating the high-side switch of a first power-switching phase, including coupling the voltage source to a first coil element, energizing a magnetic core of the directly coupled inductor, and providing, via the first coil element, output current to the filter and load;
activating a low-side switch of the first power-switching phase, including coupling the first coil element to the ground voltage and providing, via a second coil element and the energized magnetic core, output current to the filter and load; activating a high-side switch of a second power-switching phase, including coupling the voltage source to the second coil element, re-energizing the magnetic core of the directly coupled inductor, and providing, via the second coil element, output current to the filter and load; and activating a low-side switch of the second power-switching phase, including coupling the second coil element to the ground voltage and providing, via the first coil element and the energized magnetic core, output current to the filter and load.
The method of FIG. 8 also includes measuring (804) a current sensing signal provided by a current sensor. Such a current sensor may include a single current amplifier having a plurality of inputs and an output, where the output provides the current sensing signal. The current sensor may also include a single RC network coupled to one of the power-switching phases and a first input of the current amplifier, where the RC network includes a resistor in series with a capacitor. The current sensor may also include a resistive ladder. Such a resistive ladder may include, for each of the other power-switching phases, a resistor coupled in parallel to the resistor of the RC network and coupled to a second input of the current amplifier.
It will be understood from the foregoing description that modifications and changes may be made in various embodiments of the present invention without departing from its true spirit. The descriptions in this specification are for purposes of illustration only and are not to be construed in a limiting sense. The scope of the present invention is limited only by the language of the following claims.

## Claims

1. A current sensor for a DC (‘Direct Current’)-DC converter, the DC-DC converter comprising:
a directly coupled inductor comprising a plurality of coil elements; and
a plurality of power-switching phases, each power-switching phase coupled to a different one of the plurality of coil elements, each phase comprising: a high-side switch and a low-side switch, wherein the high-side switch is configured, when activated, to couple a voltage source to one of the coil elements and the low-side switch is configured, when activated, to couple the coil element to a ground voltage, wherein each switch of the DC-DC converter is configured to be alternately activated and no two switches are activated at the same time;
the current sensor comprising:
a single current amplifier comprising a plurality of inputs and an output, wherein the output provides a current sensing signal;
a single RC network (Resistor-Capacitor network') coupled to one of the power-switching phases and a first input of the current amplifier, the RC network comprising a resistor in series with a capacitor; and
a resistive ladder comprising: for each of the other power-switching phases, a resistor coupled in parallel to the resistor of the RC network and coupled to a second input of the current amplifier.

2. The current sensor of claim 1 wherein the DC-DC converter further comprises a filter and a load coupled to the directly coupled inductor and each switch of the DC-DC converter is configured to be alternately activated by:
activating the high-side switch of a first power-switching phase, including coupling the voltage source to a first coil element, energizing a magnetic core of the directly coupled inductor, and providing, via the first coil element, output current to the filter and load;
activating the low-side switch of the first power-switching phase, including coupling the first coil element to the ground voltage and providing, via a second coil element and the energized magnetic core, output current to the filter and load;
activating the high-side switch of a second power-switching phase, including coupling the voltage source to the second coil element, re-energizing the magnetic core of the directly coupled inductor, and providing, via the second coil element, output current to the filter and load; and
activating the low-side switch of the second power-switching phase, including coupling the second coil element to the ground voltage and providing, via the first coil element and the energized magnetic core, output current to the filter and load.

3. The current sensor of claim 1 wherein each switch of the DC-DC converter is configured to be alternately activated and no two switches are activated at the same time by:
activating each high-side switch for a period of time according to:





  D
  N




where D represents a duty cycle and N represents the number of power-switching phases; and
activating each low-side switch for a period of time according to:





  
    
      (
      
        1
        -
        D
      
      )
    
    N
  
  .

4. The current sensor of claim 1 wherein the number of power-switching phases is inversely proportional to the duty cycle of activating the switches and thereby inversely proportional to the inductance of the directly coupled inductor.

5. The current sensor of claim 1 wherein the DC-DC converter further comprises a filter and a load coupled to the directly coupled inductor and current ripple experienced by the filter and the load comprises:




  
    
      1
      
        
          f
          *
        
        ⁢
        
          L
          OL
        
      
    
    *
    
      (
      
        1
        -
        
          
            V
            OUT
          
          
            V
            IN
          
        
      
      )
    
    *
    
      
        V
        OUT
      
      N
    
  
  ,




where f represents the frequency of alternatively activating each switch, LOL represents the open loop inductance of the directly coupled inductor, N represents the number of power-switching phases, VIN represents the voltage of the voltage source and VOUT represents the voltage experienced at the filter and load.

6. The current sensor of claim 1 wherein each high-side switch and each low-side switch comprises a Field Effect Transistor.

7. A method of sensing current of a DC (‘Direct Current’)-DC converter, the DC-DC converter comprising:
a directly coupled inductor comprising a plurality of coil elements; and
a plurality of power-switching phases, each power-switching phase coupled to a different one of the plurality of coil elements, each phase comprising: a high-side switch and a low-side switch, wherein the high-side switch is configured, when activated, to couple a voltage source to one of the coil elements and the low-side switch is configured, when activated, to couple the coil element to a ground voltage, wherein each switch of the DC-DC converter is configured to be alternately activated and no two switches are activated at the same time; and
the method comprises:
measuring a current sensing signal provided by a current sensor, wherein the current sensor comprises:
a single current amplifier comprising a plurality of inputs and an output, wherein the output provides the current sensing signal;
a single RC network (‘Resistor-Capacitor network’) coupled to one of the power-switching phases and a first input of the current amplifier, the RC network comprising a resistor in series with a capacitor; and
a resistive ladder comprising: for each of the other power-switching phases, a resistor coupled in parallel to the resistor of the RC network and coupled to a second input of the current amplifier.

8. The method of claim 7 wherein the DC-DC converter further comprises a filter and a load coupled to the directly coupled inductor and each switch of the DC-DC converter is configured to be alternately activated by:
activating the high-side switch of a first power-switching phase, including coupling the voltage source to a first coil element, energizing a magnetic core of the directly coupled inductor, and providing, via the first coil element, output current to the filter and load;
activating the low-side switch of the first power-switching phase, including coupling the first coil element to the ground voltage and providing, via a second coil element and the energized magnetic core, output current to the filter and load;
activating the high-side switch of a second power-switching phase, including coupling the voltage source to the second coil element, re-energizing the magnetic core of the directly coupled inductor, and providing, via the second coil element, output current to the filter and load; and
activating the low-side switch of the second power-switching phase, including coupling the second coil element to the ground voltage and providing, via the first coil element and the energized magnetic core, output current to the filter and load.

9. The method of claim 7 wherein each switch of the DC-DC converter is configured to be alternately activated and no two switches are activated at the same time by:
activating each high-side switch for a period of time according to:





  D
  N




where D represents a duty cycle and N represents the number of power-switching phases; and
activating each low-side switch for a period of time according to:





  
    
      (
      
        1
        -
        D
      
      )
    
    N
  
  .

10. The method of claim 7 wherein the number of power-switching phases is inversely proportional to the duty cycle of activating the switches and thereby inversely proportional to the inductance of the directly coupled inductor.

11. The method of claim 7 wherein the DC-DC converter further comprises a filter and a load coupled to the directly coupled inductor and current ripple experienced by the filter and the load comprises:




  
    
      1
      
        
          f
          *
        
        ⁢
        
          L
          OL
        
      
    
    *
    
      (
      
        1
        -
        
          
            V
            OUT
          
          
            V
            IN
          
        
      
      )
    
    *
    
      
        V
        OUT
      
      N
    
  
  ,




where f represents the frequency of alternatively activating each switch, LOL represents the open loop inductance of the directly coupled inductor, N represents the number of power-switching phases, VIN represents the voltage of the voltage source and VOUT represents the voltage experienced at the filter and load.

12. The method of claim 7 wherein each high-side switch and each low-side switch comprises a Field Effect Transistor.