# Patent

## Title
Step-down voltage converter

## Abstract

A step-down voltage converter (100) for generating an output voltage (VOUT) from an input voltage (VIN) is provided. The converter (100) includes a switch (111) having a first terminal (112) and a second terminal (114), wherein the second terminal (114) is electrically coupled with the output voltage (VOUT). Also included is a rectifier (117) having a first terminal (118) and a second terminal (120), wherein the second terminal (120) is electrically coupled with the output voltage (VOUT). A first inductor (124) electrically couples the first terminal (112) of the switch (111) with the input voltage (VIN). A second inductor (126) magnetically coupled with the first inductor (124) electrically couples the first terminal (118) of the rectifier (117) with a voltage reference (128). A switch controller (110) coupled with the output voltage (VOUT) is configured to control the switch (111).

## Background
FIELD OF THE INVENTION
Aspects of the invention relate generally to electrical voltage converters, and more particularly to electrical step-down voltage converters producing a direct-current (DC) voltage from an alternating-current (AC) voltage or a DC voltage.
BACKGROUND OF THE INVENTION
Various electrically-powered equipment within an industrial environment often depend upon any of a variety of AC and/or DC voltages for power. More specifically, DC-oriented systems tend to utilize relatively low DC voltages, typically ranging from 12 to 50 volts DC (VDC). AC-oriented systems, however, often employ higher AC voltages, sometimes ranging between 100 and 250 volts root-mean-square (VRMS). Other AC or DC voltages outside of these ranges may be employed as well. However, industrial instrumentation, such as a Coriolis flowmeter for measuring mass flow and other information concerning a material flowing through a conduit, often employ electrical components that require a low DC voltage, such as 1.2-24 VDC, as an electrical power source, and thus are not capable of withstanding such a large range of AC or DC input voltages. Thus, a step-down converter capable of producing a substantially fixed low DC output voltage from either an AC or DC input voltage is often used to great advantage in such an environment.
A simplified schematic diagram of one particular type of step-down, or “buck,” converter or regulator 1 currently in use for converting a positive DC input voltage VIN to a DC output voltage VOUT is provided in FIG. 1. The input voltage VIN is asserted across an input capacitor CA coupled with a ground reference, and is coupled with the drain terminal of an n-channel power field-effect transistor (FET) switch Q. The input capacitor CA acts as a filter to help maintain the voltage level seen by the drain of the switch Q in the presence of changes in the input voltage VIN by providing additional current on a temporary basis to the drain of the switch Q. Similar functionality for the output voltage VOUT is provided by an output capacitor CB.
The gate of the switch Q is driven by a switch controller 2, which turns the switch Q ON and OFF depending on the voltage level of the output voltage VOUT compared to the desired or target output voltage VOUT level. Some other measurable quantity at the output, such as current, may be employed by the switch controller 2 alternatively or additionally. By turning the switch Q ON and OFF substantially periodically, the switch controller 2 is normally capable of maintaining the output voltage VOUT at a desirable level in the presence of changes in both the input voltage VIN level and the load driven by the output voltage VOUT. Generally, the switching period is the sum of the ON time and the OFF time of the switch Q during one operation cycle. Accordingly, the duty cycle of the switch Q is the ratio of the ON time to the period. Thus, by way of any of a number of techniques, the switch controller 2 controls the duty cycle and period of the switch Q to maintain the output voltage VOUT at a satisfactory level.
During operation of the converter 1, when the switch Q is ON, electrical current flows from the input voltage VIN, through the drain and source terminals of the switch Q, and through an inductor L, to the output voltage VOUT. As a result of electrical current flowing through the inductor L, electrical energy is stored in the inductor L. Typically, the ON time of the switch Q, as set by the switch controller 2, is constrained by the component values of the inductor L and the output capacitor CB such that the voltage VL across the inductor is nearly constant during the ON time. Under these conditions, the terminal of the inductor L connected to the source terminal of the switch Q remains near the input voltage VIN while the switch Q is ON, and the remaining terminal of the inductor L is at the output voltage VOUT level. As a result, the voltage at the cathode 3 of a diode D coupled at the source terminal of the switch Q causes the diode D to be reverse-biased, and therefore not conducting, when the switch Q is ON, since the anode 4 of the diode D is connected to ground.
When the switch Q is then turned OFF, the voltage VL across the inductor L reverses polarity in order to maintain the continuity of the electrical current through the inductor L. That “flyback” in voltage causes the voltage at the cathode 3 of the diode D to drop below ground, thereby forward-biasing the diode D into conduction. Thus, electrical energy stored in the inductor L while the switch Q is ON is transferred as current through the diode D and the inductor. L to the output voltage VOUT. At some point determined by the switch controller 2, the switch Q is once again turned ON, and the above cycle repeats. Current thus flows into the voltage output VOUT when the switch Q is either ON or OFF.
One potential drawback of the step-down converter 1 of FIG. 1 is the large voltage swing required from the switch controller 2 to drive the gate of the switch Q to turn the switch Q ON and OFF. More specifically, to turn the switch Q ON and maintain that state, the switch controller 2 must drive the gate to a voltage level higher than the input voltage VIN, since the gate voltage must be above that of the source, which is close to the input voltage VIN during the ON state. To turn the switch Q OFF, the gate voltage must be near ground, since the source is driven to slightly below ground due to the diode D becoming forward-biased at that time due to the flyback of the inductor L. When the input voltage VIN is a relatively low DC voltage, generation of the proper gate voltage for the switch Q to be turned ON may be accomplished by way of a readily-available voltage “boost” circuit. However, when the input voltage VIN is a large AC voltage on the order of 265 VRMS, which translates to a maximum DC voltage level of about 375 VDC, timely and accurate control of the gate voltage while providing extremely large voltage swings at the gate of hundreds of volts typically requires a relatively complex circuit design for the switch controller 2 involving specialized electrical components.
SUMMARY OF THE INVENTION
Generally, embodiments of the present invention provide a step-down voltage converter for generating an output voltage from an input voltage. The converter includes a switch having first and second terminals, wherein the second terminal is electrically coupled with the output voltage. A rectifier has first and second terminals, wherein the second terminal is electrically coupled with the output voltage. A first inductor electrically couples the first terminal of the switch with the input voltage. A second inductor magnetically coupled with the first inductor electrically couples the first terminal of the rectifier with a voltage reference. Also, a switching controller coupled with the output voltage is configured to control the switch.
Additional embodiments and advantages of the present invention will be realized by those skilled in the art upon perusal of the following detailed description, taken in conjunction with the accompanying drawings.
Aspects
One aspect of the invention includes a step-down voltage converter for generating an output voltage from an input voltage, comprising:
a switch comprising a first terminal and a second terminal, wherein the second terminal of the switch is electrically coupled with the output voltage;
a rectifier comprising a first terminal and a second terminal, wherein the second terminal of the rectifier is electrically coupled with the output voltage;
a first inductor electrically coupling the first terminal of the switch with the input voltage;
a second inductor magnetically coupled with the first inductor, the second inductor electrically coupling the first terminal of the rectifier with a voltage reference; and
a switching controller coupled with the output voltage and configured to control the switch.
Preferably, the first inductor and the second inductor each comprise an inductance of 1.7 millihenries.
Preferably, the step-down voltage converter further comprises:
a first capacitor electrically coupling the input voltage with the voltage reference; and
a second capacitor electrically coupling the output voltage with the voltage reference.
Preferably, the first capacitor comprises a capacitance of 22 microfarads.
Preferably, the second capacitor comprises a capacitance of 120 microfarads.
Preferably, voltage reference is ground.
Preferably, a number of turns of the first inductor and a number of turns of the second inductor comprise a ratio of 1:1.
Preferably, the first inductor comprises a first winding of a transformer, wherein the second inductor comprises a second winding of the transformer, and wherein the first inductor and the second inductor are wound about a core.
Preferably, the core is a ferrite core.
Preferably, the input voltage and the output voltage are positive direct-current voltages;
the switch comprises an n-channel field-effect transistor, the first terminal of the switch comprises a drain terminal of the FET, the second terminal of the switch comprises a source terminal of the FET, and the switch controller controls the FET by way of a gate terminal of the FET; and
the rectifier comprises a diode, the first terminal of the rectifier comprises an anode of the diode, and the second terminal of the rectifier comprises a cathode of the diode.
Preferably, the input voltage and the output voltage are negative DC voltages;
the switch comprises a p-channel field-effect transistor, the first terminal of the switch comprises a drain terminal of the FET, the second terminal of the switch comprises a source terminal of the FET, and the switch controller controls the FET by way of a gate terminal of the FET; and
the rectifier comprises a diode, the first terminal of the rectifier comprises a cathode of the diode, and the second terminal of the rectifier comprises an anode of the diode.
Preferably, the switch controller is configured to control the switch by turning the switch on and off substantially periodically.
Preferably, the switch controller is configured to control the switch based on the output voltage.
Preferably, the switch controller is configured to control the switch based on a current at the output voltage.
Preferably, the input voltage is an alternating current input voltage; and
the step-down voltage converter further comprises an AC rectification circuit coupling the AC input voltage with the first inductor.
Preferably, the AC rectification circuit is configured to convert the AC input voltage to a first positive DC voltage; and
the output voltage is a positive DC output voltage having a lower magnitude that the first positive DC voltage.
Preferably, the AC rectification circuit is configured to convert the AC input voltage to a first negative DC voltage; and
the output voltage is a negative DC output voltage having a lower magnitude that the first negative DC voltage.
Preferably, an item of industrial instrumentation comprises the step-down voltage converter.
Preferably, a Coriolis flowmeter comprises the step-down voltage converter.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a step-down voltage converter according to the prior art.
FIG. 2 is a block diagram of a step-down converter according to an embodiment of the invention.
FIG. 3 is a schematic diagram of a step-down converter according to an embodiment of the invention for generating a positive DC output voltage from a positive DC input voltage.
FIG. 4 is timing diagram of the current through a first inductor and a second inductor, the voltage at the drain of a switch, and the voltage at the anode of a diode, as implemented in a particular embodiment of the step-down voltage converter of FIG. 3.
FIG. 5 is a schematic diagram of a step-down converter according to an embodiment of the invention for generating a negative DC output voltage from a negative DC input voltage.
FIG. 6 is block diagram of the step-down converter of FIG. 2 further employing an AC rectification circuit for an AC input voltage.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 is a simplified block diagram of a step-down voltage converter 100 for generating an output voltage VOUT from an input voltage VIN according to an embodiment of the invention. Generally, the converter 100 includes a switch 111 having a first terminal 112 and a second terminal 114, wherein the second terminal 114 is coupled with the output voltage VOUT. The first terminal 112 of the switch 111 is electrically coupled with the input voltage VIN by way of a first inductor 124. The switch 111 is controlled by way of a switch controller 110 coupled with the output voltage VOUT. In addition, a second inductor 126 magnetically coupled with the first inductor 124 electrically couples a first terminal 118 of a rectifier 117 with a voltage reference 128, while a second terminal 120 of the rectifier 117 is electrically coupled with the output voltage VOUT.
FIG. 3 is a simplified schematic diagram of a specific example of the step-down voltage converter 100: a voltage converter 200 for generating a positive DC output voltage VOUT from a positive DC input voltage VIN according to an embodiment of the invention. The converter 200 includes a switch Q1 having a first terminal 212 and a second terminal 214, wherein the second terminal 214 is coupled with the output voltage VOUT. The first terminal 212 of the switch Q1 is electrically coupled with the input voltage VIN by way of a first inductor L1. The switch Q1 is controlled by way of a switch controller 210 coupled with the output voltage VOUT. In addition, a second inductor L2 magnetically coupled with the first inductor L1 electrically couples an anode 218 of a rectifier or diode D1 with a voltage reference, while a cathode 220 of the diode D1 is electrically coupled with the output voltage VOUT.
More specifically regarding the particular example of the converter 200 of FIG. 3, the switch Q1 may be a FET, such as an n-channel power FET, having a drain terminal 212, a source terminal 214, and a gate terminal 216. As is described in greater detail below, the switch controller 210 controls the FET Q1 by turning the FET Q1 ON and OFF by way of the gate 216. In one embodiment, the switch controller 10 turns the FET Q1 ON and OFF substantially periodically, based at least in part on the voltage level of the output voltage VOUT. In other embodiments, the switch controller 210 may employ another characteristic of the output, such as current, to control the FET Q1. In another example, a combination of output characteristics, such as voltage and current, may be employed to control Q1. In alternative embodiments, other types of electronic components, such as bipolar junction transistors (BJTs), may be employed in lieu of the FET Q1 to similar end.
In the particular embodiment of FIG. 3, a first capacitor C1 electrically couples the input voltage VIN with the voltage reference, while the output voltage VOUT is electrically coupled to the reference by way of a second capacitor C2. In one embodiment, the voltage reference is ground, or 0 V. The first capacitor C1 and the second capacitor C2 are employed as filter capacitors to help supply short-term electrical current needs in order to support the voltage level of both the input voltage VIN and the output voltage VOUT, as well as to remove high-frequency noise. In one particular implementation, the first capacitor C1, has a capacitance of 22 microfarads (μF), while the second capacitor C2 has a capacitance of 120 μF.
In one embodiment, the first inductor L1 and the second inductor L2 form first and second windings of a transformer sharing a single core 222, such as a ferrite core, about which both of the inductors L1, L2 are wound. Cores comprised of other materials may be implemented in other embodiments of the invention. Also, in one implementation, the number of turns of the first inductor L1 and the number of turns of the second inductor L2 about the core form a 1:1 ratio. Other ratios may be possible in alternative embodiments, although a 1:1 ratio is assumed in the following discussion of the converter 200 operation. In one example, the inductors L1, L2 each have an inductance of 1.7 millihenries (mH).
The operation of the converter 200 is dependent upon the state of the switch or FET Q1. The switch controller 210 turns ON the FET Q1 by raising the voltage of the gate 216 sufficiently above the voltage at the source 214, which is the output voltage VOUT, to turn ON the switch Q1. When the FET Q1 is ON, the voltage VD at the drain 212 of the FET Q1 is also approximately equal to the output voltage VOUT, and electrical current flows from the input voltage VIN, through the first inductor L1, the drain 212 and the source 214 of the FET Q1, to the output voltage VOUT. As a result, electrical energy is stored in the first inductor L1, typically within the core 222 about which the first inductor L1 is wound. Also, due to the 1:1 magnetic coupling of the first inductor L1 and the second inductor L2, the voltage VL1 across the first inductor L1 equals the voltage VL2 across the second inductor L2. Thus, since the voltage across the first inductor L1 is essentially the input voltage VIN minus the output voltage VOUT, the voltage VA at the anode 218 of the diode D1 becomes −(VIN−VOUT). Therefore, with the cathode 220 coupled with the output voltage VOUT, the anode 218 voltage VA is less than that at the cathode 220, causing the diode D1 to be reverse-biased, and hence nonconducting. Therefore, essentially no current flows through the second inductor L2 while the switch Q1 is ON, and electrical energy is stored in the core 222 while current flows through the first inductor L1.
When the switch controller 210 turns the switch Q1 OFF, the voltage VL1 across the first inductor L1 becomes negative in an attempt to maintain its former electrical current level, thus driving the drain 212 of the switch Q1 above the input voltage VIN. Due to the magnetic coupling between the two inductors L1, L2, the voltage V1 across the second inductor L2 matches the voltage VL1 across the first inductor L1. As a result, the voltage VL2 across the second inductor L2 approaches the negative of the output voltage −VOUT, at which point the diode D1 becomes forward-biased and conducting. Ignoring a typically small voltage drop across the diode D1, the voltage VL1 across the first inductor L1 is thus also limited to −VOUT, thus clamping the voltage VD at the drain 212 of the switch Q1 at the sum of the input voltage VIN and the output voltage VOUT (i.e., VIN+VOUT). As a result of this clamping, energy previously stored in the core 222 of the inductors L1, L2 is delivered in the form of current through the second inductor L2 and the diode D1 to the output voltage VOUT. After a period of time, the switch controller 210 once again turns ON the switch Q1, and the process is repeated. Whether the switch Q1 is ON or OFF, current flows from the converter 200 toward the output voltage VOUT.
FIG. 4 illustrates, by way of a simplified timing diagram, waveforms of an electrical current IL1 through the first inductor L1 and a current IL2 through the second inductor L2 according to a particular embodiment of the invention. Also, the voltage VD at the drain 212 of the switch Q1 and the voltage VA at the anode 218 of the diode D1 during the same time period are also illustrated. In this example, the input voltage VIN is 50 VDC, the output voltage VOUT is 12 VDC, and the load (not shown) driven by the output voltage VOUT is 40 ohms (Ω). Also in this specific example, the first capacitor C1 has a capacitance of 22 μF, the second capacitor C2 is 120 μF, and the inductors L1, L2 each exhibit an inductance of 1.7 mH. In addition, the switch Q1 is an n-channel power FET, part number STD5NM50, and the diode D1 is a MURS160.
The timing diagram of FIG. 4 depicts the typically periodic nature in which the converter 200 operates. For a time tON during which the switch Q1 is ON, the current IL1 through the first inductor L1 increases essentially linearly from a level I1 to a higher level I2. During that same time period, the diode D1 is reverse-biased, and thus essentially no current flows through the second inductor L2. Also, the voltage VD at the drain 212 of the switch Q1 remains at about VOUT due to the switch Q1 being ON, and the voltage VA at the anode 218 of the diode D1 is −(VIN−VOUT), as described earlier. In other words, the voltages VL1, VL2 across the inductors L1, L2 are equal at (VIN−VOUT), with the drain voltage VD offset higher than the anode voltage VA by the value of the input voltage VIN.
This equality of the voltages VL1, VL2 across the inductors L1, L2, and the relative offset of VIN between the drain voltage VD and the anode voltage VA, hold true when the switch Q1 is OFF. During the switch Q1 OFF time tOFF, with the diode D1 conducting, the anode voltage VA is clamped at the output voltage VOUT. With the drain voltage VD offset higher than the anode voltage VA by the input voltage VIN, the drain voltage VD is clamped at (VIN+VOUT), as described above. Also, since the switch Q1 is OFF, the current in the first inductor L1 is essentially zero, while the current in the second inductor L2 decreases substantially linearly from I2 to I1 due to constant voltage −VOUT across the second inductor L2.
In the particular embodiment illustrated in FIG. 4, the upper current level I2 is approximately 350 milliamps (mA), while the lower current level I1 is about 250 mA. Since the current IL1 through the inductor L1 and the current IL2 through the inductor L2 are provided at the output voltage VOUT, the average current delivered to the 40-Ω load is (12 VDC)/(40Ω)=300 mA. The switch controller 210 adjusts tON and tOFF based on the component values of the inductors L1, L2, the tolerance of the load being driven to variations in the output voltage VOUT, and other factors. In the case of FIG. 4, tON is about 5 microseconds (μS), while tOFF is about 15 μS. Since the voltage across an inductor L is equal to L(di/dt), a constant voltage VL1 across the first inductor L1 during tON of VIN−VOUT=(50 VDC)−(12 VDC)=38 VDC is approximately three times the magnitude of the voltage VL2 across the second inductor L2 during tOFF of −VOUT=−12 V, thus accounting for tOFF being approximately three times as long as tON in this particular case. Of course, with a different combination of input voltage VIN and output voltage VOUT, a different ratio of tON to tOFF would likely be implemented by the switch controller 210.
Given the various embodiments of the converter 200 disclosed herein, a wide range of positive DC voltages may be employed as the input voltage VIN in order to produce a lower-magnitude positive DC output voltage VOUT. As mentioned above, the various components employed, such as the inductors L1, L2, the capacitors C1, C2, the diode D1, the switch Q1, and the switch controller 210, determine in part the limits of the input and output voltages VIN, VOUT allowed for a particular embodiment of the invention.
A distinct advantage of various embodiments of the converter 200 described above is the limited voltage swing of the gate 216 of the switch Q1 required to turn ON and OFF the switch Q1. Since the source 214 of the switch Q1 is coupled directly to the output voltage VOUT, the voltage of the gate 216 is required to move only between the output voltage VOUT and a few volts higher to operate the switch Q1. Thus, the gate 216 of Q1 may be driven by standard, readily-available electronic components, thereby simplifying the design of the switch controller 210. One or more of these advantages, or others, may also be realized in other applications employing one or more embodiments of the present invention.
Similar advantages may also be realized by another voltage converter 300 according to another embodiment of the invention. Shown in FIG. 5, the converter 300, which operates in a manner analogous to that of the converter 200 described above, is configured to convert a negative DC input voltage VIN into a lower-magnitude negative DC output voltage VOUT. While most of the components of the converter 200 and the converter 300, such as the inductors L1, L2, the core 222, and the capacitors C1, C2, are the same, a few modifications are utilized to process the negative DC input voltage VIN. In place of the switch Q1 of the converter 200 is a switch Q2, which is a p-channel power FET in the particular example of FIG. 5. The switch Q2 includes a drain terminal 312 coupled with the first inductor L1, a source terminal 314 coupled with the output voltage VOUT, and a gate terminal 316. A switch controller 310, operating in a similar fashion to that of the switch controller 210 of the converter 200, controls the operation of the switch Q2 via the gate 316. The switch controller 310 need only move the voltage of the gate 316 between the output voltage VOUT and a few volts lower to operate the switch Q2, thus simplifying the design of the switch controller 310 compared to some prior art converters.
The converter 300 of FIG. 5 also includes a diode D2 having a first terminal 318 and a second terminal 320. Due to the negative polarity of the input and output voltages VIN, VOUT, the first terminal 318 is the cathode, while the second terminal 320 is the anode, opposite the orientation of the diode D1 of the converter 200. Operation of the converter 300 is analogous to that described above in conjunction with the converter 200 of FIG. 3, with the polarity of all voltages and currents essentially inverted.
A further embodiment of a voltage converter 400 according to an embodiment of the invention for converting an AC input voltage VIN to a DC output voltage VOUT is presented in FIG. 6. In addition to the components described above in conjunction with the converter 100 of FIG. 2, an AC rectification circuit 430 for converting the AC input voltage VIN to a DC voltage usable by the remainder of the voltage converter 400 is utilized to generate the desired DC output voltage VOUT. In one embodiment in which a positive DC output voltage VOUT is required, the AC rectification circuit 430 may be configured to convert the AC input voltage VIN to a first positive DC voltage, which may then be converted to a lower-magnitude DC output voltage VOUT by way of the converter 200 of FIG. 3. In another embodiment, if a negative DC output voltage VOUT is needed, the AC rectification circuit 430 may be configured to convert the AC input voltage VIN to a first negative DC voltage, which in one implementation is subsequently converted to a lower-magnitude negative DC output voltage VOUT via the converter 300 of FIG. 5.
While several embodiments of the invention have been discussed herein, other embodiments encompassed within the scope of the invention are possible. For example, different AC and DC voltage levels may be involved in alternative embodiments, thus possibly indicating the use of component values other than those specifically disclosed herein. Further, references to positive and negative voltage polarities are provided for reference only, and other embodiments of the invention may utilize a different voltage referencing scheme. In addition, components that are electrically coupled may not necessarily be directly interconnected in alternative embodiments. Also, aspects of one embodiment may be combined with those of alternative embodiments to create further implementations of the present invention. Thus, while the present invention has been described in the context of specific embodiments, such descriptions are provided for illustration and not limitation. Accordingly, the proper scope of the present invention is delimited only by the following claims.

## Claims

1. A step-down voltage converter (100) for generating a direct-current (DC) output voltage (VOUT) from a direct-current (DC) input voltage (VIN), comprising:
a field-effect transistor (FET) (Q1) switch (111) comprising a drain terminal (112) and a source terminal (114), wherein the source terminal (114) of the switch (111) is electrically coupled with the output voltage (VOUT);
a diode (117) comprising an anode (118) and a cathode (120), wherein the cathode (120) of the diode (117) is electrically coupled with the output voltage (VOUT);
a first inductor (124) electrically coupling the drain terminal (112) of the switch (111) with the input voltage (VIN);
a second inductor (126) magnetically coupled with the first inductor (124), the second inductor (126) electrically coupling the anode (118) of the diode (117) with a voltage reference (128); and
a switching controller (110) coupled with the output voltage (VOUT) and configured to control the switch (111) by way of a gate terminal (216) of the FET (Q1).

2. The step-down voltage converter (100) of claim 1, wherein the first inductor (124) and the second inductor (126) each comprise an inductance of 1.7 millihenries.

3. The step-down voltage converter (100) of claim 1, further comprising:
a first capacitor (C1) electrically coupling the input voltage (VIN) with the voltage reference (128); and
a second capacitor (C2) electrically coupling the output voltage with the voltage reference (128).

4. The step-down voltage converter (100) of claim 3, wherein the first capacitor (C1) comprises a capacitance of 22 microfarads.

5. The step-down voltage converter (100) of claim 3, wherein the second capacitor (C2) comprises a capacitance of 120 microfarads.

6. The step-down voltage converter (100) of claim 1, wherein the voltage reference (128) is ground.

7. The step-down voltage converter (100) of claim 1, wherein a number of turns of the first inductor (124) and a number of turns of the second inductor (126) comprise a ratio of 1:1.

8. The step-down voltage converter (100) of claim 1, wherein the first inductor (124) comprises a first winding of a transformer, wherein the second inductor (126) comprises a second winding of the transformer, and wherein the first inductor (124) and the second inductor (126) are wound about a core (222).

9. The step-down voltage converter (100) of claim 8, wherein the core (222) is a ferrite core.

10. The step-down voltage converter (100) of claim 1, wherein:
the input voltage (VIN) and the output voltage (VOUT) are negative DC voltages; and
the switch (111) comprises a p-channel field-effect transistor (FET)(Q2).

11. The step-down voltage converter (100) of claim 1, wherein the switch controller (110) is configured to control the switch (111) by turning the switch (111) on and off substantially periodically.

12. The step-down voltage converter (100) of claim 1, wherein the switch controller (110) is configured to control the switch (111) based on the output voltage (VOUT)

13. An item of industrial instrumentation comprising the step-down voltage converter (100) of claim 1.

14. A Coriolis flowmeter comprising the step-down voltage converter (100) of claim 1.