Current sensing circuit and corresponding method

Abstract

A current sensing circuit for sensing an intermittent current having a Zero Current Period includes: an amperometric transformer having a primary winding for the current to be sensed to flow therethrough and a secondary winding, a sensing resistor coupled to the secondary winding of the transformer, an offset capacitor coupled with sensing resistor between the sensing resistor and ground, and a switch element acting across the coupling of the sensing resistor and the offset capacitor, the switch element being electrically conductive during the zero current period or a fraction thereof.

Claims

1. A current sensing circuit for sensing an intermittent current having a zero current period, the current sensing circuit comprising: an amperometric transformer having a primary winding for intermittent the current to flow therethrough and a secondary winding, a sensing resistor connected across terminals of the secondary winding of the amperometric transformer, an offset capacitor coupled with the sensing resistor at one of the terminals of the secondary winding of the amperometric transformer, the offset capacitor connectable to ground with the offset capacitor set between the sensing resistor and ground, and a switch element acting across an RC network formed by the sensing resistor and the offset capacitor, the switch element electrically conductive during the zero current period or a fraction thereof.

2. The current sensing circuit of claim 1, wherein the switch element comprises an electronic switch driveable to become electrically conductive during the zero current period or the fraction thereof.

3. The current sensing circuit of claim 2, wherein the electronic switch further comprises a MOSFET or a JFET.

4. The current sensing circuit of claim 1, wherein the switch element comprises a diode.

5. The current sensing circuit of claim 4, wherein the diode has its anode coupleable to ground.

6. A method of sensing an intermittent current having a zero current period, the method comprising: feeding the intermittent current to a primary winding of an amperometric transformer, connecting a sensing resistor across terminals of a secondary winding of the amperometric transformer, coupling with the sensing resistor at one of the terminals of the secondary winding of the amperometric transformer an offset capacitor with the offset capacitor set between the sensing resistor and ground, and providing a switch element across an RC network formed by the sensing resistor and the offset capacitor, the switch element electrically conductive during the zero current period or a fraction thereof.

7. The method of claim 6, further comprising: providing an electronic switch as the switch element, and driving the electronic switch by rendering it electrically conductive during the zero current period or the fraction thereof.

8. The method of claim 6, further comprising providing a diode as the switch element.

9. The method of claim 8, further comprising coupling the anode of the diode to ground.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various aspects are described with reference to the following drawings, in which:

(2) FIG. 1 is a circuit diagram exemplifying embodiments, and

(3) FIG. 2, which comprises three overlying portions respectively denoted as a), b), c) and d), exemplifies possible signal graphs of one or more embodiment.

DETAILED DESCRIPTION

(4) In the following description, numerous specific details are given in order to provide a thorough understanding of various exemplary embodiments. The embodiments may be practiced without one or several of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring the various aspects of the embodiments.

(5) Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the possible appearances of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring exactly to the same embodiment. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

(6) The word exemplary or exemplify is used herein to mean serving as an example, instance, or illustration. Any embodiment or design described herein as exemplary is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

(7) The references provided herein are given for convenience only, and therefore do not interpret the extent of protection or the scope of the embodiments.

(8) In the diagram of FIG. 1, reference 10 denotes an (amperometric) transformer adapted to receive a current I.sub.HF to be sensed on its primary winding 10a.

(9) This current may be e.g. a high-frequency discontinuous current, which must be measured with good accuracy. Such a need may be felt e.g. in switch mode power supplies.

(10) In one or more embodiments, current I.sub.HF may be fed to primary winding 10a of amperometric transformer 10 by flowing between one of the terminals of primary winding 10a and the opposed terminal of such a winding, which may be considered as connected to ground.

(11) In one or more embodiments, transformer 10 may have a secondary winding 10b coupled to a sensing resistor 12.

(12) In one or more embodiments, resistor 12 may simply be connected across the secondary winding 10b.

(13) Reference 14 denotes a capacitor which, in one or more embodiments, may include a first terminal connected to resistor 12 at one of the ends of the secondary winding 10b of transformer 10, and a second terminal connectable to ground, so that capacitor 14 is adapted to be interposed between resistor 12 and its ground, i.e. between one of the terminals of secondary winding 10b of transformer 10 and ground.

(14) In one or more embodiments, a component 16 having the function of a switch may be connected across the RC network comprising resistor 12 and capacitor 14 (i.e. between the other end of secondary winding 10b of transformer 10 and the second terminal of capacitor 14, intended to be connected to ground).

(15) In one or more embodiments, switch 16 may be an active switch, such as, for example, an electronic switch, e.g. a MOSFET or a JFET.

(16) One or more embodiments, wherein the component acting as a switch 16 is an active switch, may include a control logic CL (which in itself may be absent in some embodiments).

(17) In one or more embodiments, logic CL may be implemented as a function of the control logic of a switch mode power supply (not visible in the Figures).

(18) In one or more embodiments, logic CL may also be connected to a line S, adapted to sense the voltage across capacitor 14, such a voltage being adapted to represent the (average) measured value of current I.sub.HF.

(19) According to the operation principle of an arrangement as exemplified in FIG. 1, the sensing function of current I.sub.HF is performed by resistor 12, which acts as a sensing element (directly) coupled to secondary winding 10b of the amperometric transformer.

(20) Capacitor 14 performs an offset blocking function between sensor 12 and reference ground, and therefore keeps the DC offset of the measured current, therefore making it possible to sense, e.g. at the terminal of resistor 12 opposed to offset capacitor 14, the instantaneous value of the current to be sensed, including the DC component, with the possibility of obtaining an average over the whole switching period, due to the magnetizing inductance of transformer 10, through which a direct current flows which equals the average of the measured current.

(21) Switch 16 (e.g. an active switch) may be turned on (i.e. rendered conductive) for a given period of time, which may be short, e.g. a fraction of the period during which current I.sub.HF (which is discontinuous, i.e. intermittent) is at zero, during the so-called Zero Current Period (ZCP). It is therefore possible to store the voltage offset, related to the zero current level of the measured current, into capacitor 16, which performs a sort of Sample & Hold (S&H) function.

(22) One or more embodiments, therefore, lead to an improvement of the usage conditions of the magnetic core of transformer 10, by reducing the flux and decreasing the number of turns, i.e. the size and the cost, of amperometric transformer 10.

(23) It will be appreciated that, in one or more embodiments, component 16 is not directly traversed by the measured current. This enables the use of a higher on-resistance, with a corresponding cost reduction.

(24) One or more embodiments may omit damping elements and are free of discontinuities in the core flux.

(25) Across capacitor 14 (e.g. via a sensing line S) a signal may be sensed which represents the (average) value of current I.sub.HF, no additional filters being required.

(26) In the diagrams of FIG. 2: the upper portion, denoted as a), exemplifies a possible behaviour of the current I.sub.HF to be sensed/measured, the Zero Current Period (ZCP) being denoted with a corresponding reference; the portion denoted as b) exemplifies a possible behaviour of voltage V.sub.12, which may be sensed on resistor R12 (average value=0); the portion denoted as c) exemplifies a possible behaviour of voltage V.sub.14 across offset capacitor 14 (reversed line), representative of the average value of the measured current, and the portion denoted as d) represents a possible command signal S16 of the gate of switch 16.

(27) In one or more embodiments, when switch 16 is an active switch, the respective driving function may be such that, when switch 16 is active, the zero level across sensing resistor 12 is sampled on offset capacitor 14.

(28) The command on the driving electrode (e.g. gate) of switch 16 may be any signal synchronized with current I.sub.HF. The driving signal may be derived from logic CL, e.g. on the basis of the command signal for driving the switching of the power electronic switches of the corresponding power supply. As a non-limiting example, the active signal may be controlled by the gate command signal on the low side of a resonant converter.

(29) In one or more embodiments, switch 16 may be activated (e.g. rendered conductive) during the Zero Current Period (ZCP), e.g. for a fraction thereof, for the period of time during which the current value across current sensing resistor 12 is stored into the offset capacitor 14.

(30) One or more embodiments, therefore, may envisage the presence of a switch element (e.g. switch 16) acting across the coupling of sensing resistor 12 and offset capacitor 14, the switch element 16 being electrically conductive during at least a fraction of the zero current period, i.e. during the ZCP or a fraction thereof.

(31) In one or more embodiments, the function of component 16 may be performed not by an active switch as previously exemplified, but by a diode, e.g. a diode having the same connection polarity of body diode 16a of switch 16 exemplified as a MOSFET in the diagram of FIG. 1, i.e. with the cathode and the anode respectively facing towards capacitor 14 and resistor 12, so that such diode may act as a switch which becomes electrically conductive automatically, when the (direct) voltage thereacross exceeds the threshold voltage of the diode. In that case, one or more embodiments may take into account a possible measurement offset or error e.g. amounting to 0.5 V and depending on the temperature. The obtained advantages include simplicity and cost reduction, due to the use of a diode instead of an active switch.

(32) Of course, without prejudice to the basic principle, the implementation details and the embodiments may vary, even appreciably, with respect to what has been described herein by way of non-limiting example only, without departing from the extent of protection.

(33) While specific aspects have been described, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the aspects of this disclosure as defined by the appended claims. The scope is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.