Abstract:
A switch mode power converter configured for operation with a plurality of outputs is disclosed. The switch mode power converter includes an inductive element and a resistance in series with the inductive element. The resistance is series with the inductive element is used for determining a current through the inductive element. The resistance is a resistance between the main terminals of a switch in an on-state. The switch have two main terminals and a control terminal and being arranged for directing current through the inductive element to a one of the plurality of outputs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority under 35 U.S.C. §119 of European patent application no. 13154564.2, filed on Feb. 8, 2013, the contents of which are incorporated by reference herein. 
     FIELD OF THE INVENTION 
     This invention relates to switched power converters and to methods of operating switched mode power converters. 
     BACKGROUND OF THE INVENTION 
     In many switch mode power converters, it is necessary or desirable to measure a current through an inductive element in the power converter. 
     In conventional switch mode power converters, measurement of this current is achieved by means of a sense resistor, located in series with the inductive element. The sense resistor is usually a separate component, and typically has a relatively low resistance of the order of 0.01 to 1 ohms, in order to minimise associated resistive losses. Nonetheless, and particularly for relatively low power outputs, these losses are undesirable. 
     One known solution to this problem is to position the sense resistor on the input side of the switch mode converter, in series with a power switch. Although positioning the resistance in series with the switch will cause lower losses, it may introduce other problems, particularly those associated with high-voltage operation. In order to determine the currents from the sense resistor, a current sense amplifier such as a trans-impedance amplifier is connected across the sense resistor: if the converter has a high input voltage, the current sense amplifier input needs to withstand that voltage, and also has two withstand any noise on the input supply line. A level shifter may be required in order to bring the output voltage of the amplifier down to ground level. Furthermore, any ringing on the switching node may introduce ringing at the current sensor, which may place a further high demand on the current sense amplifier. 
     A further, known, solution is to measure the voltage drop across the on-state drain-source resistance (Rdson) of the power switch, typically a high side switch in the case of a half bridge converter. However, such a solution is difficult to implement, and there is an ongoing need for an alternative solution. 
     SUMMARY OF THE INVENTION 
     According to a first aspect there is disclosed a switch mode power converter configured for operation with a plurality of outputs and comprising: an inductive element, and a resistance in series with the inductive element and being for determining a current through the inductive element, wherein the resistance is a resistance between the main terminals of a switch in an on-state, the switch having two main terminals and a control terminal and being arranged for directing current through the inductive element to a one of the plurality of outputs. 
     By sensing the voltage across the switch in its on-state, and with knowledge of the switch&#39;s “on-resistance” Rds-on, the current through the switch may be directly determined, and it may be possible to avoid the requirement for a separate sense resistor together with the ohmic loss associated therewith. 
     In embodiments, the switch is a relatively low power reference switch, either being configured for operation in parallel with and in synchronisation with or being a part of a relatively high power power switch. The effects of variation in Rds-on, caused either by temperature variation, or variation in other operating conditions, which are known to occur in high power power switches, may thus be avoided or reduced. 
     In embodiments, the relatively high power power switch is arranged for directing current through the inductive element to a one of the plurality of outputs. In embodiments, the switch mode power converter further comprises an amplifier connected across the main terminals of the switch, for determining a current through the switch from the voltage across the main terminals. 
     According to another aspect there is provided a solar inverter comprising a switch mode power converter as claimed in any preceding claim. Solar inverters are one application in which the invention may be used, although the skilled person will appreciate that the invention is not limited thereto. 
     According to a further aspect, there is provided a method of operating a switch mode power supply having an inductive element and a plurality of outputs, the method comprising directing current through the inductive element to a one of the plurality of outputs by means of a switch having two main terminals and a control terminal, and determining a current through the inductive element by measuring a resistance in series with the inductive element, wherein the resistance is a resistance between the main terminals of the switch in an on-state. 
     These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which 
         FIG. 1  illustrates a half bridge converter having input side current sensing by means of a series resistor in series with the high side switch; 
         FIG. 2  illustrates a half bridge converter having a sense resistor in series with the inductor; 
         FIG. 3  shows an example of a multi-output down converter according to embodiments; 
         FIG. 4  shows an example of a buck converter operable according to embodiments; 
         FIG. 5  shows timing diagram of the inductor current and switches of  FIG. 4 ; 
         FIG. 6  shows a dual output buck converter according to embodiments; and 
         FIG. 7  shows a dual output but converter according to other embodiments. 
     
    
    
     It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a conventional half bridge converter  10  having input side current sensing by means of a series resistor in series with the high side switch. The converter comprises a high side switch S 1 , and a low side switch S 2  connected in series with a half bridge node page be there between. The switches are connected across an input voltage Vin, with a sense resistor  12  between the high side switch and the input rail. A current sense amplifier, or trans-impedance amplifier,  13  is connected across the sense resistor  12  An input capacitance Cin is also connected across the input. High side switch S 1  and low side switch S 2  are controlled by means of drivers  14  and  16  respectively. On the output side is an inductor  18 , connected between the half bridge node HB and the output Vout. A output capacitor Cout is also connected across the output. 
       FIG. 2  illustrates a half bridge converter  20  having a sense resistor in series with the inductor. This converter is similar to that shown in  FIG. 1 , except that the current through the inductor  18  is sensed by means of a sense resistor  22  connected between the inductor  18  and the output rout, similarly to the sense resistor shown in  FIG. 1 , and a current sense amplifier  23  is connected across the sense resistor. 
       FIG. 3  shows, in schematic form, an example of a multi-output down converter  30  according to embodiments. Similarly to the converter shown in  FIGS. 1 and 2 , this converter is a half bridge converter having switches S 1  and S 2  with a half bridge node of therebetween. However, the current through the inductor  18  may be directed to one or other of two outputs, shown as a first output at 3.3 V, and a second output at 8V. The outputs are connected to ground by respective capacitors Cout 1  and Cout 2 . The inductor current is routed to the respective outputs by means of first output switch S 3  and second output switch S 4 . According to embodiments, a separate sense resistor is not included in the circuit; rather a current sense amplifier  33  is connected directly across the switch S 3 . Further, a second sense current amplifier  34  is connected across the second output switch S 4 . Advantageously, the losses associated with a separate sense resistor are avoided. Rather, when switch S 3  is closed, the current sense amplifier  33  measures the finite voltage drop across the switch S 3 , and provided that the switch resistance Rswitch is known, the current through the switch, and thus the current through the inductor  18  (when S 3  is closed), can be calculated. Similarly, when switch S 4  is closed the current sense amplifier  34  measures the finite voltage drop across the switch S 4 , and provided that the switch resistance Rswitch is known, the current through the switch, and thus the current through the inductor  18  (when S 4  is closed), can be calculated. 
     Generalising from the example shown in  FIG. 3 , if a buck converter has more than one output, it will have multiplexing switches, one per output on the output side. Normally only one switch will be closed at a time, directing the inductor current to the output corresponding to that switch. Whenever the switch is closed, the voltage across it will be a measure for the current through it. Thus there is no need for separate sense resistors, and this advantageously may increase efficiency and reduce cost and/or space on the circuit board. Furthermore, since the switches are on the output side of the circuit, they will generally be at a low and DC voltage level so, no specific requirements relating to high-voltage capability is placed on the sense amplifiers, which may thus be lower cost components. 
       FIG. 4  shows an example of a buck converter  40  operable according to embodiments and  FIG. 5  shows timing diagram of the inductor current and switches of  FIG. 4 . Similarly to the circuit shown in  FIG. 3 , the buck converter  40  has an input IN connected across two switches S 1  and S 2  with a half bridge node therebetween. An inductor  18  is connected to the half bridge node, and the output from the inductor is directed towards one or other of two outputs OUT 1  one and OUT 2  by means of switches S 3  and S 4  respectively. Also shown is a separate switch S 4 , which connects the output of the inductor  18  to ground and may be used to avoid ringing, and a bootstrap arrangement of a diode Db and capacitor Cb, connecting a supply voltage Vcc to the half-bridge node in order to generate a floating supply for the high-side switch S 1 , as will be familiar to the skilled person. 
     In  FIG. 5 , the timing diagram, for a buck converter  40  as shown in  FIG. 4  operated in discontinuous conduction mode (DCM) is shown. The traces show the inductor current  55 , the open/closed status of switches S 1  and S 2  at  51  and  52  respectively, and the open/closed statuses of switches S 3  and S 4  at  53  and  54 . When S 1  is closed, the inductor current will increase. S 1  turns off and S 2  turns on, so the inductor current decreases again. When it reaches zero, S 3  or S 4  can change state. After that, a new cycle can start. The inductor current is flowing through either S 3  or S 4 . S 4   a  is an optional switch that can be closed if both S 3  and S 4  are open, to prevent ringing at the right side of the inductor. If S 4   a  is closed, S 2  has to be closed as well. 
       FIG. 6  shows a dual output buck converter according to embodiments. In this embodiment switch S 3  is implemented as a series connection of diode D 3  and NMOS S′ 3 , and switch S 4  is implemented as a series connection of diode D 4  and NMOS S′ 4 . The current sense amplifiers measure the voltage across the switches S′ 3  and S′ 4 , rather than the respective switch-diode pair D 3 +S′ 3 , D 4 +S′ 4 . Optional switch S 4   a  may be a simple NMOS. In other embodiments, such as that shown in  FIG. 7 , diode D 3  and D 4  may be replaced by separates FETs S″ 3  and S″ 4 , either NMOS or PMOS, to further reduce the losses.  FIG. 7  is otherwise similar to  FIG. 6 . The skilled person would appreciate that the diode D 3  and D 4  (or switches S″ 3  and S″ 4 ) are generally required in multiple output buck converters, in order to avoid unintentional currents in the wrong direction. The skilled person further appreciated the output with the lowest voltage does not require such a diode. 
     The skilled person will be familiar that if switches are directly used as sense resistors, there may be a large influence of temperature and process spread on the measured voltage, which may impact the accuracy of the current sensing. However, the skilled person will equally appreciate that such influences might be mitigated by using a part of the switch, as a reference switch, or indeed providing a higher currents power switch in parallel and to operate agree with the switch S 3  or S 4  respectively. Such a reference switch integrated into a higher power switch is known, for instance from United States patent application publication number US2011/0181323. 
     Power converters employing such switches for current sensing may be used in a wide range of applications, including without limitation solar inverters and uses in automotive fields, as examples of higher voltage applications, and mobile applications such as cell-phones and smart-phones as examples of—typically—lower voltage applications. 
     From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of current sensing in power converters, and which may be used instead of, or in addition to, features already described herein. 
     Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
     Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 
     For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.