Abstract:
A transformer driver circuit couples to a transformer having a primary winding, a secondary winding, and a transformer tap that is connected to a first voltage source. The primary winding electrically connects at its ends to respective unipolar controllable current sinks that form part of an integrated circuit. The transformer driver circuit operates by each current sink selectively sinking current from the end of the primary winding to which it is connected so as to cause current to flow in the secondary winding in a push-pull fashion. The transformer driver circuit further includes a load electrically connected to the secondary winding and protection circuitry operative to protect the integrated circuit from input levels greater than it can withstand.

Description:
CROSS REFERENCE TO RELATED PATENTS/PATENT APPLICATIONS 
       [0001]    The present U.S. Utility Patent Application claims priority pursuant to 35 U.S.C. §120 as a continuation of U.S. Utility application Ser. No. 13/515,464, entitled “Transformer Driver Circuit with IC Protection Circuitry,” filed Jun. 26, 2012, which is the national stage entry under 35 USC 371(c) of PCT/GB2010/052132, filed Dec. 17, 2010, entitled “Current Measuring Apparatus,” which claims priority to GB0922381.9 filed Dec. 22, 2009, entitled “Current Measuring Apparatus,” all of which are hereby incorporated herein by reference in their entirety and made part of the present U.S. Utility Patent Application for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present disclosure relates to a transformer driver circuit and in particular but not exclusively networking apparatus comprising such a transformer driver circuit. 
       BACKGROUND TO THE INVENTION 
       [0003]    A known transformer driver, in this case a Class-D amplifier,  10  is shown in  FIG. 1A . The Class-D amplifier makes use of a P/N totem pole H-bridge configuration of transistors  12 ,  14 ,  16 , and  18  to drive two load terminals  20 ,  22  to alternate supply voltages in an active fashion. In  FIG. 1A  the alternate supply voltages are ground and VDD. The voltage between the load terminals  20 ,  22  is applied to an inductor  24  of an LC filter by way of a transformer  26  to induce current flow in the inductor in accordance with I=(1/L)*integral(Vout−Vfilt_out), where L is the inductance of the inductor, Vout is the voltage across the secondary winding of the transformer and Vfilt_out is the voltage across the capacitor  28  of the LC filter. A resistive load  30  is present at the output of the LC filter. The induced current flow is drawn through the transformer from whichever supply, i.e. ground or VDD, is presently driving the output. Thus energy is transferred from the supply to the inductor  24  as is shown in  FIG. 1B  or from the inductor back to the supply as is shown in  FIG. 1C  depending on the direction of the load current. The components in  FIGS. 1B and 1C  have the same reference numerals as the corresponding components in  FIG. 1A . Assuming the transistors  12 ,  14 ,  16 , and  18  to be performing as ideal switches and an ideal inductor, capacitor and resistor, the only energy loss is that which results from the ripple voltage present across the load resistor  30 . 
         [0004]    Where an increase in injected power is required, the load resistance  30  is reduced. However, at some point during the course of reducing the load resistance the parasitic resistances in the circuit become significant compared to the load resistance and efficiency decreases rapidly. The parasitic resistances are present as transistor on-state resistance and parasitic series resistance in the LC filter and the transformer. If the load resistance is further reduced it becomes impossible to achieve the required injected power because the parasitic resistances are too large. 
         [0005]    It is an object of the disclosure to address one or more of the above issues and provide a transformer driver of acceptable efficiency and able to provide for increased injection of power between a primary winding and a secondary winding of the transformer to handle increased output power. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0006]    Further features and advantages of the present disclosure will become apparent from the following specific description, which is given by way of example only and with reference to the accompanying drawings, in which: 
           [0007]      FIG. 1A  is a circuit diagram of a known Class-D amplifier; 
           [0008]      FIG. 1B  represents the circuit of  FIG. 1A  when energy flows from the supply to the inductor; 
           [0009]      FIG. 1C  represents the circuit of  FIG. 1A  when energy flows from the inductor to the supply; 
           [0010]      FIG. 2A  is a first embodiment of the present disclosure; 
           [0011]      FIG. 2B  is an alternative form of protection circuit for the first embodiment; 
           [0012]      FIG. 3  is a second embodiment of the present disclosure; 
           [0013]      FIG. 4  is a third embodiment of the present disclosure; 
           [0014]      FIG. 5  is an embodiment of the present disclosure configured to determine an impedance at the output from the secondary winding; and 
           [0015]      FIG. 6  is a representation of a network of consumer products in a building. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A bipolar output stage can generate more power than a unipolar output stage for the same supply voltage. However, a unipolar output stage can be adapted to higher output voltages more easily than a bipolar output stage. Therefore, to achieve high output power, it is proposed to combine these ideas by using two unipolar sources to provide a bi-polar output signal. The unipolar sources are unipolar current sources, and may be made using a variety of implementations, including approaches such as Class-D outputs, or multi-level current-drive circuits such as transconductor amplifiers or digital-to-analog converters. 
         [0017]      FIG. 2A  shows a first embodiment of the present disclosure, using a switched class (Class D) transformer driver circuit  60 . The transformer driver circuit  60  comprises a transformer  62  comprising a primary winding  64  and a secondary winding  66 . The primary winding  64  comprises a centre tap  68  (which constitutes a transformer tap) and first  70  and second  72  ends. The first end  70  of the primary winding is electrically connected to a first current source  74  by way of a first series connected protection circuit  76 . The second end  72  of the primary winding is electrically connected to a second current source  78  by way of a second series connected protection circuit  80 . The center tap  68  is connected to a high voltage supply, which in a first form is an integrated circuit high voltage supply and in a second form is of a higher voltage level than the integrated circuit high voltage supply. Each of the first and second current sources  74 ,  78  is constituted as a MOS transistor, which is driven by an output signal from a modulator circuit  82 . Each of the first and second protection circuits  76 ,  80  is constituted as a MOS transistor with its gate voltage maintained at a bias voltage determined such that a voltage across the associated current source does not exceed a safe operative limit. A first feedback path  84  conveys a first feedback signal from the first end  70  of the primary winding to the modulator circuit  82  by way of a first operational amplifier attenuator  86 . An input resistance and a feedback resistance of the first operational amplifier attenuator  86  are selected to reduce the voltage swing of the first feedback signal to an acceptable level for the modulator circuit. A second feedback path  88  conveys a second feedback signal from the second end  72  of the primary winding to the modulator circuit  82  by way of a second operational amplifier attenuator  90 . An input resistance and a feedback resistance of the second operational amplifier attenuator  90  are selected to reduce the voltage swing of the second feedback signal to an acceptable level for the modulator circuit. An LC filter  92  is provided at an output of the secondary winding  66  with a load  94  (e.g. the communications medium  140  described below with reference to  FIG. 6 ) being present at the output of the LC filter. The first and second current sources  74 ,  78 , the modulator circuit  82  and the first and second operational amplifier attenuators  86 ,  90  with the exception of the input resistors form part of a low voltage CMOS integrated circuit. The remaining components of the circuit of  FIG. 2A  are external to the integrated circuit. 
         [0018]    In use, the modulator circuit  82  alternately operates the first  74  and second  78  current sources by means of digital control signals. When the first current source  74  is operated, the first end  70  of the primary winding is electrically connected to the low voltage supply. Hence, a current flows from the centre tap  68  to the first end  70  of the primary winding in a first direction, which is indicated by the single arrow in  FIG. 2A . When the second current source  78  is operated, the second end  72  of the primary winding is electrically connected to the low voltage supply. Hence, a current flows from the centre tap  68  to the second end  72  of the primary winding in a second direction opposite the first direction; the second direction is indicated by the double arrow in  FIG. 2A . It should be appreciated that energy can be transferred to and fro between the primary winding  64  and the secondary winding  66 , and therefore between the load and power supply, the actual direction of transfer at any one time depending on factors such as the voltage/current flows at that particular time and the load characteristics (an inductive load is required for energy to flow back to the supply). 
         [0019]    With a centre-tap voltage higher than the on-chip voltage, the circuit of  FIG. 2A  is capable of injecting a higher level of power than the circuit of  FIG. 1A  before encountering a roll-off in efficiency. The feedback circuitry provides for a reduction in the output signals from the modulator circuit  82  to thereby compensate for signals in the transformer-current source circuitry that are liable to cause saturation and to thereby cause distortion. Such saturation causing signals are liable to arise where the load is unknown or the load varies where the circuit of  FIG. 2A  is used in certain applications, such as in multi-media networking in residential or commercial premises as is described below with reference to  FIG. 6 . 
         [0020]      FIG. 2B  shows an alternative form of protection circuit  100 , which is used when the protection circuits form part of the integrated circuit. Components in common with the circuit of  FIG. 2A  are indicated by like reference numerals. The protection circuit  100  of  FIG. 2B  comprises the first and second MOS transistors  76 ,  80  and an adaptive biasing circuit  102 . The adaptive biasing circuit is operative to vary the gate voltage of each of the first and second MOS transistors  76 ,  80  in dependence on the voltages at the sources and drains of the first and second MOS transistors  76 ,  80 . Thus, the adaptive biasing circuit is operative to keep the voltage across each of the current sources  74 ,  78  and the first and second MOS transistors  76 ,  80  within tolerable limits. The design of an adaptive biasing circuit  102  will be within the ordinary design skills of the person skilled in the art. 
         [0021]    A second embodiment of the present disclosure is shown in  FIG. 3 , namely a Class AB transformer driver circuit  110 . Components in common with the circuit of  FIG. 2A  are indicated by like reference numerals and therefore the reader&#39;s attention is directed to the description for  FIG. 2A  with respect to such common components. The circuit of  FIG. 3  operates in the same push-pull fashion as the circuit of  FIG. 2A  with the exception that the first and second current sources  74 ,  78  are operated by analogue drive signals so that the circuit operates in a linear fashion. Instead of the modulator circuit of the circuit of  FIG. 2A , each of the two drive signals are received at respective non-inverting inputs of first  112  and second  114  operational amplifiers. The first  84  and second  88  feedback signals are received at respective inverting inputs of the first  112  and second  114  operational amplifiers. Each of the first  112  and second  114  operational amplifiers is configured as an attenuator with an input resistance and a feedback resistance selected to reduce the voltage swing of the feedback signal to an acceptable level for the integrated circuit. The outputs of the first  112  and second  114  operational amplifiers provide the drive signals for the first and second current sources  74 ,  78 . Each operational amplifier adjusts its output voltage so that the inverting and non-inverting inputs are substantially equal. Hence, each operational amplifier adjusts the current sunk by the current source until the output at the load matches the input to the circuit. 
         [0022]    A third embodiment of the present disclosure is shown in  FIG. 4 , namely a direct drive transformer driver circuit  120 . Components in common with the circuit of  FIG. 2A  are indicated by like reference numerals and therefore the reader&#39;s attention is directed to the description for  FIG. 2A  with respect to such common components. The circuit of  FIG. 3  comprises first  122  and second  124  current mode Digital to Analogue Converters (DACs) instead of the current sources shown in  FIGS. 2A and 4 . Each of the first  122  and second  124  current mode Digital to Analogue Converters provide for digitally controlled current sinking from its respective end of the primary winding  64 . Each DAC  122 ,  124  may comprise the structure shown on the left of the Figure. A digital code input to each DAC provides for variation in the level of current sunk by the DAC. Hence, the circuit of  FIG. 4  operates in the same push-pull fashion as the circuit of  FIG. 2A . As can be seen, the circuit of  FIG. 4  lacks the feedback circuitry of the circuits of  FIGS. 2A and 4 , although the circuit could be modified to provide for feedback. Such modification would be within the scope of the ordinary design skills of the person skilled in the art. 
         [0023]      FIG. 5  shows a non-switching transformer driver circuit  150 , i.e. the embodiment of  FIG. 3  or  4 , which is configured to determine the impedance at the output from the secondary winding. Components in common with the circuit of  FIG. 2A  are indicated by like reference numerals and therefore the reader&#39;s attention is directed to the description for  FIG. 2A  with respect to such common components. The circuit of  FIG. 5  comprises a gain control circuit  152  and an impedance monitoring circuit  154 . In use, the transformer driver circuit is operative to source a known current by means of each of the first and second current sources and the impedance monitoring circuit  152  is operative to determine a voltage applied to the load  94  and to estimate an impedance of the load in dependence on the determined voltage and the current sourced by the first and second current sources, which are known. The impedance monitoring circuit  152  is then operative to change a gain of the gain control circuit  152  to thereby change the current sourced by the first and second current sources  74 ,  78  so that a desired or required amount of power is injected by the transformer driver circuit  150 . Adjusting the current sourced by the first and second current sources  74 ,  78  in this fashion can prevent the transformer driver circuit from saturating and thereby causing distortion. 
         [0024]    In another embodiment, which has the same components and configuration as the circuit shown in  FIG. 5 , the impedance monitoring circuit  154  is operative to measure a voltage signal across the load over a period of time, such as during a data frame, during ordinary use of the transformer driver circuit. The impedance monitoring circuit  154  is then operative to estimate an impedance of the load in dependence on the measured voltage and the current at the output from the secondary winding, which is known. The impedance monitoring circuit  152  is then operative to change a gain of the gain control circuit  152  to thereby change the current sourced by the first and second current sources  74 ,  78  so that a desired or required amount of power is injected by the transformer driver circuit  150 . Adjusting the current sourced by the first and second current sources  74 ,  78  in this fashion can prevent the transformer driver circuit from saturating and thereby causing distortion. This approach is described in more detail in WO 2008/013857 (to the present applicant). 
         [0025]    In an un-illustrated embodiment the gain of the gain control circuit  152  of  FIG. 5  is adjusted in dependence on a quality measure made at the output from the secondary winding. More specifically, a signal at the output from the secondary winding is measured and analyzed by means of a Digital Signal Processor (DSP), which is operative to perform a Fast Fourier Transform (FFT) on the measured signal to determine a signal to noise ratio of the measured signal. If the determined signal to noise ratio is not at a desired or required level, the gain of the gain control circuit  152  is adjusted to change the current sourced by the first and second current sources  74 ,  78  to meet the desired or required level of signal to noise ratio. 
         [0026]      FIG. 6  shows a network  130  of consumer products in a building. The network comprises first  132 , second  134 , third  136  and fourth  138  nodes. Adjacent pairs of nodes are connected to each other by an already installed communications medium  140 , such as mains power wiring, which provides for communication between and amongst a plurality of rooms in the residential building. Thus, for example, each of the first to fourth nodes may be located in a different room of the residential building. Each of the first to third nodes comprises a different multi-media device (which constitutes a consumer product). Thus, for example, the first node  132  comprises a Home Gateway (HGW), the second node  134  comprises Personal Computer (PC), the third node  136  comprises audio-visual entertainment apparatus and the fourth node  138  comprises network attached storage. In the network  130  of  FIG. 6  the first node  132  is configured to operate as a communications controller, the second  134  and third  136  nodes are configured to operate as repeater nodes and the fourth node  138  is configured to operate as a standard network node. A communications controller controls the function of the network to which it belongs. Normally there is only one communications controller in a network. A standard node provides for communication of data from the branch of the communications medium leading to the node to the branch of communications medium leading from the node and for communication with the multi-media device connected to the node. A repeater node provides for communication of data from the branch of the communications medium leading to the node to the branch of communications medium leading from the node but provides for no communication with the multi-media device connected to the node, e.g. where the multi-media device is not being used. Referring to  FIG. 6 , the configuration of the nodes might be such that the HGW connected to the first node is streaming a film from an external source to the network and the NAS connected to the fourth node  138  might be saving the film. Network node apparatus is present at each of the first to fourth nodes  132 ,  134 ,  136 ,  138  of  FIG. 6 . 
         [0027]    The network node apparatus of  FIG. 6  comprises a home networking integrated circuit (a GGL541 from Gigle Networks Ltd of Capital House, 2 Festival Square, Edinburgh, EH3 9SU, UK) provided within an appropriate enclosure. Network node apparatus is operative to provide for communication with a consumer product by way of an Ethernet communications controller and with the other nodes in the network over at least one of mains power wiring, co-axial cable and phone line. Reference should be made to publicly available product data from the vendor of the GGL541; such product data provides sufficient information for the skilled person to implement the network shown in  FIG. 6  without resorting to any more than ordinary design skill. A transformer driver circuit according to the present disclosure forms part of the home networking integrated circuit. More specifically, the transformer driver circuit is operative to drive the communications channel to the other nodes within the network. As described above, the transformer driver circuit of the present disclosure affords for increased power injection into the load be it in the form of mains power wiring, co-axial cable or phone line depending on the medium used for communication between and amongst nodes. The network of  FIG. 6  may have communications channels of undetermined length and hence undetermined load resistance. Hence, it is advantageous to have the capability to regulate the transformer driver circuit to prevent its saturation as described above.