Abstract:
An embodiment of an apparatus, such as a circuit breaker, includes an input node, an output node, and a digital circuit. The input node is configured to receive an input voltage, and the output node is coupled to the input node and is configured to carry an output current. And the digital circuit is configured to uncouple the output node from the input node in response to a power drawn from the input node exceeding a threshold.

Description:
PRIORITY CLAIM 
       [0001]    The instant application claims priority to Italian Patent Application No. MI2013A001238, filed 24 Jul. 2013, which application is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to circuits for disconnecting a load, and more particularly to a circuit breaker, that implements a related method, adapted to control a power transistor for disconnecting a supplied load from a power line when the power absorbed by the load exceeds a maximum value. 
       SUMMARY 
       [0003]    Circuit breakers are widely used for protecting circuits against excessive power delivery, for example, due to accidental short-circuits of a supplied load. A basic circuit breaker is depicted in  FIG. 1 . It includes:
       a power transistor M 1 , that may be a MOSFET or a BJT, inserted in a power line from an input terminal IN to an output terminal OUT of the circuit breaker,   a current sensor, that may be realized as shown in the figure with a sense resistor Rsense, inserted such as to be crossed by the current delivered by the power transistor M 1 , and with a reader CURRENT SENSE of the voltage drop V 1  on the sense resistor,   a resistive voltage divider R 1 , R 2  coupled between the power line and a ground node to generate on its center tap a scaled replica V 2  of the supply voltage on the power line (i.e., of the voltage on the input terminal IN),   an analog multiplier X configured to generate an analog product voltage V 3 , representative of the power entering in the circuit breaker as the product of the analog voltage V 1 , proportional to the current IN entering in the circuit breaker through the input terminal IN, by the scaled replica voltage V 2 ,   an analog comparator with hysteresis COMP, configured to compare the analog product voltage V 3  with a threshold voltage VTHR representative of a pre-established maximum power level, generating a control signal for turning off the power transistor M 1  when the pre-established maximum power level is exceeded.       
 
         [0009]    Optionally, the threshold voltage VTHR may be generated by a current-to-voltage converter I_TO_V_CONVERTER that converts into voltage the current flowing throughout a reference resistor Rset coupled between the input terminal IN and a reference terminal RSET of the circuit breaker, as shown in the figure. 
         [0010]    The analog multiplier X is typically realized with the architecture of  FIG. 2 . Two functional blocks LOG(.) with a logarithmic characteristic are configured to receive in input the voltages V 1  and V 2  and to generate logarithmic replicas of the voltages V 1  and V 2 , respectively; an analog adder Σ generates the sum of the logarithmic replica voltages, and a functional block E(.) with an exponential characteristic generates the product voltage V 3 . The functional blocks LOG(.) and E(.) are typically based on diodes or BJTs and exploit voltage-current characteristics of P-N junctions, that is: 
         [0000]        i=I   S ·(exp(ν/ V   T )−1)
 
         [0000]    wherein i is the current throughout the P-N junction, I S  is the saturation current, v is the direct voltage across the P-N junction and V T  is the voltage-equivalent of temperature. 
         [0011]    The functional blocks are relatively inaccurate, because of mismatches between the blocks LOG(.) with a logarithmic characteristic and also because of intrinsic offsets of analog operational amplifiers included in the functional blocks, and have a typical accuracy of about 3%. Accuracy may be improved by adding trimming cells to the shown architecture and by performing a trimming step. 
         [0012]    But it has been found that it may be inconvenient to use the above architecture for realizing circuit breakers of enhanced accuracy, for example, of 1% or better, as requested, for example, in applications for notebooks. Indeed, for obtaining a good accuracy, a great number of trimming cells would be requested and they would occupy a relatively large silicon area. Moreover, the time required for testing the whole circuit breaker would increase rapidly with the number of trimming cells. 
         [0013]    Instead of following the actual trend of research, it has been found expedient to pursue a different strategy, a circuit breaker capable of sensing with great accuracy the power absorbed by a supplied load without requiring area consuming trimming cells. 
         [0014]    This outstanding result has been attained with a circuit breaker having a substantially digital architecture instead of an analog architecture. More precisely, an embodiment of the circuit breaker of this disclosure includes: 
         [0015]    an input terminal, 
         [0016]    an output terminal, 
         [0017]    a power transistor inserted in a power line of the circuit breaker between the input terminal and the output terminal, controlled in operation by a control signal, 
         [0018]    a current sensor and a voltage sensor coupled to the power line such to generate respective voltages first and second representing a delivered current and a supply voltage on the power line, respectively, 
         [0019]    an internal line on which a reference voltage representative of a maximum power is made available, 
         [0020]    an analog-to-digital converter configured to convert in parallel or in a serial fashion the voltages first and second and the reference voltage, 
         [0021]    respective memory devices functionally coupled to the analog-to-digital converter to store respective digital words representing said voltages first and second and the reference voltage, and 
         [0022]    a digital decision circuit configured to process the digital words stored in the memory devices and to generate the control signal for turning on/off the power transistor. 
         [0023]    A method of controlling a power transistor with a circuit breaker according to an embodiment is also disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  depicts a known circuit breaker composed of analog components. 
           [0025]      FIG. 2  depicts functional circuit blocks of a typical analog multiplier. 
           [0026]      FIG. 3  depicts a circuit breaker of this disclosure according to an embodiment. 
           [0027]      FIG. 4  is a simulation circuit of a circuit breaker according to this disclosure, used for simulations, according of an embodiment. 
           [0028]      FIGS. 5 and 6  are time graphs that illustrate the functioning of the simulation circuit of  FIG. 4 , according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    An exemplary block architecture of a circuit breaker of this disclosure is shown in  FIG. 3 . It includes a power transistor M 1  inserted in a power line of the circuit breaker between an input terminal IN and an output terminal OUT, a current sensor and a voltage sensor coupled to the power line such as to generate voltages V 1  and V 2  representing the current and the voltage relative to ground, respectively, provided in input to the circuit breaker. 
         [0030]    It is also shown an optional reference circuit for generating on an internal line a reference voltage VTHR, representative of a maximum power, by cooperating with a resistor Rset coupled between the input terminal IN and a reference terminal RSET of the circuit breaker. As an alternative, the reference voltage may be provided on the reference terminal or may be internally generated by a dedicated circuit embedded in the circuit breaker. 
         [0031]    Differently from the circuit breaker of  FIG. 1 , the architecture of  FIG. 3  includes an analog-to-digital converter ADC configured to convert in parallel or in a serial fashion the voltages V 1 , V 2  and VTHR; respective memory devices CURRENT MEMORY, VOLTAGE MEMORY, POWER LIMIT MEMORY, functionally coupled to the analog-to-digital converter ADC to store respective digital values; and a digital decision circuit, enclosed in dashed line, that processes the digital words stored in the memory devices and generates a control signal for the power transistor M 1  with a level adapted to turn it on/off depending on the values stored in the memory devices and on a desired control action to be implemented. 
         [0032]    In the shown exemplary embodiment, the digital decision circuit is composed of a digital multiplier X, configured to generate a product value representative of the input power as the product between the delivered current and the supply voltage on the power line, and a hysteresis comparator COMP for generating the control voltage of the power transistor M 1  depending on the comparison of the input power with its maximum level stored in the respective memory device POWER LIMIT MEMORY. 
         [0033]    The reference voltage VTHR may be generated by a band-gap circuit Vbg coupled to a bias resistor R 3  and to an external reference resistor Rset coupled to a reference terminal RSET of the circuit breaker. 
         [0034]    In the embodiment shown in  FIG. 3 , the voltages V 1 , V 2 , and VTHR are processed in a serial fashion using a multiplexer ANALOG MUX that provides in input to the analog-to-digital converter ADC the voltages to be converted, and with a demultiplexer DEMUX that stores the digital words representing the voltages V 1 , V 2 , and VTHR in the corresponding memory device CURRENT MEMORY, VOLTAGE MEMORY, and POWER LIMIT MEMORY, respectively. A counter COUNTER  0 - 2  generates appropriate selection commands for the analog multiplexer and the digital demultiplexer for storing the digital words in the respective memory devices. 
         [0035]    The digital part of the circuit breaker, that includes the multiplexer, the analog-to-digital converter, the demultiplexer, the memory devices, the counter, and the decision circuit, occupies a negligible silicon area in respect to the power transistor M 1  and in respect to the set of trimming cells that would be required in an analog circuit breaker of  FIG. 1  for obtaining the same accuracy. Moreover, it is possible to implement any kind of control characteristic for deciding in which condition the power transistor M 1  is to be turned off/on by substituting the shown decision circuit with a programmable device. 
         [0036]    The circuit breaker shown in  FIG. 3  implements cyclically the following method steps: 
         [0037]    sensing a delivered current flowing throughout the power line of the circuit breaker generating a corresponding analog current sense signal; 
         [0038]    converting the analog current sense signal into a corresponding digital value by means of the analog-to-digital converter and storing the digital value in the first memory device; 
         [0039]    sensing a supply voltage on the power line of the circuit breaker generating a corresponding analog voltage sense signal; 
         [0040]    converting the analog voltage sense signal into a corresponding digital value by means of the analog-to-digital converter and storing the digital value in the second memory device; 
         [0041]    converting a reference voltage on the internal line of the circuit breaker into a corresponding digital value by means of the analog-to-digital converter and storing the digital value in the third memory device; 
         [0042]    generating a product value representative of a value of power provided in input to the circuit breaker as the product of the digital values stored in the memory devices first and second; 
         [0043]    comparing the product value with the digital value stored in the third memory device; and 
         [0044]    generating the control signal for the power transistor with a level adapted to turn it off when the product between the delivered current and the supply voltage exceeds the maximum power level. 
         [0045]      FIG. 4  shows a simulation circuit of the circuit breaker of  FIG. 3  in which the values of the components used for obtaining simulation graphs are shown merely by way of example. There is also a clock input PWM for providing from outside a clock signal for clocking the digital part of the circuit breaker. The depicted circuit breaker is configured to stop supplying the load, that in the shown exemplary embodiment is a 1 Ω resistor, when a power greater than 7.5 W is absorbed by the load. 
         [0046]    Simulation time graphs of the circuit of  FIG. 4  are shown in  FIGS. 5 and 6 . In the interval from 0 to 420 μs, the power absorbed by the load Load Power is smaller than 7.5 W, thus the load is supplied. The product value Power, calculated from the digital values corresponding to the load current Iload and the input voltage Vin (represented in digital format by the signals Current and Vin on the top graph of  FIGS. 5 and 6 ), is compared with the maximum power level Power Limit, defined by the reference resistor Rset. 
         [0047]    When Power exceeds Power Limit, that in the considered case is 7.5 W, the load is disconnected and the value of Power becomes 0 at the next PWM period. After a fixed delay, the load is coupled again for another PWM period. Since the power absorbed by the load is greater than the maximum power level, the value of Power will exceed again the maximum power level Power Limit and the load is disconnected again at the next PWM period. 
         [0048]    An embodiment of the herein disclosed circuit breaker has numerous advantages, among which:
       Improved threshold precision: it depends only by the accuracy of the sensing circuitry (Current Sense) and of the analog-to-digital converter ADC. In the prior analog approach, the accuracy of the comparison with the maximum power level depends upon offsets of operational amplifiers used in the voltage multiplier and in the hysteresis comparator. By contrast, in an embodiment of the circuit breaker disclosed herein, all operations are executed in a digital domain, relatively free of errors;   High flexibility of the design: an eventual modification of the control action in a digital environment is typically faster and more secure than a modification in an analog environment;   Silicon area reduction: no trimming cell is required for attaining an enhanced accuracy;   No need of an external appliance to program the circuit breaker, since it may be done by means of a digital interface (SPI, I2C).       
 
         [0053]    From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated.