Patent Publication Number: US-2005135124-A1

Title: DC-to-DC converter for a car electrical system object of the invention

Description:
OBJECT OF THE INVENTION  
      This application is based on and claims the benefit of European Patent Application No. 03293296.4 filed Dec. 23, 2003&#39;, which is incorporated by reference herein.  
      The present invention relates to a current sensing device for a bi-directional power converter in a motor vehicle, and more particularly to DC-to-DC switching power supply that regulates the current flow provided to the car electrical systems, DC-to-DC this switching power supply including electrical protections and sensors for current measurement.  
     STATE OF THE ART  
      In view of ever increasing number of electronic devices that modem motor vehicles require, a dual architecture 42/12V car system has been proposed. It is well known in the art that the dual architecture 42/12V may have a DC/DC uni- or bi-directional switching power supply coupled to convert electrical energy from a first bus to a second bus or vice versa. Note that, for example, the first bus is of lower voltage than the second bus.  
      Typically, the conversion process is performed by means of a variable buck or/and boost converter which can be operated to receive electrical energy from the first bus and convert it to the corresponding voltage of the second bus or vice versa.  
      Due to internal or external conditions, a malfunction may occur that can damage the converter or parts of the car electrical system. For the power converter, these conditions require that it be protected. Usually, these conditions are referred to as input overvoltage, input short-circuit, output overvoltage, output short-circuit, reverse voltage polarity or short-circuit between both buses. These conditions are not described but are well known to those skilled in the field.  
      Referring to  FIG. 1 , under certain operational conditions such as a short-circuit at any bus for example, an excessive current (in value and flow sense) is produced that can endanger the DC/C power converter itself or the car electrical systems. Then, this high current surpasses the technical ratings of the switches  14 ,  15 , or inductor  16 . To avoid these dangers the DC/DC converter has several protecting means  2 ,  3  for detecting dangerous voltages at input or output and current sensing means  1 ,  4  that are placed at both ends of the DC/DC power converter, near each bus. Although workable, this is a cost and packaging-inefficient use of the additional hardware represented by means used for detecting the abnormal operation condition described above because of the electrical circuitry required.  
      In addition, the use of current sensing elements means losses, being especially critical with the high currents circulating in these DC/DC power converters, increasing complexity and associated costs due to the need for a greater heat-sink for dissipation of the energy losses.  
      In addition to these elements for protection, sometimes is required that the converter monitors and reports the current circulating at input or output, so that extra elements need to be added.  
      There is a need in the automobile supplier for a DC to DC switching power system comprising reduced additional hardware means to detect such abnormal operation condition of the dual architecture 42/12V and allowing easy and reliable operation of the electrical loads of the motor vehicle.  
      Consequently, there is also a need for a DC/DC power converter design which will overcome protection difficulties and current monitoring with the DC/DC power converter and which will do so in a cost-effective and packaging-efficient manner that will provide advantages over alternative designs.  
     CHARACTERISATION OF THE INVENTION  
      The present invention seeks to overcome or reduce one or more of the above problems by means of a DC-to-DC switching power converter coupled between a higher-voltage bus and lower-voltage bus that can be operated as an up or/and down converter comprises a first switching element, a second switching element, a sensing means connected in series between an end of an inductor and an end of a capacitor, a third element connected in series between the first element and the higher-voltage bus, a fourth element connected in series between other the end of the sensing means and the lower-voltage bus such that both the third and fourth switches are controlled, open or closed, as a function of a predetermined current level signal provided from a logic control circuit when the DC/DC switching power converter operates under anomalous condition.  
      One object of the present invention is the simplification of the electrical circuitry for controlling the protections and output/input currents monitoring, due to the combination of several elements.  
      Another object is to achieve a reduced cost because fewer elements are used, together to a reduced size.  
      Consequently, a further object of the present invention is the elimination of at least a sensing resistance in series for reducing energy losses in order to optimise conversion efficiency. Furthermore, a reduction in the size of the required heat-sink is achieved.  
      An additional object of the present invention is the simplification of the electrical circuitry for controlling the protecting means, due to the combination of several elements. This simplified circuitry means a reduced cost because fewer elements are used, but also a reduced size. 
    
    
     BRIEF EXPLANATION OF THE FIGURES  
      A more detailed explanation of the invention is given in the following description based on the attached figures in which:  
       FIG. 1  is a schematic diagram of a switching power converter according to the prior art, and  
       FIG. 2  is a schematic diagram of a switching power converter used in a car electrical system according to the invention. 
    
    
     DESCRIPTION OF THE INVENTION  
       FIG. 2  illustrates a DC-to-DC switching power converter  11  that is coupled between a higher-voltage bus  13  and lower-voltage bus  18 . Further on the higher-voltage bus is a first battery  13  for energy storage at the higher voltage, preferably having a nominal voltage of 42V; and on the lower-voltage bus there is a second battery  18  for energy storage at the lower voltage, preferably having a nominal voltage of 14V. Each energy distribution bus can be connected to a plurality of electric loads  12 ,  19 , respectively.  
      The DC-to-DC converter  11  includes a first element  14  for switching control in the event of operating in down conversion mode and a second element  15  for freewheeling switch, such that it is possible to control the switching of each switched transistor by means of a switching signal generated by a pulse width modulator PWM, not shown. Note that the DC/DC converter  11  may be a variable boost or/and buck converter without electric insulation, but in this description a “step-down” (i.e. buck) mode is considered, only for explanation.  
      The DC/DC converter  11  also includes a third element  14 - 1  which is connected in series between the first element  14  and an end of the electrical loads  12 , a sensing means  17  is connected in series between an end of an inductor  16  and an end of a capacitor  17 - 2 , and a fourth element  17 - 1  is connected in series between the other end of the sensing means  17  and an end of the electric loads  19 . These are the traditional electrical loads on a motor vehicle. For example, each switching element  14 ,  15 ,  14 - 1  and  17 - 1  can be a field effect transistor MOSFET, including the representation of the parasitic diodes which are known for the skilled person.  
      In normal operation, for example, when the first battery  13  is not providing sufficient power to the higher-voltage bus, DC/DC converter  11  is operated as an up-converter to provide high-voltage energy for operating electrical loads  12 . Once the higher-voltage bus has sufficiently powered operating, this up-conversion is no longer performed, and the first load  12  can be powered from the output provided by the first battery  13  to high-voltage bus.  
      Conversely, the DC/DC converter  11  is operated as a down-converter, converting the high voltage from higher-voltage bus  13  to the relatively lower voltage of the second battery  18 . Power is thus provided to charge the second battery  13  and provide energy for the traditional loads  19 .  
      During operation of the DC/DC converter  11  under normal conditions, both switches  14 - 1 ,  17 - 1  are held closed, i.e. both switches  14 - 1 ,  17 - 1  will remain switched-ON (i.e. conducting).  
      However, when the conditions of operation of the energy conversion system diverge from normal operation as explained above (output short-circuit, for example), the current flow through the inductor  16  is then increased, i.e. current flow from higher-voltage bus towards lower-voltage bus, the sensing resistor  17  providing a current sensing signal that reflects this event to a logic control circuit  21 . The latter circuit  21  receives a fast and clean indication of the output current through the inductor  16  which is used to open both the third and fourth switches  14 - 1 ,  17 - 1 , respectively. That is, the power switches are OFF (i.e. non-conducting).  
      Depending on the specific failure condition, it is also possible to open a single switch,  14 - 1  or  17 - 1 . If the current flow under abnormal condition is not stopped, it can damage the DC/DC converter  11  and the car energy system. Then, the third and fourth switches  14 - 1 ,  17 - 1  are added to prevent such damage.  
      It should be observed that the current level signal provided from the sensing resistor  17  must be higher than a predetermined threshold value, and positive or negative depending on the flow of the current (forward or reverse).  
      When the sensing resistor  17 , providing a current sensing signal that detects the current through the inductor  16 , is reversed, current flow, from the lower-voltage bus  18  towards the higher-voltage bus  13  (for example, a short-circuit at higher-voltage bus). The current sensing signal is used by the logic circuit  21  to open third switches  14 - 1 . Then the third switch  14 - 1  provides important reverse-protection for the DC/DC converter  11  and the car electrical system.  
      Consequently, to detect these anomalous situations that could damage the DC/DC converter  11  and the electrical system of the motor vehicle as a whole, the DC/DC converter  11  includes a first 35 voltage means for measuring the input voltage, a second 36 voltage means for also measuring the output voltage, and the sensing resistor  17 . The combination of these measuring means provides the corresponding control signals to the logic control circuit  21  for operating the third and fourth switches  14 - 1 ,  17 - 1 , respectively.  
      Furthermore, the conduction duty cycle of the first switch  14  is continuously monitoring by the logic circuit  21 . Using an adequate control algorithm, the logic circuit  21  provides a highly accurate measurement of the current flow from the higher-voltage bus  13 , using the basic expression I HV-bus =I Lv-bus *D, being I Lv  calculated based on the information of sensing element  17  when the power converter is operating in “step-down”. Therefore, this logic circuit  21  is able to provide an accurate measurement of both currents (I HV-bus  and I LV-bus ) using a single current sensing element, thus simplying circuitry, size and losses.  
      In opposition, when the converter is operating in step-up or “boost” mode, the same principle can be used, the equation in this case being I HV-bus =I Lv-bus *(1-D), being in this case “1-D” the duty cycle of first switch  14 , and standing “HV” higher-voltage bus and “LV” lower-voltage bus.  
      Then, the conduction duty cycle of the first switch  14  provides a fast and clean indication of the input current, that is, current provided from the higher-voltage bus  13 .  
      It should be appreciated that this DC to DC converter  11  does not require other sensing means since the current through the sensing resistor  17  is continuous. In addition, the current sensor signal from  17  is used to manage the activation or deactivation of the protection devices  14 - 1  and  17 - 1 .