Abstract:
An embodiment of the present invention provides for a precharge circuit comprising a power source, a first transistor, a second transistor, and an input node. The power source is in electrical communication with the first transistor where the first transistor selectively conducts current from the power source during a precharge phase. The second transistor is in electrical communication with the first transistor and configured to provide negative feedback to the first transistor. Further, the second transistor is configured to adjust current flowing through the first transistor based on the temperature of the first transistor. The input node is in electrical communication with the first and second transistor and provides an initial signal to activate the precharge circuit.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a pre-charge circuit that adjusts current flow based on temperature.  
         [0003]     2. Description of Related Art  
         [0004]     High power modules that utilize large reservoir capacitors are widely used in the automotive industry. In high power modules, such as electric power assist steering (EPAS), large bus capacitors are used to reduce current ripples and provide energy storage. When the module is turned on, the capacitors act as a short circuit and this results in a large current in-rush. The value of the current in-rush will depend on the size of the capacitors, but could be hundreds of amperes. Conventionally, a resistor or thermal resistor is connected in series with the capacitors. The resistor limits the in-rush current to the capacitors, but increases recovery time.  
         [0005]     Clearly, it is desirable for the precharge time to be as short as possible. Minimizing precharge time while utilizing a resistor to minimize current in-rush requires a resistor with a high power rating, often a few watts. Resistors with high power ratings, however, are much more expensive than resistors with standard ratings. Further, thermal resistors do not allow a short recovery time due to their thermal time constant.  
         [0006]     Alternatively, resistors may be selected with a large enough power rating and resistance value to limit in-rush current in conjunction with a feedback transistor as shown in FIG. 3.72 on page 167 of Horowitz and Hill, “The Art of Electronics”. The drawback of the Horowitz circuit is a long precharge time because current through the MOSFET is limited to be harmless at the highest possible temperature. However, the circuit is designed such that the MOSFET works most of the time at normal and low temperature.  
         [0007]     In view of the above, it is apparent that there exists a need for an improved pre-charge circuit that adjusts current with temperature.  
       SUMMARY  
       [0008]     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a precharge circuit comprising a power source, a first transistor, a second transistor, and an input node. The power source is in electrical communication with the first transistor where the first transistor selectively conducts current from the power source during a precharge phase. The second transistor is in electrical communication with the first transistor and configured to provide negative feedback to the first transistor. Further, the second transistor is configured to adjust current flowing through the first transistor based on the temperature of the first transistor. The input node is in electrical communication with the first and second transistor and provides an initial signal to activate the precharge circuit.  
         [0009]     In another embodiment of the present invention, the second transistor is configured to decrease the current flowing through the first transistor as the temperature of the first transistor increases. Further, the first transistor is a P-channel MOSFET and the second transistor is a bipolar PNP transistor having a negative base emitter voltage temperature coefficient.  
         [0010]     In yet another embodiment of the present invention, the precharge circuit includes a thermal conductor connected between the first and second transistor and configured to transfer heat between the first and second transistor. Further, the circuit trace can be used as a thermal conductor, preferably with a circuit trace width of greater than about 2 mm. The second transistor has to be located as close to the first transistor as possible, preferable not farther than about 5 mm from the first transistor allowing the rise in temperature of the first transistor to raise the temperature of the second transistor thereby adjusting current flow through the first transistor.  
         [0011]     Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic view of a pre-charge circuit that adjusts current with temperature in accordance with the present invention;  
         [0013]      FIG. 2  is the layout view of the preferable thermal path; and  
         [0014]      FIG. 3  is another embodiment of a pre-charge circuit that adjusts current with temperature in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Referring now to  FIG. 1 , a circuit embodying the principles of the present invention is illustrated therein and designated at  10 . The circuit  10  generally includes a power source  14 , a first transistor  20 , a second transistor  18  and an input node  12 .  
         [0016]     The circuit  10  receives an input signal at the input node  12 , which, in cooperation with other components, causes transistor  20  to conduct. Transistor  18  is configured to provide negative feedback to transistor  20 . Further, the circuit  10  is designed such that the location and properties of transistor  18  adjusts the current flowing through transistor  20 , inversely proportional to the temperature of transistor  18 . This provides a very short precharge time without increasing power dissipation requirements for various components and protects transistor  20  at high temperature.  
         [0017]     Now referring to  FIG. 1  in detail, an input signal is received at input node  12 . Resistor  26  and resistor  28  form a voltage divider between the input node  12  and electrical ground  40 . Transistor  22  is connected to the voltage divider such that the input signal causes transistor  22  to conduct. Transistor  22  is shown as an NPN bipolar transistor having its base connected to resistor  26  and resistor  28 , its emitter connected to electrical ground and its collector connected to the gate of transistor  20  through resistor  30 . As transistor  22  turns on, current begins to flow from the power source  14 , through resistor  36  and transistor  20 . Although transistor  20  is shown as a P-channel MOSFET, other transistors are contemplated and may be readily used.  
         [0018]     When the voltage drop across resistor  36  reaches the base emitter threshold voltage of transistor  18 , transistor  18  turns on. Turning on transistor  18  keeps transistor  20  turned on with a constant current flow. As seen in  FIG. 1 , the base of transistor  18  is connected to the source of transistor  20 . The emitter of transistor  18  is connected to power source  14  and its collector is connected to the gate of transistor  20  along circuit trace  24 .  
         [0019]     Resistor  32  is connected between the gate of transistor  20  on one side, and the emitter of transistor  18  and power source  14  on the other side. The value of resistor  36  can be modified to adjust the initial charging current.  
         [0020]     Transistor  18  has a negative base emitter voltage temperature coefficient. Although transistor  18  is shown as a bipolar PNP transistor, other transistors having similar thermal properties may be substituted. One such transistor is commercially available as part number MMBTA56 from Fairchild Semiconductor of South Portland, Me. This particular transistor has a base-emitter voltage equal to 0.72 volts at 27° C. In addition, it has a base emitter voltage of 0.88 volts and 0.52 volts at 40° C. and 125° C. respectively. Therefore, using the specified component, transistor  18  adjusts current in an inversely proportional manner to the temperature of transistor  18 , thereby increasing durability of circuit  10 .  
         [0021]     Placing transistor  18  close to transistor  20 , the current flow and power dissipation of transistor  20  will decrease as the temperature of transistors  20  and, therefore  18  rises, in effect providing over temperature protection for transistor  20 . Preferably, transistor  18  will be located within 5 mm or closer of transistor  20  to thermal path  21  facilitating heat transfer between transistor  18  and transistor  20 . In addition, connecting transistor  18  to transistor  20  with a thermal conductor will facilitate the influence of transistor  20  on transistor  18 . The circuit traces between transistor  18  and transistor  20  may be used as a thermal conductor such as circuit traces  23  and  24 . If the circuit traces are used as a thermal conductor, it is preferred, although not necessary, that the circuit traces are at least 2 mm wide. An example of the Printed Circuit Board layout when the thermal path provided by copper traces for STD10PF06 (transistor  20 ) and MMBTA56 (transistor  18 ) is shown in  FIG. 2 . Other transistors may have different layouts. In addition, a thermal conductor  25  made of metal, preferably copper, or other common conductive materials, may be connected between the packages of transistor  18  and transistor  20  to facilitate heat transfer.  
         [0022]     When a predetermined voltage threshold has been exceeded across capacitor  38 , relay  16  is energized to connect power source  14  directly to the drain of transistor  20 . Therefore, relay  16  bypasses transistor  20  providing a parallel power connection. At low ambient temperatures transistor  20  can dissipate more power than at high temperatures. Transistor  20  can sustain more drain current at lower ambient temperatures before reaching its maximum rated junction temperature.  
         [0023]     Now referring to  FIG. 3 , a higher dependence of transistor drain current on temperature can be achieved by replacing resistor  36  in  FIG. 1  with diode  76  as shown in circuit  50 . Similar to the previous embodiment, an input signal is received at input node  52 . Resistor  66  and resistor  68  form a voltage divider between the input node  52  and electrical ground  80 . Transistor  62  is connected to the voltage divider such that the input signal causes transistor  62  to conduct. Transistor  62  is shown as an NPN bipolar transistor having its base connected to resistor  66  and resistor  68 , its emitter is connected to electrical ground and the collector of transistor  62  is connected to the gate of transistor  60  through resistor  70 . As transistor  62  turns on, current begins to flow from power source  54 , through diode  76  in the source of transistor  60 , and out the drain of transistor  60 . Although transistor  60  is shown as a P-channel MOSFET, other transistors may be used.  
         [0024]     When the voltage drop across resistor  75  reaches the base-emitter threshold voltage of transistor  58 , transistor  58  turns on. Turning on transistor  58  keeps transistor  60  turned on with a constant current flow. Transistor  58  is shown as a bipolar PNP transistor; however, other transistors having similar thermal properties may be substituted. The base of transistor  58  is connected to the source of transistor  60  and the cathode of diode  76  through the voltage divider formed by resistors  77  and  75 . Further, the emitter of transistor  58  is connected to power source  54  and the collector of transistor  58  is connected to the gate of transistor  60  along circuit trace  64 . Resistor  72  is connected between the gate of transistor  60  on one side, and the emitter of transistor  58  and power source  54  on the other side. The values of resistor  77  and resistor  75  can be modified to adjust the degree of dependence of transistor  60  on transistor  58 .  
         [0025]     Transistor  58  has a negative base emitter voltage temperature coefficient. Therefore, transistor  58  adjusts current inversely proportional to the temperature of transistor  58 , thereby increasing durability of circuit  50 . Placing transistor  58  close to transistor  60 , the current flow and power dissipation of transistor  60  will decrease as the temperature of transistor  58  rises, in effect providing over temperature protection for transistor  60 . Preferably, transistor  58  will be located within 5 mm or closer of transistor  60  to facilitate heat transfer due to the change in transistor  60  temperature. In addition, connecting transistor  58  to transistor  60  with a thermal conductor will facilitate the influence of transistor  60  on transistor  58 . The circuit traces between transistor  58  and transistor  60  may be used as a thermal conductor as indicated by circuit trace  63  and  64 . If the circuit traces are used as a thermal conductor, it is preferred, although not necessary, that the circuit traces are at least 2 mm wide. Alternatively, a thermal conductor may be connected between the packages of transistor  58  and transistor  60  to facilitate heat transfer.  
         [0026]     Circuit  50  allows more drain current through transistor  60  at lower ambient temperatures than higher ambient temperatures. The voltage drop on diode  76  is applied to the base of transistor  58  through the voltage divider formed by resistor  75  and resistor  77 . The forward voltage drop on diode  76  and the base-emitter on voltage of transistor  60  have a negative temperature coefficient. However, the temperature coefficient of transistor  58  is greater than the temperature coefficient of diode  76  because the collector current of transistor  58  is much lower than the forward current of diode  76 . The voltage drop on sense resistor  36  ( FIG. 2 ) changes with temperature, following base-emitter threshold voltage. The voltage drop on diode  76  has a lower dependence on temperature. Therefore the sensitivity of current limiting in relation to ambient and/or transistor temperature increases, and current will be limited to much lower levels than in the previous embodiment. Other voltage sources can be used instead of the diode  76 .  
         [0027]     As the transistor  60  or/and ambient temperature increases, the threshold voltage of transistor  58  decreases. Therefore, transistor  58  will turn on at lower current as temperature increases. Transistor  58  will keep transistor  60  in a linear mode close to gate threshold voltage at lower current causing the drain current to decrease. Therefore, for circuit  50  the drain current of transistor  60  decreases as the ambient temperature is elevated.  
         [0028]     At low ambient temperatures transistor  60  can dissipate more power than at high temperatures. Accordingly, transistor  60  can sustain more drain current at lower ambient temperatures before reaching its maximum rated junction temperature. When a predetermined voltage threshold has been exceeded across capacitor  78 , relay  56  is energized to connect power source  54  to the drain of transistor  60 , thereby bypassing transistor  60 .  
         [0029]     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.