Abstract:
The present invention provides an electronic circuit and EMI countermeasure for an automotive electronic control unit that provides a maximum amount of EMI protection under normal operating conditions, i.e., ambient temperatures. The effectiveness of the EMI countermeasure is limited at extremely high temperatures. The electronic circuit and EMI countermeasure optimizes the effectiveness of EMI countermeasures as a function of temperature to improve performance under normal conditions.

Description:
FIELD OF THE INVENTION 
   The invention generally relates to methods and systems for ensuring electromagnetic compatibility (“EMC”) in automotive systems. More specifically, the invention relates to adjusting the operation of EMC devices and EMI countermeasures to account for or otherwise take into consideration temperature and thermal effects. 
   BACKGROUND OF THE INVENTION 
   The reliability, low cost, and ability to provide control over a wide variety of devices, systems, and processes has made electronic circuits and devices ubiquitous. However, like any other technology, electronic devices are designed as a trade-off among mutually exclusive constraints. One weakness of electronic devices is their susceptibility to performance degradation when subjected to electromagnetic interference (“EMI”). EMI is produced by a variety of sources (e.g., transformers, stray radio signals, motors, heat, and other machines and sources that generate electric or magnetic fields.) Most, if not all, electronic devices are susceptible to EMI from one source or another under the right conditions. 
   Electronic circuits and devices are utilized in many automotive applications. The electronic circuits utilized in automobiles are generally designed to be electromagnetically compatible with each other, such that one electronic circuit does not interfere with the functioning of a near-by electronic circuit. Electromagnetic compatibility relates to the ability of electronic and electrical equipment and systems to operate without adversely affecting other electrical or electronic equipment or being affected by other sources of interference. 
   In many cases EMC is achieved using what are called EMI countermeasures or sometimes EMC countermeasures. Such countermeasures either reduce or eliminate the creation of interfering signals or counterbalance or cancel the signals or their effects. As noted, EMI countermeasures may cause excessive heat. Generally, electronic circuits are designed to accommodate a maximum thermal load. If the circuits are subjected to conditions beyond their limits, they will malfunction. In addition to heat caused by the operation of electronic and electric devices, the environmental temperature and other heat sources may combine to cause malfunctioning of electronic components and electronic circuits. 
   SUMMARY OF THE INVENTION 
   Accordingly, there is a need for improved EMI countermeasures, particularly countermeasures that perform in environments where excess heat can be problematic. The present invention provides an electronic circuit and EMI countermeasure for an automotive electronic control unit that provides a maximum amount of EMC protection under normal operating conditions, i.e., a range of typically encountered ambient temperatures. The effectiveness of the EMI countermeasure is reduced at extremely high temperatures. The electronic circuit and EMI countermeasure enhance the effectiveness of EMI countermeasures as a function of temperature to improve performance under normal conditions. This feature allows for a less-expensive housing to be used for an electronic control unit in which the electronic circuit (such as a control circuit) and EMI countermeasure are placed. 
   In one embodiment, the invention includes a method of controlling a device or module (generically referred to as an actuator). The method includes the acts of providing a control signal to an input circuit of a switch, the input circuit including a filter, the control signal having a plurality of peaks and a plurality of troughs and a plurality of edges, each edge running from one of the peaks to one of the troughs or one of the troughs to one of the peaks, controlling the speed of the plurality of edges by processing the control signal in the filter, the filter operable to maintain the speed of the edges at a relatively low value or rate in an ambient temperature range and to increase the speed of the edges at temperatures that are higher than the ambient temperature range, and providing the processed control signal to the switch. 
   In another embodiment, the invention includes a controller for controlling an actuator. The controller includes an input circuit having a first component that provides a resistance, a second component coupled in a parallel path with the first component, the second component providing a temperature-dependent resistance, and a third component that provides a capacitance. The controller also includes a switch operable to change from a first state to a second state according to a time constant determined by the input circuit. 
   In another embodiment, the invention includes a method of controlling a system. The method includes the acts of providing a control signal to an input circuit of a switch; calculating a time constant determined by the input circuit; processing and modifying the control signal according to the time constant; triggering a switch to adjust from a first state to a second state according to the modified control signal; and providing a current to the system when the switch is in the second state. 
   Additional objects and features of the invention are illustrated in the drawings and provided in the subsequent disclosure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary vehicle and one embodiment of the invention located in the vehicle. 
       FIG. 2  illustrates an exemplary schematic of an electronic circuit of one embodiment of the invention. 
       FIG. 3  illustrates an electronic input signal of one of the components in the electronic circuit. 
       FIG. 4  illustrates the effective gate resistance of a switch versus temperature in one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the examples set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as detector, or a crystal. The resistor  42  is positioned in path  54 . The temperature-dependent resistor  46  is in a parallel path  58  with respect to the resistor  42 . The resistor  42  and temperature-dependent resistor  46  form a network  60 . In one embodiment, the resistor  42  has a value of 1 KΩ, the temperature-dependent resistor  46  is a thermistor with a value of 56 KΩ, and the capacitor  50  is a Miller capacitor with a value of 2.2 nF. The input circuit  34  may include additional elements or components in different configurations than shown in  FIG. 2 , and the values for each element or component may differ than as specified. 
   The electronic circuit  30  includes an actuator which in the embodiment shown takes the form of a module  62 . More specifically, in the embodiment illustrated in  FIG. 2 , module  62  represents a power steering module. As is known, most modem vehicles have a power steering system. In general, the amount of power steering assist required at low speeds is significantly more than at high speeds. To achieve this, some automotive manufacturers use an electronic control unit to regulate the amount of power steering assist based on vehicle speed.  FIG. 2  represents a simplified schematic of the output driver for one specific implementation. The module  62  is modeled as having an internal inductance and resistance indicated by an inductor  66  (L) and a resistor  70  (R 2 ). More specifically, the resistor  70  represents the estimated internal resistance of the inductor  66 . In the embodiment shown, the inductor  66  has a value of 6.35 mH and the resistor  70  has a value of 2.2Ω. 
   The electronic circuit  30  includes a driver circuit  74  having a transistor  78  (T 1 ) positioned in path  82  and a diode  86  (D 1 ) positioned in series in the path  82 . The diode may be implemented by using a FET, where the gate of the FET is tied to ground (not shown). The driver circuit  74  includes a first input  90  (UZ), which is connected to the ignition switch (not shown) of the vehicle  10 . The driver circuit  74  also includes a second input  94  (V in ), which is connected to the vehicle battery (not shown). The electronic circuit  30  includes a switch  98  (T 2 ). Preferably, the switch  98  is a MOSFET transistor having an on and an off state. 
   In operation, when the vehicle  10  is running, power is supplied at the first input  90  to the electronic circuit  30 . Power is also supplied from the vehicle battery (not shown) to the second input  94 . The transistor  78  receives a voltage such that transistor  78  is in the on state during the operation detailed in this description. 
   The processor  26  receives data  106  from other systems in the vehicle, e.g., speed and steering wheel angle. The processor  26  processes the data and transmits a control signal  110  to the input circuit  34  of the electronic circuit  30 . The control signal  110  is generally characterized as a pulse width modulated (“PWM”) signal having a plurality of peaks, a plurality of troughs, and a plurality of edges.  FIG. 3  illustrates an enlarged section of a control signal  110  having a peak  114 , a trough  118 , and an edge  122 . 
   The input circuit  34  receives the control signal  110 , filters and modifies the control signal  110 , and delivers the control signal  110  to the switch  98 . The input circuit  34  includes a filter  38 . During the filtering and modification of the control signal  110 , the control signal  110  is partially diverted to the capacitor  50  to decrease the power of the signal to the switch  98 . In the embodiment shown, the resistor  42 , the temperature-dependent resistor  46 , and the capacitor  50  behave as an RC filter, which has an RC time constant. The resistance of the temperature-dependent resistor  46  varies with the air or surrounding temperature. At normal or ambient temperatures and as may be best seen by reference to  FIG. 4 , the value of the temperature-dependent resistor  46  is maintained at or near its given value. At higher temperatures, the temperature-dependent resistor  46  value decreases. A typical ambient range is from about −40° C. to about 85° C. 
   The temperature-dependent resistor  46  and resistor  42  may be viewed as a single Thevenin equivalent resistor having a resistance determined by Equation 1 below. 
                   Equation   ⁢           ⁢   1   ⁢     :       ⁢     
     ⁢         R   E     =       R1   ×   RT       R1   +   RT         ,                             
where R E  is the equivalent resistance and R 1  is the value of resistor  42  and RT is the value of temperature-dependent resistor  46 . As the value of the temperature-dependent resistor  46  varies, the RC time constant, where R E =R and C is the capacitance of the capacitor  50 , also varies. At higher temperatures, the RC time constant is lower in value than at normal or ambient temperatures. A lower RC time constant indicates that the time to discharge a capacitor is relatively short or fast. Likewise, a higher RC time constant indicates that the time to discharge a capacitor is long or slow. A lower RC time constant translates to a faster edge transition of the control signal  110 , i.e., the time from peak to trough or trough to peak is shorter.
 
   The RC time constant affects the switching time (i.e., the time it takes to transition from a peak to a trough or trough to a peak) of the switch  98 . As the switching time increases (i.e., the transition from peak to trough or vice versa gets slower) the amount of heat generated by the switch  98  increases. At a higher temperature, the switch  98  is transitioning from an on/off state more quickly (due to the decrease in value of the RC time constant) than at a lower temperature. The switching time is deliberately slowed down to limit electromagnetic radiation, which may cause interference with various devices in the vehicle  10  such as the radio  18 , i.e., as an EMI countermeasure. But this improvement in the EMC behavior causes additional heat to be dissipated in the electronic control unit  22 . In embodiments where the switch  98  is located in the housing of the electronic control unit  22 , and at high ambient temperatures the heat accumulated in the electronic control unit  22  may increase to an unacceptable level. At higher temperatures, less natural heat dissipation is possible. The additional heat may exceed the maximum thermal load that the electronic circuit  30  is designed to handle. Under these circumstances, the switching transition times may be decreased, which will prevent the ECU from exceeding its thermal limits, but may increase radiated emissions. 
   When the switch  98  is in the on state, current (as modulated by the control signal  110 ) flows from the first input  90  in the path  82  through the transistor  78 , the inductor  66 , the resistor  70 , and switch  98  to ground. The current through the inductor  66 , the resistor  70 , and the module  62  adjusts the power steering control of the vehicle  10 . When the switch  98  is in the off state, the current continues to travel through the inductor  66  in the direction of the path  82  from the transistor  78  through the inductor  66  and the resistor  70  to the diode  86 . The diode  86  acts as a re-circulation diode to maintain current flow in the driver circuit  74 . 
   To summarize the operation of the electronic control unit used in a power steering system, at extremely high environmental or surrounding temperatures, which are less likely to be encountered, the value of the temperature-dependent resistor  46  decreases, which also decreases the resulting RC time constant. The lower RC time constant value translates into a faster edge transition of the control signal  110 , which results in a quicker switching time of the switch  98 . At less extreme operating conditions, the switching time of the switch  98  is slowed, reducing potential EMI with the radio  18  or other vehicular systems (not shown). Under extreme operating temperatures, the faster switching time of the switch  98  creates less heat than when the switch operates at lower transition speeds, but increases the likelihood of EMC issues with the radio  18  for example. 
   At normal, ambient, or slightly elevated temperatures, the value of R E  of the temperature-dependent resistor network  60  is maintained at or around its given value, leading to a higher RC time constant value than at higher temperatures. The higher RC time constant translates into a slower edge transition of the control signal  110 , which results in a slower switching time of the switch  98 . The slower switching time of the switch  98  generates more heat, but since the ambient temperature is lower, the electronic circuit  30  can dissipate the entire thermal load through heat sinks, cooling fins, natural convection, and the like. 
   In another embodiment, the electronic control unit may be used in a braking system of the vehicle  10 . Instead of the module  62  being used for a power steering application, the module  62  is used for a braking system. The processor  26  receives data  106 , e.g., vehicle speed, braking input, and wheel slippage. Based on the data  106 , the processor  26  transmits a control signal  110  to the input circuit  34 . The input circuit  34  filters and modifies the control signal  110 , and delivers the control signal  110  to the switch  98 . 
   As can be seen from the above, one embodiment of the invention provides an effective EMI countermeasure at normal or ambient temperatures, but one that is less effective at extremely high temperatures. The trade-off between temperature and EMC leads to a less expensive housing for the electronic control unit  22  because the electronic circuit  30  requires less heat dissipation measures. The invention also allows for a less expensive or smaller heat sink. The invention may also allow for a less expensive or smaller transistor (T 2 ) and driver circuit. Various features and aspects of the invention are set forth in the following claims.