Abstract:
A pulse width modulation apparatus and method operates a switch at one of four slew rates to minimize electromagnetic and harmonic interference or switching losses. One of the four slew rates is selected based on a detected over-current condition, with or without a start up operation condition, an over-temperature condition and a normal working mode.

Description:
FIELD OF THE INVENTION 
     Pulse width modulation (PWM) is a method to control the root mean square voltage (V RMS ) across loads by dividing a DC voltage into pulses. Varying the pulse width in a PWM period controls the V RMS  across loads. To limit the electromagnetic interference (EMI) of a PWM system, the PWM pulses are not perfectly squared signals. EMI is reduced by having the rise and fall time of the pulses to be as long as possible. The long rise/fall times permit the switching components (SCs), such as FETs, to be turned on and off smoothly and reduce harmonic interference. This usually does not create a problem for a switching component even though power dissipation in the form of heat results from a prolonged turn on/turn off time. Heat sinks can be used in conjunction with the SCs to dissipate thermal effects. However, in high current applications, such as short circuit protection or a running motor condition, the small turn-off slew rate creates losses in the form of excessive heat, which can damage the switching component in the event of an abnormally high current cut-off. In such situations, a rapid turn off is the only way to protect the switching component. 
     Conventional PWM systems apply a fixed turn on/off slew rate such that either the EMI is reduced or the switching loss is reduced, but not both. An attempt to minimize both EMI and switching losses has generally been viewed as contradictory. Usually, EMI reduction receives higher priority. To counter the effects of switching losses when EMI reduction is optimized, PWM system designers have turned to larger switching components and larger heat sinks to handle unusually high current levels. The larger switching components and heat sinks require additional space and weight to be added to the PWM system. These larger components may also increase the price of the PWM system. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above deficiencies by employing an adaptive switching speed control (ASC) for PWM. Different switching slew rates are used for different working modes, which include the normal mode where EMI reduction is given priority and an over-current mode, where reduction of switching losses is given priority. In the over-current mode, the invention further takes into consideration whether a start up operation condition exists to provide a third mode of operation. In the start up operation mode, the ASC PWM provides a semi-large switching slew rate and a certain amount of current inrush. In particular, for an automotive lighting system, a long start up time is not acceptable to bring an automotive lamp, such as a braking-indicator lamp, a turn signal-indicating lamp or a high beam lamp, into a luminous condition. To shorten this start up time, a certain amount of inrush current (e.g., 150%-200% of normal operating current) is needed during start up. However, inrush current is highly dependent on the turn-off slew rate of the control signal of a switching component. A fast turn off will result in a smaller inrush current during the start up period. Therefore, when a certain amount of inrush current is necessary for a start up operation of a lamp, the ASC PWM operates the switching component with a semi-large turn off slew rate to provide a certain amount of inrush current during the start up period. This transition only exists for certain PWM channels where a certain amount of inrush current is necessary. The present invention also provides an over-temperature mode as a fourth mode of operation. The over-temperature mode is characterized by an intermediate switching slew rate that expands the working temperature range of a device controlled by a PWM circuit in the event of high ambient temperatures. In the normal mode, the switching slew rates are small, providing a long switching transition to reduce EMI and detrimental harmonic effects. In the over-current mode, the switching slew rates for the falling edge are large, providing a short switching transition to reduce switching losses and avoid damage to the switching component. In the start up operation mode, the switching slew rate is slightly smaller than in the in over-current mode where no start up condition exists. In the over-temperature mode, an intermediate switching slew rate between that of the normal mode and the over-current modes is used. 
     Functionally, an adaptive switching speed control PWM (ASC PWM) system includes three sections: The controller and drivers, the power switches and loads, and the ASC controller. The present invention is directed to the ASC controller of the system. Since all components of the ASC controller are smaller power components, it is possible to integrate the ASC controller into a trigger driver of switch components, e.g., a high side N channel power MOSFET driver. This would provide an efficient implementation for a practical ASC PWM system. The integration is helpful to reduce the number of components in the system, reduce the size of the system and increase the system reliability as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described below with references to the drawings in which: 
     FIG. 1 is a flowchart illustrating the operation of the adaptive switching speed control system according to a preferred embodiment of the present invention; 
     FIG. 2 is a block diagram of a system including an adaptive switching speed controller of the present invention; 
     FIGS. 3A-3H are equivalent diagrams for explaining the operation of the adaptive switching speed controller according to a preferred embodiment of the invention; 
     FIGS. 4A-4C are timing diagrams for describing the operation of the adaptive switching speed controller; and 
     FIG. 5 is a schematic diagram of the adaptive switching speed controller illustrated in FIG.  2 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 1, the overall operation of a preferred embodiment of the invention will be explained. At step  101 , system operation commences. This may correspond to a power-on reset, a trigger signal check or channel selection. This step prepares the system for PWM operation. At step  102 , when the system is ready for PWM operation, the system controller checks the ambient temperature. Based on the detected temperature, the system selects one of the normal working mode featuring a small switching on/off slew rate or an over-temperature mode, featuring a more rapid switching on/off slew rate. When the detected temperature is higher than a predetermined threshold, the controller selects the more rapid switching mode to reduce switching loss, consequently widening the working temperature range of switching components operating on the system. After temperature detection step  102 , the process branches based on the appropriate mode. Steps  10 X. 1  correspond to the small slew rate switching mode to reduce EMI, and steps  10 X. 2  correspond to the intermediate slew rate switching mode for over-temperature operation. The two branches are described in turn. 
     If the detected temperature is equal to or below the predetermined threshold, the main switches are turned on at a small slew rate (small di 1 /dt or small dv 1 /dt). Step  104 . 1 . At step  105 . 1 , the system controller checks for the condition of whether the on-time for the PWM pulse has run out. The on-time for the PWM pulse depends on the pre-set duty cycle for PWM. If it is determined that the on time has lapsed for the pulse, the procedure goes to step  110 . 1  to turn off the main switch at a small slew rate (small di,/dt or small dv 1 /dt) according to the normal working mode and returns to the detection of the over-temperature condition in step  102 . 
     Otherwise, it is determined that the on time for the PWM pulse has not yet lapsed, and the system proceeds to step  106 . 1  to determine whether an over-current condition exists. If there is no over-current condition, the procedure returns to step  105 . 1  to again determine whether the on-time for the PWM pulse has lapsed. If the over current condition is detected, the fast response procedure begins at step  107 . 1 . A start up condition is next detected at step  107   a .  1 . The start up condition corresponds to a time where a certain amount of inrush current is intentionally applied to a load, such as a vehicle headlamp (which works as a passing lamp), to stabilize its operation just as the headlamp is switched on. 
     Depending on whether a start up condition is present for the PWM channel, the fast turn off can be enabled in at least two ways. The ASC controller can cut off the switching components immediately or the PWM pulse can be turned on and off rapidly during the duration of the on time of the PWM pulse. The second implementation is described in more detail below. In normal working mode, the fast turn-off is active for the over-current condition only. In this mode, the turn off slew rate is approximately 40 times larger than in the normal working mode. Over-current mode, without the start up condition, has a large di 2 /dt or a large dv 2 /dt. The fast turn-off has no effect on normal falling PWM edges. 
     Step  108 . 1 . 1  turns off the main switch at a large slew rate or large di 2 /dt or dv 2 /dt, thereby commencing current discharge. 
     In the start up operation, step  108 . 1 . 2  turns off the main switch in a semi-large turn off mode at di 3 /dt or dv 3 /dt, where di 2 /dt&gt;di 3 /dt&gt;di,/dt, or dv 2 / dt&gt;dv 3 /dt&gt;dv 1 /dt. The smaller slew rate in the start up operation permits a larger current inrush during the start up time of the vehicle lamps. 
     At step  109 . 1 , the system determines whether the decreasing current has reached a preset lower limit. During this time, the main switch remains off. When the current reaches the lower limit, the procedure returns to step  104 . 1  to turn on the main switch. The switch on slew rate will be performed according to the normal switch on rate corresponding to di 1 /dt or dv 1 /dt. The steps for determining whether the on-time of the pulse width modulation cycle has run (step  105 . 1 ), determining whether over-current exists (step  106 . 1 ), and determining whether a start up condition exists (step  107   a .  1 ) are performed reiteratively until the on-time of the pulse cycle has lapsed. When the cycle has lapsed, the main switch is turned off at the small slew rate di 1 /dt or dv 1 /dt, and the procedure returns to step  102  to check the ambient temperature. The above steps describe the case where no over-temperature condition exists. 
     If at step  102 , it is determined that the over-temperature condition exists, then the intermediate slew rate over-temperature switching mode commences at step  103 . 2 . Since the detected ambient temperature exceeds the threshold value, to keep the main switches operational in safe thermal circumstances, it is necessary to decrease the switching loss (power dissipation) on the main switches. When the switching frequency (PWM frequency) is fixed, the only way to decrease switching loss is to shorten the rise and fall time of the PWM pulses. 
     At step  104 . 2 , the main switches are turned on at an intermediate slew rate characterized by di 4 /dt or dv 4 /dt, where di 2 /dt&gt;di 3 /dt&gt;di 4 /dt&gt;di 1 /dt, or dv 2 /dt&gt;dv 3 /dt&gt;dv 4 /dt&gt;dv 1 /dt. 
     At step  105 . 2 , the system controller determines whether the on time for a PWM pulse has lapsed. The on-time depends on a preset duty cycle. If the on time has lapsed, the procedure proceeds to step  110 . 2  to turn off the main switch and back to step  102  to determine whether the over-temperature condition exists. This turn off at step  110 . 2  is performed according to di 4 /dt or dv 4 /dt. 
     If the PWM on time has not lapsed, then a determination is made as to whether the over-current condition exists. If there is no over-current condition, the procedure returns to step  105 . 2  to determine again whether the PWM on time has lapsed. If the over-current condition does exist, then the enable fast turn-off procedure is implemented at step  107 . 2  At step  107   a . 2 , it is further determined whether the start up condition exists. 
     If the start up condition does not exist, then the main switch is turned off at the large slew rate (di 2 /dt or dv 2 /dt) in step  108 . 2 . 1 . If the start up condition does exist, then start up operation for semi-large switch off occurs at step  108 . 2 . 2 , at di 3 /dt or dv 3 /dt. 
     At step  109 . 2 , the decreasing current is detected, and it is determined whether the current has reached a bottom limit. If not, step  109 . 2  is repeated reiteratively until the bottom limit is reached. Once the bottom limit is reached, the procedure returns to step  104 . 2  to turn on the main switch at an intermediate rate di 4 /dt or dv 4 /dt corresponding to the over-temperature condition. As in the normal mode situation, the steps of determining whether the pulse width on-time has lapsed (step  105 . 2 ), determining whether the over-current condition exists (step  106 . 2 ), and determining whether the start up operation condition exists (step  107   a . 2 ) are performed reiteratively until the on-time for the PWM pulse lapses. At that time, the main switch is turned off at the intermediate slew rate di 4 /dt or dv 4 /dt, and the procedure returns to step  102  to detect the ambient temperature. 
     FIG. 2 illustrates an overall system in which the adaptive switching speed controller is used. The adaptive switching speed controller (ASC)  2  receives signals from a main controller  1 , which detects the temperature and current conditions according to temperature sensor  3  and current sensor  4 . The ASC PWM supplies switching signals to a main switch  5  controlling a load  6  based on the detected temperature and current. 
     A preferred embodiment of the circuit is illustrated in FIG. 5, which shows a temperature sensor  3  providing an input to the main controller  1  including a microcontroller  1   a  and a MOSFET driver U 1 . The controller system, in turn, provides an output to the adaptive switching speed controller  2 . The microcontroller  1   a  generates PWM pulses (TTL levels) and transmits the pulses to driver U 1 . The transistor driver shifts the received signal to a trigger signal S TP2 , which is typically about ten volts over the supply voltage to operate the main switch  5 . On the gate of the main switch  5 , the straight rise edge of S TP2  is slowed down by R 8  and C 1  to turn on the switch slowly. The logic circuit  8  cannot use S TP2  directly. Therefore, a level shifting network  7  (R 1 , R 11  and D 1 ) transfers S TP2  to S TP2 ′ which is a TTL level. 
     The various modes of operation for the above circuit will be described below with reference to the equivalent circuits  3 A- 3 H and the timing diagrams  4 A- 4 C. 
     Normal operation. As shown in FIG.  4 A and FIG. 5, at time t 1 , the output of the microcontroller  1   a  S TP1  goes high based on a preset duty cycle. S TP2  remains low because of the delay imparted by the driver U 1 . With S TP1  in a high state and S TP2  in a low state, S TP5  goes high turning on transistor Q 11  (and Q 4 , except in the case of a start up operation). The activation of Q 11  and Q 4  has no effect on the main switch since the output of U 1  at this time is zero volts. An equivalent circuit is shown in FIG.  3 A. After a delay, S TP2  goes high at time t 2 . S TP5  goes low and remains low, thereby turning off Q 11  and Q 4  off until the next occurrence of t 1  at the beginning of the next PWM cycle. S TP2  passes the slow-down network (R 8 , C 1 ) at S TP3  which turns on the main switch slowly. The equivalent circuit is shown in FIG.  3 B. At time t 3 , S TP1  goes low according to a preset PWM duty cycle. S TP2  remains high because of delay. At t 4 , S TP2  goes low. C 1  discharges through R 8  and C 1 . This makes S TP3  go low slowly, thereby turning off the main switch slowly. The equivalent circuit is shown in FIG.  3 C. 
     Over-Current Mode. Referring to FIG.  4 B and FIG. 5, at time t 1 , the output of the microcontroller  1 A S TP1  goes high based on a preset duty cycle. S TP2  remains low due to delay. S TP5  goes high and turns on Q 11  and Q 4 . At this time, the activation of Q 11  and Q 4  has no effect on the main switch operation since there is no signal on the U 1  pin OUT or on the gate of the main switch. The equivalent circuit is shown in FIG.  3 A. At time t 2 , S TP2  goes high and S TP5  goes low, turning off Q 11  and Q 4 . S TP3 , which is a slowed down signal of S TP2  turns on the main switch slowly. The equivalent circuit is shown in FIG.  3 B. The drain current of the main switch starts to go up. This current flow is detected by the current sensor  4 , which supplies an input to the driver U 1 . 
     At time t 3 , the current reaches the current limit I H . Driver U 1  receives an indication that the drain current has reached this threshold via the current sensor output. The current limit function inside of U 1  forces S TP2  to go low, making S TP4  go high immediately. Since S TP1  is still high, the condition of S TP5  is only decided by S TP4 . Therefore, S TP5  goes high, turning on Q 11  and Q 4 . The capacitor C 1  discharges through R 16 -Q 11  and R 9 -Q 4  at a speed approximately 40 times that of the normal working mode, which only discharges through R 8 . The speed at which discharge occurs can be controlled by selection of the resistors R 8 , R 9  and R 16 . The main switch is thus turned off very quickly. The equivalent circuit is shown in FIG.  3 D. 
     The current starts going down in a smooth manner due to the parasitic inductance in the circuit. At time t 4 , the current reaches the low current limit threshold I L . The current limit function in U 1  makes S TP2  go high. S TP5  goes low turning off Q 11  and Q 4 . S TP3  goes up turning on the main switch smoothly. The current continues to go up until reaching I H , where U 1  again forces S TP2  to a low value. The system repeats the operations described above until time t 8 . At t 8 , S TP1  goes low. The low value of STP, keeps S TP5  low until the next time t 1 . Because of the delay, S TP2  goes high as the detected drain current reaches IL at time t 9 . S TP3  continues to go up until time t 10 . At t 10 , the U 1  delay time is over, and S TP2  goes low. In this case, Q 11  and Q 4  are off (S TP5  is low). C 1  discharges through R 8  at a slow rate. The equivalent circuit is shown in FIG.  3 C. 
     High Ambient Temperature Operations. The microcontroller  1   a  monitors ambient temperature. If the temperature is higher than a predetermined temperature threshold, the control sets the Temperature Enable signal (T.EN, S TP7 ) to high. As illustrated in FIG. 4C, some time before t 1 , the controllers makes T.EN (S TP7 ) high. This turns on Q 10 . At time t 1 , S TP1  goes high. S TP2  remains low because of delay. S TP5  goes high turning on Q 11  and Q 4 . The turning on of these transistors has no effect on the main switch operation because both U 1  pin OUT and the gate of the main switch are zero volts at this time. The equivalent circuit is shown as in FIG.  3 A. At time t 2 , S TP2  goes high, S TP4  goes low, S TP5  goes low turning off Q 1  and Q 4 . S TP5  remains low until the next occurrence of t 1 . 
     Since S TP2  is high while Q 10  is turned on, some voltage drop across D 8  and R 17  is incurred. The voltage drop across D 8  and R 17  turns on Q 8 . Current goes through Q 8 , R 19  and the parasitic diode of Q 9  (Q 9 -D) and R 8  also charging C 1 . The main switch is turned on about two times faster than normal (through R 8  only). The equivalent circuit is shown in FIG.  3 E. As apparent from FIG. 3E, the speed of charging can be controlled by selection of the resistance values for R 8  and R 19 . At time t 3 , S TP1  goes low. Because of delay, S TP2  remains high until t 4 . At time t 4 , S TP2  goes low. Since the voltage on C 1  cannot go down to zero immediately, some current goes through D 9 , R 20 , R 18  and Q 10  to ground. Some voltage drop is incurred across D 9  and R 20 . The voltage drop across D 9  and R 20  turns on Q 9 . C 1  discharges through R 8  as well as through Q 9 , R 19  and parasitic diode of Q 8  (Q 8 -D) and turning off the main switch at a double slew rate when compared to the normal working mode. The equivalent circuit is shown in FIG.  3 F. The diodes D 8  and D 9  function as breaks to prevent current from going through R 17  and R 20  directly. In case of over-current in the high ambient temperature mode, the system operation for the over-current condition is handled in the same manner as an over-current condition detected in the normal mode. 
     Start Up Operation. In some applications, such as automotive headlamps using halogen bulbs, concerns should be given not only to current controls but also to the start up time as well. Start up circuitry may be embedded in the ASC controller to shorten the starting period. As shown in FIG. 5, during the start period, the channel enable (CH.EN, S TP8 ) supplied by the PWM controller charges C 2  through R 21 . The potential level of C 2  (S TP10 ) goes up from zero volts. STP 10  remains logical “0” until the voltage on C 2  reaches the logical “1” voltage threshold of U 2 B. The “0” level of S TP10  makes S TP11 =“0” which turns off Q 4  regardless of the level of S TP5 . Therefore, within the start up period, over current operation (S TP5 ) can only turn on Q 11 . C 1  discharges through R 16  and Q 11  at about ten times faster than normal speed. The slower response for over current (start up) makes inrush current in the main circuit about 150% of these in fast response operations for over-current operations without the start up condition. The equivalent circuit is shown in FIG. 3G and 3H. 
     Generally the ASC controller turns off the main switch quickly only during the over current operation (during the on portion of the PWM duty cycle). The normal turn-off (corresponding to PWM falling edges) always keeps a smooth turn-off. This keeps EMI and harmonic interference as small as possible while keeping fast response for current control. The system allows the switches and devices to be configured with lower capacity heat sinks and smaller switching components to reduce the bulk, weight and cost of an electrical system. 
     Though the present invention has been described above with reference to a preferred embodiment, one skilled in the art can make modifications thereto without departing from the spirit and the scope of the present invention. For instance, although the apparatus has been described as providing slew rates for various conditions corresponding to normal, over-temperature and over-current conditions, with and with out a start up mode, one skilled in the art would understand that corresponding rise and fall slew rate can also be set for these conditions. In particular, the over current mode would have the largest fall slew rate, the start up mode would have a semi-large fall slew rate, the over-temperature condition would have an intermediate fall slew rate which is larger than in the over-current and start up conditions. Finally, the normal working mode would have the smallest fall slew rate. With regard to rise slew rate, one skilled in the art would recognize that the normal working mode that would have a larger rise slew rate than the over-temperature mode.