Abstract:
A system and method to eliminate the influence of Dead Time delay in a PWM driven system. The system is implemented using a switching control circuit to operate a pair of upper and lower switching devices in a standard half-bridge arrangement, wherein both a PWM pulse train input and the direction of current flow (current command direction) are both utilized to operate the switching devices. Depending on the direction of the current command, the PWN pulse train input is directly applied to control one of the switching devices (the first switching device), while the second switching device is kept open at least during the closing time of the first switching device. Thus, a precise average PWM output volgate can be obtained, independent of the Dead Time delay, so that a large Dead Time can be used, to provide a precise voltage output over the full range of the DC power supply, while maintaining high reliability. The system can also be implemented using a microprocessor-based control system, for control of the switching devices, for betetr integration in a digital system.

Description:
This application claims the benefit of Provisional Application No. 60/247,936, filed Nov. 14, 2000. 

   FIELD OF THE INVENTION 
   The present invention relates to PWM motor control systems, and more particularly, to a motor control system and more generally, to an inductive load voltage controller. 
   BACKGROUND OF THE INVENTION 
   The power output to Inductive loads such as Motors is commonly applied by using the well-known technique of Pulse Width Modulation (PWM). This technique is well explained in chapter 5 of the text “Power Electronics”, by Joseph Vithayathil, McGraw Hill Series in Electrical and Computer Engineering. 
   In a system using the PWM technique, voltage is applied to one or several points of the load by means of two switching devices such as transistors, or more precisely IGBTs (Insulated Gate Bipolar Transistors), in a bridge arrangement. One Switching device is connected from the positive pole of the power supply to the load, and the other one from the negative pole of the same power supply to the same load connection. 
   An inherent problem related to the PWM technique, is the introduction of a so-called Dead Time delay. This delay is introduced between the opening of one switch and the closing of the other switch. Such a delay is commonly introduced in order to avoid the possibility that the two switches will be activated together. Should both switches be activated together, then the power supply would be shorted and the high current would cause the destruction of the switches or of the power supply. Also, the switching time of the switching devices has some finite value, and that value may vary under different conditions, like temperature, load current, etc. Therefore, the Dead Time delay should have a higher value than the maximum switching time delay. 
   During the Dead Time delay, both switches are non-conducting. As a result, the voltage set on the load connection depends on the load condition, and in particular on the load current direction. If the current is positive (flowing toward the load), then it will be flowing through the lower leg bridge diode, and the voltage will be almost equal to that of the negative pole of the power supply. If the current is negative (flowing from the load to the switches), then the current will be flowing through the upper leg bridge diode, and the voltage will be almost equal to that of the positive pole of the power supply. This situation creates a non-deterministic relation between PWM duty cycle and voltage applied, and large imprecision in voltage output is observed. 
   Another problem created by the Dead Time delay is that voltages having a pulse width smaller than the Dead Time, cannot be output. Thus a voltage value close to Negative or Positive Power supply line voltage, which have respectively, a small on-time or a small off-time pulse width, cannot be output, thus creating a discontinuity in the voltage close to the negative or positive power supply voltage output. 
   When designing a PWM system, the engineer must make a difficult evaluation of the Dead Time influence. Too short a Dead Time delay will reduce the reliability of the system, i.e. under some conditions a short circuit can be obtained, while too long a Dead Time delay will spoil the system performance. 
   Switching schemes have been described in prior art patents which compensate for the voltage imprecision, however they do not resolve the discontinuity of average voltage output when the desired output voltage is near zero or near the DC power supply voltage. In U.S. Pat. No. 5,930,132 to Watanabe et al., an electronic system is shown with the purpose of minimizing the dead time delay. In this patent, the dead time delay is not eliminated, but only minimized. In U.S. Pat. No. 5,859,770 to Takada et al., an electronic arrangement is used that comprises a P channel transistor in order to reduce to almost zero the dead time. P channel transistors have the disadvantage of being limited in their power rating, so that in most modern systems only N channel IGBT&#39;s are used at high power. Thus the Takada et al. patent is not advantageous for medium and high power systems. 
   Therefore, it would be desirable to provide a system in which the Dead Time delay will not influence the precision of the system, which is applicable for the Power switching transistors commonly used today. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a principal object of the present invention to overcome the problems associated with Dead Time delays. 
   The present invention provides a system and method to resolve the Dead Time delay problem. The Dead Time influence is canceled, i.e. the precise value of the voltage output is not dependent on the dead time value; furthermore, the whole range of voltage output values of the DC power supply is made available, in a continuous way. 
   In accordance with a preferred embodiment of the present invention, there is provided a system for producing an output voltage for supplying current to a load, by controlling the switching of a pair of electronic switches in order to eliminate the influence of dead time between switching them, the pair of electronic switches being connectable in a half-bridge arrangement across power supply terminals, with the first switch being connected to a positive power supply terminal, wherein a positive current flow direction is defined with respect to the load, when current flows to the load, the system comprising: 
   first means operable to close and open the first switch of the electronic switch pair in response, respectively, to the ON and OFF times of an input signal waveform; 
   second means operable to open and close the second switch of the electronic switch pair in response, respectively, to the ON and OFF times of the input signal waveform; 
   such that in the case where a positive current command is provided to establish a positive current, the first means closes the first switch during the ON time of the input signal waveform, and opens it during the OFF time thereof while the second means operates to open the second switch at least during the closing time of the first switch, and 
   in the case where a negative current command is provided to establish a negative current, the second means opens the second switch during the ON time of the input signal waveform, and closes it during the OFF time thereof, while the first means operates to open the first switch at least during the closing time of the second switch, 
   the first and second electronic switches operating such that the average output voltage will be precisely in accordance with the average width of the input signal waveform. 
   The system is implemented using a switching control circuit to operate a pair of upper and lower switching devices in a standard half-bridge arrangement, wherein both a PWM pulse train input and the desired direction of current flow (current command direction) are both utilized to operate the switching devices. In the case where the current command direction is positive, the PWM pulse train input is directly applied to control the upper switching device, while the lower switching device is kept open at least during the closing time of the upper switching device. In the case where the current command direction is negative, the PWM pulse train input is first inverted, and then applied to the lower switching device, while the upper switching device is kept open at least during the closing time of the lower switching device. 
   In accordance with the present invention, a precise average PWM output voltage can be obtained, independent of the Dead Time delay, so that a large Dead Time can be used, to provide a precise voltage output over the fill range of the DC power supply, while maintaining high reliability. 
   The inventive method can be implemented using a microprocessor-based control system, for control of the switching devices, for better integration in a digital system. 
   Other features and advantages of the invention will become apparent from the following drawings and description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference is made to the accompanying drawings, in which like numerals designate corresponding elements or sections throughout, and in which: 
       FIG. 1  shows a prior art PWM control system, to apply a PWM voltage to one side of a load connection; 
       FIG. 2  shows a timing chart for such a prior art system; 
       FIG. 3  shows a general block diagram of the inventive PWM control system; 
       FIG. 4  shows a timing chart for the inventive system in the case of a positive current command; 
       FIG. 5  shows a timing chart for the inventive system in the case of a negative current command; 
       FIG. 6  shows a hardware embodiment for generating the inventive PWM pattern; 
       FIG. 7  shows an alternative embodiment using a microprocessor; 
       FIG. 8  shows an example of a software flowchart used in the embodiment of  FIG. 7  to generate the inventive PWM pattern. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A typical system of the prior art is shown in FIG.  1 . The two switching devices are shown here as two transistors, Q 1  (upper) and Q 2  (lower). Transistor Q 1  is connected on one leg to the positive pole of the DC power supply, with the other leg connected to the load. Transistor Q 2  is connected on one leg to the negative pole of the DC power supply, and on the other leg to the load  9 , where transistor Q 1  is connected. Each transistor functions as a switch for current flowing from the upper leg to the lower leg. The transistor can be operated in a conducting state (ON) representing a closed switch, or it can be operated in a non-conducting state (OFF), representing an open switch. Connected, respectively, in parallel to each transistor are diodes D 1  and D 2  which conduct current in the opposite direction. This arrangement is further referred to herein as a half-bridge arrangement. 
   A Voltage Command is applied to the system, and may be implemented in the form of an input signal waveform having two logical states, ON and OFF. In  FIG. 1 , this Voltage Command is applied in the form of a PWM pulse train. This pulse train (see  FIG. 2 ) is a pulse with Tpwm on-time and Tcycle period, representing:
 
Voltage Command value= Vc=E*Tpwm/Tcycle. 
 
   This pulse train is transferred to the gate of the power switches through a delay circuit  12 . When the pulse train goes ON, then lower transistor Q 2  is immediately switched OFF by Gate Driver  14 , and the upper transistor Q 1  is switched ON by Gate Driver  16  after a Dead Time delay (Td). 
   When the pulse train goes from ON to OFF, then the upper transistor Q 1  is immediately switched OFF, while lower transistor Q 2  is switched ON after a Dead Time delay. 
   The timing chart for the gate signals C high  ( 18 ) and C low  ( 20 ) to transistors Q 1 -Q 2  is shown in FIG.  2 . 
   Ideally, if the dead time delay (Td) were infinitely small, then the average voltage (V) obtained on the load would be (where E represents the DC power supply voltage): 
   V=E*Tpwm/Tcycle, which is the ideal case where the average voltage output is directly proportional to the Voltage Command value. 
   However, if the dead time delay has a non-zero value, as in a practical case, then the average voltage has a different value. Upper transistor Q 1  will conduct for a period of Tpwm−Td. Lower transistor Q 2  will not conduct during a period Tpwm+Td. So, before and after the conducting time of upper transistor Q 1 , there will be a delay period during which no transistor will conduct (Dead Time delay). 
   The time continuity of the current flow implies that during the Dead Time delay, the Voltage will be equal to that of the Positive pole of the power supply (+E) if the current is negative (flowing from load to transistors). This is because current will flow through the upper diode D 1  to that pole. Symmetrically, if the current is positive, the voltage will be equal to that of the negative pole of the power supply (0 Volts), since current will flow through diode D 2 . 
   Taking into account these Dead Time delays, the average voltage applied to the load during one cycle will be: 
   If current is positive:
 
 V=E*[Tpwm−Td]/T cycle
 
   If current is negative:
 
 V=E*[*Tpwm+Td]/T cycle
 
   Some compensation methods have been proposed in the prior art (e.g., in the paper “Pulse Based Dead Time Compensator for PWM Voltage Inverter, David Leggate, IEEE Transactions on IE, Vol. 44, No. 2, 1997). These compensation methods add or subtract the Dead Time delay to or from the Tpwm value command, adding if the current is positive, subtracting if negative. These methods can compensate for the imprecision of the PWM voltage output, however they cannot be used when PWM voltage command is close to the voltage of one of the power supply poles, either 0 volts or E volts. 
   For example, with a negative current in order to output the small voltage value
         V=(E*Td/Tcycle)/2, without compensation, and the output value would be Tpwm=Td/2. As shown before, this will result in voltage   V=E*[Td/2+Td]/Tcycle, which is three times larger than the desired value.       

   If prior art compensation methods are used, the value Td is subtracted from the Tpwm command, and a negative value (−Td/2) is obtained for Tpwm, meaning that the switches will not be activated in that cycle, thus the PWM average value obtained will be V=0, for that cycle. 
   So both systems, with or without compensation, produce an imprecision in PWM voltage output. 
   Referring now to  FIG. 3 , there is shown a general block diagram of the inventive PWM system. 
   In most modern control systems, the current in the load is controlled, and the PWM voltage is the means used to control that current. As a consequence, the Current Command value is defined. Typically, the control system will set the PWM voltage in order to produce a change in the current value, according to the present needs of the control system. For example, in a servo control system for a motor, the velocity is controlled; in that system a speed sensor is used to measure the actual speed of a motor. If the speed is different from a presently desired speed (Command Speed), then the servo system will change the current command to a new value, in order to modify the torque developed on the motor, and thus correct the speed. 
   Another inner control loop (current control loop) of the servo amplifier will check the difference between the actual current flowing in the motor and the current command. If a difference is found, then the current control loop will change the PWM command, in order to change the average voltage applied to the motor, resulting in a new current with a value closer to the command value. 
   As a result, in most modern control systems, both the PWM Voltage Command and Current Command are defined. In particular, the Current Command direction is known. 
   Based on this consideration, the inventive method uses both Voltage Command (Vc) and Current Command (Cc) direction as input signals  22  and  24 , as shown in FIG.  3 . 
   The Voltage Command Vc and the Current Command direction Cc are input to a Transistor Command Generator (TCG)  26 . The TCG  26  functions as a switching controller for the switching devices Q 1  and Q 2 , so as to precisely control the switching intervals, in order to apply an exact PWM voltage (V) to the load, without being influenced by the Dead Time delay. 
   The TCG  26  creates two different gate signals  18  and  20  (C high  and C low ) which are fed to Gate Drivers  16  and  14 , respectively, for upper and lower transistors Q 1  and Q 2 . 
   In a first step, the Voltage Command (Vc) signal  22  is interpreted as a PWM pulse train of width Tpwm and period Tcycle. The pulse width Tpwm is calculated so that for the DC power supply E and for an ideal system without Dead Time delay, the pulse width Tpwm would exactly result in an average output voltage value matching that of the Voltage command, Vc, in accordance with the equation:
 
 Tpwm=T cycle* Vc/E 
 
   This PWM pulse train is an interior variable or signal of the TCG  26  implemented in hardware or software, so that it does not need to actually be output. 
   Then the TCG generates the two transistor command signals  18  and  20  (C high  and C low ) according to the following charts, as shown in FIGS.  4  and  5 :
         a) In the case where the Current Command direction is positive (FIG.  4 ):
           The upper transistor command signal (C high ) is identical to the PWM pulse train, i.e. the transistor Q 1  is set conducting (ON) for Tpwm time, at each Cycle of the PWM, and not conducting otherwise.   The lower transistor command signal (C low ) maintains Q 2  not conducting (OFF) for Td Dead Time before Q 1  is switched ON, all the time while Q 1  is ON, and for Td Dead Time after Q 1  is ON. The state of Q 2  is unimportant in other portions of the cycle, and is preferably OFF.   
           b) In the case where the Current Command direction is negative (FIG.  5 ):
           The lower transistor Q 2  is kept not conducting (OFM) for Tpwm time during each PWM cycle, and conducting (ON) otherwise.   The upper transistor command signal (C high ) maintains Q 1  not conducting for Td Dead time before Q 2  is switched ON, all the time while Q 2  is ON, and for Td Dead time after Q 2  has been switched OFF. The state of Q 1  is unimportant in other portions of the cycle, and is preferably OFF.   
               

   The purpose of the inventive system is to apply to the load an average voltage value equal to the Voltage Command. Another purpose is also to maintain the real current equal to the Command Current. As will be shown here, an additional benefit of the inventive system is that in the case where the real current has opposite direction to the Command Current, then the PWM pattern of this invention will force the real current to reverse direction, in-line with the command direction, at the maximum possible speed of the system, which is defined by the DC bus voltage and the load. 
   In a first case, in which the Current Command direction is Positive and the Real Current direction is positive, while Q 1  is switched ON, the load  9  is directly connected to the positive pole of the power supply, and the voltage applied to the load  9  is equal to that of the Power Supply (E) positive voltage. 
   During the time where Q 1  is switched OFF, the Real Current is positive, and the necessary continuity of current will force the current to flow toward the load  9 . Since the path through Q 1  is not available, then current will flow through diode D 2 , and the voltage applied to the load  9  will be that of the negative pole of the power supply. It is remarkable that this situation is not dependent on the state of Q 2 . As a result, the voltage applied to the load during the OFF phase of the upper transistor will be always zero (i.e. at the potential of the negative pole of the power supply). 
   Thus, in the case where both Real Current and Current Command are Positive, the load will be brought to a voltage E for Tpwm time and to voltage zero for Tcycle−Tpwm time. As a result, the average output voltage will be
 
 V=E*Tpwm/T cycle= Vc 
 
   In a second case, in which the Current Command direction is positive, and the real Current direction is negative, while Q 1  is switched ON, the load  9  is directly connected to the positive pole of the power supply, and the voltage applied to the load  9  is equal to that of the Power Supply (E) positive voltage. 
   During the OFF time of Q 1 , current will flow through diode D 1  in order to maintain continuity. In that case, the voltage will be equal to the Power Supply voltage. As a result, as long as Real Current is negative, then the system will directly apply the power supply to the load, thus causing the Real Current to rapidly decrease in absolute value, until it reaches zero or negative value. In that case, the average voltage will be equal to the value of the power supply voltage, which is desirable in order to return the Real Current direction to the Current Command direction. 
   In a third case, in which the Current Command direction is negative and the Real Current direction is also negative, while the lower transistor Q 2  is switched ON, then zero voltage is applied to the load, since Q 2  directly connects the load to the negative pole of the power supply. While the lower transistor Q 2  is switched off, the Real Current is flowing from the load  9  to the system, then the current continuity will cause the current to flow through the diode D 1 , and thus the voltage applied to load  9  will be the Power Supply voltage (E). It is remarkable that the voltage applied to the load  9  is not dependent on the state of the upper transistor Q 1 . 
   Thus, in the case where both Real Current and Current Command are negative, the load will be brought to a voltage E for Tpwm time and to voltage zero for Tcycle−Tpwm time. As a result, the average output voltage will be
 
 V=E*Tpwm/T cycle= Vc 
 
   In a fourth case, in which the Current Command direction is negative but the Real Current direction is positive, while the lower transistor Q 2  is switched ON, then zero voltage is applied to the load, since Q 2  directly connects the load  9  to the negative pole of the power supply. When the lower transistor Q 2  is switched OFF, then the continuity of current will force it through the diode D 2 , and thus the voltage applied to load  9  will be zero. 
   As a result, as long as Real Current is positive, then the system will directly apply a zero voltage to the load, thus causing the Real Current to rapidly decrease in absolute value, until it reaches zero or negative value. In that case, the average voltage will be equal to a zero voltage, which is desirable in order to return the Real Current direction to the Current Command direction. 
   Resuming all cases, if the Current Command and Real Current have the same direction, then the Average Output Voltage is exactly equal to the Voltage command. 
   If the Current Command and Real Current have opposite directions, then the system sets the maximum voltage possible in the appropriate direction, that will return the real current in the direction of the command. 
   The system thus allows the control of the Load Voltage, with an additional benefit for the Current Control. Most modern systems aim to control the load current as a first priority and the voltage as a second priority. A system designed according to this invention will improve the precision of the control of both the Voltage and Current output. 
   In  FIG. 6 , there is shown a preferred embodiment for implementation of the TCG  26 . TCG  26  comprises a comparator  28 , a pair of OFF delay circuits  30 ,  32 , and logic gates  34 ,  36 ,  38 ,  40 ,  42 , and  44 . 
   A voltage command signal  22  (Vc), and saw tooth (St) signal  47  from a saw tooth signal generator  48 , are input to comparator  28 . If the voltage command signal  22  exceeds the saw tooth signal  47 , then the output signal  54  of the comparator is a logic level 1. Otherwise the comparator output signal  54  is a zero logic level. This technique is a standard one for generating a PWM signal (see Vithayathil reference in Background). The output signal  54  of comparator  28  is now a pulse. The ON time of that pulse is in linear relation with the value of the voltage command signal  22  (Vc). The ON time of the pulse is represented herein by Tpwm, and Tcycle represents the period of the saw tooth signal. 
   The voltage command signal  22 , the saw tooth signal  47  and comparator  28  may be analog or digital signals or devices. If digital then voltage command (Vc) signal  22  and saw tooth signal  47  will be held in registers (in a programmable logic device for example), and the comparator  28  will be a digital comparator. In the case of analog devices, voltage command and saw tooth signals will be the electrical level of the respective input lines. In all cases, comparator  28  will have a digital signal output, i.e. with only two values. 
   A Current Command direction signal  24  is also input to the TCG  26 , in the form of a logic signal. In the preferred embodiment, the logic level is set to 1 if the current is positive. 
   Each of the pair of OFF delay circuits  30 ,  32  generates a respective output signal  50 ,  52  that has logic level 1 when its input has logic level 1, but when its input logic level returns to zero, its output signal  50 ,  52  returns to a logic level 0 only after a time delay Td. These delayed output signals  50 ,  52  are input at the inverting inputs of respective AND gates  36  and  38 . By its operation, OFF Delay circuits  30 ,  32  together with AND gates  36 ,  38  provide protection against simultaneous conduction of transistors Q 1 -Q 2 . 
   Both Current Command direction signal  24  and output signal  54  of comparator  28  are input to an AND logic gate  34 . The result is that the output signal  54  of comparator  28  passes through AND gates  34  and  36  only if Current Command direction signal  24  is positive, and the OFF delay circuit output  30  is inverted to logical 1 (from logic 0). This occurs only when a sufficient time interval has elapsed since the prior ON operation of Q 2 . 
   If this is the case, the activating gate signal C high  is transmitted to Gate Driver  16  in order to switch on transistor Q 1 . If however, the elapsed time since the last OFF switching operation of lower transistor Q 2  is shorter than a safe Dead Time delay Td, then AND gate  36  blocks transmission of the comparator  28  output signal  54 , so that Gate Driver  16  does not switch transistor Q 1  ON. 
   Normally, if the Current Command direction signal  24  is positive, and sufficient time has elapsed since the switching OFF of transistor Q 2 , then transistor Q 1  will be switched ON for a time interval of Tpwm, and switched OFF during the rest of the cycle. 
   In the case that the Current Command direction signal  24  is negative, the AND gate  34  has a zero logic output, and transistor Q 1  is never switched ON. 
   In symmetrical fashion, the switching operation of transistor Q 2  is controlled by AND logic gates  38 ,  40 . However, AND gate  40  receives the inverted output of the comparator  28  output signal  54  via inverter  42 , and the inverted Current Command direction signal  24  via inverter  44 . 
   Normally, if the Current Command direction signal  24  is negative, then transistor Q 2  will be switched OFF for a time interval of Tpwm, and switched ON during the rest of the cycle, if sufficient time has elapsed since the switching OFF of transistor Q 1 . 
   In the case that the Current Command direction signal  24  is positive, the AND gate  40  has a zero logic output, and transistor Q 2  is never switched ON. 
   In  FIG. 7 , there is shown an implementation of the TCG  26  using a microprocessor  60 . Microprocessor  60  enables control of switching signals used to control the ON time and OFF time of switching devices Q 1  and Q 2 . In this fashion, a PWM switching pattern can be produced according to the method of the invention. 
     FIG. 8  shows a possible flow chart for a microprocessor program, that will create a PWM pattern according to the invention. 
   In block  70 , the system initiates a new PWM cycle, and in block  72 , the Voltage Command Vc is received. The PWM pulse width Tpwm is calculated in block  74 , and the Current Command direction is received in block  76 . In decision block  78 , if the Current Command direction is positive, the operation continues in block  80 , where the state of transistor Q 2  is checked. If Q 2  is OFF, in block  82 , the delay from the last switch off time is recorded and represented as “Tloff”. If transistor Q 2  was in the ON state, in block  84  a command is generated to switch transistor Q 2  OFF for the remaining time of the present cycle, and the switch OFF time of transistor Q 2  is recorded in block  86  and represented as “Tloff”. In block  88 , following either of blocks  82  or  86 , the switching ON and OFF times of transistor Q 1  are calculated, and represented as “Thon” and “Thoff”. 
   In block  90 , the OFF time of transistor Q 2  is checked, to see if the time remaining until the switching ON of transistor Q 1  is greater than the Dead Time delay Td. If it is not greater, then in block  92  the switching ON time of transistor Q 1  is recalculated in order to comply with the Dead Time delay Td. Following block  90  or  92 , in block  94  the calculated switching ON time of transistor Q 1  is checked to see if it is prior to the calculated switching OFF time of transistor Q 1 , as calculated in block  88 . 
   If this condition is verified, then a command is generated in block  96  to switch ON transistor Q 1  at time “Thon”, and to switch it OFF at time “Thoff”. If this condition is not verified, then in block  98 , a command is generated to keep transistor Q 1  OFF during the remaining time of the cycle. 
   In block  100 , the software routine is completed, and will be re-activated at the beginning of the next PWM cycle. 
   Returning to decision block  78 , in the case where the Current Command direction is negative, then symmetric operation of the system commences with block  102 , and all of the remaining steps are performed, by replacing transistor Q 2  by Q 1 , and interchanging for Q 2  the ON state and OFF state when performing the step described. 
   Having described the invention with regard to certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fill within the scope of the appended claims.