Abstract:
Dead time compensated complementary pulse width modulation (PWM) signals are derived from a PWM generator by first applying time period compensation to the PWM generator signal based upon the direction of current flow in an inductive load being controlled by the PWM generator. Dead time is then applied to the compensated PWM generator signal for producing complementary dead time compensated PWM signals for controlling power switching circuits driving the inductive load.

Description:
TECHNICAL FIELD 
   The present disclosure relates to digital devices having digital pulse width modulation (PWM) capabilities, and more particularly, to digital devices having dead time compensation of the PWM waveforms when controlling inductive loads such as motors. 
   BACKGROUND 
   Pulse width modulation (PWM) controllers are effectively being used to control voltage levels in power supplies and to control rotational speed and direction of motors. For motor control, a direct current (DC) power source is switched on and off at various rates to produce an alternating current (AC) waveform that is used to control the speed and rotational direction of the motor. Referring to  FIG. 1 , depicted is a schematic block diagram of a pulse width modulation (PWM) power controller  102  and a schematic connection diagram of a power driver circuit  106 . Typically a “half-bridge” power transistor configuration (power transistors  110  and  112 ), e.g., power MOSFET, SCR, triac, etc., are controlled by two complementary PWM signals, PWMH  220  and PWML  222 , only one of which is on at any time. To insure that there can be no “on” overlap of the two complementary PWM signals, “dead time” is introduced between these two complementary PWM signals, PWMH  220  and PWML  222  (generally refer to  FIG. 2 ). The “dead time” function insures that no current spikes are generated when one transistor is turned off while the other transistor is turned on (e.g., no “on” overlap that would cause a short circuit between +V and −V). While dead-time circuits are a required function, they create their own “issues.” The biggest problem with dead-time circuits is that the resultant transistor waveforms are distorted by the inductance of the motor windings. This waveform distortion causes the controlled motor to run roughly at slow motor speeds. A dead-time compensation circuit may be used to correct for the distortion caused by the motor induction. 
   Referring to  FIG. 3 , depicted are schematic block diagrams of specific example prior technology embodiments of digital PWM generation systems for generating complementary PWM waveforms having dead time compensation. These prior technologies for dead-time compensation select between two different PWM duty cycles depending on the direction of current flow in the motor load. As shown in  FIG. 3(   a ), a simple dead time compensation circuit uses a multiplexer  306  to select one of two duty cycle values from different duty cycle control registers  302  and  304 , depending on the state of a device input  340  (determined by direction of motor current flow). This prior technology embodiment requires a large amount of software “overhead” to constantly compute and load new duty cycle values into the device&#39;s duty cycle control registers  302  and  304 . As shown in  FIG. 3(   b ), another technology uses a “brute force” method that requires a hardware adder/subtractor  310  that automatically computes the desired compensated dead time values based upon the direction of current flow through the controlled motor and adds/subtracts from the duty values stored in the duty cycle control registers  302  and  304 . The hardware implementation of  FIG. 3(   b ) is expensive and complex to implement. 
   SUMMARY 
   Therefore a need exists for a simple, cost effective and efficient way to implement dead time compensation of complementary PWM signals that are appropriate for both directions of load current flow, e.g., the load may be a motor or any other type of inductive load. 
   Therefore according to teachings of this disclosure, dead time compensated complementary pulse width modulation (PWM) signals are derived from a PWM generator by first applying time period compensation to the PWM generator signal based upon the direction of current flow in an inductive load being controlled by the PWM generator. Dead time is then applied to the compensated PWM generator signal for producing complementary dead time compensated PWM signals for controlling power switching circuits driving the inductive load. 
   According to another specific example embodiment of this disclosure, a method of providing dead time compensation to a pulse width modulation (PWM) signal and generating complementary PWM signals therefrom comprises the steps of: compensating time periods of a pulse width modulation (PWM) signal, wherein the step of compensating the time periods of the PWM comprises the steps of: stretching the time periods of the PWM signal if a current of an inductive load is flowing in a first direction; or shrinking the time periods of the PWM signal if the current of the inductive load is flowing in a second direction; adding dead times to the compensated time periods of the PWM signal; and generating complementary PWM signals from the dead time compensated PWM signal. 
   According to another specific example embodiment of this disclosure, an apparatus for providing dead time compensation to a pulse width modulation (PWM) signal and generating complementary PWM signals therefrom comprises: a first circuit for compensating time periods of a pulse width modulation (PWM) signal, wherein the time periods are stretched when an inductive load current is flowing in a first direction and shrunk when the inductive load current is flowing in a second direction; a second circuit for adding dead times to the compensated time periods of the PWM signal; and a third circuit for generating complementary PWM signals from the dead time compensated PWM signal. 
   According to yet another specific example embodiment of this disclosure, an apparatus for providing dead time compensation to a pulse width modulation (PWM) signal and generating complementary PWM signals therefrom comprises: a first delay circuit ( 402 ) for delaying a PWM signal ( 450 ) and having an output of a delayed PWM signal ( 452 ); a first edge detector circuit ( 404 ) for generating a start signal for each logic level transition of the PWM signal ( 450 ); a first timer ( 406 ) coupled to the first edge detector circuit ( 404 ), wherein the first timer ( 406 ) generate a compensation time period each time the PWM signal ( 450 ) makes a logic level transition, the first timer ( 406 ) has a first output of a compensation time period signal ( 454 ) and a second output of a complementary compensation time period signal ( 456 ); an OR gate ( 408 ) having a first input coupled to the delayed PWM signal ( 452 ), a second input coupled to the first output of the compensation time period signal ( 454 ) and an output of a stretched PWM signal ( 458 ); an AND gate ( 410 ) having a first input coupled to the delayed PWM signal ( 452 ), a second input coupled to the second output of the complementary compensation time period signal ( 456 ) and an output of a shrunk PWM signal ( 460 ); a multiplexer ( 412 ) having a first input coupled to the stretched PWM signal ( 458 ) and a second input coupled to the shrunk PWM signal ( 460 ), wherein an output of the multiplexer ( 412 ) is coupled to the first input when a current of an inductive load is flowing in a first direction and to the second input the current of the inductive load is flowing in a second direction, whereby the output of the multiplexer ( 412 ) generates a compensated PWM signal ( 462 ); a second delay circuit ( 422 ) for delaying the compensated PWM signal ( 462 ) and having an output of a delayed compensated PWM signal ( 464 ); a second edge detector circuit ( 424 ) for generating a start signal for each logic level transition of the delayed compensated PWM signal ( 464 ); a second timer ( 426 ) coupled to the second edge detector circuit ( 424 ), wherein the second timer ( 426 ) generate a dead-time time period each time the delayed compensated PWM signal ( 464 ) makes a logic level transition, the second timer ( 426 ) has an output of a dead time compensated PWM signal ( 466 ); an AND gate ( 414 ) having a first input coupled to the compensated PWM signal ( 462 ), a second input coupled to the dead time compensated PWM signal ( 466 ) and an output of a first complementary dead time compensated PWM signal (PWMH  468 ); and an AND gate ( 416 ) having a first input coupled to the delayed compensated PWM signal ( 464 ), a second input coupled to the dead time compensated PWM signal ( 466 ) and an output of a second complementary dead time compensated PWM signal (PWML  470 ). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  illustrates a schematic block diagram of a pulse width modulation (PWM) power controller and a schematic connection diagram of a power driver circuit; 
       FIG. 2  illustrates timing diagrams for (a) complementary PWM waveforms, (b) complementary PWM waveforms having dead time between the PWM and complementary PWM waveforms, and (c) complementary PWM waveforms having dead time compensation; 
       FIG. 3  illustrates schematic block diagrams of prior technology digital PWM generation systems for generating complementary PWM waveforms having dead time compensation; 
       FIG. 4  illustrates a schematic block diagram of a digital PWM generator for generating complementary PWM waveforms having dead time compensation, according to a specific example embodiment of this disclosure; and 
       FIG. 5  illustrates schematic timing diagrams of signal waveforms generated during operation of the PWM generator shown in  FIG. 4 . 
   

   While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
   DETAILED DESCRIPTION 
   Referring now to the drawing, the details of specific example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     FIG. 1  illustrates a schematic block diagram of a pulse width modulation (PWM) power controller and a schematic connection diagram of a power driver circuit. The PWM power controller  102  may comprise a digital device  104  having complementary PWM signal generation and power driver circuits  106  used to drive a load, e.g., motor, inductive heater, etc. The power driver circuits  106  may comprise power driver transistors  110  and  112  that are used to alternately connect the load (not shown) to either +V (transistor  110  on) or −V (transistor  112  on). Both of the transistors  110  and  112  cannot be on at the same time, otherwise current shoot-through can occur which can be very destructive to the power circuits. Turning the transistors  110  and  112  on and off are controlled by the complementary PWM signals  220  and  222 , respectively, from the digital device  104 . The transistors  110  and  112  represent a driver circuit  106   a  for a single phase of an inductive load. For a multi-phase inductive load, e.g., a polyphase motor, a pair of the transistors  110  and  112  would be used for each of the phases, e.g., three phases. 
   Referring to  FIG. 2 , depicted are timing diagrams for (a) complementary PWM waveforms, (b) complementary PWM waveforms having dead time between the PWM and complementary PWM waveforms, and (c) complementary PWM waveforms having dead time compensation. The PWML waveform signal  222   a  will be at a logic level low (“0”), e.g., controlling transistor  112  off, and the PWMH waveform signal  220   a  is at a logic level high (“1”), e.g., controlling transistor  112  on, and visa-versa. Therefore, these two signals  220   a  and  222   a  are “complementary” or “inverse” to each other so as to insure that only one of the transistors  110  and  112  is on at a time. 
   To further guarantee that only one of the transistors  110  and  112  is on at a time, dead times  202  are introduced to at least one of the PWM waveform signals  220   b  and  222   b . The dead times  202  effectively prevent current shoot-through occurring as one of the transistors  110  or  112  turns on and the other turns off, similar to a break before make switch. However, introducing dead times  202  creates voltage and current distortion of the power to the motor. This is because motors are inductive and therefore current continues to flow in the motor windings during the dead-time period. Thus the applied voltage to the motor is distorted, causing current distortion in the motor. This undesirable current distortion in the motor may be compensated for by pre-distorting the PWM waveforms depending upon the motor current direction. Pre-distortion of the PWM waveforms may be accomplished by stretching one of the PWM waveform signals, e.g., PWM waveform signal  222   c , represented by the numeral  208 , and shrinking the other one, e.g., times  204  and  206  of PWM waveform signal  220   c , as more fully described herein. 
   Referring now to  FIGS. 4 and 5 , wherein  FIG. 4  depicts a schematic block diagram of a digital PWM generator for generating complementary PWM waveforms having dead time compensation, according to a specific example embodiment of this disclosure, and  FIG. 5  depicts schematic timing diagrams of signal waveforms generated during operation of the PWM generator shown in  FIG. 4 . According to the teachings of this disclosure as applied to the specific example embodiment of  FIG. 4 , the PWM signal  462  is a “pre-distorted” version of the PWM signal  450 , and the PWMH signal  468  and PWML signal  470  comprise complementary pre-distorted with dead time (blanking) signals of the original PWM signal  450 , as more fully described hereinafter. One of ordinary skill in the art of digital circuit design and having the benefit of this disclosure could design other logic circuit configurations that would still be within the spirit and scope of this disclosure. 
   A PWM signal  450  from the PWM generator (e.g., part of the digital device  104 ) is applied concurrently to the inputs of a one clock delay circuit (register)  402  and an edge detector  404 . The one clock delay circuit  402  delays the PWM signal  450  by one clock time period to produce a delayed PWM signal  452  which is a replica of the PWM signal  450  that has been delayed by one clock period. Whenever the edge detector  404  detects a logic level transition, e.g., low-to-high or high-to-low, the Q output of the edge detector  404  will start a dead time compensation timer  406 , e.g., a one-shot timer. The on-time duration of the dead time timer  406  may be programmable through digital input  472  to provide a desired amount of dead-time compensation, e.g.,  204  and  206 , (see  FIG. 2 ). Upon receiving this start signal from the edge detector  404 , the dead time timer  406  produces a logic high (“1”) on its Q-output and a logic low (“0”) on its Q/not-output, as timing signals  454  and  456 , respectively. The delayed PWM signal  452  is logically or&#39;ed with the timing signal  454  in OR gate  408  to produce a “stretched” PWM signal  458 . The delayed PWM signal  452  is logically and&#39;ed with the timing signal  456  in AND gate  410  to produce a “shortened” PWM signal  460 . The one clock period time delay of the delayed PWM signal  452  insures that the PWM signal  452  and the output timing signals  454  and  456  arrive at the proper times to the inputs of the OR gate  408  and the AND gate  410 , respectively. 
   A multiplexer  412  selects, depending on the correction input signal  440  logic level, either the “stretched” PWM signal  458  on input a, or the “shortened” PWM signal  460  on input b to produce a stretched or shortened PWM signal  462 , respectively. The correction input signal  440  logic level is determined by the direction of current flow in the motor. The stretched or shortened PWM signal  462  is then sent to inputs of the AND gate  414  and the inverter  418 . An output from the inverter  418  is applied concurrently to the inputs of a one clock delay circuit (register)  422  and an edge detector  424 . The one clock delay circuit  422  delays the PWM signal from the inverter  418  one clock time period to produce a delayed PWM signal  464  which is an inverted replica of the stretched or shortened PWM signal  462  that has been delayed by one clock period. The stretched or shortened PWM signal  462  is the “pre-distorted” version of the original PWM signal  450  prior to the application of a standard dead-time blanking function, as discussed more fully hereinafter. 
   Whenever the edge detector  424  detects a logic level transition, e.g., low-to-high or high-to-low, the Q output of the edge detector  424  will start a dead time timer  426 , e.g., a one-shot timer. The on-time duration of the dead time timer  426  may be programmable through digital input  474  to provide the desired dead time  202  (see  FIG. 2 ). Upon receiving this start signal from the edge detector  424 , the dead time timer  426  produces a logic low (“0”) on its Q/not-output, as timing signal  466 . The stretched or shortened PWM signal  462  is logically and&#39;ed with the timing signal  466  in AND gate  414  to produce a compensated dead time PWMH signal  468 . The delayed PWM signal  464  is logically and&#39;ed with the timing signal  466  in AND gate  416  to produce a compensated dead time PWML signal  470 . The one clock period time delay of the delayed PWM signal  464  insures that the PWM signal  464  and the output timing signal  466  arrive at the proper times to the inputs of the AND gate  416 . The dead-time  202  is thereby integrated with the stretched or shortened (compensated) PWM  462  signal to produce the PWMH output signal  468 , and the PWML output signal  470 . 
   While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.