Abstract:
The present invention relates to a system and method for compensating the voltage distortion and minimizing the available voltage range loss both caused by switching dead-time of solid switch components in power conversion devices. As a result, the power quality supplied from the power conversion devices can be improved and the output voltage of such devices can be increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit under 35 U.S.C. § 119(e) of the currently co-pending U.S. Provisional Patent Application, entitled “DEAD-TIME COMPENSATION WITH NARROW PULSE ELIMINATION IN SOLID-SWITCH DEVICES,” converted on Nov. 20, 2002 from U.S. Utility patent application Ser. No. 09/683,202, filed Nov. 30, 2002, via petition filed on Jun. 26, 2002, such applications being hereby incorporated by reference in their entireties. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to dead-time compensation of solid power switch components in electronic devices such as inverters and converters. Specifically, the preferred embodiments of the present invention relate to means that compensate the voltage distortion and minimize the available voltage range loss of electronic devices both caused by switching dead-time of solid power switches in such devices.  
           [0004]    2. Description of the Related Art  
           [0005]    In modern power inverter/converter technology, a switching power device, such as an insulated gate bipolar transistor (IGBT), is often utilized to convert direct-current (DC) power into alternating-current (AC) power and pulse width modulated (PWM) method is widely adopted in switching pattern control. FIG. 1A shows part of a one-phase configuration of a DC-to-AC power converter (inverter)  100 . Although not shown in the figure, it should be understood by those skilled in the art that the current la is going out to a load having one end connected to node a; with the other end of the load connected to another pair of transistors and diodes that are arranged in a similar manner to solid power switch components  110 ,  130  and diodes  120 ,  140 . The switches  110  and  130  can be any solid-state transistors, such as IGBTs.  
           [0006]    In a power conversion application such as shown in FIG. 1A one of the big concerns is the harmonic voltage generated from the inverter  100 . The harmonic voltage is caused by the non-linearity behaviors of the switching operation of the inverter  100 . A major non-linearity is introduced by the dead-time required for the solid power switches  110  and  130 . This is because it is well known that there is no ideal switching component that can turn on and turn off instantaneously. To guarantee that both switches  110  and  130  in an inverter such as the one shown in FIG. 1A never conduct simultaneously a small blanking time, conventionally called dead-time, is inserted between the gate signals of the turning-off and turning-on switches to avoid a so-called shoot-through of the DC power source. This dead-time is used to delay for a short period of time for the coming-on switch to be turned on from the moment when the coming-off switch is turned off (i.e., when the falling edge of the coming-off switch occurs). FIG. 1B shows the dead-time between the off edge of the switch or IGBT  130  and the on edge of the switch or IGBT  110 . FIG. 1C shows the inverter output voltage V an  in a PWM cycle with gate drive signal as in FIG. 1B at the conditions of output current la≧0 and la&lt;0. Because of the dead time shown in FIG. 1B, a voltage waveform distortion is induced as shown in FIG. 2. In the figure, curve  211  shows the average output voltage waveform from an inverter, such as inverter  100  shown in FIG. 1A, with ideal switching components having no dead-time requirement. As a result, the curve  211  has a very good sinusoidal waveform. Curve  212 , however, shows the average output voltage from an inverter with dead time added as required for actual switching components. As seen, curve  212  is severely distorted around the zero crossing (of the phase current la shown by curve  213 ) when compared to the ideal sinusoidal waveform of curve  211 . Curve  212  also shows that the inverter output AC voltage is lower than the nominal voltage (of ideal curve  211 ) in the half cycle corresponding to positive half cycle of current la shown in curve  213 ; whereas, the inverter output AC voltage is higher than the nominal voltage in the voltage half cycle corresponding to negative half cycle of the phase current la. Due to the dead-time effect, the voltage distortion becomes more severe at the point of current polarity change, i.e., zero crossing.  
           [0007]    The voltage loss (or gain) between the nominal or ideal voltage curve  211  and the distorted voltage curve  212  in FIG. 2 can be compensated by a compensation voltage waveform as shown by Curve  214 . The compensation voltage  214  is the amount of voltage loss, as defined by the difference between the nominal voltage  211  and the distorted voltage  212 . However, the attention should not only be put to the exact amount of voltage loss compensation but also to the right moment of the compensation. FIG. 3 shows the inverter voltage distortion with correct voltage amount compensation but not at the right moment. Again, curve  311  is a copy of the phase current la waveform  213  shown in FIG. 2. Curve  312  is a copy of the distorted voltage waveform  212  shown in FIG. 2 without any dead-time compensation. Curve  313  is a copy of the compensation voltage  214  shown in FIG. 2. Curve  314  shows the resulting voltage waveform which is even more distorted than the distorted voltage waveform  212  because the compensation was not done at the right moment. For a three-phase inverter with three legs, each as shown in FIG. 1A, the voltage distortions as shown in curve  212  of FIG. 2 and curve  314  in FIG. 3 will generate severe 5 th  and 7 th  harmonics that will deteriorate the power quality for a three phase power system.  
           [0008]    Besides the voltage distortion effect, the dead-time has another negative impact on the performance of the inverter  100 . This dead-time takes part of a PWM cycle time and reduces the portion in a PWM cycle used to control IGBT&#39;s on or off time. In other words, when the dead time in a PWM cycle gets larger, the available time range for IGBT&#39;s on or off gets smaller, and a smaller output voltage range can be obtained from the inverter  100  with certain DC voltage. This dead-time effect is illustrated in FIGS. 1B and 1C. As shown in FIG. 1C, because of the dead time effect, V an  when la≧0 has a smaller voltage width than V an  when la&lt;0. Thus, the average voltage magnitude of V an  when la≧0 is less than the average voltage magnitude of V an  when la&lt;0. The following example is used to further explain this dead time effect.  
           [0009]    Let&#39;s assume the inverter PWM cycle is 100 μs (microseconds) and the maximum or nominal AC output voltage from the inverter with 0 μs dead-time is 1.0. Table 1 shows the maximum inverter output voltages when various amounts of dead time is added.  
                                   TABLE 1                                       Dead time (μs)   2.0   4.0   6.0           Per unit output voltage   0.96   0.92   0.88                      
 
           [0010]    For instance, when the added dead time is 2.0 μs, the actual amount of dead time in a PWM cycle of 100 μs is twice the amount of the dead time. This is because a dead time is added to each side of the pulse in the PWM cycle, as shown in FIG. 1B. Thus, with 4 μs out of 100 μs attributed to dead time in a PWM cycle, the resulting output voltage can only be obtained from the remaining 96 μs at 0.96.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    The above background introduction shows that switching dead-time of solid power switch components, such as IGBTs, in an electronic device, such as an electronic stationary inverter, can cause output voltage distortion and voltage utilization reduction for the inverter from a direct-current (DC) power source. The inventors have found that the time delay (phase lagging) between the actual current changing polarity and the compensation voltage changing polarity is very important to effectively compensate the voltage distortion caused by the dead-time. Furthermore, not only the exact amount of voltage drop due to dead-time needs to be compensated to obtain high quality power, but also the voltage drop needs to be compensated at the right moment.  
           [0012]    Accordingly, the preferred embodiments of the present invention provides a system and method of “quadrant PWM cycle sampling” to compensate the dead-time of solid power switch components such as IGBTs.  
           [0013]    The preferred embodiments of the present invention also provide a system and method for shortening the compensation delay time from the moment when the output current of a power converter changes its polarity to minimize the voltage distortion around the time point of current polarity change.  
           [0014]    The preferred embodiments of the present invention also provide a system and method of “narrow pulse elimination” to expand the voltage utilization range for solid-switch power converters with certain DC voltage.  
           [0015]    The preferred embodiments of the present invention also provide a system and method for improving the power quality supplied from solid-switch power converters and increasing the output AC voltage magnitude range of such power converters.  
           [0016]    Additional aspects and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the disclosure. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]    The preferred embodiments are illustrated by way of example and not limited in the following figures, in which:  
         [0018]    [0018]FIG. 1A depicts a circuit for power conversion application to which the present invention is beneficial;  
         [0019]    [0019]FIG. 1B depicts the on time, off time, and dead time characteristics of the switching devices shown in FIG. 1A;  
         [0020]    [0020]FIG. 1C depicts the output voltage waveform of the power conversion application shown in FIG. 1A;  
         [0021]    [0021]FIG. 2 depicts the dead time voltage distortion and voltage compensation for the power conversion application shown in FIG. 1A;  
         [0022]    [0022]FIG. 3 depicts the effect of the compensation time alignment between the current and compensation voltage waveforms for the power conversion application shown in FIG. 1A;  
         [0023]    [0023]FIG. 4 depicts a dead time compensation architecture in accordance with an embodiment of the present invention;  
         [0024]    [0024]FIG. 5 depicts the timing diagram used for the dead time generator and compensator shown in the dead time compensation architecture depicted in FIG. 4;  
         [0025]    [0025]FIG. 6 depicts the results of the narrow pulse elimination technique for short incoming PWM pulse width when sgn(i)&lt;0 (i.e., current is negative), in accordance with an embodiment of the present invention;  
         [0026]    [0026]FIG. 7 depicts the results of the narrow pulse elimination technique for long incoming PWM pulse width when sgn(i)≧0 (i.e., current is not negative), in accordance with an embodiment of the present invention;  
         [0027]    [0027]FIG. 8 depicts the dead time compensation algorithm provided by the dead time compensation architecture depicted in FIG. 4 for the first half PWM cycle, in accordance with an embodiment of the present invention;  
         [0028]    [0028]FIG. 9 depicts the dead time compensation algorithm provided by the dead time compensation architecture depicted in FIG. 4 for the second half PWM cycle, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Reference is now made in detail to various preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. FIG. 4 shows a dead-time compensation architecture  400  for use with an electronic switching device such as the inverter  100  shown in FIG. 1, in accordance to one embodiment of the present invention. The architecture  400  comprises a switching control module  450  to the gates of switches  110  and  130 , respectively, a zero crossing detector  420 , and a low pass filter  410 . The switching control module  450  includes: a dead-time generator and compensator (DTGC)  451 , a ¼ PWM cycle delay component  452 , a time delay component  453 , and outputs TA 1  signal  471  and TA 2  signal  472  that are connected to the gates of switches  110  and  130 , respectively, of the inverter  100  to control the inverter switching. The architecture  400  receives a feedback of the load current, i.e., the output current, la of the inverter  100  in order to provide dead-time compensation to the inverter switches  110  and  130 .  
         [0030]    As known in the art, the actual load current la of an inverter is not a pure Sine wave as depicted by curve  213  in FIG. 2. Rather, the load current la may be a sinusoidal signal with switching noise added. For instance, the load current la may comprise a fundamental component at 60 Hz with 4 KHz and 8 KHz switching noise components riding on top of the fundamental component. Thus, the low pass filter  410  is used to separate the fundamental component from the switching noise. Because the load current la has more than one frequency component, it may experience a phase shift due to the interaction of the multiple frequency components. Thus, to ensure that the zero crossing detector  420  senses true zero crossings, a time delay component  453  is used as common done in the art to compensate for the phase shift.  
         [0031]    The various components of the architecture  400  is synchronized by a synchronization PWM signal SYNCH_ 1   462  from a pulse width modulator or microprocessor. The DTGC  451 , synchronized by the SYNCH_ 1  signal, creates the TA 1  signal  471  and TA 2  signal  472  based on the input SIG_ 1  signal  461  and the detected sgn(i) current polarity signal  454 . The SIG_ 1  PWM signal  461  also comes from the pulse width modulator or microprocessor that generates the SYNCH_ 1  signal  462 . The SYNCH_ 1  has twice the PWM carrier frequency, i.e., it has a cycle that is half of the PWM cycle. The sgn(i) signal  454  tracks the polarity changes in the current feedback of the load current of the inverter  100  (FIG. 1). The TA 1  and TA 2  signals at  471  and  472 , which have been properly offset for the necessary dead-time according to the current polarity, are sent to the gate drivers of switches or IGBTs  110  and  130  (FIG. 1).  
         [0032]    [0032]FIG. 5 shows the time diagram of the DTGC  451  and other components shown in FIG. 4. TA 1 + and TA 2 + are the waveforms for TA 1  and TA 2  signals when the current polarity of the inverter load current la is positive (la≧0). TA 1 − and TA 2 − are the waveforms for TA 1  and TA 2  signals when the current polarity of la is negative (la&lt;0). For the figure: DELAY_ 1  is equal to ¼ of PWM cycle of the SIG_ 1  signal, i.e., ½ of SYNCH_ 1  signal cycle; DELAY_ 1  is the delay generated by the time delay component  453 ; t 0  corresponds to the start of a first cycle of the SYNCH_ 1  PWM synchronization signal  462 ; ts 1  corresponds to the time of a first current polarity detection; t 1  corresponds to the time of the rising edge of SIG_L PWM signal  461 ; t 2  corresponds to the start of a second cycle of the SYNCH_ 1  PWM synchronization signal (as mentioned earlier, the SYNCH_ 1  signal  462  has a cycle that is half that of the SIG_ 1  PWM signal  461 ); t 3  corresponds to the falling edge of the SIG_ 1  PWM signal  461 .  
         [0033]    From the timing diagram in FIG. 5, the compensation scheme according to an embodiment of the present invention provides a predetermined time delay, preferably ¼ of the PWM cycle of the SIG_ 1  signal, to change the compensation voltage polarity when the detected current feedback changes its polarity for dead-time compensation. As seen in FIG. 5, the TA 1  and TA 2  signals are delayed by a ¼ of PWM cycle because of the ¼ PWM cycle delay component  452 . For instance, the TA 1  signal waveform  530  for turning on switch  110  (FIG. 1), TA 1 +, at la≧0 is delayed by a ¼ PWM cycle from the rising edge of SIG_ 1  PWM signal waveform  510 . Likewise, the TA 2  signal waveform  560  for turning off switch  130  (FIG. 1), TA 2 −, at la&lt;0 is delayed by ¼ PWM cycle from rising edge of SIG_ 1  PWM signal waveform  510 . The knowledge of the timing for the TA 1 + signal waveform  520  allows for the determination of the timing for the TA 2 + signal  530  because the dead-time requirement is known from the technical specifications of the solid switches  110  and  130  (FIG. 1). FIGS. 8 and 9 show the algorithm or process for generating the compensating scheme and associated time diagram in FIG. 5 at the first and second halves of the PWM cycle of the SIG_ 1  signal. For the first half of the PWM cycle beginning at the start (t 0 ) of a cycle of the SYNCH_ 1  synchronization signal  462 , blocks S 1 -S 5  and S 8 -S 9  of FIG. 8 provide explanation to actions to be done at various different time points such as t 0 , ts 1 , and t 1 . For the second half of the PWM cycle beginning at the start (t 2 ) of a second cycle of the SYNCH_ 1  synchronization signal  462 , blocks S 13 -S 17  and S 20 -S 21  of FIG. 9 provide explanation to actions to be done at various different time points such as t 2 , ts 2 , and t 3 .  
         [0034]    According to another embodiment of the present invention, there is also provided a method and means used to reduce the voltage range loss caused by the dead-time. This “narrow pulse elimination” method is explained next also in reference to the time diagram of FIG. 5. First, let assign a value of “T PWM ” for the PWM cycle (period) of the SIG- 1  signal  510  and a value of “WIDTH” to its pulse width. Thus, from FIG. 5, the pulse width of:  
         [0035]    TA 1 +=WIDTH,  
         [0036]    TA 2 +=(T PWM-WIDTH-2*dead-time),  
         [0037]    TA 1 −=(WIDTH-2*dead-time), and  
         [0038]    TA 2 −=(T PWM-WIDTH).  
         [0039]    Also, let minimum pulse width of the switches or IGBTs  110  and  130  (FIG. 1) be W min  to avoid gate driver and/or IGBT mis-triggering. Then, without the technique of narrow pulse elimination of the present invention, the possible range of the pulse width of the SIG_ 1  PWM signal  510  is from (W min +2*dead-time) to (T PWM -2*dead-time-W min ) to accommodate the dead-time requirement. As a result, the dead-time dramatically reduces the usable range of the PWM pulse width of the SIG_ 1  signal  510 .  
         [0040]    According to an embodiment of the present invention, the “narrow pulse elimination” scheme can maximize the usable range of the PWM pulse width. With this compensation scheme, the usable range of the PWM pulse width is from W min  to (T PWM -W min ), and therefore the power source can be fully utilized. FIGS. 8 and 9 show the algorithm or process for the “narrow pulse elimination” scheme in accordance with one embodiment of the present invention. For the first half of the PWM cycle beginning at the start (t 0 ) of a cycle of the SYNCH_ 1  synchronization signal  462 , blocks S 6 -S 7  and S 10 -S 12  provide explanation to actions to be done for the “narrow pulse elimination” scheme. Likewise, for the second half of the PWM cycle beginning at the start (t 2 ) of a next cycle of the SYNCH_ 1  synchronization signal  462 , blocks S 18 -S 19  and S 22 -S 24  provide explanation to actions to be done for the “narrow pulse elimination” scheme. FIGS. 6 and 7 show examples of waveforms of narrow pulse elimination. It should be noted that the such scheme continues to provide the desirable voltage output but avoids the possibility of mis-triggering the IGBTs  110  and  130  (FIG. 1).  
         [0041]    Consequently, the effect of dead-time is removed by the technique of narrow pulse elimination. For example, if T PWM =100 μs, dead-time=2.5 μs, W min =1.0 μs, without narrow pulse elimination technique, the usable pulse width range for the SIG_ 1  PWM signal  510  is from 6 μs to 94 μs. With narrow pulse elimination technique, the usable pulse width range is from 1 μs to 99 μs. Therefore the gain of the voltage utilization is ( 99 - 1 )/( 94 - 6 )=1.11. Also, the output voltage magnitude ratio is dramatically improved as shown below,  
         [0042]    Ratio=94/6=15.7 if without the narrow pulse elimination technique,  
         [0043]    Ratio=99/1=99 if with the narrow pulse elimination technique.  
         [0044]    This ratio is very important for variable speed drive (V/f) application because it determines the variable speed adjustable range. FIG. 6 and FIG. 7 show examples of waveforms of narrow pulse elimination. Note that narrow pulse elimination still provides the desirable voltage output but avoids the possibility of mis-triggering IGBT.  
         [0045]    Together, blocks S 1 -S 12  in FIG. 8 provide a dead-time compensation algorithm for the first half of PWM cycle that minimizes the voltage range loss and voltage distortion of the inverter&#39;s voltage output. Likewise, together, blocks S 13 -S 24  provide a dead-time compensation algorithm for the second half of PWM cycle that minimizes the voltage range loss and voltage distortion of the inverter&#39;s voltage output. At block  25  in FIG. 9, the algorithms of FIGS. 8 and 9 restart for next PWM cycles of the SIG_ 1  PWM signal  510 .  
         [0046]    Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. Furthermore, any means-plus-function clauses in the claims (invoked only if expressly recited) are intended to cover the structures described herein as performing the recited function and all equivalents thereto, including, but not limited to, structural equivalents, equivalent structures, and other equivalents.  
         [0047]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0048]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.