Abstract:
A buck/boost rectifier. The rectifier is connectable to an alternating current power source and includes an upper bus, a lower bus, an upper rectifier, a lower rectifier, a pulse-width-modulation (PWM) controller, a phase-angle (PA) controller, and a capacitor. The upper rectifier is coupled to the upper bus, and the lower rectifier is coupled in a series-type relationship with the upper rectifier and to the lower bus. The PWM controller is coupled to the lower rectifier and is configured to boost a direct current (DC) voltage output by the rectifier. The PA controller is coupled to the lower rectifier and is configured to buck the DC voltage output by the rectifier. The capacitor is coupled between the upper bus and the lower bus.

Description:
BACKGROUND 
     The invention relates to a buck-boost rectifier, specifically a three-phase pulse width modulated (PWM) rectifier incorporating both buck and boost properties. 
     Three-phase PWM rectifiers convert three-phase alternating current (AC) power into direct current (DC) power. The voltage of the DC power can be boosted and exceed the voltage of the AC power. 
     In vehicle refrigeration systems, three-phase PWM rectifiers are used to convert AC power received from an alternator into DC power to power the refrigeration system. The voltage of the AC power received from the alternator varies based on the speed of the alternator. Under most circumstances, it is necessary to boost the DC voltage. However, under certain circumstances, the AC voltage can be greater than the desired DC voltage. Under these circumstances, power is generally disconnected from the refrigeration system to prevent damaging the system. 
     SUMMARY 
     The three-phase PWM rectifier of the invention incorporates a novel buck circuit. The buck circuit enables the three-phase PWM rectifier to reduce the DC voltage generated, and allows DC power to be provided to the refrigeration system continuously, regardless of the magnitude of AC voltage provided by the alternator and without completely disconnecting power to the refrigeration system. 
     In one embodiment, the invention provides a buck/boost rectifier. The rectifier is connectable to an alternating current power source, and includes an upper bus, a lower bus, an upper rectifier, a lower rectifier, a pulse-width-modulation (PWM) controller, a phase-angle (PA) controller, and a capacitor. The upper rectifier is coupled to the upper bus, and the lower rectifier is coupled in a series-type relationship with the upper rectifier and to the lower bus. The PWM controller is coupled to the lower rectifier and is configured to boost a direct current (DC) voltage output by the rectifier. The PA controller is coupled to the lower rectifier and is configured to buck the DC voltage output by the rectifier. The capacitor is coupled between the upper bus and the lower bus. 
     In another embodiment, the invention provides a method of providing DC power to a refrigeration unit. The method includes receiving three-phase AC power from a belt-driven alternator, rectifying the AC power into DC power, boosting the DC power when the AC power is not sufficient to produce DC power having a voltage that exceeds a threshold by rectifying alone, and bucking the DC power when the AC power has a magnitude which would produce a DC voltage that exceeds a second threshold when rectified alone. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a block diagram of a first construction of an electrical system for a refrigeration unit. 
         FIG. 1   b  is a block diagram of a second construction of an electrical system for a refrigeration unit. 
         FIG. 2  is a schematic diagram of a full-controlled buck/boost rectifier, a phase angle controller, and a PWM controller. 
         FIG. 3  is a schematic diagram of a first construction of a half-controlled buck/boost rectifier, a phase angle controller, and a PWM controller. 
         FIG. 4  is a schematic diagram of a second construction of a half-controlled buck/boost rectifier, a phase angle controller, and a PWM controller. 
         FIGS. 5   a  and  5   b  show the operation of a full-controlled buck/boost rectifier operating in a boost mode. 
         FIGS. 6   a A and  6   b  show the operation of a full-controlled buck/boost rectifier operating in a buck mode with phase angle control. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1   a  shows a supply system  100  for providing power to a refrigeration unit  105  (e.g., for a refrigerated truck or trailer). The system  100  includes an engine  110  driving an alternator  115  (e.g., by a belt). The alternator  115  produces three-phase AC power, which is provided to a power converter  120 . The power converter  120  includes a buck/boost rectifier  125 , a DC intermediate circuit  130 , and an output frequency inverter  135 . The buck/boost rectifier  125  is controlled by a PWM controller  140  and a phase angle controller  145 . The power converter  120  converts the AC power to a DC power  145  suitable for the refrigeration unit  105 . The three-phase AC power varies in frequency and voltage based upon the rotational speed of the engine. The frequency of the AC power varies by the relationship
 
Revolutions per Minute=120×Frequency/Number of poles
 
     Thus, a six-pole alternator coupled to an engine that operates from 600 rpm to 4000 rpm would produce an output AC signal having a range of frequencies of 30 Hz to 200 Hz. The voltage of the AC signal also varies based on the speed of the engine. In some constructions, the voltage can range from about 150 volts to about 600 volts. 
       FIG. 1   b  shows an alternate construction of a supply system  100 ′. In this construction, the three-phase AC power is supplied by a secondary source  155  (e.g., shore power) instead of the alternator  115 . The power converter  120  of  FIG. 1   a  uses the inductance of the alternator  115  to convert the three-phase AC power in the DC power  145  for the refrigeration unit  105 . When the power is supplied by a secondary source, the system  100 ′ requires the addition of accumulation chokes  200  to provide this inductance. 
     Referring to  FIGS. 2 ,  3 , and  4 , the three-phase AC power is provided to the buck/boost rectifier  125  from an accumulation choke  200  or the alternator  115 . The buck/boost rectifier  125  can be a full-controlled PWM rectifier  205  ( FIG. 2 ) or a half-controlled PWM rectifier  205 ′ ( FIG. 3 ) or  205 ″ ( FIG. 4 ). 
     The rectifiers  205  include a first (or upper) DC bus  210  and a second (or lower) DC bus  215  across a capacitor  220 . Each rectifier  205  also includes a first-phase rectifier  225 , a second-phase rectifier  230 , and a third-phase rectifier  235 . The first-phase rectifier  225  has an upper rectifier  240  and a lower rectifier  245 , both of which are electrically connected in a series-type relationship between the first DC bus  210  and the second DC bus  215 . The term “series-type” relationship is used herein since the connection is not a strict series electrical connection where all current through the upper rectifier  240  passes through the lower rectifier  245 . The second-phase rectifier  230  has an upper rectifier  250  and a lower rectifier  255 , both of which are electrically connected in a series-type relationship between the first DC bus  210  and the second DC bus  215 . The third-phase rectifier  235  has an upper rectifier  260  and a lower rectifier  265 , both of which are electrically connected in a series-type relationship between the first DC bus  210  and the second DC bus  215 . 
     A first phase input  270  is connected between the upper and lower phase rectifiers  240  and  245  of the first-phase rectifier  225 . A second phase input  275  is connected between the upper and lower phase rectifiers  250  and  255  of the second-phase rectifier  230 . A third phase input  280  is connected between the upper and lower phase rectifiers  260  and  265  of the third-phase rectifier  235 . 
     The lower rectifiers  245 ,  255 , and  265  of the full-controlled PWM rectifier  205  ( FIG. 2 ) and the half-controlled PWM rectifier  205 ′ ( FIG. 3 ) include a respective switch  290 . In the construction shown, the switch  290  is an insulated gate bipolar transistor (IGBT) having a collector coupled to a respective upper phase rectifier  240 ,  250 , or  260  and an emitter coupled to the second DC bar  215  via diode  285 . Each lower rectifier  245 ,  255 , and  265  also includes a diode  295  having a cathode coupled to a respective IGBT  290  collector, and an anode coupled to an emitter of a respective second switch  300 . Again, in the construction shown, the second switch  300  is an IGBT. A collector of the second IGBT  300  is coupled to the second DC bus  215 . 
     The lower rectifiers  245 ′,  255 ′, and  265 ′ ( FIG. 4 ) of the half-controlled PWM rectifier  205 ″ each include a respective diode  295  having a cathode coupled to the respective upper rectifiers  240 ,  250 , or  260 , and an anode coupled to the second DC bus  215 . The lower rectifiers  245 ′,  255 ′, and  265 ′ do not include any switches. 
     The upper rectifiers  240 ,  250 , and  260  ( FIG. 2 ) of the full-controlled PWM rectifier  205  and the half-controlled PWM rectifier  205 ″ ( FIG. 4 ) each include a respective diode  310  having an anode coupled to their respective lower rectifiers  245 ,  255 , or  265 , and a cathode coupled to an emitter of a respective switch  315 . In the construction shown, the switch  315  is an IGBT. A collector of the IGBT  315  is coupled to a respective lower rectifier  245 ,  255 , or  265 . In addition, a second switch (e.g., an IGBT)  320  is coupled between the first DC bus  210  and the respective lower rectifier  245 ,  255 , and  265 , its emitter coupled to the respective lower rectifier  245 ,  255 , or  265  via diode  325 , and its collector coupled to the first DC bus  210 . 
     The upper rectifiers  240 ′,  250 ′, and  260 ′ ( FIG. 3 ) of the half-controlled PWM rectifier  205 ′ each include a respective diode  310  having an anode coupled to the respective lower rectifiers  245 ,  255 , or  265 , and a cathode coupled to the first DC bus  210 . The upper rectifiers  240 ′,  250 ′, and  260 ′ do not include any switches. 
     The rectifiers  205  are controlled by a PWM controller  140  and a phase angle (PA) controller  145 . In the construction shown, the controllers  350  and  355  monitor the power received from the alternator  115 , and control the rectifiers  205  to output about 500 volts DC (e.g., 450 VDC to 550 VDC). The PWM controller  140  drives the gates of the IGBTs  290  and  320  in a known manner to work in combination with the accumulator chokes  200  to boost the output voltage when the monitored input voltage is not sufficient to generate the desired 500 VDC output voltage via rectification alone. 
     The PA controller  145  drives the gates of the IGBTs  300  and  315  to control the phase angle of the input voltage, and to reduce (i.e., buck) the output voltage when the monitored input voltage has a magnitude, that if left unchecked, would result in an output voltage above 500 VDC. 
       FIGS. 5   a  and  5   b  show an embodiment of the operation of the PWM controller  140  and the phase angle controller  145  for the rectifier  225  to boost the output of the U phase input voltage. During the positive cycle of the U phase input voltage, the PWM controller  140  provides a PWM signal D to the gate of the IGBT  290  and the phase angle controller  145  provides a PWM signal G to the gate of IGBT  315  of the rectifier  225  as shown in  FIG. 5   a . In the embodiment shown, the PWM signals D and G have a frequency equal to the carrier frequency of the rectifier  125  (e.g., 8 kHz). The U phase input voltage has a frequency between about 30 and 400 Hz. When the PWM signal D at the gate of IGBT  290  is on, current flows through IGBT  290  to the second DC bus  215 . When the PWM signal D at the gate of IGBT  290  is off and the PWM signal G at the gate of IGBT  315  is on, current flows to the first DC bus  210  through the IGBT  315 , boosting the voltage across capacitor  200 . 
     Similarly, during the negative cycle of the U phase input voltage, the PWM controller  140  provides a PWM signal A to the gate of the IGBT  320  and the phase angle controller  145  provides a PWM signal J to the gate of IGBT  300  of the rectifier  225  as shown in  FIG. 5   b . Again, in the embodiment shown, the PWM signals A and J have a frequency equal to the carrier frequency of the rectifier  125  (e.g., 8 kHz), and the U phase input voltage has a frequency between about 30 and 400 Hz. When the PWM signal A at the gate of IGBT  320  is on, current flows through IGBT  320  to the first DC bus  210 . When the PWM signal A at the gate of IGBT  320  is off and the PWM signal J at the gate of IGBT  300  is on, current flows to the second DC bus  215  through the IGBT  300 , boosting the voltage across capacitor  200 . 
     The PWM controller  140  and the phase angle controller  145  control rectifiers  230  and  235  in a similar manner for the V and W phase input voltages. The resulting DC voltage across the capacitor  200  is boosted relative to the three-phase input voltage. 
       FIGS. 6   a  and  6   b  show the reduced output of the rectifier  225  as a result of two embodiments of phase angle control.  FIG. 6   a  shows an embodiment of 60° phase angle control and  FIG. 6   b  shows an embodiment of 90° phase angle control. During phase angle control, the PWM controller  140  turns off the gates of IGBTs  290  and  320  (signals D and A), and the phase angle controller  145  controls the gates of IGBTs  300  and  315  (signals J and G) as shown in the figures. For the 60° phase angle control shown, the phase angle controller  145  drives the gate of the IGBT  315  (signal G) on for the last 60° of the positive cycle of the U-V voltage  500  and the last 60° of the positive cycle of the U-W voltage  505 . In addition, the phase angle controller  145  drives the gate of the IGBT  300  (signal J) on for the last 60° of the negative cycle of the U-V voltage  500  and the last 60° of the negative cycle of the U-W voltage  505 . The resulting reduced voltage output  510  of the rectifier  225  is shown in  FIG. 6   a.    
     The other switch pairs—inputs H/K and I/L—are driven in a similar manner 120° and 240° out of phase with the G/J switch pair. 
     For 90° phase angle control, the phase angle controller  145  drives the gate of the IGBT  315  (signal G) on for the last 30° of the positive cycle of the U-V input voltage  515  and the last 30° of the positive cycle of the U-W input voltage  520 . In addition, the phase angle controller  145  drives the gate of the IGBT  300  (signal J) on for the last 30° of the negative cycle of the U-V input voltage  515  and the last 30° of the negative cycle of the U-W input voltage  520 . The resulting reduced voltage output  525  of the rectifier  225  is shown in  FIG. 6   b.    
     The PWM controller  140  and the phase angle controller  145  control rectifiers  230  and  235  in a similar manner for the V and W phase input voltages, reducing the effective DC voltage across capacitor  200 . 
     Various features and advantages of the invention are set forth in the following claims.