Abstract:
A control methodology for a two-stage PWM DC-DC conversion system, with transformer-isolation, in which the converter circuit input voltage is compared to a set voltage calibrated as a function of the desired output voltage and the maximum voltage conversion ratio provided by the second-stage converter. When the input voltage is above the set voltage, the second-stage converter is controlled to provide both output voltage regulation during normal operation and output current limiting during over-current conditions. However, when the input voltage is below the set voltage, the first-stage converter is controlled to provide output voltage regulation with minor output current limiting, and the second-stage converter is controlled to provide extended output current limiting independent of the input voltage.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a two-stage DC-DC conversion system, and more particularly to a novel control methodology providing improved operation thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    DC-DC converter circuits are routinely used to power DC loads in systems where the source voltage is variable and/or not matched to the loads. The converter circuit can be controlled both to regulate the output voltage (i.e., the load voltage) during normal conditions, and to limit the output current (i.e., the load current) during over-current conditions. These two control functions can be segregated to a large extent with a two-stage converter topology in which one stage is controlled to provide output voltage regulation and the other stage is controlled to provide current limiting. For example, the U.S. Pat. No. 7,336,057 to Hirabayashi discloses a two-stage converter topology in which the first-stage provides output voltage regulation, the second-stage provides output current limiting, and transformer isolation allows the second-stage to limit output current independent of the input voltage if necessary. However, the voltage regulating capability of such two-stage converters is relatively limited, and the converter must be uniquely configured for nearly every application. Accordingly, what is needed is a two-stage DC-DC conversion system having both effective current limiting capability and enhanced voltage regulating capability. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention is directed to an improved control methodology for a two-stage PWM DC-DC conversion system, with transformer-isolation, where the control methodology provides both effective current limiting and an extended range of voltage regulation for suitability to a variety of applications. The converter input voltage is compared to a set voltage calibrated as a function of the desired output voltage and the maximum voltage conversion ratio provided by the second-stage of the converter circuit. When the input voltage is above the set voltage, the second-stage of the converter circuit is controlled to provide both output voltage regulation during normal operation and output current limiting during over-current conditions. However, when the input voltage is below the set voltage, the first-stage of the converter circuit is controlled to provide output voltage regulation with minor output current limiting, and the second-stage converter circuit is controlled to provide extended output current limiting independent of the input voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a block diagram of a two-stage PWM DC-DC conversion system according to this invention, including an electronic controller functioning as a PWM duty-cycle controller. 
           [0005]      FIG. 2  is a flow diagram describing a control methodology carried out by the electronic controller of  FIG. 1  according to this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0006]    Referring to the drawings, and particularly to  FIG. 1 , the reference numeral  10  generally designates a PWM DC-DC conversion system for supplying power from a DC source  12  to a DC load  14 . For example, the DC source  12  may be a battery pack of a hybrid electric vehicle, and the load  14  may be an automotive accessory. In general, the DC-DC conversion system  10  includes a PWM converter circuit  16  and an electronic controller  18  that supplies PWM duty-cycle commands to the converter circuit  16  via lines  20  and  22 . The PWM converter circuit  16  is configured as a two-stage converter, with a first-stage converter  24  receiving an input voltage V_IN from the DC source  12 , and a second-stage converter  26  coupling the first-stage converter&#39;s output to the load  14 . The first-stage converter  24  is configured as a boost converter, while the second-stage converter  26  is configured as a transformer-isolated buck converter. Both converter topologies may be conventional in design, as shown for example in the aforementioned U.S. Pat. No. 7,336,057 to Hirabayashi. 
         [0007]    The duty-cycle outputs of electronic controller  18  coordinate the operation of the first-stage and second-stage converters  24 ,  26  based on a set of calibrated control parameters and a pair of feedback signals. In the illustrated embodiment, the calibrated control parameters include a desired load voltage V_OUT_DES, an output current limit value I_SET, and a maximum PWM duty-cycle DC_BUCK_MAX for the second-stage converter  26 . The feedback parameters include an output current signal I_OUT developed by current sensor  28  on line  30 , and an output voltage feedback signal V_OUT developed by voltage sensor  32  on line  34 . 
         [0008]    In general, the electronic controller  18  coordinates the operation of the first-stage and second-stage converters  24 ,  26  to regulate the output voltage supplied to load  14  and to limit the current supplied to load  14  in the event of an over-current condition. The novelty of the control methodology lies in its ability to regulate (output voltage or output current) with either the first-stage converter  24  or the second-stage converter  26  as conditions require, and to seamlessly transition between voltage regulation and current regulation. The ability to regulate with either of the converters  24  or  26  allows the DC-DC conversion system  10  to be used in a range of applications with widely varying voltage conversion requirements, and even in applications where the source voltage V_IN is subject to substantial variation. The control methodology is described herein in the context of the flow diagram of  FIG. 2 , as though carried out by a microprocessor-based controller executing an embedded software routine. However, it will be understood that the methodology is quite independent of the implementation strategy, and that the electronic controller  18  may be implemented with analog or digital circuit elements as desired. 
         [0009]    Referring now to the flow diagram of  FIG. 2 , the block  42  is initially executed to define a set of initial conditions. As indicated, the variables DC_BOOST and DC_BUCK for the first-stage and second-stage converters  24  and  26  are initialized to 0%, and the calibrated control parameters V_OUT_DES, I_SET and DC_BUCK_MAX are dialed in. At a 0% duty cycle, the first-stage converter  24  has a voltage conversion ratio of one-to-one, and the second-stage converter  26  has a voltage conversion ratio of zero due to the operation of its input transformer. The block  42  additionally computes a set voltage V_SET corresponding to the expected voltage at the input of second-stage converter  26  when the output voltage V_OUT is equal to V_OUT_DES and the second-stage converter  26  is operated at the specified maximum duty-cycle DC_BUCK_MAX. Accordingly, the computation requires foreknowledge of the maximum voltage conversion ratio (i.e., when DC_BUCK=DC_BUCK_MAX) of second-stage converter  26  and the turns-ratio of its input transformer, in addition to the desired output voltage V_OUT_DES. 
         [0010]    Following initialization, the blocks  44 - 48  and  52 - 56  are executed to start-up the converter circuit  16 . First, the blocks  44 - 48  ramp-up the duty-cycle command DC_BUCK of the second-stage converter  26  until the maximum duty-cycle DC_BUCK_MAX or the desired output voltage V_OUT_DES is reached. Initially, of course, block  46  will be answered in the negative, and block  44  will be iteratively executed to increase DC_BUCK, as indicated by block  48  and flow line  50 . If the desired output voltage V_OUT_DES is reached before DC_BUCK=DC_BUCK_MAX, the start-up portion of the control is concluded, and the control proceeds to block  60  and the regulating portion of the control. However, if the maximum duty-cycle DC_BUCK_MAX is reached before V_OUT increases to the desired value V_OUT_DES, block  46  will be answered in the affirmative, and the blocks  52 - 56  will be executed to ramp-up the duty-cycle command DC_BOOST of the first-stage converter  24 . As with DC_BUCK, the blocks  52 - 58  ramp-up DC_BOOST until DC_BOOST reaches a maximum value DC_BOOST_MAX (95%, for example) or V_OUT increases to the desired value V_OUT_DES. Initially, of course, block  54  will be answered in the negative, and block  52  will be iteratively executed to increase DC_BOOST, as indicated by block  56  and flow line  58 . Typically, the desired output voltage V_OUT_DES is reached before DC_BOOST reaches DC_BOOST_MAX; and at such point, the start-up portion of the control is concluded, and the control proceeds to block  60  and the regulating portion of the control. 
         [0011]    In the regulating portion of the control methodology, the block  60  is initially executed to compare the source or input voltage V_IN to the set voltage V_SET computed during initialization. If V_IN is less than V_SET, the first-stage boost converter  24  is required to satisfy the desired output voltage V_OUT_DES; in this case, the blocks  62 - 74  are repeatedly executed to regulate V_OUT, and limit the load current I_OUT if required. If V_IN is greater than V_SET, the first-stage boost converter  24  is not required to satisfy the desired output voltage V_OUT_DES; in this case, blocks  66  and  76 - 82  are repeatedly executed to maintain DC_BOOST at 0%, regulate V_OUT, and limit the load current I_OUT if required. In some applications, the source voltage V_IN will always be either above or below the set voltage V_SET, but in other applications, the source voltage V_IN will vary both above and below the set voltage V_SET, depending on operating conditions, and the control method outlined in  FIG. 2  accommodates any of these applications. 
         [0012]    In the case where V_IN is less than V_SET, the block  62  is executed to detect the presence of an over-current condition, as may be signified, for example, when the average value of feedback current I_OUT exceeds a calibrated set-point, or the temperature of load  14  exceeds a calibrated value. If block  62  is answered in the negative, the system is operating normally, and blocks  64 - 66  are executed to maintain the second-stage buck converter  26  at its maximum duty-cycle (DC_BUCK_MAX), and to set the duty-cycle of the first-stage boost converter  24  as required to regulate V_OUT at the desired value V_OUT_DES. As noted in block  64 , the duty-cycle command DC_BOOST for the first-stage boost converter  24  is determined as a proportional-plus-integral (PI) function of the output voltage error V_OUT_ERROR, where V_OUT_ERROR is simply the difference between V_OUT_DES and the feedback voltage V_OUT. 
         [0013]    However, if an over-current condition occurs, block  62  is answered in the affirmative, and the blocks  66  and  70 - 74  are executed to limit the output current by suitably controlling the first-stage and second-stage converters  24  and  26 . In most cases, the duty-cycle of the first-stage boost converter  24  will initially be greater than 0%, and block  70  will direct the execution of block  72  to set the duty-cycle DC_BOOST of the first-stage boost converter  24  as required to regulate I_OUT at the over-current set value I_SET. As noted in block  72 , DC_BOOST in this case is determined as a proportional-plus-integral (PI) function of the output current error I_OUT_ERROR, where I_OUT_ERROR is simply the difference between the feedback current I_OUT and the set value I_SET. If the duty-cycle regulator function of block  72  drives the duty-cycle DC_BOOST of the first-stage boost converter  24  down to 0%, however, the block  70  will direct the execution of block  74  instead of block  72 . Block  74  maintains DC_BOOST at 0%, and sets the duty-cycle DC_BUCK of the second-stage buck converter  26  as required to regulate I_OUT at the calibrated limit value. In this case, DC_BUCK is determined as a proportional-plus-integral (PI) function of the output current error I_OUT_ERROR, as noted at block  74 . If and when the over-current condition is resolved, block  62  will again be answered in the negative, and blocks  64 - 66  will be executed as discussed above to resume output voltage regulation. 
         [0014]    In the case where V_IN is greater than V_SET, block  76  sets the duty-cycle DC_BOOST of first-stage converter  24  to 0%, and block  78  determines if an over-current condition is in effect. As mentioned above in respect to block  62 , the presence of an over-current condition may be signified, for example, when the average value of feedback current I_OUT exceeds a calibrated set-point, or the temperature of the load  14  exceeds a calibrated value. If block  78  is answered in the negative, the system is operating normally, and blocks  80  and  66  are executed to set the duty-cycle of the second-stage buck converter  26  as required to regulate V_OUT at the desired value V_OUT_DES. As noted in block  80 , the duty-cycle command DC_BUCK for the second-stage buck converter  26  is determined as a proportional-plus-integral (PI) function of the output voltage error V_OUT_ERROR, where V_OUT_ERROR is simply the difference between V_OUT_DES and the feedback voltage V_OUT. 
         [0015]    However, if an over-current condition occurs, block  78  is answered in the affirmative, and the blocks  82  and  66  are executed to regulate the output current at the set value I_SET by suitably controlling the second-stage buck converter  26 . As indicated at block  82 , the duty-cycle DC_BUCK of the second-stage buck converter  26  is determined as a proportional-plus-integral (PI) function of the output current error I_OUT_ERROR, where I_OUT_ERROR is simply the difference between the feedback current I_OUT and the set value I_SET. If and when the over-current condition is resolved, block  78  will again be answered in the negative, and blocks  80  and  66  will be executed as discussed above to resume output voltage regulation. 
         [0016]    In summary, the control disclosed methodology retains the effective current limiting capability of the prior art, while substantially enhancing the conversion system&#39;s range of voltage regulation. This not only allows the conversion system  10  to be used in a wide variety of applications, but in also allows the system to operate effectively in applications where the source voltage V_IN is subject to substantial variation. While the present invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the converter controls may be of the PID or peak-current type instead of the illustrated PI controls, if desired. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.