Abstract:
Apparatuses and methods are disclosed for controlled parallel operation of direct current converters, particularly in a multi voltage vehicle electrical system, with two voltage ranges (U 1 ) and (U 2 ). The parallel-connected voltage converters are disposed between the voltage ranges; in this connection, only one of the voltage converters is embodied as an active voltage regulator that functions all the time. The other (n−1) converters function either in full load operation or no load operation depending on the required power demand.

Description:
PRIOR ART 
     In order to increase the power of DC/DC Converters, they are operated in master/slave mode. In master/slave mode, the so-called master executes overriding control of the tasks in regulating the overall system. The subordinate converter(s) (slaves) function in controlled operation, which results in a multiplication of the electrical output potential. The control of the slave converter can be achieved, for example, through the transmission of switching commands for electrical valves (e.g. control signals of the switching transistors). 
     THE PROBLEM 
     DC/DC converters are used to produce one or more voltage potentials or for energy transmission between two different voltage potentials (e.g. 14V/42V dual-voltage vehicle electric systems). The output side parallel connection of n individual converter modules is suitable for flexible reaction to the desired maximal output power. FIG. 1 shows an apparatus of this kind. 
     For example, the input voltage side is connected to the generator of the vehicle. If each converter has a separate voltage regulator, then coupling oscillations can occur as a result of the output side connection. 
     OBJECT OF THE INVENTION 
     The object of the invention is comprised of increasing the power of direct current converters (DC/DC converters) by means of controlled parallel operation of a number of individual converters while simultaneously preventing coupling oscillations. This object is attained by means of the apparatuses and methods for controlled parallel operation of direct current converters disclosed in the main claim and the dependent claims. 
     ADVANTAGES OF THE INVENTION 
     The principle of load sharing demonstrated and claimed in claim  1  advantageously prevents the danger of coupling oscillations in voltage regulators operating in parallel. The disposition of the converters can be embodied both close together or spaced apart. In contrast to the known method, a data exchange only has to take place between the converters when they are switched. A low bandwidth connection (e.g. CAN) is sufficient for this purpose. The serial control structure has the advantage that it permits an identical design of the converters. 
     Other advantages of the invention are achieved by means of the measures taken in the dependent claims. 
    
    
     DRAWINGS 
     Exemplary embodiments of the invention are shown in the drawings and will be explained in detail in the subsequent description. 
     FIG. 1 shows the modular parallel connection of a number of converters, 
     FIG. 2 shows an example of the distribution of an instantaneous power demand, 
     FIG. 3 shows a hierarchical control structure, and 
     FIG. 4 shows the apparatus with a serial control structure. 
    
    
     DESCRIPTION 
     FIG. 1 shows how a modular parallel connection of a number of DC/DC converters,  1 ,  2 , . . . , n, which is suitable for the flexible reaction to a desired maximal output power and can be used for the invention. The input voltage, for example the electrical system voltage of a vehicle, is labeled U 1 , the output voltage of the DC/DC converter is labeled U 2 . In a dual-voltage vehicle electrical system, the associated voltages are 14 and 42 volts, for example. 
     If each of the DC/DC converters  1 ,  2 , . . . , n has a separate voltage regulator, coupling oscillations can occur as a result of the output-side connection. A reduction to a single active voltage regulator is recommended in order to prevent reciprocal regulator influence. The required power demand is shared among (n−1) converters, which function either in full load operation or in no load operation, and an additional converter, which carries out the voltage regulation in the partial load range. 
     FIG. 2 shows an example of the load distribution among five parallel converters. The converters  1 ,  2 , and  3  function in full load operation and supply their maximal output current on the output side. The converter  4  takes over the voltage regulation and covers the “active” power range. The converter  5  functions in no load operation. 
     The regulation strategy can be expressed as follows: 
     If a single converter is sufficient to cover the power demand, then it carries out the regulation of the output voltage. The other converters are not operational. 
     If the power demand exceeds the capacity of an individual converter (converter  1 ), this converter transfers the voltage regulation to the next converter (converter  2 ). Converter  1  switches into full load operation and supplies its maximal output current. 
     With a further increase in power demand, successively more converters are switched into full load operation. Another converter respectively assumes the remaining partial load. 
     In the opposite case of a decreasing power demand, the converters are successively switched from full load operation back into partial load operation. 
     The following control structures can be used in order to achieve this: 
     Hierarchical Control Structure 
     When using a hierarchical control structure, a set of central control electronics carry out the coordination of individual converters. From the respectively active voltage-regulating converter, it receives information regarding its instantaneous output power and switches the remaining converters into full load or no load operation. FIG. 3 shows a hierarchical control structure of this kind. The associated control electronics  6  can, for example, also be integrated into a converter, for example into the converter  1 . The exchange of data can take place bidirectionally via a serial bus (e.g. CAN) or via separate signal lines  7 ,  8 ,  9 . 
     Some DC/DC converters cannot function in no load operation. They require a base load on the output side. In this case, the switching thresholds for the transfer of the voltage regulation from one converter to the next must be adapted. Then the switch-on threshold when there is increasing demand is not P=P max , but is P=P max −P base . In this case, the switch-off threshold when there is decreasing demand is identical to the base load P base . 
     If the maximum efficiency of the converter used lies in the partial load range, then in order to optimize efficiency, the controlled converters can also be operated in this partial load range. The control signal for this is predetermined by the central control electronics  6 . If the total power output is insufficient, then the output of the controlled converters can subsequently be increased to the maximal power. 
     Serial Structure (Control Structure) 
     When using a serial structure as shown in FIG. 4, the central control electronics according to FIG. 3 can be omitted. All of the converters can be embodied identically. The connections are produced solely by means of the external wiring. 
     The starting of the system can take place, for example, by means of a switch-on command ON for the first converter in the sequence. This converter takes over the voltage regulation. If converter  1  reaches its power limit, the transfer of the voltage regulation to converter  2  is initiated by the signal P=P max . This is a signal for the arrival at maximal power. This means that the converter  1  is producing its maximal power or its maximal current at the output. Increasing power demand results in a successive transfer of the voltage regulation by means of the signal P=P max . 
     In the event of decreasing power demand, the instantaneously active voltage regulating converter signals the preceding converter that it has reached its no load state by means of the datum P=0. The preceding converter then functions as a voltage regulator. The converters are thus gradually switched into no load operation, starting from converter  3  down to converter  1  in the example shown. 
     As has already been explained in connection with the embodiment according to FIG. 3, the base load or the point of maximal efficiency can also be used as a switching threshold. 
     The exchange of data can likewise take place via a serial bus (e.g. CAN) or via separate signal lines. 
     The apparatus demonstrated and/or the associated methods for controlled parallel operation of voltage converters (DC/DC converters) can be used, for example, in vehicle electrical systems; however, they are also suitable for other applications which depend on the conversion of high power outputs. It is essential that one of the converters operates in the active range and all the other converters operate either in the full load range or in the no load range.