Patent Publication Number: US-2015062982-A1

Title: Circuit Arrangement and Method for Actuating at Least One Switching Element of a Voltage Converter

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation of international application number PCT/EP2013/059405 filed on May 6, 2013, which claims priority to German application number 10 2012 104 103.2 filed on May 10, 2012. 
    
    
     FIELD 
     The disclosure relates to a circuit arrangement for actuating at least one switching element of a voltage converter. The circuit arrangement has a DC (direct current)-isolating signal transformer, with a primary winding and at least one secondary winding, wherein an actuating signal for the switching element can be applied to the primary winding, and the secondary winding is connected to a switching input of the at least one switching element. The disclosure further relates to a method for actuating a switching element of a voltage converter and to a driver circuit for providing the actuating signal, which driver circuit can be used in connection with the circuit arrangement. 
     BACKGROUND 
     Voltage converters that are generally clocked can also be referred to as DC/DC converters or can be implemented as voltage converters with an AC input, which are also referred to as AC/DC converters. DC/DC converters find widespread use in power supply applications, for example as part of power supply units or as an input stage of an inverter or for providing an on-board power supply to an inverter. Both DC/DC converters with a fixed potential reference, such as step-up converters, step-down converters etc., and DC-isolating DC/DC converters are conventional. The latter are in this case configured as flyback converters or forward converters, for example, and have at least one switching element, whose switching path is arranged in a power circuit of the DC/DC converter. Examples of switching elements which are used are MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), JFETs (Junction FETs) or IGBTs (Insulated-Gate Bipolar Transistors) or other transistors. 
     In the case of a DC/DC converter, the switching element is typically actuated by a pulse-width-modulated (PWM) actuating signal, wherein various, usually integrated driver circuits are available for this purpose. In order to be able to operate the driver circuits in DC-isolated (also called galvanically isolated) fashion from the power circuit of the DC/DC converter, the actuating signal can be transmitted from the driver circuit to a switching input of the switching element in a known manner via a DC-isolating transformer. Such actuation via a signal transformer is described, for example, in document U.S. Pat. No. 6,169,681 B1 in connection with a switched mode power supply. A further switched mode power supply with a transformer for the actuation signal is described in document U.S. Pat. No. 4,744,020. The power supply implements a so-called “proportional base driving circuit”, where an additional winding on the transformer is part of the switching path and is used to keep the switching element conductive. 
     DC/DC converters are often operated in the so-called “current mode”, in which a switch current flowing in the power circuit through the switching element of the DC/DC converter is used as an actuating variable for the generation of the actuating signal. As is known, for example, from the document U.S. 2012/0020121 A1, a further transformer can be provided in the power circuit for this purpose in order to be able to measure the switch current with DC isolation and to provide the measured value to the driver circuit. 
     An arrangement with a signal transformer for the actuating signal and a transformer for the DC-isolated measurement of the switch current is disadvantageous owing to the number of transformers required. This applies in particular when the DC/DC converter has a plurality of primary power circuits. A plurality of primary power circuits provide the possibility of supplying the output of the DC/DC converter from a plurality of voltage sources at different potentials simultaneously or alternatively. 
     SUMMARY 
     One embodiment of the present disclosure comprises a circuit arrangement for actuating at least one switching element of a voltage converter of the type mentioned at the outset, in which a current measurement is realized in a simple manner without any considerable additional complexity in terms of circuitry, in particular without any additional transformers. Another embodiment comprises a method for actuating a switching element in such a circuit arrangement and providing a driver circuit which can be used in connection with the circuit arrangement. 
     A circuit arrangement according to the disclosure of the type mentioned at the outset is characterized by the fact that at least part of the at least one secondary winding is connected in series with a switching path of the switching element in such a way that a switch current flowing through a switching path of the switching element flows through this part, and that a current measurement device is arranged in a series circuit with respect to the primary winding for determining the switch current. 
     Owing to the fact that the signal transformer acts with its secondary winding not only on the control input of the switching element but is part of the power circuit and has the switch current flowing through it, the signal transformer can be used for the back-transformation of the switch current onto the primary side of the signal transformer, and thus a measurement of the switch current can take place on the primary side via the current measurement device. The signal transformer therefore acts simultaneously as an actuating transformer for the actuating signal of the switching element and as a measured value transformer for the switch current measurement. This dual use saves on material and costs and can contribute to a reduction in the physical size of the circuit arrangement and therefore of the voltage converter. 
     In an advantageous configuration, the circuit arrangement comprises at least two power circuits, each comprising at least one switching element for connection to different voltage sources. In this case, a signal transformer with a primary winding and separate secondary windings for the switching elements is provided or separate signal transformers each comprising a primary winding and a secondary winding are provided. In both cases, the back-transformed switch currents are added together and can easily be measured jointly. Control of a voltage converter with two voltage sources can then take place in the “current mode” in a simple manner using the measured total signal, for example. 
     In a further advantageous configuration of the circuit arrangement, the at least one secondary winding is a winding with a tap, wherein the secondary winding is connected in series with the switching path of the at least one switching element from an end connection up to the tap, whereas a remaining part or the entire secondary winding is operatively connected to the switching input of the at least one switching element. This makes it possible to reduce the level of the back-transformed current in the primary winding by a freely selectable ratio, which is dependent on the turns ratio of the various parts of the secondary winding, given the same value for the control voltage. This can be used, for example, to reduce the current loading in a circuit for generating the actuating signal or to increase the measurement accuracy during the current measurement. 
     A driver circuit is used for providing a pulse-width-modulated actuating signal for at least one switching element of a voltage converter at a primary winding of a DC-isolating signal transformer, which comprises at least one secondary winding connected to a control input of the switching element. According to the disclosure, the driver circuit is configured to evaluate a current flowing through the primary winding as a signal corresponding to a switch current flowing through the switching element. The driver circuit is thus designed for use with the abovementioned circuit arrangement and makes it possible to measure the switch current which has been back-transformed onto the primary side of the signal transformer. This results in the advantages mentioned in connection with the circuit arrangement. 
     Advantageously, the driver circuit is in the form of an integrated circuit and can therefore be used in a voltage converter in a space-saving manner and with little complexity. 
     In other advantageous configurations, the driver circuit comprises a current measurement device for detecting the current flowing through the primary winding or has terminals to an external current measuring device for detecting the current flowing through the primary winding. In both cases, terminals for an external shunt can be provided as part of the current measurement device. 
     A method according to the disclosure for actuating at least one switching element of a voltage converter via a DC-isolating signal transformer, which has a primary winding and at least one secondary winding, wherein an actuating signal for the switching element is applied to the primary winding, and the secondary winding provides a switching signal for the at least one switching element, is characterized by the fact that a switch current flowing through the switching path of the switching element is transferred to the primary winding via the signal transformer, and in that a current flowing through the primary winding is detected as a measure of the switch current. 
     As a result, according to the disclosure, an actuating signal is therefore generated and transmitted from a primary side to a secondary side via the signal transformer. The actuating signal controls the switching element and therefore influences the switch current. The resultant change in the switch current is transmitted from the secondary side of the signal transformer to the primary side as a current signal and is detected on the primary side. This again results in the advantages mentioned in connection with the circuit arrangement. 
     The value of the switch current detected on the primary side can then be used as an actuating variable for generating the actuating signal, in an advantageous configuration of the method. 
     According to the disclosure, the circuit arrangement and/or the driver circuit and/or the method is/are used for providing an operating supply voltage to an inverter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be explained in more detail below with reference to embodiments with the aid of four figures, in which: 
         FIGS. 1 to 3  each show an embodiment of a circuit arrangement in accordance with the application in a DC/DC converter, and 
         FIGS. 4   a - 4   c  show various configurations of a signal transformer for a circuit arrangement in accordance with the application. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic diagram of a DC/DC converter with a circuit arrangement in accordance with the application in a first embodiment. 
     The DC/DC converter comprises a power circuit  10 , in which a voltage source  11  with a switching element  13  and a primary winding  17  of a power transformer  20  are arranged. A switch current I 10  flows in the power circuit  10 . The switching element  13  has a switching path between switching connections  14  and  15 , and a control input  16 . In the example illustrated, the switching element  13  is a MOSFET with a source and a drain as switching connections  14 ,  15  of the switching path and a gate as control input  16 . For reasons of a simplified illustration, the connections of the switching element  13  are referred to as source  14 , drain  15  and gate  16  below. 
     The power transformer  20  also has a secondary winding  21 , which is connected via a diode  22  to a smoothing capacitor  23  and to output connections  24 , which are in parallel with said smoothing capacitor. Given clocked actuation of the switching element  13  and correspondingly pulsed current flow through the primary winding  17  of the power transformer  20 , an output voltage is induced in the secondary winding  21 , which output voltage is provided, rectified by the diode  22 , via the smoothing capacitor  23  as an output voltage of the DC/DC converter at the output connections  24 . 
     The voltage converter illustrated in  FIG. 1  is a DC-isolating flyback converter. However, the disclosure can likewise be used for other types of DC/DC converters, for AC/DC converters and also for DC-to-AC converters. 
     For the pulsed actuation of the switching element  13 , in addition a secondary winding  12  of a signal transformer  30  is provided between the source  14  and the gate  16 . This secondary winding, in contrast to that known from the prior art, is arranged in the power circuit  10 , i.e. the switch current I 10  flows through said secondary winding. On the primary side, the signal transformer  30  has a primary winding  31 , which is connected to a PWM driver circuit  40  which in one embodiment is implemented as an integrated circuit. In order to actuate the primary winding  31  of the signal transformer  30 , a bridge circuit is provided in the PWM driver circuit  40 , which bridge circuit is in the form of an H-bridge with two actively actuable bridge switches  41  and two passive bridge diodes  42 . The H-bridge and also the further components in the PWM driver circuit  40  are supplied with current via power supply connections  32 . In the figure, the power supply is symbolized by a positive potential V+ and a ground potential GND at the power supply connections  32 . The power supply can take place, for example, during operation via the output voltage of the DC/DC converter which is present at the output connections  24 . In order to start the DC/DC converter, an additional auxiliary voltage source is then generally provided, for example a linear controller, which is fed from the voltage source  11 . 
     The bridge switches  41  are switched via driver modules  43  of a PWM generator  44 , which is in turn connected to a control device  45  controlling the PWM generator  44 . The PWM driver circuit  40  furthermore comprises a comparator  46 , which has an actual voltage input  47  and a setpoint voltage input  48 . A desired output voltage of the DC/DC converter at the output connections  24  is preset at the setpoint voltage input  48 . The actual voltage input  47  is correspondingly connected to one of the outputs  24  of the DC/DC converter. The function of the comparator  46  for regulating the output voltage of the DC/DC converter can also be integrated into the control device  45  and take place there in analog or digital fashion. 
     During operation of the DC/DC converter, the output voltage of the DC/DC converter is therefore applied to the actual voltage input  47 , and the output voltage is compared with the setpoint voltage. The output of the comparator  46  controls the PWM generator  44  via the control device  45  in such a way that the bridge switches  41  are switched in clocked fashion such that the desired setpoint voltage is set at the output connections  24  of the DC/DC converter. In this case, the bridge switches  41  are each switched on or switched off in parallel. A current flow in the reverse direction through the primary winding  31  via the bridge diodes  42  is possible in the switch-off phases of the bridge switches  41 . The AC voltage resulting from the clocking at the primary winding  31  of the signal transformer  30  is transmitted to the secondary winding  12  of the signal transformer  30  and results in an AC voltage signal between the source  14  and the gate  16  of the switching element  13 . Said switching element  13  is switched on and off in pulsed fashion corresponding to the operation of the DC/DC converter. 
     With the pulsed switching of the switching element  13 , an alternating current I 10  flows in the power circuit  10 . Since the secondary winding  12  of the signal transformer  30  is part of the power circuit  10 , the alternating switch current I 10  brings about an additional current flow in the primary winding  31  of the signal transformer  30 . A current measurement device  33  is integrated in the bridge circuit of the PWM driver circuit  40 , where the current measurement device comprises a shunt, for example. There is a voltage drop across the shunt which is representative of the switch current I 10  in the power circuit  10 . Instead of a shunt, another current measurement device, for example a Hall sensor, can also be provided. The current measurement device can in this case be incorporated in the PWM driver circuit  40  or else be arranged external thereto. 
     The voltage drop across the shunt is supplied to the PWM generator  44  as a measured variable via a current measurement input  34  for said PWM generator being able to monitor the actuation of the switching element  13 , for example in the abovementioned “current mode”. Owing to the fact that the signal transformer  30  with its secondary winding  12  not only acts on the control input (gate)  16  of the switching element  13  but is part of the power circuit  10 , the signal transformer  30  can be used for the back-transformation of the switching current I 10  onto the primary side of the signal transformer  30  and thus result in a measurement of the switch current I 10  on the primary side via the current measurement device  33 . The signal transformer  30  therefore simultaneously acts as an actuating transformer for the actuating signal of the switching element  13  and as a measured value transformer for the switch current measurement. 
       FIG. 2  illustrates a further embodiment of a DC/DC converter as an extension of the DC/DC converter in the first embodiment in  FIG. 1 . In this embodiment, identical reference symbols denote identical or functionally identical elements to those in the example in  FIG. 1 . 
     In contrast to the embodiment in  FIG. 1 , in this case two power circuits  10 ,  10 ′ are provided, which, in this example, are each designed identically and as shown in the first embodiment in  FIG. 1 . The two power circuits  10 ,  10 ′ make it possible for two voltage sources  11 ,  11 ′ to be used for supplying power to one output circuit with the output connections  24 . In order to distinguish between the two power circuits  10 ,  10 ′, all of the components in the power circuit  10 ′ have been provided with an apostrophe in their reference symbol. 
     In the DC/DC converter, a common power transformer  20  is provided which has separate primary windings  17 ,  17 ′. Furthermore, separate signal transformers  30 ,  30 ′ are provided, which correspondingly have separate secondary windings  12 ,  12 ′. Owing to the separate primary windings  17 ,  17 ′ of the power transformer  20  and the separate signal transformers  30 ,  30 ′, the power circuits  10 ,  10 ′ are completely DC-isolated from one another and from the rest of the components in the circuit. 
     A PWM driver circuit  40 , which is configured as in the first embodiment in  FIG. 1 , is provided for both power circuits  10 ,  10 ′ and therefore for actuating the two switching elements  13 ,  13 ′. Reference is hereby made to the corresponding description. The primary windings  31 ,  31 ′ of the signal transformers  30 ,  30 ′ are connected in parallel, with the result that, firstly, the two switching elements  13 ,  13 ′ are switched simultaneously and, secondly, the back-transformed switch currents I 10 , I 10 ′ in the bridge circuit of the PWM driver circuit  40  are added to one another, as a result of which a total current across the shunt is measured. Irrespective of whether a load at the connections  24  of the DC/DC converter is supplied from the first voltage source  11 , from the second voltage source  11 ′ or from both voltage sources  11 ,  11 ′ to identical or unidentical parts, the total switch current (I 10 +I 10 ′) is measured by the current measurement device  33 . 
     Given different dimensioning of the signal transformers  30  and  30 ′, the contributions of the current sources  11  and  11 ′ to the switch current measured by the current measurement device  33  can be weighted differently. It is likewise conceivable for the actuation to be matched to different switch types by individual dimensioning of the signal transformers  30 ,  30 ′. 
     In the embodiment illustrated, two separate signal transformers  30 ,  30 ′ are used. However, it is likewise possible to use a signal transformer  30  with a common primary winding  31  and two separate secondary windings  12 ,  12 ′. 
       FIG. 3  shows a further DC/DC converter with a circuit arrangement in accordance with the application. Identical reference symbols denote identical or functionally equivalent elements to those in the preceding embodiments in this case too. 
     In the same way as in the embodiment in  FIG. 2 , two power circuits  10 .  10 ′ are provided, but in this case said power circuits are not completely DC-isolated, but have series-connected voltage sources  11 ,  11 ′. Such a situation is often present in the case of inverters with symmetrical intermediate circuits, wherein the two voltage sources  11 ,  11 ′ represent the intermediate circuits or intermediate circuit capacitances. For reasons of clarity, only the primary side with the primary windings  17 ,  17 ′ of the DC/DC converter is illustrated; a secondary winding  21  with a downstream diode  22  and buffer capacitor  23  can be provided, for example, in similar fashion to the embodiments in  FIGS. 1 and 2 . 
     With respect to the PWM driver circuit  40 , reference is again made to the above embodiments. In this case, a common signal transformer  30  with a primary winding  31  is provided. On the secondary side, two secondary windings  12 ,  12 ′ are arranged in the signal transformer  30  in order to be able to compensate for the potential difference in the power circuits  10 ,  10 ′. 
     In this case, the secondary windings  12 ,  12 ′ are in the form of windings with a tap, wherein the tap is connected in each case to the voltage source  11 ,  11 ′ and one of the end connections is connected to the respective source connection  14 ,  14 ′ and the other end connection is connected to the respective gate  16 ,  16 ′. 
     By way of example,  FIG. 4   a  illustrates the signal transformer  30  or  30 ′ with its tap, in enlarged form for illustrative purposes. Only one part  12   a  or  12   a ′ of the secondary winding  12 ,  12 ′ is arranged in the power circuit  10 ,  10 ′, whereas the entire secondary winding  12 ,  12 ′ is present between the source  14 ,  14 ′ and the gate  16 ,  16 ′, however. As shown in  FIG. 4   a , the entire secondary winding  12 ,  12 ′ comprises the mentioned part  12   a,    12   a ′ and a part  12 ,  12   b ′. In this case, the number of winding turns in the power circuit  10 ,  10 ′ (part  12   a ) is in one embodiment lower than the number of remaining winding turns (part  12   b ). For example, a ratio of  1  to  10  can be selected. This means that, given the same value for the control voltage between the source  14 ,  14 ′ and the gate  16 ,  16 ′, the level of the back-transformed current in the primary winding  31 ,  31 ′ is reduced by this turns ratio. This reduces the current loading in the bridge switches  41  and the bridge diodes  42 . 
     Further possible configurations of the secondary winding  12  or  12 ′ of the signal transformer  30 ,  30 ′ are illustrated in  FIGS. 4   b  and  4   c . Instead of the respective secondary winding  12 ,  12 ′ with the tap, the parts  12   a,    12   b  or  12   a ′,  12   b ′ can be formed as separate windings with terminals connected to one another correspondingly outside the signal transformer  30 ,  30 ′. In the example in  FIG. 4   b , the part  12   a,    12   a ′ is in the power circuits  10 ,  10 ′ and the series circuit comprising the parts  12   a  and  12   b  or  12   a ′ and  12   b ′ is used for actuating the gate  16 ,  16 ′. In the example in  FIG. 4   c , however, the winding  12   a,    12   a ′ is in the power circuit  10 ,  10 ′, but the actuation of the gate  16 ,  16 ′ only takes place by means of the part  12   b,    12   b ′. In the embodiment of the signal transformer  30 ,  30 ′ with the tap or with isolated parts  12   a,    12   b  or  12   a ′,  12   b ′, said parts can have a different wire cross section in a simple manner. It is thus possible for different current loadings in the two parts  12   a,    12   b  or  12   a ′,  12   b ′ of the secondary winding  12 ,  12 ′ to be taken into consideration and for material to be saved in the winding part which is not subjected to such severe current loading, generally the part  12   b,    12   b ′ used for the actuation. 
     The configurations of the secondary windings  12 ,  12 ′ shown in  FIGS. 3 and 4   a - 4   c  make it possible, furthermore, to use a power transformer  20  with primary windings  17 ,  17 ′ with different turns numbers, for example in order to feed different voltages to the DC/DC converters from voltage sources  11 ,  11 ′ and/or to favor one of the voltage sources  11 ,  11 ′. Since primary windings  17 ,  17 ′ of the power transformer  20  with different turns numbers transmit different powers with the same current, the current transformation ratio which can be matched via the tap of the secondary windings  12 ,  12 ′ can be used at this point as possible correction means, even when the actuating signals for switching the switching elements  13 ,  13 ′ are transmitted with the same transformation ratio. 
     Conversely, a design of the signal transformer  30 ,  30 ′ or an arrangement of the tap of the secondary windings  12 ,  12 ′ is also possible, in which the current back-transformed onto the primary winding  31  of the signal transformer  30  is increased in order to increase the accuracy during measurement of the switch current(s) I 10 , I 10 ′. 
     The embodiments in  FIGS. 2 and 3  each illustrate two power circuits  10 ,  10 ′. The number of possible power circuits  10  and therefore also the number of possible voltage sources feeding the DC/DC converter can also be greater, however. 
     In addition to the two voltage sources  11 ,  11 ′ shown, in the case of a DC/DC converter for providing an on-board power supply system for an inverter for a photovoltaic system, for example, a rectifier connected to an energy supply system with a buffer capacitor can be used as further voltage source. In this way, a supply of operating current to the inverter is possible at night via the energy supply system and during the day from the intermediate circuit and therefore a photovoltaic generator of the photovoltaic system.