Abstract:
A driving circuit is provided for a control terminal of a bipolar transistor in an emitter-switching configuration. The emitter-switching configuration is between a resonant load and a voltage reference. The driving circuit includes at least one capacitor between the control terminal of the bipolar transistor and the voltage reference. The driving circuit further includes an additional resonance capacitor between a collector terminal of the bipolar transistor and a circuit node, a first diode between the circuit node and the control terminal, and a second diode between the circuit node and the voltage reference.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a driving circuit for a control terminal of a bipolar transistor in an emitter-switching configuration with a resonant load. More specifically, the present invention relates to a driving circuit for a control terminal of a bipolar transistor inserted, in an emitter-switching configuration, between a resonant load and a voltage reference. The driving circuit comprises at least one capacitor connected between the control terminal and the voltage reference.  
           [0002]    The present invention particularly relates, but not exclusively, to a high breakdown voltage bipolar transistor cascode-connected to a low voltage MOS transistor in an emitter-switching configuration, and the following description is made with reference to this field of application for convenience of illustration only.  
         BACKGROUND OF THE INVENTION  
         [0003]    An emitter-switching circuit configuration comprises a bipolar transistor having a high breakdown voltage connected to a low voltage power MOSFET transistor. Such a configuration is schematically shown in FIG. 1 and is indicated with reference numeral  1 . The emitter-switching configuration  1  comprises a bipolar transistor T 1  and a MOS transistor M 1  cascode-connected together between a load  3  and a voltage reference, such as ground GND.  
           [0004]    The emitter-switching configuration  1  provides that the bipolar transistor T 1  is of the high voltage (HV) type, i.e., a high breakdown voltage transistor, while the MOS transistor M 1  is of the low voltage (LV) type, i.e., a low breakdown voltage transistor. The bipolar transistor T 1  has a collector terminal connected to the load  3 , and a control terminal or base B 1  connected to a driving circuit  2 .  
           [0005]    The load  3  is of the resonant or quasi-resonant type and comprises an inductor L 1  corresponding to the primary of a transformer, and a capacitor C 1  inserted in parallel with the inductor L 1 . The capacitor C 1  is between the collector terminal of the bipolar transistor Ti and a supply circuit node X 1 . The supply node X 1  is connected to a generator GB that provides a supply voltage Vcc, which is applied to the supply circuit node X 1 .  
           [0006]    The capacitor C 1  is chosen so that it resonates with the inductor L 1  based upon the operating frequency of the intended application. The MOS transistor M 1  has a control terminal or gate connected to the driving circuit  2 .  
           [0007]    The driving circuit  2  comprises a first resistive element R 1  connected to the gate terminal G 1  of the MOS transistor M 1 , and to ground GND via a voltage pulse generator GA. An electrolytic capacitor C 2  is connected between the base terminal B 1  of the bipolar transistor T 1  and ground GND, and has across its terminals a voltage value equal to VB. A second inductor L 2  corresponding to the secondary of a transformer is inserted between a second circuit node X 2  and ground GND. A diode D 1  is inserted between the base terminal B 1  of the bipolar transistor T 1  and the second circuit node X 2 .  
           [0008]    The emitter-switching configuration is particularly interesting at the present time due to the marketing of bipolar transistors having a square RBSOA (Reverse Biased Safe Operating Area) with a current near the peak current. It also has a voltage equal to the breakdown voltage BVCES between the collector and emitter terminals when the base terminal is short-circuited with the emitter terminal (Breakdown Voltage Collector-Emitter Short), as well as of MOS power transistors having a very low drain-source resistance value in conduction conditions RDSON and thus being almost similar to ideal switches.  
           [0009]    The main advantages of an emitter-switching configuration are an extremely low in-conduction voltage fall (typical of bipolar transistors) and a high turn-off speed, as readily known by those skilled in the art. When turning off, the current output from the bipolar transistor base terminal B 1  is equal to the collector terminal current of this transistor, i.e., a very high value current. This causes a drastic reduction in both the storage time and the fall time, allowing the emitter-switching configuration to operate even at frequencies of 150 kHz.  
           [0010]    All the applications satisfying the following relation will now be considered:  
           
         I 
         BOFF 
         *t 
         storage 
         &gt;&gt;I 
         BON 
         *t 
         ON  
       
           [0011]    Where:  
           [0012]    I BOFF  is the base current value of the bipolar transistor T 1  in the turn-off step;  
           [0013]    t storage  is the storage time;  
           [0014]    I BON  is the base current value of the bipolar transistor T 1  in the conduction step; and  
           [0015]    T on  is the conduction time.  
           [0016]    The above relation occurs when the operating frequency is relatively low (i.e., lower than 60 kHz), and particularly if high currents (higher than ten Amperes) are being dealt with. This is the case for rice cooker devices, for example, whose waveforms are indicated by way of example in FIG. 2 for an operating frequency of 35 kHz and with a highest collector current of about 40 A. In this case, the driving circuit of FIG. 1 is straightforward but expensive. In fact, it is necessary to provide a middle power supply (about 10 W).  
           [0017]    Only the secondary part of the relevant power supply has been indicated in FIG. 1 for convenience of illustration. The power supply is essential since the base of the bipolar transistor T 1  must be supplied with a current IB, which in the present application and in all similar applications, cannot be provided by recovering energy during the turn-off, as it happens instead in other applications.  
           [0018]    For a better understanding of this concept it must be observed in FIG. 2 that the area A 2 , representing the amount of charge recovered during the turn-off, is far lower than the area A 1  representing instead the amount of charge required by the base terminal B 1  to make the switching configuration  1  operate correctly. This known approach is thus effective from a performance point of view, but is very expensive. Other known approaches provide for the use of IGBT or MOS power transistors.  
           [0019]    When the following conditions occur: resonant or quasi-resonant load; high collector current (&gt;10 A); relatively low frequency (&lt;60 kHz); and not too low a duty cycle (&gt;15%), the devices formed by IGBT power transistors are particularly suitable. In fact, in view of voltage levels, current values and the relatively low frequency, choosing a MOS power transistor (PowerMOS) would be particularly expensive.  
           [0020]    A MOS power transistor able supporting voltages higher than 1000 V and conducting a high current would involve a silicon area from 5 to 10 times higher than an IGBT transistor or a bipolar transistor having the same breakdown voltage value and current capacity value. Moreover, a MOS power transistor would be unnecessary since its switching speed thereof, which is instead essential at higher frequencies, would not be fully exploited.  
           [0021]    A bipolar transistor in the emitter-switching configuration has highly competitive turn-off times, and in particular, fall times comparable to those obtained with a MOS power transistor. It also has a very low in-conduction fall, the lowest obtainable with power devices, as illustrated in FIG. 4.  
           [0022]    By using an IGBT transistor, a relatively low in-conduction voltage fall is similarly obtained, as indicated by the output features shown in FIG. 3. However, the IGBT transistor has a current queue when turning off, a phenomenon which can be completely deleted by using a bipolar transistor in the emitter-switching configuration (by conveniently driving the base terminal), as shown in FIG. 2. The use of a bipolar transistor in the emitter-switching configuration is thus limited with respect to the base terminal driving current.  
         SUMMARY OF THE INVENTION  
         [0023]    An object of the present invention is to provide a driving circuit for a control terminal of a bipolar transistor in an emitter-switching configuration with a resonant load. The driving circuit has structural and functional characteristics for allowing the base current of the bipolar transistor to be correctly modulated, thus overcoming the limits and drawbacks still effecting configuration driving circuits for resonant applications according to the prior art.  
           [0024]    This and other objects, advantages and features in accordance with the present invention are provided by exploiting the energy usually being transferred from the inductor to the resonance capacitor in the first part of the turn-off step when supplying the bipolar transistor base terminal.  
           [0025]    On the basis of this approach, the technical problem is solved by a driving circuit as previously indicated and defined in the characterizing part of claim  1 .  
           [0026]    The technical problem is also solved by a driving method as previously indicated and defined in the characterizing part of claim  1 . 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The features and advantages of the driving circuit and associated methods according to the invention will be apparent from the following description of an embodiment thereof given by way of a non-limiting example with reference to the attached drawings. In the drawings:  
         [0028]    [0028]FIG. 1 schematically shows a driving circuit for an emitter-switching configuration for resonant applications according to the prior art;  
         [0029]    [0029]FIG. 2 schematically shows the waveforms typical of a rice cooker device according to the prior art;  
         [0030]    [0030]FIGS. 3 and 4 schematically show the output features of an IGBT transistor and an emitter-switching configuration according to the prior art;  
         [0031]    [0031]FIG. 5 schematically shows a driving circuit for a bipolar transistor in the emitter-switching configuration for resonant applications according to the present invention;  
         [0032]    [0032]FIGS. 6 and 7 show the waveforms obtained with a known driving circuit according to the present invention; and  
         [0033]    [0033]FIGS. 8 and 9 show the waveforms obtained by rice cooker devices using the driving circuit as shown in FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    With reference to the figures, and particularly to FIG. 5, a driving circuit for an emitter-switching configuration with a resonant load according to the invention is being schematically indicated with reference numeral  10 . Elements that are structurally and functionally identical to those described with reference to FIG. 1 have been given the same reference numerals.  
         [0035]    The driving circuit  10  is associated with an emitter-switching configuration  1  comprising a bipolar transistor T 1  and a MOS transistor M 1  cascode-connected to each other. The emitter-switching configuration  1  is connected between a load  3  and a voltage reference, such as ground GND.  
         [0036]    As stated above, the emitter-switching configuration  1  provides that the bipolar transistor T 1  is of the HV (High Voltage) type, i.e., a high breakdown voltage transistor, while the MOS transistor M 1  is of the LV (Low Voltage) type, i.e., a low breakdown voltage transistor. The bipolar transistor T 1  has a collector terminal connected to the load  3 , and a control or base terminal B 1  connected to the driving circuit  10 .  
         [0037]    The load  3  is of the resonant type, and comprises an inductor L 1  corresponding to the primary of a transformer, and a first capacitor Cla. The first capacitor Cla is parallel with the inductor L 1  between the collector terminal of the bipolar transistor T 1  and a first circuit node X 1 . The first circuit node X 1  is connected to a generator GB providing a supply voltage Vcc, which is applied to the supply circuit node X 1 . The MOS transistor M 1  has a control or gate terminal connected to the driving circuit  10 .  
         [0038]    The driving circuit  10  comprises a first resistive element R 1  connected to the gate terminal G 1  of the MOS transistor M 1 . The first resistive element R 1  is also connected to ground GND via a voltage pulse generator GA. An electrolytic capacitor C 2  is connected between the base terminal B 1  of the bipolar transistor T 1  and ground GND, and across its terminals a voltage equal to VB.  
         [0039]    Advantageously according to the invention, the driving circuit  10  further comprises a capacitor Clb between the collector terminal TC 1  of the bipolar transistor T 1  and a second circuit node X 3 . First and second diodes D 2 , D 3  are inserted between the second circuit node X 3  and respectively, the base terminal B 1  of the bipolar transistor T 1  and ground GND.  
         [0040]    The first diode D 2  has a cathode terminal connected to the base terminal B 1 , and an anode terminal connected to the second circuit node X 3 . The second diode D 3  has a cathode terminal connected to the second circuit node X 3 , and an anode terminal connected to ground GND.  
         [0041]    Therefore, the driving circuit  10  advantageously exploits the energy transferred from the inductor L 1  to the capacitor C 1  in known devices in the first part of the turn-off step [OFF] in order to supply the base terminal B 1  of the bipolar transistor T 1 .  
         [0042]    With reference to the circuit of FIG. 1, it is possible to connect the first capacitor C 1  between the collector terminal TC 1  of the bipolar transistor T 1  and ground GND without making any changes concerning the system operation comprising the emitter-switching configuration  1  and the resonant load  3 .  
         [0043]    It must be also observed that a capacitor inserted between the collector terminal and ground GND is equal to a capacitor inserted between the collector terminal and the base terminal since the capacitor C 2  is a high electrolytic capacitor with respect to capacitors Cla and Clb. Therefore, in the case shown in FIG. 5, the base terminal B 1  can be considered as being connected to ground GND.  
         [0044]    It is thus evident that capacitors Cla and Clb are in parallel and that they are chosen so that they correspond to the capacity value of the capacitor C 1  of the resonant load  3  according to known approaches, such as the one shown in FIG. 1.  
         [0045]    Advantageously according to the invention, dividing the prior art resonant capacitor C 1  into capacitors Cla and Clb is used to conveniently supply the base terminal.  
         [0046]    To achieve the correct operation of the bipolar transistor T 1 , the driving circuit  10  causes the current flowing in the capacitor Clb during the charge step to flow also in the electrolytic capacitor C 2 , while the opposite sign current, which discharges the capacitor Clb, does not discharge that electrolytic capacitor C 2  due to the presence of diodes D 2  and D 3  being connected to the second circuit node X 2  oppositely to each other.  
         [0047]    Always with reference to FIG. 5, it is possible to describe the driving method according to the invention.  
         [0048]    Particularly, the method according to the invention allows the control terminal B 1  of the bipolar transistor T 1  in the emitter-switching configuration to be driven. The method provides the following steps of applying a first voltage value VB via a capacitor C 2  to the control terminal B 1 ; keeping the electrolytic capacitor C 2  charged via a current flowing in the capacitor Clb due to the presence of diode D 2 ; and preventing a capacitor Clb discharge current from flowing also in the electrolytic capacitor C 2  due to the presence of diode D 3 .  
         [0049]    Advantageously according to the invention, diodes D 2  and D 3  are chosen with a low breakdown voltage value (equal to about 20 V).  
         [0050]    By way of example, in order to show the efficacy of the driving circuit according to the invention, a rice cooker device has been chosen with an operating frequency of 35 kHz and with a highest collector current reaching about 40 A, but the following considerations can be applied to any circuit having similar features to those indicated with reference to the prior art.  
         [0051]    It is worth noting that in the system of FIG. 1 according to the prior art, during the discharge step [OFF], the magnetic energy stored during the conduction step [ON] in the inductor L 1  is transferred to the capacitor C 1  as electrostatic energy so that it can then be transferred again to the inductor L 1 .  
         [0052]    [0052]FIGS. 6 and 7 represent waveforms concerning a driving circuit according to the prior art in the case in which the capacitor C 1  is in parallel with the inductor L 1 , or between the collector terminal and ground, respectively.  
         [0053]    Particularly, it can be observed that the current flowing in the charge step on the capacitor C 1  is the same in both cases.  
         [0054]    It can also be noted in FIG. 6 that the charge serving to supply the base terminal B 1  of the bipolar transistor T 1  for each cycle during the conduction is in the range of 70 μC (area A 1 ), while the amount of charge available is the one charging the capacitor C 1  of FIG. 1 and it is equal to about 200 μC (area A 2 ). In other words, the amount of charge required to supply the base terminal B 1  is about a quarter of the total charge stored on the capacitor C 1 .  
         [0055]    Advantageously according to the invention, the driving circuit  10  replaces the capacitor C 1  with the parallel connection of two capacitors Cla and Clb, both being smaller than C 1  and particularly the one having a value equal to a quarter of the other, so that the sum gives however the value of the capacitor C 1  of the driving circuit according to the prior art. The capacitor Clb connected between the collector and base terminals will have to be the lower capacitor.  
         [0056]    In this way, the situation of FIGS. 8 and 9 is achieved, wherein the curve F 1  is the current flowing on the capacitor Clb, whose positive part charges the capacitor C 2  and allows thus the base terminal B 1  of the bipolar transistor T 1  to be supplied. It is worth noting that, advantageously according to the invention, areas A 3  and A 4  are in this case identical, as underlined in FIG. 8.  
         [0057]    Particularly, the driving circuit  10  according to the invention provides the base terminal B 1  with an amount of charge being proportional to the highest current of the collector terminal TC 1 , as shown in FIGS. 8 and 9 for a driving circuit used in a rice cooker device.  
         [0058]    In FIG. 8, for a high power rice cooker device, the highest collector current is equal to about 38 A and the charge stored on the electrolytic capacitor C 2 , and then used to drive the base terminal B 1 , is equal to 62 μC. In FIG. 8, for a low power rice cooker device, the highest collector current is equal to 30 A and the charge stored on the electrolytic capacitor C 2  is equal to about 50 μC.