Patent Publication Number: US-8970263-B2

Title: Semiconductor device driving unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit and priority of Great Britain Patent Application No. 1216743.3 filed Sep. 19, 2012. The entire disclosure of the above application is incorporated herein by reference. 
     BACKGROUND 
     Semiconductor device driving units are conventionally used to generate a three phase AC voltage to drive a synchronous motor from a variable speed drive. Such a drive unit may be used in electric vehicles, although they have other uses in other fields. Semiconductor device driving units may employ a number of gate driven semiconductor devices, for example IGBTs (Insulated-Gate Bipolar Transistor) or MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistor). 
     SUMMARY 
     IGBTs and MOSFETs are semiconductor devices that are driven by a gate-emitter voltage Vge and can be turned on and off by an input voltage signal at the gate. The term “turn on switching” means switching from a collector-emitter cut off state to a collector-emitter conductor state. The term “turn-off switching” means switching from a collector-emitter conductor state to a collector-emitter cut off state. 
     Examples of known driving units to control the switching trajectories of the semiconductor switching device in the driving unit may be found in the following documents: European patent application no. 2418776; International patent application no. WO2008/153631, European patent application no. EP1881587 and European patent application no. 0817381. 
    
    
     
       DRAWINGS 
       The technique will be described further, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  shows an example of a semiconductor device driving unit; 
         FIG. 2  shows in greater detail an example of a first embodiment of a semiconductor device driving unit; 
         FIG. 3  shows an example procedure to be executed to determine the gate impedance circuits to be connected; 
         FIG. 4  shows an example of the collector current Ic during the turn-on switching of the semiconductor switching device; 
         FIG. 5  shows an example of the collector-emitter voltage Vce during the turn-on switching of the semiconductor switching device; 
         FIG. 6  shows an example of the gate-emitter voltage Vge respectively during the turn-on switching of the semiconductor switching device; 
         FIG. 7  shows an example of the collector current Ic during the turn-off switching of the semiconductor switching device; 
         FIG. 8  shows an example of the collector-emitter voltage Vce during the turn-off switching of the semiconductor switching device; and 
         FIG. 9  shows an example of the gate-emitter voltage Vge during the turn-off switching of the switching semiconductor device. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG. 1 , a semiconductor device driving unit  100  comprises a semiconductor switching device  2  (for example an insulated gate bipolar transistor IGBT or a MOSFET), a switching unit  4 , a drive voltage  6  and a microprocessor  8 . The microprocessor  8  controls the switching of the switching unit  4  and the voltage source  6  to effect switching of the semiconductor switching device  2  and produce a drive signal for a load  10 . For simplicity, the driving unit  100  of  FIG. 1  comprises a single semiconductor switching device  2  (for example an insulated gate bipolar transistor IGBT or a MOSFET). However it will be appreciated that the driving unit  100  may comprises a plurality of semiconductor switching devices  2  (for example insulated gate bipolar transistor IGBTs or MOSFETs), with associated switching units  4  and drive voltages controlled by the microprocessor  8 . 
       FIG. 2  shows an embodiment with further details regarding the switching unit  4  and the voltage source  6 . It is proposed that multiple gate resistors and capacitor networks be used for each IGBT gate drive. 
     The switching unit  4  comprises a plurality of gate impedance circuits  40 ,  42  and diodes  44 ,  46 . The diodes  44 ,  46  control the current flow when the switching device  2  is turned on or off. In the on state, current flows via the gate impedance circuits  40  in the on loop of the switching device  2 . In the off state, current flows via the gate impedance circuits  42  in the off loop of the switching device  2 . Each gate impedance circuit  40 ,  42  comprises one or more resistors R and one or more capacitors C. For instance first gate impedance circuit  40   a  comprises resistors Ron_ 1 _a and Ron_ 1 _b and the capacitor C 1 _on. Resistors Ron_ 1 _a and Ron_ 1 _b may comprise a single resistor or two resistors in series as shown. The second gate impedance circuit in the on loop, gate impedance circuit  40   b , comprises resistors Ron_ 2 _a and Ron_ 2 _b and capacitor Con_ 2 . A third gate impedance circuit  40   c  is shown in dotted line and comprises resistors Ron_n_a and Ron_n_b and capacitor Con_n to show that as many preset gate impedance circuits  40  as required may be provided. Similarly in the off loop, a first gate impedance circuit  42   a  comprises resistors Roff_ 1 _a and Roff_ 1 _b and capacitor Coff_ 1 . A second gate impedance circuit  42   b  comprises resistors Roff_ 2 _a and Roff_ 2 _b and capacitor Coff_ 2 . Similarly, in dotted lines, a third off gate impedance circuit  42   c  is shown comprising resistors Roff_n_a and Roff_n_b and capacitor Coff_n. As many off gate impedance circuits  42  may be provided as necessary. 
     In use, the microprocessor  8  provides a first control signal (indicated by box  12 ) to a microprocessor controllable switch  18  to control the switching voltage  6  for the semiconductor switching device  2 . Voltage  6   a  is the on voltage and voltage  6   b  is the off voltage. The voltage determines whether the semiconductor switching device  2  is in the on or the off condition. The control signal  12  determines whether the on drive voltage  6   a  or the off drive voltage  6   b  is provided to the semiconductor switching device  2 . The microprocessor  8  provides a second control signal (indicated by box  14 ) to a microprocessor controllable switch  48  to control which of the gate impedance circuits  40  are connected during the on cycle of the semiconductor switching device  2 . The microprocessor  8  provides a third control signal (indicated by box  16 ) to a microprocessor controllable switch  49  to control which of the off gate impedance circuits  42  are connected during the off cycle of the switching. The selection unit comprising the control signals  14  and  16  and associated switches  48 ,  49  respectively therefore determines the selectable impedance gate drive circuit(s)  40 ,  42  selected at a given time. 
     The semiconductor device driving unit  100  is used to supply a drive signal to a gate of a semiconductor switching device  2 . The semiconductor device driving unit  100  comprises a plurality of gate impedance circuits  40 ,  42  connectable in a selectable manner to the semiconductor switching device  2 . Data is stored defining which one or more gate impedance circuits  40 ,  42  to connect to the semiconductor switching device  2  based on specific intended operating conditions of the semiconductor device driving unit  100 . A processor  8  selects one or more of the gate impedance circuits  40 ,  42  to connect to the semiconductor switching device based on the specific intended operating conditions and the stored data. 
     The choice of gate impedance circuit  40 ,  42  used to drive the semiconductor switching device  2  (e.g. an IGBT) strongly influences the measured radiated emissions, conducted emissions, and switching loss of the driving unit. The proposed gate drive can control the selected gate drive impedance to suit the needs of the application. 
     The selection of the appropriate network(s)  40 ,  42  can take place using a command from the variable speed drive (VSD) microprocessor  8 . This may be for the purpose for instance of any of the following functions:
     1) Reduction of radiated emissions at low output phase currents.   2) Increase gate resistance across entire operating range to reduce conducted emissions and allow the use of longer drive—motor cables. This can be selected based on the operating IGBT switching frequency.   3) Selection of optimum gate drive for use with second sourcing IGBT manufacturers   4) Reduction of the thermal fatigue due to thermal cycling.   

     A positive voltage  6   a  (V G     —     on ) is applied to the gate drive of the semiconductor switching device  2  to initiate the switching on of the IGBT with the microprocessor (uP) on/off control illustrated with the box  12  and switch  18 . The current from the gate power supply flows through the selected impedance network(s)  40 , as determined by the microprocessor selectable switches  48  controlled by the signal from the microprocessor as indicated by box  14 . During turn off, the uP on/off control signal indicated by box  12  causes a negative voltage  6   b  (V G     —     off ) to be applied to the gate drive of the switching device  2 . This removes the charge from the gate of the switching device  2  via the selected network(s)  42   x . The number of networks  40 ,  42  used can be increased as required for both turn on and off operation. 
       FIG. 2  shows an implementation in which a plurality of gate impedance networks are provided in both the on loop and the off loop, with each gate impedance network  40  or  42  being in a full bridge arrangement (i.e. the gate impedance circuit  40   a  is always connected and the additional gate impedance circuits  40   b ,  40   c  may be connected in parallel with the first gate impedance circuit  40   a , either separately or in parallel with each other). However it is also envisaged that the gate impedance networks  40 ,  42  may be connected in other arrangements, e.g. in series, in parallel, in combination with one or more other gate impedance networks, in isolation of other gate impedance networks etc. 
     Reduction of Radiated Emissions 
     It has been identified through research that the radiated emissions from a single fixed gate resistance reduce with increasing IGBT collector current. The proposed technique utilises this discovery by allowing the selection from a number of defined gate impedance networks  40 ,  42  by a microprocessor  8  from a knowledge of the output phase current I c . The appropriate gate impedance over the operating current range may be selected from the gate impedance circuits  40 ,  42 . 
     Altering the gate drive impedance circuit  40 ,  42  adjusts the transient behaviour of the collector current Ic and voltage during switching. With the proposed switching unit, the optimum gate drive impedance can be selected for each output phase current to minimise switching loss while maintaining compliance with radiated emissions limits. 
     To this end, the microprocessor has stored procedures to determine the selection of the gate impedances based on the required output phase current of the switching device. For instance, for the required output phase current the microprocessor stores a look up table, an example of which is shown in Table 1 for two output phase currents, although data may be stored for other output phase current: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Required output 
                   
                   
               
               
                   
                 phase current 
                 On gate impedance 
                 Off gate impedance 
               
               
                   
                   
               
             
            
               
                   
                 Ia 
                 40a 
                 42a 
               
               
                   
                 Ib 
                 40b 
                 42b 
               
               
                   
                   
               
            
           
         
       
     
     The device drive unit  100  may then be set up by a user in dependence on the required output phase current. For instance, on set up a user enters the required output phase current (say Ib) and the microprocessor looks up in the look up table which gate impedances are required for this output phase current. In the example above, this causes gate impedance circuits  40   b  and  42   b  to be selected in the on and off part of the switching cycle respectively. 
     Increase Cable Length at High Switching Frequencies 
     The length of cable which can be used with a load is often restricted by the conducted emissions. The conducted emissions increase with switching frequency for a given length of cable. The conducted emissions are known to be related to the dV/dt of the IGBT switching transients. 
     Using the proposed gate drive, the choice of gate impedance circuit  40 ,  42  may be selected for the operational switching frequency and cable length. When details of the connected cable are entered, the microprocessor  8  recalculates the change in switching loss to compensate ensuring the product remains protected. It is possible to further expand this to select the gate impedance circuit  40 ,  42  for the output phase current. 
     To this end, the microprocessor has stored procedures to determine the selection of the gate impedances based on the operational switching frequency and cable length. For instance, for the operational switching frequency and cable length the microprocessor stores a look up table, an example of which is shown in Table 2 for two operational switching frequencies and two cable lengths. Of course the stored data may relate to many more combinations of operational switching frequency and cable length: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 operational 
                   
                   
                   
               
               
                 switching 
               
               
                 frequency 
                 cable length 
                 On gate impedance 
                 Off gate impedance 
               
               
                   
               
             
            
               
                 A 
                 X 
                 40a 
                 42a 
               
               
                 B 
                 X 
                 40b 
                 42a + 42b 
               
               
                 A 
                 Y 
                 40a + 40b 
                 42a 
               
               
                 B 
                 Y 
                 40a + 40b 
                 42b 
               
               
                   
               
            
           
         
       
     
     The device drive unit  100  may then be set up by a user in dependence on the required operational switching frequency and cable length. For instance, on set up a user enters the required operational switching frequency and cable length and the microprocessor looks up in the look up table which gate impedances are required for these intended conditions. In the example above, for an operational switching frequency of B and a cable length of X this causes gate impedance circuit  40   b  to be selected in the on part of the switching cycle and impedance circuits  42   a  and  42   b  to be selected in the off part of the switching cycle. 
     Selection of Optimal Gate Drive for Use with Second Source IGBT Manufacturers 
     With the requirements for security of component supplies, is it often required to have a second manufacture supply a similar semiconductor switching device (e.g. a IGBT) for drive production. To achieve the optimum switching performance from both IGBTs from different manufacturers, different impedance networks  40 ,  42  can be selected determined by the installed semiconductor switching device  2 . 
     To this end, the microprocessor has stored procedures to determine the selection of the gate impedances based on the switching device used. For instance, for each defined switching device the microprocessor stores a look up table, an example of which is shown in Table 3 for three switching devices. Of course the stored data may relate to any number of switching devices: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Switching device 
                 On gate impedance 
                 Off gate impedance 
               
               
                   
                   
               
             
            
               
                   
                 IGBT1 
                 40a 
                 42a 
               
               
                   
                 IGBT2 
                 40b 
                 42b 
               
               
                   
                 MOSFET1 
                 40c 
                 42b 
               
               
                   
                   
               
            
           
         
       
     
     The device drive unit  100  may then be set up by a user in dependence on the installed switching device. For instance, on set up a user enters the switching device used and the microprocessor looks up in the look up table which gate impedances are required for these intended conditions. In the example above, for a switching device IGBT2 this causes gate impedance circuits  40   b  and  42   b  to be selected in the on and off part of the switching cycle respectively. 
     Reduction of Thermal Fatigue Due to Thermal Cycling 
     It is also possible that a selectable gate impedance could be used to reduce the change in temperature across the junction (delta Tj) in the IGBT for specific applications to increase product lifetime. When a low collector current is measured, the gate impedance circuit  40 ,  42  may be altered so that the power loss of the semiconductor switching device  2  is maintained relatively constant over the cyclic application. This aims to reduce thermal fatigue of the semiconductor switching device. 
     To this end, the microprocessor has stored procedures to determine the selection of the gate impedances based on the measured collector current Ic. For instance, for each defined collector current Ic the microprocessor stores a look up table, an example of which is shown in Table 4 for three collector currents. Of course the look up table may relate to any number of collector currents: 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Measured collector 
                   
                   
               
               
                 current Ic 
                 On gate impedance 
                 Off gate impedance 
               
               
                   
               
             
            
               
                 A 
                 40a 
                 42a 
               
               
                 B 
                 40b 
                 42b 
               
               
                 C 
                 40c 
                 42b 
               
               
                   
               
            
           
         
       
     
     The device drive unit  100  may then be set up by a user in dependence on the measured collector current. For instance, on set up a user enters the details of the measured collector current and the gate impendence to be selected when that collector current is measured and, in operation, the microprocessor looks up in the look up table which gate impedances are required for these intended conditions. In the example above, when a measured collector current in a device equals C this causes gate impedance circuits  40   c  and  42   b  to be selected in the on and off part of the switching cycle respectively. 
     The above examples of look up tables are examples only. It will be clear to a skilled person that the look up tables may include combinations of variables not disclosed above (e.g. defined gate impedances for all combinations of the variables discussed above) and that the data may be stored in others formats than look-up tables. For instance, the data may be stored in the form of procedures or routines to be executed. For example,  FIG. 3  shows an example procedure to be executed to determine the gate impedance circuits to be connected for a required output phase current (similar to the example discussed above in Table 1). At operation  301 , the required output phase current IΦ is read. If IΦ is less than a value A (operation  302 ) then the gate impedances  40   a + 40   b  and  42   a + 42   b  are connected (operation  303 ). If the required output phase current IΦ is greater than a value A and less than a value B (operation  304 ) then the gate impedances  40   a  and  42   a  are connected (operation  305 ). If the required output phase current IΦ is greater than a value B (operation  304 ) then the gate impedances  40   b  and  42   b  are connected (operation  306 ). Similar routines may be stored for other intended operating conditions, for example those operating conditions discussed above in relation to Tables 2 to 4. 
     A conflict may arise between gate impedance circuit selections for different intended operating conditions. In the case of conflict, the highest gate impedance option for the on loop and the off loop may be selected. 
     Selection of the gate impedance circuit(s) and connection of the selected gate impedance circuit(s) to the gate of the semiconductor switching device  2  are typically preformed before switching of the device occurs. 
       FIGS. 4 to 9  show example outputs possible with selecting gate impedances based on certain conditions of the switching device.  FIGS. 4 to 6  show the collector current Ic, the collector-emitter voltage Vce and the gate-emitter voltage Vge respectively during the turn-on switching of the switching device.  FIGS. 7 to 9  show the collector current Ic, the collector-emitter voltage Vce and the gate-emitter voltage Vge respectively during the turn-off switching of the switching device. Vt is the threshold voltage of the respective diode  44 ,  46 , VF is the forward voltage of the respective diode  44 ,  46 , Vge, IL is the gate-emitter voltage Vge at load current IL. 
     During turn on, the ramp rate of Vge strongly influences the collector current Ic ramp rate and the Vce ramp rate (as shown in  FIGS. 4 to 6  between time t 1  and t 6 ). The gate impendence  40  switched in during turn-on switching is therefore selected to control the ramp rates of both Ic and Vce to limit radiated and conducted emissions, based on the operating characteristics of the driving unit (for instance: the required output phase current of the switching device; the length of cable connecting the semiconductor device driving unit to a load; the required operational switching frequency of the switching device; the switching device of the driving unit; the collector current of the switching device to maintain a relatively constant the power loss during switching). 
     During turn off switching, Vce tends to overshoot the required level (as shown in  FIG. 8  after time t 3 ). The gate impedance  42  switched in during turn-off switching is therefore selected to minimise this overshoot, based on the operating characteristics of the driving unit (for instance: the required output phase current of the switching device; the length of cable connecting the semiconductor device driving unit to a load; the required operational switching frequency of the switching device; the switching device of the driving unit; the collector current of the switching device to maintain a relatively constant the power loss during switching). 
     The data stored for specific intended operating conditions are typically determined empirically by experimenting with combinations of resistors and capacitors for each given operating condition to be considered. Clearly the more gate impedance circuit options there are, the more complicated and device intensive the switching circuit  4  becomes. Each specific implementation will determine the number of on gate impedance circuits  40  and the number of off gate impedance circuits  42  that are justified. Once determined, the microprocessor may be programmed with the relevant procedures and stored data (e.g. look up tables) for the specific intended operating conditions.