Patent Publication Number: US-2023152827-A1

Title: Mirror clamp circuit

Description:
TECHNICAL FIELD 
     The present invention relates to a mirror clamp circuit which prevents erroneous turning-on of a transistor. 
     BACKGROUND ART 
     A method called mirror clamping is known as a method for preventing erroneous turning-on of a transistor such as a MOSFET (metal-oxide-semiconductor field-effect transistor), an IGBT (insulated gate bipolar transistor), or the like. In mirror clamping, if the target transistor is a MOSFET, a switch for mirror clamping is inserted between the gate of the transistor, of which the source is fed with a reference potential, and the reference potential. The switch is configured with, for example, a MOSFET. By keeping the switch on when the transistor is off, the Vgs of the transistor (the voltage between its gate and source) is forcibly kept at approximately 0 V to prevent a rise in the gate voltage of the transistor. If the target transistor is an IGBT, the switch is inserted between the gate of the transistor, of which the emitter is fed with the reference potential, the reference potential. 
     Moreover, in mirror clamping, the switch is turned on with a comparator that compares the gate voltage of the transistor and a predetermined reference voltage. Such a mirror clamp is disclosed in, for example, Patent Document 1. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2016-171720 (FIG. 9) 
     SUMMARY OF INVENTION 
     Technical Problem 
     Inconveniently, the known mirror clamp configuration suffers from increased power consumption, because it keeps feeding a relatively large current to the comparator mentioned above to achieve high operation speed. 
     Against the background discussed above, an object of the present invention is to provide a mirror clamp circuit that permits reduction of power consumption. 
     Solution to Problem 
     According to one aspect of the present invention, a mirror clamp circuit includes: 
     a comparator having:
         a first input terminal connectable to a first control terminal of a transistor having
           the first control terminal connected to the other terminal of a resistor of which one terminal is fed with an output voltage and   a first terminal fed with a reference potential; and   
           a second input terminal fed with a reference voltage;       

     a transistor switch having a second control terminal fed with a control terminal voltage based on a comparison signal output from the comparator, the transistor switch being inserted between the first control terminal and the reference potential; 
     an OR circuit fed with a signal based on the control terminal voltage and the output voltage; and 
     a current feeder configured to change the amount of current fed to the comparator based on the output of the OR circuit (a first configuration). 
     In first configuration described above, there may be further provided a latch circuit configured to latch the control terminal voltage at low level when the output voltage is at high level (a second configuration). 
     In first or second configuration described above, there may be further provided a delay circuit configured to delay the control terminal voltage and output, as a result of the delaying, a delayed signal, and the signal based on the control terminal voltage may be the delayed signal (a third configuration). 
     In any of the first to third configurations described above, the current feeder may include: 
     a first constant-current source; 
     a first current mirror configured to generate a first output current based on a first input current generated by the first constant-current source; 
     a second current mirror configured to generate a second output current based on the first input current; 
     a switch disposed in a path across which the second output current flows, the switch being configured to be turned on and off based on the output of the OR circuit. 
     Here, when the switch is on, a current may be supplied to the comparator based on a current generated by combining the first and second output currents together (a fourth configuration). 
     In the fourth configuration described above, the first current mirror, the second current mirror, and the switch may be configured with PMOS transistors (a fifth configuration). 
     In any of the first to fifth configurations described above, the transistor switch may be an NMOS transistor (sixth configuration). 
     According to another aspect of the present invention, an IC package includes: the mirror clamp circuit according to any of the configurations described above; an output terminal configured to output the output voltage to the outside; a mirror clamp terminal configured to permit the first control terminal to be connected to the outside; and a reference potential terminal configured to permit the reference potential to be applied to it (a seventh configuration). 
     In the seventh configuration described above, there may be further provided: a pulse generator configured to generate a pulse based on an input signal; a logic circuit; an isolation transformer configured to transmit the pulse to the logic circuit; and a switching arm configured with a switching element connected in series between a supply voltage and the reference potential, the switching arm being configured to generate the output voltage by being driven by the logic circuit (an eighth configuration). 
     Advantageous Effects of Invention 
     With a mirror clamp circuit according to the present invention, it is possible to reduce power consumption. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram showing a configuration of a gate driver according to an exemplary embodiment of the present invention. 
         FIG.  2    is a diagram showing an example of a specific circuit configuration of a comparator and a current feeder. 
         FIG.  3    is a timing chart showing an example of operation of a mirror clamp circuit according to an exemplary embodiment of the present invention. 
         FIG.  4    is a diagram showing a configuration of a gate driver according to a modified example. 
         FIG.  5    is a timing chart showing an example of operation of a mirror clamp circuit according to the modified example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an exemplary embodiment of the present invention will be described with reference to the drawings. 
     &lt;Configuration of a gate driver&gt;  FIG.  1    is a diagram showing the configuration of a gate driver  10  according to the exemplary embodiment of the present invention. As shown in  FIG.  1   , the gate driver  10  is a device that drives the gate of an NMOS transistor M 1 . 
     The gate driver  10  includes a primary circuit  1 , a secondary circuit  2 , and an isolation transformer  3 . The gate driver  10  is an IC package including a GND 1  terminal, a VCC 1  terminal, an INA terminal, an INB terminal, a GND 2  terminal (reference potential terminal), a VCC 2  terminal, an OUT terminal (output terminal), and an MC terminal (mirror clamp terminal) which are external terminals (lead terminals) for establishing electrical connection with the outside. 
     The primary circuit  1  includes a first Schmitt trigger  11 , a second Schmitt trigger  12 , an AND circuit  13 , a pulse generator  14 , and a first UVLO (undervoltage lock-out) circuit  15 . 
     The secondary circuit  2  includes a logic circuit  21 , a PMOS transistor  22 , a NMOS transistor  23 , a mirror clamp circuit  24 , a second UVLO circuit  25 , and an OVP (overvoltage protection) circuit  26 . 
     The isolation transformer  3  is provided to connect together the primary and secondary circuits  1  and  2 . The isolation transformer  3  isolates the primary and secondary circuits  1  and  2  from each other while transmitting a signal from the primary circuit  1  to the secondary circuit  2 . 
     The first UVLO circuit  15  monitors a supply voltage Vcc 1 , which is applied to the VCC 1  terminal, and when the supply voltage Vcc 1  drops below a specified voltage, the first UVLO circuit  15  shuts down the primary circuit  1 . 
     The first Schmitt trigger  11  transmits a first input signal In 1 , which is input from the outside to the INA terminal, to a first input terminal of the AND circuit  13 . The second Schmitt trigger  12  transmit a second signal In 2 , which is input from the outside to the INB terminal, to a second input terminal the AND circuit  13 . 
     The AND circuit  13  outputs the AND of the signal level input to its first input terminal and the level resulting from inverting the signal level input its the second input terminal. As a result, the output of the AND circuit  13  is at low level when the first and second input signals In 1  and In 2  are both at low level, or the first input signal In 1  is at low level and the second input signal In 2  is at high level, or the first and second input signals In 1  and In 2  are both at low level, and is at high level when the first input signal In 1  is at high level and the second input signal In 2  is at low level. 
     When triggered by a fall of the output of the AND circuit  13  from high level to low level, the pulse generator  14  generates a pulse with a smaller width than the output of the AND circuit  13  and outputs the pulse to the primary side of the isolation transformer  3 . A change in current caused by the pulse fed to the primary side of the isolation transformer  3  produces a current in the secondary side of the isolation transformer  3 , and this current is fed to the logic circuit  21 . In this state, the logic circuit  21  outputs a high-level signal, which is input to the gate of PMOS transistor  22  and to the gate of NMOS transistor  23 . 
     Here, the PMOS transistor  22  (switching element) and the NMOS transistor  23  (switching element) constitute a switching arm by being connected in series between the supply voltage Vcc 2 , which is applied to the VCC 2  terminal, and the second ground GND 2 , which is applied to the GND 2  terminal. Specifically, the source of the PMOS transistor  22  is connected to an application terminal for the supply voltage Vcc 2 . The drain of the PMOS transistor  22  is connected to the drain of the NMOS transistor  23  at a node N 2 . The source of the NMOS transistor  23  is connected to an application terminal for the second ground GND 2 . 
     A node N 1 , at which the gates of the PMOS and NMOS transistors  22  and  23  are connected together, is connected to the output terminal of the logic circuit  21 . 
     The node N 2  is connected to the OUT terminal. One terminal of a resistor R 1  is externally connected to the OUT terminal. The other terminal of the resistor R 1  is connected to the gate of the NMOS transistor M 1  (first control terminal). The source of the NMOS transistor M 1  (first terminal) is externally connected to the GND 2  terminal. The second ground GND 2 , which serves as the reference potential for in the secondary circuit  2 , differs from the first ground GND 1 , which is applied to the GND 1  terminal and which serves as the reference potential for the primary circuit  1 . 
     Here, with the high-level signal from the logic circuit  21  applied to the node N 1 , the PMOS transistor  22  is off, the NMOS transistor  23  is on, and an output voltage Out, which is the voltage at the OUT terminal, is equal to the second ground GND 2  (low level). As a result, the NMOS transistor M 1  is off. 
     By contrast, then triggered by a rise of the output of the AND circuit  13  from low level to high level, the pulse generator  14  generates a pulse with a smaller width than the output of the AND circuit  13  and outputs the pulse to the primary side of the isolation transformer  3 . A change in current caused by the pulse fed to the primary side of the isolation transformer  3  produces a current in the secondary side of the isolation transformer  3 , and this current is fed to the logic circuit  21 . In this state, the logic circuit  21  outputs a low-level signal, which is applied to the node N 1 . 
     In this state, the PMOS transistor  22  is on, the NMOS transistor  23  is off, and an output voltage Out is equal to the supply voltage Vcc 2  (high level). As a result, the NMOS transistor M 1  is on. 
     The transistor to be driven by the gate driver  10  can be configured with an IGBT instead of the NMOS transistor M 1 . In that case, the other terminal of the resistor R 1  is connected to the gate of the IGBT and the GND 2  terminal is connected to the emitter of the IGBT. 
     The second UVLO circuit  25  monitors the supply voltage Vcc 2 , which is applied to the VCC 2  terminal. When the supply voltage Vcc 2  drops below a specified voltage, the second UVLO circuit  25  shuts down the secondary circuit  2 . The OVP circuit  26  is a circuit that detects an overvoltage in the supply voltage Vcc 2 . 
     &lt;Configuration of the mirror clamp&gt; Next, the configuration of the mirror clamp circuit  24  in the secondary circuit  2  will be described. The mirror clamp circuit  24  is a circuit that prevents erroneous turning-on of the NMOS transistor M 1  in the off state. 
     As shown in  FIG.  1   , the mirror clamp circuit  24  includes a comparator  241 , a latch circuit  242 , an NMOS switch  243 , an OR circuit  244 , and a current feeder  245 . 
     The MC terminal is externally connected to a node N 3 , to which the other terminal of the resistor R 1  and the gate of the NMOS transistor M 1  are connected. The inverting input terminal (—) of the comparator  241  is connected to the MC terminal. Thus, the inverting input terminal (—) of the comparator  241  is fed with the voltage at the MC terminal, that is, a first gate voltage Gt for the NMOS transistor M 1 . A predetermined reference voltage Vref is applied to the non-inverting input terminal (+) of the comparator  241 . 
     The comparator  241  outputs, as the result of comparing the first gate voltage Gt with the reference voltage Vref, a comparison signal Cmp to the latch circuit  242 . According to the voltage level at the node N 1 , that is, the level of the output voltage Out, the latch circuit  242  switches between outputting the comparison signal Cmp as it is with the same level as the second gate voltage Mcg and outputting the second gate voltage Mcg latched at low level regardless of the comparison signal Cmp. 
     The NMOS switch  243 , which is an NMOS transistor, is inserted between the MC terminal and the GND 2  terminal. Specifically, the drain of the NMOS switch  243  is connected to the MC terminal. The source of the NMOS switch  243  is connected to the GND 2  terminal. That is, the NMOS switch  243  is inserted between the gate of the NMOS transistor M 1  and the second ground GND 2  (reference potential). 
     The second gate voltage (control terminal voltage) Mcg is applied to the gate of the NMOS switch  243  (second control terminal). The second gate voltage Mcg is input to the first input terminal of the OR circuit  244  and the output voltage Out is input to the second input terminal of the OR circuit  244 . The current feeder  245  switches between feeding a current with a reference amount of current to the comparator  241  and feeding a current with an amount of current increased from the reference amount the comparator  241 , depending to the output level of the OR circuit  244 . 
     &lt;Configuration of the comparator and the current feeder&gt;  FIG.  2    is a diagram showing an example of a specific circuit configuration of the comparator  241  and the current feeder  245 . 
     The comparator  241  includes PMOS transistors  241 A and  241 B, a current mirror CM 241 , a buffer BF, a clamper CLP, and voltage division resistors R 21  to R 24 . 
     The source of the PMOS transistor  241 A is connected to the current feeder  245 . The gate of the PMOS transistor  241 A is fed with a predetermined reference voltage REF resulting from dividing, with the voltage division resistors R 21  and R 22 , a voltage higher than the second ground GND 2  by a reference voltage (as an example, 5V) (hereinafter, the upper voltage). 
     The source of the PMOS transistor  241 B is connected to the current feeder  245 . The gate of the PMOS transistor  241 B is fed with a division voltage McD resulting from dividing, with the voltage division resistors R 23  and R 24 , the first gate voltage Gt (see  FIG.  1   ). 
     The source and gate of the clamper CLP, configured with a PMOS transistor is connected to the application terminal for the upper voltage. The drain of the clamper CLP is connected to the gate of the PMOS transistor  241 B. 
     The current mirror CM 241  is configured with NMOS transistors N 1  and N 2 . More specifically, the drain and the gate of the NMOS transistor N 1  is short-circuited. The gates of the NMOS transistors N 1  and N 2  are connected together. The sources of the NMOS transistors N 1  and N 2  are both connected to the application terminal for the second ground GND 2 . 
     The drain of the PMOS transistor  241 A is connected to the drain of the NMOS transistor N 1 . The signal appearing at a node N 241 , at which the drains of the PMOS and NMOS transistors  241 B and N 2  are connected together, is output as the comparison signal Cmp (see  FIG.  1   ) via the buffer BF. 
     The current feeder  245  includes a constant-current source  245 A, a PMOS transistor (switch)  245 C, a current mirror CM 1  configured with PMOS transistors, and a current mirror CM 2  configured with PMOS transistors. 
     The current mirror CM 1  includes a PMOS transistor PM 1  as an input-side transistor, and multiplies by one the input current I 1  generated by the constant-current source  245 A, to output the result as an output current I 2 . 
     The current mirror CM 2  includes the PMOS transistor PM 1  as an input-side transistor, and multiplies the input current I 1  by X (X&gt;1) to output the result as an output current I 3 . To the source of the output-side PMOS transistor PM 2  of the current mirror CM 2 , the drain of the PMOS transistor  245 C is connected, and the source of the PMOS transistor  245 C is connected to the application terminal for the upper voltage. The gate of the PMOS transistor  245 C is fed with the output signal of the OR circuit  244 . Thus, when the output of the OR circuit  244  is at low level and the PMOS transistor  245 C is on, the output current I 3  is generated. When the output of the OR circuit  244  is at high level and the PMOS transistor  245 C is off, the output current I 3  is not generated. 
     The output currents I 2  and I 3  are combined together to generate a feed current I 4 . When the output current I 3  is not generated, the output current I 2  as it is becomes the feed current I 4 . The feed current I 4  is distributed between and supplied to PMOS transistors  241 A and  241 B. 
     Here, depending to the reference voltage REF and the division voltage McD, how the feed current I 4  is distributed between the currents flowing through the PMOS transistors  241 A and  241 B of the feed current I 4  changes. 
     When the division voltage McD is lower than the reference voltage REF, the PMOS transistor  241 B allows more current than the PMOS transistor  241 A, and thus the comparison signal Cmp is at high level. 
     By contrast, when the division voltage McD is higher than the reference voltage REF, the PMOS transistor  241 A allows more current than the PMOS transistor  241 B, and thus the comparison signal Cmp is at low level. 
     Therefore, in the configuration shown in  FIG.  1   , the reference voltage Vref compared with the first gate voltage Gt by the comparator  241  is defined by the reference voltage REF and the voltage division ratio at which the division voltage McD is generated. 
     When the PMOS transistor  245 C is off according to the output of the OR circuit  244 , the output current I 3  is not generated, and thus the feed current I 4  with the reference amount of current generated based on the output current I 2  is distributed between and supplied to the PMOS transistors  241 A and  241 B. 
     By contrast, when the PMOS transistor  245 C is on according to the output of the OR circuit  244 , the output current I 3  is generated, and thus the feed current I 4  with an amount of current increased from the reference amount, that is, the current generated by combining the output currents I 2  and I 3  together, is distributed between and supplied to the PMOS transistors  241 A and  241 B. Thus, the operating speed of the comparator  241  increases. 
     &lt;Operation of the mirror clamp circuit&gt; Next, the operation of the mirror clamp circuit  24  will be described based on the configuration in  FIG.  1    described previously and a timing chart in  FIG.  3   .  FIG.  3    shows, from top down, the output voltage Out, the first gate voltage Gt, the comparison signal Cmp, the second gate voltage Mcg, an OR output A, which is the output of the OR circuit  244 , and a feed current I from the current feeder  245 . In the configuration in  FIG.  2    described previously, the feed current I correspond to the feed current I 4 . 
     In  FIG.  3   , immediately before timing t 1 , at which the output voltage Out turns from high level to low level, the first gate voltage Gt is at high level and thus the comparison signal Cmp is at low level. When the output voltage Out is at high level, if the NMOS switch  243  is on, an overcurrent flows from the OUT terminal to the NMOS switch  243  via the resistor R 1  and the MC terminal, and thus it is preferable that the NMOS switch  243  be kept off. Accordingly, when the output voltage Out is at high level, the latch circuit  242  latches the second gate voltage Mcg at low level. Since the output voltage Out is at high level, the OR output A is at high level. As a result, the PMOS transistor  245 C in  FIG.  2    is off and the feed current I is at a reference level. 
     When the output voltage Out turns from high level to low level at timing t 1 , owing to the resistor R 1 , the first gate voltage Gt starts to fall slowly from the output voltage Out. When the output voltage Out is at low level, the latch circuit  242  outputs the level of the comparison signal Cmp as it is as the second gate voltage Mcg. 
     Thus, the output voltage Out and the second gate voltage Mcg are both at low level and hence the OR output A is at low level. Thus, the PMOS transistor  245 C in  FIG.  2    is turned on and the feed current I now has a level increased from the reference level. In this way, the operation speed of the comparator  241  can be increased in advance in preparation for detection, by the comparator  241 , of the first gate voltage Gt having reached the reference voltage Vref as a result of a fall in the first gate voltage Gt. 
     After that, the first gate voltage Gt falls and reaches the reference voltage Vref at timing t 2 , when the comparator  241  turns the comparison signal Cmp to high level, and the second gate voltage Mcg is turned to high level. Thus, the NMOS switch  243  is turned on, the Vgs of the NMOS transistor M 1  is forced to be at approximately 0 V, and a rise in the gate voltage of the NMOS transistor M 1  can be prevented. In this way, erroneous turning-on of the NMOS transistor M 1  can be prevented. 
     Since the second gate voltage Mcg is at high level, the OR output A is at high level, and the feed current I is dropped to the reference level. 
     After timing t 2 , the first gate voltage Gt falls to reach low level. 
     After that, when at timing t 3  the output voltage Out turns from low level to high level, the first gate voltage Gt starts to rise slowly, and the second gate voltage Mcg is latched at low level by the latch circuit  242  as mentioned previously. Thus, the NMOS switch  243  is turned off. Since the output voltage Out is at high level, the OR output A is at high level, and the feed current I is at the reference level. 
     After that, when at timing t 4  the first gate voltage Gt rises to reach the reference voltage Vref, the comparison signal Cmp turns to low level. After timing t 4 , the first gate voltage Gt rises to reach high level. 
     As described above, the feed current I is increased only during a limited period in which the output voltage Out and the second gate voltage Mcg are both at low level (the period from timing t 1  to t 2 ). It is thus possible to reduce power consumption while improving the operating speed of the comparator  241  only when necessary. 
     &lt;Modified example&gt;  FIG.  4    is a diagram showing the configuration of a gate driver  10  including a mirror clamp circuit  24 X according to a modified example of the embodiment described previously. 
     A difference of the configuration of the mirror clamp circuit  24 X shown in  FIG.  4    from that in the previously-described embodiment ( FIG.  1   ) is that the mirror clamp circuit  24 X includes a delay circuit  246 . 
     More specifically, the delay circuit  246  is disposed in a stage succeeding the latch part  242 , and delays the second gate voltage Mcg, which is output from the latch circuit  242 , to output a delayed signal Dly to the first input terminal of the OR circuit  244 . 
       FIG.  5    is a timing chart showing the operation of the mirror clamp circuit  24 X according to the so modified example. 
     At timing t 1  shown in  FIG.  5   , when the output voltage Out turns from high level to low level, the OR output A of the OR circuit  244  turns from high level to low level and the feed current I is increased above the reference level. After that, when the output voltage Out falls to reach the reference voltage Vref at timing t 2 , the comparison signal Cmp turns to high level and the second gate voltage Mcg turns to high level. Thus, the NMOS switch  243  is turned on. 
     After that, at timing t 21 , which is delayed from timing t 2  by a delay time Td, the delayed signal Dly turns to high level. Thus, the OR output A turns to high level and the feed current I falls to the reference level. 
     Here, shown in  FIG.  5   , the delay time Td is set longer than the time Tgt that the first gate voltage Gt requires to fall from the reference voltage Vref to low level. In this way, until the first gate voltage Gt completely turns to low level, the feed current I can be kept stable so as not to vary. 
     &lt;Modifications&gt; It should be understood that the above-described embodiment is in every aspect illustrative and not restrictive. The scope of the present invention is defined not by the description of the embodiment given above but by the appended claims, and encompasses any modifications made without departure from the scope and sense equivalent to those claims. 
     INDUSTRIAL APPLICABILITY 
     The present invention finds applications in, for example, gate drivers. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  gate driver 
               1  primary circuit 
               11  first Schmitt trigger 
               12  second Schmitt trigger 
               13  AND circuit 
               14  pulse generator 
               15  first UVLO circuit 
               2  secondary circuit 
               21  logic circuit 
               22  PMOS transistor 
               23  NMOS transistor 
               24  mirror clamp circuit 
               241  comparator 
               242  latch circuit 
               243  NMOS switch 
               244  OR circuit 
               245  current feeder 
               246  delay circuit 
               25  second UVLO circuit 
               26  OVP circuit 
             R 1  resistor 
             M 1  NMOS transistor