Abstract:
According to one aspect, a transistor gate drive comprises a first input configured to be coupled to a DC voltage source, a second input configured to receive a control signal, a third input configured to couple to a ground connection, a transformer, a first switch configured to couple the first input to a first end of a primary winding of the transformer in response to receipt of the control signal, and to decouple the first input from the first end of the primary winding in response to the receipt of the control signal, a second switch configured to couple a second end of the primary winding to the third input in response to receipt of the control signal, and to decouple the second end of the primary winding from the third input in response to the receipt of the control signal.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of Invention 
         [0002]    At least some embodiments described herein relate generally to high-speed isolated transistor gate drives, and more particularly to the coupling of electronic pulses between two isolated circuits in such applications. 
         [0003]    2. Discussion of Related Art 
         [0004]    In various applications it may be advantageous to isolate high side switches in off-line circuits or similar high voltage circuits. For example, a common isolated transistor gate drive circuit includes a transformer to provide galvanic isolation between an input of the gate drive circuit and an output of the gate drive circuit. A primary winding of the transformer is coupled to an input side of the circuit and a secondary winding of the transformer is coupled to an output side of the circuit providing isolation between the input and the output. One or more additional primary and secondary windings may be included in the transformer and may drive a number of additional output side switching circuits. 
       SUMMARY OF INVENTION 
       [0005]    At least one aspect of the invention is directed to a transistor gate drive. The transistor gate drive comprises a first input configured to be coupled to a DC voltage source, a second input configured to receive a control signal, a third input configured to couple to a ground connection, a transformer having a primary winding and at least one secondary winding, the primary winding having a first end and a second end, a first switch having a control input and configured to couple the first input to the first end of the primary winding in response to receipt of the control signal having a first level, and to decouple the first input from the first end of the primary winding in response to the receipt of the control signal having a second level, a second switch having a control input and configured to couple the second side of the primary winding to the third input in response to receipt of the control signal having the first level, and to decouple the second side of the primary winding from the third input in response to the receipt of the control signal having the second level, a first diode coupled between the first switch and the first end of the primary winding and further coupled to the third input, and a second diode coupled to the first input and further coupled between the second switch and the second end of the primary winding. 
         [0006]    According to one embodiment, the first diode and the second diode are configured to provide a path for a reset of the primary winding. In another embodiment, the first switch includes a control pin coupled to the second input, a collector coupled to the first input, and an emitter coupled between the first diode and the primary winding of the transformer. In still another embodiment, the second switch is a MOSFET having a drain coupled between the primary winding and the second diode and a source coupled to the third input. 
         [0007]    According to another embodiment, the primary winding and the secondary winding have a same polarity. In this embodiment, the transistor gate drive further comprises at least one secondary circuit coupled to the at least one secondary winding of the transformer and having an output configured to drive a control pin of a transistor based on a level of the control signal. In addition, the at least one secondary circuit may comprise a turn-off switch, and a turn-on diode, wherein the turn-off switch and the turn-on diode are coupled in parallel with the at least one secondary winding of the transformer, and wherein a junction point between the turn-on diode and the turn-off switch are coupled to the output of the at least one secondary circuit. Further, the at least one secondary circuit may comprise a pull-down resistor coupled between the output of the at least one secondary circuit and an output ground connection. 
         [0008]    In another embodiment, the PWM controller may be coupled to the first switch and the second switch, and the PWM controller may be configured to generate the control signal. In this embodiment, the PWM controller may be configured to generate a control signal having a duty cycle of 50% or less. In addition, the duty cycle may be a variable duty cycle based on input signal provided to the PWM controller. 
         [0009]    Another aspect of the invention is directed to a method of operating a transistor gate drive circuit. The method comprises receiving a DC input voltage at a first input of the transistor gate drive circuit, receiving, in a turn-on mode, a control signal at a second input of the transistor gate drive circuit, the control signal having a first level, coupling, responsive to receipt of the control signal having the first level, the first input to a first end of a primary winding of a transformer, coupling, responsive to receipt of the control signal having the first level, a second end of the primary winding of the transformer to a ground connection, receiving, in a turn-off mode, the control signal at the second input of the transistor gate drive circuit, the control signal having a second level, and discharging, in response to the receipt of the control signal having the second level, the transformer. 
         [0010]    According to one embodiment, the method further comprises inducing, in the turn-on mode, a current in at least one secondary winding of the transformer; and using the current, driving at least one primary switch of at least one output circuit to turn on. 
         [0011]    According to another embodiment, the method further comprises decoupling, responsive to receipt of the control signal having the second level, the first input from the first end of the primary winding of the transformer, decoupling, responsive to receipt of the control signal having the second level, the second end of the primary winding of the transformer from the ground connection, and resetting the primary winding of the transformer through a first diode and a second diode. 
         [0012]    According to one embodiment, resetting includes limiting a voltage across the first switch and the second switch. 
         [0013]    According to another embodiment, the method further comprises driving, in response to resetting the primary winding, a turn-off switch to turn on, and coupling, in response to the turn-off switch turning on, a control pin of at least one primary switch of at least one output circuit to a second ground connection, and driving the at least one primary switch to turn off. 
         [0014]    According to one embodiment, receiving further includes receiving the control signal from a pulse-width modulation (PWM) controller, the control signal having a duty cycle. In this embodiment, the method further comprises changing the duty cycle of the control signal from a first duty to a second duty cycle. 
         [0015]    Still another aspect of this invention is directed to a transistor gate drive circuit. The gate drive circuit comprises a first input configured to be coupled to a DC voltage source, a second input configured to receive a control signal, a third input configured to couple to a ground connection, a transformer, the transformer having a primary winding and at least one secondary winding, and means for inducing a current on the secondary winding, in a turn-on mode of operation, by coupling the primary winding of the transformer to the first input and the ground connection creating a first current path that includes the primary winding of the transformer, and in a turn-off mode of operation, resetting the primary winding of the transformer through a second current path, wherein neither the first current path nor the second current path includes a capacitor. 
         [0016]    In one embodiment, the transistor gate further comprises means for limiting voltage across the first switch and the second switch during the turn-off mode. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]    The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
           [0018]      FIG. 1  is a schematic diagram of a transistor gate drive according to one approach; and 
           [0019]      FIG. 2  is a schematic diagram of a unipolar, isolated transistor gate drive circuit according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Various embodiments and aspects thereof will now be discussed in detail with reference to the accompanying drawings. It is to be appreciated that this invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing”, “involving”, and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0021]    As discussed above, a typical isolated transistor gate drive circuit includes a transformer coupled between an input side and an output side to provide isolation.  FIG. 1  shows one prior approach to an isolated transistor gate drive  100 . This approach may be used in conjunction with a pulse width modulation (PWM) current controller to drive a high side-switch. The isolated transistor gate drive  100  will couple an AC signal from an input side  134  to an output side  136  through a transformer  110 . The transistor gate drive  100  operates on a theoretical maximum duty cycle of 50%. This insures that over any given period of time the integral of the positive voltages plus the integral of the negative voltages will be zero across the transformer  110 . As will be discussed below, the introduction of a DC component may cause saturation of the transformer  110 . In addition, an asymmetrical pulse, or sudden change in duty cycle, may generate a shift in voltage level at the gate  122  of a primary switch  124 . Therefore, in typical prior art circuits any voltage shift or DC component is blocked or filtered. 
         [0022]    The isolated transistor gate drive  100  includes the input side  134 , an input  108 , a first switch  102 , a second switch  104 , a DC power source (V DR )  116 , a ground connection  114 , a DC blocking capacitor  112 , a primary winding  118 , the transformer  110 , the output side  136 , a secondary winding  120 , the primary switch  124 , the gate  122 , a pull-down resistor  126 , a turn-off diode  128  and a PWM current controller  150 . 
         [0023]    The output of the PWM controller  150  is coupled to the input  108 . The input  108  is coupled to the base of the first switch  102  and a base of the second switch  104 . The collector of first switch  102  is coupled to the V DR    116 . As shown in  FIG. 1 , the first switch  102  and the second switch  104  are arranged in a common bipolar, non-inverting totem-pole configuration. The intersection between the first switch  102  and the second switch  104  is coupled to the primary winding  118  of the transformer  110 . An emitter of the second switch  104  is coupled to the ground connection  114  and also coupled to one end of the DC blocking capacitor  112 . The other end of DC blocking capacitor  112  is coupled to the primary winding  118  of the transformer  110 . The secondary winding  120  of the transformer  110  is coupled to a cathode  132  of the turn-off diode  128 . An anode  130  of a diode  128  is coupled to the pull-down resistor  126  and to the gate  122  of the primary switch  124 . Although one particular input side  134  switching circuit is shown in  FIG. 1 , various other prior approaches to isolated transistor gate drives are well known in the art. Likewise, although one particular output side  136  switching circuit is shown in  FIG. 1 , a number of additional output side switching circuits may be utilized and are well known in the art. 
         [0024]    During the turn-on stage (e.g., a rising edge of a square wave from the PWM current controller  150 ), a voltage is applied to the input  108  driving the base of the first switch  102  and the second switch  104  high. The first switch  102  is a NPN type transistor which turns on and begins to conduct from emitter to collector when driven high. The second switch  104  is a PNP transistor that is turned off when driven high. Current from V DR    116  through the first switch  102 , and the primary winding  118  of transformer  110 , induces a positive current in the secondary winding  120  of the transformer  110 . At the same time, the current through the primary winding  118  of the transformer  110  charges the DC blocking capacitor  112 . The induced current in the secondary winding  120  of the transformer  110  causes the primary switch  124  to turn on. 
         [0025]    During the turn-off stage (e.g., a falling edge of a square wave from the PWM current controller  150 ), the base of the first switch  102  and the base of the second switch  104  are driven low. As a result, the first switch  102  is turned off and the second switch  104  is turned on. The DC blocking capacitor  112  then provides a reset voltage to the primary winding  118  of the transformer  110 . The transformer  112  then discharges inducing a negative current on the secondary winding  120 . As a result, the turn-off diode  128  becomes forward-biased and begins to conduct, providing a path for the negative current to drive gate  122  of the primary switch  124  to a negative potential (low) causing the primary switch  124  to turn off. 
         [0026]    The aforementioned approach accomplishes galvanic isolation of the input side  134  and the output side  136  through the coupled transformer  110  using the DC blocking capacitor  112  coupled to the primary winding  118  of the transformer  110 . Without the DC blocking capacitor  112  in the circuit  100 , a duty cycle dependent DC voltage will exist across the transformer  110 , causing the transformer  110  to saturate over time. However, the inclusion of the DC blocking capacitor  112  limits the overall speed at which the primary winding  118  of the transformer  110  resets. Also, the inclusion of the DC blocking capacitor  112  necessitates that the duty cycle remain relatively constant. A sudden change in the duty cycle will cause a proportional shift in voltage supplied to the gate  122 . In addition, sudden changes in the duty cycle may excite the resonant tank (or LC resonant tank) formed by the magnetizing inductance of the transformer  110  and the DC blocking capacitor  112  and adversely affect the shape of the output waveform. 
         [0027]    At least some embodiments described herein provide a unipolar, isolated transistor gate drive circuit with protective clamp and reset circuit which eliminates the need for a DC blocking capacitor. According to some embodiments, the transistor gate drive circuit with protective clamp and reset circuit may operate on a pulse with a constant duty cycle of 50%. According to other embodiments, the transistor gate drive circuit with protective clamp and reset circuit may operate on a pulse with a variable duty cycle. In these embodiments, the duty cycle may change at any point and rate during the operation of the transistor gate drive circuit without adverse effect on the output waveform. 
         [0028]      FIG. 2  is a block diagram of a unipolar, isolated transistor gate drive circuit with protective clamp and reset circuit  200  according to aspects described herein. According to one embodiment, the transistor gate drive circuit  200  is a transformer coupled gate drive with an input side  202  coupled to a transformer  220  to induce a current to drive an output side  204 . The gate drive circuit  200  may include a PWM current controller  206 , an input  208 , a first switch  210 , a second switch  212 , a first diode  214 , a second diode  216 , a DC voltage (V DRV )  218 , the transformer  220 , a primary winding  222 , and a ground connection  224 . It should be understood that the PWM current controller  206  may have a totem pole output (or buffer) similar to the totem pole configuration of  102  and  104  of  FIG. 1 . 
         [0029]    The input  208  is configured to be coupled to the PWM current controller  206  and a base  226  of the first switch and a gate  228  of the second switch. As shown in  FIG. 2 , the PWM current controller  206  is a UC2845 current mode PWM controller; however, in other embodiments the PWM current controller may be a different type. According to one embodiment, the first switch  210  is a NPN transistor; however, in other embodiments a different type of switch may be used. Likewise, according to one embodiment the second switch  212  device is a MOSFET; however, in other embodiments a different type of switch may be used. An emitter  230  of the first switch  210  is coupled between a cathode  232  of the first diode  214  and a first end  236  of the primary winding  222  of the transformer  220 . An anode  234  of the first diode  214  is coupled to the ground connection  224 . A collector  238  of the first switch  210  is coupled between a cathode  240  of the second diode  216  and the V DRV    218 . The anode  242  of the second diode  216  coupled to a drain  244  of the second switch  212 . A drain  244  of the second switch  212  is coupled between the anode  242  of the second diode  216  and the second end  246  of the primary winding  222  of the transformer  220 . A source  248  of the second switch  212  is coupled to the ground connection  224 . 
         [0030]    Still referring to  FIG. 2 , included is two output side switching circuits indicated at  250  coupled to the transformer  252  via secondary windings. It should be understood that the output side switching circuit  250  is not limited to the circuit shown in  FIG. 2 , as described further below, and may be configured in various alternative configurations which enable a gate on each output side switching circuit  250 , or a plurality of gates on each output side switching circuit  250 , to be driven via an induced current. According to one embodiment, each of the output side switching circuits  250  includes a turn-on diode  254 , a turn-off switch  256 , a pull down resistor  258  and a primary switch  260 . In certain other embodiments, only one output side switching circuit  250 , with a single gate  272 , may be coupled to a transformer  220  via secondary windings. 
         [0031]    According to one embodiment, a first end  262  of the secondary winding  252  is coupled to an anode  264  of the turn-on diode  254 . The first end  262  of the secondary winding  252  is also coupled to a base  266  of the turn-off switch  256 . A collector  268  of the turn-off switch  256  is coupled between a cathode  270  of the turn-on diode  254  and to the gate  272  of the primary switch  260 . One side of the pull down resistor  258  is coupled between the gate  272  of the primary switch  260  and the second end  276  of the primary winding  252 . Another side of the pull down resistor  258  is coupled to the second side  276  of the secondary winding  252 . The collector  274  of the turn-off switch  256  is coupled to a second side  276  of the secondary winding  252 . The second end  276  of the secondary winding  252  may be coupled to the ground connection  224 . In one embodiment, the ground connection  224  of the output side  204  is isolated from the ground connection  224  of the input side  202 . In other embodiments, the ground connection  224  may be common among the input side  202  and the output side  204 . 
         [0032]    As discussed above, although  FIG. 2  shows one particular type of output side  204  circuit (i.e., one skilled in the art would recognize as a PNP turn-off circuit), it should be understood that any number of circuits may be utilized including the output side  136  ( FIG. 1 ) discussed above. The output side  204  circuit may have one or more gates (or control pins) which are driven by the input side  202 . For example, and not intended to be limiting, the output side switching circuit  250  is a NMOS turn-off circuit. In other examples, the switching circuit  250  may be comprised of other electronic switches such as a biopolar transistor, a MOSFET transistor, and an insulated-gate bipolar transistor (IGBT). Still further examples include electronic switches for positive and negative logic (e.g., PNP, NPN, Nmos, Pmos, and IGBT). According to one embodiment, there may be one output side switching circuit  250  coupled to the transformer  220  via secondary windings. In other embodiments there may be a plurality of output side switching circuits  250  coupled to the transformer  220  via secondary windings. It should be recognized that each output side switching circuit  250  may be identical to one another, or each may be a different circuit configuration. 
         [0033]    In a turn-on mode of the transistor gate drive  200 , a signal is applied (e.g., rising edge of a square wave from the PWM controller  206 ) to the input  208  for an interval of time. In one embodiment, the control signal is provided by the PWM current controller  206 ; however, in other embodiments, the control signal is provided by any other signal generator known in the art. As discussed above, the duty cycle of the control signal provided by the PWM current controller  206  may be constant, or variable. In this embodiment, a width of a pulse provided by the PWM current controller  206  determines the interval of time the transistor gate drive  200  operates in the turn-on mode. As shown in  FIG. 2 , the PWM current controller  206  supplies the control signal to the base  226  of the first switch  210  and the gate  228  of the second switch  212  to selectively couple voltage of V DRV    218  to the first side  236  of the primary winding  222  of the transformer  220 . The current from the V DRV    218  induces a positive current in the secondary winding  252  at a level sufficient to drive the gate  272  of the primary switch  260  to change state (e.g., close). 
         [0034]    When the PWM current controller  206  applies a control signal (e.g., a rising edge of a square wave from the PWM current controller  206 ), the first switch  210  and the second switch  212  are simultaneously turned on (e.g., driven high). As a result, a current from the V DRV    218  flows from the collector  238  of the first switch  210  to the emitter  230  of the first switch  210 , providing the current to the first end  236  of the primary winding  222 . As the current passes from the first end  236  to the second end  246  of the primary winding  222 , current and magnetic flux in the primary winding  222  increases, resulting in energy being stored in the transformer  220 . In one embodiment, the turns ratio of the transformer  220  is 1:1; however, in other embodiments, the turns ratio of the transformer  220  may be configured differently. In the embodiment shown, the primary winding  222  and the secondary winding  252  have the same polarity. Therefore, as the current and magnetic flux in the primary winding  222  increases, a positive current is induced in the first end  262  of the secondary winding  252 . As a result, turn-on diode  254  becomes forward-biased providing a path for the positive current to the gate  272  of the primary switch  260 . At the same time, the turn-off switch  256  is turned off. The current is then applied to the gate  272  of the primary switch  260 , causing the primary switch  260  to change state (e.g., to close). 
         [0035]    In a turn-off mode of the transistor gate drive  200 , another control signal is applied (e.g., a falling edge of a square wave from the PWM current controller  206 ) at input  208 , simultaneously driving the first switch  210  and the second switch  212  off (e.g., low). When the first switch  210  and the second switch  212  turn off, back electro motive force (EMF) in a reverse polarity (i.e., negative current) is formed across the primary winding  222 . As a result, the first diode  214  and the second diode  216  become forward-biased and begin to conduct, supplying the negative current with a path from the ground connection  224  to the V DRV    218 , resetting the primary winding  222 . The negative current induces a negative voltage across the secondary winding  252  of the transformer  220 . As a result, the base  266  of turn-off switch  256  is driven low, causing the turn-off switch  256  to change state (e.g., close). As a result the turn-off switch  256  couples the gate  272  of the primary switch  260  to the ground connection  224 , causing the primary switch  260  to change state (e.g., open). In one embodiment, the first diode  214  and the second diode  216  protect the first switch  210  and the second switch  212  from excessive voltages. 
         [0036]    As described above, the PWM current controller  206  is a UC2845 current mode PWM controller; however, in other embodiments, the input side  202  may include another type of controller (e.g., a different type of current mode controller or an average current mode controller). 
         [0037]    Embodiments described herein provide a unipolar, isolated transistor gate drive circuit with protective clamp and reset circuit. The transistor gate drive circuit resets a primary winding of a transformer through a series of diodes which resets the transformer and protects input side switches from excessive voltage. Because the resetting of the transformer obviates the need for a DC blocking capacitor, the speed at which the transformer resets is enhanced. Moreover, the transistor gate drive withstands sudden changes in duty cycle because the resetting of the primary winding occurs fully between the turn-off mode and the turn-on mode of the transistor gate drive. 
         [0038]    Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.