Abstract:
Particular embodiments generally relate to driver structures. In one embodiment, an apparatus includes a first driver that drives a first current for a transistor. The first driver drives the first current during a first portion of a drive time of driving the transistor. The first driver is OFF during a second portion. A second driver drives a second current for the transistor during the second portion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional App. No. 61/167,479 for “MOSFET Driver Method and Circuit” filed Apr. 7, 2009, the contents of which is incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    Particular embodiments generally relate to short circuit protection. 
         [0003]    Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
         [0004]    Many applications may use metal oxide semiconductor field effect transistors (MOSFETs) in switching configurations where the transistors are driven either on or off with a very short time in between the on state and the off state. The MOSFETs are essentially charge controlled devices where an equivalent capacitor between the gate and source can be approximated for the MOSFET input. The equivalent capacitor may be referred to as a gate capacitor, or Cgs. 
         [0005]    In one conventional example, the MOSFET is turned on when the capacitor is fully charged. That is, a current can flow between the drain and source of the MOSFET. The MOSFET is turned off when the capacitor is empty and thus no current flows between the drain and source. The transient time to switch the MOSFET between the on and off states is determined by a speed at which necessary electric charge is loaded or unloaded into the gate capacitor. To load or unload the necessary electric charge, MOSFET gate drivers need to use charge and discharge currents that are very large. This causes the MOSFET to change state very fast. Implementations of MOSFET gate drivers use structures that are capable of supplying large peak currents. Because the MOSFET gate drivers have to drive essentially a capacitor, the MOSFET gate drivers are not designed to support the large currents for long periods of time. This saves area in the silicon&#39;s die. 
         [0006]    In some applications, the MOSFET gate driver&#39;s outputs are short-circuited to ground. This causes stressful conditions and could damage the driver, which is not designed to support the large currents short circuited to ground. Accordingly, different short circuit protection schemes are used to protect the driver. However, the schemes are complex and increase the die size of the driver chip. Also, the schemes may not always cover all the conditions that may arise. 
         [0007]      FIG. 1  depicts a conventional system  100  for short circuit protection. A driver  102  drives MOSFET  104  with a large current during a drive time. The drive time is the time when the gate capacitor Cgs is charged and consequently MOSFET  104  is conducting. The stop time is when the gate capacitor Cgs is discharged and MOSFET  104  is not conducting. During drive time of system  100 , a current limiter  106  may pull the gate of MOSFET  104  to ground to discharge capacitor Cgs. For example, current limiter  106  may be part of a power factor correction circuit. In one example, power factor correction is being performed for a load, such as a switch mode power supply  208 . Driver  102  may be damaged if the large current is short-circuited to ground. Additionally, a large amount of power is dissipated by driver  102  when the gate is short-circuited to ground, which may also damage driver  102 . 
         [0008]    A short circuit protection block  110  is included in system  100  to protect driver  102  when a short circuit occurs. Short circuit protection block  110  senses a current across a resistor  112 . If the current exceeds a threshold, a control signal is sent to driver  102  to shut driver  102  off. The use of thresholds requires complex logic, which increases the area used in the chip. Other more complicated schemes may be used that also increase the area used. 
       SUMMARY 
       [0009]    Particular embodiments generally relate to driver structures. In one embodiment, an apparatus includes a first driver that drives a first current for a transistor. The first driver drives the first current during a first portion of a drive time of driving the transistor. The first driver is OFF during a second portion. A second driver drives a second current for the transistor during the second portion. 
         [0010]    In one embodiment, an apparatus is provided comprising: a first driver driving a first current for a transistor, wherein the first driver is ON for a first portion of a drive time to drive the transistor and the first driver is OFF for a second portion of the drive time; and a second driver driving a second current for the transistor, wherein the second driver is driving the second current during the second portion of the drive time to drive the transistor. 
         [0011]    In another embodiment, a method is provided comprising: driving a first current to a transistor for a first portion of a drive time to drive the transistor; not driving the first current for a second portion of the drive time; and driving a second current for the transistor during the second portion of the drive time. 
         [0012]    The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  depicts a conventional system for short circuit protection. 
           [0014]      FIG. 2  depicts a system for providing short circuit protection according to one embodiment. 
           [0015]      FIG. 3  depicts a graph of a timing diagram according to one embodiment. 
           [0016]      FIG. 4  shows an example of applying switching logic to a sustaining driver and a full driver according to one embodiment. 
           [0017]      FIG. 5  depicts an example showing the sustaining driver and the full driver according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Described herein are techniques for short circuit protection. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. Particular embodiments as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein. 
         [0019]      FIG. 2  depicts a system  200  for providing short circuit protection according to one embodiment. In system  200 , a chip includes a first driver circuit  201   a  including sustaining driver  202   a,  a full driver  203   a,  and switching logic  206   a  and a second driver circuit  201   b  including sustaining driver  202   b,  a full driver  203   b,  and switching logic  206   b.  A MOSFET  204 , a current limiter  206 , a switch mode power supply (SMPS)  208 , and a resistor  210  may be included off the chip. 
         [0020]    First driver circuit  201   a  and second driver circuit  201   b  may be implemented in separate circuits in chip  200 . First driver circuit  201   a  may drive current in a first direction, such as supply current to MOSFET  204 . Second driver circuit  201   b  may drive current in a second direction, such as drawing current. 
         [0021]    To drive MOSFET  204  on and off, a large current is needed to charge or discharge the gate capacitor Cgs across MOSFET  204  in a required transient time. The required transient time may be a time that is desired to switch MOSFET  204  between on and off states. In one embodiment, full driver  203   a  provides a necessary full current to charge and full driver  203   b  provides a necessary full current to discharge the gate capacitor for achieving the required transient time to drive MOSFET  204  between ON and OFF states. Sustaining drivers  202   a  and  202   b  supply a smaller sustaining current than full drivers  203   a  and  203   b . The smaller current is sufficient to sustain the charge of the gate capacitor in a charged state once full driver  203   a  charges the gate capacitor. Also, sustaining driver  203   b  keeps a low impedance across the gate capacitor Cgs thereby keeping the gate capacitor in a discharged state. 
         [0022]    In some cases, the gate of MOSFET  204  may be pulled to ground. For example, current limiter  206  may be part of a power factor correction circuit. In one example, power factor correction is being performed for switch mode power supply  208 . Although switch mode power supply  208  is described, other loads may be used. For example, any powered coil may be used for the load. 
         [0023]    Power factor correction shapes an input current to be in phase and sinusoidal with an input voltage. In power factor correction, the current through MOSFET  204  is monitored and if it exceeds a threshold, the current may be cut off. In this case, the gate of MOSFET  204  is pulled to ground by current limiter  206 . Although power factor correction and current limiting is described, other events may be the cause of the gate of MOSFET  204  being pulled to ground. 
         [0024]    Particular embodiments provide short circuit protection by not having full driver  203   a  be ON for an entire drive time for MOSFET  204  The drive time is when MOSFET  204  is turned on, conducts current from the drain to source, and then is turned off. Full driver  203   a  supplies current to fully charge the gate capacitor. Once the gate capacitor is charged, the full current required to turn MOSFET  204  on in the required transient time is not needed to sustain the charge of the gate capacitor. Thus, the full current from full driver  203   a  is not needed. The sustaining current provided by sustaining driver  202   a  can be used to sustain the charge of the gate capacitor until MOSFET  204  needs to be turned OFF. Accordingly, particular embodiments turn full driver  203   a  off during a portion of the drive time. 
         [0025]    When it is time to discharge the gate capacitor, full driver  203   b  may be turned on to draw current to discharge the gate capacitor, which turns MOSFET  204  off. Once MOSFET  204  is off, sustaining driver  202   b  is used to draw a sustaining current. 
         [0026]    Particular embodiments according to this disclosure provide natural short circuit protection by recognizing that the full current is not needed to drive the gate capacitor once it is charged. Because full driver  203   a  is not needed to sustain the charge, full driver  203   a  is turned OFF during at least a portion of this time. If a short circuit occurs when full driver  203   a  is OFF, sustaining driver  202   a  is configured to withstand the short circuit condition without damaging the chip. Also, the power dissipated by the short circuit is significantly less than if full driver  203   a  was ON. For example, if sustaining driver  202   a  is supplying a very small sustaining current, then the power dissipated is much lower than if the larger full current of full driver  203   a  is being supplied. 
         [0027]    The above is also true for discharging the gate capacitor. The full current does not need to be drawn once the gate capacitor is discharged. Full driver  203   b  is turned OFF after this time. If a short circuit occurs when full driver  203   b  is OFF, sustaining driver  202   b  is configured to withstand the short circuit condition without damaging the chip. 
         [0028]      FIG. 3  depicts a graph  300  of a timing diagram according to one embodiment. A waveform  301  shows drive times  302   a  and  302   b  for charging the gate capacitor. First driver circuit  201   a  is used to supply current. For a first portion  304   a  of drive times  302   a  and  302   b , the gate capacitor is being charged by full driver  203   a.  Also, sustaining driver  202   a  may also be supplying current. At a point  306 , the gate capacitor is fully charged. For first portion  304   a,  the full current of full driver  203   a  is needed to charge the gate capacitor in the required transient time to turn MOSFET  2 - 204 ON. In this case, both sustaining driver  202   a  and full driver  203   a  may be ON. However, at point  306 , the full current of full driver  203   a  is no longer needed. Thus, full driver  203   a  may be turned OFF around this time. 
         [0029]    For a second portion  304   b,  only sustaining driver  202   a  is on. During this time, only a small current is being supplied. The current to sustain the charge is based on resistor  210 , which may be a large value, such as 100 kOhm-1 Mohm. The sustaining current may thus be low to sustain the charge. If a short circuit condition occurs, only a small amount of power will be dissipated due to the small amount of sustaining current being supplied. 
         [0030]    At a point  308 , the gate capacitor is discharged for a required transient time. Driver circuit  201   b  may be used to draw current to discharge the capacitor Cgs. The full current of full driver  203   a  is needed to switch MOSFET  2 - 204  off in the required transient time during a third portion  304   c.  Full driver  203   b  is turned on around this point and draws the full current to discharge the gate capacitor. In one embodiment, full driver  203   b  and sustaining driver  202   b  are both on during third portion  304   c  of the drive time to discharge the gate capacitor. Also, only full driver  203   b  is on during third portion  304   c.  Once the gate capacitor has been discharged, full driver  203   b  may then be off during a fourth portion  303   a.    
         [0031]    Because full drivers  203   a  and  203   b  are off for a portion of the drive time, the time in which a short circuit may damage system  200  is reduced. For example, if a short circuit occurs during portions  304   a  or  304   c,  there is a slight chance that system  200  may be damaged. However, portions  304   a  and  304   c  are short periods of time compared to second portion  304   b  and fourth portion  303   a.  For example, second portion  304   b  and fourth portion  303   a  may both be 3 microseconds as compared to 200 nanoseconds for portions  304   a  and  304   c.  Thus, for a majority of the drive time, sustaining drivers  202   a  and  202   b  is only on. Sustaining drivers  202   a  and  202   b  are designed such that they are able to sustain a short circuit condition without any damage or significant power dissipation increase for system  200  to limit damage from a short circuit. 
         [0032]    A margin of time greater than the time required to turn MOSFET  204  on and off may be used to determine when to turn full drivers  203   a  and  203   b  OFF and ON. For example, full driver  203   a  may be switched OFF after point  306  and full driver  203   b  switched on before point  308 . This allows for some variance in the time taken to turn MOSFET  204  ON and OFF. For example, portions  304   a  and  304   c  may be 200 nanoseconds. The times that full driver  203   a  is on for first portion  304   a  and full driver  203   b  is on third portion  304   c  may be set at 400 nanoseconds. This allows a 200 nanosecond margin for turning MOSFET  204  on and off. 
         [0033]    Referring back to  FIG. 2 , switching logic  206   a  or  206   b  is used to turn ON and OFF sustaining driver  202   a  and full driver  203   a  or sustaining driver  202   b  and full driver  203   b.  For example, sustaining driver  202   a  may be turned ON for the entire drive time. However, full driver  203   a  is turned ON for first portion  304   a,  but not second portion  304   b.  Although sustaining driver  202   a  is described as being ON during the full drive time, sustaining driver  202   a  may be turned OFF during first portion  304   a    
         [0034]    Switching logic  206  may be used to generate signals that turn ON and OFF sustaining driver  202  and full driver  203  in different periods. This concept may be applied to both first driver circuit  101   a  and second driver circuit  101   b.  Full driver  200  may reference either full driver  203   a  or  203   b  and sustaining driver  202  may reference either sustaining driver  202   a  or  202   b  in the following description.  FIG. 4  shows an example of applying switching logic  206  to sustaining driver  202  and full driver  203  according to one embodiment. As shown, a first signal  402   a  is sent to an amplifier  404   a  for sustaining driver  202 . Also, a second signal  402   b  is input through an amplifier  404   b  for full driver  203 . Signal  402   a  is ON for 3.4 microseconds, OFF for 3.4 microseconds, and ON for 3.4 microseconds. However, signal  402   b  is ON for 400 nanoseconds, OFF for 2.6 microseconds, and ON for 400 microseconds. After being OFF for 3.4 microseconds, signal  402   b  is again ON for 400 nanoseconds, OFF for 2.6 microseconds, and ON for 400 nanoseconds. 
         [0035]    Signal  402   a  drives amplifier  404   a  for the full drive time for sustaining driver  202 . Also, signal  402   b  drives amplifier  404   b  for a portion of the full drive time for full driver  3 - 302 . 
         [0036]    Switching logic  206  may generate signals  402   a  and  402   b.  A person of skill in the art will appreciate how to generate signals  402   a  and  402   b  in accordance with the teachings and disclosure herein. In one example, a series of flip-flops may be used to generate the pulses of signals  402   a  and  402   b.    
         [0037]    In one embodiment, full driver  203  may be implemented using multiple current sources. For example, multiple MOSFETs may be used.  FIG. 5  depicts an example showing sustaining driver  202  and full driver  203  according to one embodiment. This concept may be applied to both first driver circuit  101   a  and second driver circuit  101   b.  In one embodiment, sustaining driver  202  and full driver  203  are made in different structures of a chip. 
         [0038]    Full driver  203  may include multiple current sources. For example, each finger  502  may be a MOSFET that is configured to supply or draw a certain amount of current. In one example, each finger  502  may supply 100 milliamps. If 10 fingers are provided, then 1 amp of current may be supplied by full driver  203 . 
         [0039]    Sustaining driver  202  may be a single finger  504  that supplies or draws the sustaining current. For example, finger  504  may supply a 1 milliamp current. Although a single finger is described, any number of fingers  504  may be used for sustaining driver However, the amount of current supplied by fingers  504  of sustaining driver  202  is less than the amount of current supplied by fingers  502  of full driver  203 . 
         [0040]    Switching logic  206  is applied to fingers  502  and  504  such that they are switched ON and OFF as described above. Accordingly, if a short circuit occurs with only finger  504  ON, the power dissipated with a 1 milliamp is a lot less than the power dissipated if 1 amp of current is ON during the short circuit. 
         [0041]      FIG. 6  depicts a simplified flow chart  600  of a method for short circuit protection according to one embodiment. 
         [0042]    At  602 , a full current is driven for MOSFET  204  for a first portion for turning MOSFET  204  ON. For example, the full current may be supplied. The full current is sufficient to turn MOSFET  204  ON. 
         [0043]    At  604 , a sustaining current may or may not be driven to MOSFET  204  during the first portion. For example, the sustaining current may be supplied. In one embodiment, the sustaining current is driven during the first portion of turning the MOSFET ON. In another embodiment, the sustaining current may not be on during the first portion. 
         [0044]    At  606 , after the first portion is over, the full current is not driven for a second portion of the full drive time. At  608 , the sustaining current is driven for MOSFET  204  during the second portion. For example, the sustaining current may be supplied. The sustaining current is sufficient to keep the gate capacitor charged during the second portion while MOSFET  204  is ON. 
         [0045]    At  610 , after the second portion is over, the full current is driven to MOSFET  204  for a third portion. For example, the full current may be drawn. The full current is sufficient to turn MOSFET  2 - 204  OFF. 
         [0046]    At  612 , the sustaining current may or may not be driven to MOSFET  204  during the third portion. For example, the sustaining current may be drawn. In one embodiment, the sustaining current is driven during the third portion of turning the MOSFET OFF. In another embodiment, sustaining current may not be on during the third portion. 
         [0047]    At  614 , after the third portion is over, the full current is not supplied or drawn for a fourth portion. At  616 , the sustaining current is driven for MOSFET  204  during a fifth portion. For example, the sustaining current may be supplied. The sustaining current is sufficient to keep the gate capacitor discharged during the fifth portion while MOSFET  204  is OFF. 
         [0048]    Switching logic  206  can be implemented in an area-efficient design that does not require as complex of logic as short circuit protection block  110  of  FIG. 1 . By switching full driver  203   a  OFF when it is not needed during second portion  304   b,  natural short circuit protection for the chip is provided. The same is true for full driver  203   b.  Short circuit protection is thus inherent in the design. Thresholds are not needed to determine if a short circuit condition is occurring, which is less complex, saves area on the silicon&#39;s die, and also decreases cost. 
         [0049]    As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
         [0050]    The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the invention as defined by the claims.