Quick turn on apparatus and method for a NMOSFET switch

Abstract

A quick turn on apparatus and method for a NMOSFET switch are used to maintain the gate voltage of the NMOSFET switch non-zero but not enough to turn on the NMOSFET switch, such that the NMOSFET switch turns on more quickly when it is to be turned on. Seamless transition can be further achieved in a single pole double throw switching circuit by using the quick turn on apparatus and method.

1 . A quick turn on apparatus for a NMOSFET switch having a drain coupled to an input and a source coupled to an output, the apparatus comprising:
 a control switch for coupling a voltage to a gate of the NMOSFET switch when the NMOSFET switch is off, such that a gate-to-source voltage of the NMOSFET switch is nonzero but not enough to turn on the NMOSFET switch; said control switch distinct from said NMOSFET switch and controlled by a control signal; said voltage including an existing voltage tied to a voltage of said output.   
   
   
       2 . The quick turn on apparatus of  claim 1 , further comprising a diode serially coupled to the switch for producing a forward bias thereon when the NMOSFET switch is off. 
   
   
       3 . A quick turn on apparatus for a NMOSFET switch having a drain coupled to an input and a source coupled to an output, the apparatus comprising:
 an operational amplifier having a non-inverting input coupled with a voltage and an inverting input coupled to a gate of the NMOSFET switch; said voltage including an existing voltage tied to a voltage of said output; and   a control switch coupled directly between an output of the operational amplifier and the gate of the NMOSFET switch;   wherein the switch is on when the NMOSFET switch is off, such that a gate-to-source voltage of the NMOSFET switch is nonzero but not enough to turn on the NMOSFET switch; said control switch distinct from said NMOSFET switch and controlled by a control signal.   
   
   
       4 . A quick turn on apparatus for a NMOSFET switch having a drain coupled to an input, a source coupled to an output, and a gate coupled to a first switch, the apparatus comprising:
 a second switch for switching the first switch;   a hysteresis comparator for monitoring a gate voltage of the NMOSFET switch to produce a comparison output; said comparator having a non-inverting input coupled with a voltage, said voltage including an existing voltage tied to a voltage of said output; and   an OR gate directly coupled to said hysteresis comparator, for producing a control signal according to the comparison output and an enable signal to switch the second switch;   wherein when the NMOSFET switch is off, if the gate voltage is lower than a threshold, the first switch turns on for the gate of the NMOSFET switch to be charged.   
   
   
       5 . A quick turn on method for a NMOSFET switch having a drain coupled to an input and a source coupled to an output, the method comprising the step of:
 operating a control switch, maintaining a gate-to-source voltage of the NMOSFET switch nonzero but not enough to turn on the NMOSFET switch when the NMOSFET switch is off.   
   
   
       6 . The method of  claim 5 , further comprising the step of:
 monitoring the gate-to-source voltage when the NMOSFET switch is off; and   charging the gate of the NMOSFET switch once the gate-to-source voltage is lower than a threshold.   
   
   
       7 . A switching circuit comprising:
 a first input;   a second input;   an output;   a first NMOSFET switch having a drain coupled to the first input and a source coupled to the output;   a first quick turn on apparatus coupled to a gate of the first NMOSFET switch for maintaining a gate-to-source voltage of the first NMOSFET switch to be nonzero but not enough to turn on the first NMOSFET switch when the first NMOSFET switch is off;   a second NMOSFET switch having a drain coupled to the second input and a source coupled to the output; and   a second quick turn on apparatus coupled to a gate of the second NMOSFET switch for maintaining a gate-to-source voltage of the second NMOSFET switch to be nonzero but not enough to turn on the second NMOSFET switch when the second NMOSFET switch is off; wherein the first quick turn on apparatus comprises a first control switch for coupling a voltage to the gate of the first NMOSFET switch when the first NMOSFET switch is off; said first control switch distinct from said first and second NMOSFET.   
   
   
       8 . The switching circuit of  claim 7 , wherein the first quick turn on apparatus further comprises a diode serially coupled to the first control switch for producing a forward bias thereon when the first NMOSFET switch is off. 
   
   
       9 . The switching circuit of  claim 7 , wherein the first quick turn on apparatus comprises:
 an operational amplifier having a non-inverting input coupled with a voltage and an inverting input coupled to the gate of the first NMOSFET switch; said voltage including an existing voltage tied to a voltage of said output; and   a second control switch coupled between an output of the operational amplifier and the gate of the first NMOSFET switch;   wherein the switch is on when the NMOSFET switch is off; said second control switch distinct from said first NMOSFET.   
   
   
       10 . A switching circuit comprising:
 a first input;   a second input;   an output;   a first NMOSFET switch having a drain coupled to the first input and a source coupled to the output;   a first control switch coupled to a gate of the first NMOSFET switch;   a first quick turn on apparatus for monitoring a gate voltage of the first NMOSFET switch when the first NMOSFET switch is off, so as to turn on the first control switch for charging the gate of the first NMOSFET switch once the gate voltage of the first NMOSFET switch is lower than a first threshold;   a second NMOSFET switch having a drain coupled to the second input and a source coupled to the output;   a second control switch coupled to a gate of the second NMOSFET switch; and   a second quick turn on apparatus for monitoring a gate voltage of the second NMOSFET switch when the second NMOSFET switch is off, so as to turn on the second control switch for charging the gate of the second NMOSFET switch once the gate voltage of the second NMOSFET switch is lower than a second threshold;   
     said first and second control switch distinct from said first and second NMOSFET switch. 
   
   
       11 . The switching circuit of  claim 10 , wherein the first quick turn on apparatus comprises:
 a hysteresis comparator for comparing the gate voltage of the first NMOSFET switch and the first threshold to produce a comparison output; said comparator coupled to a voltage; said voltage including an existing voltage tied to a voltage of said output;   an OR gate, directly coupled to said hysteresis comparator, for switching the first control switch according to the comparison output and an enable signal;   wherein the enable signal maintains the first control switch on when the first NMOSFET switch is on.