Turn off delay reduction circuit and method

A gate driver circuit for switching a MOSFET on and off while reducing the turn off delay of the MOSFET without effecting the turn off slew rate thereof includes a low impedance circuit path between the gate and drain of the MOSFET which is responsive to a control signal for providing discharge of the gate capacitance and a controlled current discharge path for controlling the slew rate of the drain voltage. The low impedance circuit path is automatically disabled once the threshold voltage of the MOSFET is reached and the MOSFET begins to turn off as the drain voltage reaches a predetermined level. As the low impedance circuit path is disabled the controlled current discharge path fixes the slew rate or dv/dt of the drain to source voltage during turn off of the MOSFET.

BACKGROUND OF THE INVENTION 
The present invention relates to switching circuits and, more particularly, 
to a circuit that drives a switching device such as the gate of either an 
integrated circuit or discrete power MOSFET for reducing the turn off 
delay of the MOSFET while limiting its gate current to maintain a 
controlled rise (dv/dt) of the drain voltage. 
A myriad of circuit applications can be found in the prior art which 
utilize a gate driven MOSFET that is switched between conducting and 
non-conducting operating states. The MOSFET, which may be either a 
discrete device or an integrated device typically has its drain-source 
electrodes coupled in series with a load element between a power supply. 
The MOSFET has its gate driven by a controlled current switching circuit 
such that the load element is connected to the power supply through the 
low on-resistance of the MOSFET when the latter is turned on as is well 
known. 
Most, if not all, such circuit applications suffer from a time delay 
resulting as the MOSFET is turned off to disconnect the load element from 
the power supply. Turn-off delay is a problem for controlled current gate 
drive circuits because the gate must be discharged from a relatively high 
V.sub.GS (typically 10 volts) to the threshold V.sub.GS (typically 4.0 
volts) before the drain voltage begins to rise as the device is turned 
off. If a constant current is used to control the dv/dt voltage rise at 
the drain of the MOSFET, then the time required to slew the gate from 10 
volts to 4 volts is excessive. Previous attempts to reduce the delay have 
used rapid discharge circuits that discharge the gate to a predetermined 
voltage. These discharge circuits must initially conduct relatively high 
currents but stop conducting when the gate reaches the threshold voltage. 
However, the threshold voltage of a given MOSFET varies in production of 
such applications with the load element, temperature and FET V.sub.T, 
hence, it is impossible to design a common discharge circuit of the 
aforementioned type that works well under all conditions. 
Thus, a need exists for providing a circuit and method for overcoming the 
problems of prior art circuits and to reduce the turn-off delay time 
required in circuit applications utilizing MOSFET switched devices while 
controlling the dv/dt rise at the drain thereof. 
SUMMARY OF THE INVENTION 
In accordance with the above there is provided a driver circuit and method 
for driving the gate of an MOSFET which is responsive to an applied input 
control signal for turning on and off a MOSFET and which reduces the turn 
off delay of the MOSFET without effecting the rise time of the drain 
voltage, comprising a current boost circuit responsive to the control 
signal for providing a low impedance path between the gate and drain of 
the MOSFET to rapidly discharge the gate capacitance and which is 
automatically disabled when the drain voltage exceeds a predetermined 
value and a controlled current supply coupled to the gate which is 
responsive to the control signal for providing a predetermined current to 
turn off the MOSFET at a controlled rate once the current boost circuit is 
disabled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning to FIG. 1 there is illustrated integrated driver circuit 10 of one 
embodiment of the present invention that reduces the turn off delay of 
MOSFET switching transistor 12. MOSFET 12 has its drain and source coupled 
in series with load element R.sub.L between V+ and ground 22 at terminals 
14 and 16 respectively. It is understood that N-channel MOSFET 12 can 
either be a discrete or integrated device. Driver circuit 10 is coupled 
between the drain and gate of MOSFET 12 at terminals 14 and 18 and, as 
will be described, is responsive to a control signal, V.sub.CS, applied at 
input 20 for switching the MOSFET on and off accordingly while reducing 
the turn off delay of the MOSFET without effecting the rise time (dv/dt) 
of the drain voltage, V.sub.DS. Input 20 is coupled though inverter 24 to 
control current supply 26 and is also coupled to switching means 28. 
Controlled current supply 26 is coupled between node 30 and ground 22 with 
node 30 being coupled to the gate of MOSFET 12 at terminal 18. Switch or 
switching means 28 is also coupled to node 30 which supplied a gate 
voltage and current charge path thereto when closed. A current boost 
circuit 32 is shown coupled between the drain and gate of MOSFET 12 
between terminals 14 and 18 which when rendered operative provides a low 
impedance path therebetween for rapidly discharging the gate capacitance 
of MOSFET 12. Current boost circuit 32 includes MOSFETs 34 and 36. The 
drain and source of transistor 34 is coupled in series with the source and 
drain of transistor 36 between terminal 14 and node 30 respectively while 
the gate of transistor 34 is coupled to the gate of MOSFET 12 and the gate 
of transistor 36 is coupled to the output of inverter 24. The respective 
dashed in diodes are the intrinsic body diodes associated with the 
respective MOS transistors 34 and 36. 
Referring to FIG. 2 the operation of driver circuit 10 is described 
assuming that steady state switching operation has occurred prior to time 
t.sub.0. Hence, with V.sub.CS being in a high level state, wave form 40, 
switch 28 is closed and MOSFET 12 is turned on as maximum gate voltage, 
wave form 42, is applied to the gate and, as shown by wave form 44, its 
drain to source voltage, V.sub.DS, is at a low potential. In this mode, 
the on resistance of MOSFET 12 is very low, typically a few ohms, and 
maximum current flows through R.sub.L. Further, the output of inverter 24 
is low such that both current supply 26 and circuit 32 are disabled. 
However, transistor 34 is biased on since its gate is coupled to node 30 
whereby the drain voltage appearing at terminal 14 also appears at the 
source of the transistor while transistor 36 is off. 
At t.sub.1, control signal, V.sub.CS, goes to a low level state to initiate 
turn off of MOSFET 12 and switch 28 opens, thereby disconnecting gate 
drive to MOSFET 12. Simultaneously, the output of inverter 24 goes high to 
render both current supply 26 and circuit 32 operative. Hence, at t.sub.1, 
both transistors 34 and 36 are turned on to provide a low impedance 
conduction path between the drain and gate of MOSFET 12 since the gates 
thereof are at a positive potential initially. A high discharge current, 
I.sub.S, relative to Id, flows through transistors 34 and 36 and the drain 
of MOSFET 12 to rapidly discharge the gate capacitance of the MOSFET while 
rapidly decreasing V.sub.GS to the threshold value. When V.sub.GS reaches 
the threshold value of MOSFET 12, the drain voltage thereof appearing at 
terminal 14 begins to rise. Thus, the time delay between t.sub.1 and 
t.sub.2 is reduced by the low impedance discharge path of circuit 32. 
Circuit 32 is automatically disabled once the threshold voltage of MOSFET 
12 is reached and V.sub.DS has reached a potential to reverse bias the 
body diode associated with transistor 34 thus turning this device off. 
This results in the discharge path through transistors 34 and 36 being 
broken allowing current supply 26 to continue discharge of the gate of 
MOSFET 12 at a controlled rate such that V.sub.DS rises at a predetermined 
slew rate (dv/dt). Note, the flattening (wave form portion 46) of V.sub.GS 
is a natural occurrence due to the well known Miller capacitance effect 
associate with parasitic capacitance of MOSFET 12. 
It is significant to note that if the V.sub.GS threshold voltage of MOSFET 
12 changes for any reason, current boost or discharge circuit 32 responds 
properly and is disabled at the proper time whenever the drain voltage of 
MOSFET 12 begins to increase. Thus, discharge circuit 32 is not 
susceptable to process variations in MOSFET 12 or temperature or load 
conditions as are prior art driver circuits. In addition, it is well 
within the skill of those in the art to recognize that transistors 34 can 
be replaced with a diode oriented in the same manner as shown by the body 
diode associated with this transistor. 
Turning now to FIG. 3, driver circuit 10 is shown in more detail wherein 
like components are designated with the same reference numbers as in FIG. 
1. In this embodiment inverter 24 is realized by a MOSFET device having 
its gate connected to input 20, its drain coupled to the gates of MOSFETs 
36 and 56 and its source coupled to ground. Controlled current supply 26 
includes constant current source 62 which sources current I.sub.d to diode 
connected MOSFET 60 which biases the gate of MOSFET 58 such that it sinks 
the current equal to I.sub.d through its drain-source. MOSFET 56 which has 
its drain-source coupled between node 30 and the drain of MOSFET 58 is 
turned off whenever V.sub.CS is in a high level state and is turned on 
when V.sub.CS is in a low level state as its gate is coupled to the drain 
of MOSFET device 24. 
Switch circuit 28 includes inverter 54 coupled between input 20 and the 
gate of MOSFET 52 the latter of which has its drain-source conduction path 
coupled in series with the source-drain conduction path of MOSFET 50 
between V.sub.G and ground. The gate of MOSFET 50 is returned to the 
output of inverter MOSFET 24 the latter of which is also coupled in series 
with the source-drain conduction path of MOSFET 48 to V.sub.G. The gate of 
MOSFET 48 is coupled both to the drain of MOSFET 52 and the gate of MOSFET 
68 whose source is coupled in series with the drain-source of MOSFET 70 to 
ground. A known current turn around circuit comprising MOSFETs 64 and 66 
is coupled between the drain of MOSFET 68 and node 30. 
Thus, with V.sub.CS in a high level state transistor 50 is turned on while 
transistor 52 is turned off as their gate electrodes are both pulled low. 
In this state transistor 48 is turned off while transistor 68 is turned 
on. Hence, transistor 70, which is biased on by diode connected transistor 
60, sinks a current through transistors 68 and 66 which is mirrored 
through transistor 64 to node 30. MOSFET 12 is therefore turned on by 
transistor 64. Meanwhile transistors 56 and 36 are maintained in an off 
state as the drain of inverter MOSFET 24 is low. In response to V.sub.CS 
going to a low level state, the output or drain of inverter MOSFET 24 goes 
high which enables or turns on transistors 56 and 36. Hence, the 
controlled current I.sub.d is pulled from node 30 as described above while 
the current I.sub.S flows through transistors 36 and 34. Simultaneously, 
transistors 50 and 52 are turned off and on respectively which causes 
transistor 68 to be turned off. Transistors 66 and 64 are thereby 
effectively disabled to disconnect node 30 from V.sub.G. Therefore, MOSFET 
12 is alternately turned on and off as V.sub.CS alternates between high 
and low level states. 
Turning now to FIG. 4 there is shown driver circuit 80 of another 
embodiment of the present invention that can drive either MOSFET 82 in a 
high side driver application or in conjunction with MOSFET 118 can be 
utilized in a push-pull driver application for sinking and sourcing load 
current to a load element (not shown) coupled at output 86. 
In a high side driver application MOSFET 118 is not required and the driver 
circuit is utilized to turn MOSFET 82 on and off in accordance with the 
present invention to source current only to the load connected to output 
86. In this application, the current discharge or boost circuit for 
reducing the turn off delay of MOSFET 82 comprises MOSFETs 104, 106 and 
108 as will be described. 
Thus, MOSFET 82 is turned on responsive to V.sub.CS being in a low level 
state in the following manner. As V.sub.CS is low, the drain of MOSFET 96 
will float as this device is turned off. Simultaneously, inverter 116 
turns on MOSFET 114 as its gate is placed at a high voltage level. In this 
state, MOSFET 110 is turned on to drive the input to inverter 94 high 
which in turn places a low signal at the bases of transistors 88 and 100. 
Transistor 88 is then turned on, while transistor 100 is turned off, to 
pull current from node 92 via resistor 90. MOSFET 82 is thus turned on as 
node 92 is driven to a low voltage state to source current to a load 
connected at output 86. Concurrently, MOSFET 108 is turned on to maintain 
MOSFET 106 in a nonconducting state while MOSFET 104 is turned off. 
Incidentally if MOSFET 118 is used it will be turned off at this time. 
Responsive to V.sub.CS going high MOSFET 82 will be turned off in the 
following manner. As V.sub.CS goes high, MOSFET inverter 96 is turned on 
while MOSFET 114 is turned off thereby turning off MOSFET 110. The drain 
of MOSFET 96 being low turns on MOSFET 104 while MOSFET 108 is turned off. 
Since the potential at output 86 is high the gate of MOSFET 106 is also 
high via MOSFET 104. A low impedance path is provided through MOSFET 106 
to rapidly discharge the gate capacitance of MOSFET 82 in a similar manner 
as discussed with respect to FIG. 1. Simultaneously, transistor 100 is 
rendered conductive as its base is driven high by inverter 94 to source a 
controlled discharge current to the gate of MOSFET 82 via resistor 102 
thereby pulling the gate towards V.sub.DD. As the drain voltage decreases 
to a predetermined level after the threshold voltage, V.sub.GS, of MOSFET 
82 is reached MOSFET 106 is turned off via MOSFET 104 thereby disabling 
the fast discharge circuit and further decrease in the drain voltage is 
controlled by resistor 102 to a fixed slew rate. 
Hence, what has been described above is a novel method and circuit for 
reducing the turn off delay of a switching MOSFET which circuit includes a 
fast discharge circuit which is automatically disable wherein the turn off 
slew rate of the MOSFET is thereafter controlled. The circuit is 
independent of process and temperature variations.