Optically coupled fast turn off load switch drive

A drive ciruit for a load switch (104) controlling current flow through an electrical load (106) in accordance with the present invention includes a switching signal source (10), a first optical coupler (110), coupled to the switching signal source which turns on to produce an output drive signal in response to a first level of a switching signal (100) from the switching signal source to control turning on of the load switch; and a second optical coupler (114) coupled to the switching signal source which turns on to produce an output drive signal in response to a second level of a switching signal from the switching signal source to control turning off of the load switch.

TECHNICAL FIELD 
The present invention relates to drive circuits for load switches which 
utilize optical couplers to electrically isolate a signal drive source 
from a load switch drive. 
BACKGROUND ART 
Optical couplers are well known which are photon-coupled devices in which 
an electrical signal is converted into light that is projected through an 
insulating interface and reconverted to an electrical signal. Various 
forms of optical couplers are well known, such as a light emitting diode 
and photo diode combination, a light emitting diode and photo-transistor 
combination, a light emitting diode and photo-Darlington combination, a 
light emitting diode and LDR combination, a neon light and LDR 
combination, a lamp and LDR combination and a light emitting and diode 
photo diode driving a transistor switch combination. Optical couplers have 
a switching characteristic having a fast turn on and a slower turn off. As 
a result, optical couplers require less time to turn on than to turn off. 
Diverse switching applications exist in which the electrical isolation 
provided by an optical coupler is desirable to prevent undesired 
electrical coupling between a switching signal source and a load switch 
whose conductivity is being controlled by the switching signal source. An 
example of such an application is in the drive circuit for a variable 
reluctance multiple phase electrical motor. Many switching applications in 
which an optical coupler may be utilized require both a fast turn on and a 
fast turn off characteristic to optimize performance. Undesirably, a fast 
turn off characteristic may not be obtained with a single optical coupler 
controlling both the turning on and the turning off of a load switch (a 
switch controlling current flow through an electrical load). 
FIG. 1 illustrates an example of a prior art drive circuit for a load 
switch using an optical coupler. A switching signal source 10 provides 
switching pulses which control the conductivity of a Darlington amplifier 
12 comprised of a first bipolar transistor 14 and a second bipolar 
transistor 16. The switching pulses produced by the switching signal 
source 10 vary between a first low level and a second high level. The 
Darlington amplifier 12 is conductive when the switching signal produced 
by the switching signal source 10 is at the second level. The Darlington 
amplifier 12 controls the flow of current through an electrical load 18 
which may be diverse in nature such as the windings of a single phase of a 
variable reluctance motor. The switching signal from the switching signal 
source 10 is electrically coupled to optical coupler 20 which may be any 
known optical coupler configuration. As described above, the optical 
coupler provides electrical isolation between the switching signal source 
and the Darlington amplifier 12. When the switching signal reaches the 
high level, the optical coupler turns on to produce a high level output 
signal The turn on characteristic of the optical coupler is relatively 
fast when compared to the turn off characteristic which occurs when the 
switching pulse falls from the second level to the first level. The output 
of the optical coupler is coupled to a first channel 24 which is comprised 
of amplifier 26 and switch 28. The switch 28 is conductive when the 
switching signal is at the second level which causes the Darlington 
amplifier 12 to turn on with a relatively fast turn on characteristic. The 
output of the optical coupler 20 is coupled to a second channel 30 
comprised of inverting amplifier 32 and bipolar transistor 34. The 
inverting amplifier 32 produces a high level output signal when the 
switching signal pulse is at the first low level which causes the 
Darlington amplifier to turn off. However, as a consequence of the 
relatively slow turn off characteristic of the optical coupler 20, the 
optical coupler 20 does not rapidly change state in response to the 
switching signal source falling from the higher second level to the first 
level which slows down the turn off characteristic of the Darlington 
amplifier 12. 
DISCLOSURE OF INVENTION 
The present invention is a drive circuit and a method of turning on and off 
a switch which provides electrical isolation through optical coupling 
between a switching signal source and the switch being turned on and off 
under the control of a signal produced by the switching signal source 
which has a rapid turn on and turn off characteristic. With the invention, 
first and second optical couplers are coupled to the switching signal 
source with the switching signal controlling the turning on and off of the 
first optical coupler and an inversion of the switching signal controlling 
the turning on and off of the second optical coupler. Output drive signals 
produced by the first and second optical couplers are coupled to a control 
terminal of a load switch for controlling the conductivity of the load 
switch. As a consequence of an inversion of the switching signal 
controlling the turning on of the second optical coupler, a rapid turn off 
characteristic is produced by the load switch which is faster than the 
turn off characteristic produced by the prior art using a single optical 
coupler for controlling both the turning on and turning off of a load 
switch as described above with respect to FIG. 1. 
A drive circuit for a load switch controlling current flow through an 
electrical load in accordance with the invention includes a switching 
signal source; a first optical coupler coupled to the switching signal 
source which turns on to produce an output drive signal on an output in 
response to a first level of a switching signal from the switching signal 
source to control turning on of the load switch; a second optical coupler 
coupled to the switching signal source which turns on to produce an output 
drive signal on an output in response to a second level of a switching 
signal from the switching signal source to control turning off of the load 
switch and logic circuitry coupling the output drive signal from each of 
the optical couplers to the load switch. An inverter is coupled to the 
switching signal source and to an input of the second optical coupler for 
inverting a switching signal from the switching signal source which is 
applied to the second optical coupler. A load switch drive is coupled to 
outputs of the optical couplers and to the load switch which produces an 
output signal of a first level when the first optical coupler turns on and 
an output signal of a second level when the second optical coupler turns 
on. The optical couplers comprise a light emitting diode coupled to the 
switching signal source and a photo diode optically coupled to light 
emitted from the light emitting diode and electrically coupled to a 
transistor which turns on in response to the photo diode receiving light 
from the light emitting diode with conduction of the transistor 
controlling turning on of the load switch. The optical couplers require 
less time to turn on than to turn off. 
In a drive circuit for a load switch controlling current flow through an 
electrical load an improvement in accordance with the invention includes a 
first optical coupler controlling turning on of the load switch with an 
output signal on an output in response to a first level of a switching 
signal turning on the first optical coupler; a second optical coupler 
turning off of the load switch with an output signal on an output in 
response to a second level of a switching signal turning on the second 
optical coupler; the optical couplers require less time to turn on than to 
turn off and logic circuitry couples the output drive signal from each of 
the optical couplers to the load switch. The switching signal source 
provides the switching signal having the first and second levels. An 
inverter is coupled to the switching signal source and to an input of the 
second optical coupler for inverting a switching signal from the switching 
signal source which is applied to the second optical coupler. A load 
switch drive is coupled to outputs of the optical couplers and to the load 
switch which produces an output signal of a first level when the first 
optical coupler turns on and an output signal of a second level when the 
second optical coupler turns on. The optical couplers comprise a light 
emitting diode coupled to the switching signal source and a photodiode 
optically coupled to light emitted from the light emitting diode and 
electrically coupled to a transistor which turns on in response to the 
photodiode receiving light from the light emitting diode with conduction 
of the transistor controlling turning on of the load switch. 
A method of turning on and off a switch with a switching signal having 
first and second levels with a switch drive having a first optical coupler 
having an input coupled to a switching signal source and producing a first 
output drive signal on an output coupled to a control terminal of the 
switch and a second optical coupler having an input coupled to the 
switching signal source and producing a second output drive signal on an 
output other and coupled to the control terminal of the switch in 
accordance with the invention includes providing the control signal which 
is a pulse having a signal level of a time duration; applying the control 
signal to the first optical coupler to turn on the first optical coupler 
to produce the first output drive signal applied to the control terminal 
of the switch through logic circuitry to cause the switch to turn on; 
applying an inversion of the control signal to the second optical coupler 
to turn on the second optical coupler to produce the second drive signal 
applied to the control terminal of the switch through logic circuitry to 
cause the switch to turn off; and wherein the switch is turned on for the 
time duration. The invention further includes logic gates respectively 
coupled to the outputs of the first and second optical couplers. The 
optical couplers require less time to turn on than to turn off.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 2 illustrates a block diagram of the present invention. A switching 
signal source 10 produces an output pulse 100 having a second high level 
of a time duration. The output pulse 100 is coupled to a first channel 102 
which controls the turning on of load switch 104 and a second channel 106 
controlling the turning off the load switch 104. The load switch 104 
controls the flow of electrical current to an electrical load 106 which 
may be any electrical load to which switched current is applied such as, 
but not limited to, the windings of a phase of a variable reluctance 
motor. The first channel is comprised of a non-inverting amplifier 108 and 
an optical coupler 110 which may be any known optical coupler having a 
relatively fast turn on characteristic and a slower turn off 
characteristic as discussed above. The second channel is comprised of an 
inverting amplifier 112 and a second optical coupler 114 having the same 
characteristic as the optical coupler 110. As a consequence of the optical 
couplers requiring less time to turn on than to turn off, the second 
channel 106 including the second optical coupler 114 which turns on to 
cause the load switch to turn off produces a more rapid turn off 
characteristic than the prior art of FIG. 1 which the relatively slower 
turn off characteristic of the optical coupler 20 controlled the turning 
off the Darlington amplifier 12. 
The block diagram of FIG. 2 operates as follows. The switching pulse 100 
produced by the switching signal source 10 having a first lower level and 
a higher second level is coupled to both the first and second channels 102 
and 106. The pulse 100 is amplified by an amplifier 108 and causes the 
first optical coupler 110 to turn on to produce a first output drive 
signal in phase with the switching pulse 100 which is applied to a control 
terminal (not illustrated) of the load switch to cause the load switch to 
turn on. The application of the switching pulse 100 to the second channel 
106 produces an inversion of the switching pulse 100 which is 180.degree. 
out of phase with the switching signal produced by the switching signal 
source 10. As a result, when the switching pulse 100 is falling from the 
second higher level to the lower first level, the output of the inverter 
112 goes high which causes the optical coupler 114 to rapidly turn on to 
produce the second drive signal applied to the control terminal of the 
switch to cause the switch to turn off. As a consequence of the inverter 
driving the optical coupler 114 to turn on when the pulse 100 is falling 
from the first higher level to the lower second level, the second drive 
signal has a more rapid rise time producing a faster turn off of the load 
switch 104 than in the prior art of FIG. 1. 
FIG. 3 illustrates a circuit implementation of a drive circuit in 
accordance with the present invention. Like reference numerals identify 
like parts in FIGS. 2 and 3. Individual circuit components have been 
labelled with component values and/or industry or manufacturer's part 
numbers. Logic gates 128, 130, 132 and 134 isolate the outputs 136 and 138 
respectively of the optical couplers 110 and 114. Input 120 may be taken 
from the collector of a bipolar transistor with outputs 122 and 124 of 
switch drive 126 being applied to the base and emitter of the bipolar 
transistor load switch 104. It should be understood that the invention is 
not limited to the circuit implementation illustrated in FIG. 3. 
While the invention has been described in terms of its preferred 
embodiments, it should be understood that numerous modifications may be 
made thereto without departing from the spirit and scope of the invention 
as defined in the appended claims. It is intended that all such 
modifications fall within the scope of the appended claims.