U.S. patent number RE35,836 [Application Number 08/511,747] was granted by the patent office on 1998-07-07 for solid state optically coupled electrical power switch.
This patent grant is currently assigned to C. P. Clare Corporation. Invention is credited to Edward T. Rodriguez.
United States Patent |
RE35,836 |
Rodriguez |
July 7, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
Solid state optically coupled electrical power switch
Abstract
Solid state optically coupled power switch with light induced or
modified voltage applied or removed at one or more MOSFET gate and
source electrode pairs to shift each MOSFET between its high and
low impedance states and in various circuit arrays for ac or dc
switching and/or cross points switching or mechanical Form C relay
substitution or other purposes
Inventors: |
Rodriguez; Edward T.
(Winchester, MA) |
Assignee: |
C. P. Clare Corporation
(Beverly, MA)
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Family
ID: |
26745174 |
Appl.
No.: |
08/511,747 |
Filed: |
August 7, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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65072 |
Aug 9, 1979 |
4296331 |
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Reissue of: |
186419 |
Sep 12, 1980 |
04390790 |
Jun 28, 1983 |
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Current U.S.
Class: |
250/551;
327/514 |
Current CPC
Class: |
H01L
31/167 (20130101); H03K 17/79 (20130101); H03K
17/785 (20130101) |
Current International
Class: |
H03K
17/785 (20060101); H03K 17/79 (20060101); H01L
31/167 (20060101); H01L 31/16 (20060101); G02B
027/00 () |
Field of
Search: |
;250/551,214R,214SW
;327/509,514 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2910748 A1 |
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Sep 1979 |
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DE |
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30 00 890 |
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Sep 1980 |
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DE |
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3000890 A1 |
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Sep 1980 |
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DE |
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Sho 55-1756 |
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Jan 1980 |
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JP |
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2 017 297 A |
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Mar 1979 |
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GB |
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Other References
"Designs for VMOS Push-Pull Converters," Electronic Engineering,
MAy 1980, pp. 87-93, Regan et al..
|
Primary Examiner: Allen; Stephone
Attorney, Agent or Firm: Lappin & Kusmer LLP
Parent Case Text
.Iadd.RELATED U.S. APPLICATIONS
This application is a reissue application of U.S. Pat. No.
4,390,790, based on U.S. patent application Ser. No. 186,419, filed
Sep. 12, 1980, by Edward T. Rodriguez, which in turn is a
continuation-in-part of U.S. patent application Ser. No. 65,072,
filed Aug. 9, 1979, now U.S. Pat. No. 4,296,331. .Iaddend.
Claims
What is claimed is: .[.1. A solid state optically-coupled relay
comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay switch with gate, source
and drain electrodes, the source and drain electrodes thereof being
connected to said output terminal means and the source and gate
electrodes being connected directly
to the ends of the first diode array..].2. A solid state
optically-coupled relay comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay .[.switch.]. with gate,
source and drain electrodes, the source and drain electrodes
thereof being connected to said output terminal means and the
source and gate electrodes being connected directly to the ends of
the first diode array,
(e) a depletion-mode junction FET (JFET) with source, drain and
gate electrodes .[.of the DMOS and.]. the gate and source
electrodes of the JFET being connected to the ends of the second
diode array,
the JFET providing a maximum turn-off bias to the DMOS when the
JFET is at a low impedance, such low impedance condition of the
JFET occurring when the second diode array is not illuminated, the
JFET providing a turn on bias to the DMOS when the JFET is at a
high impedance, such high impedance
occurring when the second diode array is illuminated. 3. .[.Relay
switch apparatus in accordance with either of claims 1 or 2.].
.Iadd.A solid state optically-coupled relay comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay with gate, source and drain
electrodes, the source and drain electrodes thereof being connected
to said output terminal means and the source and gate electrodes
being connected directly to the ends of the first diode array;
wherein said relay is .Iaddend.in directional configuration with
two such output FETs connected on series opposition, the source
electrodes of said two FETs being directly connected and the gate
electrodes of said two FETs
being directly connected. 4. .[.Relay switch apparatus in
accordance with any of claims 1 or 2.]. .Iadd.A solid state
optically-coupled relay comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay with gate, source and drain
electrodes, the source and drain electrodes thereof being connected
to said output terminal means and the source and gate electrodes
being connected directly to the ends of the first diode array;
.Iaddend.
wherein the diode array and light emitting means are arranged in
the same plane and further comprising.Iadd.:
(e) .Iaddend..[.(f).]. thin film means overlying the array and
emitting
means and providing a refractive optical coupling path
therebetween. 5. A solid state optically coupled relay
comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) a depletion-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay .[.switch.]. with gate,
source and drain electrodes, the source and drain electrodes
thereof being connected to said output terminal means and the
source and gate electrodes being connected directly to the ends of
the first diode array,
(e) a depletion mode bias FET with source, drain and gate
electrodes,
(e') source and drain electrodes of the bias FET being connected to
the drain and gate electrode of the output FET,
(e") source and gate electrodes of the bias FET being connected to
ends of
the diode array. .Iadd.6. A solid state optically-coupled relay
comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay with gate, source and drain
electrodes, the source and drain electrodes thereof being connected
to said output terminal means and the source and gate electrodes
being connected directly to the ends of the first diode array;
(e) a depletion-mode junction FET (JFET) with source, drain and
gate electrodes, the gate and source electrodes of the JFET being
connected to the ends of the second diode array,
wherein said relay is in directional configuration with two such
output FETs connected on series opposition, the source electrodes
of said two FETs being directly connected and the gate electrodes
of said two FETs being directly connected. .Iaddend..Iadd.7. A
solid state optically-coupled relay comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay with gate, source and drain
electrodes, the source and drain electrodes thereof being connected
to said output terminal means and the source and gate electrodes
being connected directly to the ends of the first diode array;
(e) a depletion-mode junction FET (JFET) with source, drain and
gate electrodes, the gate and source electrodes of the JFET being
connected to the ends of the second diode array,
wherein the diode array and light emitting means are arranged in
the same plane and further comprising:
(f) thin film means overlying the array and emitting means and
providing a
refractive optical coupling path therebetween. .Iaddend..Iadd.8. A
solid state optically-coupled relay comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay with gate, source and drain
electrodes, the source and drain electrodes thereof being connected
to said output terminal means and the source and gate electrodes
being connected directly to the ends of the first diode array,
and
(e) a bias switching device connected between the gate and source
electrodes of the output FET, the gate and source electrodes having
an equivalent capacitance therebetween, the bias switching device
being operable in a first mode so as to provide a relatively high
impedance path between the gate and source electrodes of the output
FET when the light emitting means is illuminated and in a second
mode so as to provide a relatively low impedance path between the
gate and source electrodes of the output FET when the light
emitting means is not illuminated, such that the turn-on of the
output FET takes place with a time constant substantially
determined by the equivalent capacitance between the gate and
source electrodes of the output FET and the impedance path of the
bias switching device. .Iaddend..Iadd.9. A solid state
optically-coupled relay comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay with gate, source and drain
electrodes, the source and drain electrodes thereof being connected
to said output terminal means and the source and gate electrodes
being connected directly to the ends of the first diode array,
and
(e) a bias switching device connected across the gate and source
electrodes of the output FET, the gate and source electrodes having
an equivalent capacitance therebetween, the bias switching device
being operable in a first mode so as to provide a relatively high
impedance path between the gate and source electrodes of the output
FET when the light emitting means is illuminated and in a second
mode so as to provide a relatively low impedance path between the
gate and source electrodes of the output FET when the light
emitting means is not illuminated, such that the turn-off of the
output FET takes place with a time constant substantially
determined by the equivalent capacitance between the gate and
source electrodes of the output FET and the impedance path of the
bias switching device. .Iaddend..Iadd.10. A relay according to
claim 9, wherein the bias switching device is a transistor.
.Iaddend..Iadd.11. A relay according to claim 10, wherein the bias
switching device is a JFET. .Iaddend..Iadd.12. A relay according to
claim 10, wherein the bias switching device is a depletion-mode
JFET. .Iaddend..Iadd.13. A relay according to claim 9, wherein the
inherent turn-off time of the output FET, when unbaised, is a
function of the time it takes to discharge the equivalent
capacitance, wherein the turn-off time of the output FET in
response to the change of the bias switching device from the second
mode of operation to the first mode of operation is faster than the
inherent turn-off time of the
unbiased output FET. .Iaddend..Iadd.14. A relay according to claim
9, further including means for biasing said bias switching device.
.Iaddend..Iadd.15. A relay according to claim 14, wherein said
means for biasing said bias switching device includes a diode
string. .Iaddend..Iadd.16. A relay according to claim 15, wherein
the diode string biases the bias switching device to the second
mode of operation when the light emitting means is illuminated.
.Iaddend..Iadd.17. A solid state optically-coupled relay
comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) an enhancement-mode vertical conduction (DMOS) MOSFET,
constituting an output FET of the relay with gate, source and drain
electrodes, the source and drain electrodes thereof being connected
to the output terminal means and the source and gate electrodes
being connected directly to the ends of the first diode array, the
output FET being characterized as (i) having an equivalent
capacitance between the source and gate electrodes of the output
FET so that the inherent turn-on time of the out FET, when
unbiased, is a function of the time it takes to charge the
equivalent capacitance and (ii) being operable between an ON state
wherein the signal path between the source and drain electrodes of
the output FET is a relatively low impedance path and an OFF state
wherein the signal path between the source and drain electrodes of
the output FET is a relatively large impedance path, and
(e) a biasing switch connected between the source and gate
electrodes of the output FET, the bias switch being operable in a
first mode to provide a relatively low impedance path between the
source and gate electrodes of the output FET when the light
emitting means is not illuminated so as to maintain the output FET
in an OFF state, and in a second mode to provide a relatively high
impedance path between the two electrodes when the light emitting
means is illuminated so as to maintain the output FET in an ON
state, such that the turn-off time of the output FET in response to
the change of the bias switch from the second mode of operation to
the first mode of operation is faster than the inherent turn-off
time of the unbiased output FET. .Iaddend..Iadd.18. A relay
according to claim 17, wherein the biasing switch is a transistor.
.Iaddend..Iadd.19. A relay according to claim 18, wherein said
biasing switch is a JFET. .Iaddend..Iadd.20. A relay according to
claim 18, wherein said biasing switch is a deletion-mode JFET.
.Iaddend..Iadd.21. A relay according to claim 17, further including
means for biasing said biasing switch.
.Iaddend..Iadd.22. A relay according to claim 21, wherein said
means for biasing includes a diode string. .Iaddend..Iadd.23. A
relay according to claim 22, wherein the diode string biases the
biasing switch to the second mode of operation when the light
emitting means is illuminated. .Iaddend..Iadd.24. A solid state
optically-coupled relay comprising,
(a) solid state light emitting means,
(b) principal solid state photodetector means comprising a first
diode series array optically controllable by the light emitting
means,
(c) output terminal means,
(d) a first enhancement-mode vertical conduction (DMOS) MOSFET,
constituting a first output FET of the relay with gate, source and
drain electrodes, a second enhancement-mode vertical conduction
(DMOS) MOSFET, constituting a second output FET of the relay with
gate, source and drain electrodes, wherein the source electrodes of
the first and second output FETs are coupled together, the drain
electrodes of the first and second output FETs are respectively
coupled to said output terminal means and the source and gate
electrodes of the first and second output FETs are coupled to the
ends of the first diode array. .Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to solid state switching devices employing
optical isolation. Depending on the application and power level,
these devices can be characterized as solid state relays (SSR's) or
analog switches. In the invention, field-effect transistor (FET)
technology is combined with certain optoelectronic means alone or
supplemented by electromagnetic means to achieve both superior
switching characteristics and fabrication means compatible with
low-cost mass production.
It is common in the electronics industry, particularly in modern
telecommunications, test or computer equipment, to have control
circuits operating at different voltage levels from load circuits.
For example, it is common to operate 120 VAC industrial machines or
48 VDC telephone apparatus from 5 VDC computer logic.
As a result, the need arises to transfer data or control signals
between such dissimilar voltage references without physically
connecting the two circuits, i.e. with isolation.
In the evolving 1960's and 1970's state of the art, the isolation
functions have had increasingly stringent criteria of
miniaturization, long life under repetitive conditions, and low
operating power. Satisfying a large part of these needs are four
categories of isolated switching elements:
(a) Optical Couplers
These are generally low current (under 100 mA) elements employing
an LED as a light emitter and a variety of semiconductor types as
the sensor.
(b) Solid State Relays (SSR's) with Optical Isolation
These generally employ an internal optical coupler in combination
with other semiconductor elements to switch power.
(c) Solid State Relays with Transformer Isolation
(d) Reed Relays
While the primary purpose of couplers is only to isolate, the
purpose of reed relays and SSR's is not only to isolate but also to
provide power gain.
Over the past ten years, as isolators and SSR's have achieved a
degree of maturity and widespread acceptance, it has also become
apparent that a variety of limitations exist.
For example, couplers generally are limited in current and voltage.
They are usually limited to DC loads and are particularly
susceptible to damage when switching inductive loads or when
operating in an overload current mode. Furthermore, they exhibit a
voltage offset of about one volt because of the use of bipolar
photo-semiconductors. This means that voltages under one volt
cannot be switched and that audio signals cannot be faithfully
reproduced without the use of biasing.
While SSR's can offer considerable power gain and isolation, they
are generally relegated to being designed for AC or DC operation,
but not both. Also, an AC-load SSR employing a thyristor as the
output element is subject to false latchup. In any event, such an
SSR is not capable of being turned off in mid-cycle.
Reed relays experience difficulties with reactive loads and,
although faster than conventional mechanical relays, are still
slower than solid state switches. Also, reed relays are limited in
the degree they can switch momentarily high currents or voltages.
Finally, under very high switching rates, their mechanical life has
a severe limitation.
Because or these limitations, increasing attention is being given
to field effect transistors (FETs). Modern FETs offer the absence
of significant voltage offset and thermal runaway effects,
extremely high power gain, ability to operate at very high
frequencies, and under appropriate conditions, the ability to
switch AC or DC with equal ease.
The idea of FET switches for relay-type functions and related
developments go back to as early as 1955 and have been made since
by several skilled laboratories.
In spite of these many efforts going back over two decades,
optically activated FET power switches have not proven commercially
significant because of their inability to meet the specific cost
and performance requirements of certain defined industrial and
telecommunications applications.
It is a principal object of the invention, therefore, to provide
commercially useful, optically isolated power FET switches capable
of
1. operating at low (millivolt) or high (above 300 V) load
voltage,
2. operating at low (picoamps) or high (above several amperes)
current,
3. switching DC of either polarity or AC up to at least 100 KHz
(i.e., capability of sub-microsecond switching speeds),
4. immunity to false latchup (as with thyristors),
5. immunity to secondary breakdown (as with bipolar
transistors),
6. switching speeds controllable over wide range (for switching
transient control),
7. operating from very low input signals,
8. operating at high temperature,
9. being readily arranged in series or parallel arrays for extended
current or voltage capability,
10. duplicating the break-before-make configuration of Form C
mechanical relays,
11. exhibiting ON resistance under 100 milliohms and OFF resistance
over 10,000 megohms so to approach the isolation of reed
relays.
SUMMARY OF THE INVENTION
In accordance with the invention, a light emitting means--e.g., a
diode (LED) chip--is placed adjacent to a photovoltaic
device--e.g., an integrated circuit sensor chip. When activated by
light from the LED, the photovoltaic device becomes a voltage
source (of at least 8 volts). This voltage is applied between the
source and gate of a power MOSFET transistor, thereby causing the
transistor's impedance to change from many megohms down to a few
ohms. The photovoltaic integrated circuit chip may contain 16
diodes connected in series with connections at either end or the
midpoint of the series string. A low voltage MOSFET may be used to
enhance the overall unit's switching speed.
Light is coupled from the LED to the sensor through a thin, clear
film. Coupling is by refractive means, as described in the
above-cited U.S. application. The diode string, when activated by
light, generates slightly more than 0.5 volts per diode, or
slightly above 8 volts total.
The chosen sensor chip size can deliver only several micro-amperes.
(Larger chips could, of course, deliver more current.) However,
this is adequate to drive the gate of the FET because of its high
input impedance. The junction FET source and drain are across the
sensor and the parallel output FET source and gate. When the sensor
is activated, the FET is pinched off, allowing the .[.snesor.].
.Iadd.sensor .Iaddend.to deliver full current to the output chip.
When the light is removed, the FET is no longer pinched off and
drops to a low impedance. This, in turn, serves to significantly
speed up turnoff time of the output device.
The light emitter may consist of a liquid epitaxy gallium or
gallium aluminum arsenide LED to provide greater brightness than a
simple diffused LED. The sensor diode array is fabricated using
dielectric isolation techniques. This eliminated parasitic
light-generated voltages which could exist at various PN junctions
if conventional isolation-diffusion integrated circuit processing
were used.
The output consists of a pair of vertical-conduction,
double-diffused enhancement-mode MOS chips (sometimes referred to
as vertical DMOS). The use or depletion mode MOS output chips would
result in a normally on switching device which would turn off when
driven by the photovoltaic chip. The output chips are connected in
relation to each other and the sensor chip such that a two terminal
output is created that is capable of switching or blocking high
voltage AC or DC.
Other objects, features and advantages of the invention will be
apparent from the following detailed description of preferred
embodiments thereof taken in connection with the accompanying
drawing, in which,
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1A, 1B, 1C, 6, 7, 8.Iadd., 10B .Iaddend.and 11 are circuit
diagrams of preferred embodiments or the invention;
FIGS. 2, 3, 5, 9, 10A, .[.10B.]. and 10C are plan or cross section
views of physical constructions usable in one or more of said
circuits; and
FIG. 4 is a voltage vs. on-resistance trace for an operating
example of the present invention.
FIG. 11A is an equivalent circuit diagram of the circuit of FIG.
11.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 1A shows a simple form of the invention in which LED, L-1,
activated by a control current of about 10 mA, generates infrared
radiation which is sensed simultaneously by integrated diode array,
D-V. FIG. 2 depicts the coupling method. The output of D-V is
connected as shown to the gate and source of a MOSFET Q-1. The
cross-sectional geometry of Q-1 is shown in FIG. 3. Q-1 exhibits an
equivalent capacitance, C.sub.E, between gate and source of the
MOSFET. This capacitance is on the order of several dozen
picofarads for a chip 0.050 inches by 0.050 inches.
When D-V is activated, C.sub.E charges to the D-V generated voltage
through the D-V series equivalent resistance, R.sub.E, which, for
the chosen sensor chip of about 0.040 inches by 0.040 inches, is
about 1 Megohm.
Turn-on time for Q-1, is the time it takes for C.sub.E to charge to
a high enough voltage to where Q-1 has approached its lowest ON
resistance. In this case, two R.sub.E /C.sub.E time constants put
Q-1 gate voltage above 6 volts, a value where Q-1 is essentially
full on. FIG. 4 depicts the effect of gate voltage on ON
resistance.
It may take, for example, about 20-50 microseconds to charge
C.sub.E and turn on Q-1. When the signal to L-1 is removed, D-V
stops generating a voltage. However, the charge remains on C.sub.E,
thereby holding Q-1 on until stray leakage paths eventually provide
a discharge of C.sub.E. Since several seconds could be required for
discharge, alternate means for faster turn-off are normally
required.
Placing a bias resistor of several .[.megonms.]. .Iadd.megaohms
.Iaddend.as shown in FIG. 1B speeds the discharge of C.sub.E, but
has the effect of increasing turn-on time. This is acceptable where
switching speed is not critical. Dropping R to an even lower value
would theoretically discharge C.sub.E in 20-50 microseconds but can
unfortunately load down D-V excessively to where full or adequate
turn-on does not exist.
The ideal arrangement, then, is one where the full D-V voltage is
applied during turn-on with minimal biasing, but where maximum
biasing is attained during turnoff. Such an arrangement is provided
in the embodiment of the invention illustrated in FIG. 1C, where
the bias resistor is replaced by an N-channel depletion mode
junction FET, Q-2.
A second diode array, D-V.sub.2, is connected to the gate of Q-2.
Q-2, being a depletion mode FET, is normally in a low impedance
state, well under 1,000 ohms. When L-1 activates D-V.sub.2, Q-2
sces D-V.sub.2 total voltage at its gate, causing Q-2 to rapidly be
"pinched off" to a high impedance, thereby allowing enhanced
turn-on time.
When L-1 is deactivated, D-V.sub.2 voltage collapses and Q-2 is no
longer pinched off and drops to a low impedance, thereby causing
rapid C.sub.E discharge and Q-1 turn-off.
Photovoltaic diode generators, D-V and D-V.sub.2, and FET, Q-2, are
fabricated on the same chip using dielectric isolation. Each diode
is electrically isolated from the next except for surface
interconnect metallurgy, which proceeds from one anode to the next
cathode. The oxide "tub" surrounding each diode prevents electrical
interaction except between the P and N regions of each diode except
through surface interconnect metalization.
Such an array produced by conventional integrated circuit diffusion
isolation technology would result in undesirable photovoltaic
action at each isolation junction, cancelling out the desired
effects. The dielectric process provides near perfect isolation and
eliminates the many external-wire connections associated with
individual diode chips. It also provides the maximum diode element
location concentration for receiving light from L-1.
FIG. 5 depicts the cross-sectional geometry of the diodes and the
bias FET, Q-2. While FIGS. 1A, 1B and 1C reflect basic
configurations, a more preferred embodiment of the invention for
many practical applications is shown in FIG. 6. In the FIG. 6
circuit, the voltage generated by D-V is simultaneously applied to
the gates and sources of Q-1 and Q-2, which are connected
source-to-source. In operation, Q-1 and Q-2 can each block voltage
in one direction, but can withstand only about one volt in the
reverse direction. In FIG. 6, however, either Q-1 or Q-2 is always
in the full blocking mode, thereby blocking load voltage regardless
of the polarity.
With the vertical DMOS structure, the conductivity is modulated
with either polarity of load voltage. Hence, with a relatively high
AC load voltage, either Q-1 or Q-2 bears the majority of the
blocking requirement, but both drop to a low impedance with an L-1
signal. The final ON resistance, then, is the sum of Q-1 and
Q-2.
In this approach, no steering diodes are required and, therefore,
an ideal condition of no voltage offset is maintained. The
elimination of steering diodes means that dissipation and signal
distortion are minimized. Furthermore, high voltage AC (above 300
V) switching can be achieved. (Steering diodes or the low forward
drop of Schottky or gold-doped type are not available today in
high-voltage, low-leakage versions.)
It has been noted that the use of a depletion mode FET such as Q-2
can enhance turn-off. It can also be noted that the addition of a
photovoltaically activated FET, Q-2, as in FIG. 7, can enhance
turn-on speed as follows:
L-1 is so arranged as to simultaneously activate D-V. The reduced
impedance of Q-2 causes Q-1 gate capacitance to suddenly begin
charging toward V.sub.B through Q-2. Q-2 has an input capacitance
much smaller than that of Q-1 by an order to magnitude equal to the
degree of turn-on speed enhancement. Since Q-2 handles negligible
current, it can be a much smaller chip than Q-1, thereby
facilitating a geometry contributing less capacitance.
The result is the rapid charge of Q-1 gate capacitance and faster
turn-on, which could not occur were gate capacitance to charge
directly through the high impedance D-V voltage source.
With the preceding means for turn-on and the means of FIG. 1-C for
turn-off, it is possible to fabricate a bidirectional,
distortion-free AC power amplifier or switch capable of operating
at frequencies above 1 Megahertz.
Unlike thyristors, the switch exhibits no regenerative mechanism
and hence, cannot be latched on by a voltage transient.
With vertical DMOS technology, it is possible to integrate all
elements, except for the LED, on a single chip using dielectric
isolation technology. The single chip would include the
photovoltaic diodes (FIG. 7).
The circuit of FIG. 8 comprises a unique configuration in which
transformer action augments the isolated photovoltaic drive scheme
in order to achieve very fast turn-on and turn-off times.
In the FIG. 8 circuit, a LED is in parallel with the primary of a
pulse transformer T-1. When the input signal abruptly rises to a
positive level, LED-1 is illuminated. This would .[.noramlly.].
.Iadd.normally .Iaddend.cause voltage generator, D-V to begin
charging C.sub.E, in turn causing Q-1 to turn on in a time
comparable to the time constant of C.sub.E and internal equivalent
resistance of V.sub.G.
However, the rise in input signal is seen at the secondary of T-1
as a positive pulse which rapidly charges C.sub.E through forward
biased zener diode, D.sub.Z. Once the input signal reaches an
on-quiescent state, the positive pulse at the T-1 secondary
disappears, but the charge on C.sub.E .[.in.]. .Iadd.is
.Iaddend.maintained by D-V.
When the input signal drops to zero, L-1 ceases its illumination
and D-V discharge through available leakage paths, or, if one were
connected, a gate-to-source bias resistor. Turn-off time under this
condition would, in certain applications, be excessive.
With transformer augmentation, the loss of input signal and
cessation of T-1 primary current not only causes L-1's output to
cease, but also causes a negative pulse to appear at the T-1
secondary, causing D.sub.Z to break over in the reverse direction,
effectively grounding the gate of Q-1. As a result, C.sub.E is
discharged, abruptly causing extremely fast Q-1 to turn-off.
FIG. 8 depicts a similar configuration in which Q-1 and Q-2 are
connected to form an AC switch with high-speed drive circuit. It is
desirable in the circuits of FIG. 8 that the zener diode be
specified at a voltage approximately equal to that of the
photovoltaic generator and that the voltage appearing at the
secondary of T-1, especially at turn-off, be greater than the
breakdown voltage of D.sub.Z.
The result is a magnetically and optically isolated switch capable
of switching speeds on the order of hundreds of nanoseconds or
better. Since the T-1 is needed only during switching and depends
on D-V during the on period, T-1 can be sharply reduced in
size.
In various prior art technologies, it his been demonstrated that a
DC voltage could be used to operate an oscillator. The oscillator's
high frequency output could be transformer coupled, rectified,
filtered and applied to a transistor base or FET gate to turn it
on.
Such approaches, however, are more complex and introduce
filter-induced delays along with an oscillation signal which can be
undesirable, particularly when switching low-level or
high-frequency signals.
The circuit of FIGS. 1, 6, 7 and 8 provide simplified switching
without the hazard of spurious signals or beat frequencies.
While the primary mode of fabrication is based on thick film hybrid
circuit technology, variations of the preceding circuits can, with
the exception of the LED's, be readily fabricated totally on a
single integrated circuit chip; in particular, by means of
dielectric isolation technology. An IC employing the LEDS's, as
well, is compatible with present knowledge of processing
techniques, but is not yet economically feasible. Such a chip for
example could be fabricated having all elements fabricated with
Gallum Arsenide rather than having the diode and FET's processed
with silicon as is currently the practice.
A major consideration in activating the photovoltaic array is that
the maximum output current can be no greater than the current
generated by the least illuminated diode in the series string. In a
straightforward matrix of series-connected diodes (FIG. 9), the
diode farthest from the LED becomes a limiting factor. For
comparable efficiency (comparable impedance in the illuminated
state), the diode area should be larger as the distance from the
LED increases.
The optimized configuration, then, for a series string of
photovoltaic diodes is the "wagon wheel" configuration of FIG. 10A
(the equivalent circuit of which is shown in FIG. 10B and a
cross-section of which is shown in FIG. 10C), with the LED mounted
in the center so as to equally illuminate all diodes. The wagon
wheel surface geometry, coupled with the hemispherical refractive
film, provides maximum response to the LED hemispherical radiation
pattern across the entire surface of each diode.
FIG. 11 depicts such a circuit, capable of switching AC or DC, in
which the switching elements are depletion mode V-MOS chips. Bias
resistors R-1 and R-2 are selected to be above 5 .[.megohm.].
.Iadd.megaohms.Iaddend.. The result is that when the photovoltaic
generators DV-1 or DV-2 are activated, turnoff is rapid because of
minimal shunt resistance. Turn-on is slower because of the slow
discharge of MOSFET gate capacitance through the large bias
resistor value.
In the circuit shown, LED, L-1, is normally on, thereby maintaining
Q-2 and Q-3 in an OFF condition. When a control signal is applied
to terminal 1, Q-1 becomes saturated, thereby shunting current
around L-1. Base current to Q-1 passes through L-2, thereby rapidly
turning off Q-4 and Q-5 before Q-2 and Q-3 turn on. When the
control signal is removed, L-1 initiates rapid turn off of Q-2 and
Q-3 before Q-4 and Q-5 turn on.
FIG. 11A shows the equivalent circuit for such a physical circuit
as shown in FIG. 11.
It is evident that those skilled in the art, once given the benefit
of the foregoing disclosures, may now make numerous other uses and
modifications of, and departures from, the specific embodiments
described herein without departing from the inventive concepts.
Consequently, the invention is to be construed as embracing each
and every novel feature and novel combination of features present
in, or possessed by, the apparatus and techniques herein disclosed
and limited solely by the scope and spirit of the appended
claims.
* * * * *