Note: Descriptions are shown in the official language in which they were submitted.
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BALLAST WITH PROTECTION CIR(:UIT FOR PREVENTING INVERTER
STARTUP DURING AN OUTPUT GROUND-FAULT CONDITION
Field of the Invention
The present invention relates to the general subject of circuits for
powering discharge lamps. More particularly, the present invention relates to
a
ballast that includes a circuit for preventing startup of the inverter when
one or
more of the ballast output wires is shorted to earth ground.
Related Applications
This application is a continuation-in-part of application Ser. No.
09/967,192, filed September 28, 2001 and entitled "Ballast with Protection
Circuit for Preventing Inverter Startup During an Output Ground-Fault
Condition."
Background of the Invention
A number of existing electronic ballasts have non-isolated outputs. Such
ballasts typically include circuitry for protecting the ballast inverter from
damage in the event of lamp fault conditions such as lamp removal or lamp
failure.
Occasionally, the output wiring of a ballast becomes shorted to earth
ground in the lighting fixture. Such a condition can arise, for example, due
to
the wires becoming loose or pinched. For ballasts with non-isolated outputs,
if
the inverter begins to operate while an earth ground short is present at one
or
more of the output wires, a very large low frequency (e.g., 60 hertz) current
will
flow through the inverter transistors and cause them to fail.
Thus, a need exists for a ballast with a protection circuit that prevents the
inverter from starting when an output ground-fault condition is present. A
ballast with such a protection circuit would represent a significant advance
over
the prior art.
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Brief Description of the Drawings
FIG. 1 describes a ballast with a half bridge inverter and a protection
circuit for preventing inverter startup during an output-to-ground fault
involving
a first output connection, in accordance with a first preferred embodiment of
the
present invention.
FIG. 2 describes a ballast with a half bridge inverter and a protection
circuit for preventing inverter startup during an output-to-ground fault
involving
the first output connection, in accordance with a second preferred embodiment
of the present invention.
FIG. 3 describes a ballast with a half bridge inverter and a protection
circuit for preventing inverter startup during an output-to-ground fault
involving
a first output connection or a second output connection, in accordance with a
third preferred embodiment of the present invention.
FIG. 4 describes a ballast with a half bridge inverter and a protection
circuit for preventing inverter startup during an output-to-ground fault
involving
a first output connection or a second output connection or a third output
connection, in accordance with a fourth preferred embodiment of the present
invention.
FIG. 5 describes a ballast with a full-bridge inverter and a protection
circuit for preventing inverter startup during an output-to-ground fault
involving
a first output connection or a second output connection, in accordance with a
fifth preferred embodiment of the present invention.
FIG. 6 describes a ballast with a with a half bridge inverter and a
protection circuit for preventing inverter startup during an output-to-ground
fault involving a first output connection, in accordance with a sixth
preferred
embodiment of the present invention.
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Detailed Description of the Preferred Embodiments
A first preferred embodiment of the present invention is described in
FIG. 1. Ballast 10 includes a rectifier circuit 100, an inverter 200, an
output
circuit 300, and a protection circuit 400.
Rectifier circuit 100 has first and second input terminals 102,104 for
receiving a source of conventional alternating current, such as 120 volts AC
at
60 hertz, and first and second output terminals 106,108. Second output
terminal
108 is coupled to a circuit ground node 60. Rectifier circuit 100 includes a
full-
wave diode bridge 110 and a capacitor 112. During operation, capacitor 112 is
sufficiently large (e.g., on the order of tens of microfarads) such that a
substantially direct current (DC) voltage is provided between output terminals
106,108. Alternatively, and as known in the prior art, a boost converter may
be
inserted between output terminals 106,108 and inverter 200 so as to provide
power factor correction and other benefits, in which case capacitor 112 is
selected to be relatively small (e.g., on the order of tenths of a microfarad)
and
the voltage between output terminals 106,108 is substantially unfiltered, full-
wave rectified AC (i.e., "pulsating DC''). In either case, a substantially DC
voltage is provided to inverter 200.
Significantly, the voltage that exists between second output terminal 108
and earth ground (or, equivalently, the voltage that exists between second
output
terminal 108 and second input terminal 104; second input terminal 104 is
coupled to the neutral wire of AC source 20, which is at the same potential as
earth ground) is low frequency (e.g., 60 hertz) half wave rectified AC.
Inverter 200 includes first and second input terminals 202,204, an output
terminal 206, first and second inverter switches 210,220, a drive circuit 230,
and
a DC supply circuit that includes resistor 240, capacitor 250, capacitor 260,
diode 262, and a zener diode 264. First input terminal 202 is coupled to first
output terminal 106 of rectifier circuit 100. Second input terminal 204 is
coupled to second output terminal 108 of rectifier circuit 100. First inverter
switch 210 is coupled between first input terminal 210 and output terminal
206.
Second inverter switch 220 is coupled between output terminal 206 and circuit
ground 60. As depicted in FIG. 1, inverter switches 210,220 are preferably
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implemented as field-effect transistors. Drive circuit 230 is coupled to
inverter
switches 210,220, and includes a DC supply input 232. Drive circuit 230 may
be implemented using any of a number of circuits known to those skilled in the
art, such as the IR2155 high-side driver integrated circuit manufactured by
International Rectifier. Alternatively, although not explicitly shown or
described in the drawings, drive circuit 230 may be implemented using any of a
number of a self oscillating drive arrangements known to those skilled in the
art; for example, drive circuit 230 may include a diac-based startup circuit
for
initiating inverter operation and a feedback circuit that uses signals from
output
circuit 300 to provide inverter switching once the inverter begins to operate.
During operation, drive circuit 230 turns inverter switches 210,220 on
and off in a substantially complementary fashion and preferably at a high
frequency rate in excess of 20,000 hertz. Drive circuit 230 initially turns on
when the voltage at DC supply input 232 exceeds a startup threshold (e.g., 10
volts), and remains on as long as the voltage at DC supply input 232 remains
above a turn-off threshold (e.g., 8 volts). Resistor 240 and capacitor 250 are
coupled to DC supply input 232 and provide energy for initially turning on
drive
circuit 230. Once inverter 200 begins to operate, energy from output circuit
300
is delivered, via capacitor 260 and diode 262, to capacitor 250 and drive
circuit
230. This low-impedance "bootstrapping" circuit supplies the operating current
required by drive circuit 230 and maintains the voltage across capacitor 250
at a
value (e.g., 15 volts) well above the turn-off threshold (e.g., 8 volts) of
drive
circuit 230. Zener diode 264 protects drive circuit 230 from overvoltage
and/or
excessive power dissipation by ensuring that the voltage at DC supply input
230
does not exceed a specified level (e.g., 15 volts).
Output circuit 300 includes first and second output connections 302,
306, a resonant inductor 320, a resonant capacitor 330, and a direct current
(DC)
blocking capacitor 340. First and second output connections 302,306 are
adapted for connection to a lamp load comprising at least one gas discharge
lamp 30. Resonant inductor 320 is coupled between inverter output terminal
206 and first output connection 302. Resonant capacitor 330 is coupled
between first output connection 302 and circuit ground 60. DC blocking
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capacitor 340 is coupled between second output connection 306 and circuit
ground 60. During operation, resonant inductor 320 and resonant capacitor 330
function in a well-known manner as a series resonant circuit having a natural
resonant frequency that is typically at or near the frequency at which
inverter
5 switches 210,220 are turned on and off. Output circuit 300 supplies a high
voltage for igniting lamp 30, as well as a magnitude-limited current for
operating lamp 30 in a controlled manner. DC blocking capacitor 300 blocks
the DC component in the inverter output voltage (which is equal to half of the
rectifier output voltage) and thus prevents substantial DC components from
appearing in the voltage and current provided to lamp 30 during steady-state
operation.
Protection circuit 400 includes an input 402 coupled to inverter output
206, and an output coupled to DC supply input 232 of drive circuit 230. During
operation, protection circuit 400 prevents inverter 200 from starting if first
1 S output connection 302 is shorted to earth ground.
As described in FIG. l, in a first preferred embodiment of the present
invention, protection circuit 400 includes a first resistor 420, a second
resistor
440, an electronic switch 450, and a third resistor 460. First resistor 420 is
coupled between input 402 and a first node 430. Second resistor 440 is coupled
between first node 430 and circuit ground 60. Electronic switch 450 is
preferably implemented as a NPN bipolar junction transistor having a base 452,
a collector 454, and an emitter 456. Base 452 is coupled to first node 430.
Emitter 456 is coupled to circuit ground 60. Third resistor 460 is coupled
between output 410 and the collector 454 of transistor 450.
In a prototype ballast configured substantially as shown in FIG. 1, the
components of protection circuit 400, and selected components of the DC
supply circuit of inverter 200, were sized as follows:
Resistor 240: 220 kilohms
Capacitor 250: 22 microfarads
Resistor 420: 220 kilohms
Resistor 440: 2.2 kilohms
Transistor 450: 2N3904
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Resistor 460: 2.2 kilohms
The detailed operation of protection circuit 400 is now explained with
reference to FIG. 1 as follows. When AC power is initially applied to ballast
10,
drive circuit 230 and inverter 200 are off and remain off until such time as
the
voltage at DC supply input 232 reaches the predetermined startup threshold
(e.g., 10 volts) of drive circuit 23U. In the absence of a ground fault
condition at
output connection 302, protection circuit 400 will exert no effect upon
inverter
startup because transistor 450 will be non-conductive prior to inverter
startup.
With transistor 450 off, capacitor 250 charges up via resistor 240. Once the
voltage across capacitor 250 reaches the startup threshold (e.g., 10 volts),
drive
circuit 230 turns on and begins to turn inverter switches 210,220 on and off
in a
periodic manner.
At this point, with inverter 200 operating, the voltage between inverter
output 206 and circuit ground 60 varies between zero and a high DC value
(i.e.,
the DC voltage provided between inverter input terminals 202,204) at a high
frequency rate, which causes two things to occur. First, the voltage at
inverter
output 206 excites output circuit 300. Consequently, bootstrapping energy is
fed back from output circuit 300 to capacitor 250 and drive circuit 230 via
capacitor 260 and diode 262, thereby keeping drive circuit 230 active. Second,
during those intervals when the voltage at inverter output 206 is high,
sufficient
voltage is developed across resistor 440 to turn on transistor 450. Thus,
transistor 450 turns on and off at a high frequency rate. However, this exerts
no
substantial effect on the operation of inverter 200 because, even with
transistor
450 on and resistor 460 coupled to circuit ground 60, abundant bootstrapping
current is provided to maintain the voltage at DC supply input 232 well above
the turn-off threshold (e.g., 8 volts) of drive circuit 230; for this reason,
resistor
460 is sized sufficiently large (e.g., 2.2 kilohms) so as not to present so
great a
load upon the bootstrapping circuit. Thus, once inverter operation commences,
protection circuit 400 has no effect on the continued operation of inverter
200.
If, on the other hand, a ground fault condition is present at first output
connection 302 prior to inverter startup, the following events occur. As
previously discussed, once AC power is initially applied to ballast 10, the
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voltage between circuit ground 60 and earth ground is low frequency (e.g., 60
hertz) half wave rectified AC. More specifically, during the negative half
cycles of the voltage provided by AC source 20 (i.e., when a negative voltage
exists between first input terminal 102 and second input terminal 104;
equivalently, when a positive voltage exists between second input terminal 104
and first input terminal 102), the lower left-hand diode in bridge rectifier
110 is
forward-biased and the voltage between earth ground (i.e., the neutral wire at
the
lower end of AC source 20) and circuit ground 60 has a positive polarity.
Consequently, under a fault condition wherein first output connection 302 is
connected to earth ground, a positive current flows up from earth ground, into
first output connection 302, through resonant inductor 320, into input 402,
through resistors 420,440, into circuit ground 60, through the lower left-hand
diode of bridge rectifier 102, out of first input terminal 102, through AC
source,
and back to the neutral wire of AC source 20 (which is at the same potential
as
earth ground). This positive current produces sufficient voltage (e.g.,
greater
than 0.7 volts) across resistor 440 to activate transistor 450. With
transistor 450
turned on, DC supply input 232 is coupled to circuit ground 60 via resistor
460.
Because resistor 460 has a resistance (e.g., 2.2 kilohms) that is very low
relative
to that of resistor 240 (e.g., 220 kilohms), the voltage across capacitor 250
is
limited to a low value that is less than the startup threshold of drive
circuit 230.
Transistor 450 will be on during only the negative half cycles of the AC
source
voltage (during the positive half cycles of the AC source voltage, the voltage
between earth ground and circuit ground 60 is negative, and thus incapable of
keeping transistor 450 on), but that is still sufficient (provided that the RC
time
constant of resistor 240 and capacitor 250 is sufficiently large) to prevent
the
voltage across capacitor 250 from reaching the startup threshold. In this way,
inverter 200 is prevented from starting when an earth ground fault condition
is
present at output connection 302 prior to inverter startup.
It should be appreciated that protection circuit 400 does not necessarily
require a true short (i.e., zero ohm impedance) between first output
connection
302 and earth ground in order to prevent inverter startup. For example, with
the
component values discussed above, protection circuit 400 will prevent inverter
CA 02429424 2003-05-23
startup as long as the impedance between first output connection 302 and earth
ground is less than about 100,000 ohms. Given that inverter damage may occur
even for earth ground faults in which there is a substantial impedance between
first output connection 302 and earth ground, this added capability of
protection
circuit 400 is a potentially significant advantage.
Turning now to FIG. 2, in a second preferred embodiment of the present
invention, protection circuit 400 is configured in substantially the same
manner
as previously described with reference to FIG. 1, except that input 402 is
coupled to first output connection 302 instead of inverter output 206. Even
with
this modification, the operation of protection circuit 400 remains
substantially
unchanged from that which was previously described. More specifically,
because the voltage that exists between circuit ground 60 and earth ground is
low frequency (e.g., 60 hertz) half wave rectified AC, the impedance of
resonant
inductor 320 is negligible compared to that of resistor 420. Thus, it makes no
significant functional difference whether input 402 is coupled to inverter
output
206 (as in FIG. 1) or first output connection 302 (as in FIG. 2); either way,
protection circuit 400 will respond to occurrence of an earth ground fault at
first
output connection 302. However, because the maximum voltage at first output
connection 302 is (due to resonant voltage gain that occurs prior to ignition
of
lamp 30) substantially greater than the maximum voltage at inverter output
206,
it may be necessary to increase the voltage rating of resistor 420 accordingly
if
the embodiment of FIG. 2 is employed.
Referring now to FIG. 3, in a third preferred embodiment of the present
invention, protection circuit 400' includes a second input 404 and a fourth
resistor 422, in addition to the components present in protection circuit 400
in
FIG. 1. Second input 404 is coupled to second output connection 306. Fourth
resistor 422 is coupled between second input 404 and first node 430. The
addition of fourth resistor 422 allows protection circuit 400' to monitor both
output connections 302,306 and correspondingly prevent the inverter from
starting if an earth ground fault is present at either (or both) of the output
connections 302,306.
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Because resistor 422 is coupled, via input 404, to DC blocking capacitor
340 (which, during operation of lamp 30, has a large positive DC voltage
across
it all of the time), it is likely that transistor 450 will remain on all of
the time
after lamp 30 begins to operate following inverter startup. This should be
contrasted with what was previously described with reference to the circuit of
FIG. 1, where it was explained that transistor 450 will turn on and off at a
high
frequency rate (when input 402 is coupled to inverter output 206). Although
this
behavior in the circuit of FIG. 3 does not impact the desired functionality of
protection circuit 400' in preventing inverter startup under an output ground-
fault condition, it is relevant from a design standpoint because the designer
must
be sure that resistor 460 is large enough so as not to present an unduly large
load
that interferes with proper bootstrapping during normal operation of the
inverter.
Although not explicitly shown in the drawings, it should be appreciated
that first resistor 420 in FIG. 3 may alternatively be coupled to first output
connection 302 rather than inverter output 206, along the same lines as
previously discussed, without substantially affecting the desired operation of
protection circuit 400'.
Turning now to FIG. 4, in a fourth preferred embodiment that is suited
for a ballast that powers a lamp load comprising two lamps 30,32, protection
circuit 400" includes three resistors 420,422,424, each of which is coupled to
a
corresponding output connection 302,304,306. More specifically, the output
circuit includes first, second, and third output connections 302,304,306.
First
and second output connections 302,304 are adapted for connection to a first
lamp 30, while second and third output connections 304,306 are adapted for
connection to a second lamp 32. Second output connection 304 is coupled to a
junction 34 between first lamp 30 and second lamp 32. Protection circuit 400"
includes first, second, and third inputs 402,404,406, and first, fourth, and
fifth
resistors 420,422,424. First input 402 is coupled to inverter output 206.
Second
input 404 is coupled to second output connection 304. Third input is coupled
to
third output connection 306. First resistor 420 is coupled between first input
402 and first node 430. Fourth resistor 422 is coupled between second input
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404 and first node 430. Finally, fifth resistor 406 is coupled between third
input
406 and first node 430.
In the circuit of FIG. 4, protection circuit 400" monitors all three output
connections 302,304,306 and correspondingly prevents the inverter from
5 starting if an earth ground fault is present at any one (or any pair, or all
three) of
the output connections 302,304,306. As previously discussed, first input 402
may alternatively be coupled to first output connection 302 (rather than
inverter
output 206) without affecting the desired operation of protection circuit
400".
It should be appreciated that protection circuit 400" may be further
10 modified, in like fashion, to accommodate more than two lamps (i.e., more
than
three output connections) simply be adding additional inputs and resistors to
protection circuit 400"
Turning now to FIG. 5, in a fifth preferred embodiment of the present
invention, inverter 500 is a full-bridge inverter comprising first and second
input
terminals 502,504, first and second output terminals 506,508, first, second,
third, and fourth inverter switches 510,512,516,518, a drive circuit 530, and
a
DC supply 570. Input terminals 502,504 are intended for connection to either a
rectifier or a rectifier followed by a boost converter. Output terminals
506,508
are adapted for connection to a lamp load comprising at least one gas
discharge
lamp 30. First inverter switch 510 is coupled between first input terminal 502
and second output terminal 508. Second inverter switch 512 is coupled between
second output terminal 508 and circuit ground 60. Third inverter switch 516 is
coupled between first input terminal 502 and first output terminal 506. Fourth
inverter switch 518 is coupled between first output terminal 506 and circuit
ground 60. Drive circuit 530 is coupled to each of the inverter switches
510,512,516,518, and includes a DC supply input 532. During operation, drive
circuit 530 turns each opposing pair of inverter switches (i.e., switches
510,518
are one pair, switches 512,516 are the other pair) on and off in a
substantially
complementary fashion and preferably at a high frequency rate in excess of
20,000 hertz. Drive circuit 530 initially turns on when the voltage at DC
supply
input 532 exceeds a startup threshold (e.g., 10 volts), and remains on as long
as
the voltage at DC supply input 532 remains above a turn-off threshold (e.g., 8
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volts). DC supply 570, which is coupled to DC supply input 532, provides
energy for initiating operation of drive circuit 530 and maintaining operation
of
drive circuit 530 after inverter switching commences.
Protection circuit 600 includes a first input 602 coupled to first output
terminal 506, a second input 604 coupled to second output terminal 508, and an
output 610 coupled to DC supply input 532 of drive circuit 530. During
operation, protection circuit 600 prevents inverter 500 from starting if
either
one, or both, of output terminals 506,508 is shorted to earth ground.
As described in FIG. 5, protection circuit 600 includes a first resistor
620, a second resistor 622, a third resistor 640, an electronic switch 650,
and a
fourth resistor 660. First resistor 620 is coupled between first input 602 and
a
first node 630. Second resistor 622 is coupled between second input 604 and
first node 630. Third resistor 640 is coupled between first node 630 and
circuit
ground 60. Electronic switch 650 is preferably implemented as a NPN bipolar
junction transistor having a base 652, a collector 654, and an emitter 656.
Base
652 is coupled to first node 630. Emitter 656 is coupled to circuit ground 60.
Fourth resistor 660 is coupled between output 610 and the collector 654 of
transistor 650.
The detailed operation of protection circuit 600 is substantially similar to
that which was previously described with reference to the other preferred
embodiments disclosed herein.
As previously discussed with reference to FIG. 1, resistor 240 and
capacitor 250 function as a startup circuit for initially turning on drive
circuit
230. In those applications where resistor 240 and capacitor 250 have suitably
large values (e.g., 220 kilohms and 22 microfarads, respectively), the
arrangement of FIG. I works well. If, however, resistor 240 and/or capacitor
250 are substantially lowered in value (e.g., to 120 kilohms and 2.2
microfarads,
respectively) in order to accommodate "low-line" operation where the AC line
voltage is considerably lower than its nominal value (e.g., 90 volts instead
of the
nominal 120 volts), it is possible that the inverter will start even if an
output
fault is present. More particularly, as previously discussed, when an output
fault is present, transistor 450 will be on only during the negative half
cycles of
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the AC line voltage. However, with resistor 240 coupled to a source of full-
wave rectified AC voltage, capacitor 2S0 will be allowed to charge up during
the positive half cycles when transistor 450 is off. If the RC time constant
of
resistor 240 and capacitor 250 is very short (i.e., small enough to allow the
voltage across capacitor 250 to reach the startup threshold of 10 volts during
one
positive half cycle), the inverter may momentarily start even if an output
fault is
present. The possibility of this occurring becomes even greater when operating
under a "high line" condition where the AC line voltage may exceed its nominal
value by as much as twenty percent (e.g., 144 volts instead of the nominal 120
volts). Although increasing the resistance of resistor 240 and/or the
capacitance
of capacitor 250 may solve the problem, that is not a feasible design option;
for
example, the resistance of resistor 240 must be low enough to ensure normal
inverter startup under low-line conditions.
In order to properly solve this problem, and thereby ensure that the
inverter does not start up when a fault is present at the ballast output, the
startup
circuit may be modified by changing the connection of the startup resistor.
More specifically, in a sixth preferred embodiment as described in FIG. 6,
startup resistor 242 is coupled to the second input terminal 104 of rectifier
circuit 100 (as opposed to the arrangement in FIG. 1, in which startup
resistor
240 is coupled to the first output terminal 106). Because the voltage between
input terminal 104 and circuit ground 60 is half-wave rectified AC that is
substantially in phase with the voltage that activates transistor 450 when an
output fault is present, resistor 242 will supply charging current to
capacitor 250
only during the same half of the AC line cycle as the fault signal. Thus, when
transistor 450 is off, no charging current is provided to capacitor 250, and
when
transistor 450 is on, charging current flows through resistor 242 but
capacitor
250 is prevented from charging up. In this way, inverter startup is prevented
under a fault condition, even if the RC time constant of resistor 242 and
capacitor 250 is very short.
In a prototype ballast configured substantially as shown in FIG. 6, the
components of protection circuit 400, and selected components of the DC
supply circuit of inverter 200', were sized as follows:
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Resistor 242: 120 kilohms
Capacitor 250: 2.2 microfarads
Resistor 420: 200 kilohms
Resistor 440: 10 kilohms
Transistor 450: 2N3904
Resistor 460: 4.7 kilohms
The modified startup circuit described in FIG. 6 is equally applicable to
the embodiments previously described with reference to FIGs. 2-5.
Although the present invention has been described with reference to
certain preferred embodiments, numerous modifications and variations can be
made by those skilled in the art without departing from the novel spirit and
scope of this invention. For example, although the preferred embodiments
disclosed herein describe inverters 200,500 as a driven-type inverter, it
should
be understood that inverter need not be a driven-type inverter, and that
protection circuits 400, 400', 400" may be used in conjunction with a self
oscillating type inverter (e.g., to prevent triggering of a diac in a diac-
based
inverter starting circuit). As another example, although all of the preferred
embodiments disclosed herein relate to a discrete circuit implementation of
protection circuits 400, 400', 400", it should be appreciated that each
protection
circuit may alternatively by realized using a non-discrete means, such as a
microcontroller or custom integrated circuit along with peripheral components
that is programmed or configured to provide the input/output functionality of
protection circuits 400, 400', 400" as described herein.
What is claimed is:
