U.S. patent application number 09/998992 was filed with the patent office on 2002-07-04 for fault protection system for a respiratory conduit heater element.
This patent application is currently assigned to Fisher & Paykel Limited. Invention is credited to Seakins, Paul John.
Application Number | 20020083947 09/998992 |
Document ID | / |
Family ID | 19927088 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020083947 |
Kind Code |
A1 |
Seakins, Paul John |
July 4, 2002 |
Fault protection system for a respiratory conduit heater
element
Abstract
A fault protection circuit for a respiratory conduit heater
element in a respirator humidification system is disclosed. The
circuit includes a spark detector as well as overcurrent detector.
Several variations are included for the spark detector including a
two winding transformer, a centre tapped two winding transformer),
and a high pass filtered inductor. A semiconductor switching
configuration is also disclosed. Once the protection circuit
detects a change in current over a certain level, or the average
level raises above a threshold, then the current in the heater
element is interrupted for a preset period.
Inventors: |
Seakins, Paul John;
(Auckland, NZ) |
Correspondence
Address: |
Trexler, Bushnell, Giangiorgi,
Blackstone & Marr, Ltd.
36th Floor
105 West Adams Street
Chicago
IL
60603
US
|
Assignee: |
Fisher & Paykel Limited
78 Springs Road East Tamaki
Auckland
NZ
|
Family ID: |
19927088 |
Appl. No.: |
09/998992 |
Filed: |
November 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09998992 |
Nov 30, 2001 |
|
|
|
09464495 |
Dec 15, 1999 |
|
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Current U.S.
Class: |
128/204.17 ;
128/205.23 |
Current CPC
Class: |
H02H 5/10 20130101; H02H
3/44 20130101; H02H 1/0015 20130101; A61M 16/1095 20140204; H03K
2217/0027 20130101; A61M 16/1075 20130101; H02H 3/07 20130101 |
Class at
Publication: |
128/204.17 ;
128/205.23 |
International
Class: |
C10B 047/00; C10B
051/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 1998 |
NZ |
NZ 333552 |
Claims
1. A fault protection system for a respiratory conduit heater
element comprising: detecting means which include means to detect a
rapid change of current in said heater element, current interruptor
means in series with said heater element, and timer means adapted
to control the action of said interruptor means and thereby in use
determining the duration of current interruption, said timer means
being triggered by said detecting means.
2. A fault protection system for a respiratory conduit heater
element as claimed in claim 1, wherein said detecting means also
includes a peak current detector for detecting current in the
heater element and said timing means is also triggered by said peak
current detector when a predetermined threshold current is
exceeded.
3. A fault protection system for a respiratory conduit heater
element as claimed in either of claims 1 or 2, wherein said
detecting means comprises: a transformer including a primary and
secondary winding, said primary winding adapted to be connected in
series with said heater element and a power source, a full wave
rectifier including rectifier input means and rectifier output
means, said rectifier input means connected across said secondary
winding of said transformer, a low pass filter including filter
input means and filter output means, said filter input means
connected to said rectifier output means, and a comparator
including comparator input means and comparator output means, said
comparator input means connected to said filter output means,
wherein when the output of said low pass filter reaches the first
predetermined threshold, said current interrupter means is
controlled to interrupt the current in said heater element.
4. A fault protection system for a respiratory conduit heater
element as claimed in either of claims 1 or 2, wherein said
detecting means comprises: a transformer including a centre tapped
pi winding and secondary winding, each end of said primary winding
adapted to be connected in series with at least one circuit of said
heater element and said centre tapp adapted to be connected to a
power source, a full wave rectifier including rectifier input means
and rectifier output means, said rectifier input means connected
across said secondary winding of said transformer, a low pass
filter including filter input means and filter output means, said
filter input means connected to said rectifier output means, and a
comparator including comparator input means and comparator output
means, said comparator input means connected to said filter output
means, wherein when the output of said low pass filter reaches the
first predetermined threshold, said current interrupter means is
controlled to interrupt the current in said heater element.
5. A fault protection system for a respiratory conduit heater
element as claimed in either of claims 1 or 2, wherein said
detecting means comprises: an inductor adapted to be connected in
series with said heater element and a power source, a first high
pass filter including first filter input means and first filter
output means, said first filter input means connected to a first
end of said inductor, a second high pass filter including second
filter input means and second filter output means, said second
filter input means connected to the other end of said inductor, a
full wave rectifier including rectifier input means and rectifier
output means, said rectifier input means connected to said first
filter output means and said second filter output means
respectively, a low pass filter including filter input means and
filter output means, said filter input means connected to said
rectifier output means, and a comparator including comparator input
means and comparator output means, said comparator input means
connected to said filter output means, wherein when the output of
said low pass filter reaches the first predetermined threshold,
said current interrupter means is controlled to interrupt the
current in said heater element.
6. A fault protection system for a respiratory conduit heater
element, as claimed in either of claims 1 or 2 wherein said current
interruptor means comprises: two same channel MOSFETS connected in
series with their source and gate electrodes respectively tied
together, said circuit adapted to receive a switching voltage
between the commoned gate and source electrodes.
7. A fault protection system for a respiratory conduit heater
element, as claimed in either of claims 1 or 2 such that in use
when said detecting means determines a fault is present in said
heater element said timing means controls said current interruptor
means to interrupt the current in said heater element for a
predetermined period after said fault is detected, whereby when
said predetermined period expires said timing means controls said
current interruptor means to allow current to flow through said
heater element.
8. A semiconductor switching circuit for rapidly controlling the AC
supply current through a load from a power supply comprising: two
same channel MOSFETs connected in series with their source and gate
electrodes respectively tied together, said circuit adapted to
receive a switching voltage between the commoned gate and source
electrodes, their drain electrodes adapted to be connected to a
load and a power supply respectively.
9. A semiconductor switching circuit for rapidly controlling the AC
supply current through a load from a power supply comprising: two
same channel MOSFETs connected in series with their drain and gate
electrodes respectively tied together, said circuit adapted to
receive a switching voltage between the commoned gate and drain
electrodes, their source electrodes adapted to be connected to a
load and a power supply respectively.
10. A fault protection system for a respiratory conduit heater
element comprising: detecting means which includes a peak current
detector for detecting current in the heater element current
interruptor means in series with said heater element, and timer
means adapted to control the action of said interruptor means and
thereby in use determining the duration of current interruption,
said timer means being triggered by said detecting means, when a
predetermined threshold current is exceeded.
11. In a respiratory humidification system wherein a conduit
connects a patient to a humidifier, said conduit being heated by a
respiratory conduit heater element controlled by said humidifier,
the improvement comprising that said humidifier includes a fault
protection system for said heater element comprising: detecting
means which include means to detect a rapid change of current in
said heater element, current interruptor means in series with said
heater element, and timer means adapted to control the action of
said interruptor means and thereby in use determining the duration
of current interruption, said timer means being triggered by said
detecting means.
12. In a respiratory humidification system as claimed in claim 11,
the improvements further comprising that said detecting means also
includes a peak current detector for detecting current in the
heater element and said timing means is also triggered by said peak
current detector when a predetermined threshold current is
exceeded.
13. In a respiratory humidification system as claimed in either of
claims 11 or 12, the improvements further comprising that said
detecting means comprises: a transformer including a primary and
secondary winding, said primary winding adapted to be connected in
series with said heater element and a power source, a full wave
rectifier including rectifier input means and rectifier output
means, said rectifier input means connected across said secondary
winding of said transformer, a low pass filter including filter
input means and filter output means, said filter input means
connected to said rectifier output means, and a comparator
including comparator input means and comparator output means, said
comparator input means connected to said filter output means,
wherein when the output of said low pass filter reaches the first
predetermined threshold, said current interrupter means is
controlled to interrupt the current in said heater element.
14. In a respiratory humidification system as claimed in either of
claims 11 or 12, the improvements further comprising that said
detecting means comprises: a transformer including a centre tapped
primary winding and secondary winding, each end of said primary
winding adapted to be connected in series with at least one circuit
of said heater element and said centre tapp adapted to be connected
to a power source, a full wave rectifier including rectifier input
means and rectifier output means, said rectifier input means
connected across said secondary winding of said transformer, a low
pass filter including filter input means and filter output means,
said filter input means connected to said rectifier output means,
and a comparator including comparator input means and comparator
output means, said comparator input means connected to said filter
output means, wherein when the output of said low pass filter
reaches the first predetermined threshold, said current interrupter
means is controlled to interrupt the current in said heater
element.
15. In a respiratory humidification system as claimed in either of
claims 11 or 12, the improvements further comprising Mat said
detecting means comprises: an inductor adapted to be connected in
series with said heater element and a power source, a first high
pass filter including first filter input means and first filter
output means, said first filter input means connected to a first
end of said inductor, a second high pass filter including second
filter input means and second filter output means, said second
filter input means connected to the other end of said inductor, a
full wave rectifier including rectifier input means and rectifier
output means, said rectifier input means connected to said first
filter output means and said second filter output means
respectively, a low pass filter including filter input means and
filter output means, said filter input means connected to said
rectifier output means, and a comparator including comparator input
means and comparator output means, said comparator input means
connected to said filter output means, wherein when the output of
said low pass filter reaches the first predetermined threshold,
said current interrupter means is controlled to interrupt the
current in said heater element.
16. In a respiratory humidification system as claimed in either of
claims 11 or 12 the improvements further comprising that said
current interruptor means comprises: two same channel MOSFETS
connected in series with their source and gale electrodes
respectively tied together, said circuit adapted to receive a
switching voltage between the commoned gate and source
electrodes.
17. In a respiratory humidification system as claimed in either of
claims 11 or 12 the improvements further comprising that in use
when said detecting means determines a fault is present in said
heater element said timing means controls said current interruptor
means to interrupt the current in said heater element for a
predetermined period after said fault is detected, whereby when
said predetermined period expires said timing means controls said
current interruptor means to allow current to flow through said
heater element.
18. In a respiratory humidification system wherein a conduit
connects a patient to a humidifier, said conduit being heated by a
respiratory conduit heater element controlled by said humidifier,
the improvement comprising that said humidifier includes a fault
protection system for said heater element comprising: detecting
means which includes a peak current detector for detecting current
in the heater element current interruptor means in series with said
heater element, and timer means adapted to control the action of
said interrupter means and thereby in use determining the duration
of current interruption, said timer means being triggered by said
detecting means, when a predetermined threshold current is
exceeded.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the invention
[0002] This invention relates to respiratory humidifiers and heated
breathing conduits used to couple a patient to the humidifier. A
fault protection system for the conduit heater wire is
disclosed.
[0003] (2) Description of the Prior Art
[0004] In order to supply gases to a patient or a person needing
such gases, it may sometimes be necessary to first humidify those
gases, for example using a respiratory humidifier/ventilator system
In such a case where the gases have been humidified, and therefore
laden with water, it is likely that during transport through a
conduit to the patient, condensation of that water vapour will
occur. In order to overcome this disadvantage it is known to
associate a heater wire with respiratory humidifier breathing
conduits to avoid condensation. Examples of such a heated breathing
conduit are disclosed in U.S. Pat. No. 5,537,996 (McPhee) and U.S.
Pat. No. 5,392,770 (Clawson et al.).
[0005] However there are safety concerns with using a heated wire
system, especially when the gas in the breathing conduit contains a
high concentration of oxygen, which may be a common condition in
hospitals. It is possible for ignition of the heater wire and
conduit materials to occur if certain fault conditions are present
Thus physicians may be hesitant to use a humidifier with an
associated heater wire, due to the perceived risks to the patient.
However if the a humidifier is not used various respiratory
problems can occur due to the lack of controlled humidity.
BRIEF SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a fault
protection system for a respiratory conduit heater element which
goes some way towards overcoming the above mentioned
disadvantages.
[0007] Accordingly, in a first aspect, the present invention
consists in a fault protection system for a respiratory conduit
heater element comprising:
[0008] detecting means which include means to detect a rapid change
of current in said heater element,
[0009] current interrupter means in series with said heater
element, and
[0010] timer means adapted to control the action of said
interruptor means and thereby in use determining the duration of
current interruption, said timer means being triggered by said
detecting means.
[0011] In a second aspect, the present invention consists in a
semiconductor switching circuit for rapidly controlling the AC
supply current through a load from a power supply comprising:
[0012] two same channel MOSFETs connected in series with their
source and gate electrodes respectively tied together, said circuit
adapted to receive a switching voltage between the commoned gate
and source electrodes, their drain electrodes adapted to be
connected to a load and a power supply respectively.
[0013] In a third aspect, the present invention consists in a
semiconductor switching circuit for rapidly controlling the AC
supply current through a load from a power supply comprising:
[0014] two same channel MOSFETs connected in series with their
drain and gate electrodes respectively tied together, said circuit
adapted to receive a switching voltage between the commoned gate
and drain electrodes, their source electrodes adapted to be
connected to a load and a power supply respectively.
[0015] In a fourth aspect, the present invention consists in a
fault protection system for a respiratory conduit heater element
comprising:
[0016] a detector means which includes a peak current detector for
detecting current in the heater element
[0017] a current interruptor in series with said heater element,
and
[0018] a timing circuit adapted to control the action of said
current interruptor and thereby in use determining the duration of
current interruption, said timing circuit being triggered by said
detector, when a predetermined threshold current is exceeded.
[0019] In a fifth aspect, the present invention consists in a
respiratory humidification system wherein a conduit connects a
patient to a humidifier, said conduit being heated by a respiratory
conduit heater element controlled by said humidifier, the
improvement comprising that said humidifier includes a fault
protection system for said heater element comprising:
[0020] detecting means which include means to detect a rapid change
of current in said heater element,
[0021] current interruptor means in series with said heater
element, and
[0022] timer means adapted to control the action of said
interruptor means and thereby in use determining the duration of
current interruption, said timer means being triggered by said
detecting means.
[0023] In a sixth aspect, the present invention consists in a
respiratory humidification system wherein a conduit connects a
patient to a humidifier, said conduit being heated by a respiratory
conduit heater element controlled by said humidifier, the
improvement comprising that said humidifier includes a fault
protection system for said heater element comprising:
[0024] detecting means which includes a peak current detector for
detecting current in the heater element
[0025] current interruptor means in series with said heater
element, and
[0026] timer means adapted to control the action of said
interrupter means and thereby in use determining the duration of
current interruption, said timer means being triggered by said
detecting means, when a predetermined threshold current is
exceeded.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Preferred forms of the invention will now be described with
reference to the accompanying drawings:
[0028] FIG. 1 is a schematic of a prior art heated breathing
conduit to be used with a respiratory humidifier,
[0029] FIG. 2 is a block diagram of a spark detector using a two
winding transformer,
[0030] FIG. 3 is a block diagram of a spark detector using a two
winding transformer with a centre tapped primary,
[0031] FIG. 4 is a block diagram of a spark detector using a single
coil and high pass filter,
[0032] FIG. 5 is a block diagram of a current level detector,
[0033] FIG. 6 is a circuit diagram of a prior art single MOSFET
switching circuit, and
[0034] FIG. 7 is a circuit diagram of a pair of back to back
MOSFETs used for switching the heater wire.
DETAILED DESCRIPTION
[0035] An example of a prior art heated breathing conduit 102 for
use with a respiratory humidifier/ventilator, is shown generally in
FIG. 1. The heated breathing conduit ordinarily comprises an
inspiratory conduit 101 connected at its proximal end 102 to the
gases outlet of a respiratory humidifier (not shown) and at its
distal end 103 to a "Y" shaped connector having three inlet/outlet
ports. One port 104 of the "Y" shaped connector directs the
inspiratory gases to the patient and also receives exhaled air from
the patient. The expired air is channelled by the "Y" shaped
connector to an expiratory conduit 105 via the third port 106 of
the "Y" shaped connector so that the expiratory gases may be
returned to the humidifier/ventilator (not shown) from the end 107
of the expiratory conduit 105.
[0036] The conduits 101, 105 are heated by a heater wire a located
within the inspiratory conduit 101 and a second heater wire 108A is
located within the expiratory conduit 105. In this example the two
heater wires are configured in parallel such that the second heater
wire 108A shares connection 109 with the first heater wire 108 and
is connected at point 123A to the second earth return conductor
121A which extends from point 123A to connection 110, although
other arrangements are equally possible.
[0037] Power is supplied via standard domestic or industrial supply
114. The heater wire 108 is supplied with power from the secondary
side of a step down transformer 118 which is connected to the
external voltage supply across the phase 115 and neutral 116
conductors. A controller 119 controls a switch 120 which, when
closed, energises the heater wire.
[0038] The controller can determine if there is no heater wire
connected, and provides an audible alarm if this is detected.
[0039] As with all electrical installations there exists fault
conditions which potentially can ignite a fire. Trials have
indicated that two fault conditions in particular appear to be
especially important in starting a fire in a heated respiratory
conduit. These are:
[0040] 1. A break in the heater wire, leading to repeated sparks
which cause ignition.
[0041] 2. Excessive current in the breathing circuit, leading to
melting or ignition of the breathing circuit materials. This can be
caused by incorrect breathing circuit design or assembly, or by a
short circuit.
[0042] The present invention may be retrofitted to existing
respiratory humidifier/ventilator systems or included as part of
the humidifier controller. It detects sparking and over current in
the heating wire 103 as detailed in the following embodiments.
[0043] According to the present invention spark detection is
accomplished by detecting the rapid change in current that occurs
following disconnection of the heater wire load. An inductor is
used because a rapid change in current induces a voltage spike
across the inductor, which can easily be detected. Since sparks (or
disconnections of the heater wire) can happen at any part of the
mains cycle (including those times when the mains voltage is near
zero) the spark detection circuitry cannot hope to pick up every
single spark. In practice though disconnections which occur near
the mains zero voltage do not have significant energy for ignition.
It is practical to detect 75% of all heater wire disconnections,
and this provides the required degree of safety.
[0044] In a first embodiment of the invention a two winding
transformer is connected in series with the heater wire. As can be
seen in FIG. 2 during normal operation the current flows from the
AC supply 1 through the primary 2 of the spark detection
transformer then through the heater wire 3. If a break 4 occurs in
the heater wire 3 (causing sparking), a rapid change in current
will occur in the heater wire 3. Any rapid change in current
through the primary winding 2 of the transformer creates a voltage
spike, due to the action of its inductance. The voltage spike is
passed onto the secondary winding 5 of the transformer and is
multiplied by the turns ratio.
[0045] The voltage spike on the secondary winding may be either
positive or negative, but the four diodes form a bridge rectifier
6, so that the voltage spike always charges the capacitor 7 with a
positive voltage. If the magnitude of the voltage spike is large
enough, then the capacitor will charge above the threshold voltage
8 of the comparator 9, which causes the output of the comparator 9
to go high, enabling the timer 10. This comparator drives a timer
10 which turns off the power to the heater wires for a period of
time (e.g. several seconds) using a switching circuit I 1. The
resistor 12 across the capacitor 7 allows the voltage to decay away
to zero if no sparks are detected.
[0046] In a second embodiment a two winding transformer with a
centre tapped primary winding is connected in series with the
heater wire. In order to reject mains voltage spikes as a source of
false triggering, the primary winding can be centre tapped 1 as
shown in FIG. 3, and the currents from the inspiratory 13 and
expiratory 14 heater wires are each passed through a different half
of the primary winding. In this way, the current of any mains borne
interference passes through both halves of the primary winding, and
the resultant magnetic fields (flowing through the core of the
transformer) cancel out. A spark 4 will only occur in one heater
wire limb at a time (shown here as the inspiratory 13), and
therefore is not cancelled out. The remainder of the circuit
operates in the same way as the first embodiment, with the
capacitor 7 voltage compared against a threshold 8, and the
comparator 9 driving a timer 10.
[0047] In a third embodiment shown in FIG. 4, as an alternative to
using a transformer a coil 15 is connected in series with the
heater wire 3. The coil 15 is used instead of the transformer
primary utilised in the first and second embodiments. A high pass
filter (resistors 17 and capacitors 16) is used to reject mains
eg:50-60 Hz frequencies. The remainder of the circuit operates in
the same way as the first embodiment Similarly to the first
embodiment this embodiment does not include specific rejection of
mains borne interference eg: spikes. However variations in this
embodiment can be envisaged which do incorporate rejection of mains
borne interference.
[0048] The technique used for detecting excess current is common to
all three embodiments of the invention described above. The
threshold for current detection is set to be the maximum current
that will be drawn by the lowest foreseeable resistance heater
wire, at the highest rated mains voltage +10%. The current detector
is designed to respond to the peak current instead of the average
current for two reasons: (a) peak current is faster responding than
average, (b) the peak current is independent of the duty cycle that
the controlling humidifier is supplying.
[0049] Referring to FIG. 5 the heater wire current to be measured
is passed through a low value resistor 24. The voltage which
appears across the resistor is proportional to the current flowing
in the heater wire 23. This voltage is passed through an amplifier
25, then passes to a peak detection circuit, where a capacitor 27
is charged up by a diode 26 to the maximum peak of the AC voltage.
If the peak voltage is higher than the threshold voltage 8 then the
comparator 29, operates a timer 30, which removes power from the
heater wire for a period of time. At the end of this time period
the current is restored, but if the current is still too high then
the peak is detected very quickly and power removed again. Using
this circuit, a completely short-circuited heater wire can be
tolerated without blowing the heater base fuse 60. The resistor 28
slowly discharges the capacitor
[0050] The control strategy for all faults involves disconnecting
the power from the heater wire for a period and then reapplying it.
This is to avoid shutting the system down in response to a non
critical event.
[0051] Common to all embodiments of spark detection is the control
strategy used for removing and reapplying power to the heater. The
timing circuit (10) must operate for long enough that any heat that
is generated by a spark has time to dissipate before the heater
wire power is reapplied. In practice a time period of 1 second has
been found to be sufficient. As a further aspect, the timing
circuit could also be made to count up the number of sparks
detected, and then disconnect the heater wire permanently. This is
to discriminate against one of the heater wires being disconnected
while the system is in use.
[0052] If overcurrent is detected, then the average power being
dissipated in the heater wire is determined by the time period that
the device takes to detect the high current (the "on" time) and the
period that the current is removed ("off"). With the circuit
described, the maximum time it will take to detect a high peak
current is one AC cycle (i.e. 20 msec at 50 Hz). So long as the
"off" time is many times longer than the "on" time then the heater
wire will not dissipate excessive power and will be safe. In the
preferred embodiment the heater wire is turned off for 2 secs, so
power is applied for less than 1% of the time.
[0053] The `off` time period should be more than, say, 10 mains
cycles to avoid the power dissipated getting too high. Also it
should not be too long otherwise the operator loses the useful
alarm feedback. For instance an operator removing the faulty heater
wire would expect the humidifier heater wire alarm condition to
cancel promptly. Too long a period may confuse the operator.
[0054] Of importance to both spark detection and current detection
is the ability to disconnect the heater wire quickly when one of
these conditions is detected. Conventionally a triac is used to
switch off current in an AC circuit such as this, but triacs cannot
be turned off instantly--turn off occurs at the AC zero crossing,
which may take up to 10 msec at 50 Hz mains. Triacs also have a 1-2
V saturation voltage, which results in power loss. This is to allow
the user to connect or disconnect the heater wires a limited number
of times without causing a permanent disconnection or audible
alarm. However, repeated sparks would cause this to occur.
[0055] Another prior art alternative (shown in FIG. 6) is to use an
N-channel MOSFET 31 with a separate substrate connection 32.
MOSFETs have a very fast switching time (less than 1 microsecond).
They can also have a very low "on" resistance (e.g 0.03 ohm) which
results in very low power dissipation. However a single MOSFET
configuration has two disadvantages. Firstly, it requires a MOSFET
with a separate connection to the substrate, instead of having the
substrate 32 connected to the source 35. Secondly, the substrate 32
must be connected to a bias voltage 36 which is more negative than
the peak negative AC voltage (from the source 39) that will be
switched. This is necessary because the construction of a MOSFET
involves two intrinsic diodes 37, 38 between the substrate 32 and
the drain 34 and source 35. If these diodes 37, 38 are not kept
reverse biased then they will conduct, and the switching action of
the MOSFET 31 will be lost. The negative voltage 36 applied to the
substrate 32 keeps these diodes 37, 38 reverse biased.
[0056] In the preferred embodiment of this invention (shown in FIG.
7) the heater wire 40 is switched by two back to back N-channel
MOSFETs 42, 43 which have their source connections 44, 45 connected
together. The MOSFETs have their substrates 48, 49 internally
connected to their source leads, as is common. As previously
described there are intrinsic diodes 50, 51 connected between the
source (substrate) and the drain of each MOSFET. These diodes are
connected back-to-back and do not conduct. The gates 52, 53 of both
MOSFETs are connected together, and a voltage 41 is applied between
the gate connections 52, 53 and the source connections 44, 45 to
turn the MOSFETs on and off.
[0057] To turn the AC current off the gate-source voltage 41 is set
to zero, and the MOSFETs stop conducting. To turn the current on,
the gate-source voltage is increased above the threshold voltage of
the MOSFET, and they conduct. In the preferred embodiment of the
invention the "on" resistance is chosen such that at the highest
rated current the drain-source voltage of the MOSFETs never exceeds
0.6 V, so that the intrinsic diodes are never allowed to
conduct.
[0058] This AC switching configuration overcomes the disadvantages
of the AC switch in FIG. 6, while still providing a rapid switching
time. By using two MOSFETs connected in reverse the intrinsic
diodes cannot conduct, without the use of an external negative bias
voltage. Also a separate substrate connection is not required on
the MOSFETs.
* * * * *