Internal-combustion engines – Spark ignition timing control – Electronic control
Reexamination Certificate
2000-07-24
2002-03-26
Yuen, Henry C (Department: 3747)
Internal-combustion engines
Spark ignition timing control
Electronic control
C123S630000, C123S644000
Reexamination Certificate
active
06360720
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to circuitry for controlling automotive ignition systems, and more specifically to circuitry for compensating for undesirable high temperature operating effects associated with such systems.
BACKGROUND OF THE INVENTION
Modern inductive-type automotive ignition systems typically control the ignition coil such that coil current is allowed to increase to a level high enough to guarantee sufficient spark energy for properly igniting an air/fuel mixture. The inductive nature of an ignition coil dictates that the coil current will increase over time, wherein a control circuit is typically operable to terminate coil charging after a so-called “dwell time” and thereby initiate a spark event, or to dynamically maintain the coil current at a predefined current level for a predefined time period before initiating a spark event.
In either case, ignition control circuits typically include a protection feature operable to prevent damage to the ignition controller circuitry or to the ignition coil itself in the event of a fault that could cause the coil to remain in a conductive state for prolonged periods of time. Such a protection feature is commonly implemented by a simple timing function that shuts off the drive signal to the coil current switching device after a predetermined time period has elapsed since activation thereof.
This “over-dwell” protection time must be guaranteed to be longer than the longest expected dwell period required by the ignition system for proper charging of the ignition coil. If the over-dwell protection period is too short, there may be insufficient energy in the ignition coil to ignite the air-fuel mixture, or the engine spark timing may be compromised in a fashion that creates emission problems. On the other hand, if the over-dwell protection period is too long, the ignition coil and/or controlling electronics may over heat and consequently become damaged. In either case, the protection circuitry has failed at its primary purpose.
Due to the relatively long over-dwell protection times required for engines operating in very low RPM or “crank” modes; e.g., several tens of milliseconds, the over-dwell protection circuit may require a capacitor external to the integrated ignition control circuit. One known example of an ignition system
10
of the type just described is illustrated in
FIG. 1
, wherein system
10
includes an ignition control circuit
14
receiving an electronic spark timing (ES) signal from a control circuit
12
such as a microprocessor or microprocessor-based control circuit. The ignition control circuit
12
is responsive to the EST signal to supply a gate drive signal GD to a gate
16
of at least one insulated gate bipolar (IGBT) transistor
18
or other coil switching device. A collector
20
of IGBT
18
is connected to one end of a primary coil
32
forming part of an automotive ignition coil
30
having an opposite end connected to battery voltage V
BATT
. The primary
coil
30
is coupled to a secondary coil
34
having opposite terminals connected to opposing electrodes of an ignition plug
36
defining a spark gap therebetween. An emitter
22
of IGBT
18
is connected to one end of a sense resistor Rs having an opposite end connected to ground potential, and to circuit
14
. System
10
may include additional IGBT and ignition coil pairs, as is known in the art, and circuit
14
is also connected to an external capacitor C
EXT
referenced at ground potential.
In the operation of system
10
, the ignition control circuit
14
is responsive to a rising edge of an EST signal to supply a full gate drive signal GD to the gate
16
of IGBT
18
. As IGBT
16
begins to conduct in response to the gate drive signal GD, a coil current Ic begins to flow through primary coil
32
, through IGBT
18
and through Rs to ground, thereby establishing a “sense voltage” Vs across resistor Rs. As the coil current Ic increases due to the inductive nature of coil primary
32
, the sense voltage Vs across Rs likewise increases until it reaches an internal voltage VREF. At this point, the ignition control circuit
14
causes the gate drive circuit
20
to turn off or deactivate the gate drive voltage GD so as to inhibit the flow of coil current Ic through the primary coil
32
and coil current switching device
18
. This interruption in the flow of coil current Ic through primary coil
32
causes primary coil
32
to induce a current in the secondary coil
34
, wherein the secondary coil
34
is responsive to this induced current to generate an arc across the electrodes of the ignition plug
36
. The ignition control circuit
14
further includes over-dwell protection circuitry operable to selectively charge and discharge capacitor C
EXT
at a rate defined by the EST signal. If EST remains in an active state for an excessive, or over-dwell time period, the charge on C
EXT
reaches a level that causes the ignition control circuit to gradually deactivate IGBT
18
to thereby gradually decrease the coil current Ic so as not to generate a spark event. Further details relating to the structure and operation of one known ignition control circuit of the foregoing type are provided in U.S. Pat. No. 5,819,713 to Kesler, which is assigned to the assignee of the present invention, and the contents of which are incorporated herein by reference.
A common type of capacitor implemented as C
EXT
in the system
10
illustrated in
FIG. 1
; i.e., one that is available in “chip” form with desirable values and voltage ratings, uses a dielectric of the type known in the art as “X7R.” While such capacitors provide desirable parametric behavior at relatively low cost, however, they have the undesirable characteristic of a significant fall-off in capacitance at high temperatures. Referring to
FIG. 2
, for example, a waveform
40
of % capacitance variation over temperature is illustrated for a known and commonly used X7R capacitor having a room temperature (e.g., 25 degrees C) value of 0.22 microfarads. Waveform
40
exhibits a slightly rounded characteristic from approximately −40 degrees C to approximately 60 degrees C, and exhibits a steep roll-off in capacitance starting at approximately 125 degrees C. An over-dwell protection circuit of the type described hereinabove that utilizes a capacitor of the type illustrated in
FIG. 2
would accordingly exhibit an increasingly significant reduction in the over-dwell protection time as temperature increases beyond 125 degrees C.
What is therefore needed is a capacitor charging circuit operable to charge capacitor C
EXT
with a current that compensates for undesirable temperature characteristics of C
EXT
to thereby minimize timing errors at any given temperature.
SUMMARY OF THE INVENTION
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention a temperature dependent current generating circuit comprises a first circuit producing a first voltage that is substantially constant over a range of temperatures, a second circuit producing a second voltage as an increasing function of temperature over the range of temperatures, a third current producing a charging current, and a comparator circuit responsive to the first and second voltages to draw a compensation current away from the charging current when the second voltage increases with temperature above the first voltage, wherein the compensation current increases with increasing temperature over the range of temperatures.
In accordance with another aspect of the present invention, a temperature dependent current generating circuit comprises a first circuit producing a compensation current as a function of temperature, and a second circuit producing a charging current, wherein the charging current is a function only of a base charging current below a first temperature and otherwise a function of the base charging current and the compensation current.
One object of the present invention is to provide a temperature dependent current generating circ
Funke Jimmy L.
Gimie Mahmoud
Yuen Henry C
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