Internal-combustion engines – High tension ignition system – Current or voltage sensing in coil primary
Reexamination Certificate
2000-06-30
2003-12-30
Mohanty, Bibhu (Department: 3747)
Internal-combustion engines
High tension ignition system
Current or voltage sensing in coil primary
C123S609000
Reexamination Certificate
active
06668811
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to circuitry for controlling automotive ignition systems, and more specifically to circuitry for detecting and terminating ignition coil current.
BACKGROUND OF THE INVENTION
Modem 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 either 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. The former technique, commonly referred to as “ramp and fire”, is often preferable over the latter technique, commonly known as “ramp and hold”, in that closed-loop stability is typically not an issue for concern in a ramp and fire system. Moreover, power dissipation in a coil current switching device is substantially reduced in a ramp and fire system since the switching device is only required to operate in a “saturated” mode with low voltage across its terminals. By contrast, a ramp and hold system requires linearly controlling the coil current such that the coil current becomes limited by the resistance of the ignition coils and the voltage across it. This requires increasing the voltage drop across the coil current switching device which then corresponds to a proportional increase in switching device power dissipation.
One known example of a “ramp and fire” ignition system
10
of the type just described is illustrated in
FIG. 1
, wherein system
10
includes an ignition control circuit
12
having an electronic spark timing (EST) buffer circuit
14
receiving an EST control signal from a control circuit
16
via signal path
18
. The EST buffer circuit
14
buffers the EST control signal and provides a buffered EST control signal ESTB to a gate drive circuit
20
. The gate drive circuit
20
is responsive to the ESTB signal to supply a gate drive signal GD to a gate
22
of an insulated gate bipolar (IGBT) transistor
24
or other coil switching device via signal path
26
. A collector
28
of IGBT
24
is connected to one end of a primary coil
30
forming part of an automotive ignition coil having an opposite end connected to battery voltage V
BATT
. An emitter
32
of IGBT
24
is connected to one end of a sense resistor R
S
having an opposite end connected to ground potential, and to a non-inverting input of a comparator
36
via signal path
38
. An inverting input of comparator
36
is connected to a reference voltage VREF, and an output of comparator
36
supplies a trip voltage V
TRIP
to gate drive circuit
20
.
In the operation of system
10
, gate drive circuit
20
is responsive to a rising edge of an ESTB signal to supply a full gate drive signal GD to the gate
26
of IGBT
24
. As IGBT
24
begins to conduct in response to the gate drive signal GD, a coil current I
C
begins to flow through primary coil
30
, through IGBT
24
and through R
S
to ground, thereby establishing a “sense voltage” V
S
across resistor R
S
. As the coil current I
C
increases due to the inductive nature of coil primary
30
, the sense voltage V
S
across R
S
likewise increases until it reaches the comparator reference voltage VREF. At this point, the comparator
36
switches state and the corresponding change in state of the trip voltage V
TRIP
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 I
C
through the primary coil
30
and coil current switching device
24
. This interruption in the flow of coil current I
C
through primary coil
30
causes primary coil
30
to induce a current in a secondary coil coupled thereto (not shown), wherein the secondary coil is responsive to this induced current to generate an arc across the electrodes of a spark plug connected thereto (not shown in FIG.
1
).
One drawback to a ramp and fire ignition system of the type illustrated in
FIG. 1
is that under low vehicle battery voltage (V
BATT
) conditions, the resistance of the primary ignition coil
30
may limit the ability to achieve maximum coil current I
C
. The resistance of primary coil
30
is typically a function of the physical construction of the coil
30
, and is also a function of temperature with the resistance of coil
30
increasing as temperature increases. Under certain high temperature and low battery voltage operating conditions, the coil current I
C
therefore may not be able to increase to the level at which the corresponding sense voltage V
S
reaches the comparator reference voltage VREF. In operation under such conditions, the coil current I
C
may thus increase only to its resistively limited level with V
S
<VREF, and remain at that level until some other control mechanism terminates the current ignition dwell event. For example, in some known ignition systems, such backup control is effectuated by a so-called “over-dwell” or “dwell timeout” timing circuit that commands the coil current switching device (e.g., IGBT
24
) to turn off after some predetermined time period. However, in some ignition systems, such a dwell time extension may not be an acceptable strategy for addressing low coil current conditions that result in V
S
<VREF.
What is therefore needed is an improved automotive ignition control strategy that addresses the foregoing drawbacks of known automotive ignition control systems.
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, an ignition control circuit comprises a comparator circuit defining a first input receiving a variable input signal, a second input and an output producing a trip signal, a first circuit producing a first current as a function of temperature, and a second circuit producing a second current, wherein the second current is a function of battery voltage below a predefined battery voltage threshold and otherwise zero, and wherein the first and second currents combine at the second input of the comparator circuit to define a reference level at which the trip signal changes state in response to the variable input signal.
In accordance with another aspect of the present invention, an ignition control circuit comprises a comparator circuit defining a first input receiving a variable input voltage, a second input and an output producing a trip signal, a first circuit supplying a reference voltage to the second input of the comparator, wherein the reference voltage is a function of temperature and of battery voltage and defines a reference level at which the trip signal changes state, and a second circuit responsive to a control signal to reduce the reference voltage to a predefined fraction thereof.
In accordance with a further aspect of the present invention, a method of producing a reference voltage for an ignition control circuit comprises the steps of establishing a first current as a function of temperature, establishing a second current, wherein the second current is a function of battery voltage below a battery voltage threshold and otherwise zero, combining the first and second currents and producing a reference voltage therefrom, and comparing a variable input voltage with the reference voltage and producing a trip signal based thereon.
One object of the present invention is to provide an improved automotive ignition control system by implementing an ignition control circuit defining a coil current trip level reference as a function of temperature and battery voltage.
Another object of the present invention is to provide such a circuit further defining the coil current trip level reference as a function of engine speed.
These and other objects of the present invention will become more apparent from the
Chmielewski Stefan V.
Delphi Technologies Inc.
Mohanty Bibhu
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