Circuitry for maintaining a substantially constant sense...

Details Circuitry for maintaining a substantially constant sense... Circuitry for maintaining a substantially constant sense...

1999-11-23

2001-02-27

Sterrett, Jeffrey (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S351000

Reexamination Certificate

active

06194884

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to circuitry for driving electrical loads, and more specifically to such circuitry operable to limit or regulate load current flowing through an inductive load.
BACKGROUND OF THE INVENTION
Heretofore, various circuits have been designed for controlling load current in electrical load driving systems, wherein such circuits have typically been constructed of discrete electrical components, so-called hybrid circuits and integrated circuits. Oftentimes, particularly in the internal combustion engine industry, such circuitry is used in inductive load driving applications such as ignition control systems, fuel control systems and the like.
An example of one known ignition control system includes a low-valued sense resistor disposed in series with a coil current switching device which is itself series-connected to a low side of a primary coil forming part of an automotive ignition coil, wherein the opposite side of the primary coil is connected to a supply voltage. The coil current switching device may be, for example, an insulated gate bipolar transistor (IGBT) having a collector connected to the low side of the coil primary, a gate, and an emitter coupled to ground through the sense resistor. The IGBT is responsive to a gate drive signal to conduct coil current therethrough as is known in the art. As the coil current increases, a voltage is developed across the sense resistor, wherein this voltage is provided to an input of a closed-loop current control circuit operable to modulate the gate drive signal so as to limit and maintain the coil current at a desired coil current limit level. The coil current limit level guarantees sufficient energy in the ignition coil to create a spark for igniting the air/fuel mixture while preventing damage to, or destruction of, the ignition coil or IGBT due to excessive coil current levels.
One drawback associated with ignition control systems of the foregoing type is that the sense resistor must be constructed in such a manner that it is capable of withstanding the high coil current levels and corresponding power levels associated with the typical operation of an automotive ignition coil. This constraint requires a physically large resistor regardless of whether it is provided as a discrete, printed or integrated resistor. Moreover, since the voltage drop across the sense resistor adds to the voltage developed at the low side of the coil primary, the minimum supply voltage at which the ignition control system can achieve the desired coil current limit level is thereby increased. This condition is undesirable since automotive ignition control systems are typically required to be functional at very low battery voltages. Thus, to minimize voltage drop across the sense resistor, it must have a very low resistance value. Low-valued precision resistors, however, are expensive in both discrete and integrated form. Additionally, the power dissipation requirements of the sense resistor typically cause device heating that may lead to changes in the resistor value, ultimately resulting in undesirable corresponding changes in the coil current limit level.
To overcome at least some of the foregoing drawbacks, ignition control systems have heretofore been developed that implement a so-called “sense IGBT”; i.e., an IGBT having a second emitter configured to conduct an output current that is proportional to the “primary” emitter.
One particular example of a known ignition control system
10
implementing a sense IGBT is illustrated in FIG.
1
. Referring to
FIG. 1
, ignition control system
10
includes ignition control circuitry
12
connected to a voltage source VBATT via signal path
14
. In the application shown in
FIG. 1
, VBATT is a conventional automotive battery typically producing an output potential of approximately 14 volts. In any case, a voltage line VIGN is connected between ignition control circuitry
12
and one end of an ignition coil primary
18
, wherein the ignition control circuitry
12
is typically operable to switchably provide the VBATT voltage on voltage line VIGN to thereby controllably provide a suitable voltage potential to the coil primary
18
. The opposite end of the coil primary
18
is connected to one input of a suitable coil switching device such as, for example, the collector
28
of an IGBT
20
. A gate
26
of IGBT
20
is connected to a gate drive output of ignition coil circuitry
12
via signal path
34
, and a primary emitter
22
is connected to ground potential. A second “sense” emitter
24
of IGBT
20
is connected to a first end of a sense resistor R
S
30
, the opposite end of which is connected to ground potential. The first end of resistor R
S
is further connected to an input of known gate control circuitry
32
, wherein an output of gate control circuitry
32
is connected to the base
26
of IGBT
20
.
With VIGN=VBATT, ignition control circuitry
12
is operable to impress a gate drive voltage GD at the base
26
of IGBT
20
. In response to the gate drive voltage GD, IGBT
20
is operable to turn on and conduct a coil current I
L
therethrough to ground potential via emitters
22
and
24
. The sense emitter
24
is typically sized relative to the primary emitter
22
so that only 1-2% of the total coil current I
L
flows through the sense emitter with the remaining coil current IL flowing through the primary emitter
22
. As the coil current I
L
increases through the inductive load of the coil primary
18
, a voltage is developed across the sense resistor R
S
, wherein this voltage is supplied to the input of gate control circuitry
32
. The gate control circuitry
32
forms a closed-loop current control circuit that is typically operable to compare the voltage drop across R
S
with a predefined reference voltage, and to control the gate drive voltage GD at a level sufficient to maintain the coil current I
L
at a desired current limit level when the voltage drop across R
S
reaches the predefined reference voltage.
Since only a small percentage of the total coil current I
L
flows through sense emitter
24
, the “sense” current flowing through R
S
is much less than with the single emitter IGBT-based ignition control system described hereinabove. Accordingly, the sense resistor R
S
in system
10
may be larger in value, smaller in physical size and have less power dissipation capability than the sense resistor previously described herein. Such resistors can be easily created in integrated circuit form, thereby permitting R
S
to be fabricated on the same semiconductor device as the gate control circuitry
32
.
An alternate use of an IGBT, such as IGBT
20
, with a sense emitter, such as sense emitter
24
, for limiting current through a load is described in U.S. Pat. No. 5,396,117 to Housen et al. The Housen et al. circuit is described as having two modes of operation. In a first mode, “on/off” circuitry is provided that turns the IGBT completely off if a sense current flowing through the sense emitter and sense resistor connected thereto exceeds a predetermined value, thereby providing over-current protection capability. In a second mode, short circuit detection circuitry is provided that steps the IGBT gate drive voltage down to a fixed voltage level, defined by a zener diode breakdown voltage, upon detection of a short circuited load condition. It is important to note, however, that the Housen et al. circuitry does not attempt to otherwise modulate the IGBT gate voltage in a manner that would allow for stable, dynamic current limiting/maintaining of an inductive load.
In any case, while the ignition control system
10
illustrated in
FIG. 1
overcomes some of the problems associated with the single-emitter IGBT ignition control system previously described hereinabove, system
10
has certain drawbacks associated therewith. For example, as with dynamic current limit control of any electrical load, and with inductive loads in particular, the control of sense current flowing through sense emitter
24
and resistor R
S
is subject to the pos

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