Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
1999-05-18
2001-03-06
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S423000, C327S424000
Reexamination Certificate
active
06198315
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a current detection circuit for detecting current flowing to a load and, more specifically, to the current detection circuit capable of eliminating a dead band in which current detection is not possible.
2. Description of Related Art
A common method of controlling the various motors used in automotive and other applications today is to monitor the current flowing to the motor for use in pulse width modulation (PWM) control. To accomplish this, the current flowing to the motor must be accurately detected for feedback to a microprocessor or other control circuit. Methods for detecting the current value include detecting a change in field strength and converting this change to a current value, and converting the potential difference at both ends of a low resistance shunt resistor to a current value. In the latter case current is monitored by converting the current flowing to the shunt resistor to a voltage, and an operational amplifier is used for this purpose.
FIG. 5
is a circuit diagram of a current detection circuit for detecting current flowing to a motor according to the related art. Note that the current detection circuit shown in
FIG. 5
is described below as applied to a motor control circuit for controlling a motor by means of PWM control.
Referring to
FIG. 5
, this motor control circuit
100
comprises a driver circuit
101
, current detection circuit
102
, and control circuit
103
. The driver circuit
101
comprises a plurality of power elements
101
a
to
101
d
in an H-bridge construction for driving a motor M. In
FIG. 5
these power elements
101
a
to
101
d
are MOSFETs by way of example only. The current detection circuit
102
detects the current flowing to the motor M. Based on the current detected by the current detection circuit
102
, the control circuit
103
, which is typically a microprocessor, duty controls the MOSFETs
101
a
to
101
d
of the driver circuit
101
to achieve a specific current flowing to the motor M.
A shunt resistor
111
in the current detection circuit
102
converts current flowing to the motor M to a voltage. An operational amplifier
112
, npn-type transistor
113
, and resistors
114
to
116
form an amplifying circuit which amplifies the potential difference between the ends of the shunt resistor
111
and converts the potential difference to a voltage Va referenced to the ground potential. This voltage Va is then output from buffer
117
to the control circuit
103
.
Based on this supplied voltage Va, the control circuit
103
duty controls the power elements of the driver circuit
101
so that the current flowing to the motor M is maintained at a set value. For example, if the control circuit
103
determines that the current flowing to the motor M is below this setting based on this voltage Va, it increases the duty cycle of the control signals to the MOSFETs
101
a
to
101
d
in a feedback control loop until the current flowing to the motor M is adjusted to the setting.
Let us assume here that MOSFETs
101
a
and
101
d
have been turned on to drive the motor M, and the load current IL flowing to the motor M thus passes from power supply terminal Vb through shunt resistor
111
and to MOSFET
101
a,
then from MOSFET
101
a
through the motor M to MOSFET
101
d
and ultimately to ground. If the resistance of resistor
114
,
115
, and
116
is R
114
, R
115
, and R
116
, respectively, at this time, the potential difference between the ends of the shunt resistor
111
is converted by the operational amplifier
112
to voltage Va with a gain of R
116
/R
114
referenced to the ground. The voltage Va can thus be expressed in this case by the following equation (A):
Va=IL×R
111
×
R
116
/
R
114
(A)
where R
111
is the resistance of shunt resistor
111
.
For example, if R
114
=R
115
=5 k&OHgr;, R
116
=100 k&OHgr;, IL=20 A, and R
111
=5 m&OHgr;, equation (A) above shows that Va=2 V. The voltage Va thus obtained is then output through buffer
117
to the control circuit
103
.
In the motor control circuit
100
thus comprised, however, there is a dead zone in which a low load current IL cannot be detected depending upon the value of the input offset voltage of the operational amplifier
112
. That is, when the resistance of the shunt resistor
111
is low and the potential difference between the ends is low, the offset voltage of the operational amplifier
112
has a relatively greater effect on operation, and the current cannot be detected even though current is flowing to the motor M. For example, if the input offset voltage Vos of the operational amplifier
112
, the potential difference of the non-inverting input to the inverting input, is 10 mV, the operational amplifier
112
will not operate unless the potential difference of the shunt resistor
111
is 10 mV or greater.
When the resistance R
111
of the shunt resistor
111
is 5 m&OHgr;, the potential difference of the shunt resistor
111
is 10 mV at a 2 A load current IL. The voltage Va is therefore 0 V when the load current IL is less than 2 A, the input voltage Vm to the control circuit
103
is therefore also 0 V, and the load current IL cannot be detected. The relationship between the load current IL and input voltage Vm of the control circuit
103
depends upon the input offset voltage Vos of the operational amplifier
112
as shown in
FIG. 6
, and when the input offset voltage Vos is in the range 0 V to 10 mV, there is a dead zone in which the load current IL cannot be detected.
Although the object and configuration differ from those of the present invention, Japanese Patent Laid-Open Publication No. 10-117112 teaches a circuit in which a non-inverting input side
31
of an operational amplifier
3
is connected to a first reference potential P
1
, and an inverting input side
32
of the operational amplifier
3
is connected to a second reference potential P
2
through the constant current source of a current copying circuit
8
. Likewise, Japanese Patent Laid-Open Publication No. 10-51246 teaches a circuit in which the base of a current sink transistor
18
in a low voltage operational amplifier
10
is biased by connection to a current sink
15
, and a dc loop through a sink control circuit
14
and source control circuit
22
produces a base drive current bias in transistors
18
and
24
.
Furthermore, although the object and configuration again differ from those of the present invention, Japanese Patent Laid-Open Publication No. 10-41759 teaches a circuit in which a bias current I
p
is supplied from a transistor
26
forming a current mirror circuit to the bases of transistors
12
and
13
in the power output stage
6
of an operational amplifier
1
, and when biased to an AB-class state a quiescent current is controlled by a transistor
18
in gain stage
5
.
Yet further, and while again the object and configuration differ from those of the present invention, Japanese Patent Laid-Open Publication No. 10-127091 teaches a circuit in which a mirror current reflecting the current of a drive transistor for driving a motor is produced in a detection transistor, and the amplitude of current flowing to the drive transistor is controlled according to the mirror current of the detection transistor. Likewise, Japanese Patent Laid-Open Publication No. 10-90312 teaches a current detection circuit
100
for supplying an output signal I
out
, which is indicative of a current signal I
d
flowing to an FET output device
101
, to an output
110
irrespective of change in the drain-source voltage V
DS
of the FET output device
101
.
SUMMARY OF THE INVENTION
The present invention is therefore directed to a means for resolving the aforementioned problems by providing a current detection circuit for detecting a load current with good precision, and eliminating the dead zone in which current detection is not possible due to the input offset voltage of an operational amplifier, by connecting a constant current cir
Lam Tuan T.
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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