Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2002-05-16
2003-04-08
Nuton, My-Trang (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S109000, C323S274000
Reexamination Certificate
active
06545513
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electric load drive apparatus for supplying a trapezoidal wave current to an electric load.
An electric load, such as a lamp or a coil, has an impedance (i.e., a resistance value) varying in response to heat generation during its activation. In the beginning of activation, the electric load has a relatively small resistance value. A relatively large current will flow across the electric load. Accordingly, the electric load will cause significant noise. To solve this problem, U.S. Pat. No. 6,184,663 corresponding to the unexamined Japanese patent publication 2000-138570 discloses an electric load drive apparatus shown in FIG.
5
.
A conventional electric load drive circuit
1
shown in
FIG. 5
comprises a resistor
4
and a MOS transistor
5
serially interposed between a battery
2
and an electric load
3
(such as a lamp), a trapezoidal wave generating circuit
6
which generates a trapezoidal wave signal Sb in accordance with a drive command signal Sa, and a current control circuit
7
which compares the trapezoidal wave signal Sb with a voltage value detected by the resistor
4
to control a gate voltage of MOS transistor
5
.
Although not shown in the drawing, the trapezoidal wave generating circuit
6
consists of a capacitor, a charging constant-current circuit, and a discharging constant-current circuit. A trapezoidal wave signal Sb is produced between both terminals of the capacitor of trapezoidal wave generating circuit
6
. The upper edge voltage of trapezoidal wave signal Sb is controlled to be a constant value. The current control circuit
7
consists of a voltage conversion circuit
8
and an error amplification circuit
9
. The voltage conversion circuit
8
produces a trapezoidal wave signal Sd by inverting the trapezoidal wave signal Sb. The trapezoidal wave signal Sb has an electric potential defined with respect to a ground potential, while the trapezoidal wave signal Sd has an electric potential defined with respect to the reference battery voltage VB. The error amplification circuit
9
compares the inverted trapezoidal wave signal Sd with the voltage applied to the resistor
4
. The error amplification circuit
9
controls the gate potential of MOS transistor
5
so as to equalize the voltage applied to the resistor
4
with the inverted trapezoidal wave signal Sd.
According to this arrangement, a lamp current (i.e., the current flowing across the electric load
3
(=lamp)) linearly increases in accordance with voltage increase of trapezoidal wave signal Sb in the beginning of activation of this lamp. The lamp current linearly decreases in accordance with voltage decrease of trapezoidal wave signal Sb after deactivating the lamp. When the drive command signal Sa is a cyclic pulse signal, the brightness of lamp can be adjusted by changing the duty ratio of drive command signal Sa.
When the electric load
3
is a lamp installed in an automotive vehicle, it is not assured that the lamps exchanged by a user or a car repairer are always the same type. For example, there is the possibility that a user may install another type of lamp which has a different rated current (i.e., impedance).
According to the above-described conventional electric load drive circuit
1
, the upper edge of trapezoidal wave signal Sb is determined in the following manner. It is now assumed that the installed electric load
3
is a specific lamp having the largest rated current (e.g., rated current=6A). Under this condition, the upper edge of trapezoidal wave signal Sb is determined so that MOS transistor
5
operates in a saturation region until the trapezoidal wave signal Sb reaches the upper edge voltage and MOS transistor
5
operates in a linear region after the trapezoidal wave signal Sb has reached the upper edge voltage.
By determining the upper edge voltage in this manner, it becomes possible to suppress drain loss of MOS transistor
5
.
However, if an installed lamp has a rated current having a rated current smaller (i.e., an impedance larger) than that of the above-described specific lamp, brightness adjustment of a lamp may cause a trouble as explained hereinafter with reference to FIG.
6
.
FIG. 6
is a time chart showing the waveforms of various portions in the conventional electric load drive circuit in the case an electric load is a specific lamp having a rated current of 6A or another type of lamp having a rated current of 6A. The waveforms shown in
FIG. 5
are (a) drive command signal Sa, (b) trapezoidal wave signal Sb, (c) lamp current (rated current=6A), (d) lamp voltage (rated current=6A), (e) lamp current (rated current=3A), and (f) lamp voltage (rated current=3A).
When a lamp having a rated current of 6A is connected to the electric load drive circuit
1
, the lamp voltage (i.e., a voltage applied to the lamp) increases or decreases in accordance with ascent or descent of trapezoidal wave signal Sb. The trapezoidal wave signal Sb is derived from drive command signal Sa. The lamp voltage is equal to the battery voltage VB during a term the trapezoidal wave signal Sb is equal to the upper edge voltage. For example, a current supply threshold can be set to a mid point (3A) of the current amplitude with respect to a trapezoidal wave lamp current (as shown by an alternate long and two short dashes line in FIG.
6
(
e
)). The duty ratio of lamp current, being set based on the current supply threshold, can be always equalized with the duty ratio of drive command signal Sa. It becomes possible to adjust the brightness of lamp according to a given command.
On the other hand, when a lamp having a rated current of 3A is connected to the electric load drive circuit
1
, the lamp current increases in accordance with ascent of trapezoidal wave signal Sb. However, the lamp current becomes equal with the battery voltage VB at time tb and stops increasing before the trapezoidal wave signal Sb reaches the upper side voltage. Furthermore, the trapezoidal wave signal Sb starts decreasing at time td. Then, waiting for passage of a time lag, the lamp voltage starts decreasing at time te. As a result, significant deviation is caused between the waveform of the lamp current and the waveform of trapezoidal wave signal Sb at each of time durations tb-tc and td-te.
Hence, the above-described duty ratio of lamp current (i.e., effective current supply time) becomes larger than the duty ratio of drive command signal Sa (i.e., commanded current supply time).
In this manner, when a lamp having a rated current smaller than that of the above-described specific lamp is connected to the electric load drive circuit
1
, the duty ratio of lamp current (i.e., effective current supply time for determining the brightness of lamp) cannot agree with the light adjusting command given by drive command signal Sa. Furthermore, the duty ratio of lamp current cannot be lowered sufficiently. The brightness of lamp may not be reduced sufficiently.
SUMMARY OF THE INVENTION
In view of the foregoing problems of the prior art, the present invention has an object to provide an electric load drive apparatus which is capable of equalizing an actual current supply time with a commanded current supply time designated by a drive command signal irrespective of impedance of this electric load.
In order to accomplish the above and other related objects, the present invention provides an electric load drive apparatus comprising a switching element provided in an electric power supply path extending from a direct-current power source to an electric load. A detecting resistor, connected in series with the switching element, detects a voltage value representing a load current flowing across the electric load via the switching element. A saturation state detecting section is provided for outputting a current saturation signal during a term an upper limit current flows across the electric load. The upper limit current is a maximum current flowable via the switching element. A signal generating section is provided for producin
Fujimoto Hiroshi
Koyama Yoshiki
Tsuchida Masahiro
Ueda Nobumasa
Denso Corporation
Harness Dickey & Pierce PLC
Nguyen Hiep
Nuton My-Trang
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