Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2001-09-04
2003-03-25
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S427000, C327S380000
Reexamination Certificate
active
06538480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a load driving device for driving a load such as a lamp, an LED, or an inductor, and particularly to a technique for preventing thermal destruction of a load driving device due to heat of a power switch element composing the load driving device when an overcurrent or a short-circuit current is generated.
2. Related Background Art
Conventionally, according to a technique for driving a load such as a lamp or a coil, as shown in
FIG. 4
, a power source
9
is connected to a high-potential side of a load
17
while a load driving device
1
′ is provided on a low-potential side of the load
17
, and the load
17
is driven by turning a power switch element
2
of the load driving device
1
′ ON/OFF according to a signal supplied to an input terminal
22
from the outside so that current from the power source
9
is passed/stopped. Such a load driving device
1
′ generally has various protective functions. To realize functions of load short-circuit protection and overcurrent protection among the protective functions, it is necessary to protect the load driving device
1
′ from thermal destruction by detecting a voltage applied to the low-potential side of the load
17
, that is, a potential of a drain terminal
21
of the load driving device
1
′, with current detecting resistors
13
and
14
, and turning the power switch element
2
OFF according to the potential detected.
The following description will depict specific load short-circuit protection and overcurrent protection, while referring to FIG.
5
.
FIG. 5
is a waveform diagram illustrating voltages and currents at respective parts of the load driving device
1
′ upon the occurrence of a load short-circuit or an overcurrent.
First of all, when a voltage V
IN
is applied to an input terminal
22
from the outside and a voltage V
T
obtained by dividing the voltage V
IN
with resistors
10
,
11
and a resistor
12
exceeds a threshold value of a gate voltage detecting element
16
, the gate voltage detecting element
16
is turned ON, thereby being in a state of monitoring a voltage V
D
at a drain terminal
21
.
In this state, when something abnormal occurs to the load
17
thereby causing a large current to flow, the voltage V
C
obtained by dividing the voltage V
D
with the current detecting resistors
13
and
14
rises according to the current I
L
, and a current detecting element
15
is turned ON when the voltage V
C
exceeds a threshold value (V
TH
) of the current detecting element
15
. Therefore, a gate voltage V
G
of the power switch element
2
drops to a ground level since the gate is grounded via a grounding terminal
23
, and the power switch element
2
is turned OFF.
When the lowering of the gate voltage V
G
of the power switching element
2
causes the voltage V
T
obtained by dividing the gate voltage V
G
with the resistors
11
and
12
to drop to a level lower than the threshold value of the gate voltage detecting element
16
, this causes the gate voltage detecting element
16
to be turned OFF, and also the current detecting element
15
to be turned OFF. As a result, again the gate voltage V
G
of the power switch element
2
rises, thereby again causing the power switch element to be turned ON.
The rise of the gate voltage V
G
of the power switch element
2
causes a voltage V
T
obtained by dividing the voltage V
G
with the resistors
11
and
12
to rise, and when the voltage V
T
exceeds the threshold value of the gate voltage detecting element
16
, the gate voltage detecting element
16
is turned ON.
In the case where the abnormality in the load
17
causes a large current to continue flowing, the device is in an oscillation state in which the foregoing sequence of actions is repeated. Thus, to prevent thermal destruction of the load driving device
1
′ from occurring when a large current flows through the load
17
, the functions of load short-circuit protection and
25
overcurrent protection are provided. It should be noted that the load short-circuit protection and the overcurrent protection has a difference therebetween only in a value of a current flowing through the load
17
, which results in a difference in the voltage detected by the current detecting resistors
13
and
14
, and the two functions are identical in the actions per se.
As shown in
FIG. 5
, while the voltage V
IN
continues to be applied to the input terminal
22
, the oscillation state of the load current I
L
continues due to the load short-circuit protection and the overcurrent protection. In such an oscillation state of the load current I
L
, the load driving device
1
′ generates heat, thereby causing a temperature to rise, but by providing an overheat protection circuit
18
, the gate voltage V
G
of the power switch element
2
is caused to drop when the temperature of the load driving device
1
′ reaches a heating set temperature, whereby the power switch element
2
is turned OFF. Therefore, thermal destruction of the device is prevented.
The heating set temperature of the overheat protection circuit
18
usually is set in the vicinity of 150° C., which is an operating upper limit temperature of the load driving device
1
′. It, however, possibly is close to 200° C. due to variation of circuit constants, and in the case where a load short-circuit or an overcurrent occurs repeatedly, the load driving device
1
′ possibly undergoes thermal destruction. Thus, in some cases this raises a problem in reliability of the device.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a load driving device capable of avoiding thermal destruction even when a load short-circuit or an overcurrent occurs, thereby having improved reliability.
To achieve the foregoing object, a load driving device according to the present invention includes a power switch element for driving a load and a circuit for controlling the power switch element according to a signal supplied from the outside, the power switch element and the circuit being provided on one chip, and the load driving device comprises a switching time changing means for changing a time to turn the power switch element ON/OFF, according to a result of detection of a current flowing through the load and an input signal level of the power switch element. Here, the power switch element is an N-channel MOSFET or an insulated-gate bipolar transistor, each of which is a voltage-driven type, or a normal bipolar transistor, which is a current-driven type.
In the load driving device, the switching time changing means preferably includes an OFF-time delaying circuit for delaying an OFF-time transition of the input signal level at which the power switch element makes the transition from an ON state to an OFF state.
Furthermore, the OFF-time delaying circuit preferably draws its power from the signal supplied from the outside.
Furthermore, the OFF-time delaying circuit preferably delays the OFF-time transition of the input signal level for a period according to a temperature rise of the load driving device.
In this case, the load driving device preferably further includes an overheat protection circuit that detects a temperature and turns the power switch element OFF according to the detected temperature, wherein the OFF-time delaying circuit delays the OFF-time transition of the input signal level according to the temperature detected by the overheat protection circuit.
Furthermore, the OFF-time delaying circuit draws its power from a connection that connects a resistor to which the signal is supplied from the outside and a plurality of diodes connected in series in a forward direction.
With the foregoing configuration, the OFF time of the power switch element is prolonged when a load short-circuit or an overcurrent occurs, and the OFF time is set shorter at normal temperature and is increased as the temperature rises. By so doing, it is possible to surely
Takada Kouji
Takahashi Satoru
Yamaguchi Seiki
Callahan Timothy P.
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Nguyen Hai L.
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