Switching device with break detecting function

Details Switching device with break detecting function Switching device with break detecting function

2000-02-11

2002-03-12

Tran, Toan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Gating

Utilizing three or more electrode solid-state device

C327S378000, C327S389000, C327S108000, C323S282000, C307S125000

Reexamination Certificate

active

06356138

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
The subject application claims benefit of the earlier filing dates of Japanese Patent Application Nos.Hei 11-74258 and 2000-32359 filed on Feb. 14, 1999 and Feb. 9, 2000 under the Paris Convention, the entire contents of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching device provided with a function for detecting a break in a load of lights.
2. Description of the Related Art
FIG. 1
shows a switching device according to a prior art used for a load such as a lamp, having a function for detecting a break in the load. The switching device has an element (an FET in this example) QF for controlling the supply of power from a power source such as a battery of an automobile to a load such as a lamp, The power source
101
provides an output voltage VB. The power source
101
is connected to an end of a shunt resistor RS. The other end of the shunt resistor RS is connected to a drain D of the element QF. A source S of the element QF is connected to the load
102
. A driver
901
detects a current flowing through the shunt resistor RS and drives the element QF accordingly. An A/D converter
902
and a microcomputer (CPU)
903
receive a curre.nt value detected by the driver
901
, and according to the current value, determine whether or not an overcurrent is flowing to the element QF and whether or not the load
102
involves a break. The element QF may have a temperature sensor to achieve an overheat breaking function.
A zener diode ZD
1
keeps a voltage of 12 V between the gate G and source S of the element QF, to bypass an overvoltage so that the overvoltage may not be applied to the gate of the element QF. The driver
901
includes differential amplifiers
911
and
913
serving as a current monitor circuit, a differential amplifier
912
serving as a current limiter, a charge pump
915
, and a driver
914
. The driver
914
receives an ON/OFF control signal from the microcomputer
903
and an overcurrent signal from the current limiter and drives the gate of the element QF through an internal resistor RG (not shown). A voltage drop occurring at the shunt resistor RS is detected by the microcomputer
903
through the differential amplifiers
911
and
913
and A/D converter
902
. If the voltage drop is above a normal level corresponding to a normal current value, the microcomputer
903
determines an overcurrent, and if it is below the normal level, the microcomputer
903
determines a break in the load
102
.
This prior art must have the shunt resistor RS connected in series to a power line, to detent a current in the power line. The shunt resistor RS causes a large heat loss if a current passing through the power line is large.
The shunt resistor RS, A/D converter
902
, microcomputer
903
, etc., that are imperative for the prior art require a large space and are expensive, thereby increasing the size and cost of the switching device.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor switching device that is easy to integrate, inexpensive, and capable of detecting a current in a power line without a shunt resistor connected to the power line, minimizing a heat loss, and detecting a break in a load such as a lamp.
In order to accomplish the objects, an aspect of the present invention provides a semiconductor switching device having a first semiconductor element, a second semiconductor element, and a comparator. The first semiconductor element has a first main electrode, a second main electrode, and a control electrode. This second main electrode is connected to lamps serving as a load. The second semiconductor element has a first main electrode connected to the first main electrode of the first semiconductor element, a control electrode connected to the control electrode of the first semiconductor element, and a second main electrode connected to a circuit that is composed of a resistor and a constant current source that are connected in parallel with each other. The comparator compares potentials of the second main electrodes of the first and second semiconductor elements with each other. If the potential of the second main electrode of the first semiconductor element is higher than the potential of the second main electrode of the second semiconductor element, it is determined that there is a break in the lamps. The first and second semiconductor elements may be FETS, SITs (static induction transistors), or BJTs (bipolar junction transistors). Alternatively, the first and second semiconductor elements may be MOS composite elements such as ESTs (emitter switched thyristors) and MCTs (MOS controlled thyristors), or insulated gate power elements such as IGBTs. These elements may be any one of n- and p-channel types. The first main electrode is one of the emitter and collector electrodes of a BJT or an IGBT, or one of the source and drain electrodes of an IGFET such as a MOSFET and MOSSIT. The second main electrode is the other of the emitter and collector electrodes of the BJT or IGBT, or the other of the source and drain electrodes of the IGFET. If the first main electrode is an emitter electrode, the second main electrode is a collector electrode. If the first main electrode is a source electrode. the second main electrode is a drain electrode. The control electrode is a base electrode of the BJT, or a gate electrode of the IGBT or IGFET.
According to the aspect, a resistance value of the resistor and a current value of the constant current source are set such that the potential difference between the second main electrodes of the first and second semiconductor elements will be zero if a current flowing through the first semiconductor element has an intermediate value between a normal current value and a break current value.
A break in the lamps is detected according to the value of a current passing through the lamps. The intermediate current value between a normal current value and a break current value is IDAS, which is used as a break reference value. If the first semiconductor element is a power MOSFET, a terminal voltage (drain-source voltage) of the power MOSFET is expressed as “RonA×IDA” where RonA is, the ON resistance of the power MOSFET and IDA is a drain current thereof. Similarly, a terminal voltage of the second semiconductor element is expressed as “RonB×IDB” where RonB is the ON resistance of the second semiconductor element and IDB is a drain current thereof. The drains of the first and second semiconductor elements are connected to each other, and the gates thereof are also connected to each other. When the break reference current IDAS is flowing to the first semiconductor element, the current value IDB of the second semiconductor element is so set as to zero the potential difference between the sources (the second main electrodes) of the first and second semiconductor elements, and the following is established:
R
on
B×IDB=R
on
A×IDA
  (1)
If the lamps have no break, the following is established:
R
on
A×IDA>R
on
B×IDB
  (2)
If the lamps have a break, the following is established:
R
on
A×IDA<R
on
B×IDB
  (3)
Namely, the potential difference between the sources (the second main electrodes) of the first and second semiconductor elements is indicative of a break in the lamps.
The drain current IDB is expressed as follows:
IDB
=(
R
on
A/R
on
B
)
IDAS
  (4)
The break reference current IDAS changes according to a power source voltage. As the power source voltage increases, the break reference current IDAS increases. However, as the source voltage increases, the temperature of lamp filaments increases to increase the resistance of the lamp filaments, and therefore, the break reference current IDAS is not simply proportional to the. power source voltage. To realize IDB that satisfies the expression (4), the circuit composed of the resistor and constant current source that are connected in p

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