Power source system for vehicular electronic device

Herein disclosed is a vehicular power source system which uses as its load a memory-arithmetic circuit (5) such as a microcomputer. An electric power is supplied to the load (5) through a diode (6) and a constant voltage circuit (4), when a switch (2) for running the vehicle is open, but a booster (3) is operated to supply the power when the switch (2) is turned on. The voltage drop of a battery (1), which may be caused during the running operation of the vehicle, is not transmitted to the load (5), but a constant voltage is supplied to the load (5) at all times so that the storage data of the memory unit (5) is always held.

DESCRIPTION 
1. Technical Field 
The present invention relates to a power source system for vehicles and, 
more particularly, to a power source system for a vehicular electronic 
device, which is equipped with memory-arithmetic units such as a 
microcomputer. 
2. Background Art 
Recent automobiles are being equipped with electronic devices, which are 
required to store and hold data, such as clocks or trip computers. In an 
electronic device having built therein a memory unit for storing data, if 
a voltage to be applied for operating an electronic device is blocked or 
dropped to a level lower than a predetermined value, the storage data held 
in the memory unit until that block or drop in voltage is lost so that the 
electronic device which relies for its operation on that data may possibly 
malfunction or be rendered inoperative. If a clock exemplifies that 
electronic device, it may indicate an improper time. If a trip meter is 
another example, it may improperly indicate a running distance, a runnable 
distance and/or an elapse time. 
In the system thus far described, a serious problem for causing the storage 
data of the memory unit to be lost is a drop of the terminal voltage of a 
battery at the time of start of the engine. The voltage to be applied to a 
microcomputer is generally required to be held at about 5.+-.0.5 [V]. For 
this requirement, the applied voltage is held constant by the action of a 
constant voltage circuit. If the battery terminal voltage ranges from 6 to 
20 [V], the required voltage is ensured in a normal manner so that the 
voltage to be applied to the microcomputer is held at about 5.+-.0.5 [V]. 
At the time of start of the engine, however, especially when the engine is 
started at the time when the ambient temperature is low, the battery 
terminal voltage is frequently dropped to a level lower than 5 [V] to 
invite a state in which the voltage to be applied to the microcomputer 
becomes too low so that the storage data cannot be held. In order to 
eliminate such problem, there has been adopted in the prior art a method 
by which the power source is backed up by means of a battery or by which a 
DC-DC converter is used at all times to boost the voltage. The former 
method has a defect in that it is required to retain a space for arranging 
the battery or to maintain the battery. The latter method has a defect 
that, since the DC-DC converter is operating even when the engine is 
stopped, the discharge of the battery is so increased that not only the 
engine cannot be started but also the storage data is lost if the 
automobile is left as it is for a long time. 
DISCLOSURE OF INVENTION 
One object of the present invention is to provide a power source system for 
a vehicular electronic device, which is free from the aforementioned 
defects while having its power consumption reduced. 
Another object of the present invention is to provide a power source system 
which is able to reliably hold the storage data of a memory unit to be 
built in a vehicular electronic device. 
Still another object of the present invention is to provide a power source 
system for a vehicular electronic device, which is small, light and 
inexpensive. 
The present invention is characterized in that the power source system of a 
vehicular electronic device is equipped with control means for controlling 
a booster circuit. By the action of this control means, the terminal 
voltage of the battery is applied to the electronic device, without being 
boosted by the booster circuit, in case the load upon the battery is 
limited to the electronic device including a memory unit, i.e., in case an 
ignition switch is left open. As a result, the power to be consumed in the 
booster circuit can be reduced to reduce the discharge of the battery.

BEST MODE FOR CARRYING OUT THE INVENTION 
The embodiments of the present invention will be described in detail in the 
following with reference to the accompanying drawings. 
FIG. 1 is a block diagram showing one embodiment of the power source system 
of a vehicular electronic device according to the present invention. In 
FIG. 1, the main voltage of a battery 1 is applied to an ignition circuit 
system (although not shown) by closing an ignition switch 2. When the 
ignition switch 2 is closed, moreover, the aforementioned voltage of the 
battery 1 is boosted by a DC-DC converter 3 and is then applied to a 
constant voltage circuit 4, the output voltage of which is fed to an 
electronic device 5 having a memory unit built therein. When the ignition 
switch 2 is open, on the other hand, the DC-DC converter 3 is left 
inoperative so that the voltage of the battery 1 is not boosted but 
applied to the input terminal of the constant voltage circuit 4 through a 
diode 6. The DC-DC converter 3 also has a function to hold the voltage 
constant so that its output voltage is substantially constant, e.g., about 
8 [V] when the ignition switch 2 is left closed. When the voltage of the 
battery 1 is at 12 [V], the potential at a point P.sub.2 is at about 11 
[V] if the diode 6 has a forward voltage drop of about 1 [V]. Since the 
output voltage of the DC-DC converter 3 is at about 8 [V], on the other 
hand, a voltage of about 11 [V] is then applied to the input terminal of 
the constant voltage circuit 4. The voltage of the battery 1 is boosted, 
when it is dropped to a level, e.g., 5 [V], to about 8 [V] by the action 
of the DC-DC converter 3, but the diode 6 is left non-conductive so that 
the voltage of about 8 [V] is applied to the input terminal of the 
constant voltage circuit 4. As a result, the voltage to be applied to the 
input terminal of the constant voltage circuit 4 is held at a level higher 
than about 8 [V] so that the output voltage of the constant voltage 
circuit 4, i.e., the voltage to be fed to the electronic device 5 having 
the memory unit built therein is held at a proper level. When the ignition 
switch is open, the load to be connected to the battery 1 is limited to 
the electronic device 5 having the memory device, and the electronic 
device of that kind has a small power consumption. As a result, no drop of 
the voltage of the battery 1 is caused if the load is high. At this time, 
therefore, the DC-DC converter 3 is not operated, but the voltage of the 
battery 1 is applied to the input terminal of the constant voltage circuit 
4 through the diode 6. These operations are realized by equipping the 
power source system with both a control terminal 7 for controlling the 
DC-DC converter 3 and the diode 6. 
With the construction thus far described, the supply voltage to the 
electronic device 5 having the memory unit is maintained at about 5.+-.0.5 
[V] even if the load is abruptly increased with the ignition switch 2 
being closed, e.g., even if the engine starter starts so that the voltage 
of the battery 1 is dropped to a level lower than about 5 [V]. When the 
ignition switch 2 is open, the power consumption can be reduced to about 
one fortieth as high as that of the prior art. 
FIG. 2 is a block diagram showing another embodiment of the power system of 
a vehicular electronic device according to the present invention. Parts 
designated at the same reference characters as those of FIG. 1 indicate 
the same units or devices. The electronic device 5 shown in FIG. 1 to have 
the memory unit is generaly divided roughly into a microcomputer 50 having 
storing and computing functions, a peripheral circuit 51 of the 
microcomputer 50, and a fluorescent tube driver 54 which is made operative 
to drive a fluorescent tube for displaying the storage data or the 
computed result. It is only the microcomputer 50 having the memory 
function that should be supplied with the voltage at all times. It is 
sufficient to supply the peripheral circuit 51 and the fluorescent tube 
driver 54 with the required voltage only during the running operation of 
the vehicle, i.e., while the ignition switch 2 is closed. FIG. 2 is a 
block diagram showing the construction for accomplishing that supply of 
power. As shown in FIG. 2, the microcomputer 50 is supplied through the 
diode 6 with the voltage of the battery 1 even when the ignition switch 2 
is left open, i.e., when the DC-DC converter 3 is not operated. On the 
other hand, the peripheral circuit 51 and the fluorescent tube driver 54 
are supplied with that voltage when the ignition switch 2 is closed. The 
output voltage of the DC-DC converter 3 is supplied through a constant 
voltage circuit 41 to the peripheral circuit 51. On the other hand, the 
drive of the fluorescent tube requires a voltage higher than the voltage 
of the battery, e.g., about 40 [V]. For this requirement, the output 
voltage of the DC-DC converter 3 is boosted by a DC-DC converter 31 having 
a large capacity and is supplied to the fluorescent tube driver 54. With 
this construction, the electronic circuit other than the microcomputer 50 
consumes no electric power when the ignition switch 2 is open, so that the 
discharge of the battery can be restricted to a remarkably low extent even 
if the vehicle is left undriven. 
FIG. 3 is a block diagram showing still another embodiment of the power 
source system of a vehicular electronic device according to the present 
invention. In the boosting DC-DC converter, there is used a transformer 
for the voltage conversion. Although the two DC-DC converters are used in 
the embodiment of FIG. 2, they can be replaced by a single one if they 
have their transformer cores shared inbetween. In FIG. 3, a DC-DC 
converter 800 is integrated from the DC-DC converters 3 and 4 shown in 
FIG. 2. In the DC-DC converter 800, there is built a trans-core 81, on 
which are wound a primary winding 82 and two secondary windings 83 and 84. 
With the primary winding 82, there is connected a DC-AC inverter 85 which 
has the control terminal 7. When the ignition switch 2 is closed, the 
DC-AC inverter 85 operates to invert the d.c. voltage of the battery 1 
into an a.c. voltage, which is applied to the primary winding 82. The a.c. 
voltage to be induced in the secondary winding 83 is rectified by a 
rectifier 86 into a d.c. voltage, which is supplied through the constant 
voltage circuit 4 to the microcomputer and its peripheral circuit 52. The 
secondary winding 84 has a larger number of turns than the secondary 
winding 83 so as to induce therein an a.c. voltage sufficient for driving 
the fluorescent tube. This a.c. voltage is rectified by the action of a 
rectifier 87 into a d.c. voltage, which is supplied to the fluorescent 
tube driver 54. When the ignition switch 2 is left open, the DC-AC 
inverter is inoperative. As a result, the DC-DC converter 800 generates no 
output, but the microcomputer and its peripheral circuit 52 is supplied 
with the voltage from the battery 1 through the diode 6. 
With the construction thus far described, the storage data of the 
microcomputer can be held with a slight power consumption even when the 
ignition switch 2 is open, and the DC-DC converter 800 can be made small, 
light and inexpensive. 
FIG. 4 is one circuit diagram of the DC-DC converter 800 constituting the 
embodiment of the present invention shown in FIG. 3. In FIG. 4, units or 
elements designated by the same reference characters as those of FIG. 3 
indicate the same unit or elements. The primary winding 82 of the 
transformer and a transistor Tr.sub.2 are connected in series with the 
battery 1, and a transistor Tr.sub.1 is connected between one terminal 91 
of the primary winding 82 and the base of the transistor Tr.sub.2. A 
resistor R.sub.1 is connected through the ignition switch 2 between the 
node P.sub.1 of the primary winding 82 and the battery 1 and the base of 
the transistor Tr.sub.1. On the other hand, the secondary winding 83 has 
its one terminal 93 grounded through a diode D.sub.1 and a capacitor 
C.sub.3 and its other terminal 95 grounded directly. The secondary winding 
84 has its one terminal grounded directly and its other terminal 97 
grounded through a diode D.sub.2 and a capacitor C.sub.4, and a series 
circuit of a resistor R.sub.2 and a capacitor C.sub.2 is connected between 
an intermediate tap 99 and the base of the transistor Tr.sub.1. Moreover, 
a constant voltage diode Dz is connected between the node P.sub.5 of the 
diode D.sub.2 and the capacitor C.sub.4 and the base of the transistor 
Tr.sub.1. The node P.sub.2 is connected to input terminal of the constant 
voltage circuit 4 whereas the node P.sub.5 is connected to the input 
terminal P.sub.4 of the fluorescent tube driver 54. Thus, the diode 
D.sub.1 and the capacitor C.sub.3 constitute the rectifier 86 whereas the 
diode D.sub.2 and the capacitor C.sub.4 constitute the rectifier 87. 
When the ignition switch 2 is closed, a current flows from the battery 1 
through the resistor R.sub.1 to the bases of the transistors Tr.sub.1 and 
Tr.sub.2 so that the transistors Tr.sub.1 and Tr.sub.2 turn on. The 
current further flows through the primary winding 82 so that a postive 
potential is induced in the secondary wiring termina1 93 whereas a 
negative potential is induced in the wiring terminals 99 and 97. Then, the 
current made to flow through the resistor R.sub.1 by the negative 
potential at the secondary wiring terminal 99 passes through the capacitor 
C.sub.2 and the resistor R.sub.2 to charge up the capacitor C.sub.2 so 
that the base current of the transistor Tr.sub.1 disappears to render the 
transistors Tr.sub.1 and Tr.sub.2 non-conductive. When these transistors 
Tr.sub.1 Tr.sub.2 become non-conductive, no current flows through the 
primary winding 82, but a voltage is induced in the secondary windings 83 
and 84 in a direction to cause the current of the primary winding to 
continuously flow (i.e., in the polarity opposite to that when the 
transistors Tr.sub.1 and Tr.sub.2 are rendered conductive) so that the 
charges which have been stored in the capacitor C.sub.2 are discharged. As 
a result, the discharge current flows from the secondary winding terminal 
99 to the base of the transistor Tr.sub.1, and its composed current with 
the current flowing from the battery 1 through the resistor R.sub.1 flows 
through the bases of the transistors Tr.sub.1 and Tr.sub.2. As a result, 
the transistors Tr.sub.1 and Tr.sub.2 restore their conductive states so 
that the current begins to flow through the primary winding 82 to 
establish a positive potential at the secondary winding terminal 93 and 
negative potentials at the secondary winding terminals 99 and 97. These 
operations are repeated to generate a.c. voltages at the secondary 
windings 83 and 84. These a.c. voltages are rectified by the actions of 
the rectifiers 86 and 87 so that a positive d.c. voltage is obtained at 
the terminal P.sub.2 whereas a negative d.c. voltage is obtained at the 
terminal P.sub.5. Since the secondary windings 83 and 84 have a larger 
number of turns than that of the primary winding 82, the d.c. voltages to 
be induced at the secondary windings 83 and 84 are higher than that at the 
primary winding 82. As a result, the d.c. voltage of the battery 1 is 
converted into a higher d.c. voltage. On the other hand, the potential at 
the node P.sub.5 connected with the input terminal P.sub.4 of the 
fluorescent tube driver is depressed to a certain level (e.g., -40 [v]) by 
the action of the constant voltage diode Dz. The potental at the node 
P.sub.2 to the input terminal of the constant voltage circuit 4 is also 
depressed to a certain level (e.g., 8 [V]) because it is in proportion to 
that at the node P.sub.5. As a result, if the voltage of the battery 1 has 
a normal value, e.g., about 12 [V] even if the ignition switch 2 being 
closed, the diode 6 is brought into its conducting state so that a voltage 
of about 11 [V] is applied to the input terminal of the constant voltage 
circuit 4. When the voltage of the battery 1 is dropped to about 6 [V], 
for example, as at the time of start of the engine, the diode 6 is not 
rendered conductive so that the output voltage of the DC-DC converter 800 
is applied to the input terminal of the constant voltage circuit 4. When 
the ignition switch 2 is open, the transistors Tr.sub.1 and Tr.sub.2 are 
held in their non-conducting states so that the DC-DC converter 800 is 
left inoperative so as to consume no electric power. 
With the circuit construction shown in FIG. 4, both the positive d.c. 
voltage to be applied to the microcomputer and its peripheral circuit 52 
and the negative d.c. voltage to be applied to the fluorescent tube driver 
54 can be simultaneously generated to make the power source system small, 
light and inexpensive. 
As has been described hereinbefore, according to the present invention, 
there is provided a power source system which can reliably hold the 
storage data of an electronic device having storing and computing 
functions without inviting any increase in the power consumption.