Power supply device having low internal power consumption and electronic appliances using said power supply device

A power supply device includes a secondary battery and a charger, and can further utilize an external D.C. power supply. A switch is interposed between a negative electrode of the secondary battery and the ground. This switch is turned OFF when the external D.C. power supply is connected to the power supply device, and is turned ON when the external D.C. power supply is removed from the power supply device. The charger controls the charge of the secondary battery from the external D.C. power supply by regulating the potential of the negative electrode of the battery. A contact remains OFF while the external D.C. power supply is connected to the power supply device. In consequence, a current does not directly flow from the external D.C. power supply to the secondary battery. Therefore, a back-flow prevention diode, which otherwise results in power consumption, need not be disposed between the secondary battery and the load.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a power supply device for efficiently supplying 
power to a load by reducing the internal power consumption of the device, 
and to an electronic appliance equipped with such a power supply device. 
2. Description of the Related Art 
An electronic appliance such as a notebook type personal computer contains 
a secondary battery such as a NiCd battery as a power supply device. The 
appliance can also utilize a D.C. voltage, obtained by rectifying a 
commercial A.C. power supply, as an external D.C. power supply. A charger 
is provided to the appliance so as to charge the secondary battery from 
the external D.C. power supply. When the external D.C. power supply is 
utilized, an A.C. adaptor for rectifying the commercial A.C. power supply 
and obtaining the D.C. voltage is connected to the power supply device. 
The secondary battery and the external D.C. power supply are connected in 
parallel across the load. Back-flow prevention diodes are interposed 
between the secondary battery and the load and between the external D.C. 
power supply and the load lest a current flows from the secondary battery 
to the external D.C. power supply or from the external D.C. power supply 
to the secondary battery. When the external D.C. power supply is inputted, 
the voltage of the external D.C. power supply is set to a level higher 
than that of the voltage of the secondary battery. Therefore, the supply 
of power to the load is made from the external D.C. power supply. When the 
external D.C. power supply is inputted and the voltage of the secondary 
battery drops below a predetermined value, the charger charges the 
secondary battery. When the A.C. adaptor is removed and the input of the 
external D.C. power supply is cut off, the supply of power to the load is 
made from the secondary battery. 
In the power supply device according to the prior art described above, the 
back-flow prevention diode is interposed between the load and the 
secondary battery. Therefore, a load current flows through this back-flow 
prevention diode when power is supplied from the battery to the load, and 
a power loss develops in the back-flow prevention diode. This power loss 
in the back-flow prevention diode cannot be neglected in consideration of 
the battery capacity. The voltage drop of the diode is as great as from 
0.55 to about 0.7 V. When six NiCd batteries are used as the secondary 
battery, the power loss reaches 7.6 to 10% of the battery capacity. When 
the battery consists of two NiCd batteries, the proportion of the power 
loss becomes further higher, and reaches 23 to 30% of the battery 
capacity. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a power 
supply device having a low power loss and an electronic appliance using 
the power supply device. 
To accomplish this object, the power supply device according to the present 
invention includes a secondary battery and a charger, and can further 
utilize an external D.C. power supply. A switch is disposed between the 
negative electrode of the secondary battery and the ground. This switch is 
turned OFF when the external D.C. power supply is connected to the power 
supply device and is turned ON when the latter is removed from the power 
supply device. The charger controls charge from the external D.C. power 
supply to the secondary battery by regulating the potential of the 
negative electrode of the battery. 
When an A.C. adaptor is connected to the power supply device and power is 
supplied from the external D.C. power supply to the load, this switch is 
turned OFF and the negative electrode of the secondary battery is cut off 
from the ground. Therefore, a charge current does not directly flow from 
the external D.C. power supply to the battery. When the A.C. adaptor is 
removed from the power supply device, the switch is turned ON and the 
negative electrode of the secondary battery is connected to the ground. 
Consequently, power is supplied from the secondary battery to the load. 
Since the secondary battery is directly connected to the load without 
passing through a back-flow prevention diode at this time, the power loss 
due to this back-flow prevention diode does not occur, and the efficiency 
of the power supply device can be improved. 
A MOSFET is often used for the switching device for the charger. In the 
present invention, the switching device of the charger can be disposed on 
the negative electrode side of the secondary battery. Therefore, a voltage 
having a positive polarity can be used as a control voltage of the gate of 
the MOSFET. This means that an n-type MOSFET can be used as the switching 
device. Because an n-type MOSFET has a lower ON resistance than a p-type 
MOSFET, power consumption in the switching device of the charger can be 
reduced. 
Further, when a DC-DC convertor is disposed on the output side of the power 
supply device, an n-type MOSFET can be used as the switching device of the 
DC-DC convertor. Therefore, power consumption in the DC-DC convertor can 
be reduced, as well.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a circuit diagram of the first embodiment of the present 
invention. In FIG. 1, reference numeral 10 denotes a portable personal 
computer, which contains a power supply device 20 and a load 11. Reference 
numeral 21 denotes an A.C. adaptor for rectifying a commercial A.C. 
voltage to a D.C. voltage. When the A.C. adaptor 21 is fitted to a plug 
socket and connected to a connector 22 of the power supply device 20, 
power is supplied from the external D.C. power supply. The detail of this 
connector 22 will be described elsewhere. Reference numeral 23 denotes a 
filter for removing ripple from the external D.C. power supply. In the 
filter 23, symbols C.sub.2 and C.sub.4 denote capacitors, and L.sub.2 and 
L.sub.4 denote choke coils, respectively. Reference numeral 25 denotes a 
secondary battery. Symbol D.sub.1 denotes a back-flow prevention diode for 
preventing a current from flowing from the secondary battery 25 to the 
filter 23 on the external D.C. power supply side. Reference numeral 24 
denotes a DC-DC convertor. This convertor 24 converts the voltage of the 
external D.C. power supply or the secondary battery 25 to a constant 
voltage and outputs it to the load 11. In the DC-DC convertor 24, symbol 
Tr2 denotes a control transistor, L2 is a choke coil, C3 is a capacitor 
and D7 is a flyback diode. Reference numeral 31 denotes a control unit, 
which keeps the output voltage of the DC-DC convertor 24 at a constant 
voltage by executing ON/OFF control of the switching transistor Tr2 on the 
basis of the output voltage of the DC-DC convertor 24. Since the content 
of this control unit 31 is well known in the art, its detailed explanation 
will be omitted. For instance, a commercially available IC can he used for 
this purpose. Reference numeral 26 denotes a charging unit, which charges 
the secondary battery 25 from the external D.C. power supply by 
controlling the voltage at the negative electrode of the secondary battery 
25. In the description which follows, the point of the negative electrode 
of the secondary battery 25 will be referred to as the "A" point. 
In the charging unit 26, symbol Tr1 denotes a control transistor, and 
reference numeral 30 denotes a charge control unit. Since the content of 
this charge control unit is well known in the art. For instance, a 
commercially available IC can be used. Reference numeral 26' denotes a 
negative voltage generation unit. In the negative voltage generation unit 
26', symbol L1 denotes a choke coil and D4 denotes a flyback diode. Symbol 
R0 denotes a resistor for detecting a charge current. Symbol Tr3 denotes a 
switching transistor. This transistor Tr3 is turned ON when the secondary 
battery 25 is charged upon instruction from the charge control unit 30, 
and is turned OFF when the secondary battery 25 is not charged. Symbol C1 
denotes a capacitor for smoothing the negative voltage generated by the 
negative voltage generation unit 26'. 
Reference numeral 28 denotes a contact. This contact 28 is disposed inside 
the connector 22 and is connected between the negative electrode of the 
secondary battery 25 and the ground. FIG. 2 shows in detail the connector 
22 and the contact 28. 
In FIG. 2, reference numeral 51 denotes a plug, which is connected to the 
A.C. adaptor 21. Reference numeral 61 denotes a jack, which is disposed in 
the power supply device 20. FIG. 2 shows the state where the plug 51 and 
the jack 61 are separated from each other. The plug 51 comprises a center 
electrode 53 buried in an insulating body 52, an insulating collar 54 
disposed around the center electrode 53 and a sleeve electrode 55 shaped 
into a cylindrical shape around the outer periphery of the insulating 
collar 54. The jack 61 comprises an insulating housing 62, a center 
electrode 63 disposed at a holding portion 64 formed at the center of the 
housing 62, a sleeve spring 65 disposed in the space around the holding 
portion 64, and an auxiliary spring 66. When the plug 51 and the jack 61 
are cut off from each other, the sleeve spring 65 and the auxiliary spring 
66 are in mutual contact. When the plug 51 is fitted into the jack 61, the 
center electrode 53 comes into contact with the center electrode 63, and 
the sleeve electrode 55 comes into contact with the sleeve spring 65. At 
the same time, the sleeve electrode 55 of the plug 51 moves outward the 
sleeve spring 65 of the jack 61, and the contact between the sleeve spring 
65 and the auxiliary spring 66 is cut. 
The center electrode 53 of the plug 51 is connected to the positive 
electrode of the A.C. adaptor 21 and the sleeve electrode 55 is connected 
to the negative electrode side. The center electrode 63 of the jack 61 is 
connected to the positive electrode side of the power supply device and 
the sleeve spring 65 is connected to the negative electrode of the power 
supply device. The auxiliary spring 66 is connected to the negative 
electrode of the secondary battery 25. The contact 28 shown in FIG. 1 
comprises the sleeve spring 65 and the auxiliary spring 66. 
The contact 28 need not necessarily be formed inside the connector 22, and 
can be constituted by a microswitch which operates and is turned OFF when 
the plug is fitted into the jack, for example. 
Turning back to FIG. 1, reference numeral 27 denotes a switch circuit, 
which is connected between the negative electrode of the secondary battery 
25 and the ground. In the switch circuit 27, symbol D6 denotes a switching 
diode. Symbol Tr4 denotes a switching transistor, which is turned ON when 
the voltage of the external D.C. power supply is detected inside the power 
supply device 20. 
Reference numeral 40 denotes a battery voltage monitor unit 40. This unit 
40 includes a differential amplifier 41. The differential amplifier 41 
detects the voltage of the secondary battery 25 and inputs it to the 
microcomputer 46. Reference numeral 42 denotes an external input voltage 
detection unit. This detection unit 42 includes a comparator 43 and 
another comparator 44. Each comparator 43, 44 compares the voltage 
detected from a voltage divider resistor R8, R9 with a reference voltage 
V1, V2. When the A.C. adaptor 21 is connected and the voltage of the 
external D.C. power supply is detected, the comparator 43 outputs an H 
signal, and when the voltage is not detected, it outputs an L signal. The 
output signal of the comparator 43 is inputted to the microcomputer 46. 
The comparator 44 outputs the L signal when the voltage of the external 
D.C. power supply is detected, and outputs the H signal when the voltage 
is not detected. The output signal of the comparator 44 is inputted to the 
gate of the transistor Tr4 of the switch circuit 27. 
The output signal of the differential amplifier 41 of the battery voltage 
monitor unit 40 and the output signal of the comparator 43 of the external 
input voltage detection unit 42 are inputted to the microcomputer 46. The 
voltage signal from the differential amplifier 41 is inputted to an A/D 
conversion input portion (not shown) of the microcomputer 46. The 
microcomputer 46 delivers a control signal, which starts the charging 
operation, to the charge control unit 30 when the A.C. adaptor 21 is 
connected, the voltage of the external D.C. power supply is detected, and 
moreover, the voltage of the secondary battery 25 drops. 
Next, the operation of the circuit shown in FIG. 1 will be explained. 
(1) When external D.C. power supply is used 
When the A.C. adaptor 21 is fitted into the plug socket of the commercial 
power supply and is connected to the connector 22, the contact 28 is 
turned OFF. Power is supplied from the external D.C. power supply to the 
power supply device through the connector 22. Since the comparator 44 of 
the external input voltage detection unit 42 outputs the L signal to the 
gate of the switching transistor Tr4 of the switch circuit 27, the 
switching transistor Tr4 is turned OFF. Consequently, the negative 
electrode of the secondary battery 25 is cut off from the ground, and only 
the voltage of the external D.C. power supply is inputted to the DC-DC 
convertor 24. This convertor 24 converts the voltage of the external D.C. 
power supply to a constant voltage and supplies it to the load. 
While the external input voltage detection unit 42 detects the voltage of 
the external D.C. power supply, the microcomputer 46 judges whether or not 
the voltage of the secondary battery 25 from the battery voltage monitor 
unit 40 is above the reference value. When the drop of the battery voltage 
is detected, the microcomputer 46 outputs the control signal instructing 
the operation to the charge control unit 30. 
The operation of the charge control unit 30 will be explained with 
reference to the time chart of FIG. 3. In FIG. 3, (a) indicates the 
operation of the control transistor Tr1, the H level indicates ON and the 
L level indicates OFF, (b) shows the voltage change at the A point (at the 
negative electrode of the secondary battery 25), (c) shows the operation 
of the switching transistor Tr3, and (d) shows the operation of the 
switching transistor Tr4. 
When the control signal is inputted from the microcomputer 46, the charge 
control unit 30 starts the ON/OFF control of the control transistor Tr1 
(see FIG. 3(a)), and at the same time, outputs the ON signal to the gate 
of the switching transistor Tr3. The switching transistor Tr3 is turned ON 
when the voltage at the A point becomes negative, and thereafter keeps the 
ON state (see FIG. 3(c)). Detecting the voltage of the external D.C. power 
supply, the comparator 44 of the external input voltage detection unit 42 
outputs the L signal and turns OFF the switching transistor Tr4 of the 
switch circuit 27. Accordingly, the switching transistor Tr4 is kept OFF 
during the charging period as shown in FIG. 3(d). 
When the control transistor Tr1 is ON, the current flows through the choke 
coil L1 and energy is accumulated. When the control transistor Tr1 is 
turned OFF, energy accumulated in the choke coil L1 passes the current 
through the circuit comprising the capacitor C1--switching transistor 
Tr3--resistor R0--diode D4--choke coil L1. As a result, the voltage at the 
A point becomes negative with respect to the ground (see FIG. 3(b)). This 
negative voltage controls the voltage of the negative electrode of the 
secondary battery 25, and the charging current flows through the secondary 
battery 25. Since the charging current flows also through the resistor R0 
at this time, the voltage corresponding to the charging current develops 
across both ends of the resistor R0 and is inputted to the charge control 
unit 30. The charge control unit 30 regulates the ON period of the control 
transistor Tr1 in accordance with the quantity of the charging current so 
that the secondary battery 25 can be charged by a suitable charging 
current. 
While the external D.C. power supply is supplied to the power supply device 
20, the contact 28 which connects the negative electrode of the secondary 
battery 25 to the ground and the switching transistor Tr4 are both OFF. 
Therefore, the current does not directly flow from the external D.C. power 
supply to the secondary battery 25. Accordingly, the back-flow prevention 
diode need not be interposed between the load and the secondary battery 25 
in the circuit shown in FIG. 1. 
(2) When secondary battery 25 is used as power supply 
When the A.C. adaptor 21 is removed from the connector 22, the contact 28 
is turned ON, and the negative electrode of the secondary battery 25 is 
connected to the ground. Consequently, only the voltage of the secondary 
battery 25 is inputted to the DC-DC convertor 24. The DC-DC convertor 24 
converts the output voltage of the secondary battery 25 to a predetermined 
constant voltage and outputs it to the load. At this time, since the 
back-flow prevention diode D1 exists, the current does not flow from the 
secondary battery 25 to the external input voltage detection unit 42, etc. 
on the external D.C. power supply side. 
Because the secondary battery 25 is directly connected to the input side of 
the DC-DC convertor 24 without passing through the back-flow prevention 
diode, power consumption does not develop in a back-flow prevention diode 
interposed between the secondary battery and the DC-DC convertor as has 
been observed in the power supply devices according to the prior art. 
Accordingly, the battery capacity can be used without waste. 
(3) When A.C. adaptor 21 is connected to connector 22 of power supply 
device 20 but is not connected to plug socket 
In this case, although power supply from the external D.C. power supply 
does not exist, the contact 28 of the connector 22 is turned OFF, and the 
negative electrode of the secondary battery 25 is cut off from the ground. 
If the state remains as such, power is not supplied from either the 
external D.C. power supply or the secondary battery 25 to the DC-DC 
convertor 24, and the load undergoes an interruption of power. The switch 
circuit 27 is disposed in order to prevent such service interruption. 
Since the voltage of the external D.C. power supply is not detected, the 
comparator 44 of the external input voltage detection unit 42 outputs the 
H signal and turns ON the switching transistor Tr4 of the switch circuit 
27. Consequently, a circuit comprising the ground--switching transistor 
Tr4--diode D6 secondary battery 25--DC-DC convertor 24 is formed. 
Therefore, the secondary battery 25 is connected to the input side of the 
DC-DC convertor 24. The DC-DC convertor 24 converts the voltage supplied 
from the secondary battery 25 to a constant voltage and supplies it to the 
load. At this time, power consumption occurs in the switching diode D6, 
but the supply of power to the load continues as such and an interruption 
does not occur. When the AC adaptor 21 is removed from the connector 22 
under this state, the contact 28 is turned ON, the output of the secondary 
battery 25 is directly inputted to the DC-DC convertor 24, and power 
consumption in the diode D6 does not exist any longer. 
When the voltage of the external D.C. power supply is not detected, the 
comparator 43 of the external input voltage detection unit 42 outputs the 
L signal. Therefore, the microcomputer 46 does not output the control 
signal to the charge control unit 30. 
FIG. 4 is a circuit diagram of the second embodiment of the present 
invention. The circuit shown in FIG. 4 eliminates one (44) of the 
comparators of the external input voltage detection unit 42 by modifying 
the charge portion 26 of the circuit shown in FIG. 2. In the following 
description of FIG. 4, like reference numerals will be used to identify 
like constituents having the same function as in FIG. 1, and an overlap of 
explanations will be omitted. 
In the power supply device 20-1, the external input voltage detection unit 
42-1 has only one comparator 43. The comparator 43 outputs the H signal 
when the AC adaptor 21 is connected and the voltage of the external D.C. 
power supply is detected, and outputs the L signal when this voltage is 
not outputted. The output signal of the comparator 43 is inputted to the 
microcomputer 46. When the adaptor 21 is connected, the voltage of the 
external D.C. power supply is detected and moreover, the voltage of the 
secondary battery 25 drops, the microcomputer 46 outputs the control 
signal instructing the operation to the charge control unit 30-1 of the 
charge portion 26-1. The control signal outputted by the charge control 
portion 30-1 is inputted to the gate of the switching transistor Tr3 
through the inverter 32 and the transistor Tr7 of the negative voltage 
generation unit 26'-1. At the same time, the control signal is inputted to 
the gate of the switching transistor Tr4 of the switch circuit 27. 
The operation of the circuit shown in FIG. 4 is substantially the same as 
that of the circuit shown in FIG. 1. Therefore, the following explanation 
will be directed to only the difference from the circuit operation shown 
in FIG. 1. 
(1) When an external D.C. power supply is used 
When the control signal indicating charge is inputted from the 
microcomputer 46, the charge control unit 30-1 turns ON the switching 
transistor Tr3 through the inverter 32 and the transistor Tr7. At the same 
time, the switching transistor Tr4 of the switch circuit 27 is turned OFF. 
(2) When secondary cell 25 is used as power supply and 
(3) When A.C. adaptor 21 is connected to connector 22 but not connected to 
plug socket 
The switching transistor Tr4 is turned ON/OFF by the signal from the charge 
control unit 30-1 but not by the signal from the comparator 44. 
FIG. 5 is a circuit diagram of the third embodiment of the present 
invention. In FIG. 5, the circuit constituent members such as the personal 
computer 10, the load 11, the A.C. adaptor 21, the connector 22, the DC-DC 
convertor 24, the secondary battery 25 and the back-flow prevention diode 
D1 are the same as those shown in FIG. 1, and the detailed explanation 
will be omitted. In the power supply device 20-2, only the switching diode 
D6 is interposed between the negative electrode and the ground, but the 
switching transistor is not connected. 
Reference numeral 26-2 denotes a charge unit. In the charge unit 26-2, 
reference numeral 30-2 denotes a charge control unit. Since the content of 
this charge control unit 30-2 is well known in the art, its explanation 
will be omitted. A commercially available IC can be used for the charge 
control unit 30-2, for example. Reference symbol Tr1-2 denotes a control 
transistor, and a p-type MOSFET is used. Symbols L11 and L12 denote 
primary and secondary windings of a voltage conversion transformer. Symbol 
D3 denotes a rectification diode, and C1 denotes a smoothing capacitor for 
a D.C. output. Symbol R0 is a current detection resistor, and symbols R1 
and R2 denote voltage divider resistors. These resistors divide the 
voltage on the current outflow side of the current detection resistor R0. 
Symbols R3 and R4 denote voltage divider resistors, and they divide the 
voltage on the current inflow side of the current detection resistor R0. 
Symbols R5 and R6 denote voltage divider resistors for detecting the 
output voltage. A diode D8 and a resistor R7 generate a reference voltage. 
Symbol D5 denotes a back-flow prevention diode, and reference numeral 29 
denotes a fuse. 
Next, the operation of the circuit shown in FIG. 5 will be explained. 
(1) When external D.C. power supply is used 
When the A.C. adaptor 21 is fitted into the plug socket of the commercial 
power supply and is connected to the connector 22, the contact 28 of the 
connector 22 is turned OFF. The external D.C. power supply is connected to 
the input side of the DC-DC convertor 24. The secondary battery 25 is 
connected to the input side of the DC-DC convertor 24 through the route 
comprising the ground--switching diode D6--secondary battery 25--DC-DC 
convertor 24. Because the voltage of the external D.C. power supply is set 
to a higher level than the voltage of the secondary battery 25, only the 
voltage of the external D.C. power supply is inputted to the input of the 
DC-DC convertor 24. The DC-DC convertor 24 converts the voltage of the 
external D.C. power supply to the constant voltage and supplies it to the 
load. A current cannot flow from the external D.C. power supply into the 
secondary battery 25 because it is inhibited by the switching transistor 
D6. 
The charge control unit 30-2 executes ON/OFF control of the charge 
transistor Tr1-2 when the voltage of the external D.C. power supply is 
supplied thereto. Accordingly,, the external D.C. power supply is 
converted to an A.C. voltage and is supplied to the primary winding L11 of 
the transformer. The A.C. voltage transmitted to the secondary winding L12 
of the transformer is converted to a D.C. voltage by the rectification 
diode D3, and charges the secondary battery 25. The charge control unit 
30-2 regulates the ON/OFF period of the transistor Tr1-2 on the basis of 
the charging current determined by the resistors R0, R1, R2, R3 and R4, 
the voltage of the secondary battery 25 due to the resistors R5 and R6, 
and the reference voltage due to the diode D4 and the resistor R7, so that 
the charging current attains the constant value. Since the operation of 
the charge control unit 30-2 is well known, its explanation will be hereby 
omitted. The charge portion 26-2 regulates the voltage of the negative 
electrode of the secondary battery 25 and executes its charging operation 
from the external D.C. power supply. 
In the circuit explained above, the charge portion 26-2 is electrically 
insulated by the transformer. Therefore, the output voltage of the charge 
portion 26-2 is separated from the ground voltage of the device. 
Accordingly, the charging operation can be controlled by controlling the 
voltage of the negative electrode of the secondary battery 25. By the way, 
the potential of the negative electrode of the secondary battery 25 does 
not drop below the ground potential during charging; hence, the current 
does not flow from the ground through the diode D6. In other words, the 
switching transistor need not be disposed in series with the diode D6 as 
in the circuit shown in FIG. 1. 
While the external D.C. power supply is supplied to the power supply device 
20-2, the contact 28 which connects the negative electrode of the 
secondary battery 25 and the ground is turned OFF, and the switching diode 
D6 has the reverse polarity. Therefore, the current does not directly flow 
from the external D.C. power supply to the secondary battery 25, and the 
back-flow prevention diode need not be interposed between the load and the 
secondary battery 25. 
(2) When secondary battery 25 is used as power supply 
When the A.C. adaptor 21 is removed from the connector 22, the contact 28 
is turned ON, and the negative electrode of the secondary battery 25 is 
connected to the ground. Accordingly, only the voltage of the secondary 
battery 25 is inputted to the DC-DC convertor. This DC-DC convertor 24 
converts the output voltage of the secondary battery 25 to the constant 
voltage and outputs it to the load. At this time, since the back-flow 
prevention diodes D1 and D5 exist, the current does not flow from the 
secondary battery 25 to the external input voltage detection unit 42, etc. 
on the external D.C. power supply side. 
The secondary battery 25 is directly connected to the input side of the 
DC-DC convertor 24 without passing the back-flow prevention diode. 
Therefore, power consumption in a back-flow prevention diode interposed 
between the secondary battery and the load does not occur, unlike the 
prior art devices. In other words, the battery capacity can be fully used. 
(3) When the A.C. adaptor 21 is connected to connector 22 but is not 
connected to plug socket 
The contact 28 of the connector 22 is OFF. However, since the circuit 
comprising the ground--switching--diode D6--secondary battery 25--DC-DC 
convertor 24 is formed, the secondary battery 25 is connected to the input 
side of the DC-DC convertor 24. The DC-DC convertor 24 converts the 
voltage supplied from the secondary battery 25 to the constant voltage and 
outputs it to the load. At this time, power consumption occurs in the 
diode D6, but because the supply of power to the load is continued, 
interruption of power does not occur. 
FIG. 6 is a circuit diagram of the fourth embodiment of the present 
invention. In FIG. 6, the personal computer 10, the load 11, the A.C. 
adaptor 21, the connector 22, the DC-DC convertor 24, the secondary 
battery 25, the back-flow prevention diodes D1, D5 and the switching diode 
D6 are the same as those shown in FIG. 5. Therefore, the detailed 
explanation of these components will be omitted. 
In the power supply device 20-3, reference numeral 26-3 denotes a charge 
portion. In this charge portion 20-3, reference numeral 30-3 denotes a 
charge control unit. Symbol Tr1-3 denotes an n-type MOSFET control 
transistor. This transistor is controlled by the charge control unit 30-3 
and conducts the ON/OFF operation. Reference numeral 214 denotes a charge 
voltage generation unit. In this charge voltage generation portion 214, 
reference symbol L1 denotes a choke coil. Symbol C1 denotes a smoothing 
capacitor and symbol D3 denotes a flyback diode. Reference numeral 215 
denotes a current detection unit. In the current detection unit 215, 
symbols R0, R1, R2, R3 and R4 denote current detection resistors 
constituted in the same way as in the circuit shown in FIG. 5. Reference 
numeral 216 denotes a voltage detection unit. In the voltage detection 
unit 216, resistors R5 and R6 are voltage divider resistors for detecting 
the output voltage in the same way as in the circuit shown in FIG. 5. A 
resistor R7 and a diode D8 together generate a reference voltage in the 
same way as in the circuit shown in FIG. 5. 
Next, the operation of the circuit shown in FIG. 6 will be explained. 
(1) When external D.C. power supply is used 
When the A.C. adaptor 21 is fitted into the plug socket of the commercial 
power supply and is connected to the connector 22, the contact 28 of the 
connector 22 is turned OFF. The external D.C. power supply and the 
secondary battery 25 are connected to the input side of the DC-DC 
convertor 24. Because the voltage of the external D.C. power supply is set 
to a higher level than the voltage of the secondary battery 25, only the 
voltage of the external D.C. power supply is inputted to the input of the 
DC-DC convertor 24. The DC-DC convertor 24 converts the voltage of the 
external D.C. power supply to the constant voltage and supplies it to the 
load. A current cannot flow from the external D.C. power supply to the 
secondary battery 25 because it is checked by the switching transistor D6. 
The charge control unit 30-3 executes the ON/OFF control of the control 
transistor Tr1-3 when the voltage of the external D.C. power supply is 
supplied. When the control transistor Tr1-3 is ON, the current from the 
external D.C. power supply flows through the route comprising the diode 
D1--secondary battery 25--diode D5--current detection resistor R0--choke 
coil L1--control transistor Tr1-3. When the control transistor Tr1-3 is 
OFF, energy accumulated in the choke coil L1 flows through the route 
comprising the flyback diode D3--diode D1--secondary battery 25--diode 
D5--current detection resistor R0--choke coil L1, and charges the 
secondary battery 25. The charge control unit 30-3 regulates the ON/OFF 
period of the control transistor Tr1-3 on the basis of the voltage and the 
current detected by the current detection portion 215 and the voltage 
detection portion 216, and executes control so that the charging current 
attains a constant value. Since the operation of the charge control unit 
30-3 is well known in the art, an explanation will be omitted. 
Incidentally, since the voltage of the negative electrode of the secondary 
battery 25 does not become lower than the ground voltage during the 
charging operation, the current does not flow from the ground through the 
diode D6. Accordingly, the switching transistor need not be disposed in 
series with the diode D6 as was necessary in the circuit shown in FIG. 1. 
While the external D.C. power supply is supplied to the power supply device 
20-3, the contact 28 which connects the negative electrode of the 
secondary battery 25 to the ground is turned OFF, and the switching diode 
D6 has the opposite polarity. Therefore, the current does not directly 
flow from the external D.C. power supply to the secondary battery 25. This 
means that a back-flow prevention diode need not be interposed between the 
load and the secondary battery 25. 
(2) When secondary battery 25 is used as the power supply 
When the A.C. adaptor 21 is removed from the connector 22, the contact 28 
is turned ON and the negative electrode of the secondary battery 25 is 
connected to the ground. Accordingly, only the voltage of the secondary 
battery 25 is inputted to the DC-DC convertor 24. This DC-DC convertor 24 
converts the output voltage of the secondary battery 25 to a predetermined 
voltage and outputs it to the load. At this time, since the back-flow 
prevention diodes D1 and D5 exist, the current does not flow from the 
secondary battery 25 to the current detection portion 215, etc. on the 
external D.C. power supply side. 
Since the secondary battery 25 is directly connected to the input side of 
the DC-DC convertor 24 without passing through a back-flow prevention 
diode, power consumption does not occur in a back-flow prevention diode 
disposed between the secondary battery and the load, as has been observed 
in the power supply devices according to the prior art. For this reason, 
the battery capacity can be fully used without waste. 
(3) When A.C. adaptor 21 is connected to connector 22 but is not connected 
to plug socket 
Though the contact 28 of the connector 22 is OFF, the secondary battery 25 
is connected to the input side of the DC-DC convertor 24 because the 
circuit comprising the ground--switching diode D6--secondary battery 
25--DC-DC convertor 24 is formed. The DC-DC convertor 24 converts the 
voltage supplied from the secondary battery 25 to the predetermined 
voltage and outputs it to the load. At this time, power consumption occurs 
in the diode D6, but an interruption of power supply does not occur 
because the power supply to the load is continued. 
In the circuit of FIG. 6 explained above, the control transistor Tr1-3 of 
the charge portion 26-3 is connected to the negative electrode side. 
Therefore, a signal having a positive polarity can be applied to the gate. 
For this reason, an n-type MOSFET can be used as the transistor. Since the 
n-type MOSFET has a smaller ON resistance than a p-type MOSFET, power 
consumption in the charge portion 26-3 can be reduced. 
FIG. 7 is a circuit diagram of the seventh embodiment of the present 
invention. The circuit of FIG. 6 described above reduces the power 
consumption of the power supply device by using the n-type MOSFET having a 
low ON resistance for the control transistor. On the other hand, the 
circuit shown in FIG. 7 uses the n-type MOSFET for the control transistor 
of the DC-DC convertor so as to reduce power consumption in the DC-DC 
convertor, as well. 
In FIG. 7, reference numeral 200 denotes a power supply device, and 
reference numeral 201 denotes a battery. Besides the secondary battery, a 
primary battery such as a dry battery can be used for the battery 201. 
Reference numeral 202 denotes a DC-DC convertor, and reference numeral 11 
denotes a load. In the DC-DC convertor 202, symbol Tr6 denotes a control 
transistor, symbol L21 denotes a choke coil, symbol C21 denotes a 
capacitor, and symbol D21 denotes a flyback diode. Symbols R21 and R22 
denote voltage divider resistors for detecting a voltage. A constant 
voltage diode D22 and a resistor R23 generate a reference voltage. 
Reference numeral 203 denotes a control unit, which detects the output 
voltage and keeps the output voltage constant by executing the ON/OFF 
control of the control transistor Tr6. Since the content of this control 
unit 203 is well known, its detailed explanation will be omitted. For 
example, a commercially available IC can be used for the control unit 203. 
Next, the operation of the circuit shown in FIG. 7 will be explained. 
Receiving the supply of the output voltage of the battery, the control unit 
203 starts its operation. The output voltage obtained by the voltage 
divider resistors R21, R22 and the reference voltage obtained from the 
constant voltage diode D22 are compared with each other, and the control 
transistor Tr6 is turned ON when the output voltage is lower, and is 
turned OFF when the output voltage is higher. When the control transistor 
Tr6 is turned ON, power is supplied from the battery 201 to the load 11 
and at the same time, energy is stored in the choke coil 11. When the 
control transistor Tr6 is turned OFF, energy stored in the choke coil L21 
causes a current to flow through the circuit comprising the choke coil 
L21--flyback diode D21--capacitor C21, and the current is supplied to the 
load 11. The control unit 203 controls the output voltage of the voltage 
conversion portion 202 to a constant value by regulating the ON/OFF period 
of the control transistor Tr6. 
The circuit shown in FIG. 7 can reduce the internal resistance of the power 
supply device by using the n-type MOSFET as the switching device, and can 
reduce power consumption. 
FIG. 8 is a circuit diagram of the sixth embodiment of the present 
invention. In FIG. 8, the personal computer 10, the load 11, the A.C. 
adaptor 21, the connector 22, the DC-DC convertor 24, the secondary 
battery 25, the back-flow prevention diodes D1, D5 and the switching diode 
D6 are the same as those shown in FIG. 5, and their detailed explanation 
will be omitted. 
In the power supply device 20-4, reference numeral 26-4 denotes a charge 
portion. This charge portion is the same as the charge portion shown in 
FIG. 5 with the exception that the connecting position of the control 
transistor is different. The control transistor Tr1-4 shown in FIG. 8 is 
interposed between the primary winding L11 of the transformer and the 
ground. Since the rest of the circuit construction are the same as those 
of FIG. 5, an explanation will be omitted. 
Since the operation of the circuit shown in FIG. 8 is the same as the 
operation of the circuit shown in FIG. 5, it is covered by the explanation 
of the circuit operation of FIG. 5. The circuit shown in FIG. 8 can use 
the n-type MOSFET having a low ON resistance for the control transistor. 
Therefore, this circuit can reduce power consumption in the switching 
device and can improve efficiency of the power supply device 20.