JP2538480B2 - Charging circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging circuit, and more particularly to a charging circuit having a simplified structure and stabilized characteristics.

[0002]

2. Description of the Related Art Various portable devices such as cordless telephones have a built-in rechargeable secondary battery, and the secondary battery is charged by a higher voltage DC power supply. In Figure 2,
An example of a conventional charging circuit of this type is shown. A DC power source E is connected to the input terminals T1 and T2 of the secondary battery BAT via a charging circuit. The DC power source E has a resistor R11
Is connected to the light emitting side of the photocoupler 11, and a series circuit of a resistor R12 and the transistor 10 is connected in parallel to the resistor R11. Between the terminals T1 and T2 of the secondary battery BAT,
A series circuit of the thermistor TH2, the variable resistor VR, and the resistor R14 is connected in parallel. The thermistor TH2 is for compensating the temperature characteristic of the secondary battery BAT, and a voltage detector VD for monitoring the charging voltage of the secondary battery BAT is connected from the movable terminal of the variable resistor VR. Further, a series circuit of a photodetector 11 and a resistor R15 is connected in parallel to both ends of the DC power source E, and the connection point between them is a terminal T.
Connected to 3. The voltage of the DC power supply E is the secondary battery B
It is set to a voltage value higher than the end-of-charge voltage Vf of AT.

In order to charge the secondary battery BAT, the terminal T1
When T2 and T2 are connected to the DC power source E side via the charging circuit, current flows to the light emitting side of the photocoupler 11, the light emitting element on the light emitting side emits light, and current flows to the light receiving element on the light receiving side. , The potential of the terminal T3 decreases. The connection state of the secondary battery BAT is detected by this state change. The voltage detector VD has the same potential as the ground G on the output side O when the potential on the input side I is less than or equal to a predetermined threshold value, and has high impedance with respect to ground when the potential on the input side I is greater than or equal to the threshold value. That is, the voltage detector VD monitors the charging potential of the secondary battery BAT, and when the charging voltage is equal to or lower than a predetermined potential (for example, the charge end threshold Vdet) corresponding to the above threshold, the collector and the emitter of the switching transistor 10 are electrically connected. Then, the combined resistance between the DC power source E and the terminal T1 is set as the combined resistance of the resistors R11 and R12 to lower the combined resistance. As a result, the current supplied to the secondary battery BAT increases, and high-speed charging is performed. On the other hand, when the detected voltage is equal to or higher than the charge end threshold Vdet, the transistor 1 is used to prevent overcharge.
0 is set to the cutoff state, and charging is performed with a small current determined by the resistor R11. Here, the volume VR is for adjustment for correcting the variation of the voltage detector VD.

[0004]

As described above, the conventional charging circuit monitors the charging voltage of the secondary battery BAT by detecting the voltage of the variable resistor VR with the voltage detector VD, and charging is performed. The voltage is low, that is, in order to perform high-speed charging at the initial stage of charging, the transistor 10 is turned on (in the same state), and a parallel resistor of resistors R11 and R12 is inserted between the DC power source E and the secondary battery BAT. The charging current is made to flow. In addition, when the charging voltage rises to reach the end of charging, the transistor 1 is used to prevent heat generation during overcharging.
0 is turned off and only the resistor R11 is inserted between the DC power source E and the secondary battery BAT to reduce the charging current.

However, as shown in FIG. 2, in the conventional charging circuit, the charge state is confirmed by the photocoupler 11, but only the voltage drop of the light emitting diode of the photocoupler 11 (usually about 1.3 V). Since the voltage decreases, a power source that can generate a voltage higher than the charging voltage of the secondary battery BAT must be prepared as the DC power source E. In addition, in order to control the operation of the transistor 10, the volume VR is compensated in terms of compensating for variations in the threshold of the voltage detector VD.
Must be adjusted. Furthermore, the charging terminals T1 and T2 may be affected by external factors such as contact of a conductor such as a clip.
If a short circuit occurs between them, the charging voltage drops abnormally, the transistor 10 consumes an excessive amount of current in order to maintain the conductive state, and it is necessary to unnecessarily increase the DC power supply capacity.

Therefore, an object of the present invention is to provide a simple structure,
Another object of the present invention is to provide a charging circuit capable of stabilizing the characteristics.

[0007]

In order to solve the above-mentioned problems, a charging circuit according to the present invention is connected to an input terminal to which a constant voltage DC power source is connected and an output terminal to which a secondary battery is connected. A resistance means having a variable resistance value, a first A / D converter for converting the voltage of the input terminal into a digital signal, and a second A / D converter for converting the voltage of the output terminal into a digital signal.
A / D converter, and control means for determining whether or not the secondary battery is connected to the output terminal based on digital signals from the first and second A / D converters. It A charging circuit according to another aspect of the present invention is connected to an input terminal to which a constant-voltage DC power source is connected and an output terminal to which a secondary battery is connected, and a resistance means having a variable resistance value, and the input terminal. From a first A / D converter that converts the voltage of the terminal into a digital signal, a second A / D converter that converts the voltage of the output terminal into a digital signal, and the first and second A / D converters. Control means for switching at least one of the resistance value of the resistance means and determining whether or not the secondary battery is connected to the output terminal based on the digital signal, and a resistor and a thermistor connected in parallel to the input terminal. And a third A / D for converting the voltage at the connection point of the resistor and the thermistor into a digital signal
A converter, and is configured to detect the ambient temperature based on the digital signal from the third A / D converter and perform temperature compensation of the charging circuit system.

[0008]

According to the present invention, various states at the time of charging are determined based on the digital signals of respective parts obtained by the A / D converter, and optimal control is performed according to the determination result, thereby eliminating the need for a photocoupler or variable resistor. The circuit configuration is simplified and the characteristics are stabilized.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a charging circuit according to the present invention. In this embodiment, a parallel circuit of resistors R1 and R2 similar to the conventional circuit shown in FIG. 2 is inserted between the charging terminals T1 and T2 to which the secondary battery BAT is connected and the DC power source E, and is connected to the resistor R2. Is connected to the transistor 1 in series, and the resistor R is turned on / off by the control signal supplied through the resistor R3 connected to the base.
It is controlled and adjusted whether or not 2 is connected in parallel with the resistor R1. Further, a series circuit of the thermistor TH1 and a resistor R4 is connected in parallel with the DC power source E. The voltage of the DC power source E is designed to be higher than the end-of-charge voltage of the secondary battery BAT. Due to the potential difference between the voltage of the secondary battery BAT and the voltage of the DC power source E, a charging current I0 having a different current value flows through the resistor R1 or the parallel resistor of the resistors R1 and R2. The current flowing through the thermistor TH1 and the resistors R3 and R4 is designed to be sufficiently smaller than the charging current I0.

In this embodiment, the voltage of each part of the above circuit is set to A
The microprocessor 2 with a built-in / D converter monitors and performs various operation controls described below. The first A / D converter 21 digitally converts the voltage of the DC power supply E and supplies it to the microprocessor 24. The second A / D converter 22 digitally converts the charging voltage of the secondary battery BAT, and the third A / D converter 23 uses a thermistor TH1 whose resistance value changes linearly with temperature and a resistance value with temperature. Of the voltage of the DC power source E divided by the resistor R4 whose change of
4

In using the charging circuit of this embodiment,
It is necessary to register the initial value in the memory of the microprocessor 24 in advance in order to correct the variation of each circuit. Terminal T
With the secondary battery BAT opened from 1 and T2, each voltage is input by the first A / D converter 21 and the second A / D converter 22, and the digitally converted values are respectively D1a
And D2a in the memory of the microprocessor 24. Also, after setting the ambient temperature to a certain condition, the third
The voltage is input by the A / D converter 23, and the digitally converted value is stored in the memory of the microprocessor 24 as D3a. Whether or not the secondary battery BAT is connected to the terminals T1 and T2 can be determined by the calculation of the microprocessor 24 as follows. The first A / entered at the time of judgment
If the digital conversion values D1b and D2b of the D converter 21 and the second A / D converter 22 are used, the potential difference between the DC power source E and the secondary battery BAT is calculated using the initial values D1a and D2a.
That is, assuming that a value proportional to the charging current I0 is ΔD, ΔD = (D1a−D1b) − (D2a−D2b) is obtained. Whether or not the secondary battery BAT is connected is determined by comparison with a predetermined determination threshold ΔD0, and when ΔD> ΔD0, it is determined that the secondary battery BAT is connected. Here, if the load stability of the DC power source E is sufficiently high, D1a =
Although the first A / D converter 21 is no longer necessary because it becomes D1b, such a DC power source E is actually expensive, so in the present embodiment, the first A / D converter 21 is used. The load fluctuation of the DC power source E is also corrected.

Next, the determination of the charge end voltage and the charge current switching operation will be described. Until charging of the secondary battery BAT is completed, the transistor 1 is made conductive to increase the charging current, and after the completion of charging, the charging current is necessary to determine whether or not the secondary battery BAT is connected to prevent overcharging. It is necessary to reduce it to a certain degree. Second entered at the time of judgment
Assuming that the digital conversion value of the A / D converter 22 is D2b, the charging voltage VBAT of the secondary battery BAT is calculated by the following equation using the voltage VE of the DC power source E and the above-mentioned initial value D2a. VBAT = E · (D2b / D2a) Also, the charge end voltage VDET, which is the threshold for switching the charging current.
Can be expressed as a function of T because it varies with the ambient temperature T. VDET = f (T) where the ambient temperature T is the third A / D input at the time of determination.
The digital conversion value D3b of the converter 23 and the above-mentioned initial value D
It is calculated by the following equation using 3a and the coefficient k. Where the coefficient k
Is a coefficient obtained based on the temperature coefficient of the thermistor TH1. T = k (D3b-D3a) The charging voltage VBAT and the charging end voltage VDET obtained as described above are compared, and when the condition of VBAT> VDET is satisfied, the microprocessor 24 cuts off the transistor 1. . Further, due to the characteristics of the secondary battery BAT, the charging voltage decreases when the charging current decreases, so VBAT <VDET immediately after the charging current decreases, and oscillation occurs when the charging current increases again. The judgment condition is VBAT <VDET-α, and it is necessary to provide a hysteresis for α.

The operation of protecting the charging terminal from a short circuit is performed as follows. If the charging current is increased under the condition of VBAT <VDET-α, the power supply current when the terminals T1 and T2 are short-circuited becomes excessive, and the current capacity of the DC power supply E and the rated power of the resistors R1 and R2 must be increased. Occurs. Therefore, the short-circuit detection threshold Vs is determined in advance, and the transistor 1 is cut off under the condition of VBAT <Vs to reduce the short-circuit current. As described above, in the present embodiment, by entrusting the determination of each voltage to the software of the microprocessor, a function such as a short circuit prevention is added, and the determination threshold value when a secondary battery having a different charging characteristic is used is changed. However, it can be handled by changing the program, and the hardware can be simplified and shared.

In the above embodiments, the voltage detection accuracy is A
The resolution of the A / D converter is generally 8 bits, and even if the quantization error is 1 bit, the accuracy is up to a value obtained by dividing the reference voltage of the A / D converter into 128. When the reference voltage of the A / D converter is set to 5V which is equal to the voltage of the DC power source E, the voltage can be determined with an accuracy of about 40 mV. Since the variation of the voltage detector used in the conventional technique is about 200 mV, fine adjustment by the volume is necessary, but the problem is almost eliminated. Furthermore,
In recent years, microprocessors with a built-in A / D converter have become inexpensive, and the superiority of the present invention is demonstrated.

[0015]

As described above, the charging circuit according to the present invention determines various states during charging based on the digital signals of the respective parts obtained by the A / D converter, and performs the optimum control according to the determination result. Therefore, unlike the prior art, a photocoupler, a variable resistor, etc. are not required, the circuit configuration is simplified, and the characteristics can be stabilized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a charging circuit.

FIG. 2 is a circuit diagram showing an example of a conventional charging circuit.

[Explanation of symbols]

1,10 Transistor 2 Microprocessor with A / D converter 21-23 A / D converter 24 Microprocessor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Atsushi Wakamatsu 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (56) Reference JP-A-4-178121 (JP, A)

Claims (2)

(57) [Claims] 1. A resistance means connected to an input terminal to which a constant-voltage DC power source is connected and an output terminal to which a secondary battery is connected, the resistance value of which is variable, and the voltage at the input terminal is converted into a digital signal. First A to do
/ D converter, and a second A for converting the voltage of the output terminal into a digital signal
/ D converter, and control means for determining whether or not the secondary battery is connected to the output terminal based on digital signals from the first and second A / D converters. And charging circuit. 2. A resistance means, which is connected to an input terminal to which a constant-voltage DC power source is connected and an output terminal to which a secondary battery is connected, and whose resistance value is variable, and a voltage at the input terminal to a digital signal. First A to do
/ D converter, and a second A for converting the voltage of the output terminal into a digital signal
/ D converter, and based on the digital signal from the first and second A / D converter, at least one of the switching of the resistance value of the resistance means and the presence or absence of the connection of the secondary battery to the output terminal Control means for performing, a series circuit of a resistor and a thermistor connected in parallel to the input terminal, a third A / D converter for converting the voltage at the connection point of the resistor and the thermistor into a digital signal, A charging circuit, which detects an ambient temperature based on a digital signal from a third A / D converter and performs temperature compensation of the charging circuit system.