JP2001327096A - Charging control circuit of lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure a voltage required for control using a simple and inexpensive arrangement, even when the battery voltage is low. SOLUTION: When a PWM control AC/DC converter is employed for charging a lithium ion battery, the voltage of a battery 9 to be charged may be as low as 2 V. Since the secondary voltage is also close to the battery voltage in this case and charge control is disabled, a current control element (FET) 2 is connected in series between the output end Tv of a secondary rectifying/ smoothing section 1 and the battery 9, and its control (gate) terminal is controlled so as to ensure a voltage required for control.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge control circuit for a lithium ion battery which has been widely used as a power source for various portable devices.

[0002]

2. Description of the Related Art A charge control of a lithium ion battery is performed by monitoring a charge current and a voltage. Since the application of overvoltage deteriorates the battery, the voltage at the time of charging is 4.2V ± 30mV (± 0.7% in terms of the variation rate) of the battery at the time of full charge of 4.2V, and the voltage management with high accuracy. Is required. When one battery is used, the voltage is 4.1 V or 4.2 V.

FIG. 5 shows a general A as a power source for charging a battery.
3 shows a C / DC converter. This is to double-wave rectify the AC power supply 24, switch the smoothed voltage at a high frequency, and rectify and smooth the voltage induced on the secondary side via the transformer 25 for battery charging. The charging control circuit section 22 on the AC line side and the secondary side is insulated by the transformer 25 and the photocoupler 23. The secondary side voltage control is
This is performed by transmitting the signal to the controller 21 on the primary side through the photocoupler 23. The controller 21 on the primary side is a photocoupler 23
The secondary-side voltage is controlled by changing the pulse width of high-frequency switching (PWM control: changing the current supply period) according to the amount of light obtained from. That is, the primary side controls so that the output voltage is low when the light emission amount is large and the output voltage is high when the light emission amount is small according to a command from the secondary side. At the time of start-up, the control power supply on the secondary side is 0 V, and there is no feedback (no light emission) by the photocoupler 23, so the command is to increase the output voltage. As a result, the controller 22 on the secondary side can obtain the operating voltage and execute the control.

In the configuration shown in FIG. 5, it is most natural that the power of the controller 22 on the secondary side is obtained from the secondary side winding, but it is obtained from the secondary winding in accordance with the voltage of the battery to be charged. The applied voltage varies according to the battery voltage. That is, when the battery voltage is as low as about 2V, the voltage obtained from the secondary winding is also 2V.
+ Α, the voltage required by the controller (4-6V)
The challenge is how to secure these. That is, when charging the battery, the voltage on the secondary side is slightly higher than the battery voltage (= battery voltage + 0.3 V to 0.5 V: forward voltage drop of reverse blocking diode D + voltage drop at current detecting resistor R). ). In particular, when attempting to charge an excessively discharged battery, it is necessary to consider a battery voltage of at least about 2 V. When charging such a low-voltage battery, a voltage obtained by rectifying and smoothing the voltage of the secondary winding of the transformer (hereinafter also referred to as a charging voltage) is 2.3 V (= 2 V (battery voltage) +0.3 V). Only occurs to the extent.

[0005] Therefore, the charging voltage becomes 2.3 V, but the voltage is too low to use this voltage as the power supply voltage of the controller (a voltage loss for stabilizing the voltage is also required). Although it may be good for the purpose of pursuing low-voltage operation, a normal analog / digital mixed controller requires a stabilized voltage of 3 to 5V. For this reason, if a margin of 1 V is expected for stabilization, a voltage of at least 4 to 6 V is required. However, when the battery to be charged is a single battery and the battery voltage is about 2 V, the charging voltage is about 2.3 V as described above, so that the voltage (4 to 6 V) necessary for the controller is secured. Can not do.

As a countermeasure, there is the following method.
A configuration as shown in FIG. 6 is conceivable in which a control power source is not obtained from a battery charging power source but a stable voltage is secured from another route. This is because the secondary side voltage control is not performed on the primary side, but the secondary side voltage is rectified and smoothed by DC.
A switching regulator 32 having a step-down configuration is provided based on the voltage, and a DC voltage obtained by rectifying and smoothing the voltage on the secondary side or a separate winding is provided as a power supply for the controller, and the voltage is used. In this case, a controller on the primary side is not particularly required, and a power transformer configuration at a commercial power frequency can be considered. However, since the transformer is large and heavy, a high-frequency switching system is indispensable for this portion as well. As a result, a similar circuit 32 is prepared on the secondary side, causing problems such as an increase in cost and an increase in circuit size due to an increase in the number of components. 33 indicates a charge controller, and 34 indicates a lithium ion battery.

[0007]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a simple and inexpensive configuration capable of securing a voltage required for control even when a battery voltage is low.

[0008]

In order to solve such a problem, according to the first aspect of the present invention, a DC voltage on the secondary side of an AC / DC converter is used as a power supply for charging a lithium ion battery, and charge control is performed. A current control loop and a voltage control loop for detecting these values in order to obtain a charging current and a voltage according to a sequence and controlling a DC voltage so that each of them becomes a target value are provided. In a charge control circuit for a lithium ion battery which changes a pulse width by feeding back to a width modulation controller, a current control element for rectifying and smoothing a DC voltage obtained by rectifying and smoothing a secondary side voltage of the charging power source is provided by the DC Connect between the voltage and the battery, and connect the D terminal to the control terminal of the current control element.
The DC voltage is made constant by inputting a result of comparison between a voltage from a voltage detection voltage dividing circuit connected to the C voltage and a reference voltage. According to the first aspect of the present invention, a signal for forcibly turning on and off the current control element is given to its control terminal, so that charging current to the battery can be interrupted (the second aspect of the present invention). ).

[0009]

FIG. 1 is a block diagram showing an embodiment of the present invention, in which 1 is a rectifying / smoothing section, and 2 is a current control element (P).
Channel FET: PchFET), 3 is a voltage comparison amplifier, 4 is a voltage divider, 5 is a low voltage lock circuit (UV
LO, 6 is a reference power supply (VREF), 7 is a control power supply (VREG), 8A is a current control amplifier, 8B is a voltage control amplifier, 9 is a lithium ion battery, and 10 is a photocoupler.

First, a battery charge control method will be described with reference to FIG. 1A is a circuit diagram of a main part of FIG. 1, and FIG. 1B is an explanatory diagram of its operation. The detected values of the current and the voltage are compared with the command values by the current control amplifier 8A and the voltage control amplifier 8B shown in FIG. 3A, and the lower one of the output voltages is selected to select the light emitting diode of the photocoupler 10. The configuration is such that the current flowing through D is determined. In the battery voltage control, as shown in (b), when the battery voltage is low, constant current control is performed as shown by a dotted line, and the battery voltage is constant (for example, if the battery voltage is a 4.2 V type with one battery, When the charging current reaches 4.2 V), the charging is shifted to the constant voltage charging. When the charging current becomes equal to or less than a constant value (for example, 1/10 of the current at the time of the constant current charging), the charging is completed.

The mode switching from constant current charging to constant voltage charging is such that the amount of light emitted from the photocoupler 10 is controlled in accordance with the lower control voltage in each control. If the output voltage of the voltage control amplifier 8B is set to be lower than the output voltage of the current control amplifier 8A when the battery voltage at the time becomes 4.2 V, the output of each amplifier is used for control switching. Since the cathodes of the diodes D1 and D2 are connected to each other, and the anodes of the control switching diodes D1 and D2 are commonly connected to the cathode of the light emitting diode D, the voltage control to achieve an output voltage lower than the current control output is automatically performed. Can be selected and executed.

The charging voltage for the battery 9 shown in FIG. 1 is ultimately determined by the battery voltage. Even when the battery voltage is as low as about 2 V, a power supply voltage of 4 to 6 V for the controller is required.
In FIG. 1, the charging voltage which is the output of the rectifying / smoothing unit 1 is represented by Pch
The drain is connected to the source of the FET (2), and the output is connected to the charging circuit of the battery 9. The charging voltage is connected to the power supply of the controller and supplied to circuits 6 and 7 for generating a reference voltage (VREF) and an internal power supply voltage of the controller (3 to 5 V: VREG). Further, a resistor voltage dividing circuit 4 for voltage detection for securing a charging voltage is also connected.

The operation of FIG. 1 will be described. The controller's internal power supply voltage is 5 V for easy understanding, and the externally input voltage (charging voltage) is at least 6 V
(5V + 1V). Assuming that the battery 9 having a low voltage of about 2 V is connected and the AC power supply is turned on, the operation of the primary circuit of the AC is performed as a full voltage output command because the secondary photocoupler for feedback does not emit light. Be started. PchFE for control connected in series with battery
T (2) remains off in a state where the charging voltage is low and the power supply for the controller cannot be secured, so that the power supply terminal T
The voltage applied to v (= charging voltage) increases. When the charging voltage reaches a voltage at which the operation of the controller starts (a voltage slightly lower than 4 to 6), the lock of the low voltage lock circuit (UVLO) 5 inside the controller is released, and the outputs of the circuits 6 and 7 are enabled. Becomes

Thus, the voltage control of each section is started,
The power-supply-side terminal voltage of the charging current detection resistor R is about 2.3 V (2 V + 0.3 V) as described above. The added voltage comparison amplifier 3 compares the voltage obtained by dividing the charging voltage with the reference voltage, and outputs the PchFET
By controlling the gate terminal of (2), the charging voltage terminal Tv is controlled to be 6V. Here, PchF
The gate voltage of ET (2) is set to a potential (about 5V) such that the source voltage becomes 6V and the drain voltage becomes 2.3V. With this control, even when the battery voltage is lower than the power supply of the controller, the charging voltage terminal (power supply terminal to the controller) T
v can be secured at 6V. On the other hand, when the battery voltage is higher than the minimum operation voltage of the controller (charging voltage is 6 V or more), since the source voltage of the PchFET (2) exceeds 6 V, the gate is required to reduce the voltage by flowing more current. I will lower the voltage to near 0V, but PchFET
(2) is sufficiently turned on, and simply functions as a switch SW that is turned on.

FIG. 3 shows how the charging voltage and the battery voltage fluctuate. The charging voltage when the battery voltage is low is controlled by the PchFET so as to be the lower limit operation voltage of the controller set by the controller.
The hFET is turned on, and transitions at a voltage higher than the battery voltage by about 0.3 V (diode D, current detection resistor R).

The added PchFET can be turned off by an external command, and this function can be used when cutting off the current to the battery or measuring the battery voltage. FIG. 4 shows an example in that case. For example, the on / off control terminal Tc of the circuit is set to “high level”.
As a result, the voltage at which the charging voltage was divided is short-circuited by the NchFET (11) and becomes 0 V,
As a result of controlling the PchFET to be turned off (to bring the gate voltage closer to the source voltage), the PchFET is turned off. In this way, it also has a function of disconnecting the charging power supply. FIG. 3 shows a voltage when the PchFET is turned off.
When the SW 11 is turned off, the controller turns off the current and voltage control, so that the charging voltage operates at the maximum duty set on the primary side, and the voltage on the secondary side also increases to the maximum.

[0017]

According to the present invention, a current control element (FET) is connected in series to the rectification and smoothing output on the secondary side, and various controls can be performed by a simple and inexpensive configuration that only controls the current of the FET. The advantage that it is possible to secure the voltage for the above is obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory diagram for explaining a charging method in FIG. 1;

FIG. 3 is an explanatory diagram showing a relationship between a charging voltage and a battery voltage in FIG. 1;

FIG. 4 is a configuration diagram showing a second embodiment of the present invention.

FIG. 5 is a configuration diagram showing an AC / DC converter as a battery charging power supply.

FIG. 6 is a configuration diagram showing another example of a battery charging power supply.

[Explanation of symbols]

1: rectifying / smoothing section 2: current control element (P channel F
ET), 3 ... voltage comparison amplifier, 4 ... voltage dividing circuit, 5
... Low voltage lock circuit (UVLO), 6 ... Reference power supply (VR)
EF), 7: control power supply (VREG) 8A: current control amplifier, 8B: voltage control amplifier, 9: lithium ion battery, 10: photocoupler, 11: switch (N-channel FET).

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H02M 3/28 H02M 3/28 K

Claims (2)

[Claims] 1. A DC voltage on a secondary side of an AC / DC converter is used as a power supply for charging a lithium ion battery, and these values are detected to obtain a charging current and a voltage according to a charging control sequence. DC to reach the target value
A charge control circuit for a lithium-ion battery, comprising: a current control loop for controlling voltage; and a voltage control loop, the output of which is switched and fed back to a pulse width modulation controller on the AC side to change a pulse width. A current control element for making a DC voltage obtained by rectifying and smoothing the voltage on the secondary side constant between the DC voltage and the battery, and a control terminal of the current control element for detecting a voltage connected to the DC voltage. A charge control circuit for a lithium ion battery, wherein the DC voltage is made constant by inputting a comparison result obtained by comparing a voltage from a voltage dividing circuit with a reference voltage. 2. The lithium ion according to claim 1, wherein a signal for forcibly turning on and off the current control element is given to a control terminal thereof, so that charging current to the battery can be cut off. Battery charge control circuit.