Cell voltage balancer

This application incorporates by reference Taiwanese application Serial No. 89211927, Filed Jul. 11, 2000.

1. Field of the Invention

The invention relates in general to a cell voltage balancer, and more particularly to a cell voltage balancer which is capable of balancing the voltage of numerous cells which are connected in series.

2. Description of the Related Art

Unbalance charging usually occurs in cells connected in series. Cells connected in series have equal current flowing through but may not be equally charged due to various cell voltages and cell capacities of the cells. Consequently, some cells are overcharged while some are not fully charged.

The lifetime of overcharged cells could be shortened as a result of raised temperature. Irreversible chemical reaction may also occur, which reduces the performance of the cells and causes permanent damage of the cells.

Therefore, the application of a cell voltage balancer is important. FIG. 1, is a schematic diagram showing charging cells connected in series with the aid of a conventional cell voltage balancer. Four cells in charging are taken as an example. The conventional cell voltage balancer 100 includes resistances R 1 , R 2 , R 3 , R 4 , switches S 1 ,S 2 ,S 3 , and S 4 . Cells B 1 ,B 2 ,B 3 and B 4 are charged by charger 101 . Cell B 1 is connected in parallel with resistance R 1 and switch S 1 connected serially; cell B 2 is connected in parallel with resistance R 2 and switch S 2 connected serially; cell B 3 is connected in parallel with resistance R 3 and switch S 3 connected serially; and cell B 4 is connected in parallel with resistance R 4 and switch S 4 connected serially. The voltage V 1 , V 2 , V 3 , and V 4 of the respective cells B 1 , B 2 , B 3 and B 4 are inputted into the control signal generator 102 for controlling the switches S 1 , S 2 , S 3 and S 4 .

While in charging mode, The charger 101 can, for example, provide a constant current I to charge the cells. An average voltage V of the cells B 1 , B 2 , B 3 and B 4 is obtained after the voltages V 1 , V 2 , V 3 and V 4 are received by the control signal generator 102 . The voltages V 1 , V 2 V 3 and V 4 of each cell are then compared with the average voltage V. If the voltage of a cell is higher than the average voltage V, the corresponding switch is set on.

For example, while the voltage V 1 of the cell B 1 is higher than the average voltage V, and the voltages of the cells B 2 , B 3 and B 4 are all lower than the average voltage V, the control signal generator 102 sets the switch S 1 on and the switches S 2 , S 3 and S 4 off. Meanwhile, the current I will be separated in two directions. The first direction is still to charge cell B 1 . The second direction is through R 1 and S 1 . The charging current of B 1 is less then the others, so the voltage rise of B 1 will less then others.

When the charger 101 stops charging the cells B 1 , B 2 , B 3 and B 4 and these cells are not balanced in voltage, the control signal generator 102 can still receive voltages, V 1 , V 2 , V 3 and V 4 and set the corresponding switch on or off in order to balance the voltage of the cells.

However, the conventional method and mechanism stated above has the following disadvantages:

1. the resistance R 1 consumes a part of the electrical energy and therefore the efficiency of the charger is reduced and the energy dissipated in resistance R 1 is wasted; and

2. the heat produced by the resistance R 1 raises the surrounding temperature, which reduces the performance of the cells.

To reduce the disadvantage of energy consumption of the cell voltage balancer as shown in FIG. 1, the discharging resistance is replaced by a transformer. FIG. 2A is a circuit block diagram showing another conventional cell voltage balancer applicable to charging serially connected cells. And FIG. 2B illustrates the operation of the circuit as shown in FIG. 2 A. The transformer 202 includes a primary winding 204 and a secondary winding 206 and the transformer 208 includes a primary winding 210 and a secondary winding 212 . The primary windings 204 , 206 are serially connected with the switches S 1 , S 2 , respectively. The secondary windings 206 , 212 are serially connected with the diodes D 1 , D 2 , respectively. The transformers 202 , 208 , switches S 1 , S 2 , and diodes D 1 , D 2 together form a conventional cell voltage balancer 214 .

Cell B 1 and the serially connected primary winding 204 and switch S 1 are connected in parallel. Cell B 2 and the serially connected primary winding 210 and switch S 2 are connected in parallel. The secondary winding 206 and the diode D 1 connect to the secondary winding 212 and the diode D 2 in parallel. The turns of the secondary windings 206 , 212 are at least two times of that of the primary windings 204 , 210 . The following example is taken for illustration, assuming that the of the secondary windings 206 , 212 is two times of the primary windings 204 , 210 and the voltage of cell B 1 is larger than that of cell B 2 .

As shown in FIG. 2B, when the voltage of cell B 1 is larger than the average voltage of cell B 1 and cell B 2 , the switch S 1 is turned on and cell B 1 discharges. Meanwhile, current I 1 flows through the primary winding 204 . Because the voltage across the primary winding 204 is equal to the voltage of cell B 1 , the voltage across the secondary winding 206 is two times of that of the primary winding 204 ; that is two times of the voltage of the cell B 1 . Consequently, the voltage of the node N 1 is larger than the sum of the voltage of cell B 1 and the voltage of cell B 2 . So, the induced current 12 of the secondary winding 206 flows through diode D 1 and cell B 1 and flows toward cell B 2 to charge cell B 2 . Consequently, the voltage of cell B 1 is decreased to effectively avoid overcharge of cell B 1 . In addition, the energy released by cell B 1 transfers to cell B 2 .

Similarly, when the charger 200 stops charging cell B 1 and cell B 2 , the voltage unbalance of cells B 1 and B 2 may occurs and the cell voltage balancer 214 could still function to balance the voltage of the cells.

FIG. 3 shows the conventional cell voltage balancer of FIG. 2A which charges n serially connected cells. In order to recover energy to the n serially connected cells, the voltage of the secondary winding must be n times of that of the primary winding. Thus, the turns of the secondary windings 304 1 , 304 2 . . . , 304 n of the transformers 302 1 , 302 2 . . . , 302 n should be more than n times of that of the primary windings 306 1 , 306 2 . . . 306 n . Although the conventional cell voltage balancer is capable of recovering the energy of high voltage cells to others, the design of the cell voltage balancer therefor becomes much more complex and the size of the transformer is thus enlarged due to the various turns of the transformer while various number of cells are used.

It is therefore an object of the invention to provide a cell voltage balancer which effective recovers energy with the aid of a transformer. The transformer of the invention is provided in the form of a module so that the turns of the transformer will not be a function of the number of series connected cells. The turns of each transform is fixed so that the circuit design is simplified and the size of the transformer is reduced.

It is another object of the invention to provide a cell voltage balancer for balancing the voltage of the first cell and the voltage of the second cell. The cell voltage balancer includes the first input terminal, the second input terminal and the third input terminal. The cell voltage balancer includes a transformer, the first switch, the second switch, the first diode and the second diode. The transformer includes a primary winding and a secondary winding. The first switch and the primary winding are serially connected between the first input terminal and the second input terminal. The second switch and the secondary winding are serially connected between the second input terminal and the third input terminal. The first switch and the second switch are switched on and off alternatively. The first diode and the first switch are connected in parallel and the second diode and the second switch are connected in parallel. While the first switch is on and the second switch is off, the primary winding stores energy of the first cell in the transformer and while the first switch is off and the second switch is on, the secondary winding recovers energy stored in the transformer into the second cell.

It is therefore a further object of the invention to provide a cell voltage balancing system for balancing the voltage of a number of cells. The cell voltage balancing system includes a number of cell voltage balancers. Each of the cell voltage balancers includes the first input terminal, the second input terminal and the third input terminal. Each of the cell voltage balancers is connected to two adjacent cells, the first cell and the second cell, of the cells. The second input terminal is connected to the common node of the first cell and the second cell. Each of the cell voltage balancer includes a transformer, the first switch, the second switch, the first diode and the second diode. The transformer includes a primary winding and a secondary winding. The first switch and the primary winding are serially connected between the first input terminal and the second input terminal. The second switch and the secondary winding are serially connected between the second input terminal and the third input terminal. The first switch and the second switch are switched on and off alternatively. The first diode and the first switch are connected in parallel and the second diode and the second switch are connected in parallel. While the first switch is on and the second switch is off, the primary winding stores energy of the first cell in the transformer and while the first switch is off and the second switch is on, the secondary winding recovers energy stored in the transformer into the second cell.

Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The description is made with reference to the accompanying drawings in which:

FIG. 1 (Prior Art) is a schematic diagram showing charging cells connected in series with the aid of a conventional cell voltage balancer.

FIG. 2A (Prior Art) is a circuit block diagram showing another conventional cell voltage balancer charges serially connected cells.

FIG. 2B (Prior Art) illustrates the operation of the circuit as shown in FIG. 2A.;

FIG. 3 (Prior Art) shows the conventional cell voltage balancer of FIG. 2A charges n serially connected cells.

FIG. 4 shows the circuit of the cell voltage balancer according to a preferred embodiment of the invention.

FIG. 5A-5B are the equivalent circuit diagrams of the cell voltage balancer, as shown in FIG. 4, according to a preferred embodiment of the invention.

FIG. 6 is the block diagram of the cell voltage balancer according to a preferred embodiment of the invention as shown in FIG. 4 .

FIG. 7 shows the schematic diagram of applying the cell voltage balancer to more than two cells according to a preferred embodiment of the invention.

FIG. 8A and B show the cell voltage balancer using MOSFETs as switches.

The cell voltage balancer 400 as shown in FIG. 4 includes three input terminals P, G, N, a transformer 401 , switches S 1 , S 2 and diodes D 1 , D 2 . The transformer 401 includes a primary winding 402 and a secondary winding 406 . Switch S 1 and diode D 1 , which are connected in parallel, and the primary winding 402 are serially connected between the input terminal P and the input terminal G. Switch S 2 and diode D 2 , which are connected in parallel, and the secondary winding 406 are serially connected between the input terminal G and the input terminal N.

The input terminal P is connected to the positive terminal of cell B 1 ; the input terminal G is connected to the negative terminal of cell B 1 and the positive terminal of cell B 2 ; and the input terminal N is connected to the negative terminal of cell B 2 . Herein, the situation that the voltage of cell B 1 is higher than the voltage of cell B 2 is taken as an example to illustrate the preferred embodiment of the invention.

While cell B 1 and cell B 2 are charged, the cell voltage balancer 400 is capable of balancing the voltage of cell B 1 and cell B 2 so that the phenomenon of overcharge can be prevented. Cell B 1 is between the input terminal P and the input terminal G and cell B 2 is between the input terminal G and the input terminal N and they are connected in series. Switch S 1 and switch S 2 are two switching devices.

While the charging of cell B 1 and cell B 2 is ceased, the cell voltage balancer 400 is able to keep the voltage of cell B 1 and cell B 2 balanced. Thus, cell B 1 and cell B 2 can be prevented from having voltage unbalance.

Furthermore, the turns of the primary winding 402 of the transformer 401 and that of the secondary winding 406 of the transformer 401 can be equal. The primary winding 402 has a positive terminal 404 and the secondary winding 406 has a positive terminal 408 . The positive terminals 404 and 408 are the terminals, which are marked by dots in the transformer symbol. The transformer operates as follows: the positive terminal 408 of the secondary winding 406 has the same voltage polarity as the positive terminal 404 of the primary winding 402 . The positive terminal 404 connects to the positive terminal 408 through switch S 1 FIGS. 5A and 5B show the operation of the circuit of the cell voltage balancer as shown in FIG. 4 . FIG. 5A is the equivalent circuit diagram of the cell voltage balancer, as shown in FIG. 4, according to a preferred embodiment of the invention while switch S 1 is turned on and switch S 2 is turned off. FIG. 5B is the equivalent circuit diagram of the cell voltage balancer according to a preferred embodiment of the invention while switch S 1 is off and switch S 2 is on. The cell voltage of cell B 1 is V 1 , and the voltage of cell B 2 is V 2 . Switch S 1 and switch S 2 are switched on and off alternatively. While switch S 1 is on, cell B 1 discharges and generates current Ip. Meanwhile, because switch S 2 is off, current will not flow through the secondary winding 406 . Current Ip therefore flows through the primary winding 402 so the electrical energy of cell B 1 is transformed to magnetic energy and stored in the core of the transformer 401 .

Next, switch S 1 is turned off and switch S 2 is turned on. When the switch S 1 is turned off, the magnetic flux in the core of the transformer 401 disappears. According to Faraday's law of induction, an induced electromotive force (emf) is generated due to the change of magnetic flux. Consequently, the induced emf across the primary winding 402 is substantially equal to the voltage of cell B 1 , V 1 , with inverse polarity. That is to say, voltage Tp across the primary winding 402 upon switch S 2 turning on is substantially equal to voltage V 1 with inverse polarity as shown in FIG. 5 B. Voltage Ts across the secondary winding 406 changes to V 1 with inverse polarity because the turns of the primary winding 402 and the turns of the secondary winding 406 are equal. Because the voltage of V 1 across the secondary winding 406 is larger than the voltage of V 2 across the cell B 2 , current Is is therefore generated and flows out from the secondary winding 406 . The magnetic energy stored in the transformer 401 is then released in form of current Is, and transformed to cell B 2 by current Is flowing back to the cell B 2 for charging. Thus, the energy of the cell with higher voltage, cell B 1 , transfers to the cell with lower voltage, cell B 2 . The object of balancing the voltage of cell B 1 and cell B 2 is then achieved by using the cell voltage balancer of a preferred embodiment of the invention.

The principle of operation is similar, while the voltage of cell B 2 is higher than the voltage of cell B 1 . The energy of cell B 2 is effectively transferred to cell B 1 to balance the voltage of cell B 1 and cell B 2 . The transformer 401 according to a preferred embodiment of the invention can be a flyback transformer. Current flows through the primary winding 402 and the secondary winding 406 of the flyback transformer at different points in time.

Preferably, switch S 1 and switch S 2 can each be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). Body diodes of MOSFET can be used as diode D 1 and diode D 2 . Thus, the number of devices in the cell voltage balancer 400 is reduced.

Referring to FIG. 8A and 8B, the cell voltage balancer using MOSFETS as switches is illustrated. For the sake of clarity, the components of the cell voltage balancer are rearranged. FIG. 8A shows the cell voltage balancer with the first MOSFET M 1 turned on and the second MOSFET M 2 turned off. Meanwhile, FIG. 8B shows the cell voltage balancer with the first MOSFET M 1 turned off and the second MOSFET M 2 turned on. The detailed operations of the cell voltage balancer shown in FIG. 8A and 8B will be described in the following.

In FIG. 8A, when the MOSFET M 1 is turned on, the cell B 1 , primary winding 402 and MOSFET M 1 form a loop and the voltage across the primary winding 402 is about the voltage across cell B 1 which is V 1 . At the same time, although the MOSFET M 2 is turned off, the voltage across the secondary winding 406 is also about V 1 . The positive terminal 404 of primary winding 402 and the positive terminal 408 of secondary winding 406 have the same polarity which is positive polarity, as shown in FIG. 8 A.

In FIG. 8B, the MOSFET M 1 is turned off and the MOSFET M 2 is turned on. When the MOSFET M 1 is turned off, the magnetic flux in the core of the transformer 401 disappears. According to Faraday's law of induction, an induced emf is generated due to the change of magnetic flux, which results the voltage of the primary winding 402 to be equal to V 1 with inverse polarity as shown in FIG. 8 B. The voltage of the secondary winding 406 changes to V 1 with inverse polarity, too. The positive terminal 404 of primary winding 402 and the positive terminal 408 of secondary winding 406 have the same polarity, negative polarity, as shown in FIG. 8 B. Because the MOSFET M 2 is turned on, the secondary winding 406 , MOSFET M 2 , and cell B 2 form a loop. Thus, owing to the higher voltage of the secondary winding 406 comparing to cell B 2 , a current 12 is generated and flows into cell B 2 to charge the B 2 . By turning on the MOSFET M 1 and M 2 alternatively, the voltage of the primary winding 402 and the secondary winding 404 change alternatively and the energy of cell B 1 will transfer to cell B 2 . Only with simple circuit structure of the cell voltage balancer of the invention, the goal of cell balance is achieved by discharging a higher voltage cell and charging a lower voltage cell through the transformer 401 .

The cell voltage balancer according to a preferred embodiment of the invention as shown in FIG. 4 is provided in form of a module, which is shown in FIG. 6 . The cell voltage balancer 602 includes three input terminals P. G, and N.

FIG. 7 shows the schematic diagram of applying more than two cells in the cell voltage balancer according to a preferred embodiment of the invention. Cell B 1 and cell B 2 are connected to the cell voltage balancer 702 and cell B 2 and cell B 3 are connected to the cell voltage balancer 704 . Two terminals of cell B 1 are connected to the input terminals P 1 and G 1 of the cell voltage balancer 702 ; two terminals of the cell B 2 are connected to the input terminals G 1 and N 1 of the cell voltage balancer 702 and the input terminals P 2 and G 2 of the cell voltage balancer 704 ; two terminals of cell B 3 are connected to the input terminals G 2 and N 2 of the cell voltage balancer 704 .

As shown in FIG. 7, the cell voltage balancer 702 is used to balance the cell voltage of cell B 1 and cell B 2 in order to make the voltage of cell B 1 and the voltage of cell B 2 equivalent. The cell voltage balancer 704 is used to balance the cell voltage of cell B 2 and cell B 3 in order to make the voltage of cell B 2 and the voltage of cell B 3 equivalent. Consequently, the voltage of cell B 1 , the voltage of cell B 2 and the voltage of cell B 3 are balanced to be substantially equal. While the serially connected cell is more than 3 , each of the cell voltage balancers is connected to two adjacent cells, so that the cell voltage can be balanced.

The cell voltage balancer according to a preferred embodiment of the invention can be used to balance more than two serially connected cells during charging. Thus, the voltage of the cells are balanced to be substantially equal. Overcharge of the cell with higher voltage is prevented. The transformer according to a preferred embodiment of the invention has a primary winding and a secondary winding with equal turns, which makes the design of the cell voltage balancer easier without being effected by the number of serially connected cells.

While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.