Highly effective, the low cost and the reliable battery charger design available each method realizes, but uses 8 to dodge fast MCU not only to be able to reduce the design time, to reduce the cost and to provide the safe reliable product, moreover can also cause the designers to carry on the field upgrade by the least work loads. Considered the battery safe charge the cost, the rated capacity and the importance, may provide many superiority based on the MCU solution for the designers. Through chooses the belt suitable periphery with 8 MCU which dodges saves, engineers can use its superiority to design one kind of off-line lithium battery battery charger fully. Brings 2KB to dodge saves and the suitable periphery provides one kind of inexpensive solution Phillips 80C51 MCU is this kind of example. The integration dodges saves can also provide highly effective and debugs the application code conveniently and carries on the scene software to promote (, if needs) ability. Not only because design familiar and widely accepts 8 MCU, therefore the software and hardware development may carry on fast. Each kind of function which provides by the numerous manufacturers is formidable, and not expensive application development kit, is also this method another superiority. Using this method, the design team not can only reduce the design cycle enormously, moreover can also carry on a more complex design, and causes the project overall materials expenses (BOM) not to surpass the acceptable scope.
Peripheral circuit integration
For example, when the MCU integration has the internal neterodyne, the off-line lithium battery battery charger design may obtain benefits from the following two aspects. First, may save the exterior oscillator, thus saves the cost and the PCB perch; Next, the internal neterodyne may enhance when the system initiation stability. Four channel A/D the switch is project engineers should seek integrates in the chip another kind of valuable peripheral circuit. Besides can compare uses an exterior A/D switch more frugal cost, but can also use it to examine the charging voltage, the electric current and the battery temperature–Includes in the security battery charge operation nearly all key parameters. Not only below used for to realize introduces the design MCU(P89LPC916) to integrate the above all these characteristics moreover also to have may simultaneously on two clocks the executive order high performance processor construction, thus enhanced its performance to standard 80C51 component’s 6 times. Time0 (timer 0) is disposed very easily the PWM output, therefore easy to establish and to use the PWM function.
Basic battery charge standard
This design to aim at fixed 700-750mAh, the 3.6V discharge voltage and the 4.2V voltage limit lithium battery battery charger solution specially. The charge order divides into following three stages: Pre-charge stage, constant current charge stage and constant voltage charge stage. When the battery is only left over the very few electric quantities, and therefore and can only produce time the very low output voltage, must have the pre-charge stage. Under this situation, must use the low electric current charge to protect the battery. But if may produce by the rechargeable battery the high voltage (>3V), then may abbreviate falls the pre-charge stage. Certainly, this is the most universal situation. The majority of electrical energies are flow in the battery in the constant current and the constant voltage charge stage from the battery charger. Battery’s biggest permission charging current decided by this battery’s rated capacity. Regarding the rapid charge rate, for example the fixed 700mAh battery, the available 350-400mA electric current charges. In the lithium battery situation, MCU must while maintain the battery normal charging voltage also monitors the charging current, by when the battery fills can terminate the charging-up. The temperature surveillance may use for to guarantee that carries out the security charge step, because fills along with the battery, any extra electrical energy will be transformed the quantity of heat. Although MCU must complete for it the function increases the temperature surveillance, but in the market majority lithium batteries have the built-in surcharge protection now, therefore the temperature surveillance needs but actually very little to use freely.
Voltage dropping switch design
If designs one kind of belt cone-shape termination characteristic battery charger, most is effective and the most economical method is uses the voltage dropping switch to take the switch adjuster. The voltage dropping switch use inductance stores up the electrical energy. Chart 1a and 1b respectively are in passes/breaks when the position the voltage dropping switch work schematic drawing be the switch.
Figure 1:(a): The voltage dropping switch switch “starts“; (b): Voltage dropping switch switch “pass“.
From PWM signal control charge switch. When switch closed (chart 1a), electric current, because the battery charger provides the voltage (battery charger Vin) winds through the electric circuit, this time electric capacity through inductance charge. When the switch opens (for example 1b shows), the inductance attempts through the induced tension maintains the electric current is mobile, but it cannot charge immediately. Then the electric current winds through the Short base diode and to the condenser charge. This process repeat in cycles. When through reduces the PWM dutyfactor to reduce the switch “passes” the time, average voltage reduction. On the contrary, when through increases the PWM dutyfactor to lengthen the switch “breaks” the time, the average voltage increases. Therefore through controls the PWM dutyfactor to cause MCU to adjust the value of exports which the charging voltage (or electric current) may achieve needs. Before the discussion designs the detail, must discuss and the inductance and the electric capacity related two main points first:
- 1. the inductance size is not difficult to see, the definite voltage dropping switch inductance’s size is achieves the appropriate charging voltage and the electric current key. The inductance size also concerns with the cost. The inductance capacity available formula 1 calculates: Formula 1 in which: Vi: Input to switch’s battery charger voltage; Vsat: The switch “passes” when switch’s voltage drop; Vo: Voltage output; T:PWM cycle; DutyCycle:PWM dutyfactor; Io: Electric current output (i.e. constant current charge). The formula 1 demonstrated that PWM the turn-on frequency is higher (i.e. switching period T to be smaller), then needs the inductance is smaller, this is helpful to the reduced component cost.
- 2. what the electric capacity size must pay attention, in this electric circuit’s electric capacity is completely uses for to reduce the ripple electric current, therefore bigger is better, because the ripple and the capacitance value are in reverse proportion.
Design main point
This design based on Phillips P89LPC916 MCU, its overall design thought is, through uses the constant current charge, then to use the constant voltage charge first to realize as far as possible the quick charge again. MCU also controls uses in instructing the battery charger active status LED.
- 1. precision power source VDD must use the precise potential source, because the voltage serves as for this reason the DA-DA switch’s voltage reference. Low pressure drop (LDO) adjuster for this potential source best choice, and this design uses 3 carries LDO LM1117 to come is VDD provides the precise 3.31 V power sources.
- Output solution Timer0 (timer 0) a channel uses for to produce the control voltage dropping switch switch’s PWM signal. Because LPC916 has on its own piece the RC oscillator, therefore charges is stabler and is effective–Especially under voltage control working pattern. Needs the PWM frequency probably is only 14kHz, can therefore control well on the piece in oscillator’s frequency range. May “open” the time through the change voltage dropping switch to adjust the PWM dutyfactor.
System design
Figure 2 is the lithium battery battery charger system composition diagram. And PWM output control charge switch, and its dutyfactor may according to need to use the charging voltage and the electric current feedback adjusts. On the LPC916 8 pieces the high speed A/D switch has provided the high accuracy which the surveillance charging voltage needs. Avoids in the lithium ion application the surcharge being important, because will charge will maintain may lengthen battery’s service life in its maximum value. Table 1 for this electric circuit’s input/output parameter specification.
Table 1: Figure 2 electric circuit’s input, output parameter specification.
Next step calculates the inductance value, what must first point out that the formula 1 has given dutyfactor, output current, between the PWM cycle and other variable relations. The inductance value may through supposition Vi=5.1V, probably output voltage Vsat=0.5V (on Io=350mA, Vo=4.25V, must output current Io=350mA, 1/T=14.7kHz as well as dutyfactor is 50% computations). Above uses these values, 1 may calculate the inductance value with the formula is not smaller than 10 µ H. In this design, the suggestion inductance value is 33-10 µ H. Although may use is bigger than 5.1V the input voltage, but the higher input voltage request uses high-frequency PWM or a bigger inductance, thus causes the component cost enhancement.
Figure 2: Lithium battery battery charger solution which controls by LPC916
The lithium battery should charge by three independent stages. If the battery voltage is lower than 3V, then needs to have the pre-charge stage, and the charging current should maintain is 65mA. Once the battery voltage achieves 3V -1%, namely starts to enter the rapid charge rate stage, and uses 350mA the constant charging current. May cause the charging current through the adjustment steering impulse maintains constant. When the battery voltage achieves 4V -1%, namely starts to meet the constant voltage charge stage. This time the voltage maintains at 4.23V, the charging current is under the surveillance.
After constant voltage charge stage, the battery by the other overcharging 50 minutes, simultaneously maintains the charging current is smaller than 30mA. The duration of charging available timer controls, but the surveillance charge ends the method has three kinds: The examination charging current, uses the timer as well as the surveillance temperature (may elect).
Figure 3: Lithium battery charging-up
Charging-up as shown in Figure 3. Enters from a stage to the another stage accurate sign is as follows: Pre-charge stage (, when needs): If Vbat<3.0 (1%, then establishes Iout=10%; Ireg=65mA; Rapid charge rate stage (constant current charge): When Vbat<=4.00 -1%V, establishes Iout=Ireg=350mA; Timer control charge stage (constant voltage charge): When Ibat<60mA, establishes Vout=Vreg=4.23V (50 minutes) to guarantee that the battery charges fully, but causes the charging current is smaller than 30mA. The charge completes in 4 hours. Considered the end-user, in the design has used the LED condition indicating lamp, supplies the related charge sequence condition the information.
Design proposal test
May use for in the charging-up to test this design electric circuit diagram as shown in Figure 4. Surveys Vout and the Vsense_res reading with two avonmeters. Vout=Vbat Vsense_res, charging current available formula Iout=Vsense res/0.75 calculates.
Figure 4: Test circuit
When charges starts, an every 15 second record data, but works as after electric current and voltage regulation, the record cycle may reduce for every 5 second record one time. The result will possibly change along with the different battery’s chemistry characteristic, moreover battery’s initial voltage will be also influential to the result. Figure 5 and Figure 6 demonstrated that this design satisfies the target.
Figure 5: Output voltage test result
Figure 6: Output current test result
