Design of pulse fast charging system based on single chip microcomputer

The flyback switching power supply circuit is adopted. The flyback switching power supply uses one less large energy storage filter inductor and one freewheeling diode than the forward switching power supply. Therefore, the flyback switching power supply circuit is relatively simple, small in size and low in cost. In addition, the output voltage of the flyback switching power supply is modulated by the duty cycle, which is much higher than that of the forward switching power supply. Therefore, the flyback switching power supply requires the error signal amplitude of the duty cycle to be relatively low, and the gain and dynamic range of the error signal amplifier should also be small. It is precisely because of these advantages that the flyback switching power supply is widely used in the current home appliance field. Its power supply circuit is shown in Figure 1.

UC3842 is a relatively ideal new type of pulse width modulator, so the switching power supply circuit control chip uses UC3842. Compared with the voltage-controlled pulse width modulation switching power supply, the switching power supply composed of this integrated circuit has the following characteristics: 1) fine-tuned oscillator discharge current, which can accurately control the duty cycle; 2) current mode operation up to 500kHz; 3) automatic feedforward compensation; 4) latched pulse width modulation, which can limit the current cycle by cycle; 5) internal fine-tuned reference voltage with undervoltage lockout; 6) large current totem pole output; 7) undervoltage lockout with hysteresis; 8) low startup and operating current.

TL431 is called an adjustable precision shunt regulator. It can set any reference voltage value in the range of 2.5V~36V by using two external resistors. Its working principle is that when the output voltage U0 fluctuates, the sampled voltage obtained after the resistor voltage division is compared with the 2.5V bandgap reference voltage in TL431, and an error voltage is formed at the cathode, so that the working current of the light-emitting diode changes accordingly, and then the control terminal current Ic is changed by the optical coupler, and the output duty cycle of the switch tube is adjusted to keep the output voltage U0 unchanged, thereby achieving the purpose of voltage stabilization.

2. Design of charging and discharging circuit

As shown in Figure 2, the charge and discharge process in this design is mainly completed by two P-channel field effect transistors IRF9540N. Q2 controls the charging process of the system, and Q3 mainly controls the discharge pulse.

3. Collection part

There is a 12-bit precision AD conversion module ADC12 in MSP430F149. ADC12 can convert one of 8 external analog signals or one of 4 internal voltages. The ADC core converts the analog signal into 12-bit data and stores it in the conversion storage register. The core uses two reference levels, VR+ and VR-, as the upper and lower limits of the conversion range and the range value and "0" value of the reading. The conversion value is full scale when the input signal is greater than or equal to VR+, and is "0" when it is less than or equal to VR-. The battery terminal voltage is divided by two resistors R2 and R6 to limit the microcontroller sampling voltage value AD1 within the input voltage range. The charging current and the discharging current are converted into corresponding voltage values ​​through resistors and sent to the AI ​​port of the microcontroller. The three AD sampling results are stored in the corresponding ADC12MEMx registers respectively.

4. Drive part

After determining the charging status of the charger according to the three-way AD sampling results, the single-chip microcomputer outputs two PWM waveforms PWM1 and PWM2 with adjustable duty cycle and frequency of 1kHz, which are used to control the conduction degree of the two transistors 2N5551 respectively. The collector current of the transistor will be different, and then provide different voltages to the gate of the field effect tube after resistor voltage division, driving MOSFET, outputting drain current Id of varying sizes, and providing the battery with charging current and discharge pulse amplitude of varying sizes, ultimately achieving the effect of pulse charging.