Automatic charger constructed with new integrated circuit KW9712

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Automatic charger constructed with new integrated circuit KW9712 [Copy link]

summary:

The charger is designed with the complementary output voltage-controlled pulse width circuit KW9712 as the core and adopts the half-bridge switching power supply solution.

It has the functions of pulse charging, automatic stop when fully charged, charger not starting when battery is not connected or reversely connected, and overcurrent protection.

Keywords : complementary output voltage-controlled pulse width circuit IC automatic charging

1KW9712 Introduction

1.1 Product Introduction:

　　KW9712 is a bipolar integrated circuit designed by us, processed by a professional factory, and packaged in DIP-8. An external resistor and a capacitor can determine its oscillation frequency, which can be set at a very low frequency to 500kHz. It has two sets of totem pole outputs with complementary duty cycles, and its duty cycle change is controlled by the input voltage of a dedicated pin. There is also a dedicated prohibition input terminal. When its input voltage is lower than (1/20) VDD, the circuit oscillates. When it is higher than (1/20) VDD, the output is prohibited, and all output pins are low potential. With this as the core, intelligent temperature control, light controller, speed regulator, power regulator, single-ended, half-bridge or full-bridge switching power supply and special switching power supply can be manufactured. It can also be used as an inverter, charger, inverter welding machine, ultrasonic generator, etc. The basic application circuit is shown in Figure 1.

Figure 1

1.2 Pin functions:

① Pin: VDD, 5~18V; ② Pin: Oscillator resistor and capacitor access terminal; ③ Pin: Control voltage input terminal, which should vary between (1/3) VDD and (2/3) VDD; ④ Pin and ⑤ Pin: Two totem pole output terminals. When the potential of ③ Pin rises from (1/3) VDD to (2/3) VDD, the duty cycle of ④ Pin decreases from 100% to zero, and the duty cycle of ⑤ Pin increases from zero to 100%; conversely, when the potential of ③ Pin decreases from (2/3) VDD to (1/3) VDD, the duty cycle of ④ Pin increases from zero to 100%, and the duty cycle of ⑤ Pin decreases from 100% to zero; ④ and ⑤ Pins have a load capacity of 10mA for VDD and GND. ⑥ Pin: GND; ⑦ Pin: Disable terminal. When it is lower than (1/20) VDD, this circuit starts to oscillate. When it is higher than (1/20) VDD, ④ and ⑤ Pins are both at ground potential; ⑧ Pin: Oscillator resistor access terminal. When the circuit starts to oscillate, there is a square wave with a duty cycle close to 50% at pin ⑧, and an approximate triangle wave with a peak-to-valley value of (1/3~2/3) VDD at pin ②. The load capacity of the two pins is small.

2 Automatic charger

2.1 Charger logic diagram, see Figure 2

Figure 2

2.2 Main switch circuit

The principle is shown in Figure 3. This is a half-bridge switch circuit driven by IC1 oscillator and T1. IC1 is a KW9712. When Ra and Rb are equal, the potential of IC1's ③ pin is equal to (1/2) VDD, and the duty ratio of ④ and ⑤ pins is 50% respectively. At this time, the potential of ⑦ pin is low, and the full voltage of Uo2 is output; when the potential of ⑦ pin is high, Uo2=0. Controlling the high and low potential pulse width of ⑦ pin controls the pulse width of the output voltage Uo2.

2.3 Working power supply

　　The principle is shown in Figure 4. The AC mains power Uac is rectified by V1~V4 and filtered by C1 and C2 to supply the half-bridge switch circuit. The parallel regulator composed of R1, V5, V6 and C3 has an output of VDD in Figure 3.

2.4 Control Circuit

Figure 3

Figure 4

The principle is shown in Figure 5. IC2 is another KW9712. The upper terminal of the voltage regulator tube V3 is connected to the Uo2 terminal and the positive terminal of the battery to be charged. When the charging voltage is low, V3 is not turned on, and Rc and Rd determine the pulse width of the ④ pin, which is about 80% here. R and C on IC2 determine the oscillation frequency of IC2. When the ⑦ pin is low, IC2 controls the ⑦ pin of IC1 through the light-emitting tube V1, that is, controls the Uo2 terminal to charge the battery at the oscillation frequency of IC2 and a pulse width of nearly 80% and 20% intermittently. When the voltage of the charged battery rises to the set value, V3 is turned on. At this time, the Rx and Rd voltage divider controls the potential of the ③ pin of IC2, that is, the pulse width of the ④ pin. When the potential of the ③ pin of IC2 rises to (2/3) VDD, the ④ pin is at ground potential, V1 is cut off, IC1 stops oscillating, and there is no output at the Uo2 terminal. This is full automatic stop, and between the start control voltage and the stop charging voltage, a trickle charge with decreasing pulse width is performed.

Figure 5

2.5 Control circuit power supply　

　The principle is shown in Figure 6. The upper end of R2 is connected to the Uo2 end of Figure 3 and the positive end of the battery to be charged, forming a parallel regulator to provide working power for the control circuit and the protection circuit.

2.6 Protection Circuit

The principle is shown in Figure 7. IC3 uses a dual comparator LM393. The upper comparator is used to automatically shut down when the battery is removed, and the lower comparator is used for overcurrent protection. The upper end of Re is connected to the Uo2 end and the positive end of the battery.

During normal charging, the voltage at the upper end of Re is low, there is no overcurrent in the circuit, the output transistors in the upper and lower comparators are both in the off state, V1 is turned on, IC2 starts to oscillate, and the same is true for full charge automatic stop.

When the battery is fully charged and removed, the potential at the Uo2 terminal increases due to no-load, causing the upper comparator to flip, the output transistor to turn on, V1 to turn off, and then IC2 and IC1 to stop oscillating.

When overcurrent occurs, the lower comparator flips, the output transistor turns on, V1 turns off, and IC2 and IC1 stop oscillating. At this time, the battery discharges to keep IC2 and IC3 working. The lower comparator is in the state of protecting the charger from shutdown and instant inspection according to the charging time of Cg by Rg. If an alarm is required, a long-tone buzzer can be connected in series with Rg to alarm for overcurrent.

Figure 6

Figure 7

2.7 Reverse connection self-locking circuit

The principle is shown in Figure 8. When the polarity of the battery GB is connected correctly, the optocoupler V2 is turned on and the thyristor V1 is turned on. When the connection is reversed, V3 is turned on and V2 and V1 are turned off to protect the battery.

2.8 Design points

Take good care of the insulation and withstand voltage between the transformer T1 and T2 windings; pay attention to the winding process of T1 and T2 to reduce the leakage inductance between the windings; select the voltage regulator tube (V3 in Figure 5) and adjust Rx according to the set voltage of the charged battery. Make sure that V3 starts accurately, and that the charging and full-charge automatic stop are accurate; select Re in Figure 7 so that the upper comparator can flip when it is slightly higher than the full-charge automatic stop voltage.

2.9 Conclusion

Automatic charger constructed with new integrated circuit KW9712

Figure 8

The charger designed according to the above circuit has the following characteristics: ① It adopts a half-bridge switching circuit as the main power supply, which has the advantages of simple circuit, high efficiency and low cost; ② It adopts pulse charging, and the charging effect is good; ③ It adopts the voltage pulse width decreasing method to achieve automatic stop when fully charged and trickle charging; ④ It will not start if no battery is connected, it will not start if the polarity is reversed, and it will automatically shut down if the battery is removed.