The schematic diagram of the charger is illustrated in Figure 13. Unlike the half-bridge type, the single-excitation charger's startup circuit draws power directly from the smoothed DC output after the mains rectification and filtering process. Integrated circuits commonly used include UC3842, UC3845, UC3844N, and a more compact solution like the three-terminal switch-type TOP226. UC38xx series chips are specialized current-controlled PWM single-output devices widely employed in power supply products, including computer monitor power supplies and electric vehicle chargers.
Similar to TL494, UC38xx incorporates essential components for a PWM dedicated chip, such as an oscillator (OSC), error amplifier, pulse width modulation (PWM), and reference voltage generation. Its unique features include a totem pole output circuit capable of delivering up to 1A of output current, enabling direct driving of power switch VDMOS transistors. It also includes an internally adjustable reference power supply with undervoltage lockout functionality, allowing pulse-by-pulse current limiting or what is known as periodic limiting.
In Figure 13, components like R18, D5, and N5 form the startup and power supply circuit. Upon power-on, the smoothed DC power from the mains rectification and filtering process is applied to the UC3845's pin via R18, initiating the power supply. Initially, D5 is reverse-biased. Once the UC3845 begins functioning, the switching transformer's windings generate an induced voltage. The secondary winding voltage is regulated by D4 and N5, turning on D5 to provide a stable working voltage for the UC3845, completing the startup and power supply process. LM393, a deformed Schmitt voltage comparator, serves as the mains overvoltage protection mechanism. If the mains voltage exceeds a certain threshold, the comparator triggers, causing pin 1 to go low and D3 to turn on, which shuts off the UC3845. The output-regulated negative feedback system involves a photocoupler, a reference power supply N6, RV1, R27, R26, R23, and others. During the voltage regulation process, if the output voltage increases due to any reason, the current through the photocoupler’s LED increases, enhancing its light intensity and increasing the conduction of the phototransistor. This reduces the internal resistance, raising the voltage at pin 2 of UC3845, decreasing the PWM duty cycle, and pulling the output voltage down. Conversely, increasing the PWM duty cycle raises the output voltage, stabilizing it automatically.
1) Overcurrent (Overload) Protection: The switch tube overcurrent signal is obtained from resistors R3 and R4. If the switch tube experiences overcurrent, the voltage at pin 3 of UC3845 exceeds 1V, triggering the internal circuit to shut off the output, thereby implementing overcurrent (or overload) protection. Increasing the sampling resistance lowers the starting current operating point and correspondingly reduces the power output.
2) Overvoltage Protection: The power supply output uses LM339's four voltage comparators A, B, C, and D, with their inverting terminals fixed at +5V. A and B monitor the output voltage. When the output voltage is low, during the initial charging phase, A’s pin 2 is at a low level, illuminating the low-voltage indicator (LOW). Similarly, B’s pin 1 is also at a low level, lighting the high-voltage indicator (HI). As the charging voltage rises, A flips, turning off the low-voltage indicator (LOW), while the high-voltage indicator (HI) remains lit. When the battery approaches full charge, the rising battery voltage causes B to flip, setting pin 1 to a high level and extinguishing the high-voltage indicator (HI). Simultaneously, pin 13 of C and pin 14 of D go high, turning on N7, connecting J1, and disconnecting J1-1 (normally closed). This action connects the sampling resistor R4, activating the current sampling resistor to control the starting current, reducing the output current and transitioning into the float charging phase. Components N4, W1, R8, and R7 form a 12V regulated power supply, providing power for the 12V relay.
In addition to these features, the charger also integrates several safety mechanisms to ensure reliable operation. For instance, temperature sensors are embedded within the device to monitor heat dissipation. If the temperature exceeds a safe threshold, the system automatically reduces the output power or halts operations entirely. Furthermore, advanced firmware programming allows for dynamic adjustments based on real-time data, ensuring optimal performance under varying load conditions. These enhancements make the charger not only efficient but also highly dependable, catering to diverse user needs while maintaining stringent safety standards.
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