I worked on modifying an SMPS (Switched-Mode Power Supply) for charging a 14V battery with a 3.5 Å current under the guidance of my teacher, Bulent NUR. While I’ve come across several examples of SMPS Charger modifications, I hadn’t had the chance to apply them until now. The modification involves two key components: adjusting the output voltage of the SMPS adapter to the suitable level for battery charging (previously discussed in my topic on “Increasing SMPS Voltage”) and incorporating a basic current limiting circuit.
This battery charging method can be tested using common flyback SMPS adapters frequently found in the modification market. These adapters typically have an output voltage ranging from 12V to 15V, which can be adjusted by altering the TL431 voltage divider resistors. The voltage at the TL431 REF bridge should be set at 2.5V, calculated using the formula 2.5V * (1 + R1 / R2).
As depicted in the circuit diagram, SMPS integrations such as TOP2XX TopSwitch series or UC3842, UC3843, etc., are used. These integrations involve PWM and SMPS control, making them complex. Advanced SMPS designs incorporate protection mechanisms, but there is a risk of failure, so caution is necessary.
The current limit is set by the 0.65V R5 resistor, providing a charging current of 0.75A to 0.8A. For this configuration, a 0.82 Ohm 0.5W resistor is used. Alternatively, for a charging current of 3.5A, a 0.22 Ohm 5W resistor is employed, and the resistance value of T1 BC547 transistor depends on the base’s pin, typically set at 1K.
SMPS CIRCUIT SCHEMATIC ORIGINAL AND MODIFIYE
Modifying off-the-shelf switch mode power supplies for battery charging might seem straightforward. However, despite conducting thorough measurements and tests, the process requires careful attention. In my final schematics, I’ll include similar charge modification circuits employed in the UC3842, UC3843, and TOP204 integrations. Specifically, I’ll focus on utilizing the current and voltage control components from the secondary circuit of these setups.
The UC3842 and UC3843 are crucial for voltage regulation, and a transformer will be incorporated for this purpose. It’s vital to use the transformer within the specified power supply limits, ensuring it doesn’t exceed these boundaries. For instance, a 12V SMPS adapter can charge a 12V battery by adjusting the voltage to 14.2V. However, applying excessive voltage for a 6V battery can lead to malfunctioning, emphasizing the need for precise voltage control.