In modern electronics, especially in color TVs with medium and small screens, a current-limiting resistor is typically used in series with the rectifier filter circuit. This resistor, usually several ohms in value, serves to limit the initial charging current of the filter capacitor during startup, preventing damage to the rectifier bridge due to excessive inrush current. However, this approach has its limitations: it consumes energy during normal operation and increases the internal resistance of the power supply, which can negatively affect voltage regulation.
For smaller screen TVs, where the filter capacitor is relatively small, a low-value resistor can be used without significant issues. However, for larger screen models—such as 29-inch or 34-inch TVs—the situation becomes more complex. These TVs often have large filter capacitors, such as 570μF, and using a standard current-limiting resistor would lead to unnecessary energy loss and a voltage drop across the resistor, reducing the efficiency of the power supply.
Without a current-limiting resistor, the inrush current during startup could easily damage the rectifier bridge or the capacitor itself. This creates a dilemma: too small a resistor leads to high inrush current, while a larger resistor causes inefficiency and poor voltage regulation. Some TV models attempt to solve this by using a relay to short out the resistor after startup, like the Konka T2910 or T2988 models. However, this method adds complexity, increases cost, and reduces reliability.
Enter the NTC thermistor, specifically the NTC2.5 model, which is perfectly suited for this task. The following diagram shows the rectification and filtering circuit of the Konka T3498/T3898 switching power supply, where HT902 and HT903 are NTC thermistors. At room temperature, these resistors have a cold resistance of about 5Ω, effectively limiting the initial current surge when the TV is turned on. As current flows through them, they heat up, causing their resistance to drop rapidly. Once the temperature reaches around 75°C, the resistance falls to approximately 0.12Ω.
During normal operation, the resistance of the NTC thermistor is only about 0.25Ω, resulting in almost no voltage drop. This means it functions similarly to having no current-limiting resistor at all, significantly reducing power consumption while maintaining efficient voltage regulation. By using this innovative component, the conflict between inrush current protection and power efficiency is resolved, allowing for a simpler and more reliable circuit design.
This technology not only improves performance but also enhances the overall reliability of large-screen color TVs, making it a smart and effective solution for modern power supply systems.
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