In the design of medium and small-sized color TVs, a current-limiting resistor is typically connected in series with the rectifier filter circuit. This resistor, usually in the range of a few ohms, serves to limit the initial charging current of the filter capacitor during startup. Its main purpose is to prevent excessive current from damaging the rectifier bridge or causing it to fail. However, this approach has its drawbacks: it consumes energy during normal operation and increases the internal resistance of the power supply, which can negatively affect the voltage regulation performance.
For standard medium and small-screen TVs, where the filter capacitor has a relatively small capacity, using a low-value resistor for current limiting works well. However, this method becomes ineffective for larger screen TVs, such as 29-inch or 34-inch models, where the filter capacitors can be as large as 570μF. In such cases, selecting a high-value resistor for current limiting would result in unnecessary power consumption and a significant voltage drop across the resistor during operation, which degrades the power supply's performance.
Without a current-limiting resistor, the inrush current during startup could be dangerously high, potentially damaging the rectifier bridge and the filter capacitor itself. This creates a dilemma: too small or no resistor leads to high inrush current, while a larger resistor affects the power supply’s voltage regulation. Some TV models, like Konka T2910 and T2988, use a relay to short-circuit the resistor after startup. While this helps, it increases cost, complicates the circuit, and reduces reliability.
Enter the NTC thermistor—specifically the NTC2.5 model, which is ideally suited for this task. The following diagram shows the rectification and filtering circuit of the Konka T3498/T3898 switching power supply. Components HT902 and HT903 are negative temperature coefficient (NTC) thermistors. These resistors act as current limiters, overcoming the limitations of traditional resistors.
At room temperature, the NTC thermistor has a resistance of 2.5 Ω, but as it heats up, its resistance drops rapidly. At 75°C, the resistance is only about 0.12 Ω. In the circuit, two NTC thermistors with a cold resistance of 5 Ω are used to effectively limit the inrush current when the TV is first turned on. As current flows through them, they generate heat, and their resistance decreases significantly once the TV is operating normally. At that point, the resistance is around 0.25 Ω, nearly eliminating any voltage drop and behaving like a wire. This means no extra power is consumed, and the voltage regulation remains stable.
By using this smart component, the conflict between current limiting and power efficiency in large-screen TVs is resolved. The circuit becomes simpler, more reliable, and more efficient. The NTC thermistor offers a smart, adaptive solution that improves both performance and longevity of the power supply system.
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