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NTC thermistor escort lighting system to effectively limit inrush current

eepw | 08/05/2021

The lighting industry continues to promote inductive loads. What is disturbing is that the inductance generated by it is opposite to the resistance of the system, which will reduce the efficiency of the system. PFC can solve the above problems. However, when the PFC is initially charged, it will generate inrush current that damages other circuits in the system. Through the use of thermistor, the inrush current can be effectively suppressed and the circuit is prevented from being damaged.

There are many ways to build a lighting system, and good design can directly improve energy efficiency and save material costs. Today's lighting industry is gradually changing from 240V to 277V to improve efficiency. So now is the perfect time to introduce Power Factor Correction (PFC) to lighting product manufacturers. Since these lighting systems need to be updated anyway, original equipment manufacturers (OEMs) can enjoy the many advantages of PFC at the same time.

Moving towards inductive loads is the beginning of the demand for PFC. Traditional lighting applications use resistive loads, such as incandescent lamps. However, the disadvantage of resistive loads is that the resistance they introduce into the system generates heat. Thermal energy causes power loss and reduces efficiency. To avoid these losses, the lighting industry continues to promote inductive loads, such as fluorescent lamps with higher efficiency.

Unfortunately, the way in which many lighting equipment manufacturers implement inductive loads has severely reduced the efficiency of the lighting system. In many cases, they just don't realize that power factor correction can solve these problems in a simple and inexpensive way.

In terms of its nature, inductive loads convert the phases of voltage and current to each other. In particular, the inductive reactance produced is inverse to the resistance of the system. This phase difference will reduce the efficiency of the system.

The power factor (PF) is the ratio of the system's real power (Real Power) to its apparent power (Apparent Power). The apparent power is the expected system power, and the actual power is the actual power obtained. Depending on the application, the inverting system has the lowest efficiency, which may drop to 60%.

The goal of power factor correction is to minimize the phase difference between voltage and current. Capacitive reactance can be used to bring inductive reactance back to the only resistance phase of the system. Only capacitors with the right characteristics are required, that is, a high enough power ratio and a 180 degree antiphase with the inductance.

Power factor correction has many benefits

There are many advantages of applying PFC to the lighting system, which are explained separately as follows:

. Efficiency improvement

Depending on different applications, adding PFC to the lighting system can increase the efficiency as high as 80~95%. With the rising cost of public utilities, this will make PFC-based lighting systems attract a large number of end customers.

. Easy to install

As long as there is a capacitor, PFC can be introduced into the lighting system. Please note: an inrush current limiter is also required to avoid damage to the system by the initial capacitance of the capacitor when starting up.

. Reduce power supply costs

A system with a high power factor can perform the same work as a system with a low power factor through a smaller power supply. The need to carry less current means the need for smaller and cheaper generators, conductors, transformers and switches, so the machine body can be simplified and material costs can be saved.

.Stability improvement

A more efficient system needs to consume less heat, so the system can maintain stable operation within an acceptable temperature range.

. Distinguishing characteristics

Whether your design is a stand-alone product or merged into a part of a large-scale system, higher power efficiency can drive a higher-level system compared to a lower-efficiency system of the same level.

. Low operating cost

For large-scale lighting applications, the high efficiency created by PFC can substantially save the cost of public utilities.

. Industry dynamics

As early as more than ten years ago, power factor correction became a mandatory standard in Europe, China, and Japan. Although the adoption rate of PFC in the United States is not high, it continues to be applied to more and more applications, especially lighting systems. Obviously, PFC is meaningful and will eventually be used by applications that are not currently in demand. It is expected that PFC will become a company for its future needs, and it will benefit from today's PFC as one of its distinguishing features in the future. Manufacturers that cannot provide PFC will soon find themselves uncompetitive.

The lighting industry continues to promote inductive loads. What is disturbing is that the inductance generated by it is opposite to the resistance of the system, which will reduce the efficiency of the system. PFC can solve the above problems. However, when the PFC is initially charged, it will generate inrush current that damages other circuits in the system. Through the use of thermistor, the inrush current can be effectively suppressed and the circuit is prevented from being damaged.

Inrush current suppression thermistor is cheap and easy to use

When the PFC capacitor is initially charged, it will produce the maximum current that the system can withstand. This short inrush current may be much higher than the operating current of the system, and depending on the lighting application, it may damage other circuits in the system. In order to avoid such damage, a circuit that can limit the inrush current is required.

The core of the surge limiting circuit is high resistance. Placing resistors in the circuit can limit the capacitance that the capacitor can achieve. However, once the capacitor has been charged, if the resistor is left in the circuit, it will continue to cause heat energy loss and will reduce the overall efficiency. Basically, once the inrush current is limited, the switch can be used to bypass the resistor.

The most efficient way to deal with inrush current is to use a thermistor. Thermistor is a special variable resistor whose resistance depends on temperature. For example, a Negative Temperature Coefficient (NTC) thermistor can greatly and predictably reduce its resistance when its temperature rises.

In order to limit the inrush current, the NTC thermistor is placed between the power supply and the PFC capacitor and the inductive load capacitor. When starting up, the NTC thermistor has a low temperature, so it can provide high resistance. In addition to limiting the current into the capacitor, the heat generated by this high resistance will increase the temperature of the thermistor.

NTC automatically heats up while its resistance drops rapidly. When the inrush current stabilizes, the temperature of the NTC thermistor is already sufficient to minimize the resistance and allow current to pass without negatively affecting system operation or efficiency. In this way, the NTC thermistor can effectively provide the resistance required to limit the inrush current, while eliminating the need for additional circuit systems, such as bypass switches.

The durability of the NTC thermistor must be quite high, and its effective operating range is between -50°C and 250°C. At present, circuit protection component manufacturers have realized the shift to 277V, and have developed thermistors for this higher voltage level for lighting applications, and at the same time provide the industry with UL and CSA certified thermistors. Therefore, customers The power efficiency lost due to resistance heat can be minimized.

The price range of NTC thermistors suitable for lighting applications is 0.15~0.90 US dollars. Compared with those priced at 0.50 to 1 US dollars or more, NTC thermistors are rated enough to handle the large current of electric light ballasts. The price of the resistor also needs to consider the circuit used to bypass the resistor after the inrush current is limited.

The power factor correction is extremely simple and the installation price is low. In terms of improved efficiency, PFC is an inevitable new choice for many inductive lighting applications, even if the original design does not require the use of PFC. And with the negative temperature coefficient thermistor, lighting equipment manufacturers can protect the lighting system without the need for complex and expensive bypass circuits to prevent it from being affected by the surge current associated with the PFC.

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