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NTC thermistor escort lighting system effectively limits surge current

Date: 2013-11-29
Viewed: 41
案例名称: NTC thermistor escort lighting system effectively limits surge current

The lighting industry continues to push inductive loads, and the trouble is that the inductive reactance generated by the system is reversed, which reduces the efficiency of the system. PFC solves the above problems. However, when the PFC is initially charged, it will generate a surge current that damages other circuits in the system, and the use of the thermistor can effectively suppress the surge current and prevent the circuit from being damaged.

There are many ways to build a lighting system, and an excellent 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 power factor correction (Power

Factor Correction, PFC) is an excellent time to introduce 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 PFC demand. Traditional lighting applications use resistive loads such as incandescent lamps. However, the disadvantage of resistive loads is that the resistors they introduce into the system generate thermal energy. Thermal energy can cause power loss and reduce efficiency. To avoid these losses, the lighting industry continues to drive inductive loads, such as more efficient fluorescent lamps. Figure 1 is an illumination system based on an inductive load.


NTC热敏电阻护航照明系统有效限制涌浪电流


Figure 1 Adding a shunt capacitor to an inductive load

Power factor correction reduces voltage/current phase difference

Unfortunately, the way many lighting manufacturers implement inductive loads severely reduces the efficiency of the lighting system. In many cases, they just don't realize that power factor correction can solve these problems in an easy and inexpensive way.

By its nature, an inductive load converts the phase of voltage and current into each other. In particular, the resulting inductive reactance is in anti-phase with the resistance of the system. This phase difference reduces the efficiency of the system.

The power factor (PF) is the ratio of the real power of the system (Real Power) to its apparent power (Apparent Power). The apparent power is the desired system power, and the actual power is the actual power. Depending on the application, the reversed-phase system is the least efficient and can be reduced to 60%.

The goal of power factor correction is to minimize the phase difference between voltage and current. Capacitance can be used to bring the inductance back into the only phase of the resistor in the system. Only capacitors with the correct characteristics are needed, that is, there is a high power ratio and an inverse of 180 degrees with the inductance (Figure 1).

Power factor correction benefits

There are many advantages to applying PFC to lighting systems, as explained below:

. Efficiency improvement

Depending on the application, the efficiency of adding PFC to the lighting system can be as high as 80∼95%. As public utility costs soar, this will allow PFC-based lighting systems to attract a large number of end customers.

. Easy to install

As long as there is a capacitor, the PFC can be introduced into the lighting system. Please note: A surge current limiter is also required to avoid damage to the system from the initial capacitance of the capacitor when it is turned on.

. 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 small power supply. The need to carry less current represents the need for smaller and less expensive generators, conductors, transformers and switches, thus streamlining the body and saving material costs.

. Stability improvement

Efficient systems consume less heat, allowing the system to maintain stable operation over an acceptable temperature range.

. Distinctive features

Whether your design is a stand-alone product or a part of a larger system, higher power efficiency drives a higher-level system than a less efficient system.

. Low operating cost

For large-scale lighting applications, the high efficiency created by PFC can save substantial money on utility costs.

. Industrial power

As early as more than a decade ago, power factor correction became mandatory in Europe, China, and Japan. Although the adoption rate of PFC in the United States is not high, it is continuously applied to more and more applications, especially lighting systems. Obviously, PFC makes sense and will eventually be used by applications that are not currently available. PFC is expected to be the company of its future needs and will benefit from PFC today as one of its distinguishing features. Manufacturers who cannot provide PFC will soon find themselves uncompetitive.

The lighting industry continues to push inductive loads, and the trouble is that the inductive reactance generated by the system is reversed, which reduces the efficiency of the system. PFC solves the above problems. However, when the PFC is initially charged, it will generate a surge current that damages other circuits in the system, and the use of the thermistor can effectively suppress the surge current and prevent the circuit from being damaged.

Suppressing surge current thermistor is cheap and easy to use

When the PFC capacitor is initially charged, it will generate the maximum current that the system can withstand. This brief surge current may be much higher than the operating current of the system, and depending on the lighting application, other circuits in the system may be damaged. To avoid such damage, a circuit that limits the surge current is required.

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

The most efficient way to handle surge currents is to use a thermistor. The thermistor is a special type of variable resistor whose resistance depends on the temperature. For example, negative temperature coefficient (Negative

Temperature Coefficient, NTC) Thermistor, which can greatly and predictably reduce resistance when temperature rises.


To limit the surge current, place the NTC thermistor between the power supply and the PFC capacitor and the inductive load capacitor (Figure 2). When turned on, the NTC thermal resistance temperature is low, so it can provide high resistance. In addition to limiting the current entering the capacitor, the thermal energy generated by this high resistance will increase the temperature of the thermistor.


NTC热敏电阻护航照明系统有效限制涌浪电流


Figure 2 Adding an NTC thermistor to limit the surge current


When NTC is automatically heated, its resistance drops rapidly. While the surge current tends to be steady, the temperature of the NTC thermal resistor is sufficient to minimize the resistance and allow current to pass without adversely affecting system operation or efficiency. As a result, NTC thermal resistors effectively provide the resistors needed to limit surge currents while eliminating the need for additional circuitry, such as bypass switches.

The durability of NTC thermistors must be quite high, with an effective operating range of -50 ° C ∼ 250 ° C. Currently, circuit protection component manufacturers have realized the transition to 277V and developed thermistors for such higher voltage levels for lighting applications, while providing UL and CSA-certified thermistors for the industry, Power efficiency due to resistive thermal energy can be minimized.

NTC thermistors for lighting applications range in price from $0.15 to $0.90. Compared to resistors that sell for more than $0.50 to $1, NTC thermal ohms are rated to handle a large amount of current in a lamp ballast. The price of the resistor also needs to be limited by the surge current, and the circuit used to bypass the resistor is considered.

Power factor correction is extremely easy and the installation price is low. In terms of efficiency, PFC is an inevitable new choice for many inductive lighting applications, even if the original design does not require the use of PFC. With a negative temperature coefficient thermistor, the lighting equipment can protect the lighting system from the surge current associated with the PFC without the need for complicated and expensive bypass circuits.


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