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# An important indicator for evaluating the performance of LED power supply supplies-power factor PFC

2020-09-08

A very important parameter of LED power supply-power factor (PFC) is often easily overlooked by people. What is power factor and what is the significance of power factor?

1. Power factor, power factor characterizes the ability of a lamp to output active power.

In the AC system, part of the AC current circulates in the load without transmitting electrical energy. It is called reactance current or harmonic current, which makes the apparent power (voltage Volt times current Amps) greater than the actual power. The difference between apparent power and actual power leads to power factor, which is equal to the ratio of actual power to apparent power. Therefore, the actual power in the AC system is equal to the apparent power multiplied by the power factor. That is: power factor = actual power/apparent power. Only linear loads such as electric heaters and light bulbs have a power factor of 1. The difference between the actual power and the apparent power of many devices is very small and can be ignored, while the difference between capacitive devices such as lamps is very large ,Very important.

2. Apparent power is the product of AC voltage and AC current. The formula is expressed as: S=UI. In the formula, S is the rated output power, the unit is VA (Volt-Ampere); U is the rated output voltage, the unit is V, such as 220V, 380V, etc.; I is the rated output current, the unit is A. Apparent power includes two parts: active power (P) and reactive power (Q). Active power refers to the part that directly does work. For example, make the lamp light up, make the motor rotate, make the electronic circuit work, etc. Because this power becomes heat after doing work, it can be directly felt by people, so some people have the illusion that the active power is regarded as the apparent power. Don’t know that the active power is only a part of the apparent power, expressed by the formula : P=Scosθ=UIcosθ=UI•F. In the formula, P is the active power, the unit is W (watts); F=cosθ is called the power factor, and θ is the phase difference when the voltage and current are in different phases when a nonlinear load is applied. Reactive power is the part of power stored in the circuit but not directly doing work, expressed by the formula: Q=Ssinθ=UIsinθ. In the formula, Q is reactive power, and the unit is var (worry).

For lamps and all other electronic circuits that work on DC voltage, they cannot work at all without reactive power. General users think that equipment such as lamps only needs active power, not reactive power. Since reactive power does not do work, what is the use of it! So of course they think that a lamp with a power factor of 1 is the best. Because it can give the maximum output power. However, this is not the case.

If there is a lamp that is rectified after the AC mains is input, a pulsating DC voltage is obtained. If the pulsating voltage is not processed, it is directly supplied to the lamp. There is no doubt that the circuit cannot work normally at all. Although the power factor of the lamp is close to 1, what is the use? In order for the lamp circuit to work normally, it must be provided with a smoothed DC voltage. This "smoothing" work must be completed by the filter capacitor C connected to the lamp rectifier. This filter is like a reservoir. A sufficient amount of charge must be stored in the capacitor to keep the working voltage on the circuit uninterrupted and maintain a normal level when the gap between the rectification half-waves is maintained. In other words, even when there is no input power between the two pulsating half-waves, the voltage level of Uc does not change significantly. This function is realized by the energy storage in the capacitor, and this part of the energy stored in the capacitor It is reactive power. Therefore, the lamps rely on the support of reactive power to ensure that the circuit can correctly use the active power to achieve normal use. Therefore, lamps require not only active power, but also reactive power, both of which are indispensable.

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