The requirements of the power-on pulse on the selection of LED lighting fuses

There are many factors to consider when choosing a fuse for an LED lighting power supply, but one of the most important factors is the power-on pulse caused by the moment the power is turned on. If the fuse is not selected properly, the power-on pulse will break the fuse, causing the light to not light up.

1. Why is there a power-on pulse?

Since LED lighting power supplies are generally switching power supplies, and switching power supplies generally use larger capacitors for filtering, a larger current spike is formed due to the charging of the capacitor at the moment of power on.

Theoretically, there are three factors that affect capacitor charging:

2. Which components affect the power-on pulse?

Since the LED lighting circuit is similar before the rectifier bridge, the influencing factors are analyzed one by one for the LED lighting input circuit:

Fuse, PCB trace, rectifier bridge, these three parts only affect the parameter R. If the total impedance increases, the Peak value will be reduced and the pulse width will be increased.

MOV, no effect on

X capacitor, will affect C and R, but compared with the filter capacitor, the X capacitor is often very small and can be ignored. So it can also be considered to have no effect. Inductor, it has impedance and inductance; so on the one hand, it affects the Peak of the pulse, and on the other hand, it also plays a role in extending the pulse. In actual use, the impact is relatively large.

Filter capacitor, due to the existence of capacitance and ESR, the size and type of filter capacitor are different, and the Peak and actual width of the power-on pulse are also different. Filter capacitor is the power supply component with the greatest impact on the power-on pulse in actual use.

PFC: Circuits with PF value requirements often use smaller filter capacitors, and there will also be compensation circuits, so the power-on pulse is often very small. The US Energy Star has PF value requirements for LED lighting fixtures >5w; countries or regions that adopt IEC standards have requirements for 25w or more.

3. Test of actual power-on waveform

Since the grid voltage is a sine wave, the power-on pulse of the power supply varies with the phase, so we choose to test it under the condition of 90° phase.

1. As the capacitance increases, due to the combined influence of ESR and capacity, the power-on pulse Peak increases and the pulse width also increases.

2. After using color-coded inductors, the shape of the waveform is changed due to the combined effect of impedance and inductance, and the Peak current value of the pulse is reduced.

4. How to ensure that the fuse is turned on continuously?

Each time a pulse strikes the fuse, the fuse will be damaged to a certain extent. After repeated impacts, the fuse will melt. The following is a table showing the number of impacts that AEM products can withstand:

If you want to ensure that the fuse can withstand 100,000 power-on pulse shocks without breaking, then you need to select a fuse with I2t>5 times the power-on pulse energy. Taking the pulse waveform of a 2.2uF capacitor without inductance as an example, the power-on pulse energy is calculated as (37.5)2*0.000019*1/2=0.013. As long as you choose a fuse with I2t>0.065 (0.013*5), you can ensure that the fuse can withstand 100,000 pulses under cold startup. If you consider the hot startup situation, you need to consider the temperature reduction factor. Taking the AEM product as an example, at 105℃, it is only reduced by about 80%. In other words, if you want to achieve 100,000 hot startups without breaking the fuse, you only need to choose a fuse with I2t>0.081 (0.065/80%).

5. From the perspective of the startup pulse of LED lighting, both fast-break and slow-break fuses are applicable.

Many engineer friends tend to use slow-break fuses because of their usage habits. In fact, for LED lighting, both fast-break and slow-break fuses can meet the requirements. As long as you pay attention to ensuring that the fuse's I2t is 6.5 times the startup pulse when selecting the fuse, you can ensure that the hot start-up is continuous for 100,000 times. There are

many types of fuses on the market that can meet the needs of LED lighting. Common ones include traditional glass tubes, ceramic tubes, plastic-sealed plug-ins, and other types. There are also 250Vac chip fuses such as the MF series launched by AEM. If engineer friends encounter pressure on the use of space and production process costs, they can choose chip products.