How to choose suitable AC fuse LED lighting design?

Since the first indoor LED lighting version was developed, designers are faced with the technical challenges of all kinds, including AC to DC power conversion, thermal design, the current size is limited, the lighting electrical transient physical space, the basic challenge plus drive LED light.

One of the most important challenges which is upstream of LED and circuit of all active and passive components provide transient protection. These transients are usually due to lightning induced surge AC input. These threats means that the LED lamp requires overcurrent and overvoltage protection. The function of increasing demand and higher light output increases the number of components on the LED board. High light output requires a large heat sink, produced by high power LED heat dissipation. The LED light bulb shape with the incandescent lamp and CFL lamp (such as home A19 lighting lamps in common) compatible, including a AC / DC power supply circuit, so that they can work in a standard bulb socket (Figure 1). Anything is directly connected to the AC power supply circuit and overload conditions may be caused by internal components of lighting and / or circuit fault and damage. In addition, lightning surge or load switch transient (from external lighting) may produce voltage spikes or ring wave, the voltage spikes or annular wave will generate pressure and eventually damage the bulb internal components.

Figure 1: the structure of a typical household LED lamp.

The basic unit circuit block as shown in figure LED is shown based on the lamp 2, (right to left):

The single chip of LED series (LED Series)

The LED series driving circuit with protection circuit of LED controller for the corresponding string

The DC-DC converter circuit

The circuit rectifier circuit

The EMI filter assembly

The AC input circuit is composed of a circuit varistor series AC fuse and a line to neutral. Is the main AC fuse bulb current protection device. When the correct choice of all the necessary design parameters, connecting the fuse through safely disconnect all input and AC line circuit, can fully protect all downstream components from electrical overload induced transients and short-circuit / overload condition (EOS) damage. Considering the LED lighting space is narrow, so the need to enter the selection of highly compact AC AC fuse. The function of a fuse is to provide protection for components and complete circuit by melting in current overload conditions. In other words, the fuse is intended to become a weak link in the circuit. AC fuse in series with the AC line input will provide short circuit protection and overload protection. Today, AC fuse can provide minimum dimensions, can provide a variety of ampere rating and selection of rated voltage. A series of key fuse additional parameters and surface mount design is also available for design engineers.

LED lighting designers need to consider various important issues, in order to select the suitable AC fuse:

This is the first step to understand some of the basic information, such as the normal operating current, lamp application voltage, ambient temperature, overload current level and fuse must open the length of time, the maximum allowable fault current and inrush current, inrush current, pulse, starting current and transient circuit etc..

Very early in the design, it is necessary to know the light bulb at which the market or what the market sales. Because different countries or regions, such as the United States, North America, Europe and Asia, different standards will determine the requirements of design and test.

Determine the size limit. The fuse can use a variety of methods to package, but the surface mount design is the most common form of LED lighting applications. Fortunately, for circuit designers, the fuse can now use a smaller size to protect the AC input, this is just the minimum fuse size before using the half.

* produced by fuse current temperature as the ambient temperature changes increase or decrease. Please note that the fuse "temperature" and "room temperature". On the contrary, the ambient temperature fuse refers to the temperature of the surrounding air, usually much higher than room temperature. For 25 degrees around the ambient temperature, the rated current of the fuse is generally recommended not to exceed the rated current of 75%. The fuse is essentially a temperature sensitive device, so even if the temperature change is very small, life expectancy will greatly influence the fuse.

It determines the interrupting capacity of application needs. This is the maximum current safety fuses can be recognized under rated voltage interruption. In case of failure or short circuit, the fuse may receive an instantaneous overload current than the normal operating current many times. Safety requirements for fuse intact (no explosion or rupture and cut off the circuit).

* ensure that in the production before there is enough time for the application of a thorough test and verification. If the initial design did not pass the test, please make sure that the plan has enough room to modify the design and re test.

. finally, the designer must be overpressure protection and LED series drive circuit coordination with downstream fuse. At the beginning of LED lighting design process, must consider the transient suppression. The selected fuse must bear the prescribed transient energy level, which does not have a negative impact on the performance of the driving circuit on LED. AC input circuit fuse and the varistor in the correct coordination provide transient protection solutions, in the case of the fuse does not burn, have the ability to control the safety and protection of overvoltage, the downstream circuit, LED driving circuit on the minimum interference, including the LED string itself. (compiled: LED James)

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