Circuit Protection Design Analysis of Five Common LED Lighting Applications

In the past few years, LED lighting technology has become more and more mature, which has cleared away all kinds of obstacles for it to become lighting equipment. LED lighting was once the favorite of scientists and "early adopters" users, but now it has become a "mainstream product". This technology is expected to make people bid farewell to soaring electricity charges and cold fluorescent lamps, and realize long life. The replacement frequency of LED lamps is so low that guidance may be needed to replace them correctly! But does LED lighting really fulfill these commitments?

When visiting a newly renovated building earlier this year, the owner introduced in detail some characteristics of LED lamps used to provide general lighting in the factory workshop. His statement is a bit similar to "they are really good lamps - they have a seven-year warranty". There was no doubt in his voice, but he did not reply when asked about the maintenance / replacement cost if the lamp could not last for such a long time. What if these LEDs fail prematurely?

Let's take a look at five types of LED lighting applications with broad market prospects, and discuss the threats to the lighting system and various circuit protection strategies that can maximize reliability and "strengthen" your design.

This paper focuses on those valuable circuit protection parts. Of course, safety rating agencies (UL, ETL, etc.) will require minimum fire and personnel safety measures. The real purpose of this article is how to cost effectively prolong the service life of protected equipment (rather than those problems solved by security personnel). The real value of lamps exceeds their retail price (although this is certainly a factor), including valuable functions (which may put people or other equipment at risk in case of failure), inaccessibility (the proximity cost during maintenance or replacement may be one order of magnitude higher than the cost of the equipment itself), or reliability is a hot competition among suppliers in the target sales market.

AC input general lighting

The block diagram of Figure 1 shows a typical industrial lighting fixture. The third module represents the transformer free rectifier (also known as bridge rectifier, abbreviated as BR) of AC circuit, power factor correction (PFC) circuit, and a capacitor bank included in many industrial lighting devices, which can smooth 120Hz pulses to an acceptable level. Fire safety also requires adding a fuse at the input end to protect key electronic components from excessive current at the input end caused by their failure. This way of adding overvoltage protection in the overall design helps to ensure that the fuse will not blow. Diodes in bridge rectifier (BR) are often the first damaged components of voltage surge.

Without input overvoltage protection, the AC input surge will find a path into the DC / DC voltage current converter module, and the switching electronics in the converter are just responsible for converting the high DC voltage input into the current available to the LED. The rated voltage of these electronic devices in the circuit can be as high as hundreds of volts, but it is easy to be destroyed by a single voltage surge exceeding its rated value - in fact, thousands of volts peak voltage can be caused when nearby lightning strikes or industrial motors are cut off.

Setting up a suitable high-capacity metal oxide varistor (MOV) device network at the AC input is a solution to limit voltage surge. After each surge absorption, MOV devices will age, so there must be room for design. In street lamp applications, electronic components are basically attached to lightning rods. It is generally recommended to combine mov devices with line voltage silicon TVS diode devices. Littelfuse ak10-170c is one of the choices of 120V line voltage. TVs devices provide accurate voltage clamping and will not age, but TVs has the disadvantage of high cost. If the maintenance cost is expected to be very high and professional service vehicles and technicians need to be called, the additional cost caused by these circuit protection designs is usually considered reasonable. If the equipment is still under warranty, these high maintenance costs may be borne by the manufacturer.

In some designs, in fact, the LED itself is far away from the corresponding driving circuit. This helps to keep the heat generated by the led away from the electronic driver. In some street lamp applications, the electronic devices are located on the base and the LED is on the lamp pole. In these designs, it is appropriate to provide additional TVs devices to protect the LED lamp string from induced high voltage, because the long wire from the driver circuit to the LED will have antenna effect (see the TVs module after DC / DC conversion in Figure 1).

The wire bond LED structure is connected to a long high-voltage string through leads. This structure design has an additional failure mode, that is, thermal cycle or mechanical vibration may cause lead bonding fracture, resulting in open circuit of LED circuit. The solution is to use the LED open circuit bypass device (Figure 1) to bypass the LED with open circuit fault to keep the rest of the lamp string working. Without these devices, an LED that fails as an open circuit will extinguish all led strings. Some assurance measures allow a certain proportion of LED lamps to fail, as long as the lamps can maintain the specified minimum brightness output.

For more protection, please pay attention to our next article.

Editor's note: what is bridge rectifier?

Bridge rectifier is the most commonly used circuit for rectifying by using the unidirectional conductivity of diodes. It is often used to convert alternating current into direct current.

The working principle of bridge rectifier circuit is as follows: when E2 is positive half cycle, apply forward voltage to D1 and D3, and DL and D3 are on; Apply reverse voltage to D2 and D4, and D2 and D4 are cut off. The power on circuits E2, D1, rfz and D3 are formed in the circuit, and the upper positive and lower negative half wave rectified voltage is formed on rfz. When E2 is the negative half cycle, the positive voltage is applied to D2 and D4, and D2 and D4 are turned on; Apply reverse voltage to D1 and D3, and D1 and D3 are cut off. The power on circuits E2, D2, rfz and D4 are formed in the circuit, and the other half wave rectifier voltage with upper positive and lower negative is also formed on rfz. As a result, a full wave rectified voltage is obtained on rfz. The waveform diagram is the same as that of full wave rectification. It is not difficult to see from figure 5-6 that the reverse voltage borne by each diode in the bridge circuit is equal to the maximum value of the secondary voltage of the transformer, which is half smaller than that of the full wave rectifier circuit. Bridge rectifier is an improvement of diode half wave rectifier.

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