Safety and reliability protection of residential LED bulb LED bulb is a solid-state lamp with semiconductor light-emitting diodes in parallel configuration. In addition, the lamp also includes power conversion Electronics (DC / AC), LED driver chip, heat sink for thermal control and optical devices to optimize light quality. Since LED bulbs need to be compatible with current incandescent and halogen bulbs in shape factors, they will have a DC / AC circuit to enable them to be used in standard bulb “sockets”

Equipment directly connected to an AC power source (e.g. 120 / 250vacrms) may be damaged due to short circuit and overload caused by internal component or circuit failure of the bulb. In addition, lightning surges or load switch transients (generated outside the bulb) can cause voltage spikes or ring waves, put pressure on the components and eventually lead to damage, resulting in the scrapping of the bulb. Since the value orientation of LED bulb is not only to reduce energy consumption, but also to have a longer service life, it is very important to consider transient voltage protection in eliminating field faults caused by electrical environment.Moreover, the LED is extremely sensitive to high temperature and shall not exceed 85 ℃. Controlling the temperature of LED and LED driver board below this level is very important for their service life.

Littelfuse can provide perfect AC line interface protection and transient protection at DC end. For an overview of the different components and systems inside the LED bulb power supply and control circuit, see Figure 1

Design of LED bulb power supply and control circuit

Figure 1: LED driver input line coordination AC line protection recommendations.

As shown in Figure 1, a Littelfuse AC fuse connected in series with the line can provide safety protection under short circuit and overload conditions. They have a wide range of choices in shape factors, current rating, rated voltage, breaking capacity and installation options, providing design flexibility for design engineers.

Metal oxide varistors (MOVs) on the entire power input provide overvoltage clamping protection. They are cost-effective and can minimize the transient power that may be connected to downstream electronic devices. The selection of metal oxide varistors is based on a series of parameters, including rated voltage, peak pulse current, device size and wire configuration. The varistors provided by Littelfuse have a device diameter as small as 5mm and surface mount ch series, which makes them available in the power supply part of LED bulbs with limited space.

The MOV and fuse need to form a coordinated solution so that the electrical energy required to fuse the fuse element (also known as the “i2t” value) is high enough to allow the MOV to function under surge voltage conditions without tripping the fuse. In other words, under the condition of surge energy clamping and dissipation, the fuse should not be opened. It shall only trip in case of overload or short circuit.

Even if there is a varistor in the AC input, there are examples where the circuit cannot be spared in surge events. In these cases, a TVS diode can be added to the input circuit of the inverter. Referring again to figure 1, this TVS diode provides secondary protection against transients. The electric energy “allowed to pass” from the varistor will be further reduced by the TVS diode to a level that will not fail the circuit.

Littelfuse offers a wide selection of fuses, metal oxide varistors and TVs diodes in terms of shape factors, space constraints, electrical parameters and cost.

Littelfuse’s fuses, metal oxide varistors and TVs diodes play a key role in helping lamp manufacturers pass some important regulations and safety standards. The United States is taking the lead in establishing uniform performance and safety standards for some indoor commercial lighting, as well as road, parking and garage lighting. The energy star surge specification of the U.S. Environmental Protection Agency (EPA) is applicable to PAR30 / PAR38 / A-19 / MR16 / Candlestick lamp / g spherical lamp / R reflective LED lamp. These specifications are discussed below.

● Energy Star program requirements for integral LED lamps (applicable to the United States) * refer to ANSI / IEEE c.62.41-1991, class a operation (83A).

*For common mode and differential mode, the line transient shall include 7 times of 100kHz ring wave and 2.5KV strike.

● IEC 61547 general lighting equipment – Electromagnetic compatibility immunity requirements (applicable to Europe, Asia and Canada) * refer to iec61000-4-5 Surge immunity test.

*Waveform: 1.2×50 voltage / 8×20 current combination; 2 Ω impedance; 10 shocks.

*For lamps lower than 25 watts: 500V / 250A; Line to line.

*For lamps greater than 25 watts: 1000V / 500A; Line to line.

Design of LED bulb power supply and control circuit

Figure 2: U.S. Department of energy (DOE) requirements for surge testing.

Usually, engineers want to test their application and protection scheme during the qualification process. For those lacking internal surge test equipment, Littelfuse’s laboratory facilities are available to verify their application design. This test can find problems as early as possible and correct them in time to avoid any field problems.

Safety and reliability protection of LED road and parking lot lighting

The United States is taking the lead in establishing uniform performance and safety standards for some indoor commercial lighting, as well as road, parking and garage lighting. The municipal solid state street lamp alliance of the U.S. Department of energy (DOE) has issued a template specification for LED road lamps used in all cities and municipalities, detailing specifications for performance, safety and surge immunity. The replacement and transformation efforts of these cities and municipalities directly under the central government will require a large amount of initial capital and early investment. At present, the cost of LED lamps is almost twice that of high-pressure sodium lamps at this stage. However, their service life is expected to be nearly 100000 hours, which is almost four times that of high-pressure sodium lamps. The goal of the U.S. Department of energy is to protect the LED lamp investment made by these cities by establishing strong safety and surge standards, so that these lamps can be maintained for at least 5 years without any maintenance.

In the characteristic description of low voltage (1000V or lower) surge in AC power supply circuit, the requirements of the U.S. Department of energy are similar to IEEE c.62.41-2002 and IEEE Recommended practices, with only some small differences. The category corresponds to the physical location of LED lamps, whether they are placed indoors or outdoors. For example, outdoor lamps in category C (high or low) are more vulnerable to lightning strikes, so the most stringent Surge immunity test is required. The relevant tests of these are summarized by category below.

Careful selection of circuit protection elements by design engineers is particularly important for passing the surge immunity waveform test recommended by the U.S. Department of energy. The surge energy (i2t) measured by A2S generated by pulse (such as 6kV / 3KA) is a key parameter. The withstand capacity of fuse pulse cycle and its fusing energy (i2t) considered shall be coordinated with the surge energy of waveform. The selection of metal oxide varistors shall also be based on surge current (e.g. 3KA), anti repetitive surge capacity, rated voltage and clamping voltage.

Littelfuse is committed to assisting LED lighting manufacturers to meet or exceed the requirements of these specifications by providing strong overcurrent and overvoltage collaborative protection schemes for all surge levels. Littelfuse has conducted a large number of tests on the recommended components and in most cases implements more stringent standards than those required by the above specifications. Littelfuse’s fuses, metal oxide varistors, TVs diodes and other solutions meet the required test standards and will improve the reliability of overall lighting fixtures.

The adoption rate of LED lighting market is expected to soar worldwide in the next few years, bringing a “bright” prospect for this technology. As a global leader in circuit protection, Littelfuse is committed to providing application support, LED lighting sample package, standard interpretation and testing to ensure that our customers meet the requirements of end customers and prosper.

Responsible editor; zl

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