Detailed explanation of lightning protection requirements for outdoor LED driver power supplies

Outdoor lighting has been widely used in many demonstration projects at home and abroad. Due to its harsh use environment, the design of outdoor LED lighting driver power supply must focus on evaluating issues caused by factors such as waterproofing, high temperature, lightning strikes, etc. This article will focus on the lightning protection requirements.

　　Lightning strikes are a common natural phenomenon, especially in the rainy season. The damage and losses caused by lightning strikes are estimated at hundreds of billions of dollars each year worldwide. Lightning strikes are divided into direct lightning and indirect lightning. Indirect lightning mainly includes conducted lightning and inductive lightning. Since the energy impact brought by direct lightning is very large and the destructive power is extremely strong, ordinary power supplies cannot withstand it. Therefore, this article will discuss indirect lightning, including two types of lightning strikes: conducted lightning and inductive lightning.

　　The surge impact caused by lightning strike is a transient wave, a transient interference, which can be a surge voltage or a surge current. It is transmitted to the power line along the power line or other paths (conduction lightning) or through the electromagnetic field (induction lightning). Its waveform characteristic is that it rises rapidly first and then slowly falls. This phenomenon will have a fatal impact on the power supply, because the instantaneous surge impact it generates far exceeds the electrical stress of general electronic devices, and the direct result is damage to electronic components .

　　If it is a smart power supply, even if the transient surge impact does not cause damage to the device, it may interfere with the normal operation of the program, thereby issuing erroneous instructions, causing the power supply to fail to work as expected. This also explains why surge (impact) immunity belongs to the category of electromagnetic compatibility in safety certification.

　　For outdoor LED lighting driver power supply, the use environment determines that lightning protection is an important indicator to measure its performance. Therefore, lightning protection design must be considered for outdoor LED power supply. Figure 1 shows a typical lightning protection circuit at the AC input end of a power supply. Its main mechanism is to absorb the transient energy caused by lightning strike or discharge the energy to the ground through a predetermined path, thereby avoiding impact on the back end of the power supply.

　　The design includes a secondary protection circuit. The primary protection circuit consists of a fuse FUSE, a varistor MOV1, and a gas discharge tube AR3; the secondary protection circuit consists of varistors MOV2 and MOV3, and gas discharge tubes AR1 and AR2; the functions of the secondary protections are different. The primary protection circuit is mainly responsible for handling differential mode surges and instantaneously absorbing the differential mode surge voltage of the power port. The secondary protection circuit is mainly responsible for handling common mode surges. Through the effective absorption of varistors and lightning arresters and the energy discharge to the ground, the surge impact is greatly reduced after passing through this level of protection circuit.

　　Lightning protection is a commonly used term. Translated into the requirements for power supply, it means that the power supply must have the ability to prevent surge impact. LED driver power supply safety certification requires surge (impact) immunity test to ensure that the power supply has a certain anti-interference ability. The domestic standard is GB/T17626.5 "Electromagnetic compatibility test and test technology surge (impact) immunity test", which is equivalent to IEC61000-4-5: 2005.

　　LED driver exists as a component of LED lamps. When the LED driver and LED module are placed separately, the output port of the LED driver and the input port of the LED module must meet the requirements of this standard: they can withstand surge impacts of 2kV between lines and 4kV between lines and ground (i.e. 2KV for differential mode and 4KV for common mode). The same requirement also applies to the AC power port of LED lamps.

　　At present, it is still a difficult problem for outdoor LED driver power supply to protect against lightning strikes. Of course, with the development of electronic technology, it is not a big challenge from a technical perspective. It is just that LED lamps have requirements and restrictions on the overall size of the power supply. It is not so easy to design a power supply that meets the lightning protection requirements in a limited space. Generally, the current GB/T17626.5 only recommends that products meet the standards of 2KV differential mode and 4KV common mode. In fact, this specification is far from meeting the actual requirements, especially for special conditions such as ports and docks, factories with large electromechanical equipment around, or areas prone to lightning strikes.

　　In order to solve this contradiction, many engineering companies often solve it by adding an independent surge suppressor. By adding an independent lightning protection device between the input and the outdoor LED driver power supply (not limited to LED driver power supply, but generally refers to electronic equipment, the same below), the threat of lightning strikes to the outdoor LED driver power supply can be resolved, thereby greatly ensuring the reliability of the power supply. It is necessary to point out that for independent lightning arresters, the reference standard is different from the integrated power supply lightning protection standard. The reference standard is IEC61643-1 or EN61643-11. Independent lightning arresters are generally composed of multi-level protection circuits. In order to protect the power supply equipment of the subsequent stage, it must ensure that the residual voltage after its multi-level protection is less than the surge impact voltage that the subsequent power supply can withstand. The smaller the value, the better, generally required to be less than 1.5KV, and the value of the better designed products can be lower than 0.8KV.

　　In fact, in addition to being struck by lightning, the power supply will also generate very high instantaneous overvoltage (or overcurrent) when connecting and disconnecting inductors, capacitive loads or large loads, switching of power systems, resonance phenomena associated with switching devices, and various system failures such as short circuits and arc faults in the system combination to the grounding system. For example, at the moment of turning on a high-power drive power supply, especially under cold start conditions, a large surge current will flow into the power supply equipment. This is because there are many capacitors for different purposes in the power supply, especially high-voltage and large-capacity electrolytic capacitors used after the PFC (power factor adjustment) line. The voltage is very low before starting the machine, and the capacitor is quickly charged in a very short time after the mains is connected. The peak current is much larger than the input current under steady-state conditions.

　　If it is turned on at the same time when the input voltage is at a phase angle of 90 degrees, the inrush current will be even greater.

　　How to ensure that the outdoor LED driver meets the lightning protection requirements, in addition to designing the corresponding lightning protection circuit, the stress capacity of the components should be fully considered, especially the varistor, discharge tube, rectifier bridge, fuse, EMI filter and other devices at the power input end, and the possible surge level should be fully considered. At least the basic requirements of relevant standards such as GB/T17625.6 (equivalent to IEC61000-4-5:2005) or IEC61643-1 or EN61643-11 should be met.

　　In addition, there are some matters worth noting in the correct installation and use of the driver power supply. For example, the power supply must be reliably grounded to ensure that the impact energy has a fixed path to be discharged; use a dedicated line to supply power to the outdoor driver power supply, avoid large-scale electromechanical equipment around to avoid surge impact when the electromechanical equipment is started; reasonably control the total load of the lamps (or power supplies) on each branch to avoid surge impact caused by excessive load at the moment of startup; reasonably configure the switch, and open or close it step by step. These can effectively avoid the impact of operational surges, so that the LED driver power supply can work more reliably.