Commonly used devices for preventing surge or transient immunity include gas discharge tubes, metal oxide varistors, silicon transient voltage absorbing diodes, and solid discharge tubes, and combinations thereof. The LED street light lightning protection circuit and its device are generally integrated with the LED control device, and a combination of a gas discharge tube and a varistor is commonly used.

First, the suppression circuit principle composed of a combination of a gas discharge tube and a varistor

Since the varistor (VDR) has a large parasitic capacitance, it is used in an AC power system, and a considerable leakage current is generated. After using the varistor with a poor performance for a period of time, the leakage current may become hot and may self-explosion. To solve this problem, a gas discharge tube is inserted between the varistor. In Figure 1, the varistor is placed in series with the gas discharge tube. Since the parasitic capacitance of the gas discharge tube is small, the total capacitance of the series branch can be reduced to a few pF. In this branch, the gas discharge tube will act as a switch. When there is no transient voltage, it can separate the varistor from the system, leaving the varistor with almost no leakage current. However, this has the disadvantage that the reaction time is the sum of the reaction times of the devices. For example, the reaction time of the varistor is 25 ns, and the reaction time of the gas discharge tube is 100 ns. The reaction time of R2, G, and R3 in Fig. 2 is 150 ns. To improve the reaction time, the R1 varistor is added, so that the reaction time is 25ns.

The voltage-current characteristics of the metal oxide varistor (MOV) are shown in Figure 3. The metal oxide varistor (MOV) characteristics are shown in Table 1. The voltage-current characteristics of the gas discharge tube (GDT) are shown in Figure 4. The characteristics of the gas discharge tube (GDT) are shown in Table 2.

Due to the surge interference, once the voltage applied across the gas discharge tube exceeds the spark discharge voltage (u1 in Fig. 4), the gas inside the discharge tube is ionized and the discharge tube begins to discharge. The voltage drop across the discharge tube drops rapidly to the glow discharge voltage (u2 in Figure 4) (u2 is 140V or 180V in Table 2, depending on the characteristics of the tube itself), and the current in the tube begins to rise. As the discharge current is further increased, the discharge tube enters an arc discharge state. In this state, the voltage across the tube (arc voltage) drops very low (u3 in Figure 4) (u3 is 15V or 20V in Table 2, depending on the characteristics of the tube itself), and the arc voltage is quite wide The current variation range (from the i1 → i2 process in Figure 4) remains stable. Therefore, the external high-voltage surge interference is resolved into a protected condition of low voltage and large current (u3 and i2) due to the discharge of the gas discharge tube, and this current (from i2 → i3 in Fig. 4) The gas discharge tube itself flows back into the interference source, eliminating the possibility of interference to the luminaire. As the surge overvoltage subsides, the current flowing through the gas discharge tube drops below the minimum required to maintain the arc discharge state (approximately 10 mA to 100 mA, which is related to the characteristics of the tube itself), and the arc discharge stops and again After the glow discharge state, the entire discharge state (arc stop) is ended.

Second, the insulation withstand voltage problem common to LED street lamps with anti-surge interference function

1. The status quo of the lamp withstand voltage problem

In the LED street lamp using the combination of the above gas discharge tube and varistor to prevent surge interference, the insulation withstand voltage problem is that the contact between the live parts of the lamp and the metal part cannot withstand 2U+1000(V). The voltage of the insulation is usually breakdown at around 600V. The root cause of the insulation withstand voltage problem is the unreasonable selection of the withstand voltage parameters of the gas discharge tube. It is not so much the insulation withstand voltage problem of LED street lamps, but rather the insulation withstand voltage problem of LED control devices. Because the surge protection circuit is usually located in the LED control unit. The LED control device with anti-surge interference function shall comply with GB 19510.14-2009 "Lighting control device Part 14: Special requirements for DC or AC electronic control devices for LED modules" and GB19510.1-2000 "Light control devices" Part 1: General requirements and safety requirements.

2. Problems and answers to the problem of withstand voltage

Manufacturers often question the test method for insulation withstand voltage, and believe that the series circuit of the gas discharge tube and the varistor should be disconnected during the insulation withstand voltage test.

The doubts mainly come from the following two reasons: Reason for question 1: GB 7000.1-2007 "Lamps - Part 1: General requirements and tests", "10.2 Insulation resistance and electrical strength" stipulates: "When performing these tests, the following parts It should be disconnected so that the test voltage is applied to the insulation of the component, not to the capacitive or inductive functional components of these components: a) bypass connected capacitors; b) capacitors between live parts and the lamp housing; c ) Chokes and transformers connected between live parts.) Should L-R2-G-PE or N-R3-G-PE in Figure 2 be disconnected?

For the above parts of GB7000.1-2007, 10.2, the parts to be disconnected during the test refer to the additional capacitors or components in the lamp lighting circuit, and do not include the components in the LED control unit (component). The electrical strength test of the LED control device is based on the requirements of GB 19510.14-2009 and GB19510.1-2000. When the luminaire is tested for electrical strength, the capacitors or components in the control unit should not be disconnected.