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THYRISTORS HIGH SURGE LEVEL PROTECTION FOR SMART LED STREET LIGHTING

LED smart street lights are growing each year as replacements of incandescent or gas lights. According to ABI research, world’s LED street light installations are progressing from 8 per cent of total lights in 2013 to a forecasted share of 55 per cent in 2025. In India, as per EESL, the potential energy consumption savings on municipal street lighting – switching to LED – is more than 8 terawatt hours. From this assessment, the Indian government has started in 2015 the Street Lighting National Program (SLNP), to replace old lamp technologies to low consumption LED lights ranging from 18 to 190 W. Today a total of 8 million units has been replaced in the country and more than 35 million units will be installed in the next few years. Mission profile defined by SLNP demands a high-quality LED design, with a minimum 50,000-hour life span. The manufacturer has to give warranty for 7 years. To achieve this reliability, several conditions for safety and electric compliance have to be passed. One of the biggest challenges is overvoltage surge withstanding. The minimum mandatory level is 4 kV, required by the Bureau of Indian Standard for classic LED driver, as defined in IEEE C64.41.2-2002. Keeping in mind that street lights are deployed under very harsh environment which is very prone to lightning strikes on the Line, as well as the life expectancy is quite high, so, the driver should be designed to sustain the stresses it goes through. Typically, 6 or 8 kV surge test is the key recommendation to enhance the reliability of the LED power supply. STMicroelectronics proposes Thyristors SCR-based protection, with improved safety, reliability, efficiency and accuracy versus state-of-the-art solution.

Classic strategy in case of overvoltage surge is the crowbar function. Usually, with Metal Oxide Varistors (MOV), the overvoltage is clamped at a specific voltage level, allowing to protect the DC downstream of the application, for example, the MOSFETs of the converter are mainly able to withstand maximum 800 V. MOV offers limited reliability in term of maximum acceptable power and repetition. Indeed, MOVs surge capability is specified with a de-rating curve of maximum current and number of surges (ageing effect). Above this limit, MOVs show signs of electric failure, with degradation of leakage current and clamping voltage. Once the MOV clamping voltage is below the AC input line voltage, the MOV is in permanent conduction, leading to device burning. Thereby, the MOV is not recommended in application where high number of high-level surges could occur. Furthermore, the high cost of street light module replacement is of course a good incentive to improve this performance.

To face the above challenge, STMicroelectronics proposes a circuit able to avoid any risk of MOV burning for safety purpose – among others advantages. The Figure 1 shows the circuit diagram of the solution based on High temperature SCR from STMicroelectronics TN5015H-6I SCR, 50 A / 600 V. In this circuit, the SCR triggers the protection, a Transil diode controls protection voltage threshold level, and the MOV absorbs the surge energy. All are on-the-shelves devices of power electronics.

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Figure 1: High level surge protection circuit (left part) and operation principle (right part)

The operation principle is the following: when a lightning surge is applied to the AC Line, for example a standard 1.2/50 us voltage wave shape, the voltage is rising quickly across the application DC bus. While the DC voltage stays below the break over voltage of the Transil, the SCR remains in OFF-state. Once the overvoltage reaches the breakdown voltage of the Transil, the current flows through it and the SCR gate. As soon as this current is equal to the SCR minimum required gate current IGT (15 mA for the TN5015H-6I), the SCR is triggered in ON-state. Then, the overvoltage is applied to the MOV which is able to absorb the extra power from the surge. The overcurrent induced by the stress, defined as a standard 8/20 us current wave shape, flows through the MOV and the series SCR.

To withstand such a high current from the stress, the SCR offers a very good robustness to overcurrent, with a specific specified parameter called IPP; for example, IPP = 1500 A for an 8/20 us current shape for the TN5015H-6I. Once the overcurrent is reaching zero crossing, the SCR is automatically turned off thanks to its holding built-in function.

The Figure 2 highlights the IPP overcurrent surge capability of the SCR TN5015H-6I. The waveform (left part) is the 8/20 us waveform of current through a TN5015H-6I under test. The maximum gate current in this case is 350 mA (zoom at SCR triggering in the right part) and the maximum non-repetitive rate of current (dI/dt)OC is 250 A/us. The device safely withstands the current stress without any risk.

Figure 2: IPP specification of the SCR TN5015H-6I

Benefits of such a solution are the safety and the reliability of the system, which can then fulfill the life span requirement. In the same time, the solution improves the overall standby efficiency of the system and the accuracy of the protection.

Safety improvement has been previously demonstrated. If the MOV fails, i.e. if its clamping voltage is abnormally low, there is no risk to have MOV burning as the SCR in series is in OFF-state, blocking the input voltage, then there is no current through the MOV. In case of next surge occurrence, the SCR will be triggered by overvoltage through the Transil, and the protection will be ON for only a line half-cycle until the next current zero crossing. It means a maximum conduction of few milliseconds, not enough to burn the MOV.

Moreover, regarding reliability improvement, in case one of the three devices of the circuit is failed separately in short-circuit or in open circuit, the application is still working, ensuring lighting main function to fit the reliability requirement of 50 000 hours of life span.

The solution allows a better standby efficiency of the application. Higher is the surge level requirement, the higher is the MOV power ratings. High MOV power ratings induce high leakage current, which impact the stand-by losses of the LED module. Typical leakage current of a 14 mm / 391 V MOV is 1.6 µA, when applying a 320 V RMS input voltage. This leakage current can increase a lot with ageing and number of surges as explained previously. Thus, adding a series SCR allows to limit the leakage current in the MOV at 0.1 µA in the same conditions. It means 16 times less stand-by losses. Of course, as the SCR is a solid-state silicon device, leakage current is more stable and predictable with time than amorphous-state MOV.

Finally, the accuracy of the protection is well set-up as the maximum voltage across the LED DC downstream is the breakdown voltage of the Transil before SCR triggering, for example 603 V with a 0.8 A current for BZW04-376 Transil from STMicroelectronics. The maximum voltage across the downstream after SCR Triggering is the MOV clamping voltage at the overcurrent level. In any case, the voltage well manages accurately to prevent any risk of damage, particularly in case the voltage is higher than the rated voltage of the SMPS MOSFETs (Rated VDC in Figure 1).

In 2015, Indian government launched a massive project of energy savings in the country. Switching from incandescent or gas lamps to efficient and smart LED lights seemingly proves to save quite few terawatt hours. In the stringent power line conditions and to make drivers reliable with time, STMicroelectronics proposes high level overvoltage surge protection circuit, based on SCR Thyristors. The high surge current capability of SCR technology, such as TN5015H-6I High Temperature SCR, seems the perfect solution for the over voltage surge condition for the Indian street light, compare with traditional stand-alone MOV-based solution. In addition to reliability performance, the solution improves the application safety, protecting from MOV burning. Finally, the circuit also brings benefits of lower stand-by losses and accuracy level for DC/DC devices protection.