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Designing for LED electrical safety

Designing for LED electrical safety. Presentation at the Lighting Fixture Design Conference 6 th June 2013 By Ken Dale, Principal Engineer, Harvard Engineering plc. Outline. Safety with high voltage LED systems Applications and standards in practice A particular reliability consideration

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Designing for LED electrical safety

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  1. Designing for LED electrical safety Presentation at the Lighting Fixture Design Conference 6th June 2013 By Ken Dale, Principal Engineer, Harvard Engineering plc

  2. Outline • Safety with high voltage LED systems • Applications and standards in practice • A particular reliability consideration • Applicable to high voltage and low voltage systems • Design Guidelines • References • EN 60598-1 – Luminaires – Part 1: General requirements and tests • EN 62031 – LED modules for general lighting — Safety specifications • EN 61347-1 – Lamp controlgear - General and safety requirements • EN 61347-2-13 – Lamp controlgear — Particular requirements for d.c. or a.c. supplied electronic controlgear for LED modules

  3. Surely LEDs are low voltage? • Yes, individual white LED chips work at approximately 3V and many are limited to about 3 Watts operating power with a resulting light output in the region of 300 lumens • But multiple LED chips (or pre-assembled clusters) are needed for useful level of light output • Multiple LEDs usually connected in series to give same operating current • 18 white LEDs in series is the practical limit for a SELV (≤60V) driver

  4. Example Light Engine with 36 series LEDs • 36 single white LED chips on metal core PCB (MCPCB) • Series connection giving total working voltage of about 110V • Should the driver be isolated? • Should the heatsink be earthed?

  5. Series connected SELV modules • To reduce system cost, some designers wish to power 2 modules with one driver • Parallel connection not acceptable since a 3% difference in forward voltage would give about 30% difference in current • Series connection of two of these 11 LED modules takes the drive voltage over 60V • Need to enclose module so it cannot be accessed when powered • Additional insulation between module and heatsink may be needed for safety

  6. Fewer safety issues if LED voltage ≤60V • 4 parallel strings of 12 LEDs on FR4 fibreglass PCB • Series-parallel connection giving total working voltage of about 36V • Will each string take the same current? • Sharing acceptable if all LEDs from same manufacturing batch

  7. What do we mean by high voltage? Wherever contact would give a shock hazard – defined as a “live part” • Any system not isolated from the mains supply • Even if the LED voltage would be “safe” by itself • Isolated systems with LED operating voltage greater than 60V • Between 60V and 120V dc (25V to 50V ac rms) EN 60598-1 classifies the voltage as “safe” if the user is prevented (by basic insulation) from contact with the LED supply • Up to 60V dc (25V ac rms) is classed as “Safety Extra Low Voltage” (SELV) if isolated by reinforced insulation from the mains supply Reference EN 60598-1:2008 Clauses: 1.2.15, 1.2.42 and 8.2.3

  8. Determination of insulation required Part of Table M.1 extracted from EN 60598-1:2008 Refers to: - Basic insulation - Supplementary insulation - Reinforced insulation

  9. Creepage and clearance required Table 11.1 from EN 60598-1:2008 For mains connected LED PCB assemblies, the circled figures for creepage and clearance apply: -Basic insulation from live parts to earth -Reinforced insulation from live parts to exposed unearthed metal

  10. What about distances through insulation? • EN 60598-1 does not seem to specify minimum insulation thickness! • Metal Core PCB (MCPCB) insulation layer is typically 100µm • Additional measures are required for reinforced insulation. This may be 2 additional layers of thin sheet material or a thicker solid insulator. The actual thickness required will depend on the working voltage (standards not clear!) • Extra insulation will increase thermal resistance from LEDs to ambient • Consider thermally conductive ceramic sheet insulator (additional cost) • Amendment 2 of EN 61347-1 will specify insulation thickness for reinforced insulation

  11. Additional considerations with metal core PCBs • Capacitance between copper and aluminum base is typically 50pF per cm2 • With a non-isolated driver, this capacitance can contribute to touch current (earth leakage current) • The capacitance provides an unintentional path for EMI and there may be additional resonances with the output wiring inductance • MCPCBs have high breakdown voltage for dc, but may be less tolerant of ac voltages and spikes – check requirements for electric strength test Etched copper Insulating layer Aluminium base

  12. High voltage driver safety • Non-isolated or drivers with high output voltage can be hazardous, so must be treated with respect and require careful system design • Some models of the 150 Watt driver shown can have isolated output voltages up to 430V! • A special safety feature of this driver disables the output if the LED string goes open circuit to avoid high voltages in a faulty system • Full enclosure and power interlock may still be necessary

  13. High voltage LED system advantages and disadvantages Disadvantages • Lighting fixture may need additional parts to protect the user from the shock hazard • Safety ground may be required • LED PCB will need improved isolation • Thermal performance may be degraded Advantages: • Driver efficiency can be improved with use of high voltage and lower currents • For cost reduction and further efficiency gains, could use a non-isolated driver • Simpler driver can be more reliable • Very basic drive circuit, directly from the mains supply possible

  14. A story about LED luminaire unreliability • Customer using Harvard drivers and third-party LED assemblies in a street light reported field failure of one or more LEDs on several luminaires, but the drivers were not failing • They blamed the driver claiming that there must be output current peaks • We investigated and found no evidence of high currents • However, surge testing of the street light at 2kV common-mode caused some LEDs to fail and testing at 4kV caused most LEDs to fail • Concluded that insulation on the LED assemblies was breaking down and overstressing the LEDs during the surge transient • Faulty LED assemblies replaced, capacitor fitted across insulation barrier – no failures on re-test!

  15. Understanding the complete system • Common-mode surges at the mains input are passed to the driver output by the EMI suppression capacitor • The LED assembly must be designed to withstand this surge or the voltage across the insulation barrier attenuated • This requires larger LED PCB creepage distances or additional protection components Typical LED Driver topology

  16. Some design guidelines • If LED system works at more than 60V, protection will be required to prevent user contact – check by measuring touch current if apprpriate • Test the electrical strength of all safety barriers • Above 60V dc (25V ac rms), the LED assembly will need a minimum of basic insulation • Series connected SELV modules may no longer be safe without additional protection • Pay close attention to creepage, clearance and distance through insulation when designing LED assemblies – even for those working at SELV • Surge test the complete lighting fixture!

  17. Questions and further discussion

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