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

Designing for LED electrical safety

Presentation at the Lighting Fixture Design Conference6th 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

– 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

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 limitfor a SELV (≤60V) driver

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?

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

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

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

Determination of insulation requiredPart of Table M.1 extracted from EN 60598-1:2008

Refers to:- Basic insulation- Supplementary insulation- Reinforced insulation

Creepage and clearance requiredTable 11.1 fromEN 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

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

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

Aluminium base

Insulating layer

Etched copper

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

High voltage LED system advantages and disadvantagesAdvantages:•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

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

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!

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

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!

Questions and further discussion