Download References - Lighting Fixture Design

Designing for LED electrical safety
Presentation at the Lighting Fixture Design Conference
6th 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
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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 limit
for 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 required
Part of Table M.1
extracted from
EN 60598-1:2008
Refers to:
- Basic insulation
- Supplementary
insulation
- Reinforced
insulation
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
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
Etched copper
Insulating layer
Aluminium base
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 disadvantages
Advantages:
• Driver efficiency can be
improved with use of high
voltage and lower currents
• For cost reduction and further
efficiency gains, could use a nonisolated 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