Andrew Smith, Power Integrations, USA, Andrew.smith@powerint.com
Jose Del Carmen Power Integrations USA
Christian Angeles Power Integrations USA
The Power Point Presentation will be available after the conference
Abstract
There are significant challenges to overcome before LEDs can take over from existing
technologies for lighting. Operating temperatures of LED light fixtures are much lower than for
incandescent lights, yet an internal temperature of 100°C is not uncommon. This can be a
problem for the reliability and lifetime of the driver circuit. Achieving a higher efficiency for the
driver circuit would clearly be beneficial.
This paper presents two LED driver designs for LED bulb replacement that illustrate new
developments.
1. Power in the LED lighting environment
1.1 Temperature effects
LEDs are not an efficient means of turning electrical energy into light. LEDs are significantly
more efficient than incandescent lamps, but the majority of the delivered power is still lost as
heat. Ambient temperature for LED lighting typically places the power supply (LED driver) in
close proximity to the load. In addition, the lamp body surrounding both the power supply and
the LEDs themselves is used as the convective heat path for all the energy lost in the conversion
process - the result is that the power supply sees a lot of heat.
Power supply designers have been fitting power supplies into small spaces – like adapters for
a long time. A 10 W adapter power supply might dissipate between 1 and 1.5 W (65-80%
efficient) but the surface of the enclosure is usually sufficient to cool the power supply – as long
as the output power is sent down the cable to the load. Therefore, simple isolated-flyback
topologies have been extremely popular for adapter power supplies due to their small size and
low cost. In LED lighting systems power densities are also quite high. An LED lamp delivering
the equivalent illumination of a 60 W incandescent bulb needs to drive around 10 W to the LED
load – assuming approximately 80 lumens per watt from the advanced high brightness LEDS
available on the market today.
Conversion efficiency in the LEDs themselves is less than 30% whilst the best power supply
designs have been in the order of 80% efficient. Importantly, unlike an adapter all the wasted
energy must be dissipated in close proximity to the power supply. Therefore the enclosure now
needs to handle the heat dissipation of around 10 watts. Finned heatsink/enclosures help to
limit the temperature but LED bulbs routinely generate internal ambient temperatures of 100°C.
This has implications for the components used in the LED driver.
Figure 1. Temperature measurements for key elements of a 12 W PAR 38 Lamp
1.2. Size Requirements for LED Lamp Drivers
As well as the challenge caused by high internal ambient temperatures, many lamp
replacement power supplies are required to fit into very small footprints. The bulb replacement
market assigns very little space to the driver – typically just the mounting base previously used
by the light bulb. While larger PAR lamps can accommodate a solution on a single PCB, A19
and B10 sized bulbs (which are the most widely used today) leave very little space for the power
supply. With so little space available the driver topology must be kept simple. -this restricts
topology selection.
Figure 2. 5 W B10 and 7W A19 LED Lamps Highlighting Space Available for the Power Stage
1.3. Input Considerations
AC mains power (110VAC or 230VAC) for LED lighting needs high power factor (either 0.7 or
0.9)[1] and Europe sets strict THD limitations (EN61000-3-2 C and D)[2]. North America, Europe
and Asia require high performance from solid-state lighting. With an incandescent light bulb
this was not an issue – lamps loads are almost ideal resistors, LED drivers requires input current
waveform shaping, Valleyfill circuits or active correction are typically employed.
In North America in particular there are a large number of homes that employ wall mounted
dimmers to reduce output light. LED based lamps are required to work with these installed
dimmers. This can pose a challenge as the dimmers are frequently sized to support >300 W
(10 x the load applied by LED based lamps). This creates issues associated with ensuring that
the dimmers operate properly at very low currents – especially when the LED driver presents
complex input impedance to the AC line rather than that exhibited by a purely resistive
(incandescent) lamp load.
1.4. Cost Considerations
Costs needs to come down if LED lighting is to become successful. Breakthroughs in LED
technology have significantly reduced the price of the LEDs, however this means that the cost
burden of the power supply is more significant in overall BOM cost. Price reduction is the single
most pressing issue in LED lamp design today.
2. Isolated flyback as a solution for LED lighting Applications
The first solutions offered in the market place for bulb replacement were either single stage or
two stage isolated flyback. Concerns about the cost and technical difficulties of isolating the
mechanical structure of the lamp drove customers to specify non-isolated LED lamps.
The following design is typical first generation LED driver, designed for use in an isolated LED
driver for a 7 W PAR20 bulb application. [3]
Figure 3. PAR30 LED Lamp Driver (18V 200 mA)
2.1. Circuit operation
The key design feature is the use of a combined single stage PFC and CC driver stage to
position would not fit into smaller bulb footprints like the popular A19 type.
Figure 5. Power supply top view showing PAR30 compatible layout (Note : No electrolytic
capacitors used in the design)
2.2. Input Filtering
Fuse F1 fuses the input and BR1 rectifies the AC line voltage. Inductors L1-L3, C2, R2, and R3
form the EMI filter: a low value of capacitance is necessary to maintain a power factor of greater
than 0.9.
2.3. Controller Primary
This signal is used by the control IC (via the Vpin) to set the input over/undervoltage protection
thresholds and to control the average output LED current. Diode D1 and VR1 clamp the drain
voltage to below the BVDSS rating (725 V) of the internal power MOSFET in U1. Diode D5 is
necessary to prevent reverse current from flowing through the LinkSwitch-PH device (as a result
of the minimal input capacitance).
2.7. TRIAC Dimming
Components R12, R13, R20, R17, D7, Q1, C13, VR2, and Q3 in conjunction with R16 reduce
the inrush current when the TRIAC dimmer turns on. This prevents the line inductance from
peak charging input capacitance above the line voltage, causing flicker.
This circuit allows the value of R16 to be large enough to limit the initial inrush current but keeps
the power dissipation on R16 low for higher-efficiency. Capacitor C9 and R14 form a passive
bleeder circuit with keep the AC input current above the holding current threshold for the TRIAC
to prevent multiple firings on each AC cycle.
2.8. Circuit performance
This circuit drives LEDs effectively, but how does it perform with respect to the key LED driver
parameters discussed in Section 1 - temperature, size, input compatibility and cost?
2.9. Thermal performance and efficiency
Figure 4. Thermal image of test Board top-side and bottom-side at 230 VAC, Full Load
(24°C Ambient)
Thermally the board functions adequately with a maximum device temperature of 58 degrees
– a 34°C maximum rise over ambient for the hottest component. The test was performed at
local ambient temperature with no free air motion, suggestion a component temperature of
90°C in a typical down light ambient environment. A more compact design might require
additional heatsinking to reduce localized heating - adding to component and assembly cost.
One advantage of the single stage combined PFC and CC stage is the elimination of electrolytic
bulk capacitors – somewhat reducing the temperature dependence of the design.
Efficiency was also reasonable coming in at 85% at 230 VAC input (82% at 115 VAC) for full
load.
2.10. Size
The board easily fits into a PAR20 enclosure;, the power transformer (RM-6) in the offset
position would not fit into smaller bulb footprints like the popular A19 type.
Figure 5. Power supply top view showing PAR30 compatible layout (Note : No electrolytic
capacitors used in the design)
2.11. Input Compatibility
The design provided wide range input and met EMI, THD and PF requirements for lighting
applications in North America and Europe.
Figure 6. 230 VAC Harmonic Content, Room Temperature and Full Load
2.12. Cost
The design was aimed at reducing driver cost compared to comaparble 2 stage LED drivers
and as such provided a BOM with only 44 components. No heatsinks were required to meet
the thermal requirements of a typical application. The elimination of electrolytic bulk capacitors
for the design demonstrated that a viable LED driver was possible without these components.
Lifetime would be increased by this approach but at the expense of the increased cost of the
ceramic capacitors used (perhaps 1.5 x the cost of comparable aluminium electrolytic
capacitors). Features such as the use of a double-sided PCB also increased the cost of the
final solution.
At the time the design was completed, LEDs were by far the most expensive part of the design.
Over the last 12 months LED costs have reduced dramatically making the cost contribution of
the driver stage much more significant.
2.13. Summary of DER-277
The design works well, delivers acceptable performance and provides good thermal
performance for the bulb size selected. However, for price sensitive bulb designs (such as the
popular A19) this design would be challenged.
3. Non-Isolated Buck Conversion for LED lighting Applications
The costs associated with fully isolated designs plus improvements in heat sinking techniques
and materials have made the selection of non-isolated LED drivers a key decision in many LED
bulb replacement applications.
The following design is typical first generation LED driver, designed for use in a non-isolated
Buck conversion LED driver for a 7.2 W A19 bulb application. [4]
Figure 8. High-Line Non-Isolated Buck Conversion Using Integrated Controller
3.1. Circuit Operation
3.2. Input Stage
Single stage converter designed to deliver high power factor while regulating the output current
across a range of input (185 VAC to 265 VAC) in a single conversion stage (combined PFC
and CC conversion). The design also supports the output voltage variations typically
encountered in LED driver applications. All of the control circuitry responsible for these functions
plus the high-voltage power MOSFET is incorporated into the IC.
3.3. Input Circuit
Resistor R9 and R13 are fusible resistors that also serve as passive dampers to reduce input
ringing during operation with a dimmer. BR1 rectifies the AC line voltage with capacitor C2 providing
a low impedance path to decouple the primary switching current. A low value of capacitance
(sum of C2andC3) is necessary to maintain a power factor of greater than 0.9. MI filtering
is provided by inductors L1, L2 and L3, and capacitors C2 and C3. R1, R2 and R3 are resonance
damping resistors and reduce conducted EMI.
3.4. Power Circuit
The circuit is configured as a buck converter with the SOURCE (S) pin of power MOSFET in
U1 connected to the freewheeling diodes (D2 and D4) and Inductor T1. The drain pin is connected
to the positive side of the DC rectified input via D1. Diode D1 is used to prevent reverse
current from flowing through U1. An RM6 core size was selected to optimize the inductor for
highest system efficiency.
Capacitor C5 provided local decoupling for the bias supply pin of the internal controller. During
startup, C5 charged to ~6 V from an internal high-voltage current source connected to the drain
pin. Once switching begins the controller is powered via R8 and series rectifier D5 and D6.
Capacitor C7 was selected to give an output LED ripple current of ±50%.
3.5. Feedback Loop Control
Resistors R4 and R5 sense the diode current of the buck converter. Resistor R6 and C6 provide
additional filtering to lower the ripple voltage fed to the FEEDBACK (FB) pin of U1 for improved
regulation.
3.6. TRIAC Phase Dimming Control Compatibility
The large impedance presented to the line by the LED driver allowed significant ringing to occur
due to the inrush current charging the input capacitance when the TRIAC turned on. This effect
can cause TRIAC current to fall to zero and turn off resulting in the lamp flickering unacceptably
during dimming.
Passive Damper and Passive Bleeder were incorporated. The drawback of these circuits was
increased dissipation and therefore reduced efficiency of the supply. The Passive Damper
consists of components R13 and R9 to limit inrush currents and associated ringing of the input
impedance during TRIAC dimming. The passive bleeder is C8, C9 and parallel combination of
R1, R2, and R3. This arrangement kept the input current above the TRIAC holding current at
the start of each AC half-cycle preventing the TRIAC from oscillating.
3.7. Circuit performance
As previously we will review performance against the key LED driver parameters discussed in
section one - temperature, size, input compatibility and cost?
Figure 9. Thermal Performance of LED Driver – Topside and Bottom Side Respectively,
230 VAC, Full Load (25°C Ambient still air).
Maximum device temperature was recorded as 55°C – very close to that seen for the non-isolated
circuit. Efficiency was measured at 87.4%. Tests were repeated with the dimming circuitry
removed and efficiency increased to 90.3% [5]. This marked reduction of waste heat - 22%,
would be important in highly space sensitive designs.
3.9. Size: 19.6 x 55.3 mm (max)
Figure 10. DER-302 High-Line TRIAC Dimmable A19 LED Driver
The board fits into an A19 base. The power inductor (again RM-6) was repositioned to the
center line of the driver providing sufficient vertical clearance to be incorporated into the A19
bulb circular cross section.
3.10. Input Compatibility
The design provided wide range input and met EMI, THD and PF requirements for lighting
applications in Europe. It was found that widening the specification to include wide-input
performance was detrimental to device operation. A similar circuit (buck configuration) was
developed [6] to cover low-line applications and demonstrated that the non-isolated
single-stage concept was also valid for North American mains power.
THD performance for non-isolated designs is made more difficult by the direct relationship
betweeninput current flow and output voltage level. Buck designs in particular are limited in the
maximum output voltage that can be delivered whilst still meeting THD limits. In this case output
voltage was low enough for a compliant design to be implemented. Buck-boost or tapped buck
designs can be used where output voltage is too high to insure compliance with THD due to
higher output voltage requirements.
Figure 11. DER-302 THD (EN61000-3-2 D) Compliance, 230VAC Full Load, Nominal
Output Voltage
TRIAC dimmer compatibility with a highly compact design is challenging. While universal
compatibility is desirable it is not always possible to make the active and passive bleeders large
enough (with associated loss of efficiency) to make a truly universally compatible solution.
Geographically-specific dimmer sets representing a cross section of typically (worst) performing
dimmers are generally used to prove the validity of a dimmer design. DER-302 was tested with
a range of dimmers from different (voltage compatible) regions and found to work satisfactorily
- see [3] for more information.
3.11. Cost
This design reduced component count significantly –to just 34 parts. In addition the major
wound element (the output inductor) was simplified, saving perhaps 50% of the cost compared
to an isolation transformer. The reduction in component count allowed a single sided board to
be used resulting in additional cost savings.
Against these savings the cost of adding isolation to the physical structure of the bulb must be
considered, and for some manufacturers this represents a technical problem that cannot be
easily overcome. In addition the technical expertise and burden for meeting safety has moved
from the power supply maker to the bulb integrator which can lead to additional restrictions in
the use of non-isolated topologies.
4. Summary and Conclusions
The two designs presented offer different solutions to powering LED bulbs. Both deliver the
performance to meet the requirements of a deployment in an LED lighting application. Very
small lamps (such as B10 and similar candelabra types) have already made the change due to
the tiny space available for these designs
However trade-offs in cost verses production simplicity mean that no-clear winner has emerged
in the design of LED drivers. Improvements in packaging techniques and materials (such as
high density thermally conductive plastics) may assist the designer in moving to a non-isolated
design.
Universal input designs are preferred by some manufacturers as it allows for a consolidation of
production and parts inventories – this specification is easier to meet with an isolated flyback
design than a non-isolated topology.
Finally, increasing LED driver voltage which helps improve efficiency also limits the
effectiveness of buck topologies – especially where good THD and high PF is required.
5. Literature
[1] EnergyStar Voulntary US standard for consumer applications. Integral LED lamps standard
http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_cod
e= LB
[2] EN61000-3-2 C(D) (IEC6100-3-2 C(D) http://webstore.iec.ch/p-preview/info_iec61000-32%7Bed2.1%7Den_d.pdf
[3]Power Integrations reference Design DER-277, www.powerint.com
[4]Power Integrations reference Design DER-302, www.powerint.com
[5]Power Intergations reference Design DER-303, www.powerint.com
[6]Power integrations reference Design DER-306, www.powerint.com
