ST AN2042 Application note

AN2042

Application note

VIPower: dimmable driver for high
brightness LEDs with VIPer22A-E

Introduction

This application note introduces an innovative solution to drive high brightness 1W LEDs
(Light Emitting Diode), using VIP e r22A-E in flyback configuration with output current control.

The power supply is able to drive an array of 1 to 8 LEDs in European range, i.e. 185-265
VAC with no modifications. By means of an input voltage doubler, it is possible to use the
same VIPer device also in U.S. input voltage range, guaranteeing the specs. A new control
technique is used to adjust the duty cycle of the ou tput current, in or der to dim the luminosity
of the LEDs down to 10% of the maximum value (patent pending by STMicroelectronics).

The proposed driver can be suitably used in applications such as landscape lighting, street
lighting, car parks, bollards, garden lighting, large area displays and so on.

Also domestic applications such as room lighting, decorative fixtures and architectural
lighting can benefit from the advant age of this dimmable light source.

10W Dimmable LEDs driver board layout

March 2007 Rev 4 1/30

www.st.com

Contents AN2042

Contents

1 Light sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Light emitting diode and colour vision . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Commercial LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 New dimming technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5 Application description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.1 Dimming control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.2 Transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.3 DALI Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

8 EMI measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

9 Non dimmable version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

10 Input section arrangement for U.S. market . . . . . . . . . . . . . . . . . . . . . . 26

11 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

12 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

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AN2042 List of figures

List of figures

Figure 1. Light emitting diode structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. The electromagnetic spectrum and visible region of light. . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. Human relative vision curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. C.I.E. chromaticity diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. Forward current vs. forward voltage in a typical commercial LEDs . . . . . . . . . . . . . . . . . . . 8

Figure 6. PWM technique for dimming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 7. Brightness variation versus duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 8. Dimming technique using series switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 9. Dimming technique using the new methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 10. New dimming technique: typical waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 11. Converter schematic for European input voltage range . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 12. Transformer features: (a) schematic, (b) mechanical characteristics and (c) pinout . . . . . 17

Figure 13. V
Figure 14. V
Figure 15. Typical waveforms: drain voltage and outp ut curr e nt ripple at 23 0 V
Figure 16. Typical waveforms: startup at 265 V
Figure 17. Drain voltage V
Figure 18. Drain voltage V
Figure 19. Drain voltage V
Figure 20. Drain voltage V
Figure 21. Drain voltage V
Figure 22. Drain voltage V
Figure 23. Control signals at 230 V
Figure 24. Control signals at 230 V
Figure 25. Control stage at 230 V
Figure 26. Control stage at 230 V
Figure 27. Open load condition at 230 V
Figure 28. Open load condition at 230 V

Figure 29. Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 30. PCB layout (not in scale) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 31. Conducted emissions at full load: line 1 emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 32. Conducted emissions at full load: line 2 emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 33. Non dimmable solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 34. Application circuit for U.S. input volta ge range: changes on the input section. . . . . . . . . . 26

Figure 35. STEVAL-ILL001V1 Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

and ID at 230 VAC: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

DS

and ID at 230 VAC: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

DS

and output current I

DS

and output current I

DS

and output current I

DS

and output current I

DS

and output current I

DS

and output current I

DS

: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

AC

: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

AC

: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

AC

: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

AC

AC
AC

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

AC

: 1 LED. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

OUT

: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

OUT

at 50% dimming: 1 LED . . . . . . . . . . . . . . . . . 20

OUT

at 50% dimming: 8 LEDs . . . . . . . . . . . . . . . . 20

OUT

at 10% dimming: 1 LED . . . . . . . . . . . . . . . . . 20

OUT

at 10% dimming: 8 LEDs . . . . . . . . . . . . . . . . 20

OUT

: no dimming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

: minimum dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

. . . . . . . . . . . . . . . 19

AC

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Light sources AN2042

1 Light sources

Incandescent lights are basically electric space heaters that give off light as a by-product.
They are very inefficient, wasting most of the power they cons ume as heat.

An innovative light source is represented by LED technology, with very low power
consumption and virtually no heating effect, making LEDs ideal for several domestic and
commercial applications.

The long lifetime characteristic of LEDs means savings on maintenance costs. Unlike
traditional light sources, LEDs are not subject to sudden f ailure or b urnout. Since LED based
light sources last at least 10 times longer than a normal light source (up to 10 years or

100.000 hours for the higher quality products), it is possible to reduce or eliminate the
maintenance ongoing costs.

This can be useful in many critical applications where the location makes replacement
difficult (radio tower, aircraft warning lights, bridge and tunnel lights…) or in applications
where a failure of the light source is not acceptable (emergency exit lights, back up lighting,
security lighting…).

LED lighting technology features many advantages compared to conventional lighting:

● Higher energy efficiency, in terms of lumens per watt;

● Direct light beam for increasing system performance;

● Dynamic color control technology;

● Full dimmable without color variation;

● No mercury and no UV or heat in light beam;

● Low voltage operation, suitable fo r safety purpose in SELV systems.

The most important limitation for using high brightness LEDs is the manufacturing cost,
which is still relatively high.

In Table 1 a comparison between traditional light sources and a typical commercial LED is
shown.

Table 1. Performance of typical light sources compared with whi te Luxeon LEDs

Lighting source

Incandescent bulbs 18 ÷ 25 1000 – 2000 15 – 1000 W

Halogen lamps 15 – 25 2000 – 5000 5 – 2000 W

Fluorescent lamps 60 – 110 14000 – 20000 4 – 60 W

Mercury lamps 15 – 60 12000 – 24000 50 – 1000 W

LEDs (white luxeon) 25 100000 0.7 – 5 W

Luminous efficiency

(lm/W)

Lifetime (hours)

Theoretical optical

power (min and max)

4/30

AN2042 Light emitting diode and colour vision

2 Light emitting diode and colour vision

Light-emitting diodes (LEDs) used for illumination are solid-state devices that produce light
by passing electric current across layers of semiconductor chips that are housed in a
reflector , which in tu rn is encased in an epoxy lens. The semiconductor material determines
the wavelength and subsequent color of the light. The lens converts the LED into a
multidirectional or unidirectional light source based on specification.

The first generation of LED was based on Gallium Arsenide (GaAs), Gallium Arsenide
Phosphide (GaAsP), Gallium Phosphide (GaP) technology, but thanks to the growth of solid
state technology, new structures have been introduced based on Aluminum Indium Gallium
Phosphide (AlInGaP), Indium Gallium Nitride (InGaN) or Gallium Aluminum Arsenide
(AlGaAs), mainly for the high brightness LEDs branch.

In Figure 1 the basic LED structure and the energy bands are shown.

Figure 1. Light emitting diode structure

The junction in an LED is forward biased and when electrons cross the junction from the n to
the p type material, the electron-hole recombination results in a process called
electroluminescence: when the applied voltage driv es the electrons and holes into the activ e
region between the n-type and p-typ e material, the ene rgy can be converted into infrared or
visible photons. This implies that the electron-hole pair drops into a stabler bound state,
releasing energy on the order of elect ron volts by emission of a photon of energy, according
to (Equation 1).

Equation 1

h

c

Egh

-----=•=

υ

c

λ

The human eye is excited in response to electromagnetic radiat ions with wavelengths in a
tight range of the electromagnetic spectrum, as shown in Figure 2, from 400 nm to 700 nm
which corresponds to extreme red and violet respectively.

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Light emitting diode and colour vision AN2042

Figure 2. The electromagneti c spectrum and visible region of light

The red extreme of the visible spectrum, 700 nm, requires an energy release of 1.77 eV to
provide the quantum energ y of the phot on. At the other e xtreme , 400 nm in the violet, 3. 1 eV
is required.

The human vision efficacy is not constant in the entir e visible region, b ut decreases near the
edges, as shown in Figure 3 featuring a peak value for a wavelength of 555 nm (green­yellow).

Figure 3. Human relative vision curve

Wavelength can be defined in terms of dominant wavelength and x-y chromaticity
coordinates, which define the color as perceived by the human eye. The dominant
wavelength is derived from the C.I.E.

(Commission Internationale de l'Eclairage - International Commission on Illumination)
Chromaticity Diagram, as shown in Figure 4 This is an international standard for primary
colors established in 1931. Based on the f act that the human e y e is able to separat ely sense
three different portions of the spectrum (we identify these peak sensitivities as red, green
and blue), the eyes response is best described in terms of such primary colors. All the other
colors are defined as weighted sum of them.

6/30

AN2042 Commercial LEDs

Figure 4. C.I.E. chromaticity diagram

3 Commercial LEDs

In the last years, light emitting diodes can be chosen from a wide variety of products
designed to meet specific needs to provide more efficient, longer life time alternatives to
traditional incandescent lamps.

They are manufactured of GaN and related compounds of AlGaN and InGaN due to the
wide bandgap, which allows emission of light ranging from the red to the ultraviolet (UV)
wavele ngth. Blue an d green LEDs are of special int erest and are be ing used in a wide range
of applications from outdoor video displays to automotive and cell phone backlights. LEDs
for solid-state white lighting offer high efficiency, long lifetime and a high degree of design
flexibility for a variety of lighting applications.

Thanks to new solid state technology, it now delivers from 25 to more then 120 lm/W in white
and comparable light output in other colors. In Table 2 are listed the main specifications for
typical commercial high efficiency LEDs are listed, while Figure 5 shows a typical V-I
characteristic for a high efficiency LED.

Table 2. Typi cal characteristic for commercial LEDs (from Luxeon)

Color

White 3.42 350 5500 K 18

Blue 3.42 350 470 nm 5

Cyan 3.42 350 505 nm 30

Green 3.42 350 530 nm 25

Amber 2.85 350 590 nm 20

Red 2.85 350 625 nm 25

Operating

voltage (V)

Operative forward current

(mA)

Dominant wavelength/

color temperature

Typical lu minous flux (lm)

7/30

New dimming technique AN2042

Figure 5. Forward current vs. forward volta ge in a typical commercial LEDs

4 New dimming technique

Nowadays, thanks to the growth of process, packaging and therm al transfer technologies,
light output continues to evolve. This involves especially the InGaN technology, which
produces light output across blue, cyan, green and white, with high reliability and efficiency.

The wavelength of the light emitted is strongly dependent on the forward current driven
through the device and in order to avoid shifts in color the dimming strategies have to be
chosen carefully.

The most common method of dimming a LED is by varying either f o rw ard current or voltage
across it. Unfortunately, due to the characteristics of InGaN, varying current or voltage will
shift the wavelength. This effect is proportional to the wavelength, with the longer
wavelengths undergoing the strongest shift variation versus current.

In many applications this effect cannot be accepted and, employ ing a PWM technique, it is
possible to dim a LED in the right manner, without wavelength shift.

The LED is switched on and off at constant f orward current (I
shown in Figure 6.

If the PWM frequency is higher than 100 Hz, the human eyes cannot perceive the single
pulses, but they integrate and interpret those pulses as brightness, which can be changed
linearly by varying the duty cycle linearly, with no wavelength shift. Figure 7 shows the
brightness variation versus duty cycle.

) by varying the duty cycle, as

F

8/30

AN2042 New dimming technique

Figure 6. PWM technique for dimming

Figure 7. Brightness variation versus duty cycle

As shown in Figure 8, the most common method to dim LEDs consists in a series
connection of a power switch which is controlled by PWM.

Due to the relatively high operative forward current, the switch has to be selected carefully in
order to handle the conduction losses.

9/30