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 VIPer22A-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 output current, in order 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 advantage of this dimmable light source.

10W Dimmable LEDs driver board layout

March 2007

Rev 4

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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. VDS and ID at 230 VAC: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 14. VDS and ID at 230 VAC: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 15. Typical waveforms: drain voltage and output current ripple at 230 VAC . . . . . . . . . . . . . . . 19

Figure 16. Typical waveforms: startup at 265 VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 17. Drain voltage VDS and output current IOUT: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 18. Drain voltage VDS and output current IOUT: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 19. Drain voltage VDS and output current IOUT at 50% dimming: 1 LED . . . . . . . . . . . . . . . . . 20 Figure 20. Drain voltage VDS and output current IOUT at 50% dimming: 8 LEDs . . . . . . . . . . . . . . . . 20 Figure 21. Drain voltage VDS and output current IOUT at 10% dimming: 1 LED . . . . . . . . . . . . . . . . . 20 Figure 22. Drain voltage VDS and output current IOUT at 10% dimming: 8 LEDs . . . . . . . . . . . . . . . . 20 Figure 23. Control signals at 230 VAC: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 24. Control signals at 230 VAC: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 25. Control stage at 230 VAC: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 26. Control stage at 230 VAC: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 27. Open load condition at 230 VAC: no dimming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 28. Open load condition at 230 VAC: minimum dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 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 voltage range: changes on the input section . . . . . . . . . . 26 Figure 35. STEVAL-ILL001V1 Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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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 consume 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 failure or burnout. 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 for 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 white Luxeon LEDs
	Lighting source		Luminous efficiency	Lifetime (hours)		Theoretical optical
			(lm/W)			power (min and max)
	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

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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 turn 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 drives the electrons and holes into the active region between the n-type and p-type material, the energy 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 electron volts by emission of a photon of energy, according to (Equation 1).

Equation 1

Eg	= hc	hc
		• υ = -----
		λ

The human eye is excited in response to electromagnetic radiations 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 electromagnetic 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 energy of the photon. At the other extreme, 400 nm in the violet, 3.1 eV is required.

The human vision efficacy is not constant in the entire visible region, but 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 fact that the human eye is able to separately 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.

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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) wavelength. Blue and green LEDs are of special interest and are being 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.	Typical characteristic for commercial LEDs (from Luxeon)
	Color	Operating	Operative forward current	Dominant wavelength/	Typical luminous flux (lm)
		voltage (V)	(mA)	color temperature
	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

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New dimming technique	AN2042

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

4 New dimming technique

Nowadays, thanks to the growth of process, packaging and thermal 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 forward 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, employing 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 forward current (IF) by varying the duty cycle, as 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.

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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.

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