cd00043387

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
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
2/30
AN2042
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Light emitting diode structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The electromagnetic spectrum and visible region of light . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Human relative vision curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
C.I.E. chromaticity diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Forward current vs. forward voltage in a typical commercial LEDs . . . . . . . . . . . . . . . . . . . 8
PWM technique for dimming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Brightness variation versus duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Dimming technique using series switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Dimming technique using the new methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
New dimming technique: typical waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Converter schematic for European input voltage range . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Transformer features: (a) schematic, (b) mechanical characteristics and (c) pinout . . . . . 17
VDS and ID at 230 VAC: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
VDS and ID at 230 VAC: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Typical waveforms: drain voltage and output current ripple at 230 VAC . . . . . . . . . . . . . . . 19
Typical waveforms: startup at 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Drain voltage VDS and output current IOUT: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Drain voltage VDS and output current IOUT: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Drain voltage VDS and output current IOUT at 50% dimming: 1 LED . . . . . . . . . . . . . . . . . 20
Drain voltage VDS and output current IOUT at 50% dimming: 8 LEDs . . . . . . . . . . . . . . . . 20
Drain voltage VDS and output current IOUT at 10% dimming: 1 LED . . . . . . . . . . . . . . . . . 20
Drain voltage VDS and output current IOUT at 10% dimming: 8 LEDs . . . . . . . . . . . . . . . . 20
Control signals at 230 VAC: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Control signals at 230 VAC: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Control stage at 230 VAC: 1 LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Control stage at 230 VAC: 8 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Open load condition at 230 VAC: no dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Open load condition at 230 VAC: minimum dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
PCB layout (not in scale) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Conducted emissions at full load: line 1 emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Conducted emissions at full load: line 2 emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Non dimmable solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Application circuit for U.S. input voltage range: changes on the input section . . . . . . . . . . 26
STEVAL-ILL001V1 Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3/30
Light sources
1
AN2042
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.
4/30
Performance of typical light sources compared with white Luxeon LEDs
Lighting source
Luminous efficiency
(lm/W)
Lifetime (hours)
Theoretical optical
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
AN2042
2
Light emitting diode and colour vision
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
h
E g = h c • υ = -----c
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.
5/30
Light emitting diode and colour vision
Figure 2.
AN2042
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.
6/30
AN2042
Commercial LEDs
Figure 4.
3
C.I.E. chromaticity diagram
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.
Color
Typical characteristic for commercial LEDs (from Luxeon)
Operating Operative forward current
voltage (V)
(mA)
Dominant wavelength/
color temperature
Typical luminous flux (lm)
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
7/30
New dimming technique
Figure 5.
4
AN2042
Forward current vs. forward voltage in a typical commercial LEDs
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.
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
New dimming technique
Figure 8.
AN2042
Dimming technique using series switch
To overcome this problem, a patented solution has been implemented, which allows to
eliminate the series switch, with a considerable improvement in terms of efficiency.
The new technique consists in a double control loop: a current and a voltage control loops.
The first one drives the LEDs with constant current when the maximum luminosity is
required. During the dimming operation, the current control loop will still limit the maximum
output current, while the voltage loop will maintain the output voltage below the threshold
voltage of the LEDs array. Also disconnecting the LEDs, the maximum output voltage will be
limited by the voltage loop. In Figure 9 and Figure 10 the block diagram of the new dimming
technique and the temporal diagrams are respectively shown. Thanks to the absence of the
power switch, it is possible to have a more efficient and cheaper solution.
Figure 9.
10/30
Dimming technique using the new methodology
AN2042
Application description
Figure 10. New dimming technique: typical waveforms
5
Application description
The proposed converter is based on VIPer22A-E, a smart power with a current mode PWM
controller, startup circuit and protections integrated in the same monolithic chip, using
STMicroelectronics VIPower M0 Technology. The power stage consists in a vertical Power
MOSFET with 730 V breakdown voltage and 0.7 A typical peak drain current.
The application consists in an isolated constant current power supply, intended to supply an
array of eight high efficiency LEDs, as shown in Figure 11.
The board has been designed referenced to the specifications listed in Table 3 It is
important to highlight that the converter works in single range, but both U.S. and European
range can be selected, with only a few modifications in the input section.
Table 3.
SMPS Specifications
Parameters
Value
Selectable Input voltage range
85VAC÷135 VAC or 185 VAC÷265 VAC
Nominal output voltage range
3.5 V÷28 V
Maximum output voltage at open load
32 V
Output current
350 mA
Dimming range
0%÷90%
EMI Standard
EN55015:2000
11/30
Application description
AN2042
In the input stage, an EMI filter is implemented (C1, CM, C2) for both differential and
common mode noise, in order to fit the EN55015:2000 standard (limits for electrical lighting
and similar equipment). The input resistor R1, limits the inrush current of the capacitors at
plug-in and a standard fuse is also introduced to prevent catastrophic failure.
The clamping network (R2-C4-D5), limits the peak of the leakage inductance voltage spike,
assuring reliable operation of the VIPer22A-E.
The auxiliary winding on the primary side, is connected in forward mode, since the output
voltage ranges from 3.5 V to 28 V and the voltage on VDD pin varies from 17 V to 24 V.
A brown-out circuit (R3, R4, R5, Q1, Q2 and C7) is implemented in order to avoid the
flickering of the LEDs during switch off. The values of R3, R4 and R5 are chosen in order to
get the given thresholds, while C7 stabilizes the voltage on the base of Q1.
The output filter selection is a very critical point to consider during the design. Since LEDs
are switched on and off during the dimming phase the value of the output capacitor has to
be as low as possible.
Therefore, in order to avoid exceeding the maximum output current ripple, care must be paid
to design the right LC post filter.
5.1
Dimming control circuit
The current loop is controlled by the second operational amplifier of TSM104W and the
sense resistor R10. The voltage threshold is generated by means of a resistor bridge (R12,
R13 and R14) connected to the 2.5 V internal voltage reference VREF. The resistors of the
bridge should be 1% precision in order to get the best precision on the regulation. The
current control equations are given by (Equation 2) and (Equation 3).
Equation 2
V REF • R 14
V ( Iout ) = ----------------------------------------R 12 + R 13 + R 14
Equation 3
I OUT = V ( Iout ) • R 10
The sense resistor R10, is chosen taking into account the maximum dissipation during full
load. The voltage loop is controlled by the third operational amplifier and the voltage divider
R8 and R9 directly connected to the output. The values are chosen according the equations
(Equation 4) and (Equation 5).
Equation 4
V REF • ( R 13 + R 14 )
V Oref = -------------------------------------------------R 12 + R 13 + R 14
Equation 5
V OUT ( MAX )
V Oref = ----------------------------R8 + R9
Where VOUT(MAX) is the maximum acceptable output voltage, when the LEDs array is
disconnected. The transistor Q3, connected to the dimming control section, is ON during
normal operation.
12/30
AN2042
Application description
The feedback to the primary side is achieved thanks to the diodes D9 and D10, which
decouple the two loops and drive the optocoupler OPT. The legs R23-C11 and R24-C12 are
connected for feedback stabilization.
The zener diode DZ2 is connected at the non-inverting input of the voltage control
operational amplifier in order to clamp the maximum voltage on the pin in any operative
condition.
The PWM control is realized using the first operational amplifier to generate a sawtooth
waveforms at 270 Hz (given by the leg R19-C13), which is compared with a variable voltage
(set by the potentiometer R21): the generated signal will drive the NPN transistor Q3. When
the transistor is "ON", the SMPS works in "current control" mode limiting the max output
current while, when the transistor is "OFF", it works in "voltage control" mode, regulating the
output voltage below the LEDs threshold and consequently switching them off.
During the dimming operation, the transistor Q3 is switched off and the voltage on pin 11 of
IC2 is pulled up and limited to VDZ1. Consequently, the VIPer stops switching and the output
current falls to zero, while the output voltage decrease down to VOUT = n · VF(OFF), where n
is the number of LEDs and VF(OFF) is the threshold voltage. Further decrease of the output
voltage is not possible because of the high output impedance. Doing so, the output voltage
never falls to zero, resulting in a big improvement in the dynamic behavior of the dimming
function, with a slight impact on the efficiency PDISS = (VOUT-VDZ2)/R8.
In open load condition, the maximum voltage is regulated by R8, R9 and DZ2 according to
the reference voltage given by (Equation 5).
13/30
Application description
Figure 11. Converter schematic for European input voltage range
14/30
AN2042
AN2042
Application description
Table 4.
Component list
Reference
Description
Note
FS
1 A-250 V
Fuse
R1
10., 1/2 W
Metallic oxide resistor – No
flammable
R2
1M., 1/2 W
R3
560 k., 1/4 W
R4
12 k., 1/4 W
R5
24 k., 1/4 W
R6
1 k., 1/4 W
R7
150., 1/2 W
R8
5.6 k., 1/4 W
R9
220., 1/4 W
R10
0.47., 1/4 W
R11
2.7 k., 1/4 W
R12
12 k., 1/4 W
R13
10 k., 1/4 W
R14
1.5 k., 1/4 W
R15
4.7 k., 1/4 W
R16, R18, R22
22 k., 1/4 W
R17
100., 1/4 W
R19
33 k., 1/4 W
R20
15 k., 1/4 W
R21
20 k., 1/4 W
R23, R24
220 k., 1/4 W
R25
1.2 k., 1/4 W
R26
6.8 k., 1/4 W
C1
100 nF, 275 V
X2 Capacitor
C2
10 µF, 400 V
Electrolytic capacitor
C4
100 pF, 630 V
Polypropylene capacitor
C5
33 µF, 25 V
Electrolytic capacitor
C6, C13
220 nF
Polyester capacitor
C7
47 nF
Polyester capacitor
C8
33 µF, 16 V
Electrolytic capacitor
C9
1 µF, 50 V
Electrolytic capacitor
C10
3.3 µF, 50 V
Electrolytic capacitor
Sense resistor
Potentiometer
15/30
Application description
Table 4.
5.2
AN2042
Component list (continued)
Reference
Description
Note
C11, C12
2.2 nF
Polyester Capacitor
C14
2.2 nF, 250 V
Y1 Capacitor
D1, D2, D3, D4
1N4007
D5
STMicroelectronics STTH1R06
D6, D8, D9, D10, D11
1N4148
D7
STMicroelectronics STTH102
DZ1, DZ2
Zener Diode 5.1 V, 1/4 W
Q1, Q3
STMicroelectronics BC337
NPN transistor
Q2
STMicroelectronics BC327
PNP transistor
L1
47 µH
Radial
TF
TDK SRW16ES-ExxH003
CM
Coilcraft BU9-103R25B
OPT
SFH610A
IC1
STMicroelectronics
VIPer22ADIP-E
IC2
STMicroelectronics TSM104
2X10 mH Common mode choke
Transformer specifications
The transformer has four windings, included two auxiliaries. One is used to supply the VIPer
and the other one to supply the TSM104 and the dimming control circuit on the secondary
side.
Since the output voltage is variable between 3.5 V (with 1 LED) and 28 V (with 8 LEDs), the
two auxiliary windings are coupled in forward mode to the primary winding.
In order to limit the reflected voltage to a maximum value (100 V), the primary-to-secondary
turn's ratio has been set according to the maximum count of LEDs.
The transformer characteristics are listed in Table 5 and the winding arrangement as well as
the mechanical specifications are shown in Figure 12
5.3
DALI Interface
In order to control the board in remote fashion a connector has been introduced to interface
it with the DALI reference design (ST7DALI-EVAL).
Referring to the schematic in Appendix A, it is possible to move from analog control by the
trimmer R21 to the digital one by DALI, removing the jumper J1 and J2. Then, connect the
1..10 V output of the DALI interface on connector J2 of the ST7DALI-EVAL demo board to
CN1 connector of the STEVAL-ILL001V1, providing the correct voltage range, i.e. from 0 to
2.5 V.
16/30
AN2042
Application description
Figure 12. Transformer features: (a) schematic, (b) mechanical characteristics and
(c) pinout
a
Table 5.
b
c
Transformer specifications
Parameters
Value
Ferrite
PC40EF16
Core geometry
E16
Primary inductance
2.0 mH±12%
Leakage inductance
60 µH max
NP
135 turns – AWG 35
NAUX1
9 turns – AWG 35
NAUX2
5 turns – AWG 29
NSEC
36 turns – AWG 29
17/30
Experimental results
6
AN2042
Experimental results
In this section typical waveforms are given under several load conditions. In Figure 13 and
Figure 14 the drain-source voltage and the drain current at minimum load (1 LEDs) and full
load (8 LEDs), at nominal input voltage (230 VAC) are shown, respectively. In Figure 15 the
output current ripple is shown, which is fixed to about 20% IOUT, in order to keep the output
filter small and improve the output dynamic behavior.
In Figure 17 to Figure 22 the output current and drain-source voltage are shown during
dimming operations. It is important to point out that the driver is able to dim the LEDs array
down to 10% of its maximum luminosity.
In Figure 23 and Figure 24 typical waveforms of the dimming control section, as introduced
in Section 5.1, are shown: the sawtooth waveform, VSAW, defines the dimming frequency
while varying the reference voltage, VREF, by means of the potentiometer R21, it is possible
to change the PWM duty-cycle and consequently the LEDs luminosity. It is important to
point out that the output voltage never goes to zero, but is always above a minimum value
depending on the number of LEDs in the array. In Figure 25 and Figure 26 the output during
dimming is shows.
Finally, Figure 27 and Figure 28 shows the drain voltage and output voltage in open load
condition with 1 or 8 LEDs connected respectively. Under this condition the output voltage is
limited to about 33 V both in steady state and dimming operation.
Figure 13. VDS and ID at 230 VAC: 1 LED
Ch1 freq - 58.18 kHz (black)
Ch2 max - 196 mA (green)
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Figure 14. VDS and ID at 230 VAC: 8 LEDs
Ch1 freq - 58.18 kHz (black)
Ch2 max - 196 mA (green)
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Experimental results
Figure 15. Typical waveforms: drain voltage
Figure 16. Typical waveforms: startup at
and output current ripple at 230 VAC
265 VAC
Ch1 freq - 548 V (black)
Ch2 max - 348 mA (red)
Ch3Pk-Pk - 68 mA (red)
Figure 17. Drain voltage VDS and output
current IOUT: 1 LED
Ch1 max - 418 V (black)
Ch2 max - 348 mA (green)
Ch1 max - 610 V (black)
Figure 18. Drain voltage VDS and output
current IOUT: 8 LEDs
Ch1 max - 542 V (black)
Ch2 mean - 352.6 mA (green)
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Experimental results
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Figure 19. Drain voltage VDS and output
Figure 20. Drain voltage VDS and output
current IOUT at 50% dimming: 1 LED
current IOUT at 50% dimming: 8
LEDs
Ch2 mean - 170.6 mA (green)
Ch2 duty - 51.63% (green)
Ch2 freq - 246 Hz (green)
Ch2 mean - 171.1 mA (green)
Ch2 duty - 50.52% (green)
Ch2 freq - 245 Hz (green)
Figure 21. Drain voltage VDS and output
Figure 22. Drain voltage VDS and output
current IOUT at 10% dimming: 1 LED
current IOUT at 10% dimming: 8
LEDs
Ch2 mean - 33.9 mA (green)
Ch2 duty - 10.18% (green)
Ch2 freq - 252 Hz (green)
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Ch2 mean - 31.5 mA (green)
Ch2 duty - 8.8% (green)
Ch2 freq - 249 Hz (green)
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Figure 23. Control signals at 230 VAC: 1 LED
Ch2 mean - 33.9 mA (green)
Ch2 duty - 10.18% (green)
Ch2 freq - 252 Hz (green)
Figure 25. Control stage at 230 VAC: 1 LED
Ch1 mean - 200 mA (black)
Ch2 max - 3.48 V (green)
Ch2 min - 2.44 V (green)
Experimental results
Figure 24. Control signals at 230 VAC: 8 LEDs
Ch2 mean - 31.5 mA (green)
Ch2 duty - 8.8% (green)
Ch2 freq - 249 Hz (green)
Figure 26. Control stage at 230 VAC: 8 LEDs
Ch2 mean - 197.4 mA (black)
Ch2 max - 26.6 V (green)
Ch2 min - 20.2 V (green)
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Experimental results
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Figure 27. Open load condition at 230 VAC: no Figure 28. Open load condition at 230 VAC:
dimming
minimum dimming
Ch1 freq - 613 Hz (black)
Ch2 max - 32.8 V (green)
Ch1 freq - 250 Hz (black)
Ch2 max - 33.6 V (green)
The efficiency of the system, one of the key parameters of the application, has been
measured in the whole input voltage range varying the number of LEDs from 1 to 8, and the
experimental results are shown in Figure 29.
Figure 29. Efficiency
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7
Layout considerations
Layout considerations
As any switched mode power supply, for proper operations, basic rules have to be taken into
account in order to optimize the current path, especially in the routing of high current path. In
fact, since EMI issues are also related to layout, the current loop area has to be minimized.
In addition to this, in order to avoid any noise interference between the control section and
the power section, the control ground paths have to be kept separated from each other. All
the high current traces have to be as short and wide as possible, in order to minimize the
resistive and inductive effect.
A particular care has to be taken regarding the optimal routing of the input EMI filter path
and the correct placement of any single component.
A final consideration regards the thermal management: a copper area has to be provided on
the VIPer drain, in order to reduce the thermal resistance Rth and consequently keep the
device temperature reasonably low. All the aforementioned considerations have been taken
into account in the lab prototype, as shown in Figure 30.
Figure 30. PCB layout (not in scale)
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EMI measurements
8
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EMI measurements
Conducted EMI measurements have been performed according to EN55015:2000, the
specific European standard on electrical lighting and similar equipment, using a 50 LISN
and a spectrum analyzer with peak detector.
The results are shown in Figure 31 and Figure 32, for Line 1 and Line 2 respectively, under
full load condition at nominal input voltage, i.e. 230 VAC. The emissions level are well below
the Quasi Peak limit although the measurements have been performed using the Peak
detector, conforming the conducted EMI compliance of the system.
Figure 31. Conducted emissions at full load: line 1 emissions
Figure 32. Conducted emissions at full load: line 2 emissions
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9
Non dimmable version
Non dimmable version
A lower cost solution is introduced as shown in Figure 33, if the dimming function is not
required. In this case the TSM104 used for the dimming control is replaced by the simpler
TSM1011 and the brown-out circuit is not necessary anymore during the switch off of the
circuit. No other changes need to be introduced neither the transformer specifications nor
the voltage and current thresholds have to be changed.
The dimming control section is eliminated and the TSM104 is replaced by the simplest
TSM1011. Moreover, the brownout circuit is not necessary during the switch off. The same
rules to design to define the transformer specifications and voltage and current thresholds
are still valid.
Figure 33. Non dimmable solution
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Input section arrangement for U.S. market
10
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Input section arrangement for U.S. market
The proposed system has been designed for the European voltage range, i.e. 187-264 VAC,
but by means of a voltage doubler, consisting of D1-D2 and C2-C3, it can also be used with
the U.S. voltage range, i.e. 88-132 VAC. The only modification needed is related to the input
capacitor C2 which has to be replaced by two capacitors C2 and C3 with half the value of the
European voltage range, connected as shown in Figure 34.
Figure 34. Application circuit for U.S. input voltage range: changes on the input
section
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11
Conclusions
Conclusions
In this document an innovative solution for driving high efficiency LEDs has been introduced.
The power converter is based on a flyback topology with the smart power VIPer22A-E. It is
able to drive with no circuital modifications 1 to 8 LEDs array and to perform an optimal
dimming function by means of a patented PWM technique. A simplified version of the
system has also been introduced in order to address the low end applications which do not
require the dimming function.
A lab prototype has been developed and fully tested under several conditions, confirming
the suitability of the proposed approach to such an emerging application.
The reference board will be available at stock through the order code: STEVAL-ILL001V1.
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Fs
C1
CM
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D2
D1
D4
D3
C3
J3
C2
+
+
+Vcc
OPT
C6
C7
Q2
FB
R3
R5
R6
IC1
CONTROL
VDD
Q1
R4
C4
+
DRAIN
SOURCE
C5
D6
D5
R2
D8
D7
+
C8
+
R7
C9
L1
+Vcc
+Vcc
C10
Vref
C13
R19
DZ1
+
R18
8
R11
Q3
R9
R8
13
9
4
2
7
R17
3
+
-
C11
R23
D9
R10
R16
1
2
+
-
R22
15
-
5 12
CN1
1
IC2
D11
6
DZ2
14
11
+
-
+
-
4
3
J1
16
10
R25
R21
C12
R24
D10
OUT
R26
R20
R14
+Vcc
R13
R12
R15
Vref
LD8
LD2
LD1
J2
Appendix A
IN
IN
C14
STEVAL-ILL001V1 schematic
AN2042
STEVAL-ILL001V1 schematic
Figure 35. STEVAL-ILL001V1 Schematic
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12
Revision history
Revision history
Table 6.
Revision history
Date
Revision
Oct-2004
1
First issue
Feb-2005
2
D5 & Q2 values change in component list table
Feb-2005
3
– Figure in cover page changed
– Bil of material modified
4
–
–
–
–
21-Mar-2007
Changes
The document has been reformatted
Figure in cover page changed
PCB layout changed
STEVAL-ILL001V1 Schematic insertion
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AN2042
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