AN2042

AN2042 STMICROELECTRONICS | Alldatasheet

Document overview

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• PDF pages: 30

Technical content

Datasheet sections

• 1 Light sources
• 2 Light emitting diode and colour vi sion
• 3 Commercial LEDs
• 4 New dimming technique
• 5 Application description
• 5.1 Dimming control circuit
• 5.2 Transformer specifications
• 5.3 DALI Interface
• 6 Experimental results
• 7 Layout considerations
• 8 EMI measurements
• 9 Non dimmable version
• 11 Conclusions
• 12 Revision history

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

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.

• 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

2 Light emitting diode and colour vision

multidirectional or unidirectional light source based on specification. (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 which corresponds to extreme red and violet respectively.

Figure 4. C.I.E. chromaticity diagram

3 Commercial LEDs

traditional incandescent lamps. flexibility for a variety of lighting applications. characteristic for a high efficiency LED. Table 2. Typical characteristic for commercial LEDs (from Luxeon)

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

4 New dimming technique

produces light output across blue, cyan, green and white, with high reliability and efficiency. wavelengths undergoing the strongest shift variation versus current. possible to dim a LED in the right manner, without wavelength shift. brightness variation versus duty cycle.

Figure 10. New dimming technique: typical waveforms

5 Application description

MOSFET with 730 V breakdown voltage and 0.7 A typical peak drain current. array of eight high efficiency LEDs, as shown in Figure 11. range can be selected, with only a few modifications in the input section. Table 3. SMPS Specifications

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 R 1, 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 R 10. 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 Equation 3 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 R 8 and R9 directly connected to the output. The values are chosen according the equations (Equation 4) and (Equation 5). Equation 4 Equation 5 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. V Iout() VREF R14• IOUT V Iout() R10•= VOref VREF R13 R14+()• VOref VOUT MAX()

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 R 19-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 Q 3 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 P DISS = (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).

Figure 11. Converter schematic for European input voltage range

Table 4. Component list

5.2 Transformer specifications

two auxiliary windings are coupled in forward mode to the primary winding. turn's ratio has been set according to the maximum count of LEDs.

5.3 DALI Interface

it with the DALI reference design (ST7DALI-EVAL). Table 4. Component list (continued)

Figure 12. Transformer features: (a) schematic, (b) mechanical characteristics and Table 5. Transformer specifications

6 Experimental results

filter small and improve the output dynamic behavior. down to 10% of its maximum luminosity. limited to about 33 V both in steady state and dimming operation. Figure 13. V

Figure 15. Typical waveforms: drain voltage Figure 16. Typical waveforms: startup at

265 VAC

Figure 17. Drain voltage V DS and output Figure 18. Drain voltage V DS and output

7 Layout considerations

fact, since EMI issues are also related to layout, the current loop area has to be minimized. resistive and inductive effect. and the correct placement of any single component. into account in the lab prototype, as shown in Figure 30. Figure 30. PCB layout (not in scale)

8 EMI measurements

and a spectrum analyzer with peak detector. 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

9 Non dimmable version

the voltage and current thresholds have to be changed. Figure 33. Non dimmable solution

European voltage range, connected as shown in Figure 34. Figure 34. Application circuit for U.S. input voltage range: changes on the input

11 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.

Figure 35. STEVAL-ILL001V1 Schematic

Table 6. Revision history