ST AN2811 Application note

AN2811

Application note

3.5 W non-isolated offline constant-current LED driver based on VIPER17

Introduction

High brightness LEDs are becoming a prominent source of lighting. Compared to conventional incandescent bulbs, high brightness LEDs (light emitting diodes) have advantages in higher light efficacy, much longer life and faster reaction time in a smaller profile. Since LEDs cannot sustain high voltage stress directly from an AC source, providing a reliable constant-current source to drive LEDs becomes fundamental. This solution provides even luminosity, reliability, the highest efficacy and the longest operating life for LEDs.

This application note describes the non-isolated offline constant-current driver based on the VIPER17HN (high frequency version). This solution operates with an AC line input range from 176 V to 264 VAC and provides 500 mA constant current from a 7 VDC source. It can illuminate two LEDs in series.

This device is an offline converter with an 800 V rugged power section, a PWM control, twice the level of overcurrent protection, overvoltage and overload protections, hysteretic thermal protection, soft-start and also safe auto-restart after any fault condition removal. The embedded brownout function protects this switch mode power supply in case the main input voltage falls below the specified minimum level for this system.

Figure 1. STEVAL-ILL017V1 demonstration board

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June 2009

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Contents	AN2811

Contents

1	Safety instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
2	Design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	2.1	Selected topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
3	General circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8

3.1 Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3 PCB layout view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Transformer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4	Test results and waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . 13
5	Connection of AC line and LED lamp to the demonstration board	. . 21
6	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . 22
7	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . 23
8	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . 24

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AN2811	List of tables

List of tables

Table 1. Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Table 2. Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 3. Basic electrical characteristics of flyback transformer (T1). . . . . . . . . . . . . . . . . . . . . . . . . 11 Table 4. Bobbin dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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List of figures	AN2811

List of figures

Figure 1. STEVAL-ILL017V1 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Conventional buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 3. Modified buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 4. Flyback converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 5. Schematic diagram of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 6. Top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 7. Bottom view with SMD parts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 8. Winding structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 9. Bobbin outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 10. Efficiency versus input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 11. Standby power versus input voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 12. Vin and Iin at 176 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 13. Vin and Iin at 176 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 14. Vin and Iin at 264 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 15. Vin and Iin at 264 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 16. Inrush current at LINE IN, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 17. Inrush current at LINE IN, two LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 18. Vds and Id at 176 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 19. Vds and Id at 176 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 20. Vds and Id at 264 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 21. Vds and Id at 264 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 22. Vo and Io at 176 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 23. Vo and Io at 176 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 24. Vo and Io at 264 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 25. Vo and Io at 264 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 26. Startup of Vo and Io at 176 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 27. Startup of Vo and Io at 176 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 28. Startup of Vo and Io at 264 VAC, one LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 29. Startup of Vo and Io at 264 VAC, two LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 30. Vdd and Vds at 264 VAC, output in short-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 31. Io at 264 VAC, output in short-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 32. Vdd and Vds at 264 VAC, output in open-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 33. Startup of Vdd and Vds at 264 VAC, output in open-circuit . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 34. Completed demonstration board connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 35. Connection of AC line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 36. Connection of LED lamp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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AN2811	Safety instructions

1 Safety instructions

Warning: The demonstration board must be used in a suitable laboratory by only qualified personnel who are familiar with the installation, use, and maintenance of electrical systems.

Intended use

The demonstration board is a component designed for demonstration purposes only, and shall be used neither for domestic installation nor for industrial installation. The technical data as well as the information concerning the power supply and working conditions shall be taken from the documentation included with the demonstration board and strictly observed.

Installation

The installation of the demonstration board shall be taken from the present document and strictly observed. The components must be protected against excessive strain. In particular, no components are to be bent, or isolating distances altered during the transportation, handling or usage. The demonstration board contains electro-statically sensitive components that are prone to damage through improper use. Electrical components must not be mechanically damaged or destroyed (to avoid potential risks and health injury).

Electrical connection

Applicable national accident prevention rules must be followed when working on the mains power supply. The electrical installation shall be completed in accordance with the appropriate requirements (e.g. cross-sectional areas of conductors, fusing, and PE connections).

Board operation

A system architecture which supplies power to the demonstration board shall be equipped with additional control and protective devices in accordance with the applicable safety requirements (e.g. compliance with technical equipment and accident prevention rules).

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Design considerations	AN2811

2 Design considerations

2.1Selected topology

This is a 500 mA constant-current source conversion from 176 VAC ~ 264 VAC line input. The specifications shown in Table 1 are for refrigerator lighting usage.

Table 1.	Specifications
	Parameter	Value
	AC input	220 VAC ± 20%
	Output current	500 mA
	Output voltage	7 V max
	Dimensions	30 mm x 30 mm
	Isolation	Not required
	Topology	Constant-current source

According to the specifications the maximum operating power is 3.5 watts. No power factor correction circuit is required. Therefore, both buck and flyback topologies are suitable for this application. Figure 2 shows the conventional buck converter while Figure 4 illustrates the flyback converter. To convert high voltage to low voltage, a conventional buck converter just requires a few components. Output current ripple is small due to Vout obtained from inherent filter L1 and C1, thus the voltage and current stresses on these power components are small. In order to properly drive the MOSFET (Q1), a controller and an additional transformer are required. Additional winding with L1 to bias Q1 as well as a feedback current to manage output in constant-current mode are needed.

Figure 2. Conventional buck converter

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AN2811	Design considerations

Figure 3. Modified buck converter

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For ease in driving Q1 using a conventional buck converter, a modified buck converter has been introduced as shown in Figure 3. Such topology is widely used to drive LEDs. With this modified solution, the MOSFET is no longer floating. In this case the output (Vout) is not connected to ground, and it becomes quite difficult to sense the output current in the output stage directly. Compared to a buck converter, the flyback converter may be the better choice. Figure 4 shows the typical circuit of a flyback converter.

Figure 4. Flyback converter

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The auxiliary winding can be added to the transformer (T1) to provide bias for Q1. Unlike the buck converter, T1 provides isolation between Vin and Vout. Since such isolation is not required for this application, a current sense resistor can be placed across the primary ground and negative polarity of Vout. Thus, Vout shares the same primary ground. In this topology, the MOSFET is not floating. Thanks to VIPer17 the board is built with a highperformance low-voltage controller chip with an 800 V avalanche rugged power MOSFET. Designed with VIPer17, only a few external components are required which allows a smaller profile in the design.

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General circuit description	AN2811

3 General circuit description

3.1Schematic diagram

Figure 5 shows the complete schematic diagram of the demonstration board. It consists of an input full-bridge rectifier with filtering circuit, flyback converter and output stage.

Figure 5. Schematic diagram of demonstration board

AM01057v1

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