ST AN3111 Application note

AN3111

Application note

18 W single-stage offline LED driver

Introduction

With the rapid development of high brightness LEDs, SSL (solid state lighting) has begun to move from being a niche market to penetrating residential markets. There is a large potential market for the residential application of SSL, and CFL (compact fluorescent lamp) retro-fit is part of it. Standardization of SSL products is helping to lead the growth of the market. In September 2007, the US department of energy (DOE) issued ENERGY STAR® criteria for SSL products. To meet the ENERGY STAR specifications for SSL products, the power factor of power supply must be higher than 0.7 for residential applications. For CFL retro-fit applications, cost, size, and reliability are very important. To achieve a high power factor, either passive PFC (power factor correction) or active PFC can be used. Typically, passive PFC requires large passive components, which makes it difficult to maintain a small size. Traditional active PFC circuits require a two-stage topology, which entails a boost stage for PFC, and then buck or flyback for current regulation of the LEDs. The cost of the two stages is high. In this application note, a non-isolated, soft-switched, single-stage high power factor offline LED driver is introduced. The buck-boost converter is chosen for this application due to its simplicity and low cost. The converter operates with constant peak current for constant power control, and in transition mode (boundary mode between CCM and DCM) to achieve soft switching, using the L6562A controller. High power factor is achieved by reshaping the peak current near the zero crossing of the input AC line.

Figure 1. 18 W single-stage offline LED driver board

For this particular design, the LED string consists of 18, 1 W white LEDs in series. Isolation is not required. The goal of the design is high power factor, high efficiency, simplicity, and low cost.

February 2011

Doc ID 16815 Rev 2

1/23

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Contents	AN3111

Contents

1	Circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
2	STEVAL-ILL027V1 demonstration board . . . . . . . . . . . . . . . . . . . . . . . .		13
3	STEVAL-ILL027V2 demonstration board . . . . . . . . . . . . . . . . . . . . . . . .		14
	3.1	Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	3.2	BOM for the STEVAL-ILL027V2 demonstration board . . . . . . . . . . . . . . .	15
	3.3	STEVAL-ILL027V2 description for EU voltage range . . . . . . . . . . . . . . . .	15
	3.4	Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
4	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		22
5	Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		22
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		22

2/23	Doc ID 16815 Rev 2

AN3111	List of tables

List of tables

Table 1. Bill of material for the STEVAL-ILL027V1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Table 2. Bill of material for the STEVAL-ILL027V2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 3. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Doc ID 16815 Rev 2

3/23

List of figures	AN3111

List of figures

Figure 1. 18 W single-stage offline LED driver board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Block diagram for the L6562A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Schematic diagram of the single-stage LED driver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 4. Illustration of key waveforms of the converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 5. Inductor current and multiplier input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Input voltage and input current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 7. LED voltage and LED current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 8. Switching waveform of MOSFET Q1: conclusions when Vout > Vin . . . . . . . . . . . . . . . . . 11 Figure 9. Switching waveform of MOSFET Q1: conclusions when Vout < Vin . . . . . . . . . . . . . . . . . 11 Figure 10. STEVAL-ILL027V2 schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 11. Output LED current and voltage for input voltage 230 V / 50 Hz . . . . . . . . . . . . . . . . . . . . 17 Figure 12. Output LED current and voltage for input voltage 180 V / 50 Hz . . . . . . . . . . . . . . . . . . . . 17 Figure 13. Output LED current and voltage for input voltage 260 V / 50 Hz . . . . . . . . . . . . . . . . . . . . 18 Figure 14. Open load measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 15. Short-circuit measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 16. EMI measurement - detector average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 17. EMI measurement - detector quasi-peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 18. LED current vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 19. Efficiency vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 20. LED current vs. LED number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4/23	Doc ID 16815 Rev 2

AN3111	Circuit design

1 Circuit design

The L6562A is a current mode PFC controller operating in transition mode (boundary mode between CCM and DCM). Its linear multiplier enables the converter to shape the AC input current waveform following the input voltage. The block diagram of the L6562A is shown in Figure 2 below. For more detailed information regarding the L6562A, please refer to the device datasheet and application notes.

Figure 2. Block diagram for the L6562A

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Figure 3 shows the schematic of the proposed single-stage LED driver. If the inductor current is constant, the power of the converter is constant. As the LED string is

a constant voltage load, we can obtain a constant current in the LED string. This makes it possible to leave out the LED current sensing, therefore simplifying the circuit design. If the inductor current is constant, then the power factor of the circuit is very poor. The input current waveform is greatly distorted during line voltage zero crossing. If there is a way to reduce the current amplitude near the line voltage zero crossing, the power factor can be improved. The L6562A PFC controller is used to achieve this objective. The idea is to reduce the current amplitude near the line voltage zero crossing, which results in an improved power factor.

Doc ID 16815 Rev 2

5/23

Circuit design	AN3111

Figure 3. Schematic diagram of the single-stage LED driver

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6/23	Doc ID 16815 Rev 2

AN3111	Circuit design

The proposed circuit runs at constant peak current. The current sensing voltage is set for 1 V, which is the clamping voltage of the current sensing comparator of the L6562A. The voltage of the LED string is sensed on the INV pin of the L6562A through a coupled winding. The turn ratio of the coupled inductor is designed so that the feedback voltage is lower than 2.5 V in normal operating conditions, so the error amplifier is saturated at maximum level which sets the max. current sensing voltage to 1 V. Therefore, the peak current of the inductor is fixed at a level determined by the sensing resistor value. As the peak current of the inductor is constant, the input power is constant. The LED voltage is considered constant, and the current into the LED can also be considered constant. If the load (the LEDs) is open, the reflected output voltage on the INV pin is above a certain level, and the controller shuts down to provide open load protection.

If the inductor current is controlled at constant level all the time, then the power factor is very poor. The multiplier of the L6562A is used to reshape the current amplitude near the line voltage zero crossing to improve the power factor. The rectified sine waveform is sampled at the MULT pin. The output of the multiplier, which is the current setting, is lower than setting the level near the line voltage zero crossing. In this way, the power factor of the converter is improved significantly.

Design procedure:

● Input voltage: Vin( θ ) = 2 Vin sin ( θ,Vin) = 120V

●Output voltage: 18 LEDs in series, Vout=54 V

●Output current: Iout=350 mA

The design variable is the peak current of the inductor (Ipk), and the inductance (L).

When Q1 is turned on, the inductor is charged to Ipk. The ON time is:

Equation 1

θ = L *Ipk

Ton( )

Vin(t)

The OFF time is:

Equation 2

θ = L * Ipk

Toff( )

Vout

The period of the switching cycle is:

Equation 3

θ = θ + θ = L *Ipk + L * Ipk

T( ) Ton( ) Toff( )

Vin(θ) Vout

The duty cycle (D) is:

Equation 4

D(θ) =	Ton(θ)	=	Vout
			Vout + Vin(θ)
	T(θ)

Doc ID 16815 Rev 2

7/23