ST AN2928 Application note

AN2928

Application note

Modified buck converter for LED applications

Introduction

The use of high power LEDs in lighting applications is becoming increasingly popular due to rapid improvements in lighting efficiency, longer life, higher reliability and overall cost effectiveness. Dimming functions are more easily implemented in LEDs, and they are more robust and offer wider design flexibility compared to other light sources.

Applications suitable for the use of LEDs include lighting for streets, stadiums, fairs and exhibitions, shops, interiors, as well as for decorative lighting, outdoor wall lighting and consumer lighting such as lamps and ballasts. Therefore, LED use for lighting is likely to represent an increasingly large proportion of the lighting market in the future. To assist engineers in their design approach, the STEVAL-ILL013V1 80 W offline PFC LED driver demonstration board has been developed. This application note describes, step-by-step, all the principles and calculations used for a modified buck converter intended for high brightness LED applications.

The converter is designed as a constant current source to achieve the best lighting performance from the LEDs. A “modified buck" topology was chosen because the power switch is connected to ground rather than the high side switch, as in a standard buck topology, so with this solution it is easier to control the switch. The design uses a fixed offtime (FOT) network operating in continuous conduction mode (CCM), rendering the overall solution simple and cost-effective. The modified buck converter described in this document can be used for lighting applications from low power and low voltage, to high power and high voltage. This allows designers to cover a wide range of different LED systems using a single topology.

Additionally, in lighting applications where the input active power is higher than 25 W and a high power factor is required, the high PF converter can be connected as the first stage, before the modified BUCK converter. The STEVAL-ILL013V1 shows this design concept.

The STEVAL-ILL013V1 demonstration board is an 80 W offline dimmable LED driver with high power factor (PF) intended for 350 mA, 700 mA and 1 A LEDs, and is based on STMicroelectronics’ L6562A transition-mode PFC controller. The design is complaint with standard EN61000-3-2 (limits for harmonic current emissions). The order code is STEVALILL013V1 and the complete design, including schematic diagram, bill of material, calculations, measurements, etc. is described in user manual UM0670 (see Section 3: Reference and related materials).

March 2009

Rev 1

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www.st.com

Contents	AN2928

Contents

1	Modified buck converter in constant current mode . . . . . . . . . . . . . . .		. 4
2	Design equations for the modified buck converter . . . . . . . . . . . . . . . .		6
	2.1	Basic equations for the modified buck converter . . . . . . . . . . . . . . . . . . . .	6
	2.2	Fixed off-time network calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	2.3	LED current calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	2.4	Power MOSFET calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	2.5	Power diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	2.6	Inductor calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
3	Reference and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		19
4	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		20

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AN2928	List of figures

List of figures

Figure 1. Modified buck converter - tON time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 2. Modified buck converter - tOFF time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Modified buck converter - theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Sawtooth signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 5. Real drain MOSFET current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 6. Real power diode current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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Modified buck converter in constant current mode	AN2928

1 Modified buck converter in constant current mode

As stated in the introduction, the aim of this application note is to describe a modified buck converter working in FOT and CCM. The basic principle of the design using the L6562A controller is shown in Figure 1 and Figure 2. Figure 1 represents the stage when the power MOSFET Q1 is turned on. As shown by the red arrow, the current flows from the DC voltage input (VIN) through the load (LEDs), the inductor (L), the power MOSFET Q1 and the sensing resistor. Capacitor C4 is charged via diode D2 and resistor R5, since the transistor Q1 is open and its gate voltage is around 10 V. During the tON time, the load current increases and stops as soon as the voltage on the current sense resistor reaches the internal threshold on the CS pin of the L6562A. The current sense of the L6562A is clamped at 1.08 V (typ). Figure 2 shows the tOFF time, when the power MOSFET is switched off. The inductor keeps the current flowing in the same direction and the circuit is closed through diode D1. The load current is decreasing and the minimum current is set by the fixed off-time network (tOFF time is always constant), because capacitor C4 is discharged to the resistor R4. The voltage on capacitor C4 is connected to the ZCD (zero current detector) pin of the L6562A. As soon as the capacitor is discharged and its voltage falls below 0.7 V (the ZCD threshold), the L6562A switches the power MOSFET again and the load current is increased. This process repeats cycle-by-cycle, as shown in the timing diagrams in Figure 1 and Figure 2.

Figure 1. Modified buck converter - tON time

		VC				VIN
				C1		D1	LEDs
							load
I LED
I_MAX			R1	INV	VCC	tON	L
I_AVR							Q1
				L6562A
I_MIN				COMP	GD
FOT	FOT		R2	MULT	GND
						D2
							RS
tON tOFF tON tOFF		t		CS	ZCD
FOT- fixed off-time					C3	R5
				C2	R3	C4
					R4
				Fixed off-time
				network
							AM00366
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AN2928	Modified buck converter in constant current mode

Figure 2. Modified buck converter - tOFF time

		VC			VIN
			C1		D1	LEDs
						load
I LED
I_MAX		R1			tOFF	L
			INV	VCC
I_AVR						Q1
			L6562A
I_MIN			COMP	GD
FOT	FOT	R2	MULT	GND
					D2
				ZCD		RS
tON tOFF tON tOFF		t	CS
FOT- fixed off-time				C3	R5
			C2	R3	C4
				R4
			Fixed off-time
			network
						AM00367

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Design equations for the modified buck converter	AN2928

2 Design equations for the modified buck converter

This section provides all the calculations required for a designer to develop an application with the modified buck converter working in FOT and CCM. The equations are described step-by-step, following an application design procedure. First, the basic equations for this type of converter are shown, then the components for the tOFF time are calculated, the proper power diode and power MOSFET is selected, and finally the power inductor calculation is demonstrated.

2.1Basic equations for the modified buck converter

Figure 3 shows basic circuit stage during tON and tOFF time, with indicated voltage and component references used in the equations.

The voltage across the inductor L is calculated using the following equation:

Equation 1

V		= L	dIL
	L		-------
			dt

VL= inductor voltage (V)

L = inductance (H)

IL = inductor current (A)

Figure 3. Modified buck converter - theory of operation

		tON time		tOFF time
	VIN	V	VIN	V
		LED		LED
	D1	LEDs	D1	LEDs
		load		load
		L		L
	tON		tOFF
		VL = VIN - VLED		VL = VLED
	Q1		Q1
	RS		RS
				AM00368
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AN2928	Design equations for the modified buck converter

Using Equation 1, it is possible to calculate an inductor current change during tON and tOFF time:

Equation 2

		tO N	VL	(VIN	– VLED ) tON
	∆IL ON =	∫
			------dt	= -------------------------------------------------
			L		L
		0
	Equation 3
		tO N + tO FF		VL	VLED tOFF
	∆IL OFF =		∫
				------dt =	–--------------------------------
				L	L

tON

∆IL_ON = inductor current change during tON time (A)

∆IL_OFF = inductor current change during tOFF time (A) VIN = input voltage (V)

VLED = LED (load) voltage (V) tON = turn-on time (s)

tOFF = turn-off time (s)

In CCM, the inductor current change during tON and tOFF time is the same:

Equation 4

∆lL ON = ∆IL OFF

Using Equation 2 and Equation 3, it is possible to create following equations:

Equation 5

(VIN – VLED ) tON (VLED – tOFF )

------------------------------------------------- = ------------------------------------

L L

Equation 6

VIN tON–VLED tON = VLED tOFF

Equation 7

VIN tON = VLED (tOFF + tON) = VLED T

The duty cycle for the modified buck topology (also valid for a standard buck topology) converter is calculated:

Equation 8

tON VLED

D = --------= -------------

T VIN

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