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 off-
time (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 STEVAL-
ILL013V1 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
1/21
www.st.com
Contents
AN2928
Contents
1
2
Modified buck converter in constant current mode . . . . . . . . . . . . . . . . 4
Design equations for the modified buck converter . . . . . . . . . . . . . . . . 6
2.1
2.2
2.3
2.4
2.5
2.6
Basic equations for the modified buck converter . . . . . . . . . . . . . . . . . . . . 6
Fixed off-time network calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
LED current calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Power MOSFET calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Power diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Inductor calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3
4
Reference and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2/21
AN2928
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Modified buck converter - t
ON
time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Modified buck converter - t
OFF
time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Modified buck converter - theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Sawtooth signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Real drain MOSFET current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Real power diode current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3/21
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 Q
1
is turned on. As shown by the red arrow, the current flows from the DC voltage
input (V
IN
) through the load (LEDs), the inductor (L), the power MOSFET Q
1
and the
sensing resistor. Capacitor C
4
is charged via diode D
2
and resistor R
5
, since the transistor
Q
1
is open and its gate voltage is around 10 V. During the t
ON
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 t
OFF
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 D
1
. The load current is decreasing and the minimum current is set by the fixed off-time
network (t
OFF
time is always constant), because capacitor C
4
is discharged to the resistor
R
4
. The voltage on capacitor C
4
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 - t
ON
time
VC
VIN
LEDs
load
C1
I LED
I_MAX
I_AVR
I_MIN
FOT
FOT
t
R2
R1
INV
COMP
MULT
CS
VCC
GD
GND
D1
tON
L6562A
Q1
L
D2
ZCD
tON
tOFF
tON
tOFF
-
FOT- fixed off-time
RS
C2
R3
C3
R4
R5
C4
Fixed off-time
network
AM00366
4/21
AN2928
Figure 2.
Modified buck converter in constant current mode
Modified buck converter - t
OFF
time
VC
VIN
LEDs
load
C1
I LED
I_MAX
I_AVR
I_MIN
FOT
FOT
R2
t
R1
INV
COMP
MULT
CS
VCC
GD
GND
D1
tOFF
L6562A
Q1
L
D2
ZCD
tON
tOFF
tON
tOFF
FOT- fixed off-time
-
RS
C2
R3
C3
R5
C4
R4
Fixed off-time
network
AM00367
5/21
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