ST AN2928 Application note
AN2928
Applica t ion 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 connecte d 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 1/21
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
2/21
AN2928 List of figures
List of figures
Figure 1. Modified buck converter - tON time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2. Modified buck converter - t
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
time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
OFF
3/21
Modified buck converter in constant curren t 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
input (V
sensing resistor. Capacitor C
Q
is open and its gate voltage is around 10 V. During the tON time, the load current
1
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
inductor keeps the current flowing in the same direction and the circuit is closed through
diode D
network (t
R
. The voltage on capacitor C4 is connected to the ZCD (zero current detector) pin of the
4
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 pr ocess repeat s cycle-b y-cycl e, as shown in the ti ming diag rams in Figure 1
and Figure 2.
is turned on. As shown by the red arrow, the current flows from the DC volt age
1
) through the load (LEDs), the inductor (L), the power MOSFET Q1 and the
IN
. The load current is decreasing and the minimum current is set by the fixed off-time
1
time is always constant), because capacitor C4 is discharged to the resistor
OFF
is charged via diode D2 and resistor R5, since the transistor
4
time, when the power MOSFET is switched off. The
OFF
Figure 1. Modified buck converter - t
V
C
I
LED
R
I
_MAX
I
_AVR
I
_MIN
R
FOT
t
t
ON
OFF
-
FOT- fixed off-time
-
t
ON
FOT
t
OFF
t
t
t
1
2
ON
time
C
1
VCC
INV
L6562A
GND
ZCD
R
3
GD
COMP
MULT
CS
C
2
Fixed off-time
network
V
IN
Q
R
LEDs
load
L
1
S
AM00366
D
1
t
ON
D
2
C
R
3
R
5
C
4
4
4/21
AN2928 Modified buck converter in constant current mode
Figure 2. Modified buck converter - t
V
C
I
LED
R
I
_MAX
I
_AVR
I
_MIN
FOT
t
t
OFF
-
t
ON
ON
FOT-fixed off-time
FOT
t
OFF
1
R
2
t
OFF
time
C
1
INV
L6562A
COMP
MULT
CS
C
2
Fixed off-time
network
VCC
GND
ZCD
R
3
GD
V
IN
D
t
OFF
1
LEDs
load
L
Q
1
D
2
C
3
R
4
R
S
R
5
C
4
AM00367
5/21
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 t
proper power diode and power MOSFET is selected, and finally the power inductor
calculation is demonstrated.
2.1 Basic equations for the modif ied buck conver ter
time are calculated, the
OFF
Figure 3 shows basic circuit stage during tON and t
component references used in the equations.
The voltage across the inductor L is calculated using the following equation:
Equation 1
VLL
= inductor voltage (V)
V
L
L = inductance (H)
= inductor current (A)
I
L
Figure 3. Modified buck convert er - the or y of operation
⋅=
tON time t
V
IN
LEDs
D
1
LEDs
load
V
LED
V
IN
D
1
time, with indicated voltage and
OFF
dI
L
--------
dt
time
OFF
LEDs
load
V
LED
L
t
ON
VL = VIN -V
Q
1
R
S
6/21
-
LED
t
OFF
Q
R
L
VL = V
LED
1
S
AM00368
AN2928 Design equations for the modified buck converter
Using Equation 1, it is possible to calculate an inductor current change during tON and t
time:
Equation 2
t
ON
∆I
LON
V
------
∫
L
0
VINV
L
td
–()tON⋅
--------------------------------------------------==
LED
L
Equation 3
tONt
+
OFF
∆I
LOFF
∆I
= inductor current change during tON time (A)
L_ON
∆I
V
V
t
ON
t
OFF
= inductor current change during t
L_OFF
= input voltage (V)
IN
= LED (load) voltage (V)
LED
= turn-on time (s)
= turn-off time (s)
∫
t
ON
OFF
In CCM, the inductor current change during t
V
L
------
L
time (A)
ON
td
and t
V
⋅
LEDtOFF
---------------------------------–==
L
time is the same:
OFF
OFF
Equation 4
∆l
=
LON∆ILOFF
Using Equation 2 and Equation 3, it is possible to create following equations:
Equation 5
VINV
–()tON⋅
--------------------------------------------------
LED
L
V
–()
LEDtOFF
------------------------------------=
L
Equation 6
V
INtONVLEDtONVLEDtOFF
⋅=⋅–⋅
Equation 7
V
INtONVLEDtOFFtON
+()V
LED
T⋅=⋅=⋅
The duty cycle for the modified buck topology (also valid for a standard buck topology)
converter is calculated:
Equation 8
t
V
ON
---------
D
LED
------------- -==
T
V
IN
7/21
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