STEVAL-ILL013V1 Datasheet, PDF (6/43 Pages) STMicroelectronics – 80 W offline LED driver with PFC

UM0670
3
Design concept
Design concept
The STEVAL-ILL013V1 block schematic is illustrated in Figure 2. As shown, the design is
divided into two main topologies. The first is a high PF (power factor) boost converter, and
the second is a modified buck converter. As an additional board, any external PWM
generator can be used for LED brightness regulation. If no PWM generator is connected to
the STEVAL-ILL013V1, the LED brightness is pre-adjusted to 100%.
There are two main reasons the high PF boost converter is designed on the STEVAL-
ILL013V1 demonstration board. The first is the requirement for lighting equipment with an
input active power higher than 25 W to comply with standard EN61000-3-2 (harmonic
current distortion). Thanks to the high PF converter, compliance to the standard is achieved
with no difficulty. The second reason is that a high input voltage (in this case 400 V) is
needed for the modified buck converter, because it is, in fact, a buck converter and thus the
input voltage must be higher than the output voltage. The output LED voltage can be up to
228 V, as was shown in Table 1. An additional advantage of the high PF converter is its wide
input voltage range. This allows the demonstration board to be used either in either the
European or US markets. A more detailed description of the high PF boost converter is
provided in the EVL6562A-TM-80W data brief (see Section 11: References and related
materials: 1).
The second converter is designed as a constant current source, as it ensures the best
lighting performance from the LEDs. Concerning the topology, the “modified buck” has been
chosen, “modified” insofar as the power switch is connected to ground (instead of the high-
side switch, as in a standard buck topology) and therefore it is easier to control the switch.
The design uses a FOT (fixed off-time) network, operating in CCM (continuous conduction
mode) and thanks to this principle the overall solution is very simple and cost effective. All
equations needed for proper modified buck converter design are described in application
note AN2928 (see Section 11: References and related materials: 2).
Figure 2. STEVAL-ILL013V1 block schematic
STEVAL- ILL013V1
HIGH POWER FACTOR BOOST CONVERTER
STTH1L06
Input
filter
VIN = 88 V to
265 V AC
STP8NM50FP
PF controller
L6562A
MODIFIED BUCK CONVERTER
400 V
80 W LOAD
STPSC806D
PF controller
L6562A
STF9NM50N
EXTERNAL PWM GENERATOR
Microcontroller
ST7LITEU05
Bright regulation
AM00400
Doc ID 15327 Rev 2
6/43