UM0670 datasheet(6/43 Pages) STMICROELECTRONICS

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UM0670

Design concept

Doc ID 15327 Rev 2

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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

Microcontroller

ST7LITEU05

PF controller

L6562A

EXTERNAL PWM GENERATOR

MODIFIED BUCK CONVERTER

HIGH POWER FACTOR BOOST CONVERTER

Bright regulation

STTH1L06

STP8NM50FP

VIN = 88 V to

265 V AC

Input

filter

PF controller

L6562A

400 V

80 W LOAD

STPSC806D

STF9NM50N

STEVAL- ILL013V1

AM00400

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