STEVAL-ILL027V2 demonstration board

AN3111

16/23

Doc ID 16815 Rev 2

capacitor package on the original design. The capacitors C8 and C10 470 nF / 305 V were

selected, because, thanks to this value, EMI behavior is fulfilled as demonstrated in

Figure 16

and 17. The capacitor C9 was replaced by the capacitor 150 nF / 305 V AC.

Maximum input voltage can be up to 362 V (265 V x

resistors must be used for sensing the input voltage. An additional resistor, R16 = 100 k

Ω,

is connected in series with R12 and the additional resistor, R15 = 510 k

series with R9.

Also the voltage divider used for the MULT pin is recalculated in order to have the correct

voltage on the MULT pin. The highest voltage on the MULT pin is presented for maximum

input voltage 265 V AC. In this case the voltage on the MULT pin is done by the following

equation:

Equation 14

Absolute maximum rating for the MULT pin is 8 V, which means that such a margin is high

enough to correctly operate even with maximum input voltage 265 V AC.

The next modification is to change the sense resistors R6 and R11, because these resistors

are used to set the level of constant LED current. The resistors R6 and R11 were replaced

by resistor 2R7.

Regarding the power MOSFET capability, it is possible to calculate its maximum drain

source voltage. Assume that maximum output LED voltage is approximately 72 V (18 LEDs

with maximum forward voltage 4 V) and then maximum drain source voltage is:

Equation 15

For this kind of topology the switching losses are higher for higher input voltage and

therefore the power MOSFET with better current capability is selected for the input voltage

230 V AC. The STD11NM50N power MOSFET is used for the STEVAL-ILL027V2

demonstration board.

Maximum reverse voltage on diode D7 is also 445 V, when the power MOSFET is ON.

Ultrafast diode D7 STTH3R06A with maximum repetitive reverse voltage 600 V was

selected for this application.

Electrolytic capacitor C5 = 10 µF / 35 V is replaced by the small SMD ceramic capacitor

2.2 µF / 50 V as there is a size limitation on the board.

The transformer's number of turns can be used exactly the same as on the original design,

because the output LED voltage is transformed to the auxiliary winding when the MOSFET

is OFF in the same ratio as on the original design. For the output LED power 18 W and

constant LED current 350 mA, the LED voltage is 51.4 V. This voltage is reflected to the

auxiliary winding. The transformer isolation is 500 V DC and so the transformer can be used

in this application for EU input voltage range. The others components are without any

change.

Vmult

Vin max

R3

×

R3

R9

R15

++

----------------------------------------

362

12000

×

12000

510000

510000

++

---------------------------------------------------------------------

4.2 V

==

=

Vds

Vin max

2

V

+

×

led max

265

2

×

72

+

445V

==

=