AN2928

Design equations for the modified buck converter

11/21

Equation 22

●

●

●

The average inductor (LED) current is independent of the input voltage

●

The average inductor (LED) current depends slightly on the voltage across the LEDs

(i.e. number of LEDs) and therefore the design shows the best results using a fixed

number of LEDs. A variable number of LEDs results in less current precision.

2.4

Power MOSFET calculation

The power MOSFET is chosen based on maximum stress voltage, maximum peak

MOSFET current, total power losses, maximum allowed operating temperature and the

driver capability of the L6562A.

Maximum stress voltage on the power MOSFET (drain-source voltage) for this modified

buck converter is equal to the input voltage. The power MOSFET must be selected with

some voltage margin. For example, if the input voltage is maximally 400 V, then maximum

drain-source voltage should be 450 V or higher.

Maximum peak MOSFET current was selected in order to calculate the inductor size in

Equation 20. Also in this case, the power MOSFET must be chosen with some current

margin.

Total power losses on the power MOSFET must be calculated, due to the importance of

designing a proper heat sink to avoid temperature stress on the power MOSFET. Basically,

total power losses on the power MOSFET occur through conduction losses (depending on

capacitance and then discharging this capacitance to ground. The gate charge loss is very

small compared to the conduction and switching losses, so it is not used for further

calculations. For total power MOSFET loss, a valid equation is:

Equation 23

The power MOSFET conduction loss is represented by the continuous conduction current

flowing through the MOSFET during the on-time stage. Therefore, the power loss depends

properly, it is necessary to calculate the drain current RMS value. Figure 4 shows the

sawtooth signal for which the RMS value was calculated in Equation 24. Note that in this

l

AVR

l

MAX

V

LED

1D

–

()

⋅

2Lf

⋅⋅

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

l

MAX

V

LED

t

OFF

⋅

2L

⋅

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

–

=

–

=

P

TOT

P

CON

P

SW

+

=