AN4501 datasheet(23/48 Pages) STMICROELECTRONICS

23 / 48 page

DocID026394 Rev 1

23/48

AN4501

Selection of the inductor

ringing at the drain of the MOSFET with a typical frequency of some MHz. The effect is

caused by the inductor in conjunction with the parasitic capacitances of the freewheeling

diode and of the switching MOSFET, when the inductor current is close to zero.

We propose to calculate the minimum inductance value, which guarantees an operation in

CCM over the whole input voltage range.

Figure 9. Inductor current at the boundary between CCM and DCM

The Figure 9 shows the inductor waveform at the boundary between CCM and DCM. This

condition can be expressed as: IL,PP = 2 x IL.

Using Equation 3 and Equation 5, we obtain:

where LBOUNDARY is the inductance, for which the boost converter operates at the boundary

between CCM and DCM

Extracting the inductance value yields:

Equation 15:

The expression of the derived function of LBOUNDARY over D is:

This equation shows that, for a given output voltage and output current, LCRIT reaches its

maximum for a duty cycle of 33%:

•

an operation in CCM at a duty cycle of 33% guarantees the CCM over the whole duty

cycle range

•

an inductance value higher than

ensures the operation in CCM over the whole operating range

OU T

–

()

BOUNDARY

×η

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

OUT

–

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

BOUNDARY

OUT

–

()

OU T

×η

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

BOUNDARY

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

OU T

–

()

13D

–

()

OUT

×η

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

BOUNDARY

OUT

×η

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

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