UM0670 datasheet(40/43 Pages) STMICROELECTRONICS

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UM0670

Design calculation

Doc ID 15327 Rev 2

40/43

Power loss on the STPSC806D diode is:

Equation 17

where forward diode voltage was found for the average diode current 0.8 A in the datasheet

(see datasheet Section 11: References and related materials: 4.).

Calculated junction diode temperature without using heat sink is:

Equation 18

Junction-to-case thermal resistance is available in the datasheet for the STPSC806D and

case-to-ambient temperature is determined by the device package used. In this case, the

TO-220 package is used and its thermal resistance is typically 60 °C / W.

Calculated junction diode temperature without using a heat sink is much lower than the

maximum junction temperature for the STPSC806D, and therefore this diode is suitable for

the design.

One of the most important things to consider is proper inductor design. The inductor size

was calculated in Equation 8, but generally the inductor size by itself is not enough to ensure

proper inductor design and therefore several additional equations are used for completing

overall inductor construction.

First, the inductor core size must be selected and for this selection it is very helpful to

calculate the minimum area product using application parameters. Minimum required core

area product (AP), where the flux swing is limited by core saturation is:

Equation 19

where the constant is Cl = JMAX x CR x 10

–4 = 420 x 0.5 x 10–4.

The inductor core E25 from EPCOS was selected. The minimum core cross section is 51.5

mm2 and the winding cross section is 61 mm2 (see datasheet Section 11: References and

related materials: 5.) and the calculated product area is:

Equation 20

The calculated product area is bigger than the minimum required product area, and

therefore the inductor core E25 can be used.

LOSS D

AVR D

0.8

0.7

0.56W

LOSS D

thJC

thCA

()

0.56

2.4

() 30 65° C

MIN

PEAK

RMS

MAX

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

⎝⎠

⎛⎞

---

1.6

–

1.4

0.3

420

0.5

–

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

⎝⎠

⎜⎟

⎛⎞

---

0.2518cm

MIN

51.5

0 31415cm

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