SHINDENGEN

ELECTRIC MFG.CO.,LTD

- 22 -

4

Determining peripheral circuit constants

The characteristics of a PFC circuit also depend largely on peripheral components besides IC.

For optimum design, clarify the specifications for the required power supply and select

components as described below.

4.1

Selecting choke coils (L111, L121, L131…)

Choke coils are important components that can determine the performance of a PFC circuit. Work

out the ideal constants for PFC circuit specifications using the formulas below. Inquire with coil

manufacturers, if necessary.

Check for rising core and coil temperatures in an actual device before finalizing coil and core

sizes.

<Selecting the core size>

Select the core size to make sure that core gap Ig does not exceed 2 mm.

Use formulas (1) to (5) below to calculate the core gap Ig.

]

mm

[

10

Lp

4

lg

7

-

2

　　









Np

Ae



... (1)

]

mH

[

10

Idp

2

min

)

(









AC

Vin

Ton

Lp

... (2)

]

A

[

Vin(AC)min

2

2

　　











Ps

Idp

... (3)

]

[

min

max

s

f

Don

T

Don

Ton

　　







... (4)

Vo

AC

Vin

Vo

Don

2

min

)

(







... (5)

Ae represents the effective cross sectional area [mm

1.5 times as high as Po(max)) at the droop point, and fmin the minimum oscillation frequency [Hz]

(auto-sensing power supply: 40 k to 60 kHz, 100 V/ 200 V group: 50 k to 70 kHz). The figures in

parentheses are provided as guidelines only.

<Number of turns of Np coil>

The result given by formula (6), rounded down to the nearest integer, gives the number of turns of

the Np choke coil.

]

[

10

Ae

B

2

min

)

(

9

Turn

AC

Vin

Ton

Np

　　

Δ











... (6)

ΔB represents the magnetic flux density variation [mT] of the core. ΔB can vary significantly from

core to core. Inquire with the coil manufacturer.

<Number of turns of Nc coil>

Apply formula (7) to select the minimum integer for the number of turns of the Nc coil. A minimum

voltage of 1.5 V should be generated in the control coil at maximum input voltage.

]

[

max}

)

(

2

{

5

.

1

Turn

AC

Vin

Vo

Np

Nc

　　









... (7)

In the case of an auto-sensing power supply, assuming that Vin(AC)max is 264 V and that PFC

output voltage Vo is 390 V, a ratio of the numbers of turns of Np and Nc is approximately 10 to 1.

Example) The number of turns of Np with an auto-sensing power supply is 50.

Assuming that Vin(AC)max = 264 V and Vo = 390 V, Nc > 4.5 and Nc = 5 turns.