Circuit design

AN3111

8/23

Doc ID 16815 Rev 2

The frequency is:

Equation 5

The switching frequency varies during the line cycle, which is good for EMI.

The max. frequency occurs at peak input voltage:

Equation 6

The input power is:

Equation 7

Equation 8

The integration term of Equation 8 is a constant value, so the power is determined by Ipk

only. There is no simple solution form for the integration term. The average value of the input

voltage and duty cycle can be used to perform the estimation. After Ipk is calculated, the

inductance can be determined according to the desired switching frequency range.

The average input voltage over half-cycle at 120 VAC line is:

Equation 9

The average duty cycle is:

Equation 10

Therefore:

Equation 11

After the current is determined, it is necessary to choose the right inductance value. The

inductance affects the running frequency. The maximum switching frequency below 200 kHz

has been chosen.

)

Vout

)

(

Vin

)

(

Vin

*

Vout

(

Ipk

*

L

1

T

1

)

(

fsw

+

θ

θ

=

=

θ

)

Vout

Vpk

Vpk

*

Vout

(

Ipk

*

L

1

max

fsw

+

=

dt

)

(

fsw

*

Ipk

*

L

*

2

1

P

2

0

θ

=

∫

π

θ

θ

θ

=

∫

π

d

)

(

Vin

*

)

(

D

*

Ipk

*

2

1

P

0

=

θ

θ

=

∫

π

0

d

)

sin(

*

Vpk

Vave

108 V

=

+

=

Vout

Vave

Vout

Dave

0.333

=

=

Dave

*

Vave

*

2

1

Pin

Ipk

1.2 A