LV5028TT

No.A1999-10/16

Sense resistor is obtained as follows.

Rs =

VREF_IN

Ipk_p

=

VREF_IN

× Lp

Vac

× Ton_p =

VREF_IN

× Lp

Vac

× Don_p × T

(16)

[Inductance Ls of secondary side]

Since output current Iout is the average value of current flows to transformer of secondary side

Iout = Ipk_s

×

Ton_s

T

×

1

2

=

Ipk_s

× Don_s

2

(Don_s =

Ton_s

T

= Ton_s

×

ƒosc) (17)

Ipk_s =

Vout

Ls

× Ton_s =

Vout

Ls

=

Don_s

ƒosc

(18)

Ls =

Vout

2

× Iout

=

Vout

2

× Iout ×

ƒosc =

Vout

2

2

× Pout ×

ƒosc (19)

Calculation of the ratio of transformer coil on primary side and secondary side

Since ratio and inductance of transformer coil is

Ns

Np

=

√Ls

√Lp

(20)

substituted equations (15), (19) for (20)

∴

Np

Ns

=

Vac

Vout

× √

η ×

Don_p

Don_s

(21)

Calculation of transformer coil on primary side and secondary side

N =

Vac

2

× ΔB × Ae ×

ƒosc (22)

ΔB: variation range of core flux density [Gauss]

Ae: core section area [cm

To use Al (L value at 100T),

N =

Al

(23)

L: inductance [

μH]

Al: L value at 100T [uH/N

lg (Air gap) is obtained as follows:

lg =

μ

2

L

(24)

μr: relative magnetic permeability, μr = 1

N: turn count [T]

Ae: core section area [m

L: inductance [H]