AN1660 APPLICATION NOTE

6/13

Choose Cf = 220pF, then

Rfstart is 35.2Kohm , this is larger than the minimum value of Rfstart which is 25Kohm.

In this application, we use R6, R7 voltage divider to get a fixed voltage to Pin6 (OPin-) as the voltage of

Pin2 (Rfstart) is fixed at Vref = 2V. At the same time Pin7(OPin+) connected to primary current sense re-

sister. Voltage of Pin7 (Vopin+) will increase while output power increase. The output of the operational

amplifier will be high when Vopin+ tends to be higher than Vopin-. Then another voltage divider R8, R9

will send this high voltage to Pin9 (EN2). L6598 will stop the normal operation and restart as long as EN2

is high. Thus over-current-protection is achieved.

If set R6 = 27Kohm , R7 = 6.8Kohm, then the input voltage for Pin6 is 0.4V.

Now,

Rfstart = R6+R7 = 33.8Kohm

Rfmin = R11 = 100Kohm

Recalculated Fstart & Fmin. Fmin is 64KHz, Fstart is 254KHz.

The design for resonant components will be finished in the followed step.

3.2 Design transformer and resonant components

First select core size.

(3)

For half-bridge converter, k = 0.165

With Pout = 180W,

η = 94%, ∆B = 0.4T, Fmin = 64KHz,

Choose core: EC39, with Ae = 1.32cm2

(4)

(5)

Rfm in

1.41

68 10

3

220 10

12

–

⋅⋅

⋅

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

94.2 k

Ω

100

÷

k

Ω

==

Rfstart

1.41

250

68

–

() 10

3

220 10

12

–

⋅⋅

⋅

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

32.5 k

Ω

==

A

P

11.1 Pin

⋅

kB

∆

F

⋅⋅

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





1.31

=

Np mi n

Vin mi n

() 10

4

⋅

2fsm i n

B

∆

Ae

⋅⋅

⋅

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

=

Np mi n

370 10

4

⋅

26810

3

0.4 1.32

⋅⋅

⋅

⋅

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

51.5

==

n

N

p

N

s1

----------

V

in ma x

2

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

V

o1

V

F1

+

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

≥

=

n

420

2

----------

110

1

+

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

≥

1.9

=

Ns

2

V

o2

V

F2

+

()N

s1

V

o1

V

F1

+

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

=