5/14

AN1621 APPLICATION NOTE

Figure 4. Output Current Limiting Characteristic with Foldback

2.6

Output Diode selection

The reverse voltage of the output diodes is given by the formula:

Vr = Vin max/n = 375/3.3= 114V,

where n is the transformer turns ratio.

For a correct functionality a ultra-fast recovery diode is requested, mainly to limit switching losses and EMI prob-

lems.

To calculate the conduction losses the following equation has been applied to the selected type, BYV52-200.

For the single diode the formula is:

2

(RMS)

that, rearranged to take into account both the diodes, becomes:

Considering the TO247 package (1.2 °C/W total thermal resistance junction-heatsink), to ensure that the junc-

tion temperature does not exceed 100 °C at 40 °C max. ambient temperature, the heatsink has to be dimen-

sioned for about 4°C/W.

2.7

Power transformer design

The forward transformer delivers energy from the primary to the secondary without any storage.

The only consideration is with regards of the magnetising current, that has to be limited at a safety value far from

core saturation.

Our core selection is based on AP, area product, defined as AP = Aw · Ae.

a: Output current limiting characteristic

b: Schematic diagram of the modified

468

10

12

14 Io

0

5

10

15

20

25

Vo

D96IN459

13

R8

3K

D96IN460_mod

L5991A

C16

ISEN

10T

P1

R1

4

3K

C2

C3

C4

L1

1 T

D1

D5

D2

C6

R2

R11

1.2K

50T

5

FB

120K

2K

hiccup limiting current thereshold.