AN1621 APPLICATION NOTE

4/14

Figure 2. Output Current Limiting Characteristic Using Two Current Transformer

The calculated value is R1=3.5

Ω

Fig. 2 shows the constant current characteristic using two current transformers.

The difference from the two current values, at output short-circuit and at current limiting intervention, is propor-

tional to the half of chocke ripple current.A choke with higher value or higher switching frequency, can reduce

this difference. If constant current feature is not requested to be constant till the output is reaching zero V, one

single current transformer can be used.

The new limiting current characteristic and the schematic diagram are shown in Fig 3a and 3b:In order to reduce

the peak current before hiccup intervention, an offset can be superimposed to the current sensing circuit to an-

ticipate the hiccup limiting current intervention, by using the additional network shown below ( figs. 4a and 4b):

Using this solution, also the

∆I/∆Vo is higher, reducing the difference from the intervention point and the short

circuit current values.This solution can be used with two current transformers too.

Figure 3. Output Current Limiting Characteristic Using One Current Transformer

810

12

14

Io

0

5

10

15

20

25

Vo

D96IN456

a: Output current limiting characteristic

using one current transformer.

b: limiting current schematic diagram

468

10

12

14 Io

0

5

10

15

20

25

Vo

D96IN457

L1

D1

C6

10T

R2

D2

R1 3.5

Ω

R8

430

Ω

C16

ISEN

D5

13

L5991A

D96IN458_mod

1T

50T

330pF