Start-up network

AN1889

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8

Start-up network

To let the circuit start as soon as the line voltage is applied it is necessary to pre-charge both

C10 and C4 base capacitances. A resistor connected to the mid point of the input filter

capacitor divider directly makes the pre-charge. Of course an active start-up circuit, avoiding

dissipation during steady-state behavior, can improve the overall efficiency of the converter

at expenses of increased circuit complexity and cost.

threshold of 16 V, therefore the total turn-on current is:

Equation 35

and

Equation 36

R6 has been supposed equal to 56k

Ω, whose value is enough to pre-charge the base

capacitor C4, anyway it is important to check a posteriori that the time constant established

by R6 and C4 is negligible with respect to that imposed by (R1+R2) and C10.

C4 must be able to supply UC3842 till the steady-state behavior of the converter is

established. This time, from bench verification is 15ms maximum. Being under-voltage

threshold of UC3842 8.5V, the voltage across C4 must always be over 8.5V during the start-

up period. Therefore:

Equation 37

The next closed commercial value for C10 = 47

µF has been chosen.

Therefore:

Equation 38

This completely verifies our assumption.

I

tot

I

UC3842

start

–

up

–

I

R12

R13

+

I

R6

0.5mA

16V

25.9k

Ω

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

16V

56k

Ω

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

1.4mA

=

++

=

++

=

R

1

R

2

+

()

min

V

dcmin

2I

TOT

•

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

78k

Ω

=

=

C

10min

I

UC3842

quiescent

–

I

R12

R13

+

I

R6

++

() t

start

up

–

•

16

8.5

–

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

18mA

15ms

•

7.5

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

36

µF

=

≅

=

R

6

C

4

•

R

1

R

2

+

()

min

C

10

•

12.3ms

3.6s

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