AN2454

Active start-up circuit

13/24

Equation 14

7

Active start-up circuit

A non-dissipative, active start-up circuit has been implemented to optimize converter

efficiency. The alternative option to use a pure resistive start-up circuit was rejected

because total converter efficiency would have been extremely low due to the low output

power (2 W).

To select the components of the start-up circuit, the following steps should be followed:

1.

2.

driver consumption (see above) and using Equation 15 below.

3.

Set the start-up capacitor according to the start-up and UVLO threshold of the

UC3845B and total consumption during start-up time (i.e. the time between the points

when start-up threshold and output and auxiliary steady state voltage are reached).

4.

Verify the relevant start-up time and test the wake-up time.

5.

Fine tune the necessary components. For example, if wake-up time is too long the

Figure 1 may need to be increased.

Equation 15

voltage may then be calculated using Equation 16.

Equation 16

output power is 2 W.

µA. Substituting into Equation 17, Equation 18, a value of 0.66 W for the additional power

dissipation is still too high since it represents about 33% of total output power.

R

15

15V

4.4mA

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

3.4k

Ω

=

=

R

start

upmax

–

V

dcmin

I

UC3845

start

–

up

–

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

300k

Ω

=

=

P

Rstart

1200

2

300

10

3

•

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

4.8W

=

=