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DAP011

PWM Current−Mode Controller for High−Power Universal Off−Line Supplies

ON Semiconductor 

DAP011/DAP011C

Startup Sequence

When the power supply is first connected to the mains

outlet, the internal current source is biased and charges up

the VCC capacitor. When the voltage on this VCC capacitor

reaches the VCCON level (typically 12.8 V), the current

source turns off, reducing the amount of power being

dissipated. At this time, the VCC capacitor only supplies the

controller, and the auxiliary supply should take over before

VCC collapses below VCC(min). Figure 20 shows the internal

arrangement of this structure:

and IC2. At power−up, as long as VCC is below a certain

level (1.8 V typical), the source delivers IC1 (around

500 mA typical), then, when VCC reaches 1.8 V, the source

smoothly transitions to IC2 and delivers its nominal value.

As a result, in case of short−circuit between VCC and GND,

the power dissipation will drop to 370 x 500 m = 185 mW.

Figure 21 portrays this particular behavior:

VCC

+

−

+ VCC ON

VCC latch

HV

14

IC1 or 0

10

8

IC2 min

IC1 min

VCCON

CVCC = 22 mF

Vth

Figure 20. The Current Source brings VCC above 15 V

and then turns off

In some fault situations, a short−circuit can purposely

occur between VCC and GND. In high line conditions

(VHV = 370 VDC) the current delivered by the startup

device will seriously increase the junction temperature. For

instance, since IC1 equals 2 mA (the minimum corresponds

to the highest Tj), the device would dissipate

370 x 2 m = 740 mW. To avoid this situation, the controller

includes a novel circuitry made of two startup levels, IC1

t1

t2

Figure 21. The Startup Source Now Features a Dual

Level Startup Current

The first startup period is calculated by the formula C x V

= I x t, which implies a 22 m x 1.5 / 350 m = 94ms startup time

for the first sequence. The second sequence is obtained by

changing to 2 mA with a DV of VCCON – VCCth = 12.8 – 1.5

= 11.3 V, which finally leads to a second startup time of 12.8

x 22 m / 2 m = 140 ms. The total startup time becomes 94 m

+ 140 m = 235 ms. Please note that this calculation is

approximated by the presence of the knee in the vicinity of

the transition.

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