ICL8001G / ICLS8082G
Design Guidelines
Design Parameters
Application Note
13
Version 2.0, 2011-04-14
4.5
Switch-on Determination for Quasi-resonant Operation
For ideal quasi-resonant operation the switch-on of the power switch should occur when the transformer current
in the output circuit reaches zero and half of the oscillation period (oscillation 2 in
Figure 5
) defined by the leakage
inductance and the drain-source capacitance of the power switch has elapsed.
Figure 5
Determination of the switch-on instant
The delay td elapses from the instant of discharging the transformer to when the first VDS valley is reached. For
initial dimensioning of the delay circuit the relationship
(21)
can be used. For the 230 V / 10 W demo board an initial value of td ≈ 1 µs is appropriate. For output overvoltage
protection with final shutdown at Vo > 45 V it turns out to be useful to choose Rzc2 ≈ 2 kΩ. Consequently, the
capacitance of the delay circuit Czc = 580 pF (choose Czc = 470 pF) will be obtained according to
(22)
Cds
L
td
2
1
2
1
Rzc
Rzc
Rzc
Rzc
td
Czc
