AN892 Datasheet, PDF (13/16 Pages) Silicon Laboratories – DESIGN GUIDE FOR ISOLATED DC/DC USING THE

AN892
4.2. C1 Selection
When S2 is closed, a resonant current in the primary is developed with frequency given by Equation 4. To ensure
zero current switching of the diode by the time S1 closed, S2 open mode begins, C1 should be chosen so that at
least half of the resonant period is completed in (1–D)T_sw time.
_r  _---₁-----–----_D-^----_--_T----_s---_w--
Equation 41.
Combining Equation 4 and Equation 41 and solving for C1:
C1

-_L---¹_l-_k---_g-


_---1_{-----–----}D_-----_--T_{----s---w--}
2

9.1uF
Equation 42.
The next standard size capacitor 10 F was chosen.
4.3. D1 Selection
Equations 23 and 24 define the requirements for selecting D1. Substituting into Equation 23,
I_{D1  AVE1 – D}
=
I_L
O
A
D


1^{-----–-1----}D^{---}
=
0.4 _{ 0}----_.-¹⁷----₅--_
=
0.533A
Equation 43.
Diode current capacities are usually specified in rms. Assuming a half wave sinusoid current through D1, consider
the translation of average to rms:
l_{D1 RMS1 – D}
=
I_D
1
A
V
E
1
–

D_
2^{--------}2^{--}
=
0.592A
Equation 44.
Substituting
biasing:
into
Equation
24,
consider
using
the
maximum
expected
V_IN
as
the
worst
case
requirement
for
reverse
V_{D1 REVD} = V_{IN MAX1 – D}N + V_OUT = 5.50.754 + 5 = 21.5v
Equation 45.
Equations 24 and 45 do not include the voltage spike due to the interaction of the diode capacitance and leakage
inductance and as a result, a diode with a larger withstanding voltage is required in practice. When selecting D1,
Schottky diodes are the preferred choice due to their low forward voltage as it minimizes the associated power
loss.
A 1A, 40V Schottky diode was selected.
4.4. C10 Selection
C10 is inversely proportional to output voltage ripple and sets the crossover frequency of control loop gain. It is
suggested to use the minimum size capacitor to meet output voltage ripple requirements. Rearranging Equation
21,
C10 = _V---^I--_OL---^O-_U---^AT---^D_---_R-^D--_I--_P-^T---_P-^s--_Lw---_E-  -⁰---^.--⁴----^-----⁰---^.--²⁰---⁵-_.--₀--^-₅----⁴----^x---¹----⁰---^–---⁶-  8F
Equation 46.
Rev. 0.1
13