Datasheet

SEC1 SEC2
diode_max IN_max PRI IN_max PRI
OPOS ONEG
PRI PRI
N N
V = (V - V ) x + V = (V - V ) x + V
N N
( )
CPRI_ch CPRI
PRI
PRI
I × t
C =
VD
CPRI
SW SW
D (1 - D) ILpri_pospk
t + x
f f ILpri_pospk - ILpri_negpk
»
CPRI_ch
ILpri_pospk
D + (1 - D) x
ILpri_pospk - ILpri_negpk
I ILpri_pospk x
3
»
CPRI_ LS HSI rms I _rms + I _rms»
( )
1
2
2 2
LS
OPN OPN
3 × D - 1 Im_ripple 1 - D
I _ rms × I + × I + × Im_ripple
3 × 1 - D 3 12
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÷
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ç
»
÷
ç
÷
÷
ç
è ø
1
2
2 2
HS
OPN
D
I _ rms D × I + × Im_ripple
12
æ ö
÷
ç
»
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( )
IN
OPRI
sw
V × D × 1 - D
Im_ ripple =
× Lf
TPS55010
SLVSAV0A –APRIL 2011–REVISED JUNE 2011 www.ti.com
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PRIMARY SIDE CAPACITOR
The ΔV
PRI
voltage should be less than 10% of V
PRI
. The rated RMS current of C
PRI
should be greater than
Equation 49. It is desirable to have a larger primary capacitance to minimize ripple but this will slow the transient
response. For this design example, the charging current and time need to be calculated using Equation 50 and
Equation 51. The I
CPRI_ch
is 0.63 A and the t
CPRI
is 1.56 µs. Assuming the ΔV
PRI
is 0.193 V, the primary side
capacitance is 5.09 µF using Equation 49. The rms current is 1.16 A from Equation 49. A 10 µF/25 V X5R
ceramic capacitor is used.
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SECONDARY SIDE DIODE
The diodes should be selected to handle the voltage stresses and rms currents calculated in Equation 53 and
Equation 55. Typically, a low duty cycle or high turns ratio design will have a larger voltage stress on the diode
At the maximum input voltage of 5.5 V, the V
diode_max
voltage is calculated at 43.56 V. The rms current is
calculated as 0.059 A. The diode peak current is 0.130 A using Equation 54 and the power dissipated in the
diode is 0.02 W. The B1100 diode will be used which is rated for 100 V and 1 A.
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