Datasheet

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IN
OPN
SW OPRI
V × D × 1 - D
1 + D
ILpri_negpk -I x -
1 - D 2 × × L
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f
( )
IN
OPN
SW OPRI
V × D × 1 - D
ILpri_pospk I +
2 × × L
»
f
( )
( )
IN
OMIN
SW HSCL OPN
V × D × 1 - D
L =
2 × × I - If
( )
IN
OMAX
OPN SW
V × D × 1 - D
L =
2 × I × f
SEC1 SEC2
PRI PRI
OPN OPOS ONEG
N N
I = I + I
N N
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è ø
PRI
HS LS
V - 0.829V
R = R ×
0.829V
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+ x
FD
SEC1 SEC2 OPOS ONEG
PRI PRI
N + N V - V 2 V
=
N V
TPS55010
www.ti.com SLVSAV0A –APRIL 2011–REVISED JUNE 2011
Turns Ratio
The transformer turns ratio is calculated using the desired output voltages, diode voltages and the primary
voltage. Assuming diode voltages of 0.5 V, V
O
pos of 15 V, V
O
neg of -15V and a V
PRI
of 1.93 V yields a N
PRI
x
N
SEC1
x N
SEC2
turns ratio of 1:8:8. Since the TPS55010 is flexible on the adjusting the primary side, a couple
iterations of selecting turns ratio may help find a solution that is good for multiple applications with the same
transformer.
(39)
VOLTAGE FEEDBACK
Selecting 10 kΩ for the R
LS
, R
HS
is calculated to be 13.28 kΩ using Equation 40. Choose 13.7 kΩ as the nearest
standard value.
(40)
SELECTING THE SWITCHING FREQUENCY and PRIMARY INDUCTANCE
For this design example, the switching frequency is selected to be 400 kHz. Using Equation 6, the timing resistor
value is 243 kΩ. L
O
max and L
O
min are calculated to be 2.31 µH and 1.09 µH respectively assuming a current
limit of 2 A. Selecting a primary inductance of the 2 µH, the positive and negative peak current are calculated as
1.38 A and -2.19 A in the primary which do not exceed the current limits of the power switch. The rms currents
can be calculated and used to determine the power dissipation in the device. The magnetizing ripple current is
calculated as 1.48 A using Equation 46.
The highside FET and lowside FET rms currents are calculated as 0.478 A and 0.681 A, respectively using
Equation 47 and Equation 48. The sum of these currents, i.e. 1.16 A is the primary side rms current for the
magnetics.
(41)
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