Datasheet
PRI
IN
V
= D
V
O SEC
IN PRI
V N
= × D
V N
T1
C
PRI
+
_
D1
+
V
PRI
_
T1
C
PRI
+
_
D1
+
V
PRI
_
D x Ts
(1 - D) x Ts
+
V
O
V
O
_
T1
C
IN
C
IN
C
PRI
VIN
N N
PRI SEC
C
O
C
O
+
V
PRI
_
D1
S
LS
S
LS
S
HS
S
HS
C
IN
C
IN
V
O
C
O
S
LS
S
HS
N N
PRI SEC
N N
PRI SEC
TPS55010
www.ti.com
SLVSAV0A –APRIL 2011–REVISED JUNE 2011
During the off time of S
HS
, S
LS
conducts and the voltage on C
PRI
continues to increase during a portion of the S
LS
conduction time. The voltage increase is due to the energy transfer from L
PRI
to C
PRI
. For the remaining portion of
the S
LS
conduction time, the C
PRI
voltage decreases because of current in L
PRI
reverses; see the IL
PRI
and V
PRI
waveforms in Figure 23. By neglecting the diode voltage drop, conduction dead time and leakage inductance, the
input to output voltage conversion ratio can be derived as shown in Equation 7 from the flux balance in L
PRI
. It
can be seen in Equation 7 that the input to output relationship is the same as a buck-derived converter with
transformer isolation. The dc voltage V
PRI
on the primary side capacitor in Equation 8 has the same linear
relationship to the input voltage as a buck converter.
Figure 22.
The small signal model for the Fly-Buck is derived by changing the transformer to the inductor equivalent and
reflecting the output filter to the primary side for the circuit shown in Figure 22. Assuming negligible leakage
inductance and equivalent series resistance for the capacitors, the V
PRI
transfer function is similar to the current
mode control buck power stage transfer function with the exception that the C
O
and load are in parallel with the
C
PRI
only for the 1-D time. Averaging the secondary side components, an approximate transfer function is shown
in Equation 9 and pole location in Equation 10. R
O
is the secondary side load resistance and the R
LM
is the dc
resistance of the primary. R
i
is the inverse of the Comp to PH gm.
(7)
spacer
(8)
Spacer
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