Datasheet

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   
SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
21
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TYPICAL APPLICATION
II. Two Switch Forward DC−to−DC Power Stage
A two-switch forward converter topology was selected for the second stage of this design. The two-switch
forward power converter has two major advantages over a traditional forward converter, making it ideal for this
application. First, the FETs used in the two-switch forward required only one-half the maximum V
DS
as
compared to the traditional forward converter. Second, the transformer’s reset energy is returned to the input
through clamping diodes for higher efficiency.
transformer turns ratio
Equation (29) calculates the transformer turns ratio required for the two-switch forward power converter of this
design example. It can be derived from the dc transfer function of a forward converter. V
OUT
is the output voltage
of the forward converter and is 12-V for this design. V
F
is the forward voltage drop of the secondary rectifier diode
and is set to 1V. V
BOOST(min)
is the minimum input voltage to the forward converter. The level of this voltage is
determined by where the control device forces the dc-to-dc converter into undervoltage lockout (UVLO). The
UCC38500 control device is configured to drive the dc-to-dc power stage into UVLO at approximately 74% of
the nominal boost converters output voltage. V
BOOST(min)
for this design is approximately 285 V. D
MAX
is 0.44
and is the guaranteed maximum duty cycle of the forward converter. For this design example the calculated
turns ratio is approximately 0.101.
Transformer Turns +
V
OUT
) V
F
V
BOOST(min)
D
MAX
+
N
S
N
P
output inductor
The following equations can be used to calculate the inductor required for this design example. First, the
minimum duty cycle D
MIN
, which occurs at the maximum boost voltage, needs to be calculated. The maximum
boost voltage is limited by the OVP trip point, which is set to approximately 425 V. For this design D
MIN
is
approximately 31%. The output inductor ripple current (∆I
L
) for this design is given at 30% of the maximum load
current. Next calculate the output inductor (L), where the switching frequency (f
S
) is 100 kHz. The calculated
output inductor for this design is approximately 38 µH.
D
MIN
+
V
OUT
) V
F
V
BOOST(max)
N
P
N
S
DI
L
+
P
OUT
0.3
V
OUT
L +
ǒ
V
OUT
) V
F
Ǔ ǒ
1 * D
MIN
Ǔ
DI
L
f
S
(29)
(30)
(31)
(32)