LT3574
16
3574f
applications inForMation
BIAS winding turns ratio is selected to program the BIAS
voltage to 3~5V. The BIAS voltage shall not exceed the
input voltage.
The turns ratio is then selected as primary: secondary:
BIAS = 3:1:1.
2. Select the transformer primary inductance for target
switching frequency.
The LT3574 requires a minimum amount of time to sample
the output voltage during the off-time. This off-time,
t
OFF(MIN)
, shall be greater than 350ns over all operating
conditions. The converter also has a minimum current limit,
I
MIN
, of 175mA to help create this off-time. This defines
the minimum required inductance as defined as:
L
N V V
I
t
MIN
OUT F
MIN
OFF MIN
=
+( )
•
( )
The transformer primary inductance also affects the
switching frequency which is related to the output ripple. If
above the minimum inductance, the transformer’s primary
inductance may be selected for a target switching frequency
range in order to minimize the output ripple.
The following equation estimates the switching frequency.
f
t t
I
V
L
I
N V V
L
SW
ON OFF
PK
IN
PK
PS OUT F
=
+
=
+
+
1 1
( )
Table 7. Switching Frequency at Different Primary Inductance
at I
PK
L (µH)
f
SW
AT V
IN(MIN)
(kHz)
f
SW
AT V
IN(MAX)
(kHz)
30 317 373
60 159 187
120 79 93
Note: The switching frequency is calculated at maximum output.
In this design example, the minimum primary inductance is
used to achieve a nominal switching frequency of 350kHz
at full load. The PA2627NL from Pulse Engineering is
chosen as the flyback transformer.
Given the turns ratio and primary inductance, a custom-
ized transformer can be designed by magnetic component
manufacturer or a multi-winding transformer such as a
Coiltronics Versa-Pac may be used.
3. Select the output diodes and output capacitor.
The output diode voltage stress V
D
is the summation of
the output voltage and reflection of input voltage to the
secondary side. The average diode current is the load
current.
The output capacitor should be chosen to minimize the
output voltage ripple while considering the increase in
size and cost of a larger capacitor. The following equation
calculates the output voltage ripple.
DV
LI
CV
MAX
PK
OUT
=
2
2
4. Select the snubber circuit to clamp the switch voltage
spike.
A flyback converter generates a voltage spike during switch
turn-off due to the leakage inductance of the transformer.
In order to clamp the voltage spike below the maximum
rating of the switch, a snubber circuit is used. There are
many types of snubber circuits, for example R-C, R-C-D and
Zener
clamps. Among them, RCD is widely used. Figure 9
shows the RCD snubber in a flyback converter.
A typical switch node waveform is shown in Figure 10.
