Datasheet

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.
V V
V
N
D OUT
IN
= +
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.