LT3575
16
3575f
APPLICATIONS INFORMATION
The transformer turns ratio is selected such that the
converter has adequate current capability and a switch
stress below 50V. Table 6 shows the switch voltage stress
and output current capability at different transformer
turns ratio.
Table 6. Switch Voltage Stress and Output Current Capability vs
Turns-Ratio
N
V
SW(MAX)
AT V
IN(MAX)
(V)
I
OUT(MAX)
AT V
IN(MIN)
(A)
DUTY CYCLE
(%)
1:1 33.5 1.26 16~22
2:1 39 2.07 28~35
3:1 44.5 2.63 37~45
4:1 50 3.05 44~52
BIAS winding turns ratio is selected to program the BIAS
voltage to 3V~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 LT3575 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 400mA to help create this off-time. This defi nes
the minimum required inductance as defi ned as:
L
NV 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
tt
I
V
L
I
NV V
L
SW
ON OFF
PK
IN
PK
PS OUT F
=
+
=
+
+
11
()
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)
15 174 205
30 87 103
60 44 51
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 200kHz at
full load. The 750311458 from Würth Elektronik is chosen
as the fl yback 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 refl ection of input voltage to the
secondary side. The average diode current is the load
current.
VV
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.
ΔV
LI
CV
MAX
PK
OUT
=
2
2
4. Select the snubber circuit to clamp the switch
voltage spike.
A fl yback 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
