lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 16

• Manufacturer Part Number: lt3573
• Description: Isolated Flyback Converter Without An Opto-coupler
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3573
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
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
The LT3573 requires a minimum amount of time to sample
the output voltage during the off-time. This off-time,
t
conditions. The converter also has a minimum current
limit, L
defi nes the minimum required inductance as defi ned as:
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 fre-
quency.
16
OFF(MIN)
1:1
2:1
3:1
4:1
N
switching frequency.
L
f
SW
MIN
MIN
V
, shall be greater than 350ns over all operating
SW(MAX)
t
, of 250mA to help create this off-time. This
N V
ON
(
33.5
44.5
(V)
OUT
39
50
1
AT V
I
MIN
t
OFF
IN(MAX)
V
F
)
V
•
I
IN
PK
I
OUT(MAX)
t
OFF MIN
L
(
0.53
0.88
1.12
1.30
(A)
N
AT V
PS
)
IN(MIN)
(
1
V
OUT
I
PK
DUTY CYCLE
V
16~22
28~35
37~45
44~52
F
(%)
)
L
Table 7.Switching Frequency at Different Primary Inductance
at I
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 275kHz
at full load. The PA2454NL from Pulse Engineering 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
the output voltage and refl ection 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.
4. Select the snubber circuit to clamp the switch
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
PK
voltage spike.
V
D
V
MAX
L (μH)
100
25
50
V
OUT
2
LI
CV
V
2
N
OUT
IN
PK
f
SW
AT V
(kHz)
236
121
61
IN(MIN)
D
is the summation of
f
SW
AT V
(kHz)
305
157
80
IN(MAX)
3573f