Datasheet

LT3825
13
3825fe
APPLICATIONS INFORMATION
Transformer Design
Transformer design/specification is the most critical part
of a successful application of the LT3825. The following
sections provide basic information about designing the
transformer and potential trade-offs.
If you need help, the LTC Applications group is available
to assist in the choice and/or design of the transformer.
Turns Ratios
The design of the transformer starts with determining duty
cycle (DC). DC
impacts the current and voltage stress on
the power switches, input and output capacitor RMS cur-
rents and transformer utilization (size vs power).
The ideal turns ratio is:
N
IDEAL
=
V
OUT
V
IN
•
1– DC
DC
Avoid extreme duty cycles as they, in general, increase
current stresses. A reasonable target for duty cycle is 50%
at nominal input voltage.
For instance, if we wanted a 48V to 5V converter at 50%
DC then:
N
IDEAL
=
5
48
•
1– 0.5
0.5
=
1
9.6
In general, better performance is obtained with a lower
turns ratio. A DC of 45.5% yields a 1:8 ratio.
Note the use of the external feedback resistive divider
ratio to set output voltage provides the user additional
freedom in selecting a suitable transformer turns ratio.
Turns ratios that are the simple ratios of small integers;
e.g., 1:1, 2:1, 3:2 help facilitate transformer construction
and improve performance.
When building a supply with
multiple outputs derived
through a multiple winding transformer, lower duty cycle
can improve cross regulation by keeping the synchronous
rectifier on longer, and thus, keep secondary windings
coupled longer.
For a multiple output transformer, the turns ratio between
output windings is critical and affects the accuracy of the
voltages. The ratio between two output voltages is set with
the formula V
OUT2
= V
OUT1
• N21 where N21 is the turns
ratio between the two windings. Also keep the secondary
MOSFET R
DS(ON)
small to improve cross regulation.
The feedback winding usually provides both the feedback
voltage and power for the LT3825. So set the turns ratio
between the output and feedback winding to provide a
rectified voltage that under worst-case conditions is greater
than the 11V maximum V
CC
turn-off voltage.
N
SF
>
V
OUT
11+ V
F
For our example: N
SF
>
5
11+ 0.7
=
1
2.34
We will choose
1
3
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a spike after the primary-side switch
turn-off. This is increasingly prominent at higher load
currents, where more stored energy is dissipated. Higher
flyback voltage may break down the MOSFET switch if it
has too low a BV
DSS
rating.
One solution to reducing this spike is to use a snubber
circuit to suppress the
voltage excursion. However, sup-
pressing the voltage extends the flyback pulse width. If
the flyback pulse extends beyond the enable delay time,
output voltage regulation is affected. The feedback system
has a deliberately limited input range, roughly ±50mV re-
ferred to the FB node. This rejects higher voltage leakage
spikes because once a leakage spike is several volts in
amplitude, a further increase in amplitude has little
effect
on the feedback system.
Therefore, it is advisable to arrange the snubber circuit to
clamp at as high a voltage as possible, observing MOSFET
breakdown, such that leakage spike duration is as short as
possible. Application Note 19 provides a good reference
on snubber design.