LT3825
22
3525fe
Output Voltage Error Sources
The LT3825’s feedback sensing introduces additional
sources of errors. The following is a summary list.
The internal bandgap voltage reference sets the reference
voltage for the feedback amplifier. The specifications detail
its variation.
The external feedback resistive divider ratio proportional
directly affects regulated voltage. Use 1% components.
Leakage inductance on the transformer secondary reduces
the effective secondary-to-feedback winding turns ratio
(N
S
/N
F
) from its ideal value. This increases the output
voltage target by a similar percentage. Since secondary
leakage inductance is constant from part to part (with a
tolerance) adjust the feedback resistor ratio to compensate.
The transformer secondary current flows through the
impedances of the winding resistance, synchronous MOS-
FET R
DS(ON)
and output capacitor ESR. The DC equivalent
current for these errors is higher
than the load current
because conduction occurs only during the converter’s
“off” time. So divide the load current by (1 – DC).
If the output load current is relatively constant, the feedback
resistive divider is used to compensate for these losses.
Otherwise, use the LT3825 load compensation circuitry
(see Load Compensation).
If multiple output windings are used, the flyback winding
will have a signal that represents
an amalgamation of all
these windings impedances. Take care that you examine
worst-case loading conditions when tweaking the voltages.
Power MOSFET Selection
The power MOSFETs are selected primarily on the criteria of
on- resistance, R
DS(ON)
, input capacitance, drain-to-source
breakdown voltage (BV
DSS
), maximum gate voltage (V
GS
)
and maximum drain current (I
D(MAX)
).
For the primary-side power MOSFET, the peak current is
:
I
PK(PRI)
=
P
IN
V
IN(MIN)
• DC
MAX
• 1+
X
MIN
2
where X
MIN
is peak-to-peak current ratio as defined earlier.
For each secondary-side power MOSFET, the peak cur-
rent is:
I
PK(SEC)
=
I
OUT
1– DC
MAX
• 1+
X
MIN
2
Select a primary-side power MOSFET with a BV
DSS
greater
than:
BV
DSS
≥I
PK
L
LKG
C
P
+ V
IN(MAX)
+
V
OUT(MAX)
N
SP
where N
SP
reflects the turns ratio of that secondary-
to-primary winding. L
LKG
is the primary-side leakage
inductance and C
P
is the primary-side capacitance (mostly
from the C
OSS
of the primary-side power MOSFET). A
snubber may be added to reduce the leakage inductance
as discussed earlier.
For each secondary-side power MOSFET, the BV
DSS
should
be greater than:
BV
DSS
≥ V
OUT
+ V
IN(MAX)
• N
SP
Choose the primary-side MOSFET R
DS(ON)
at the nominal
gate drive voltage (7.5V). The secondary side MOSFET gate
drive voltage depends on the gate drive method.
Primary-side power MOSFET RMS current is given by:
I
RMS(PRI)
=
IN
V
IN(MIN)
DC
MAX
For each secondary-side power MOSFET RMS current is
given by:
I
RMS(SEC)
=
OUT
1– DC
MAX
Calculate MOSFET power dissipation next. Because the
primary-side power MOSFET operates at high V
DS
, a transi-
tion power loss term is included for accuracy. C
MILLER
is
the most critical parameter in determining the transition
loss, but is not directly specified on the data sheets.
APPLICATIONS INFORMATION
