13
LT1425
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
degrades load regulation (at least before load compensa-
tion is employed).
Bifilar Winding
A bifilar or similar winding technique is a good way to
minimize troublesome leakage inductances. However,
remember that this will increase primary-to-secondary
capacitance and limit the primary-to-secondary break-
down voltage, so bifilar winding is not always practical.
Finally, the LTC Applications group is available to assist
in the choice and/or design of the transformer. Happy
Winding!
OUTPUT VOLTAGE ERROR SOURCES
Conventional nonisolated switching power supply ICs
typically have only two substantial sources of output
voltage error—the internal or external resistor divider
network that connects to V
OUT
and the internal IC refer-
ence. The LT1425, which senses the output voltage in both
a dynamic and an isolated manner, exhibits additional
potential error sources to contend with. Some of these
errors are proportional to output voltage, others are fixed
in an absolute millivolt sense. Here is a list of possible
error sources and their effective contribution:
Internal Voltage Reference
The internal bandgap voltage reference is, of course,
imperfect. Its error, both at 25
°
C and over temperature is
already included in the specifications for Reference
Current.
User Programming Resistors
Output voltage is controlled by the ratio of R
FB
to R
REF
.
Both are user supplied external resistors. To the extent
that the resistor ratio differs from the ideal value, the
output voltage will be proportionally affected.
Schottky Diode Drop
The LT1425 senses the output voltage from the trans-
former primary side during the flyback portion of the cycle.
This sensed voltage therefore includes the forward drop,
V
F
, of the rectifier (usually a Schottky diode). The nominal
signal! It then reverts to a potentially stable state whereby
the top of the leakage spike is the control point, and the
trailing edge of the leakage spike triggers the collapse
detect circuitry. This will typically reduce the output volt-
age abruptly to a fraction, perhaps between one-third to
two-thirds of its correct value. If load current is reduced
sufficiently, the system will snap back to normal opera-
tion. When using transformers with considerable leakage
inductance, it is important to exercise this worst-case
check for potential bistability:
1. Operate the prototype supply at maximum expected
load current.
2. Temporarily short circuit the output.
3. Observe that normal operation is restored.
If the output voltage is found to hang up at an abnormally
low value, the system has a problem. This will usually be
evident by simultaneously monitoring the V
SW
waveform
on an oscilloscope to observe leakage spike behavior
firsthand. A final note, the susceptibility of the system to
bistable behavior is somewhat a function of the load I/V
characteristics. A load with resistive, i.e., I = V/R behavior
is the most susceptible to bistability. Loads which exhibit
“CMOSsy”, i.e., I = V
2
/R behavior are less susceptible.
Secondary Leakage Inductance
In addition to the previously described effects of leakage
inductance in general, leakage inductance on the second-
ary in particular exhibits an additional phenomenon. It
forms an inductive divider on the transformer secondary,
that reduces the size of the primary-referred flyback pulse
used for feedback. This will increase the output voltage
target by a similar percentage. Note that unlike leakage
spike behavior, this phenomenon is load independent. To
the extent that the secondary leakage inductance is a
constant percentage of mutual inductance (over manufac-
turing variations), this can be accommodated by adjusting
the R
FB
/R
REF
resistor ratio.
Winding Resistance Effects
Resistance in either the primary or secondary will act to
reduce overall efficiency (P
OUT
/P
IN
). Resistance in the
secondary increases effective output impedance which
