Datasheet

LT3574
13
3574f
applications inForMation
Figure 5. Maximum Voltages for SW Pin Flyback Waveform
Figure 4. RCD Clamp
Figure 6. Good Snubber Diode Limits SW Pin Voltage Figure 7. Bad Snubber Diode Does Not Limit SW Pin Voltage
< 50V
< 55V
< 60V
V
SW
t
OFF
> 350ns
TIME
t
SP
< 150ns
3574 F05
100ns/DIV
10V/DIV
3574 F06
100ns/DIV
10V/DIV
3574 F07
3574 F04
L
S
D
•
R
C
•
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 effectively 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 manufacturing 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 reduce
overall efficiency (P
OUT
/P
IN
). Good output voltage regula-
tion will be maintained independent of winding resistance
due to the boundary mode operation of the LT3574.
Bifilar Winding
A bifilar, or similar winding technique, is a good way to
minimize troublesome leakage inductances. However, re-
member that this will also increase primary-to-secondary
capacitance and limit the primary-to-secondary breakdown
voltage, so, bifilar winding is not always practical. The
Linear Technology applications group is available and
extremely qualified to assist in the selection and/or design
of the transformer.