10
LT3420/LT3420-1
3420fb
COMPONENT SELECTION
Choosing the Right Transformer
The flyback transformer plays a key role in any LT3420/
LT3420-1 application. A poorly designed transformer can
result in inefficient operation. Linear Technology Corpora-
tion has worked with a number of transformer manufactur-
ers to develop specific transformers for use with the
LT3420/LT3420-1. These predesigned transformers are
sufficient for a large majority of the applications that may
be encountered. In some cases, the reader may choose to
design his own transformer or may simply be curious about
the issues involved in designing the transformer. The fol-
lowing is a brief discussion of the issues relating to trans-
former design.
Transformer Turns Ratio
The turns ratio for the transformer, N, should be high
enough so that the absolute maximum voltage rating for
the NPN power switch is not exceeded. When the power
switch turns off, the voltage on the collector of the switch
(SW Pin) will “fly” up to the output voltage divided by N
plus the battery voltage (neglecting the voltage drop
across the rectifying diodes). This voltage should not
exceed the 38V (LT3420) or 50V (LT3420-1) breakdown
rating of the power switch.
Choose the minimum N by the following formula.
N
V
V
LT
N
V
V
LT
MIN
OUT
BAT
MIN
OUT
BAT
≥
≥−
38
3420
50
3420 1
–
()
–
()
For an LT3420 design, a 5V battery voltage and a 330V
output results in a N
MIN
of 10 so a turns ratio of 10 or greater
should be used.
Transformer Primary Inductance
A flyback transformer needs to store substantial amounts
of energy in the core during each switching cycle. The
transformer, therefore, will generally require an air gap.
The use of an air gap in the core makes the energy storage
ability, or inductance, much more stable with temperature
and variations in the core material. Most core manufactur-
ers will supply standard sizes of air gaps with a given type
of core, resulting in different A
L
values. A
L
is the induc-
tance of a particular core per square turns of winding. To
get a certain inductance, simply divide the desired induc-
tance by the A
L
value and take the square root of the result
to find the number of turns needed on the primary of the
transformer.
The LT3420/LT3420-1 detect the output voltage via the
flyback pulse on the SW pin. Since this can only occur
while the power switch is off, an important criteria is that
the value of the primary inductance of the transformer be
larger than a certain minimum value. The switch off time
should be 500ns or larger for the LT3420 and 350ns or
larger for the LT3420-1. The minimum inductance can be
calculated with the following formula:
L
V
NN
LT
L
V
NN
LT
PRI
OUT
PRI
OUT
≥
≥−
500 10
14 004
3420
350 10
10 0015
3420 1
9
9
••
•( . – . )
()
••
•(.–. )
()
–
–
V
OUT
: Target Output Voltage
N: Transformer Turns Ratio
Transformer Leakage Inductance
The leakage inductance of the transformer must be care-
fully minimized for both proper and efficient operation of
the part. The DC voltage rating of the SW pin on the LT3420
is 38V while on the LT3420-1 it is 50V. These ratings are
for DC blocking voltages only and additional precautions
APPLICATIO S I FOR ATIO
WUUU
