LT3575
12
3575f
APPLICATIONS INFORMATION
Turns Ratio
Note that when using an R
FB
/R
REF
resistor ratio to set
output voltage, the user has relative freedom in selecting
a transformer turns ratio to suit a given application.In
contrast, simpler ratios of small integers, e.g., 1:1, 2:1,
3:2, etc., can be employed to provide more freedom in
setting total turns and mutual inductance.
Typically, the transformer turns ratio is chosen to maximize
available output power. For low output voltages (3.3V or 5V),
a N:1 turns ratio can be used with multiple primary windings
relative to the secondary to maximize the transformer’s
current gain (and output power). However, remember that
the SW pin sees a voltage that is equal to the maximum
input supply voltage plus the output voltage multiplied by
the turns ratio. This quantity needs to remain below the
ABS MAX rating of the SW pin to prevent breakdown of
the internal power switch. Together these conditions place
an upper limit on the turns ratio, N, for a given application.
Choose a turns ratio low enough to ensure:
N
VV
VV
IN MAX
OUT F
<
+
50 –
()
For larger N:1 values, a transformer with a larger physical
size is needed to deliver additional current and provide a
large enough inductance value to ensure that the off-time is
long enough to accurately measure the output voltage.
For lower output power levels, a 1:1 or 1:N transformer can
be chosen for the absolute smallest transformer size. A 1:
N transformer will minimize the magnetizing inductance
(and minimize size), but will also limit the available output
power. A higher 1:N turns ratio makes it possible to have
very high output voltages without exceeding the breakdown
voltage of the internal power switch.
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to appear at the primary
after the output switch turns off. This spike is increasingly
prominent at higher load currents where more stored energy
must be dissipated. In most cases, a snubber circuit will
be required to avoid overvoltage breakdown at the output
switch node. Transformer leakage inductance should be
minimized.
An RCD (resistor capacitor diode) clamp, shown in
Figure 4, is required for most designs to prevent the
leakage inductance spike from exceeding the breakdown
voltage of the power device. The fl yback waveform is
depicted in Figure 5. In most applications, there will be a
very fast voltage spike caused by a slow clamp diode that
may not exceed 60V. Once the diode clamps, the leakage
inductance current is absorbed by the clamp capacitor.
This period should not last longer than 150ns so as not to
interfere with the output regulation, and the voltage during
this clamp period must not exceed 55V. The clamp diode
turns off after the leakage inductance energy is absorbed
and the switch voltage is then equal to:
V
SW(MAX)
= V
IN(MAX)
+ N(V
OUT
+ V
F
)
This voltage must not exceed 50V. This same equation
also determines the maximum turns ratio.
When choosing the snubber network diode, careful
attention must be paid to maximum voltage seen by the
SW pin. Schottky diodes are typically the best choice to
be used in the snubber, but some PN diodes can be used
if they turn on fast enough to limit the leakage inductance
spike. The leakage spike must always be kept below 60V.
Figures 6 and 7 show the SW pin waveform for a 24V
IN
,
5V
OUT
application at a 1A load current. Notice that the
leakage spike is very high (more than 65V) with the “bad”
diode, while the “good” diode effectively limits the spike
to less than 55V.
An alternative to RC network is a Zener diode clamping.
The Zener diode must be able to handle the voltage rating
and power dissipating during the switch turn-off time.
Application Note 19 has more details on Zener diode
snubber design for fl yback converters.
For applications with SW voltage exceeding 50V,
Zener diode clamp must be considered. At higher operating
primary current, the leakage inductance spike can
potentially exceed the breakdown voltage of the internal
power switch.
