Datasheet

LT3748
15
3748fb
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applications inForMation
Saturation Current
As discussed earlier in the Maximum Output Power sec-
tion, because
the core of the transformer is being used for
energy storage in a flyback, the current in the transformer
windings should not exceed their rated saturation current
as energy injected once the core is saturated will not be
transferred to the secondary and will instead be dissipated
in the core. Information on saturation current should be
provided by the transformer manufacturers and Table 1
lists the saturation current of the transformers designed
for use with the LT3748.
Leakage Inductance and Snubbers
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to appear at the
primary after the MOSFET switch turns off. This spike is
increasingly prominent at higher load currents where more
stored energy must be dissipated. Transformer leakage
inductance should be minimized.
In most cases, proper selection of the external MOSFET
and a well designed transformer will eliminate the need for
snubber circuitry, but in some cases the optimal MOSFET
may require protection from this leakage spike. An RC
(resistor capacitor) snubber may be sufficient in applica
-
tions where the MOSFET has significant margin beyond
the predicted DC drain voltage applied in flyback while a
clamp
using an RCD (resistor capacitor diode) or a Zener
might be a better option when using a MOSFET with very
little margin for leakage inductance spiking.
The recommended approach for designing an RC snubber
is to measure the period of the ringing at the MOSFET
drain when the MOSFET turns off without the snubber
and then add capacitance—starting with something in
the range of 100pF—until the period of the ringing is 1.5
to 2 times longer. The change in period will determine
the value of the parasitic capacitance, from which the
parasitic inductance can be determined from the initial
period, as well. Similarly, initial values can be estimating
using stated switch capacitance and transformer leakage
inductance. Once the value of the drain node capacitance
and inductance is known, a series resistor can be added to
the snubber capacitance to dissipate power and critically
dampen the ringing. The equation for deriving the optimal
series resistance using the observed periods (t
PERIOD
, and
t
PERIOD(SNUBBED)
) and snubber capacitance (C
SNUBBER
) is
below, and the resultant waveforms are shown in Figure 6.
C
PAR
=
C
SNUBBER
t
PERIOD(SNUBBED)
t
PERIOD






2
– 1
L
PAR
=
t
PERIOD
2
C
PAR
• 4π
2
R
SNUBBER
=
L
PAR
C
PAR
TIME (µs)
0
0
V
DRAIN
(V)
10
30
40
50
0.20
90
3748 F06
20
0.10
0.05
0.25
0.15 0.30
60
70
80
NO SNUBBER
WITH SNUBBER
CAPACITOR
WITH RESISTOR
AND CAPACITOR
Figure 6. Observed Waveforms at MOSFET Drain when
Iteratively Implementing an RC Snubber
Note that energy absorbed by a snubber will be converted
to heat and will not be delivered to the load. In high volt-
age or
high current applications, the snubber may need to
be
sized for thermal dissipation. To determine the power
dissipated in the snubber resistor from capacitive losses,
measure the drain voltage immediately before the MOSFET
turns on and use the following equation relating that volt
-
age and
the MOSFET switching frequency to determine
the expected power dissipation:
P
SNUBBER
= f
SW
• C
SNUBBER
• V
DRAIN
2
/2
Decreasing the value of the capacitor will reduce the dis-
sipated power
in the snubber at the expense of increased
peak voltage on the MOSFET drain, while increasing the
value of the capacitance will decrease the overshoot.
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